Content uploaded by Ukpong Eyo
Author content
All content in this area was uploaded by Ukpong Eyo on Jan 29, 2018
Content may be subject to copyright.
REVIEW ARTICLE
Microglia–Neuron Communication
in Epilepsy
Ukpong B. Eyo, Madhuvika Murugan, and Long-Jun Wu
Epilepsy has remained a significant social concern and financial burden globally. Current therapeutic strategies are based pri-
marily on neurocentric mechanisms that have not proven successful in at least a third of patients, raising the need for novel
alternative and complementary approaches. Recent evidence implicates glial cells and neuroinflammation in the pathogenesis
of epilepsy with the promise of targeting these cells to complement existing strategies. Specifically, microglial involvement,
as a major inflammatory cell in the epileptic brain, has been poorly studied. In this review, we highlight microglial reaction to
experimental seizures, discuss microglial control of neuronal activities, and propose the functions of microglia during acute
epileptic phenotypes, delayed neurodegeneration, and aberrant neurogenesis. Future research that would help fill in the cur-
rent gaps in our knowledge includes epilepsy-induced alterations in basic microglial functions, neuro–microglial interactions
during chronic epilepsy, and microglial contribution to developmental seizures. Studying the role of microglia in epilepsy
could inform therapies to better alleviate the disease.
GLIA 2016;00:000–000.
Key words: microglia, epilepsy, seizures, kainic acid, pilocarpine
Epilepsy: A Global Neurological Disorder
Epilepsy is a term used to describe a spectrum of neurologi-
cal disorders in which there is abnormal hypersynchrony
of neuronal activity (Fisher et al., 2005). The disorder affects
between 50 and 65 million people globally (Thurman et al.,
2011), including both children and the elderly with a myriad
of etiologies of known (such as genetic risk factors) and
unknown factors. Indeed epileptic seizures occur in other
neurological conditions being comorbid with stroke and trau-
matic brain injury (Temkin, 2009). Moreover, because of the
reorganization of neural circuits and activities in the brain in
response to seizures, patients frequently experience cognitive,
psychiatric and mood disorders (Jensen, 2011). Furthermore,
patients with epilepsy has been reported to exhibit increased
mortality of 2–10 years earlier than the general population
(Gaitatzis et al., 2004). Thus, epilepsy is a significant health
concern for the human population. There is, therefore, a crit-
ical need for the development of appropriate strategies to
ameliorate the progression and/or limit the detrimental conse-
quences of the disease.
Regarding the mechanisms underlying epilepsy, it is
generally assumed that because of the resulting hyperactivity
in neurons during the condition, there is an imbalance
wherein excitatory neurotransmission predominantly through
glutamatergic signaling is increased and inhibitory neurotrans-
mission predominantly through GABA-ergic signaling is
decreased (Dalby and Mody, 2001; Sharma et al., 2007).
Despite the widespread acknowledgement of these mecha-
nisms, therapeutic antiepileptic strategies targeting these
mechanisms have proved insufficient in a significant propor-
tion of patients (Kwan and Brodie, 2006). In recent years,
however, a role for inflammation and inflammatory mediators
has become increasingly appreciated and is a focus of current
research (Choi and Koh, 2008; Vezzani et al., 2013).
Our current understanding of epileptic mechanisms
have been especially informed by studies from human patients
with epilepsy (including examinations of excised tissues and
post-mortem studies) and experimental epilepsy models devel-
oped primarily in rodents. Although several such epilepsy
models have been developed, the most ubiquitous models
View this article online at wileyonlinelibrary.com. DOI: 10.1002/glia.23006
Published online Month 00, 2016 in Wiley Online Library (wileyonlinelibrary.com). Received Mar 14, 2016, Accepted for publication Apr 28, 2016.
Address correspondence to Dr. Long-Jun Wu, Department of Cell Biology and Neuroscience, Rutgers University, 604 Allison Road, Piscataway,
NJ 08854, USA. E-mail: lwu@dls.rutgers.edu
From the Department of Cell Biology and Neuroscience, Rutgers University, Piscataway, New Jersey
Conflict of interest: The authors declare no competing financial interests.
V
C2016 Wiley Periodicals, Inc. 1
include chemically induced models using kainic acid or pilo-
carpine, which is sometimes coupled with lithium chloride
pre-treatment (Kandratavicius et al., 2014; Leite et al., 2002).
Both chemoconvulsants induce hippocampal sclerosis, a fea-
ture that is prominent in clinical epilepsy. These models are
now recognized to mimic salient histopathological and clinical
features of human mesial temporal lobe epilepsy (MTLE), the
most common form of epilepsy in adults. Kainic acid is a
glutamate analog that preferentially activates kainate gluta-
mate receptor subtypes (Ben-Ari and Cossart, 2000) while
pilocarpine is a muscarinic receptor agonist (Vezzani, 2009).
Various kainic acid models of seizures are employed including
intraperitoneal or intracerebral (such as cerebroventricular,
striatal, hippocampal, and amygdala) injections. Kainic acid
in these different brain regions leads to hypersynchronized
excitatory neuronal activity which may persist, if prolonged,
may result in neuronal death. Although a complex interplay of
kainate and non-kainate glutamate receptors have been impli-
cated in the mechanism of kainic acid induced seizures, the
CA3 region of the hippocampus is recognized to be extremely
susceptible due to a high density of specific kainate receptors
in this region (Levesque and Avoli, 2013). The pilocarpine
model is usually induced by intraperitoneal delivery though it
is sometimes coupled with lithium pre-treatment to lower the
threshold for seizure-induction. In this model, muscarinic ace-
tylcholine receptor receptor 1 (M
1
)wasshowntobeimportant
for development of seizures, since mice lacking this receptor
failed to develop seizures following pilocarpine injection (Ham-
ilton et al., 1997). Downstream of M
1
receptor activation, the
homeostatic balance of neuronal excitation–inhibition is tipped
toward increased excitability presumably at least in part by
increase in glutamate release and sustained NMDA receptor
activation (Priel and Albuquerque, 2002; Smolders et al.,
1997). Although beyond the scope of this review, further
details of the mechanics and precise pathological features of
these models can be found in several excellent reviews of the
kainic acid and pilocarpine models of experimental seizures/
epilepsy (Curia et al., 2008; Levesque and Avoli, 2013; Lev-
esque et al., 2016; Turski et al., 1989).
In the following pages, we discuss the literature highlight-
ing (i) microglial response to epilepsy at both morphological
and molecular levels, (ii) microglial regulation of neuronal
activities during epilepsy, (iii) microglial contributions to acute
seizure phenotypes and seizure-induced neurodegeneration, and
(iv) conclude with suggesting areas for future research to
understand the role of microglia in epilepsy.
Microglial Morphological and Molecular
Activation in Response to Seizures
Microglia are highly adaptable glial cells of the central nerv-
ous stem (CNS) that are now recognized to play important
roles in the healthy CNS, as well as, during various CNS
pathologies (Morris et al., 2013; Nayak et al., 2014; Ransoh-
off and Perry, 2009; Tremblay et al., 2011; Zhuo et al.,
2011). Microglia constitutively scan the brain and spinal cord
(Davalos et al., 2005; Dibaj et al., 2010; Nimmerjahn et al.,
2005) where they interact physically with neurons (Baalman
et al., 2015; Li et al., 2012; Tremblay et al., 2010; Wake
et al., 2009) and modulate neurotransmission (Hoshiko et al.,
2012; Ji et al., 2013; Li et al., 2012; Paolicelli et al., 2011;
Pascual et al., 2012; Zhang et al., 2014a). During develop-
ment, microglia prune synapses (Paolicelli et al., 2011; Scha-
fer et al., 2012), participate in the clearance of apoptotic
neurons (Ahlers et al., 2015; Marin-Teva et al., 2004; Sierra
et al., 2010), regulate neurogenesis as well as oligodendrogen-
esis (Shigemoto-Mogami et al., 2014), promote neural pre-
cursor cell development (Arno et al., 2014; Cunningham
et al., 2013), and enhance neuronal survival (Ueno et al.,
2013). They are thus critical in the early wiring of the CNS
(Squarzoni et al., 2014; Zhan et al., 2014). In the mature
brain, although microglia do not directly respond to neuronal
stimulations (Wu and Zhuo, 2008), microglia are important
in the processes required for learning and memory (Parkhurst
et al., 2013), synaptic plasticity, and general cognitive func-
tion (Rogers et al., 2011; Sipe et al., 2016). During disease
conditions, they undergo activation where their functions are
hotly debated as neurotoxic or neuroprotective agents depend-
ing on the precise timing and disease context (Block et al.,
2007; Hanisch and Kettenmann, 2007; Morris et al., 2013;
Wu, 2014; Wu et al., 2012). Thus, microglia are very relevant
to both nervous system physiological function and pathophys-
iological processes. However, microglial roles in epilepsy have
been less studied. Therefore, in this section, we consider
some of the evidence for microglial activation in response to
seizures or epilepsy at both morphological and molecular lev-
els (Fig. 1).
Microglial Activation: Morphological Considerations
As with other neurological diseases, there are dramatic obser-
vations with microglia morphological activation in epilepsy.
Autopsy analysis from control individuals and human patients
with intractable seizures revealed a 3- and 11-fold increase in
microglial reactivity to major histocompatibility antigen in
the CA3 and CA1 regions of the epileptic hippocampus,
respectively (Beach et al., 1995). Similarly, increased micro-
glial immunoreactivity was observed in patients with focal
cortical dysplasia known to trigger epilepsy. Moreover, a cor-
relation was evident between the duration and frequency of
epilepsy and the degree of microglial activation as shown by
the immunohistochemical expression of HLA-DR, an MHC
class II cell surface receptor often used to identify activated
2 Volume 00, No. 00
microglia (Boer et al., 2006). These observations indicate that
persistent microglial reactivity is a clinical feature of epilepsy.
Consistent with human studies, experimentally induced
seizures in rodents has revealed marked morphological
changes of microglia. For example, during kainic acid-
induced seizures elicited by intracerebral drug delivery, there
is a delayed (up to 48 h) increase in hippocampal microglial
numbers following drug delivery which was presumed to
result from ensuing kainic acid-induced neuronal excitotoxic
injury in mice (Andersson et al., 1991). A more recent study
conducted in transgenic mice that express GFP selectively in
microglia corroborated these findings and microglial morpho-
logical activation (including shortening and thickening of
their processes concomitant with an enlargement of their
somata) was confirmed at 24–48 hours following intraperito-
neal delivery of kainic acid, a time point when neuronal
injury was detectable (Avignone et al., 2008). Together, these
studies indicated that microglial activation is delayed and may
result as a secondary reaction to neuronal injury following the
initial seizures.
Despite these observations, there are studies suggesting
that microglial reactivity following seizures occurs much ear-
lier than 24–48 h following kainic acid treatment. By 8 h
(but not 4 h), a time point at which overt neuronal injury is
absent, microglial activation as detected by lectin staining was
reported and suggested to proceed along propagation path-
ways of the hippocampal seizures (Taniwaki et al., 1996).
These results indicate that microglia may be activated, not
merely by neuronal degeneration, but by neuronal hyperactiv-
ity that precedes neuronal demise. Another study in rats
showed that cortical microglia exhibited a “bushy” appearance
with seemingly increased ramifications within 3 h of intraper-
itoneal drug delivery of kainic acid introducing the concept
of hyper-ramified microglia during acute seizures (Rappold
et al., 2006). In all these studies, microglial reactivity was
always reported only after the initiating seizures were
completed.
In our recent study, we investigated the acute effects of
kainic acid-induced seizures on microglial morphologies in
the CA1 region of the hippocampus (Eyo et al., 2014). Inter-
estingly, unlike the typical sequelae of microglial activation
consisting of a reduction in microglial process ramifications,
the acute microglial response to seizures caused an increase in
microglial process numbers within 40 min of kainic acid
treatment by both intraperitoneal and intracerebroventricular
delivery at a time when seizure manifestations were elaborate.
Furthermore, we showed that this early microglial response is
dependent on neuronal NMDA receptor activation. In addi-
tion, these observations were recapitulated in real time by
bath application of glutamate/NMDA to acute brain slices in
our study (Eyo et al., 2014) and independently reported by
MacVicar’s group (Dissing-Olesen et al., 2014). These results
therefore suggest that microglia respond rapidly (within 5–10
min) to acute neuronal hyperactivity triggered by activation
of NMDA glutamate receptors during seizures in vivo. The
mechanism involves the coupling of neuronal NMDA recep-
tor activation to neuronal ATP release via a pannexin 1-
independent pathway that remains to be determined. Released
ATP stimulates microglial P2Y12 receptors leading to process
extension/outgrowth (Dissing-Olesen et al., 2014; Eyo et al.,
2014).
Similar to these observations with seizures induced by
kainic acid, pilocarpine-induced seizures have been reported
to induce microglial morphological activation between 3 h
FIGURE 1: Schematic diagram depicting molecular and morphological changes of microglial activation following seizures. The molecular
consequences of seizures on microglial activation (above) include changes in the expression pattern of an array of microglial molecules
such as classical microglial markers, purinergic receptors, fractalkine receptor, and cytokines. The morphological consequences of seiz-
ures on microglial activation (below) include changes in microglial cell body size, process length, process numbers, and complexity of
branching. Please refer to section “Microglial Morphological and Molecular Activation in Response to Seizures” for references.
Eyo et al.: Microglia in Epilepsy
Month 2016 3
and 3 days in several brain regions following intraperitoneal
drug treatment (Borges et al., 2003; Kang et al., 2006; Rosell
et al., 2003). These studies also indicate that microglia are
hyper-ramified (thickened processes but increased branch
points) as early as 3 h after pilocarpine treatment. At later
time points following pilocarpine treatment, microglia exhib-
ited hypertrophic morphologies denoted as typical activation
(Jung et al., 2009; Shapiro et al., 2008). As with the kainic
acid model, microglial reactivity increased following pilocar-
pine exposure by 24 h. Taken together, both widely used
experimental models of epilepsy indicate that microglia
respond rapidly to experimental seizures in a manner that
does not merely result from neuronal demise or injury since
their activation represented by early morphological hyper-
ramification precedes neuronal death. In addition, microglial
reaction to seizures persists for days to weeks, but with differ-
ent morphological characteristics.
Although microglial surveillance is known to be critical
for microglial functions, the effect of seizures on microglial
surveillance have only been addressed by a few studies and
both were performed in excised brain slices. In the first study,
microglial chemotactic abilities were increased following
kainic acid seizures presumably as a result of upregulation of
P2Y12 receptor expression (Avignone et al., 2008). A follow-
up study confirmed this initial study but also showed that
kainic acid-induced seizures did not alter their basal surveil-
lance ability (Avignone et al., 2015). In a recent study, we
described a novel phenomenon by which microglia and neu-
rons interact during epileptiform activity induced by deple-
tion of extracellular calcium. We have referred to this
phenomenon as “microglial process convergence” and is char-
acterized by spontaneous focal targeting of microglial proc-
esses toward neuronal dendrites that seem to be independent
of metabolic astrocytic activity (Eyo et al., 2015). Although
we confirmed that the phenomena occurs both in excised and
intact brain tissue, whether the interaction occurs in the epi-
leptic brain, as well as, the functional consequence of these
interactions will need to be determined by future studies.
Resident microglia and peripheral monocytes/macro-
phages share most common molecular markers (Hickman
et al., 2013). Therefore, the increase in microglial–macro-
phage numbers following both kainic acid and pilocarpine
treatment may result from one or a combination of (i) migra-
tion/infiltration from other sites such as the blood [a phe-
nomena that does not occur in healthy brain (Ajami et al.,
2007, 2011)], (ii) proliferation of the resident pool, or (iii) de
novo generation of microglia from neural progenitor cells.
The de novo generation of microglia from nestin-positive pro-
genitors is a relatively new observation that has so far only
been described following widespread microglia depletion
(Elmore et al., 2014). Whether such a mechanism is relevant
during seizures has yet to be evidenced.
Regarding the possibility of infiltration, most studies have
failed to distinguish resident microglia and infiltrating cells
after seizures. Recent studies, however, suggest that peripheral
cells do enter the brain following both pilocarpine and kainic
acid-induced seizures in mice, as well as, in human epileptic
brain tissues without permanently taking up residence therein
(Bellavance et al., 2015; Longo et al., 2010; Ravizza et al.,
2008; Zattoni et al., 2011). These results indicate that mono-
cyte infiltration may at least in part account for the increased
microglia–macrophage numbers following epilepsy. Indeed, sei-
zure studies using chimeric PU.1
gfp
donor marrow cells trans-
planted into wildtype mice revealed an increase in engraftment
of donor cells as early as 6 h after kainic acid-induced seizures
suggesting infiltration of blood-derived monocytes (Bellavance
et al., 2015). A similar chimeric strategy has been employed
with bone marrow transplants from generic GFP mice and
peripheral cells were identified as early as 2 h after pilocarpine-
induced seizures (Longo et al., 2010). However, we have to
caution that experimental artifacts of BBB breakage from irra-
diation/bone-marrow transplantation may complicate the
observation of monocyte infiltration after epilepsy (Ajami
et al., 2007). In addition to monocyte infiltration, CD45
1
CD3
1
T-cells, but not CD45
1
B220
1
B-cells, were detected
in both the human and kainic acid-induced mouse epileptic
hippocampus that were not found in control tissues. These
cells were cytotoxic in nature being CD8
1
. Similarly, Gr-1
1
neutrophils, absent in the na€ıve brain, were found in the epi-
leptic brain (Zattoni et al., 2011). Together, these data suggest
that resident microglia undergo robust morphological activa-
tion following seizures as well as proliferation accompanied by
infiltration of circulating cells including monocytes, T cells,
and neutrophils.
Microglial Activation: Molecular Considerations
Although it is widely accepted that microglial activation
occurs following seizures, less is known about the specific
molecules of microglial activation other than classical activa-
tion markers like Iba-1, CD68, or CD11b. Currently, data is
available regarding the fractalkine receptor, purinergic recep-
tors, certain proteases and cytokine expression on microglia in
this experimental context. Investigation into the molecular
signatures for microglia in epilepsy will improve our under-
standing of not only microglial activation but also their
potential effector mechanisms in the pathogenesis of epilepsy.
Fractalkine is a chemotactic cytokine predominantly
expressed by neurons, whose receptor is selectively expressed
on microglia in the CNS (Cardona et al., 2006). Fractalkine
is upregulated in the serum and cerebrospinal fluid of epilep-
tic patients as well as in a lithium-pilocarpine rat model (Ali
4 Volume 00, No. 00
et al., 2015). Furthermore, a corresponding increase in frac-
talkine receptor expression is detected between 1 and 6 h and
begins to decline by 3 days following seizures (Ali et al.,
2015; Yeo et al., 2011). However, following intrastriatal
kainic acid treatment, fractalkine receptor expression
remained unchanged in microglia despite evident neuronal
loss (Hughes et al., 2002). Thus, the regulation and expres-
sion of the microglial fractalkine receptor needs to be further
clarified in these experimental conditions.
In addition to fractalkine signaling, purinergic signaling
is now known to be critical in epileptogenesis. An upregula-
tion of purinergic receptors have been confirmed on microglia
following experimental seizures. Rappold et al. (2006) first
reported an increased immunohistochemical expression of the
P2X7 receptor predominantly in microglia in rats following
kainic acid treatment. Similar results have been confirmed in
mice by quantitative PCR and functional electrophysiology
(Avignone et al., 2008). Like the P2X7 receptor, upregulated
microglial P2X4 expression has been confirmed in the mouse
hippocampus following seizures (Avignone et al., 2008;
Ulmann et al., 2013). In a similar manner, both P2Y6 and
P2Y12 mediated responses and mRNA are increased in hip-
pocampal microglia following kainic-acid induced seizures
(Avignone et al., 2008).
In addition to modulation of specific microglial cell sur-
face receptors, seizures also upregulated microglial cytokine
expression including TGFb(Morgan et al., 1993), IL-1b
(Eriksson et al., 2000; Vezzani et al., 1999), and TNF-a(Tur-
rin and Rivest, 2004). Finally, regarding microglial proteases,
cathepsin B, D, and S were increased following seizures (Aka-
hoshi et al., 2007; Banerjee et al., 2015). Together, these
studies highlight the dramatic upregulation of microglial
receptors, cytokines, and proteases following seizures.
Taking a cue from macrophage studies, microglial
researchers have identified microglial polarization during dis-
ease: the M1 (classical) and M2 (alternative) activation states
(Boche et al., 2013; Colton, 2009; Perry et al., 2010).
Although this M1/M2 microglia has been studied in several
brain diseases such as Alzheimer’s Disease (Tang and Le,
2016; Varnum and Ikezu, 2012), ischemia (Frieler et al.,
2011; Hu et al., 2012), multiple sclerosis (Mikita et al.,
2011; Peferoen et al., 2015; Vogel et al., 2013), and amyotro-
phic lateral sclerosis (Henkel et al., 2009; Liao et al., 2012),
less has been done to investigate this microglial polarization
during seizures/epilepsy. One recent study reported that M1
markers (IL-1b,TNFa, CD16, CD86,IL-6, IL-12, Fc recep-
tors 16, and CD86) were upregulated in both the pilocarpine
and kainic acid models (Benson et al., 2015). Interestingly,
there was a transition from M1 to M2 markers (Arg1, Ym1,
FIZZ-1, CD206, IL-4, and IL-10) after pilocarpine treatment
by 3 days of seizures (Benson et al., 2015). Here, the authors
suggested that the underlying mechanisms for this transition
may result from peripheral inflammation since pilocarpine
was administered systemically by the intraperitoneal route
while kainic acid was only administered directly to the hippo-
campus. This peripheral inflammation in the pilocarpine
model may in turn alter the brain environment and thus
microglial cells. Further work is required to ascertain the pre-
cise contributions and roles of the differently polarized micro-
glia to the progression of epilepsy. However, although this
M1/M2 polarization has been suggested for microglial pheno-
types, caution has to be employed with this type of simplistic
nomenclature as the validity characterizing microglia and
macrophages in this polarized manner has recently been ques-
tioned (Murray et al., 2014; Prinz et al., 2014).
Microglial Regulation of Neuronal Activities in
Epilepsy
Since seizures and epileptic phenotypes are rooted in aberra-
tions in neuronal function and microglia are homeostatic regu-
lators of the CNS (Hanisch and Kettenmann, 2007; Tremblay,
2011), microglial contributions to seizure phenotypes and con-
sequences can be expected. Indeed, several lines of evidence
have shown that microglia might be able to regulate neuronal
activities in the healthy and epileptic brain (Fig. 2).
Evidence for Microglial Regulation of Neuronal
Activities
Mounting evidence suggests delicate interactions between
microglia and neurons by which microglia can modulate neu-
ronal activities (Bechade et al., 2013; Eyo and Wu, 2013).
Evidence for this comes primarily from two approaches: (i)
genetic deletion of microglial-specific proteins and (ii) mecha-
nistic interrogation of neuronal activity by microglial
manipulations.
Evidence from the genetic approach has mainly been
provided for by fractalkine signaling. The fractalkine receptor
is exclusively expressed on microglial cells in the CNS paren-
chyma (Cardona et al., 2006). Since neurons express fractal-
kine, it provides an interesting signaling axis for
communication between microglia and neurons that has been
extensively investigated (Lauro et al., 2015; Limatola and
Ransohoff, 2014; Paolicelli et al., 2014). Interestingly, in
mice lacking this microglial receptor, there is a delayed func-
tional maturation of neuronal synapses and consequently syn-
aptic plasticity is perturbed. Deficiency of fractalkine
signaling was mechanistically linked to impairment in micro-
glial colonization of the CNS, the subsequent lack of develop-
mental pruning, as well as, perturbation of the developmental
switch from GluN2B to GluN2A NMDA subunits at the
synapse (Hoshiko et al., 2012; Paolicelli et al., 2011). Fur-
thermore, fractalkine receptor deficiency has also been linked
Eyo et al.: Microglia in Epilepsy
Month 2016 5
to disruptions in neurogenesis, neural connectivity, and long-
term potentiation (LTP, the cellular basis for learning and
memory), with corresponding defects in fear and motor learn-
ing (Rogers et al., 2011) as well as social behaviors (Zhan
et al., 2014). Similar to the fractalkine receptor, DAP12 is a
microglia-specific protein whose genetic disruption results in
enhanced LTP and thus synaptic plasticity. The basis for these
changes were suggested to occur as a result of alterations in
glutamate receptor content at synapses as well as through
microglial brain-derived neurotrophic factor (BDNF) (Roum-
ier et al., 2004). Indeed, microglial BDNF has also been oth-
erwise shown to be relevant for learning behaviors (Parkhurst
et al., 2013) and development of pathological pain (Coull
et al., 2005; Trang et al., 2009).
A second more widely used approach to revealing
microglial control of neuronal activities has involved probing
neurotransmission with microglial-specific manipulations.
This has especially been shown in both dissociated cultures as
well as in tissue contexts. Microglial conditioned media
enhanced excitatory postsynaptic potentials and currents in
cultured neurons and in acute hippocampal slices through
released proteins and/or glycine from microglia (Hayashi
et al., 2006; Moriguchi et al., 2003). In contrast, by combin-
ing microglial depletion strategies in organotypic cultures
with exogenous microglial addition to neuronal cultures, a
recent study showed that microglia regulate synaptic transmis-
sion by decreasing miniature excitatory postsynaptic currents
which are due to microglial pruning of synapses containing
GluA1 receptors (Ji et al., 2013).
Microglial Regulation of Neuronal Activities in
Inflammation and Epilepsy
Further evidence of microglial control of neuronal activities
has been obtained from studies using lipopolysaccharide
(LPS), a component of the outer layer of the cell membrane
of gram negative bacteria. Detection of LPS in the brain is
predominantly carried out by Toll-like receptor 4 (TLR4)
which are mostly expressed by microglia in the parenchyma
of the healthy brain (Zhang et al., 2014b). This expression
pattern suggests that microglia are the direct target of LPS
signaling. Of relevance here, LPS application induced
increases in spontaneous excitatory postsynaptic currents
(EPSCs) in hippocampal neurons from brain slices in a
microglial-dependent manner (Pascual et al., 2012). The
mechanism required microglial TLR4 activation inducing
ATP release and astrocytic P2Y1 receptor activation, which in
FIGURE 2: Microglial influence on neuronal activity. This schematic summarizes the current literature on the effects of microglia on neu-
ronal activity. The studies were broadly classified into two categories based on the experimental approach: (1) genetic deletion of
microglial-specific proteins (lower left, pink) and (2) mechanistic interrogation of neuronal activity by microglial manipulations (upper
right, blue). Arrowheads indicate an enhancing effect, while rounded head indicate an inhibitory effect. Proven pathways and proposed
pathways are represented by solid and dashed lines, respectively. Please refer to section “Microglial Regulation of Neuronal Activities in
Epilepsy” for references.
6 Volume 00, No. 00
turn regulated neuronal EPSC frequency by glutamate release
to activate presynaptic neuronal mGluR5. Consistently, in
vivo application of LPS also increases neuronal excitability
and seizures through TLR4 activation and subsequent IL-1b
signaling primarily through microglia (Rodgers et al., 2009).
Interestingly, LPS-induced microglial activation was recently
shown to dampen synaptic transmission in a hypoxic context
where coupling LPS treatment with hypoxic treatment
induced long-term depression (LTD) (Zhang et al., 2014a).
The mechanism required microglial complement receptor
function but [unlike the previous study (Pascual et al., 2012)]
did not involve TLR4 activation. Downstream of comple-
ment activation, superoxide production was induced through
activation of NADPH oxidase that subsequently led to
AMPA receptor internalization underlying observed LTD
(Zhang et al., 2014a).
In the mammalian brain, neuronal NMDA receptor
activation elicited robust microglial process outgrowth/exten-
sion in a microglial P2Y12-dependent manner which
increased microglial contact of neuronal elements (Dissing-
Olesen et al., 2014; Eyo et al., 2014). Although the precise
function of this contact was not definitively determined (as
discussed in more detail below), an abrogation of the micro-
glial response by genetic depletion of the P2Y12 receptor cor-
related with a worsened seizure phenotype (Eyo et al., 2014).
These results suggest a neuroprotective action of P2Y12-
dependent microglial contact of neurons that may happen in
epilepsy. This neuroprotective hypothesis for microglial con-
tact of neurons is supported by evidence from the developing
zebrafish brain where, during physiological activity, microglial
processes made increasing contact with hyperactive neurons
and such contact served to downregulate neuronal activity (Li
et al., 2012). Together, these studies show that in multiple
paradigms and in multiple ways, microglia are capable of reg-
ulating neuronal activities including during epilepsy (Fig. 2).
Microglial Function in Acute Seizures,
Neurodegeneration, and Neurogenesis in
Epilepsy
Microglia possess both neurotoxic and neuroprotective poten-
tial in the context of CNS diseases (Ransohoff and Perry,
2009). As discussed above, microglial reactivity during seiz-
ures occurs prior to overt neurodegeneration raising questions
as to the functional significance of microglial activation in
response to the initial events precipitating the seizures. Micro-
glial roles in two aspects of epilepsy can be distinguished: the
initial intensity of acute seizures and the subsequent delayed
neurodegeneration that occurs. Interestingly, recent studies
have also found that microglia also play an important role in
epilepsy-induced aberrant neurogenesis (Fig. 3).
FIGURE 3: Microglia at different stages after seizures. This figure highlights three keynote studies that investigated microglial activation
at different time points following seizures. In the acute phase (1–3 h), microglial P2Y12 receptor-mediated process extension attenuated
seizure outcome, playing a neuroprotective role. In the sub-acute phase (48–72 h), fractalkine signaling is one signaling axis that has
been identified that mediates microglial activation resulting in neuronal degeneration. Finally, in the chronic phase (several weeks),
microglia was shown to be capable of recognizing DNA from degenerating neurons via TLR9 and TLR9 signaling prevented aberrant
neurogenesis following seizures. Please refer to section “Microglial Function in Acute Seizures, Neurodegeneration, and Neurogenesis in
Epilepsy” for references.
Eyo et al.: Microglia in Epilepsy
Month 2016 7
Microglial Function in Acute Seizures
One approach recently used to determine general microglial
roles in experimental epilepsy has involved the ablation of res-
ident microglia followed by the subsequent exposure of ani-
mals to a seizure-inducing stimulus. At present, several
microglial ablation strategies have been developed: (1)
pharmaco-genetically inducible models using either CD11b-
HSVTK or CD11b-DTR mice (Duffield et al., 2005; Hepp-
ner et al., 2005); (2) exclusively genetic models such as PU.1
and colony stimulating factor 1 receptor (CSF1R) knockout
mice (Erblich et al., 2011; McKercher et al., 1996; Scott
et al., 1994); and (3) purely pharmacological models with the
use of drugs like clodronate (Cunningham et al., 2013; Faus-
tino et al., 2011; Marin-Teva et al., 2004) or PLX3397, a
CSFR1 inhibitor (Elmore et al., 2014), to selectively elimi-
nate brain resident microglia. Despite the availability of these
ablation techniques, only one study has attempted to investi-
gate microglial roles in pilocarpine-induced epilepsy using an
ablation strategy (Mirrione et al., 2010). Although selective
microglial ablation in the dorsal hippocampus using CD11b-
HSVTK mice tended toward a slightly worsened acute seizure
phenotype, this was not significant and led to the suggestion
that resting microglia may not play significant roles during
acute seizures (Mirrione et al., 2010). However, using this
strategy, microglia were found to be neuroprotective following
a 24 h LPS-preconditioning paradigm in the pilocarpine sei-
zure model (Mirrione et al., 2010).
In our recent study, we approached the question of
acute microglial roles during epilepsy using a genetic
approach wherein a specific microglial receptor, the P2Y12
receptor, known to be selectively expressed in the brain by
microglia (Gu et al., 2015; Haynes et al., 2006) is deleted.
This receptor is critical for acute microglial chemotactic
responses to ATP in the brain (Haynes et al., 2006; Swiat-
kowski et al., 2016; Wu et al., 2007). Since ATP is also
released during experimental seizures (Santiago et al., 2011)
and the P2Y12 receptor mediates microglial process extension
in epilepsy (Eyo et al., 2014), we investigated whether micro-
glial chemotactic responses to ATP (released presumably from
hyperactive neurons) are functionally significant during acute
seizures. Interestingly, using both intraperitoneal and intrace-
rebroventricular models of kainic acid-induced seizures, we
found a dramatic exacerbation of seizure behaviors in P2Y12
deficient mice. Consistent with a role for ATP in seizure-
induced changes in microglial morphology, P2Y12 deficient
microglia exhibited reduced primary process numbers in
response to kainic acid treatment as compared with wildtype
microglia (Eyo et al., 2014). These results are the first to
show (i) such an early (within an hour) morphological
response of microglial changes during seizures and (ii) a direct
involvement of microglia in the progression of the seizure
phenotype, presumably at least in part through their morpho-
logical dynamics. This latter point, therefore, suggest neuro-
protective roles for microglia through this receptor in acute
seizure phenotypes.
Increased seizures in P2Y12 knockout mice may be
thought to be in conflict with the other study that failed to
observe significant effects of microglial depletion on acute sei-
zure phenotypes (Mirrione et al., 2010). Although the reasons
for the seeming discrepancy between both studies are not
clear, it is possible that (i) the differences might be a result of
the different chemoconvulsants (pilocarpine vs. kainic acid)
used and can be resolved by investigating differences between
wildtype and P2Y12 knockout seizure behaviors in response
to pilocarpine treatment; (ii) microglia may possess equally
detrimental/beneficial roles during acute seizures that may be
masked by whole cell ablation but evident through selective
genetic ablation of a single receptor (in this case P2Y12)
whose function when present, limits the effect of seizures;
(iii) the lack of P2Y12 receptor function in the knockouts
through development may cause a developmental deficiency
in which the neural environment is compromised and thus
more susceptible to seizures. This explanation could be
addressed with a temporally controlled depletion or pharma-
cological inhibition of the receptor’s function; and (iv) it is
possible that there are some limitations to the method of
microglial elimination which could be addressed by employ-
ing other methods of microglial depletion (mentioned above)
during epilepsy. Also, local inflammation and astrogliosis are
concerns for microglial ablation approaches. Evidently, these
results suggest that further studies are required to adequately
understand microglial roles during acute seizures.
Although we have identified microglial P2Y12 receptors
as modulators of epileptic seizure phenotypes, whether other
microglial purinergic receptors are also involved is not
known. A recent study failed to detect a significant difference
between wildtype and P2X4 deficient animals during acute
seizures (Ulmann et al., 2013) suggesting that microglial
P2X4 receptors do not modulate acute seizure behaviors.
Finally, although P2X7 receptor function is proposed to be
neurotoxic during epilepsy (Jimenez-Pacheco et al., 2013), the
evidence for the selective contribution of microglial specific
P2X7 receptors is currently debated.
Regarding roles for other microglial proteins in acute
seizures, at least one study in rats suggest that pharmacologi-
cal manipulation of the CX3CL1-CX3CR1 signaling axis
between neurons and microglia may be neurotoxic in
pilocarpine-induced seizures (Yeo et al., 2011), an idea that is
consistent with other evidence that indicates reduced seizures
in young pentylene tetrazole (PTZ)-treated fractalkine recep-
tor knockout mice (Paolicelli et al., 2011). Nevertheless,
future studies would have to appropriately ascertain the role
8 Volume 00, No. 00
of fractalkine signaling in kainic acid- and or pilocarpine-
induced acute seizures.
Microglial Function in Delayed Neurodegeneration
In addition to acute seizure phenotypes, microglia play
important roles in seizure-induced neurodegeneration. Both
pilocarpine and kainic acid models of epilepsy result in
delayed neurodegeneration beginning at about 24 h after
drug treatment (Curia et al., 2008; Levesque and Avoli,
2013). So far, microglial function in chronic neurodegenera-
tion in seizures has largely employed a pharmacological
approach. Several studies have indicated that minocycline, a
tetracycline-derivative and microglial inhibitor, is protective in
several models of rodent (Abraham et al., 2012; Heo et al.,
2006; Wang et al., 2012, 2015) and human epilepsy (Nowak
et al., 2012). Minocycline’s neuroprotective effects include a
reduction in the degree of neurodegeneration that result from
the initial insult, mitigation of pro-inflammatory cytokines
presumable released from tissue microglia, as well as the sub-
sequent development of spontaneous recurring seizures (SRS)
(Wang et al., 2015). An alternative pharmacological approach
that has been employed involved the use of macrophage
inhibitory factor (MIF) to reduce microglial activation. This
approach revealed that under such conditions, intrahippocam-
pal kainic acid-induced neurodegeneration is dramatically
reduced (Rogove and Tsirka, 1998). The emerging data,
therefore, suggest that subsequent to the initial seizures,
microglia may play detrimental pro-convulsive roles in epilep-
togenesis because minocycline (Yrjanheikki et al., 1999) and
MIF (Thanos et al., 1993) inhibit microglial activation. How-
ever, caution needs to be taken in interpreting the neuropro-
tective effects of these drugs (especially minocycline) as the
precise mechanisms of their action are not clear and may
occur directly on neurons independent of microglial effects
due to its lack of specificity of cellular action (Domercq and
Matute, 2004; Huang et al., 2010). Alternative pharmacologi-
cal and complementary genetic approaches are thus required
to adequately ascertain microglial roles in seizure-induced
neurodegeneration.
Microglia have also been implicated in seizure-induced
neurodegeneration, e.g. through the microglial-specific fractal-
kine receptor. Inhibiting fractalkine signaling through fractal-
kine receptor antibodies reduced seizure-induced
neurodegeneration in the hippocampus (Ali et al., 2015).
Interestingly, a recent report also showed that functional
maturity of newborn neurons following kainic acid induced
seizures was delayed in fractalkine receptor knockouts (Xiao
et al., 2015). Although roles for fractalkine signaling have
been suggested in epilepsy, other molecular mechanisms, yet
to be identified are sure to be involved and should be a focus
of future studies. Together, these studies suggest that micro-
glial roles in delayed neurodegeneration following acute seiz-
ures may be complex and more stringent approaches are
necessary to resolve the current disparities in results.
Microglial Function in Aberrant Neurogenesis
Neurogenesis is a phenomenon that occurs throughout life in
the adult brain and in most cases is thought to be helpful.
For example, promoting adult neurogenesis reduces anxiety-
and depression-like behaviors in mice (Hill et al., 2015).
Moreover expanding the pool of adult-born neurons has been
shown to improve the ability to differentiate between similar
contexts in mice (Sahay et al., 2011). Thus, the importance
of lifelong neurogenesis has been recognized in both regulat-
ing mood and cognitive functions (Christian et al., 2014).
Recent studies also highlighted the roles for microglia in adult
neurogenesis including providing trophic support for neuro-
nal survival, proliferation, and differentiation as well as phag-
ocytic clearance of apoptotic neurons as new cells are birthed
(Gemma and Bachstetter, 2013; Ribeiro Xavier et al., 2015;
Sierra et al., 2010, 2014).
In addition to this ongoing adult neurogenesis, seizures
induce aberrant neurogenesis where it is thought to be dys-
functional (Cho et al., 2015). Whether microglia are involved
in this seizure-induced aberrant neurogenesis has only begun
to be investigated in recent years. Inhibiting microglial activa-
tion with minocycline reduced the pilocarpine-induced aber-
rant neurogenesis while activating microglia with LPS further
exacerbated this neurogenesis (Yang et al., 2010). More
recently, mechanisms involved in microglial regulation of
seizure-induced aberrant neurogenesis have been determined
by microglial TLR9 activation following kainic acid seizures
(Matsuda et al., 2015). TLR9 senses nucleic acids including
DNA released from damaged cells and was demonstrated to
reduce aberrant seizure-induced neurogenesis (Matsuda et al.,
2015). Together, these results indicate that microglia are crit-
ically involved in the modulation of acute seizure phenotypes
as well as the delayed consequences of such seizures on neuro-
nal survival and subsequent proliferation (Fig. 3).
Future Direction and Conclusions
Current results as discussed above show that microglia are
both activated during/following seizures at both morphologi-
cal and molecular levels and are capable of attenuating or
exacerbating seizure intensity during neuronal dysfunction fol-
lowing seizures. Yet, research into microglial function during
seizures/epilepsy has not been extensive. In this section, we
highlight three poorly studied areas of microglia roles in seiz-
ures/epilepsy that warrant future investigation.
Microglia are widely recognized to be highly morpho-
logically dynamic cells but real time in vivo or ex vivo
descriptions of microglial dynamics during seizures and
Eyo et al.: Microglia in Epilepsy
Month 2016 9
subsequent epilepsy are lacking. More importantly, detailed
descriptions and the underlying mechanisms governing their
cellular interactions with neurons under these conditions are
not known. Despite the evidence of increased P2Y12-
dependent chemotactic responses following kainic acid-
induced seizures (Avignone et al., 2008), the consequence of
this increased dynamics on neuronal function remains elusive.
Thus, we still do not actually know (i) the real-time dynamics
of microglial activities during acute seizures since no high
resolution in vivo imaging studies exist on this and (ii) the
dynamics of the physical interaction between microglia and
neuronal elements and (iii) the functional consequence of
such cellular interactions of epileptic phenotypes. Future
research should be directed toward understanding these
questions.
A second line of research that deserves further attention
is with regards to microglial function in spontaneous epilepsy.
Although chemical convulsants such as kainic acid and pilocar-
pine are well known to induce seizures acutely, seizures, in
themselves, are technically not considered epilepsy. Rather, epi-
lepsy disorders subsequently develop some time following a
latent period in these models from days to weeks after the ini-
tial insult in the form of SRS (Curia et al., 2008; Dudek
et al., 2002). Microglial roles in these delayed phenotypes that
are more reminiscent of clinical epilepsy and results from the
underlying re-organization of neuronal networks have not been
extensively studied. Following experimental seizures and during
SRS, do (and if so how do) microglia modulate neuronal net-
works? Could these mechanisms, once understood, be targeted
in the clinic to ameliorate the consequences of seizures and epi-
lepsy in general? These questions will need to be addressed.
Third, seizures are a relatively common phenomenon in
the developing brain and can occur as typically benign
“febrile” seizures or more malignant seizure disorders in
which some of the underlying mechanisms including inflam-
matory mechanisms are known (Glass, 2014; Nardou et al.,
2013; Patterson et al., 2013, 2014). However, specific micro-
glial involvement has not been determined. For example, the
pro-inflammatory cytokine IL-1bpromotes febrile seizures
(Heida et al., 2009; Yu et al., 2012) and humans with poly-
morphisms in the IL-1bgene have increased susceptibility to
febrile seizures (Kira et al., 2010; Virta et al., 2002).
Although microglia are known to release IL-1b, it is not clear
that its release under these conditions require or stem from
microglia. Therefore, the effect of developmental seizures on
microglial functions and the consequence of microglial activ-
ity on seizure behaviors need further investigation. Because
emerging data suggest that microglia as the brain resident
immune cells play crucial roles in neural development (Eyo
and Dailey, 2013; Hoshiko et al., 2012; Paolicelli et al.,
2011; Paolicelli and Gross, 2011; Pont-Lezica et al., 2011;
Schafer et al., 2012; Schlegelmilch et al., 2011; Squarzoni
et al., 2014), microglial roles in developmental seizures
should hold promise in advancing new approaches in the
treatment of such disorders.
In summary, we have reviewed some of the relevant liter-
ature with regards to the role microglia in human and experi-
mental seizure/epilepsy disorders. Microglia show
morphological/molecular alterations in response to seizures and
in turn regulate neuronal activities under these conditions.
This microglia–neuron communication plays a critical role in
acute seizures, delayed neurodegeneration, and aberrant neuro-
genesis in experimental models. Finally we highlight three areas
that demand further research in the context of microglial roles
in epilepsy including: (i) microglial dynamics and physical
interactions with neurons at the cellular level, (ii) microglial
roles in the pathogenesis of chronic epilepsy at the circuit level,
and (iii) microglial responses and contributions to the pathoge-
nesis of developmental epilepsy. In light of the current estab-
lished experimental epilepsy models and given the availability
of genetic tools (knockout mice), replacement (e.g., cell trans-
plantation) and pharmaco-genetic abilities to eliminate, inhibit,
or otherwise manipulate microglial functions, these questions
can now and should begin to be addressed.
Acknowledgment
This work is supported by grants from the National Institute
of Health (R01NS088627, and R21DE025689) and New Jersey
Commission on Spinal Cord Research (CSCR15ERG015).
References
Abraham J, Fox PD, Condello C, Bartolini A, Koh S. 2012. Minocycline
attenuates microglia activation and blocks the long-term epileptogenic
effects of early-life seizures. Neurobiol Dis 46:425–430.
Ahlers KE, Karacay B, Fuller L, Bonthius DJ, Dailey ME. 2015. Transient acti-
vation of microglia following acute alcohol exposure in developing mouse
neocortex is primarily driven by BAX-dependent neurodegeneration. Glia 63:
1694–1713.
Ajami B, Bennett JL, Krieger C, McNagny KM, Rossi FM. 2011. Infiltrating
monocytes trigger EAE progression, but do not contribute to the resident
microglia pool. Nat Neurosci 14:1142–1149.
Ajami B, Bennett JL, Krieger C, Tetzlaff W, Rossi FM. 2007. Local self-renewal
can sustain CNS microglia maintenance and function throughout adult life.
Nat Neurosci 10:1538–1543.
Akahoshi N, Murashima YL, Himi T, Ishizaki Y, Ishii I. 2007. Increased expres-
sion of the lysosomal protease cathepsin S in hippocampal microglia follow-
ing kainate-induced seizures. Neurosci Lett 429:136–141.
Ali I, Chugh D, Ekdahl CT. 2015. Role of fractalkine-CX3CR1 pathway in
seizure-induced microglial activation, neurodegeneration, and neuroblast pro-
duction in the adult rat brain. Neurobiol Dis 74:194–203.
Andersson PB, Perry VH, Gordon S. 1991. The kinetics and morphological
characteristics of the macrophage-microglial response to kainic acid-induced
neuronal degeneration. Neuroscience 42:201–214.
Arno B, Grassivaro F, Rossi C, Bergamaschi A, Castiglioni V, Furlan R, Greter
M, Favaro R, Comi G, Becher B, Martino G, Muzio L. 2014. Neural progenitor
10 Volume 00, No. 00
cells orchestrate microglia migration and positioning into the developing cor-
tex. Nat Commun 5:5611.
Avignone E, Lepleux M, Angibaud J, Nagerl UV. 2015. Altered morphologi-
cal dynamics of activated microglia after induction of status epilepticus.
J Neuroinflammation 12:202.
Avignone E, Ulmann L, Levavasseur F, Rassendren F, Audinat E. 2008. Status
epilepticus induces a particular microglial activation state characterized by
enhanced purinergic signaling. J Neurosci 28:9133–9144.
Baalman K, Marin MA, Ho TS, Godoy M, Cherian L, Robertson C, Rasband MN.
2015. Axon initial segment-associated microglia. J Neurosci 35:2283–2292.
Banerjee M, Sasse VA, Wang Y, Maulik M, Kar S. 2015. Increased levels and activ-
ity of cathepsins B and D in kainate-induced toxicity. Neuroscience 284:360–373.
Beach TG, Woodhurst WB, MacDonald DB, Jones MW. 1995. Reactive micro-
glia in hippocampal sclerosis associated with human temporal lobe epilepsy.
Neurosci Lett 191:27–30.
Bechade C, Cantaut-Belarif Y, Bessis A. 2013. Microglial control of neuronal
activity. Front Cell Neurosci 7:32.
Bellavance MA, Gosselin D, Yong VW, Stys PK, Rivest S. 2015. Patrolling
monocytes play a critical role in CX3CR1-mediated neuroprotection during
excitotoxicity. Brain Struct Funct 220:1759–1776.
Ben-Ari Y, Cossart R. 2000. Kainate, a double agent that generates seizures:
Two decades of progress. Trends Neurosci 23:580–587.
Benson MJ, Manzanero S, Borges K. 2015. Complex alterations in microglial
M1/M2 markers during the development of epilepsy in two mouse models.
Epilepsia 56:895–905.
Block ML, Zecca L, Hong JS. 2007. Microglia-mediated neurotoxicity: Uncov-
ering the molecular mechanisms. Nat Rev Neurosci 8:57–69.
Boche D, Perry VH, Nicoll JA. 2013. Review: Activation patterns of microglia
and their identification in the human brain. Neuropathol Appl Neurobiol 39:
3–18.
Boer K, Spliet WG, van Rijen PC, Redeker S, Troost D, Aronica E. 2006. Evidence
of activated microglia in focal cortical dysplasia. J Neuroimmunol 173:188–195.
Borges K, Gearing M, McDermott DL, Smith AB, Almonte AG, Wainer BH,
Dingledine R. 2003. Neuronal and glial pathological changes during epilepto-
genesis in the mouse pilocarpine model. Exp Neurol 182:21–34.
Cardona AE, Pioro EP, Sasse ME, Kostenko V, Cardona SM, Dijkstra IM,
Huang D, Kidd G, Dombrowski S, Dutta R, et al. 2006. Control of microglial
neurotoxicity by the fractalkine receptor. Nat Neurosci 9:917–924.
Cho KO, Lybrand ZR, Ito N, Brulet R, Tafacory F, Zhang L, Good L, Ure K,
Kernie SG, Birnbaum SG, et al. 2015. Aberrant hippocampal neurogenesis con-
tributes to epilepsy and associated cognitive decline. Nat Commun 6:6606.
Choi J, Koh S. 2008. Role of brain inflammation in epileptogenesis. Yonsei
Med J 49:1–18.
Christian KM, Song H, Ming GL. 2014. Functions and dysfunctions of adult
hippocampal neurogenesis. Annu Rev Neurosci 37:243–262.
Colton CA. 2009. Heterogeneity of microglial activation in the innate immune
response in the brain. J Neuroimmune Pharmacol 4:399–418.
Coull JA, Beggs S, Boudreau D, Boivin D, Tsuda M, Inoue K, Gravel C, Salter
MW, De Koninck Y. 2005. BDNF from microglia causes the shift in neuronal
anion gradient underlying neuropathic pain. Nature 438:1017–1021.
Cunningham CL, Martinez-Cerdeno V, Noctor SC. 2013. Microglia regulate
the number of neural precursor cells in the developing cerebral cortex.
J Neurosci 33:4216–4233.
Curia G, Longo D, Biagini G, Jones RS, Avoli M. 2008. The pilocarpine model
of temporal lobe epilepsy. J Neurosci Methods 172:143–157.
Dalby NO, Mody I. 2001. The process of epileptogenesis: A pathophysiologi-
cal approach. Curr Opin Neurol 14:187–192.
Davalos D, Grutzendler J, Yang G, Kim JV, Zuo Y, Jung S, Littman DR, Dustin
ML, Gan WB. 2005. ATP mediates rapid microglial response to local brain
injury in vivo. Nat Neurosci 8:752–758.
Dibaj P, Nadrigny F, Steffens H, Scheller A, Hirrlinger J, Schomburg ED,
Neusch C, Kirchhoff F. 2010. NO mediates microglial response to acute spi-
nal cord injury under ATP control in vivo. Glia 58:1133–1144.
Dissing-Olesen L, LeDue JM, Rungta RL, Hefendehl JK, Choi HB, MacVicar
BA. 2014. Activation of neuronal NMDA receptors triggers transient ATP-
mediated microglial process outgrowth. J Neurosci 34:10511–10527.
Domercq M, Matute C. 2004. Neuroprotection by tetracyclines. Trends Phar-
macol Sci 25:609–612.
Dudek FE, Hellier JL, Williams PA, Ferraro DJ, Staley KJ. 2002. The course of
cellular alterations associated with the development of spontaneous seizures
after status epilepticus. Prog Brain Res 135:53–65.
Duffield JS, Tipping PG, Kipari T, Cailhier JF, Clay S, Lang R, Bonventre JV,
Hughes J. 2005. Conditional ablation of macrophages halts progression of
crescentic glomerulonephritis. Am J Pathol 167:1207–1219.
Elmore MR, Najafi AR, Koike MA, Dagher NN, Spangenberg EE, Rice RA,
Kitazawa M, Matusow B, Nguyen H, West BL, et al. 2014. Colony-stimulating
factor 1 receptor signaling is necessary for microglia viability, unmasking a
microglia progenitor cell in the adult brain. Neuron 82:380–397.
Erblich B, Zhu L, Etgen AM, Dobrenis K, Pollard JW. 2011. Absence of colony
stimulation factor-1 receptor results in loss of microglia, disrupted brain
development and olfactory deficits. PLoS One 6:e26317.
Eriksson C, Zou LP, Ahlenius S, Winblad B, Schultzberg M. 2000. Inhibition of
kainic acid induced expression of interleukin-1 beta and interleukin-1 receptor
antagonist mRNA in the rat brain by NMDA receptor antagonists. Brain Res
Mol Brain Res 85:103–113.
Eyo UB, Dailey ME. 2013. Microglia: Key elements in neural development,
plasticity, and pathology. J Neuroimmune Pharmacol 8:494–509.
Eyo UB, Gu N, De S, Dong H, Richardson JR, Wu LJ. 2015. Modulation of
microglial process convergence toward neuronal dendrites by extracellular
calcium. J Neurosci 35:2417–2422.
Eyo UB, Peng J, Swiatkowski P, Mukherjee A, Bispo A, Wu LJ. 2014. Neuro-
nal hyperactivity recruits microglial processes via neuronal NMDA receptors
and microglial P2Y12 receptors after status epilepticus. J Neurosci 34:10528–
10540.
Eyo UB, Wu LJ. 2013. Bidirectional microglia–neuron communication in the
healthy brain. Neural Plast 2013:456857.
Faustino JV, Wang X, Johnson CE, Klibanov A, Derugin N, Wendland MF,
Vexler ZS. 2011. Microglial cells contribute to endogenous brain defenses
after acute neonatal focal stroke. J Neurosci 31:12992–13001.
Fisher RS, van Emde Boas W, Blume W, Elger C, Genton P, Lee P, Engel J
Jr. 2005. Epileptic seizures and epilepsy: Definitions proposed by the Interna-
tional League Against Epilepsy (ILAE) and the International Bureau for Epi-
lepsy (IBE). Epilepsia 46:470–472.
Frieler RA, Meng H, Duan SZ, Berger S, Schutz G, He Y, Xi G, Wang MM,
Mortensen RM. 2011. Myeloid-specific deletion of the mineralocorticoid
receptor reduces infarct volume and alters inflammation during cerebral
ischemia. Stroke 42:179–185.
Gaitatzis A, Johnson AL, Chadwick DW, Shorvon SD, Sander JW. 2004. Life
expectancy in people with newly diagnosed epilepsy. Brain 127:2427–2432.
Gemma C, Bachstetter AD. 2013. The role of microglia in adult hippocampal
neurogenesis. Front Cell Neurosci 7:229.
Glass HC. 2014. Neonatal seizures: Advances in mechanisms and manage-
ment. Clin Perinatol 41:177–190.
Gu N, Eyo UB, Murugan M, Peng J, Matta S, Dong H, Wu LJ. Microglial
P2Y12 receptors regulate microglial activation and surveillance during neuro-
pathic pain. Brain Behav Immun, in press.
Hamilton SE, Loose MD, Qi M, Levey AI, Hille B, McKnight GS, Idzerda RL,
Nathanson NM. 1997. Disruption of the m1 receptor gene ablates muscarinic
receptor-dependent M current regulation and seizure activity in mice. Proc
Natl Acad Sci USA 94:13311–13316.
Hanisch UK, Kettenmann H. 2007. Microglia: Active sensor and versatile effec-
tor cells in the normal and pathologic brain. Nat Neurosci 10:1387–1394.
Eyo et al.: Microglia in Epilepsy
Month 2016 11
Hayashi Y, Ishibashi H, Hashimoto K, Nakanishi H. 2006. Potentiation of the
NMDA receptor-mediated responses through the activation of the glycine
site by microglia secreting soluble factors. Glia 53:660–668.
Haynes SE, Hollopeter G, Yang G, Kurpius D, Dailey ME, Gan WB, Julius D.
2006. The P2Y12 receptor regulates microglial activation by extracellular
nucleotides. Nat Neurosci 9:1512–1519.
Heida JG, Moshe SL, Pittman QJ. 2009. The role of interleukin-1beta in feb-
rile seizures. Brain Dev 31:388–393.
Henkel JS, Beers DR, Zhao W, Appel SH. 2009. Microglia in ALS: The good,
the bad, and the resting. J Neuroimmune Pharmacol 4:389–398.
Heo K, Cho YJ, Cho KJ, Kim HW, Kim HJ, Shin HY, Lee BI, Kim GW. 2006.
Minocycline inhibits caspase-dependent and -independent cell death path-
ways and is neuroprotective against hippocampal damage after treatment
with kainic acid in mice. Neurosci Lett 398:195–200.
Heppner FL, Greter M, Marino D, Falsig J, Raivich G, Hovelmeyer N,
Waisman A, Rulicke T, Prinz M, Priller J, et al. 2005. Experimental auto-
immune encephalomyelitis repressed by microglial paralysis. Nat Med 11:
146–152.
Hickman SE, Kingery ND, Ohsumi TK, Borowsky ML, Wang LC, Means TK, El
Khoury J. 2013. The microglial sensome revealed by direct RNA sequencing.
Nat Neurosci 16:1896–1905.
Hill AS, Sahay A, Hen R. 2015. Increasing adult hippocampal neurogenesis is
sufficient to reduce anxiety and depression-like behaviors. Neuropsychophar-
macology 40:2368–2378.
Hoshiko M, Arnoux I, Avignone E, Yamamoto N, Audinat E. 2012. Deficiency
of the microglial receptor CX3CR1 impairs postnatal functional development
of thalamocortical synapses in the barrel cortex. J Neurosci 32:15106–15111.
Hu X, Li P, Guo Y, Wang H, Leak RK, Chen S, Gao Y, Chen J. 2012. Micro-
glia/macrophage polarization dynamics reveal novel mechanism of injury
expansion after focal cerebral ischemia. Stroke 43:3063–3070.
Huang WC, Qiao Y, Xu L, Kacimi R, Sun X, Giffard RG, Yenari MA. 2010.
Direct protection of cultured neurons from ischemia-like injury by minocy-
cline. Anat Cell Biol 43:325–331.
Hughes PM, Botham MS, Frentzel S, Mir A, Perry VH. 2002. Expression of
fractalkine (CX3CL1) and its receptor, CX3CR1, during acute and chronic
inflammation in the rodent CNS. Glia 37:314–327.
Jensen FE. 2011. Epilepsy as a spectrum disorder: Implications from novel
clinical and basic neuroscience. Epilepsia 52(Suppl 1):1–6.
Ji K, Akgul G, Wollmuth LP, Tsirka SE. 2013. Microglia actively regulate the
number of functional synapses. PLoS One 8:e56293.
Jimenez-Pacheco A, Mesuret G, Sanz-Rodriguez A, Tanaka K, Mooney C,
Conroy R, Miras-Portugal MT, Diaz-Hernandez M, Henshall DC, Engel T.
2013. Increased neocortical expression of the P2X7 receptor after status epi-
lepticus and anticonvulsant effect of P2X7 receptor antagonist A-438079. Epi-
lepsia 54:1551–1561.
Jung KH, Chu K, Lee ST, Kim JH, Kang KM, Song EC, Kim SJ, Park HK, Kim
M, Lee SK, et al. 2009. Region-specific plasticity in the epileptic rat brain: A
hippocampal and extrahippocampal analysis. Epilepsia 50:537–549.
Kandratavicius L, Balista PA, Lopes-Aguiar C, Ruggiero RN, Umeoka EH, Garcia-
Cairasco N, Bueno-Junior LS, Leite JP. 2014. Animal models of epilepsy: Use
and limitations. Neuropsychiatr Dis Treat 10:1693–1705.
Kang TC, Kim DS, Kwak SE, Kim JE, Won MH, Kim DW, Choi SY, Kwon OS.
2006. Epileptogenic roles of astroglial death and regeneration in the dentate
gyrus of experimental temporal lobe epilepsy. Glia 54:258–271.
Kira R, Ishizaki Y, Torisu H, Sanefuji M, Takemoto M, Sakamoto K, Matsumoto
S, Yamaguchi Y, Yukaya N, Sakai Y, et al. 2010. Genetic susceptibility to febrile
seizures: Case–control association studies. Brain Dev 32:57–63.
Kwan P, Brodie MJ. 2006. Refractory epilepsy: Mechanisms and solutions.
Expert Rev Neurother 6:397–406.
Lauro C, Catalano M, Trettel F, Limatola C. 2015. Fractalkine in the nervous
system: Neuroprotective or neurotoxic molecule? Ann N Y Acad Sci 1351:
141–148.
Leite JP, Garcia-Cairasco N, Cavalheiro EA. 2002. New insights from the use
of pilocarpine and kainate models. Epilepsy Res 50:93–103.
Levesque M, Avoli M. 2013. The kainic acid model of temporal lobe epilepsy.
Neurosci Biobehav Rev 37:2887–2899.
Levesque M, Avoli M, Bernard C. 2016. Animal models of temporal lobe epi-
lepsy following systemic chemoconvulsant administration. J Neurosci Meth-
ods 260:45–52.
Li Y, Du XF, Liu CS, Wen ZL, Du JL. 2012. Reciprocal regulation between rest-
ing microglial dynamics and neuronal activity in vivo. Dev Cell 23:1189–1202.
Liao B, Zhao W, Beers DR, Henkel JS, Appel SH. 2012. Transformation from
a neuroprotective to a neurotoxic microglial phenotype in a mouse model of
ALS. Exp Neurol 237:147–152.
Limatola C, Ransohoff RM. 2014. Modulating neurotoxicity through CX3CL1/
CX3CR1 signaling. Front Cell Neurosci 8:229.
Longo B, Romariz S, Blanco MM, Vasconcelos JF, Bahia L, Soares MB, Mello
LE, Ribeiro-dos-Santos R. 2010. Distribution and proliferation of bone marrow
cells in the brain after pilocarpine-induced status epilepticus in mice. Epilep-
sia 51:1628–1632.
Marin-Teva JL, Dusart I, Colin C, Gervais A, van Rooijen N, Mallat M. 2004.
Microglia promote the death of developing Purkinje cells. Neuron 41:535–547.
Matsuda T, Murao N, Katano Y, Juliandi B, Kohyama J, Akira S, Kawai T,
Nakashima K. 2015. TLR9 signalling in microglia attenuates seizure-induced
aberrant neurogenesis in the adult hippocampus. Nat Commun 6:6514.
McKercher SR, Torbett BE, Anderson KL, Henkel GW, Vestal DJ, Baribault H,
Klemsz M, Feeney AJ, Wu GE, Paige CJ, et al. 1996. Targeted disruption of
the PU.1 gene results in multiple hematopoietic abnormalities. EMBO J 15:
5647–5658.
Mikita J, Dubourdieu-Cassagno N, Deloire MS, Vekris A, Biran M, Raffard G,
Brochet B, Canron MH, Franconi JM, Boiziau C, et al. 2011. Altered M1/M2
activation patterns of monocytes in severe relapsing experimental rat model
of multiple sclerosis. Amelioration of clinical status by M2 activated monocyte
administration. Mult Scler 17:2–15.
Mirrione MM, Konomos DK, Gravanis I, Dewey SL, Aguzzi A, Heppner FL,
Tsirka SE. 2010. Microglial ablation and lipopolysaccharide preconditioning
affects pilocarpine-induced seizures in mice. Neurobiol Dis 39:85–97.
Morgan TE, Nichols NR, Pasinetti GM, Finch CE. 1993. TGF-beta 1 mRNA
increases in macrophage/microglial cells of the hippocampus in response to
deafferentation and kainic acid-induced neurodegeneration. Exp Neurol 120:
291–301.
Moriguchi S, Mizoguchi Y, Tomimatsu Y, Hayashi Y, Kadowaki T, Kagamiishi
Y, Katsube N, Yamamoto K, Inoue K, Watanabe S, et al. 2003. Potentiation
of NMDA receptor-mediated synaptic responses by microglia. Brain Res Mol
Brain Res 119:160–169.
Morris GP, Clark IA, Zinn R, Vissel B. 2013. Microglia: A new frontier for syn-
aptic plasticity, learning and memory, and neurodegenerative disease
research. Neurobiol Learn Mem 105:40–53.
Murray PJ, Allen JE, Biswas SK, Fisher EA, Gilroy DW, Goerdt S, Gordon S,
Hamilton JA, Ivashkiv LB, Lawrence T, et al. 2014. Macrophage activation and
polarization: Nomenclature and experimental guidelines. Immunity 41:14–20.
Nardou R, Ferrari DC, Ben-Ari Y. 2013. Mechanisms and effects of seizures in
the immature brain. Semin Fetal Neonatal Med 18:175–184.
Nayak D, Roth TL, McGavern DB. 2014. Microglia development and function.
Annu Rev Immunol 32:367–402.
Nimmerjahn A, Kirchhoff F, Helmchen F. 2005. Resting microglial cells are highly
dynamic surveillants of brain parenchyma in vivo. Science 308:1314–1318.
Nowak M, Strzelczyk A, Reif PS, Schorlemmer K, Bauer S, Norwood BA,
Oertel WH, Rosenow F, Strik H, Hamer HM. 2012. Minocycline as potent anti-
convulsant in a patient with astrocytoma and drug resistant epilepsy. Seizure
21:227–228.
Paolicelli RC, Bisht K, Tremblay ME. 2014. Fractalkine regulation of microglial
physiology and consequences on the brain and behavior. Front Cell Neurosci
8:129.
12 Volume 00, No. 00
Paolicelli RC, Bolasco G, Pagani F, Maggi L, Scianni M, Panzanelli P, Giustetto
M, Ferreira TA, Guiducci E, Dumas L, et al. 2011. Synaptic pruning by micro-
glia is necessary for normal brain development. Science 333:1456–1458.
Paolicelli RC, Gross CT. 2011. Microglia in development: Linking brain wiring
to brain environment. Neuron Glia Biol 7:77–83.
Parkhurst CN, Yang G, Ninan I, Savas JN, Yates JR III, Lafaille JJ, Hempstead
BL, Littman DR, Gan WB. 2013. Microglia promote learning-dependent syn-
apse formation through brain-derived neurotrophic factor. Cell 155:1596–1609.
Pascual O, Ben Achour S, Rostaing P, Triller A, Bessis A. 2012. Microglia acti-
vation triggers astrocyte-mediated modulation of excitatory neurotransmis-
sion. Proc Natl Acad Sci USA 109:E197–E205.
Patterson JL, Carapetian SA, Hageman JR, Kelley KR. 2013. Febrile seizures.
Pediatr Ann 42:249–254.
Patterson KP, Baram TZ, Shinnar S. 2014. Origins of temporal lobe epilepsy:
Febrile seizures and febrile status epilepticus. Neurotherapeutics 11:242–250.
Peferoen LA, Vogel DY, Ummenthum K, Breur M, Heijnen PD, Gerritsen WH,
Peferoen-Baert RM, van der Valk P, Dijkstra CD, Amor S. 2015. Activation sta-
tus of human microglia is dependent on lesion formation stage and remyeli-
nation in multiple sclerosis. J Neuropathol Exp Neurol 74:48–63.
Perry VH, Nicoll JA, Holmes C. 2010. Microglia in neurodegenerative disease.
Nat Rev Neurol 6:193–201.
Pont-Lezica L, Bechade C, Belarif-Cantaut Y, Pascual O, Bessis A. 2011. Physi-
ological roles of microglia during development. J Neurochem 119:901–908.
Priel MR, Albuquerque EX. 2002. Short-term effects of pilocarpine on rat hip-
pocampal neurons in culture. Epilepsia 43(Suppl 5):40–46.
Prinz M, Tay TL, Wolf Y, Jung S. 2014. Microglia: Unique and common fea-
tures with other tissue macrophages. Acta Neuropathol 128:319–331.
Ransohoff RM, Perry VH. 2009. Microglial physiology: Unique stimuli, special-
ized responses. Annu Rev Immunol 27:119–145.
Rappold PM, Lynd-Balta E, Joseph SA. 2006. P2X7 receptor immunoreactive
profile confined to resting and activated microglia in the epileptic brain. Brain
Res 1089:171–178.
Ravizza T, Gagliardi B, Noe F, Boer K, Aronica E, Vezzani A. 2008. Innate and
adaptive immunity during epileptogenesis and spontaneous seizures: Evi-
dence from experimental models and human temporal lobe epilepsy. Neuro-
biol Dis 29:142–160.
Ribeiro Xavier AL, Kress BT, Goldman SA, Lacerda de Menezes JR,
Nedergaard M. 2015. A distinct population of microglia supports adult neu-
rogenesis in the subventricular zone. J Neurosci 35:11848–11861.
Rodgers KM, Hutchinson MR, Northcutt A, Maier SF, Watkins LR, Barth DS.
2009. The cortical innate immune response increases local neuronal excitabil-
ity leading to seizures. Brain 132:2478–2486.
Rogers JT, Morganti JM, Bachstetter AD, Hudson CE, Peters MM, Grimmig
BA, Weeber EJ, Bickford PC, Gemma C. 2011. CX3CR1 deficiency leads to
impairment of hippocampal cognitive function and synaptic plasticity.
J Neurosci 31:16241–16250.
Rogove AD, Tsirka SE. 1998. Neurotoxic responses by microglia elicited by
excitotoxic injury in the mouse hippocampus. Curr Biol 8:19–25.
Rosell DR, Nacher J, Akama KT, McEwen BS. 2003. Spatiotemporal distribu-
tion of gp130 cytokines and their receptors after status epilepticus: Compari-
son with neuronal degeneration and microglial activation. Neuroscience 122:
329–348.
Roumier A, Bechade C, Poncer JC, Smalla KH, Tomasello E, Vivier E,
Gundelfinger ED, Triller A, Bessis A. 2004. Impaired synaptic function in the
microglial KARAP/DAP12-deficient mouse. J Neurosci 24:11421–11428.
Sahay A, Scobie KN, Hill AS, O’Carroll CM, Kheirbek MA, Burghardt NS,
Fenton AA, Dranovsky A, Hen R. 2011. Increasing adult hippocampal neuro-
genesis is sufficient to improve pattern separation. Nature 472:466–470.
Santiago MF, Veliskova J, Patel NK, Lutz SE, Caille D, Charollais A, Meda P,
Scemes E. 2011. Targeting pannexin1 improves seizure outcome. PLoS One
6:e25178.
Schafer DP, Lehrman EK, Kautzman AG, Koyama R, Mardinly AR, Yamasaki R,
Ransohoff RM, Greenberg ME, Barres BA, Stevens B. 2012. Microglia sculpt
postnatal neural circuits in an activity and complement-dependent manner.
Neuron 74:691–705.
Schlegelmilch T, Henke K, Peri F. 2011. Microglia in the developing brain:
From immunity to behaviour. Curr Opin Neurobiol 21:5–10.
Scott EW, Simon MC, Anastasi J, Singh H. 1994. Requirement of transcription
factor PU.1 in the development of multiple hematopoietic lineages. Science
265:1573–1577.
Shapiro LA, Wang L, Ribak CE. 2008. Rapid astrocyte and microglial activation
following pilocarpine-induced seizures in rats. Epilepsia 49(Suppl 2):33–41.
Sharma AK, Reams RY, Jordan WH, Miller MA, Thacker HL, Snyder PW. 2007.
Mesial temporal lobe epilepsy: Pathogenesis, induced rodent models and
lesions. Toxicol Pathol 35:984–999.
Shigemoto-Mogami Y, Hoshikawa K, Goldman JE, Sekino Y, Sato K. 2014.
Microglia enhance neurogenesis and oligodendrogenesis in the early post-
natal subventricular zone. J Neurosci 34:2231–2243.
Sierra A, Beccari S, Diaz-Aparicio I, Encinas JM, Comeau S, Tremblay ME,
2014. Surveillance, phagocytosis, and inflammation: How never-
resting microglia influence adult hippocampal neurogenesis. Neural Plast
2014:610343.
Sierra A, Encinas JM, Deudero JJ, Chancey JH, Enikolopov G, Overstreet-
Wadiche LS, Tsirka SE, Maletic-Savatic M. 2010. Microglia shape adult hippo-
campal neurogenesis through apoptosis-coupled phagocytosis. Cell Stem
Cell 7:483–495.
Sipe GO, Lowery RL, Tremblay ME, Kelly EA, Lamantia CE, Majewska AK.
2016. Microglial P2Y12 is necessary for synaptic plasticity in mouse visual cor-
tex. Nat Commun 7:10905.
Smolders I, Khan GM, Manil J, Ebinger G, Michotte Y. 1997. NMDA
receptor-mediated pilocarpine-induced seizures: Characterization in freely
moving rats by microdialysis. Br J Pharmacol 121:1171–1179.
Squarzoni P, Oller G, Hoeffel G, Pont-Lezica L, Rostaing P, Low D, Bessis A,
Ginhoux F, Garel S. 2014. Microglia modulate wiring of the embryonic fore-
brain. Cell Rep 8:1271–1279.
Swiatkowski P, Murugan M, Eyo UB, Wang Y, Rangaraju S, Oh SB, Wu LJ.
2016. Activation of microglial P2Y12 receptor is required for
outward potassium currents in response to neuronal injury. Neuroscience
318:22–33.
Tang Y, Le W. 2016. Differential roles of M1 and M2 microglia in neurodege-
nerative diseases. Mol Neurobiol 53:1181–1194.
Taniwaki Y, Kato M, Araki T, Kobayashi T. 1996. Microglial activation by epi-
leptic activities through the propagation pathway of kainic acid-induced hip-
pocampal seizures in the rat. Neurosci Lett 217:29–32.
Temkin NR. 2009. Preventing and treating posttraumatic seizures: The human
experience. Epilepsia 50(Suppl 2):10–13.
Thanos S, Mey J, Wild M. 1993. Treatment of the adult retina with microglia-
suppressing factors retards axotomy-induced neuronal degradation and
enhances axonal regeneration in vivo and in vitro. J Neurosci 13:455–466.
Thurman DJ, Beghi E, Begley CE, Berg AT, Buchhalter JR, Ding D,
Hesdorffer DC, Hauser WA, Kazis L, Kobau R, et al. 2011. Standards
for epidemiologic studies and surveillance of epilepsy. Epilepsia 52(Suppl 7):
2–26.
Trang T, Beggs S, Wan X, Salter MW. 2009. P2X4-receptor-mediated synthe-
sis and release of brain-derived neurotrophic factor in microglia is dependent
on calcium and p38-mitogen-activated protein kinase activation. J Neurosci
29:3518–3528.
Tremblay ME. 2011. The role of microglia at synapses in the healthy CNS:
Novel insights from recent imaging studies. Neuron Glia Biol 7:67–76.
Tremblay ME, Lowery RL, Majewska AK. 2010. Microglial interactions with
synapses are modulated by visual experience. PLoS Biol 8:e1000527.
Tremblay ME, Stevens B, Sierra A, Wake H, Bessis A, Nimmerjahn A. 2011.
The role of microglia in the healthy brain. J Neurosci 31:16064–16069.
Eyo et al.: Microglia in Epilepsy
Month 2016 13
Turrin NP, Rivest S. 2004. Innate immune reaction in response to seizures:
Implications for the neuropathology associated with epilepsy. Neurobiol Dis
16:321–334.
Turski L, Ikonomidou C, Turski WA, Bortolotto ZA, Cavalheiro EA. 1989.
Review: Cholinergic mechanisms and epileptogenesis. The seizures induced
by pilocarpine: A novel experimental model of intractable epilepsy. Synapse
3:154–171.
Ueno M, Fujita Y, Tanaka T, Nakamura Y, Kikuta J, Ishii M, Yamashita T.
2013. Layer V cortical neurons require microglial support for survival during
postnatal development. Nat Neurosci 16:543–551.
Ulmann L, Levavasseur F, Avignone E, Peyroutou R, Hirbec H, Audinat E,
Rassendren F. 2013. Involvement of P2X4 receptors in hippocampal micro-
glial activation after status epilepticus. Glia 61:1306–1319.
Varnum MM, Ikezu T. 2012. The classification of microglial activation pheno-
types on neurodegeneration and regeneration in Alzheimer’s disease brain.
Arch Immunol Ther Exp (Warsz) 60:251–266.
Vezzani A. 2009. Pilocarpine-induced seizures revisited: What does the model
mimic? Epilepsy Curr 9:146–148.
Vezzani A, Aronica E, Mazarati A, Pittman QJ. 2013. Epilepsy and brain
inflammation. Exp Neurol 244:11–21.
Vezzani A, Conti M, De Luigi A, Ravizza T, Moneta D, Marchesi F, De Simoni
MG. 1999. Interleukin-1beta immunoreactivity and microglia are enhanced in
the rat hippocampus by focal kainate application: Functional evidence for
enhancement of electrographic seizures. J Neurosci 19:5054–5065.
Virta M, Hurme M, Helminen M. 2002. Increased frequency of interleukin-
1beta (2511) allele 2 in febrile seizures. Pediatr Neurol 26:192–195.
Vogel DY, Vereyken EJ, Glim JE, Heijnen PD, Moeton M, van der Valk P,
Amor S, Teunissen CE, van Horssen J, Dijkstra CD. 2013. Macrophages in
inflammatory multiple sclerosis lesions have an intermediate activation status.
J Neuroinflammation 10:35.
Wake H, Moorhouse AJ, Jinno S, Kohsaka S, Nabekura J. 2009. Resting
microglia directly monitor the functional state of synapses in vivo and deter-
mine the fate of ischemic terminals. J Neurosci 29:3974–3980.
Wang DD, Englot DJ, Garcia PA, Lawton MT, Young WL. 2012. Minocycline-
and tetracycline-class antibiotics are protective against partial seizures in vivo.
Epilepsy Behav 24:314–318.
Wang N, Mi X, Gao B, Gu J, Wang W, Zhang Y, Wang X. 2015. Minocycline
inhibits brain inflammation and attenuates spontaneous recurrent seizures fol-
lowing pilocarpine-induced status epilepticus. Neuroscience 287:144–156.
Wu LJ. 2014. Voltage-gated proton channel HV1 in microglia. Neuroscientist
20:599–609.
Wu LJ, Vadakkan KI, Zhuo M. 2007. ATP-induced chemotaxis of microglial
processes requires P2Y receptor-activated initiation of outward potassium
currents. Glia 55:810–821.
Wu LJ, Wu G, Akhavan Sharif MR, Baker A, Jia Y, Fahey FH, Luo HR,
Feener EP, Clapham DE. 2012. The voltage-gated proton
channel Hv1 enhances brain damage from ischemic stroke. Nat Neurosci 15:
565–573.
Wu LJ, Zhuo M. 2008. Resting microglial motility is independent of synaptic
plasticity in mammalian brain. J Neurophysiol 99:2026–2032.
Xiao F, Xu JM, Jiang XH. 2015. CX3 chemokine receptor 1 deficiency leads
to reduced dendritic complexity and delayed maturation of newborn neurons
in the adult mouse hippocampus. Neural Regen Res 10:772–777.
Yang F, Liu ZR, Chen J, Zhang SJ, Quan QY, Huang YG, Jiang W. 2010.
Roles of astrocytes and microglia in seizure-induced aberrant neurogenesis in
the hippocampus of adult rats. J Neurosci Res 88:519–529.
Yeo SI, Kim JE, Ryu HJ, Seo CH, Lee BC, Choi IG, Kim DS, Kang TC.
2011. The roles of fractalkine/CX3CR1 system in neuronal death
following pilocarpine-induced status epilepticus. J Neuroimmunol 234:93–
102.
Yrjanheikki J, Tikka T, Keinanen R, Goldsteins G, Chan PH, Koistinaho J.
1999. A tetracycline derivative, minocycline, reduces inflammation and pro-
tects against focal cerebral ischemia with a wide therapeutic window. Proc
Natl Acad Sci USA 96:13496–13500.
Yu HM, Liu WH, He XH, Peng BW. 2012. IL-1beta: An important cytokine
associated with febrile seizures? Neurosci Bull 28:301–308.
Zattoni M, Mura ML, Deprez F, Schwendener RA, Engelhardt B, Frei K,
Fritschy JM. 2011. Brain infiltration of leukocytes contributes to the patho-
physiology of temporal lobe epilepsy. J Neurosci 31:4037–4050.
Zhan Y, Paolicelli RC, Sforazzini F, Weinhard L, Bolasco G, Pagani F, Vyssotski
AL, Bifone A, Gozzi A, Ragozzino D, et al. 2014. Deficient neuron-microglia
signaling results in impaired functional brain connectivity and social behavior.
Nat Neurosci 17:400–406.
Zhang J, Malik A, Choi HB, Ko RW, Dissing-Olesen L, MacVicar BA. 2014a.
Microglial CR3 activation triggers long-term synaptic depression in the hippo-
campus via NADPH oxidase. Neuron 82:195–207.
Zhang Y, Chen K, Sloan SA, Bennett ML, Scholze AR, O’Keeffe S, Phatnani
HP, Guarnieri P, Caneda C, Ruderisch N, et al. 2014b. An RNA-sequencing
transcriptome and splicing database of glia, neurons, and vascular cells of
the cerebral cortex. J Neurosci 34:11929–11947.
Zhuo M, Wu G, Wu LJ. 2011. Neuronal and microglial mechanisms of neuro-
pathic pain. Mol Brain 4:31.
14 Volume 00, No. 00
