Circadian regulation of mouse suprachiasmatic nuclei neuronal states shapes responses to orexin

Abstract Our knowledge of how circadian and homeostatic brain circuits interact to temporally organize physiology and behavior is limited. Progress has been made with the determination that lateral hypothalamic orexin (OXA) neurons control arousal and appetitive states, while suprachiasmatic nuclei (SCN) neurons function as the master circadian clock. During the day, SCN neurons exhibit heterogeneity in spontaneous resting membrane potential (RMP), with some neurons becoming severely depolarized (hyperexcited) and ceasing to fire action potentials (APs), while other neurons rest at moderate RMP and fire APs. Intriguingly, the day phase is when the SCN clock is most readily influenced by arousal, but it is unclear if and how heterogeneity in the excitability state of SCN neurons shapes their response to arousal signals, such as OXA. In whole‐cell recordings we show that during the day OXA recruits GABA‐GABAA receptor signaling to suppress the RMP of hyperexcited silent as well as moderately hyperpolarized AP‐firing SCN neurons. In the AP‐firing neurons, OXA hyperpolarized and silenced these SCN cells, while in the hyperexcited silent neurons OXA suppressed the RMP of these cells and evoked either AP‐firing, depolarized low‐amplitude membrane oscillations, or continued silence at a reduced RMP. These results demonstrate how the resting state of SCN neurons determines their response to OXA, and illustrate that the inhibitory action of this neurochemical correlate of arousal can trigger paradoxical AP firing.


Introduction
Circadian and homeostatic signals interact to orchestrate the temporal architecture of behavioral and metabolic states (Bechtold & Loudon, 2013). The arousal-promoting orexin/hypocretin neurons in the lateral hypothalamus and the circadian clock neurons within the suprachiasmatic nuclei (SCN) are two circuits involved in this process. In the SCN, activity of the circadian molecular clock, of which the Period1 (Per1) gene forms a key component, is synchronized by the environmental light-dark cycle through the glutamatergic retinohypothalamic tract. This drives daily changes in electrical states, with SCN neurons being overtly more active (upstate) across the day and less excited (downstate) at night (Brown & Piggins, 2007;Colwell, 2011;Belle, 2015). Such pronounced day-night variation in SCN electrical activity functions to communicate temporal signals to the rest of the brain and body, including orexin neurons. Indeed, the activity of orexin neurons is under circadian control (Zhang et al., 2004), with their molecular activity (Estabrooke et al., 2001;Marston et al., 2008) and orexin release elevated during the behaviorally active circadian night in nocturnal rodents (Deboer et al., 2004).
The phase of the SCN circadian clock is sensitive to feedback from arousal-promoting stimuli, particularly during the day (Mistlberger & Antle, 2011;Hughes & Piggins, 2012). Such stimuli activate orexin neurons (Estabrooke et al., 2001;Marston et al., 2008;Webb et al., 2008), whose brain-wide projection targets include several structures of the neural circadian system (McGranaghan & Piggins, 2001;Backberg et al., 2002). Indeed, orexin neurons form putative synaptic appositions onto SCN clock neurons and transcripts for orexin receptor 1 and 2 (OX 1 and OX 2 , respectively) are expressed in the SCN (Belle et al., 2014). Orexin exists as two forms, orexin-A (OXA) and orexin-B, and exogenously applied OXA acts to mainly suppress electrical activity of rodent SCN cells (Brown et al., 2008;Klisch et al., 2009;Belle et al., 2014). This suggests that OXA released in the vicinity of SCN neurons during states of arousal can influence excitability in this brain region.
The SCN neural population is both molecularly and neurochemically heterogeneous. For example, in mice in which an enhanced green fluorescent protein (EGFP) reports the activity of the Per1 promotor, many SCN neurons express the Per1-driven construct (Per1-EGFP+ve), but in some neurons this construct is not detected (EGFPÀve, presumed non-Per1 cells) (Kuhlman et al., 2000;Belle et al., 2009). This heterogeneity also extends to the neurophysiology of SCN neurons during the day, but not at night. Indeed, targeted whole-cell recordings in mouse SCN revealed that although the intrinsic membrane excitability states of Per1-EGFP+ve neurons and EGFPÀve cells are generally similar across much of the day-night cycle, during the mid-afternoon (ZT6-10) they are vastly different (Belle et al., 2009). During the mid-afternoon, Per1-EGFP+ve neurons enter hyperexcited electrical states, where their resting membrane potential (RMP) becomes severely depolarized (~À33 mV) and their input resistance (R input ) elevates to maximal values (~2-3 GO) (Belle et al., 2009). Consequently, at these hyperexcited states Per1-EGFP+ve neurons stop generating action potentials (APs) through depolarization blockade, and become completely silent or generate depolarized low-amplitude membrane oscillations (DLAMOs; Belle et al., 2009;Diekman et al., 2013). By stark contrast, EGFPÀve neurons do not enter hyperexcited states at any time across the day-night cycle, and in the mid-afternoon they exhibit lower R input (~1.5 GO) and rest at moderate RMP (~À45 mV) where they can readily discharge APs (Belle et al., 2009).
The physiological significance for this dichotomy in Per1-EGFP+ve and EGFPÀve excitability states during this time of the day remains elusive, and here we explore the possibility that their resting states shape and influence the way SCN neurons respond to inputs, specifically the actions of the arousal-evoking neuropeptide, OXA.

Animals
We used 28 male and female mice (8 weeks to 6 months old) hemizygous for the Period1::d2EGFP transgene (Per1-EGFP-expressing mice in which an enhanced destabilized green fluorescent protein (EGFP) reports the activity of the Per1 promoter (a gift from Professor D. McMahon, Vanderbilt University, USA; see Kuhlman et al., 2000). These animals were bred and supplied by the Biological Services Facility of the University of Manchester, and were grouphoused on a 12 hour light : 12 hour dark (L : D) cycle (lights on at 07:00). Food and water were available ad libitum. All experimental procedures were carried out according to the provisions of the UK Animal (Scientific Procedures) Act 1986, and approved by the Research Ethics Committee of the University of Manchester.
Preparation of living mouse SCN brain slices for in vitro whole-cell recordings Brain sections were prepared during the subjective day at Zeitgeber Time (ZT) 1-2, with ZT0 defined as the time of lights on. Coronal brain slices containing the mid-level of the rostro-caudal SCN were prepared from 28 male and female Per1-EGFP mice (one slice per animal), as previously described (Belle et al., 2009(Belle et al., , 2014. Briefly, animals were deeply anesthetized with isoflurane to minimize pain and discomfort and killed by cervical dislocation and decapitation. Brains were immediately removed and 250 lm thick coronal slices cut with a vibroslicer (Campden Instruments, Loughborough, UK) in an ice-cold (4°C) low Na + /Ca 2+ , high Mg 2+ sucrose-based artificial cerebrospinal fluid (aCSF) (in mM: NaCl 95; KCl 1.8; KH 2 PO 4 1.2; CaCl 2 0.5; MgSO 4 7; NaHCO 3 26; glucose 15; sucrose 50; Phenol Red 0.005 mg/L; oxygenated with 95% O 2 ; 5% CO 2 ; pH 7.4, measured osmolarity 300-310 mOsmol/kg). For whole-cell patch-clamp recordings, slices were transferred to a recording chamber mounted on the stage of an upright Olympus epi-fluorescence microscope (BX51WI; Olympus, Japan), where they were continuously perfused (~3 mL/min) with recording aCSF. The composition of the recording aCSF was identical to the incubation solution except for the following (mM): NaCl 127; CaCl 2 2.4; MgSO 4 1.3; sucrose 0.

Whole-cell recordings
Targeted recordings (current-or voltage-clamp) were made from Per1-EGFP positive (henceforth referred to as Per1-EGFP+ve) neurons and cells in which EGFP could not be detected (EGFPÀve), in the ventral and central sub-regions of the SCN on the coronal plane. These SCN anatomical regions were chosen as they are associated with arousal-promoting signal input to the SCN (Morin, 2013). Per1-EGFP+ve neurons were visually identified and distinguished from EGFPÀve cells with a 40 9 water immersion UV objective (LUMplanFL/IR; Olympus) using epifluorescence with a Leica camera (DFC 350 FX) and capturing software (Leica Application Suite; Leica Microsystems, UK), as previously described (Belle et al., 2009(Belle et al., , 2014Scott et al., 2010; see also Fig. 1). Giga-ohm seal and cell membrane rupture were done under infra-red video-enhanced differential interference contrast microscopy with an infra-red camera (Hitachi, Japan). An Axoclamp 2A amplifier (Molecular Devices, CA, USA) was used for current-clamp recordings as previously described (Belle et al., 2009(Belle et al., , 2014. Patch pipette electrodes (7-10 MΩ) were fashioned from thick-walled borosilicate glass capillaries (Harvard Apparatus) using a two-stage vertical micropipette puller (PB-10; Narashige, Japan). Unless otherwise stated, electrodes were filled with an internal solution containing (mM: K-gluconate 130; KCl 10; MgCl 2 2; K 2 -ATP 2; Na-GTP 0.5; Hepes 10, EGTA 0.5; pH adjusted to 7.3 with KOH, measured osmolarity 295-300 mOsmol/kg). Pipette series resistance (typically 10-30 MΩ) was corrected using bridge-balance in current-clamp experiments and was not compensated during voltage-clamp recordings. Cells were accepted for analysis only if the series resistance was below 30 MΩ, remained within 20% of this value throughout the recordings and the RMP remained stable for over 2 min before orexin-A (OXA) application. RMP, spontaneous firing rate (SFR) and input resistance (R input ) were determined within 1-2 min of membrane rupture. The average firing rate in firing cells was measured as the number of spikes per second within a 20 s window of stable firing, and average RMP was measured as the mean voltage (including AP or DLAMOs, for cells displaying such activities) over a 20 s window using a custom-written SPIKE2 script [Cambridge Electronic Design (CED), Cambridge, UK]. R input was assessed only in current-clamp recordings and was estimated by a series of hyperpolarizing current pulses (À10 to À30 pA for 500 ms). Signals were sampled at 30 kHz, and stored and analyzed on a personal computer using SPIKE2 software (Version 7; CED). All data acquisition and step protocols were generated through a micro 1401 mkII interface (CED).

Synaptic current measurement
Post-synaptic currents (PSCs) were measured with an Axopatch 200B amplifier (Molecular Devices), holding the cells at À70 mV. Patch pipettes (4-6 MΩ) were fashioned as described above and filled with an internal solution that was identical to the one used in current-clamp recordings, except for the following (mM): K-gluconate 120; KCl 20. Dialyzing SCN neurons with this concentration of Cl À in the pipette causes inhibitory PSCs to reverse between À40 to À50 mV (Itri & Colwell, 2003;Belle et al., 2014). At À70 mV this positive shift in Cl À reversal potential causes GABA and glutamate PSCs to appear as inward currents. To discriminate between excitatory and inhibitory PSCs, a cocktail of antagonists (AP5 and CNQX) for the glutamate and/or GABA A (Gabazine) receptors were used.

Drugs and analysis
Stock solutions (1000 9 concentration of working dilutions) for orexin-A (OXA), D-2-amino-5-phosphonopentanoate (AP5), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and gabazine (Tocris, Bristol, UK) were made in purified MilliQ water. Working drug dilutions were made in aCSF immediately before their application. Recording aCSF (drug-free or containing drugs) solution was bath applied to SCN slices by gravity-feed perfusion. Switching from drug-free to drugcontaining aCSF was done using an in-house built drug-switching solenoid valves system. All recording and pharmacology methods used for isolating and measuring the effects of OXA on synaptic activity, RMP and AP firing frequency in SCN neurons were as previously established (Belle et al., 2014). Analysis of PSC frequency and amplitude were conducted offline by template-based sorting in CLAMPFIT 10.2 (Molecular Devices). Synaptic events were detected with an amplitude threshold of 5 pA (see Belle et al., 2014).
Grouped data from Per1-EGFP+ve and EGFPÀve neurons were initially examined for normality using the Kolmogorov-Smirnov (K-S) test. When normality was found, data were statistically Example of typical current-clamp traces from Per1-EGFP+ve SCN neurons recorded in the morning (ZT2-5) and afternoon (ZT6-10). Note that Per1-EGFP+ve neurons recorded in the morning are at moderate resting membrane potential (RMP:~À45 mV) and discharging action potentials (APs), while in the afternoon they become hyperexcited with RMPs of À25 to À35 mV and unable to generate APs. These cells are either silent or generating depolarized low-amplitude membrane oscillations (DLAMOs). By contrast, EGFPÀve neurons (E) remain at moderate RMP throughout the day, firing APs. Dashed lines in C indicate the outline of the patch pipette. SCN, suprachiasmatic nuclei; OX, optic chiasm; P, patch pipette. [Colour figure can be viewed at wileyonlinelibrary.com]. compared using two-way ANOVA with repeated measures and/or paired/unpaired two-tailed t tests. Bonferroni correction was used for multiple comparisons. In the absence of normality, statistical comparisons were made using the Mann-Whitney U Test or related samples Wilcoxon Test (SPSS version 22; SPSS Inc., Chicago, IL, USA). For all tests, P < 0.05 was considered significant. All numerical data both in text and graphs represent mean AE SEM.

OXA suppresses the RMP of Per1-EGFP+ve and EGFPÀve neurons
During the day, OXA acts in the SCN to mainly suppress excitability in Per1-EGFP+ve neurons by elevating the pre-synaptic release of GABA onto post-synaptic GABA A receptors (Belle et al., 2014). To test if a similar mechanism of OXA suppression operates in EGFPÀve cells and to investigate whether the contrasting electrical states in mid-afternoon Per1-EGFP+ve and EGFPÀve neurons functionally determine their responses to OXA, we bath applied OXA (80 nM for 3 min) to the current-clamped Per1-EGFP+ve and EGFPÀve neurons, and compared if and how their electrical states changed in the presence of this neuropeptide.
Cell-state and cell-type determine the response magnitude of SCN neurons to OXA The above observation suggests that afternoon cell-state and/or celltype underpins the contrasting magnitude in response to OXA. To directly investigate these possibilities we next assessed the magnitude of OXA-evoked membrane suppression in Per1-EGFP+ve cells current-clamped in the morning (ZT2-5), a time when the natural intrinsic excitability state of Per1-EGFP+ve neurons is moderate and similar to that measured in EGFPÀve cells (Fig. 1D, compare with 1E), and importantly, is vastly different from afternoon Per1-EGFP+ve cells (Fig. 1D, see also Belle et al., 2009). We compared the magnitude of these morning responses to OXA with those measured in afternoon neurons. As previously described, morning Per1-EGFP+ve neurons rested at moderate RMPs (À44.6 AE 1.5 vs. afternoon À30.5 AE 1 mV; T (26) = À8.301, P < 0.0001) and can discharge APs (2.5 AE 0.4 vs. 0 Hz; Z = À3.594, P < 0.0001; Fig. 1D, 17 neurons from six animals). When challenged with OXA, the majority of these cells (12/17; 71%) reduced excitability by hyperpolarization and suppression or abolishment of AP firing (À44.6 AE 1.5 vs. À53.4 AE 1.7 mV, T (11) = 8.380, P < 0.0001; 2.6 AE 0.4 vs. 0.1 AE 0.04 Hz, Z = À2.936, P = 0.003; Fig. 2B, H and K), while 2/17 (~12%) did not respond and 3/17 (~17%) showed membrane excitation (data not shown). When the magnitude of this RMP change was compared with that measured in hyperexcited afternoon Per1-EGFP+ve neurons, a significant difference was detected (8.9 AE 1.1 vs. 14.2 AE 1 mV; T (26) = À3.654, P = 0.001), indicating that OXA evoked larger delta RMP in afternoon than in morning Per1-EGFP+ve cells (Fig. 2I). We next compared OXAevoked delta RMP measured in morning Per1-EGFP+ve with that assessed in EGFPÀve cells to test whether elements of cell-type with similar excitability state also determine the magnitude of OXA response. Interestingly, although the steady-state RMP attained during OXA application was not significantly different in these two cell populations (À53.4 AE 1.7 vs. À50 AE 1.3 mV; P > 0.05; Fig. 2H), delta RMP in Per1-EGFP+ve neurons was significantly larger than that measured in EGFPÀve cells (8.9 AE 1.1 vs. 4.5 AE 0.6 mV; T (25) = 3.822, P = 0.001; Fig. 2I). It is noteworthy, however, that when the magnitude of the T-values and mean difference between the deltas were considered across time of day (morning vs. afternoon) as well as neuronal type (Per1-EGFP+ve vs. EGFPÀve), cellstate rather than cell-type emerged as the more defining determinant of delta RMP magnitude to OXA (DRMP in afternoon Per1-EGFP+ve > morning Per1-EGFP+ve > EGFPÀve; Fig. 2I).
The trigger for the range of excitability activities seen in afternoon Per1-EGFP+ve neurons by OXA could be simply by the relief of these cells from depolarization blockade through suppressive GABA-GABA A signaling. We therefore tested whether such a common and general mechanism of OXA-dependent rescue of hyperexcited Per1-EGFP+ve neurons from depolarization blockade accounts for the array of electrical behaviors seen during their responses to OXA. To do this, we recorded from afternoon Per1-EGFP+ve neurons and mimicked the hyperpolarizing effects of OXA by applying controlled steady-state negative currents which artificially hyperpolarized and maintained their RMPs at appropriate levels (from À25 to~À35 and À45 mV; Fig. 3). From the 10 cells actively manipulated in this way, 7/10 (70%, from three animals) fired APs when maintained at~À45 mV (Fig. 3A, B and D). Of these seven cells, 3 (43%) produced DLAMOs when membrane potential was maintained at~À33 mV (Fig. 3A); observations that support our previous finding (Belle et al., 2009). The remaining cells (3/10; 30%) hyperpolarized but sustained silence throughout steady-state current injection (from À25 to À45 mV; Fig. 3C). Passage of a square depolarizing pulse during manual suppression produced APs in these neurons (Fig. 3C). This suggests that intrinsic mechanisms operate to maintain membrane silence in these cells even in the potential range at which APs could be spontaneously fired, as shown by the acute excitatory stimulation. Indeed, this observation supports previous findings that single SCN neurons do not necessarily discharge APs, even when at values of RMP that are permissive of AP firing (M. D. C. Belle and H. D. Piggins, unpublished observations and see also (Rohling et al., 2006)). Furthermore, in these hyperexcited cells a rebound spike can be triggered if the RMP is acutely removed from depolarization blockade by a brief hyperpolarizing pulse ( Fig. 3E and F). By contrast, manual steady-state hyperpolarization of morning Per1-EGFP+ve and afternoon EGFPÀve cells invariantly ceased AP generation (data not shown).

OXA-evoked increase in spontaneous post-synaptic current amplitude is significantly larger in afternoon Per1-EGFP+ve neurons than in EGFP-ve cells
When OXA-responsive Per1-EGFP+ve (n = 5) and EGFPÀve (n = 5) neurons were re-tested with OXA in the presence of the selective GABA A receptor blocker, gabazine, no effects on RMP were observed (Fig. 2F, G, and I). This replicates our earlier finding that gabazine blocks RMP effects of OXA on Per1-EGFP+ve neurons during the day (Belle et al., 2014), and extends this to show that gabazine also prevents hyperpolarizing actions of OXA in EGFPÀve cells. This indicates that in both cell types, OXA recruits indirect/pre-synaptic GABA to signal via the GABA A receptor and hyperpolarize SCN neurons during the day. Our afternoon currentclamp data also demonstrates that the magnitude of RMP suppression by this pre-synaptic action of OXA is significantly larger in hyperexcited Per1-EGFP+ve neurons than in moderately resting EGFPÀve cells (Fig. 2I). This suggests that Per1-EGFP+ve and EGFPÀve SCN neurons have differential response characteristics to OXA-evoked synaptic GABA-GABA A receptor signaling. To investigate this, we next performed voltage-clamp recordings in SCN Per1-EGFP+ve and EGFPÀve neurons (n = 7 cells each, from five animals) and compared the effects of bath-applied OXA on the frequency and amplitude of PSCs. To isolate GABAergic events, we performed these recordings in the presence of AP5 and CNQX, respective blockers of NMDA and non-NMDA receptors.

Discussion
An important quest in neuroscience is identifying processes that shape how neurons respond to inputs. Here, we show that molecular clock-driven changes in membrane potential and neural state is a key factor in shaping SCN neuronal responses to a neurochemical correlate of arousal. Specifically, we revealed that the intrinsic excitability state of Per1-EGFP+ve and EGFPÀve neurons during the day determines how these cells respond to OXA.
Our results indicate that the vast majority of moderately hyperpolarized AP-discharging EGFPÀve (~94%) and Per1-EGFP+ve (~71%) cells were hyperpolarized and silenced by OXA. This differs from hyperexcited Per1-EGFP+ve neurons in which diverse electrical activities were seen during OXA-evoked RMP suppression, ranging from DLAMO generation to paradoxical spiking. In both cell populations these OXA-dependent electrical behaviors relied on GABA-GABA A receptor signaling, but not ionotropic glutamatergic transmission. This is consistent with our previous observation that OXA-mediated pre-synaptic release of GABA in the SCN is critical for OXA's modulatory suppression of SCN neuron activity during the day (Belle et al., 2014). Here, we extend these observations to show that this OXA-GABA-GABA A receptor- Fig. 3. Manual hyperpolarization elicits a range of electrical states in hyperexcited Per1-EGFP+ve neurons that are comparable to those evoked by OXA. (A) Typical silent Per1-EGFP+ve neuron firing APs when manually maintained at~À44 mV and producing DLAMOs when transiting between silent and firing states (equivalent to OXA-evoked responses in Fig. 2E). Gradual release from forced hyperpolarization returned this cell to DLAMO and silent states. (B) A DLAMOs producing neuron forced to fire APs upon manual hyperpolarization and stopped/reduced AP firing when RMP is maintained at a more negative potential. Cell discharges APs when RMP is returned to firing range. (C) Typical silent Per1-EGFP+ve neuron that sustained silence when hyperpolarized to a RMP that promotes AP discharge (equivalent to OXA-evoked response shown in 2C). Depolarizing pulses of 10 and 20 pA elicited AP firing, demonstrating this cell's ability to produce an AP when acutely excited. (D) Silent hyperexcited Per1-EGFP+ve neuron producing APs when maintained at the firing range (~À45 mV) but in between was rendered silent by RMP maintenance below (À58 mV) firing threshold. (E and F) Typical rebound spike response in depolarized silent cell by a hyperpolarizing pulse which transiently removes the cell from depolarization blockade, re-activating an AP. [Colour figure can be viewed at wileyonlinelibrary.com]. mediated action also operates to suppress activity in EGFPÀve neurons, eliciting distinct and diverse electrical behaviors in these identified SCN cell populations. Our present results also demonstrate that the magnitude of these OXA actions depends primarily on the intrinsic membrane potential of the neurons.
Surprisingly, the suppressive action of OXA on SCN electrical activity during the day does not have such dramatic effects on clock phase, but instead accentuates the actions of NPY to potently inhibit cellular activity and phase-shift the SCN clock (Belle et al., 2014). These selective effects of OXA may be important in that, when acting alone, it relieves SCN-inhibition of behavioral activity without affecting overall circadian rhythmicity. This allows animals to respond to acute behavioral necessities, such as grooming and drinking (van Oosterhout et al., 2012), without uncoupling behavioral rhythms from prevailing light-dark cycles. Under circumstances when non-photic adjustment of the clock phase may be necessary, such as during physiological conditions that promote IGL-SCN signaling, OXA acts to reinforce these non-photic signals to the SCN. This implicates OXA signaling in the integration of non-photic communication in the SCN, and its distinct and diverse effects on the electrical activity of Per1-EGFP+ve and EGFPÀve neurons seen here may partially underpin such capacity and plasticity in its action.
At multiple brain sites OXA acts to cause excitation, but the importance of its engagement with local GABAergic circuits to suppress/modulate spiking activity in cells is becoming more prominent. For example, in the lateral hypothalamus where orexinergic and melanin-concentrating hormone (MCH) neurons are intermingled, OXA acts through local GABAergic interneurons to mainly suppress electrical activity of MCH cells. This provides a possible switching mechanism which appropriately isolates the conflicting signals of sleep and wakefulness (Apergis-Schoute et al., 2015). Here, and in our previous work, we provide evidence that this local 'switch control' also operates in the SCN circuit, which presumably acts in behaving animals to avoid conflicts between circadian and arousal signals (Belle et al., 2014). Indeed, our present data provide some insight into the complex nature of such 'switch control', indicating that investigation of how this is achieved in vivo to govern behavior will be a challenging venture.
Our results indicate that the change in GABAergic PSC amplitude elicited by OXA in Per1-EGFP+ve neurons was significantly larger than those measured in EGFPÀve cells. This likely underpins the significantly greater magnitude of OXA-evoked RMP suppression in Per1-EGFP+ve neurons than in EGFPÀve cells. As the delta PSC frequency evoked by OXA between these cell types approached but was not statistically significant, the results suggest the existence of mainly clock-driven post-synaptic membrane properties/processes that render Per1-EGFP+ve neurons more amenable to OXA-dependent GABA-GABA A receptor signaling than EGFPÀve cells. Indeed, even when comparison was made between cell-types of similar moderate RMPs and firing capability (morning Per1-EGFP+ve vs. EGFPÀve cells), OXA-induced delta RMP was unexpectedly larger in Per1-EGFP+ve neurons than in EGFPÀve cells. Several intrinsic post-synaptic membrane properties of neurons are associated with synaptic processing gain, measured as PSC amplitude and summation. In the entorhinal stellate cells, for example, depolarization and increases in input resistance give rise to a voltage-dependent increase in excitatory and inhibitory post-synaptic potential amplitude (Economo et al., 2014). In a similar fashion, alteration in input resistance can regulate synaptic transmission efficacy at single synapses (see Harnett et al., 2012 for example). Increased postsynaptic receptor density also influences the magnitude of synaptic signaling, by primarily augmenting the PSC amplitude elicited by pre-synaptic neurotransmitter release, a process that occurs at central GABAergic synapses (Nusser et al., 1998). Although the density of post-synaptic GABA A receptor expression in Per1-EGFP+ve and EGFPÀve neurons is unknown, measurement of membrane properties in these cells revealed that during the day the R input and RMP of Per1-EGFP+ve neurons are significantly higher than those measured in EGFPÀve cells (see also Belle et al., 2009). This difference in excitability states can therefore account for the distinctive response magnitude to OXA measured in these two cell populations.
Perhaps less surprising is the observation that OXA-induced delta RMP in morning Per1-EGFP+ve neurons is smaller than that elicited by this neuropeptide in afternoon Per1-EGFP+ve cells. Indeed, as is the case for morning Per1-EGFP+ve and EGFPÀve neurons, on average the intrinsic baseline R input value of Per1-EGFP+ve cells is significantly elevated in the afternoon when compared with morning measurements (Belle et al., 2009). This positive association of response magnitude with R input further reinforces the possibility that post-synaptic membrane resistance of SCN neurons largely contributes to cell-state and, to a lesser extent, cell-type response magnitude to OXA. By extension this post-synaptic property may also influence the way SCN neurons respond to other neurochemical signals conveyed to this brain structure.
More broadly, our study reveals how intrinsic electrical states can shape neural responses to incoming signals. Hyperexcited and moderately resting neurons with high input resistance are reported in a number of other brain regions, such as the cerebellum, habenula and arcuate nuclei of the mediobasal hypothalamus (Belousov & van den Pol, 1997;Raman et al., 2000;Pugh & Raman, 2009;Sakhi et al., 2014a,b). As these regions also express circadian clock genes, it is plausible that circadian-driven neural states determine how input signals, such as those associated with appetite and motor co-ordination, are received and processed in these brain tissues. Although the function of hyperexcited neurons is yet to be determined, it is likely that, as in the SCN, inhibitory signals conveyed to these brain regions may cause paradoxical generation of spiking activity. Indeed, our manual RMP manipulation in hyperexcited SCN Per1-EGFP+ve cells with steady-state current injection was able to mimic and recapitulate all of the electrical behaviors elicited by OXA in these afternoon SCN neurons. This demonstrates that by the simple removal of hyperexcitation or depolarization blockade, DLAMOs and normal spiking can be resumed in some of these cells. This argues for a general mechanism through which inhibitory signals, such as OXA acting through GABA, can evoke diverse and complex electrical activities in SCN neurons. Such neuronal state transition by inhibitory signals, as evoked here in the SCN by OXA, likely act to shape how cells respond and process information to subsequent synaptic inputs. For example, the transition from depolarization blockade to spiking would enable cells to be more responsive to subsequent excitatory signals.
The distinct responses to OXA in Per1-EGFP+ve and EGFPÀve cells may therefore be central to the way SCN neurons organize, integrate and appropriately balance incoming excitatory photic vs. suppressive arousal signals in behaving animals across the day. Indeed, photic and non-photic signals can interact with each other at the level of the SCN; the effects of non-photic resetting cues are cancelled if the non-photic signal is followed by a light pulse, or administration of glutamatergic receptor agonists (Biello & Mrosovsky, 1995;Biello et al., 1997;Gamble et al., 2004). This may permit animals to appropriately respond to potentially competing external and internal signals in order to organize physiology and behavior.