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Characterization of PICs in 5-HT neurons. (A) The transverse slices were collected from the medulla indicated by dash lines. (B) One slice cut from the sections shows the EYFP⁺ serotonin (5-HT) neurons with enhanced yellow fluorescent proteins in parapyramidal region (PPR) and midline raphe nuclei (MRN) areas. (C) Measurement of biophysical parameters of PICs by the voltage bi-ramp from −70 to 50 mV. (D) Measurement of biophysical parameters (recruitment currents, decruitment currents, and difference ΔI) from repetitive firing induced by current bi-ramp with a duration of 10 s, peak of 60–80 pA, and holding current of zero pA. (E) Distribution of 5-HT neurons with PICs in medulla (top). Proportions of PICs (solid) and Non-PICs (open) recorded from PPR and MRN 5-HT neurons (bottom). (F) Statistic results of the onset and amplitude of PICs in PPR (n = 107) and MRN (n = 91). Error bars show SD, unpaired t-test. (G) Four patterns of PICs in 5-HT neurons of medulla. (G1) a-PIC only with counterclockwise hysteresis. (G2) a-PIC > d-PIC with counterclockwise hysteresis of PICs. (G3) d-PIC only with clockwise hysteresis. (G4) d-PIC > a-PIC with clockwise hysteresis of PICs.
Source publication
Serotonergic (5-HT) neurons in the medulla play multiple functional roles associated with many symptoms and motor activities. The descending serotonergic pathway from medulla is essential for initiating locomotion. However, the ionic properties of 5-HT neurons in the medulla remain unclear. Using whole-cell patch-clamp technique, we studied the bio...
Citations
... 15,16 Previous studies have reported that persistent sodium channels regulate Vth and repetitive firing in spinal motoneurons, 36 interneurons, 76 hippocampal neurons, 77 and brainstem 5-HT neurons. 78 In addition, persistent sodium channels are critical in regulating pacemakers and locomotion speed. 79 Recent studies have discovered that 3 weeks of treadmill training increases neuronal excitability through the enhancement of persistent sodium channels in spinal interneurons 15 and midbrain 5-HT neurons in mice. ...
... 48,66 Studies of their FÀI relationship suggest persistent sodium channels also play a role in regulating output from spinal neurons during locomotion. 23,78,80 More recent studies suggest persistent sodium channels may contribute to the hour-long effects of transspinal stimulation combined with epidural polarization, suggesting they also play a role in promoting rehabilitation. 81,82 Overall, persistent sodium channels help to facilitate the adaptation of the locomotor system in response to exercise. ...
... 23 L-type calcium channels have also been shown to amplify synaptic excitatory input, enhance firing frequency, 73 and maintain continuous firing in spinal neurons. 75,78,80 Recent studies have revealed that 3 weeks of treadmill exercise enhances L-type calcium channel activity in laminar X interneurons and 5-HT neurons of the dorsal raphe nucleus. 12,16 Further studies have confirmed L-type calcium channels mainly contribute to the regulation of repetitive firing, the facilitation of spike initiation (along with persistent sodium channels), and exercise-prolonged hysteresis of firing after chronic exercise (along with persistent sodium channels). ...
Vertebrate neurons are highly dynamic cells that undergo several alterations in their functioning and physiologies in adaptation to various external stimuli. In particular, how these neurons respond to physical exercise has long been an area of active research. Studies of the vertebrate locomotor system's adaptability suggest multiple mechanisms are involved in the regulation of neuronal activity and properties during exercise. In this brief review, we highlight recent results and insights from the field with a focus on the following mechanisms: (a) alterations in neuronal excitability during acute exercise; (b) alterations in neuronal excitability after chronic exercise; (c) exercise-induced changes in neuronal membrane properties via modulation of ion channel activity; (d) exercise-enhanced dendritic plasticity; and (e) exercise-induced alterations in neuronal gene expression and protein synthesis. Our hope is to update the community with a cellular and molecular understanding of the recent mechanisms underlying the adaptability of the vertebrate locomotor system in response to both acute and chronic physical exercise.
... The liquid conjunction potential was calculated as 10.4 mV with pH value adjusted to 7.3 by KOH, osmolarity adjusted to 310 mOsm by sucrose, and the presence of 10 μM TEA in the recording solution. This value was not corrected in this study, in order to make our data comparable to those from our previous study of PICs (Cheng et al., 2021;Dai & Jordan, 2010). ...
... Incomplete space clamp is a problem for almost all studies using whole cell patch-clamp techniques (Cheng et al., 2021;Dai & Jordan, 2011). Any voltage-dependent current could be contaminated by the unclamped currents. ...
... We classified these patterns into three types based on differences in onset and offset voltages of PICs. In fact, six patterns of PICs have been described previously in locomotor-activated spinal interneurons (Dai & Jordan, 2010) and four patterns in the brainstem serotonergic neurons (Cheng et al., 2021;Ge & Dai, 2020). PICs have been shown to be responsible for variable patterns of repetitive firing induced by current bi-ramps (Harvey et al., 2006;Lee & Heckman, 1998Powers & Binder, 2003;Powers et al., 2008). ...
Chronic exercise has been shown to enhance excitability of spinal interneurons in rodents. However, the mechanisms underlying this enhancement remain unclear. In this study we investigated adaptability of lamina X neurons with 3‐week treadmill exercise in mice of P21–P24. Whole‐cell patch‐clamp recording was performed on the interneurons from slices of T12–L4. The experimental results included the following. (1) Treadmill exercise reduced rheobase by 7.4 ± 2.2 pA (control: 11.3 ± 6.1 pA, n = 12; exercise: 3.8 ± 4.6 pA, n = 13; P = 0.002) and hyperpolarized voltage threshold by 7.1 ± 1.5 mV (control: −36.6 ± 4.6 mV, exercise: −43.7 ± 2.7 mV; P = 0.001). (2) Exercise enhanced persistent inward currents (PICs) with increase of amplitude (control: 140.6 ± 56.3 pA, n = 25; exercise: 225.9 ± 62.5 pA, n = 17; P = 0.001) and hyperpolarization of onset voltage (control: −50.3 ± 3.6 mV, exercise: −56.5 ± 5.5 mV; P = 0.001). (3) PICs consisted of dihydropyridine‐sensitive calcium (Ca‐PIC) and tetrodotoxin‐sensitive sodium (Na‐PIC) components. Exercise increased amplitude of both components but hyperpolarized onset voltage of Na‐PIC only. (4) Exercise reduced derecruitment current of repetitive firing evoked by a current bi‐ramp and prolonged firing in the falling phase of the bi‐ramp. The derecruitment reduction was eliminated by bath application of 3 μM riluzole or 25 μM nimodipine, suggesting that both Na‐PIC and Ca‐PIC contributed to the exercise‐prolonged hysteresis of firing. (5) Exercise facilitated dendritic development with significant increase in dendritic length by 285.1 ± 113 μm (control: 457.8 ± 171.8 μm, n = 12; exercise: 742.9 ± 357 μm, n = 14; P = 0.019). We concluded that 3‐week treadmill exercise increased excitability of lamina X interneurons through enhancement of PICs and increase of dendritic length. This study provided insight into cellular and channel mechanisms underlying adaptation of the spinal motor system in exercise. image
Key points
Chronic exercise alters adaptability of the spinal motor system in rodents; multiple mechanisms are responsible for the adaptation, including regulation of neuronal excitability and change in dendritic morphology.
Spinal interneurons in lamina X are a cluster of heterogeneous neurons playing multifunctional roles in the spinal cord; chronic exercise in juvenile mice increased excitability of these interneurons and facilitated dendritic development.
Lamina X neurons expressed persistent inward currents (PICs) with calcium (Ca‐PIC) and sodium (Na‐PIC) components; the exercise‐increased excitability of lamina X neurons was mediated by enhancing the Ca‐PIC and Na‐PIC components and increasing dendritic length.
This study unveiled novel morphological and ionic mechanisms underlying adaptation of lamina X neurons in rodents during chronic exercise.
... Neuronal excitability is generally characterized not only by voltage threshold, but also by recruitment (I rec ) and decruitment (I dec ) currents (e.g., Cheng et al., 2020;Cheng et al., 2021). In the present study, we defined the recruitment current as the current ramp at which the first spike was initiated ( Figures 3A1,A2, red dots) and decruitment current as the current ramp at which the last spike was elicited ( Figures 3B1,B2). ...
Oscillations in membrane potential induced by synaptic inputs and intrinsic ion channel activity play a role in regulating neuronal excitability, but the precise mechanisms underlying their contributions remain largely unknown. Here we used electrophysiological and modeling approaches to investigate the effects of Gaussian white noise injected currents on the membrane properties and discharge characteristics of hypoglossal (HG) motoneurons in P16-21 day old rats. We found that the noise-induced membrane potential oscillations facilitated spike initiation by hyperpolarizing the cells’ voltage threshold by 3.1 ± 1.0 mV and reducing the recruitment current for the tonic discharges by 0.26 ± 0.1 nA, on average (n = 59). Further analysis revealed that the noise reduced both recruitment and decruitment currents by 0.26 ± 0.13 and 0.33 ± 0.1 nA, respectively, and prolonged the repetitive firing. The noise also increased the slopes of frequency-current (F-I) relationships by 1.1 ± 0.2 Hz/nA. To investigate the potential mechanisms underlying these findings, we constructed a series of HG motoneuron models based on their electrophysiological properties. The models consisted of five compartments endowed with transient sodium (NaT), delayed-rectify potassium [K(DR)], persistent sodium (NaP), calcium-activated potassium [K(AHP)], L-type calcium (CaL) and H-current channels. In general, all our experimental results could be well fitted by the models, however, a modification of standard Hodgkin-Huxley kinetics was required to reproduce the changes in the F-I relationships and the prolonged discharge firing. This modification, corresponding to the noise generated by the stochastic flicker of voltage-gated ion channels (channel flicker, CF), was an adjustable sinusoidal function added to kinetics of the channels that increased their sensitivity to subthreshold membrane potential oscillations. Models with CF added to NaP and CaL channels mimicked the noise-induced alterations of membrane properties, whereas models with CF added to NaT and K(DR) were particularly effective in reproducing the noise-induced changes for repetitive firing observed in the real motoneurons. Further analysis indicated that the modified channel kinetics enhanced NaP- and CaL-mediated inward currents thus increasing the excitability and output of HG motoneurons, whereas they produced relatively small changes in NaT and K(DR), thus balancing these two currents and triggering variability of repetitive firing. This study provided insight into the types of membrane channel mechanisms that might underlie oscillation-induced alterations of neuronal excitability and motor output in rat HG motoneurons.
... The persistent inward currents (PICs) are widely found in the mammalian spinal neurons and mainly mediated by L-type calcium channels (Schwindt and Crill 1980a, b;Crill 1996;Carlin et al. 2000;Lee and Heckman 2001;Hounsgaard and Kjaerul 2010;Powers et al. 2012) and slow inactivated sodium channels (Zhong et al. 2007;Tazerart et al. 2008;Brocard et al. 2010).These currents are involved in multiple functions including induction of plateau potential and amplification of synaptic inputs (Binder et al. 2019). The PICs are also found in 5-HT neurons of the brainstem and have been shown to upregulate excitability of the neurons (Cheng et al. 2019(Cheng et al. , 2021. Cholinergic modulation of PICs in the spinal cord and hypoglossal motoneurons in the brainstem has been reported in previous studies (Delgado-Lezama et al. 1997;Hounsgaard 1998;Revill et al. 2019). ...
... The PICs were found in 5-HT neurons of the brainstem in previous studies (Cheng et al. 2019(Cheng et al. , 2021. However, little is known about the cholinergic modulation of PICs in the 5-HT neurons. ...
... ACh-induced reduction of composite PICs (4/27) and Na-PIC (3/10) were observed in some 5-HT neurons (data not shown). ACh-induced hyperpolarization of the membrane potential of the 5-HT neurons was reported in our recent studies, and we also found that excitability of 5-HT neurons could be modulated by the PICs (Dai and Jordan 2016;Cheng et al. 2021). In hippocampal pyramidal neurons, Cantrell had reported that acetylcholine reduced Na + currents by binding to muscarinic receptors (Cantrell et al. 1996). ...
Persistent inward currents (PICs) play important roles in regulating neural excitability. Results from our previous studies showed that serotonergic (5-HT) neurons of the brainstem expressed PICs. However, little is known about cholinergic (ACh) modulation of PICs in the 5-HT neurons. The whole-cell patch-clamp recordings were performed in the brainstem slices of ePet-EYFP mice to investigate the electrophysiological properties of PICs with cholinergic modulation. PICs in 5-HT neurons were activated at − 51.4 ± 3.7 mV with the amplitude of − 171.6 ± 48.9 pA (n = 71). Bath application of 20–25 μM ACh increased the amplitude by 79.1 ± 42.5 pA (n = 23, p < 0.001) and hyperpolarized the onset voltage by 2.2 ± 2.7 mV (n = 23, p < 0.01) and half-maximal activation by 3.6 ± 2.7 mV (n = 6, p < 0.01). Muscarine mimicked the effects of ACh on PICs, while bath application of nicotine (15–20 μM) did not induce substantial change in the PICs (n = 9). Muscarine enhanced the amplitude of PICs by 100.0 ± 27.4 pA (n = 28, p < 0.001) and lowered the onset voltage by 2.8 ± 1.2 mV (n = 28, p < 0.001) and the half-maximal activation by 2.9 ± 1.4 mV. ACh-induced increase of amplitude and hyperpolarization of onset voltage were blocked by 3–5 μM atropine. Furthermore, the muscarine-induced enhancement of the PICs was antagonized by 5 μM 4-DAMP, the antagonist of M3 receptor, while the antagonists of M1 (Telenzepine, 5 μM) and M5 (VU6008667, 5 μM) receptors did not significantly affect the PIC enhancement. This study suggested that ACh potentiated PICs in 5-HT neurons of the brainstem by activating muscarinic M3 receptor.
