Differential effects of constant light on circadian clock resetting by photic and nonphotic stimuli in Syrian hamsters

Circadian rhythms in Syrian hamsters can be phase shifted by behavioral arousal during the usual rest phase of the circadian rest-activity cycle. Phase shifts can be greatly potentiated by exposing the animals to constant light for 1 or 2 cycles. This could reflect a change in a specific nonphotic input pathway to the suprachiasmatic nucleus (SCN) circadian pacemaker, or it could be caused by a change in the amplitude of the pacemaker. If the latter, then phase shifts to any stimulus, including those activating the photic input pathway, should be potentiated. This hypothesis was tested by measuring phase shifts induced by microinjections of NMDA (500 nl, 10 mM) into the SCN area of hamsters exposed to constant light or dark for 2 days. NMDA induced significant phase delay shifts that mimicked those induced by light exposure early in the night. The magnitude of these shifts did not differ by prior lighting condition. Shifts induced by NMDA (200 nl, 10 mM) microinjections on day 3 and 13 of LL also did not differ. Phase shifts induced by a nonphotic stimulus (3 h of running stimulated by confinement to a novel wheel) were significantly potentiated by 2 days of exposure to constant light. These results indicate that exposure to constant light for 2 circadian cycles differentially affects phase resetting responses to photic and nonphotic inputs to the circadian pacemaker, suggesting that potentiation of shifts to nonphotic stimuli reflect changes in a nonphotic input pathway rather than in an amplitude dimension of the circadian pacemaker.     

Blockade of the NPY Y5 receptor potentiates circadian responses to light: complementary in vivo and in vitro studies

Neuropeptide Y (NPY) is delivered to the suprachiasmatic nuclei (SCN) circadian pacemaker via an input from the thalamic intergeniculate leaflet. NPY can inhibit light-induced responses of the circadian system of Syrian hamsters. Here we studied whether an antagonist to NPY receptors can be used to potentiate photic phase shifts late in the subjective night. First we determined by in situ hybridization that both NPY Y1 and Y5 receptor mRNA are expressed in the SCN of Syrian hamsters. Second, similar to our previous findings at Zeitgeber time 14 (ZT 14, where ZT 12 was the time of lights off), we found that NPY applied at ZT 18.5 onto the SCN region of brain slices maintained in vitro could block NMDA-induced phase advances of the spontaneous firing rate rhythm, and this blocking effect was probably mediated by the Y5 receptor, since co-application of Y5 receptor antagonists completely reversed the effect of NPY, while application of a Y1 receptor antagonist had no effect under the same conditions. Third, we found that co-treatment with a Y5 receptor antagonist in vivo (s.c., 10 mg/kg) not only reversed the effect of NPY applied to the SCN in vivo through a cannula but also significantly potentiated the light-induced phase advance in the absence of NPY. This is the first report of a NPY receptor antagonist having such an effect, and indicates that NPY Y5 receptor antagonists could be clinically useful for potentiating circadian system responses to light.     

A Thalamic Contribution to Arousal-induced, Non-photic Entrainment of the Circadian Clock of the Syrian Hamster

It is well established that the circadian clock of the suprachiasmatic nuclei (SCN) is entrained by light. More recently, the potent effects of arousing, non-photic cues on the clock have been recognized. The neural mediators of non-photic entrainment are yet to be identified. To examine the contribution of the thalamic intergeniculate leaflet (IGL) and its NPY-immunopositive projection, the geniculo-hypothalamic tract to non-photic entrainment by arousal, male Syrian hamsters received lesions of the IGL (IGLX) which ablated NPY-immunoreactivity in the SCN. Their circadian responses to both photic and non-photic cues were then tested. Lesions resulted in a delay in the timing of activity onset following lights out, but had no effect on the behavioural or cellular circadian responses to phase-advancing light pulses presented at circadian time (CT) CT19 (where CT12 represents the time of activity onset). Injection with a benzodiazepine (chlordiazepoxide, 100 mg/kg) at CT6 suppressed wheel-running, increased general locomotion of intact controls and induced large phase advances of the circadian rhythm of wheel-running. Chlordiazepoxide also inhibited wheel-running in lesioned animals, but there was no significant increase in general locomotion and the lesioned animals did not phase advance. Serial arousal by injection of saline at intervals of 23.5 h for 6 days entrained the circadian rhythm of wheel-running of intact hamsters and was associated with an increase in general locomotor activity. Entrainment by serial arousal was abolished by IGLX. However, the lesioned animals did show a clear behavioural response to every presentation of the non-photic cue. These results show that the IGL is a necessary component of the neural pathways mediating both arousal- and benzodiazepine-induced non-photic entrainment.     

Effect of constant light, pinealectomy and guanosine triphosphate gamma-s on the density of melatonin receptors in the rat suprachiasmatic nucleus: a possible implication of melatonin action

We report here the effects of pinealectomy and exposition to constant light on the density of melatonin receptors in the suprachiasmatic nuclei of the rat using quantitative autoradiography. The B(max) values were significantly increased when the animals were maintained in constant light for 3 days (8.22 ± 0.95 fmol/mg protein versus 4.55±0.14 fmol/mg protein in control group, 12 h light/12 h dark cycle (12L/12D), n = 6). A similar increase was also observed in rats pinealectomized 3 days before sacrifice and then maintained either under 12L/12D (B(max) 7.56±0.80 fmol/mg protein) or in constant light (B(max) 7.85±1.02 fmol/mg protein), while K(d) values failed to show any variations after constant light and/or pinealectomy. The effect of GTPγS on the density of rnelatonin binding sites was also investigated in control animals and after 3 days of constant light. In 12L/12D animals, the B(max) shifted from 5.94 ± 0.14 fmol/mg protein in the absence of GTPγS to 3.97±0.22 fmol/ mg protein in the presence of 50μiM GTPγS. In animals maintained for 3 days in constant light, a similar decrease in the B(max) value was observed (8.95 ± 0.25 fmol/mg protein in absence and 5.95 ± 0.22 fmol/mg protein in presence of 50 μ GTPγS). In both cases, K(d) values were not affected by GTPγS. Pinealectomy and constant light exposition are known to induce a suppression of the nocturnal peak of plasma rnelatonin and to keep plasma rnelatonin concentrations at a very low level. These results could suggest a regulatory effect of rnelatonin on the density of its own receptors which are shown here to be also coupled with a G-protein.     

Metabolic influences on circadian rhythmicity in Siberian and Syrian hamsters exposed to long photoperiods

Calorie restriction and other situations of reduced glucose availability in rodents alter the entraining effects of light on the circadian pacemaker located in the suprachiasmatic nuclei. Siberian and Syrian hamsters are photoperiodic species that are sexually active when exposed to long summer-like photoperiods, while both species show opposite changes in body mass when transferred from long to short or short to long days. Because metabolic cues may fine tune the photoperiodic responses via the suprachiasmatic nuclei, we tested whether timed calorie restriction can alter the photic synchronization of the light-entrainable pacemaker in these two hamster species exposed to long photoperiods. Siberian and Syrian hamsters were exposed to 16 h:8 h light:dark cycles and received daily hypocaloric (75% of daily food intake) or normocaloric diet (100% of daily food intake) 4 h after light onset. Four weeks later, hamsters were transferred to constant darkness and fed ad libitum. The onset of the nocturnal pattern of locomotor activity was phase advanced by 1.5 h in calorie-restricted Siberian hamsters, but not in Syrian hamsters. The lack of phase change in calorie-restricted Syrian hamsters was also observed in individuals exposed to 14 h:10 h dim light:dark cycles and fed with lower hypocaloric food (i.e. 60% of daily food intake) 2 h after light onset. Moreover, in hamsters housed in constant darkness and fed ad lib., light-induced phase shifts of the locomotor activity in Siberian hamsters, but not in Syrian hamsters were significantly reduced when glucose utilization was blocked by pretreatment with 500 mg/kg i.p. 2-deoxy-D-glucose. Taken together, these results show that the photic synchronization of the light-entrainable pacemaker can be modulated by metabolic cues in Siberian hamsters, but not in Syrian hamsters maintained on long days.     

Neuroglobin expression in the rat suprachiasmatic nucleus: Colocalization,innervation, and response to light

Neuroglobin (Ngb) is a myoglobin‐like (Mb) heme‐globin, belonging the globin family located only in neuronal tissue of the central nervous system. Ngb has been shown to be upregulated in and to protect neurons from hypoxic and ischemic injury, but the function of Ngb—in particular how Ngb may protect neurons—remains largely elusive. We have previously described the localization of Ngb in the rat brain and found it to be expressed in areas primarily involved in sleep/wake, circadian, and food regulation. The present study was undertaken, using immunohistochemistry, to characterize the localization, colocalization, innervation, and response to light of Ngb‐immunoreactive (IR) cells in the rat suprachiasmatic nucleus (SCN). Our results demonstrate that the majority of Ngb‐expressing neurons in the SCN belong to a cell group not previously characterized by neurotransmitter content; only a small portion was found to co‐store GRP in the ventral SCN. Furthermore, some Ngb‐containing neurons were responsive to light stimulation at late night evaluated by the induction of cFOS and only a few cells were found to express the core clock gene PER1 during the 24‐hour light/dark cycle. The Ngb‐containing cells received input from neuropeptide Y (NPY)‐containing nerve fibers of the geniticulo‐hypothalamic tract (GHT), whereas no direct input from the eye or the midbrain raphe system was demonstrated. The results indicate that the Ngb could be involved in both photic and nonphotic entrainment via input from the GHT. J. Comp. Neurol. 518:1556–1569, 2010. © 2009 Wiley‐Liss, Inc.     

Neonatal exposure to constant light prevents anhedonia-like behavior induced by constant light exposure in adulthood

Depressive episodes are associated with disturbances in circadian rhythms, and constant illumination has been reported to induce depressive-like behavior in rodents. Rats kept in constant darkness express the endogenous circadian rhythm, and most animals under constant light conditions lose circadian locomotor rhythmicity. Exposure to constant light in rats during lactation was reported to prevent this loss of circadian rhythm in adulthood. Thus, the aim of the present study was to verify whether exposure to constant light during lactation prevents anhedonia-like behavior induced by constant light in adult rats. In experiment 1, we replicated the anhedonia-like effects of constant light in adult male rats. We showed that this effect is reversed by imipramine treatment in the drinking water. In experiment 2, we subjected rats to constant darkness (neonatal-DD), constant light (neonatal-LL) or to normal light/dark cycle (neonatal-LD) during the neonatal phase and evaluated them after constant light exposure in adulthood. The group exposed to constant light during the neonatal phase did not reduce their sucrose preference and exhibited greater locomotor activity than the other groups. The neonatal-DD group exhibited decreased sucrose preference earlier than controls and had higher serum corticosterone concentrations. Prevention of arrhythymicity might protect neonatal-LL rats from anhedonia-like behavior induced by constant light, whereas constant darkness during the neonatal phase rendered the neonatal-DD group more susceptible to depressive-like behavior. These results corroborate with the literature data indicating that circadian disruption may contribute in mood disorders and that early life stress can influence stress responsivity in adulthood.     

The effects of constant light and constant darkness on daily changes in the morphology of the pineal organ in the goldfish,Carassius auratus

Summary The fine structure of photoreceptor cells in the pineal organ of the goldfish was found to vary quantitatively over a 24-hour period. Stereological analysis revealed significant daily changes in the volume of the cell and inner segment, nuclear volume and nucleolar diameter, volume of endo-plasmic reticulum and Golgi bodies, area of both rough and smooth endo-plasmic reticulum, and number of vesicles associated with each Golgi body. Peak values of these variables occurred either during the dark phase or latter part of the light phase. These findings agree closely with those reported in higher vertebrates, and suggest that metabolic activities, and possible secretory functions, of the pineal organ of fishes are synchronized to the light: dark cycle. Daily changes in these variables generally persisted in fish exposed to constant darkness for seven days, with the peaks in these rhythms coinciding closely with those observed in fish exposed to a light: dark cycle. In contrast, the rhythms in all variables were abolished in fish kept in continual light for seven days. Photoreceptor cells from fish exposed to continuous light had larger nucleoli and greater amounts of rough endoplasmic reticulum indicating a further effect of light on pineal metabolism in lower vertebrates.     

Role for the NR2B subunit of the N-methyl-D-aspartate receptor in mediating light input to the circadian system

Light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells that utilize glutamate as a neurotransmitter. A variety of evidence suggests that the release of glutamate then activates N -methyl- d -aspartate (NMDA) receptors within the SCN and triggers a signaling cascade that ultimately leads to phase shifts in the circadian system. In this study, we first sought to explore the role of the NR2B subunit in mediating the effects of light on the circadian system of hamsters and mice. We found that localized microinjection of the NR2B subunit antagonist ifenprodil into the SCN region reduces the magnitude of light-induced phase shifts of the circadian rhythm in wheel-running activity. Next, we found that the NR2B message and levels of phospho-NR2B vary with time of day in SCN tissue using semiquantitative real-time polymerase chain reaction and western blot analysis, respectively. Functionally, we found that blocking the NR2B subunit with ifenprodil significantly reduced the magnitude of NMDA currents recorded in SCN neurons. Ifenprodil also significantly reduced the magnitude of NMDA-induced Ca²⁺ changes in SCN cells. Together, these results demonstrate that the NR2B subunit is an important component of NMDA receptor-mediated responses within SCN neurons and that this subunit contributes to light-induced phase shifts of the mammalian circadian system.     

Dim light melatonin onset and circadian temperature during a constant routine in hypersomnic winter depression

The onset of melatonin secretion under dim light conditions (DLMO) and the circadian temperature rhythm during a constant routine were assessed in 6 female controls and 6 female patients with winter depression (seasonal affective disorder, SAD) before and after bright light treatment. After sleep was standardized for 6 days, the subjects were sleep-deprived and at bedrest for 27 h while core temperature and evening melatonin levels were determined. The DLMO of the SAD patients was phase-delayed compared with controls (2310 vs 2138); with bright light treatment, the DLMO advanced (2310 to 2135). The minimum of the fitted rectal temperature rhythm was phase-delayed in the SAD group compared with the controls (0542 vs 0316); with bright light treatment, the minimum advanced (0542 vs 0336).     

Central neural mechanisms in diurnal rhythm regulation and neuroendocrine responses to light

(1) Two central neural mechanisms are neceessary for the regulation of diurnal rhythms in mammals. The first is a visual pathway to mediate the effects of light in the entrainment of such rhythms. The second is an endogenous ‘clock’ mechanism. (2) The retinohypothalamic projection terminating in the suprachiasmatic hypothalamic nuclei appears to be the visual pathway which mediates entrainment of rhythms. (3) Ablation of the suprachiasmatic nuclei results in loss of diurnal rhythms indicating that this region of the hypothalamus plays an essential role in the endogenous central generation of rhythmic functions.     

Intrinsic light responses of retinal ganglion cells projecting to the circadian system

In mammals, light entrainment of the circadian clock, located in the suprachiasmatic nuclei (SCN), requires retinal input. Traditional rod and cone photoreceptors, however, are not required. Instead, the SCN-projecting retinal ganglion cells (RGCs) function as autonomous photoreceptors and exhibit light responses independent of rod- and cone-driven input. Using whole-cell patch-clamp recording techniques, we have investigated the morphological and electrophysiological properties of this unique class of RGCs. Although SCN-projecting RGCs resemble Type III cells in form, they display strikingly different physiological properties from these neurons. First, in response to the injection of a sustained depolarizing current, SCN-projecting cells fired in a transient fashion, in contrast to most RGCs which fired robust trains of action potentials. Second, in response to light, SCN-projecting RGCs exhibited an intensity-dependent transient depolarization in the absence of rod and cone input. This depolarization reached a peak within 5 s and generated increased spiking activity before decaying to a plateau. Voltage-clamp recordings were used to characterize the light-activated conductance which generated this depolarization. In response to varying light intensities, SCN-projecting RGCs exhibited a graded transient inward current which peaked within 5 s and decayed to a plateau. The voltage dependence of the light-activated current was obtained by subtracting currents elicited by a voltage ramp before and during illumination. The light-activated current displayed both inward and outward rectification and was largely unaffected by substitution of extracellular Na+ with choline. In both respects, the intrinsic light-activated current observed in SCN-projecting RGCs resembles currents carried by ion channels of the transient receptor potential (trp) family, which are known to mediate the light response of invertebrate photoreceptors.     

Responses of cells in the rat suprachiasmatic nucleus in vivo to stimulation of afferent pathways are different at different times of the light/dark cycle

Conventional extracellular recordings were made from single cells in the suprachiasmatic nucleus (SCN) region of the anaesthetized rat. Each cell was tested for its response to stimulation at three sites; the contralateral optic nerve, the ipsilateral supraoptic nucleus (SON) or the ipsilateral arcuate nucleus (ARC) to determine whether the behaviour of the synapses in the SCN was different at different times. Responses to stimulation were tested once each hour and assessed by creating peristimulus time histograms. Excitatory, inhibitory or complex (consisting of more than one component) responses were seen. The responses of some cells that were recorded for several hours changed with time. Changes were seen in the responses of SCN cells to stimulation of the ARC (31/91 cells) and the SON (26/90 cells) regions, but only rarely to stimulation of the optic nerve (2/72 cells). Such differences in proportion are unlikely to have occurred by chance (P < 0.001; chi-square test). Changes seen included the appearance of both excitatory and inhibitory responses in cells that were initially unresponsive. In some cells, one component of a complex response remained constant while another component changed with time. When the cells in the SCN were treated as a group, the proportion of excitatory, inhibitory or complex responses to ARC stimulation did not remain constant throughout the light/dark cycle (P = 0.014; chi-square test). The proportion of excitatory, inhibitory or complex responses to SON and optic nerve stimulation showed no significant variation with the light/dark cycle. If a change in response can be interpreted as a change in the behaviour of a neural connection, the results imply that some of the projections to the SCN from within the hypothalamus change at different times of the light/dark cycle, whereas no change could be seen in the input from the optic nerve. Thus, some of the connections of the SCN appear not to be hard wired, but change rapidly with time.     

Daily exposure to cold phase‐shifts the circadian clock of neonatal rats in vivo

Maternal rhythms entrain the prenatal and neonatal circadian clock in the suprachiasmatic nucleus (SCN) before light entrainment is established. However, the responsible time cues for maternal entrainment are not identified. To examine the role of cyclic changes of ambient temperature in maternal entrainment, blind neonatal rats carrying a clock gene (Per2) bioluminescence reporter were exposed to either of three ambient temperatures (10, 20 or 30 °C) during 6‐h maternal separation in the early light phase. Cold exposure was performed from postnatal day 1 (P1) to P5. On P6, the SCN was harvested and cultured for photometric monitoring of the circadian rhythm in Per2 expression. Here we demonstrate that the daily cold exposure phase‐delayed the circadian Per2 expression rhythms at P6 in a temperature‐dependent manner. Exposure to 10 °C produced the largest phase‐shift of 12.7 h, and exposure to 20 and 30 °C yielded moderate shifts of 4.1 and 4.5 h, respectively. There was no significant difference in the phase‐shifts between the latter two temperatures, indicating that ambient temperature is not the sole factor for the phase‐shift. Behavioral rhythms that developed after weaning reflected the phase‐shift of clock gene expression rhythm in the SCN. These findings indicate that a daily exposure to an ambient temperature of 10 °C during the neonatal period is capable of resetting the circadian clock in the SCN, but other factors yet unidentified are also involved in maternal entrainment.     

Role of vasoactive intestinal peptide in the light input to the circadian system

The neuropeptide vasoactive intestinal peptide (VIP) is expressed at high levels in a subset of neurons in the ventral region of the suprachiasmatic nucleus (SCN). While VIP is known to be important for the synchronization of the SCN network, the role of VIP in photic regulation of the circadian system has received less attention. In the present study, we found that the light‐evoked increase in electrical activity in vivo was unaltered by the loss of VIP. In the absence of VIP, the ventral SCN still exhibited N‐methyl‐d ‐aspartate‐evoked responses in a brain slice preparation, although the absolute levels of neural activity before and after treatment were significantly reduced. Next, we used calcium imaging techniques to determine if the loss of VIP altered the calcium influx due to retinohypothalamic tract stimulation. The magnitude of the evoked calcium influx was not reduced in the ventral SCN, but did decline in the dorsal SCN regions. We examined the time course of the photic induction of Period1 in the SCN using in situ hybridization in VIP‐mutant mice. We found that the initial induction of Period1 was not reduced by the loss of this signaling peptide. However, the sustained increase in Period1 expression (after 30 min) was significantly reduced. Similar results were found by measuring the light induction of cFOS in the SCN. These findings suggest that VIP is critical for longer‐term changes within the SCN circuit, but does not play a role in the acute light response.     

Responses of cells in the rat supraoptic nucleus in vivo to stimulation of afferent pathways are different at different times of the light/dark cycle

To determine whether the daily rhythms of spike activity in the supraoptic nucleus (SON) were accompanied by changes in the behaviour of its inputs, we used conventional extracellular single cell recordings from cells in the SON of anaesthetized rats while stimulating the contralateral optic nerve and the ipsilateral suprachiasmatic nucleus (SCN). Neurones in the SON region were identified by antidromic activation and classified as oxytocin or vasopressin cells, on the basis of their spontaneous firing patterns. Approximately 27% of both oxytocin (29/108) and vasopressin (39/147) neurones were excited by stimulation of the optic nerve, and the majority of responses had a long latency (>20 ms). Very few oxytocin (3/108) and vasopressin cells (2/147) were inhibited by stimulation of the optic nerve. The pattern of the responses (excitatory, inhibitory or nonresponsive) of oxytocin and vasopressin cells to stimulation of the optic nerve was significantly related to the time of day (chi-square test; P = 0.012, oxytocin cells; P = 0.006, vasopressin cells). The proportion of oxytocin cells excited by stimulation of the optic nerve was highest at ZT 4-8 and lowest at ZT 20-24. For vasopressin cells, it was highest at ZT 12-16 and lowest at ZT 20-24. The proportion of excitatory, inhibitory and complex responses seen in oxytocin and vasopressin cells following stimulation of the SCN also changed and was significantly different at different times of day (oxytocin cells: highest proportion of excitatory responses at ZT 12-16, P = 0.029; chi-square test; vasopressin cells: highest proportion of excitatory responses at ZT 0-4, P = 0.005; chi-square test). Thus, inputs to oxytocin and vasopressin neurones from the optic nerve and some outputs from the SCN changed during the light/dark cycle. Such changes may contribute to the generation of 24-h rhythms in activity of oxytocin and vasopressin neurones and release of the peptides.     

Retinal ganglion cells expressing the FOS protein after light stimulation in the Syrian hamster are relatively insensitive to neonatal treatment with monosodium glutamate

In nocturnal rodents, the c-fos gene is directly involved in the light mechanism of resetting of the suprachiasmatic nucleus (circadian clock). Light also induces c-fos expression in the retinal ganglion cell layer (GCL), but no attempt has been made to study the retinal responses to the phase-shifting effects of light. The expression of the Fos protein in each of the two populations of the GCL (displaced amacrine cells [DACs] and ganglion cells [GCs]) was analyzed in hamsters after light stimulation delivered early (circadian time [CT13]) and in the middle (CT18) of the subjective night. To evaluate as accurately as possible the number of GCs able to phase shift the locomotor activity rhythm (LAR), neonatal hamsters treated with monosodium glutamate (MSG) were also used, an in vivo model which displays retinal degeneration and LAR normally entrained by light. In nontreated hamsters, the number of Fos-immunoreactive (Fos-ir⁺) nuclei in the GCL was significantly higher at CT18 than at CT13. In MSG-treated hamsters, the number of Fos-ir⁺ nuclei was the same at both CTs and nonsignificantly different as those of nontreated hamsters at CT13. MSG treatment destroyed as many Fos-ir⁺ DACs as Fos-ir^- DACs or Fos-ir⁺ GCs. Fos-ir⁺ GCs were less sensitive to neurotoxic than other GCs, as only 37% of them were destroyed by treatment versus 92% for Fos-ir^- GCs. At CT18, a maximum of 3,500 GCs expressed Fos protein in nontreated hamsters versus only 2,200 in MSG-treated hamsters. This minor subgroup was sufficiently potent to normally synchronize the circadian rhythms to the Light/dark cycle in treated hamsters. J. Comp. Neurol. 392:458–467, 1998. © 1998 Wiley-Liss, Inc.     

Sleep deprivation decreases phase-shift responses of circadian rhythms to light in the mouse: role of serotonergic and metabolic signals

The circadian pacemaker in the suprachiasmatic nuclei is primarily synchronized to the daily light-dark cycle. The phase-shifting and synchronizing effects of light can be modulated by non-photic factors, such as behavioral, metabolic or serotonergic cues. The present experiments examine the effects of sleep deprivation on the response of the circadian pacemaker to light and test the possible involvement of serotonergic and/or metabolic cues in mediating the effects of sleep deprivation. Photic phase-shifting of the locomotor activity rhythm was analyzed in mice transferred from a light-dark cycle to constant darkness, and sleep-deprived for 8 h from Zeitgeber Time 6 to Zeitgeber Time 14. Phase-delays in response to a 10-min light pulse at Zeitgeber Time 14 were reduced by 30% in sleep-deprived mice compared to control mice, while sleep deprivation without light exposure induced no significant phase-shifts. Stimulation of serotonin neurotransmission by fluoxetine (10 mg/kg), a serotonin reuptake inhibitor that decreases light-induced phase-delays in non-deprived mice, did not further reduce light-induced phase-delays in sleep-deprived mice. Impairment of serotonin neurotransmission with p-chloroamphetamine (three injections of 10 mg/kg), which did not increase light-induced phase-delays in non-deprived mice significantly, partially normalized light-induced phase-delays in sleep-deprived mice. Injections of glucose increased light-induced phase-delays in control and sleep-deprived mice. Chemical damage of the ventromedial hypothalamus by gold-thioglucose (600 mg/kg) prevented the reduction of light-induced phase-delays in sleep-deprived mice, without altering phase-delays in control mice. Taken together, the present results indicate that sleep deprivation can reduce the light-induced phase-shifts of the mouse suprachiasmatic pacemaker, due to serotonergic and metabolic changes associated with the loss of sleep.     

Dysfunctions in circadian behavior and physiology in mouse models of Huntington's disease

Many patients with Huntington's disease (HD) exhibit disturbances in their daily cycle of sleep and wake as part of their symptoms. These patients have difficulty sleeping at night and staying awake during the day, which has a profound impact on the quality of life of the patients and their care-givers. In the present study, we examined diurnal and circadian rhythms of four models of HD including the BACHD, CAG 140 knock-in and R6/2 CAG 140 and R6/2 CAG 250 lines of mice. The BACHD and both R6/2 lines showed profound circadian phenotypes as measured by wheel-running activity. Focusing on the BACHD line for further analysis, the amplitude of the rhythms in the BACHD mice declined progressively with age. In addition, the circadian regulation of heart rate and body temperature in freely behaving BACHD mice were also disrupted. Furthermore, the distribution of sleep as well as the autonomic regulation of heart rate was disrupted in this HD model. To better understand the mechanistic underpinnings of the circadian disruption, we used electrophysiological tools to record from neurons within the central clock in the suprachiasmatic nucleus (SCN). The BACHD mice exhibit reduced rhythms in spontaneous electrical activity in SCN neurons. Interestingly, the expression of the clock gene PERIOD2 was not altered in the SCN of the BACHD line. Together, this data is consistent with the hypothesis that the HD mutations interfere with the expression of robust circadian rhythms in behavior and physiology. The data raise the possibility that the electrical activity within the central clock itself may be altered in this disease.