Activity of a NA-LC neuron during the sleep-waking cycle. Note that activity is confined to W, the cell being completely silent during D, SWS, and PS. However, the cell fires during SWS just prior to the onset of W (B3, arrow). The four traces in (B) are from the periods indicated by the bars under “1– 4” in (A). In (A), the behavioral states (W, S1, S2, and PS) are determined with 4-s bins. “Stim” indicates an arousing sound stimulus (hand clapping). In (B), the arrows indicate the state transition, while the arrowhead indicates the onset of EMG activity. In this and the following figures, the scale bars ϭ 500, 500 ␮ V, and 1 mV, respectively, for the EMG, EEG, and unit recordings. 

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... single unit recordings were made from a total of 71 NA-LC neurons during the complete sleep-waking cycle, including at least one PS. In light of previous studies in rats demonstrating the presence of distinct topographic distribution within the NA-LC (Loughlin et al., 1986; España and Berridge, 2006), we divided the LC into rostral, middle, and caudal regions, as shown in Fig. 1, in which the histo- logical localization of the recording sites is depicted. As shown in this figure, recordings were made in the rostral ( n ϭ 10), middle ( n ϭ 25), and caudal ( n ϭ 36; Fig. 2) LC. All NA-LC neurons showed a triphasic and very broad action potential (Fig. 3), characterized by a shoulder on either the early ( n ϭ 52; Fig. 3A1) or late ( n ϭ 19; Fig. 3A2) descending phase of the spike potential. The mean ( Ϯ SD) spike duration measured from onset (t0) to the first (D2) or second (D4) zero crossing was 1.15 Ϯ 0.20 or 4.73 Ϯ 0.28 ms, respectively (Fig. 3B). NA-LC neurons displayed a slow tonic and irregular firing during waking and fell completely silent during both SWS and PS (Fig. 4), as seen with HA-containing tuberomammillary (TM) neurons in mice (Takahashi et al., 2006). As seen in Figs. 4B1, 5A and 6, they discharged either as single spikes or as clusters [peak instantaneous mean frequency (IMEANFR) Ͻ 80 Hz], particularly in waking periods accompanying abrupt body movements. The mean ( SD) CV for the spike interval during AW or QW was 0.90 Ϯ 0.19 or 0.68 Ϯ 0.14, respectively. No marked reduction in unit activity or clustering discharge was observed during grooming or sniffing behavior (Fig. 6). The mean spontaneous discharge rate of all NA-LC neurons during the sleep-waking cycle is shown in Fig. 5B. They fired at the highest rate during AW, showed a significant decrease in firing rate during QW, and virtually stopped firing before the onset of the drowsy state (D) (see below). The differences in firing rate between rostral, middle, and caudal LC neurons were not statistically significant in each state ( P Ͼ 0.05, Kruskal–Wallis) (Table 1). When an arousing sound stimulus (hand clapping) was applied during QW or D, NA-LC neurons responded with either a single spike or a cluster of 2–3 spikes with a short latency (mean Ϯ SD, 11.2 Ϯ 1.9 ms; n ϭ 48) (Fig. 7A1). The differences in latency between the rostral, middle, and caudal LC neurons were not statistically significant ( P Ͼ 0.05, Kruskal–Wallis) (Table 2). When the stimulus was applied during light SWS, the NA-LC neurons responded with a longer mean ( Ϯ SD) latency of 29.2 Ϯ 22.7 ms ( n ϭ 54), particularly when the stimulus elicited a clear change in EEG pattern from synchronization to desynchronization. When the stimulus did not elicit a steady state of EEG desynchronization, they either responded with a long delay (latency Ͼ 50 ms; n ϭ 10) or did not respond at all (latency Ͼ 1 s; n ϭ 8). During deep SWS, they either responded to the stimulus with a pronounced delay (246.9 Ϯ 167.8 ms; n ϭ 16) or did not respond at all ( n ϭ 19). During PS, they did not respond at all to the stimulus in the 19 cases examined. As seen with the arousing sound stimulus, application of an air puff to the mouse’s face elicited unit discharges, together with EEG and behavioral (EMG) alerting responses, during W ( n ϭ 9), D ( n ϭ 18), S1 ( n ϭ 25), S2 ( n 15), and at the beginning of PS ( n 5), but failed to elicit responses during PS ( n ϭ 15). NA-LC neurons responded to the stimulus either with a single spike or with clusters (2– 6 action potentials; IMEANFR Ͻ 70 Hz) (Fig. 7A2). Because we could not determine the exact latency to onset of unit discharge after air-puff application, we measured the interval between the onset of phasic EMG activation and the first unit discharge (Fig. 7A2). NA-LC neurons discharged prior to ( Ϫ ), at the same time as (0), or after ( ϩ ), the onset of the EMG activation. The mean ( Ϯ SD) interval observed during W, D, S1, S2, and at the beginning of PS was, respectively, 6.6 Ϯ 11.0, 53.2 Ϯ 73.4, 63.9 Ϯ 70.2, 78.7 Ϯ 72.4, and 145.1 Ϯ 155.3 ms. Transition from SWS to W. Two types of spontaneous transition from SWS to W were distinguished, that is transitions without (Fig. 7A3) or with sudden EMG activation, as seen with air-puff application (Fig. 7A2). In both cases, all NA-LC neurons exhibited firing before the onset of EEG activation. When awakening accompanied a sa- lient EMG activity, NA-LC neurons discharged in clusters (2– 6 action potentials; IMEANFR Ͻ 60 Hz). At this transition, the mean ( Ϯ SD) interval between the first spike discharge and the onset of EEG activation was 361.1 Ϯ 129.2 ms ( n ϭ 105). When awakening did not accompany an EMG change, NA-LC neurons discharged as a single spike before the onset of EEG activation, with a mean ( Ϯ SD) interval of 986.4 Ϯ 549.1 ms ( n ϭ 208). The differences in interval between the rostral, middle, and caudal LC neurons were not statistically significant ( P Ͼ 0.05, Kruskal– Wallis) (Table 2). In order to compare the activity profiles of waking- specific NA-LC neurons and those previously reported for the sleep-specific POA/BFB and waking-specific POA/BFB and PH neurons (Takahashi et al., 2006, 2008, 2009), we determined the time course of NA-LC unit discharges from SWS to W and from W to D with 0.1-s bins (Fig. 8). Because the firing rate of the NA-LC neurons was very low, we examined the transitions using all their unit recordings. At the spontaneous transition from SWS to W accompanying abrupt EMG activation ( n ϭ 74), NA-LC neurons displayed a sharp increase in discharge rate, beginning to fire as early as 1.0 s prior to the onset of W (EEG activation) (Fig. 8, upper panel, black thick line). At the transition without accompanying EMG activation ( n ϭ 183), they be- gan to fire in single spikes as early as 2.2 s prior to the onset of W, exhibiting a progressive increase in discharge rate (Fig. 8, upper panel, green thick line). Transition from W to D. During transitory periods from W to SWS in which the mouse exhibited frequent alternation between QW and ...