Tetrodotoxin blocks NPY-induced but not muscimol-induced phase advances of wheel-running activity in Syrian hamsters

Distinct feedback actions of behavioural arousal to the master circadian clock in nocturnal and diurnal mammals

The master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus provides a temporal pattern of sleep and wake that - like many other behavioural and physiological rhythms - is oppositely phased in nocturnal and diurnal animals. The SCN primarily uses environmental light, perceived through the retina, to synchronize its endogenous circadian rhythms with the exact 24 h light/dark cycle of the outside world. The light responsiveness of the SCN is maximal during the night in both nocturnal and diurnal species. Behavioural arousal during the resting period not only perturbs sleep homeostasis, but also acts as a potent non-photic synchronizing cue. The feedback action of arousal on the SCN is mediated by processes involving several brain nuclei and neurotransmitters, which ultimately change the molecular functions of SCN pacemaker cells. Arousing stimuli during the sleeping period differentially affect the circadian system of nocturnal and diurnal species, as evidenced by the different circadian windows of sensitivity to behavioural arousal. In addition, arousing stimuli reduce and increase light resetting in nocturnal and diurnal species, respectively. It is important to address further question of circadian impairments associated with shift work and trans-meridian travel not only in the standard nocturnal laboratory animals but also in diurnal animal models.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

Functional Significance of the Excitatory Effects of GABA in the Suprachiasmatic Nucleus

Over 90% of neurons within the suprachiasmatic nucleus (SCN) express γ-aminobutyric acid (GABA). Although GABA is primarily an inhibitory neurotransmitter, in vitro studies suggest that the activation of GABA_A receptors (GABA_AR) elicits excitation in the adult SCN. The ratio of excitatory to inhibitory responses to GABA depends on the balance of chloride influx by Na⁺-K⁺-Cl^- cotransporter 1 (NKCC1) and chloride efflux by K⁺-Cl^- cotransporters (KCCs). Excitatory responses to GABA can be blocked by inhibition of the inward chloride cotransporter, NKCC1, with the loop diuretic bumetanide. Here we investigated the role of NKCC1 activity in phase shifting the circadian pacemaker in response to photic and nonphotic signals in male Syrian hamsters housed in constant darkness. In the early subjective night (CT 13.5), injection of bumetanide into the SCN reduced light-induced phase delays. However, during the late subjective night (CT 19), bumetanide administration did not alter light-induced phase advances. Injection of bumetanide during the subjective day (CT 6) did not alter the phase of free-running circadian rhythms but attenuated phase advances induced by injection of the GABA_AR agonist muscimol into the SCN. These data support the hypothesis that the excitatory effects of endogenously released GABA contribute to the ability of light to induce phase delays, thereby contributing to the most important function of the circadian system, its entrainment with the day-night cycle. Further, the finding that bumetanide inhibits the phase-advancing effects of muscimol during the subjective day supports the hypothesis that the excitatory responses to GABA also contribute to the ability of nonphotic stimuli to phase shift the circadian pacemaker.

Geniculohypothalamic GABAergic projections gate suprachiasmatic nucleus responses to retinal input

KEY POINTS

Visual input to the suprachiasmatic nucleus circadian clock is critical for animals to adapt their physiology and behaviour in line with the solar day. In addition to direct retinal projections, the clock receives input from the visual thalamus, although the role of this geniculohypothalamic pathway in circadian photoreception is poorly understood. In the present study, we develop a novel brain slice preparation that preserves the geniculohypothalamic pathway to show that GABAergic thalamic neurons inhibit retinally-driven activity in the central clock in a circadian time-dependent manner. We also show that in vivo manipulation of thalamic signalling adjusts specific features of the hypothalamic light response, indicating that the geniculohypothalamic pathway is primarily activated by crossed retinal inputs. Our data provide a mechanism by which geniculohypothalamic signals can adjust the magnitude of circadian and more acute hypothalamic light responses according to time-of-day and establish an important new model for future investigations of the circadian visual system.

ABSTRACT

Sensory input to the master mammalian circadian clock, the suprachiasmatic nucleus (SCN), is vital in allowing animals to optimize physiology and behaviour alongside daily changes in the environment. Retinal inputs encoding changes in external illumination provide the principle source of such information. The SCN also receives input from other retinorecipient brain regions, primarily via the geniculohypothalamic tract (GHT), although the contribution of these indirect projections to circadian photoreception is currently poorly understood. To address this deficit, in the present study, we established an in vitro mouse brain slice preparation that retains connectivity across the extended circadian system. Using multi-electrode recordings, we first confirm that this preparation retains intact optic projections to the SCN, thalamus and pretectum and a functional GHT. We next show that optogenetic activation of GHT neurons selectively suppresses SCN responses to retinal input, and also that this effect exhibits a pronounced day/night variation and involves a GABAergic mechanism. This inhibitory action was not associated with overt circadian rhythmicity in GHT output, indicating modulation at the SCN level. Finally, we use in vivo electrophysiological recordings alongside pharmacological inactivation or optogenetic excitation to show that GHT signalling actively modulates specific features of the SCN light response, indicating that GHT cells are primarily activated by crossed retinal projections. Taken together, our data establish a new model for studying network communication in the extended circadian system and provide novel insight into the roles of GHT-signalling, revealing a mechanism by which thalamic activity can help gate retinal input to the SCN according to time of day.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

The role of retinal photoreceptors in the regulation of circadian rhythms

The circadian clock is an evolutionarily, highly conserved feature of most organisms. This internal timing mechanism coordinates biochemical, physiological and behavioral processes to maintain synchrony with the environmental cycles of light, temperature and nutrients. Several studies have shown that light is the most potent cue used by most organisms (humans included) to synchronize daily activities. In mammals, light perception occurs only in the retina; three different types of photoreceptors are present within this tissue: cones, rods and the newly discovered intrinsically photosensitive retinal ganglion cells (ipRGCs). Researchers believe that the classical photoreceptors (e.g., the rods and the cones) are responsible for the image-forming vision, whereas the ipRGCs play a key role in the non-image forming vision. This non-image-forming photoreceptive system communicates not only with the master circadian pacemaker located in the suprachiasmatic nuclei of the hypothalamus, but also with many other brain areas that are known to be involved in the regulation of several functions; thus, this non-image forming system may also affect several aspects of mammalian health independently from the circadian system.

Regulation of light's action in the mammalian circadian clock: role of the extrasynaptic GABAA receptor

GABA(A) receptor agonists act in the suprachiasmatic nucleus (SCN) to reset circadian rhythms during the day but inhibit the ability of light to reset rhythms during the night. In the present study, we examined whether these paradoxical differences in the effect of GABA(A) receptor stimulation on the circadian system are mediated by separate GABA(A) receptor subtypes. 4,5,6,7-Tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), a GABA(A) receptor agonist, preferentially activates GABA(A) receptors in extrasynaptic locations. THIP, muscimol (a GABA(A) agonist), or vehicle were microinjected into the SCN region of Syrian hamsters free-running in constant darkness during the mid-subjective day, early subjective night, or late subjective night. The subjective night injections were followed by a light pulse or sham control. Behavioral phase shifts of wheel running rhythms and both Period1 (Per1) and Per2 mRNA levels in the SCN were assessed. Animals that received THIP during the subjective day did not exhibit significant phase alterations. During the early and late subjective night, however, THIP abolished the phase-shifting effects of light and the ability of light to increase Per1 and Per2 mRNA levels. The ability of N-methyl-d-aspartic acid to phase-shift wheel running rhythms was also attenuated by THIP. Together these data demonstrate that THIP does not produce phase shifts during the subjective day, but does inhibit the ability of light to produce phase shifts. Thus, extrasynaptic GABA(A) receptors appear to play a role in regulating light input to the SCN, while a different population of GABA(A) receptors appears to be responsible for daytime effects of GABA.

Light and GABAAreceptor activation alterPeriodmRNA levels in the SCN of diurnal Nile grass rats

We examined Period (Per) mRNA rhythms in the suprachiasmatic nucleus (SCN) of a diurnal rodent and assessed how phase-shifting stimuli acutely affect SCN Per mRNA using semiquantitative in situ hybridization. First, Per1 and Per2 varied rhythmically in the SCN over the course of one circadian cycle in constant darkness: Per1 mRNA was highest in the early to mid-subjective day, while Per2 mRNA levels peaked in the late subjective day. Second, acute light exposure in the early subjective night significantly increased both Per1 and Per2 mRNA. Third, Per2 but not Per1 levels decreased 1 and 2 h after injection of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol into the SCN during the subjective day. Fourth, muscimol also reduced the light-induced Per2 in the early subjective night, but Per1 induction by light was not significantly affected. Consistent with previous studies, these data demonstrate that diurnal and nocturnal animals show very similar daily patterns of Per mRNA and light-induced Per increases in the SCN. As with light, muscimol alters circadian phase, and daytime phase alterations induced by muscimol are associated with significant decreases in Per2 mRNA. In diurnal animals, muscimol-induced decreases in Per are associated with phase delays rather than advances. The direction of the daytime phase shift may be determined by the relative suppression of Per1 vs. Per2 in SCN cells. As in nocturnal animals, changes in Per1 and Per2 mRNA by photic and non-photic stimuli appear to be associated with circadian phase alteration.

GABAAreceptor activation suppressesPeriod 1mRNA andPeriod 2mRNA in the suprachiasmatic nucleus during the mid-subjective day

The mammalian circadian clock can be entrained by photic and nonphotic environmental time cues. gamma-aminobutyric acid (GABA) is a nonphotic stimulus that induces phase advances in the circadian clock during the middle of the subjective day. Several nonphotic stimuli suppress Period 1- and Period 2 mRNA expression in the suprachiasmatic nucleus (SCN); however, the effect of GABA on Period mRNA is unknown. In the present study we demonstrate that microinjection of the GABA(A) receptor agonist muscimol into the SCN region suppresses the expression of Period 1 mRNA in the hamster. A significant suppression of Period 2 mRNA following microinjection of muscimol was not observed in free-running conditions. However, Period 2 mRNA was significantly reduced following muscimol treatment when animals were maintained under a light cycle and transferred to constant darkness 42 h prior to treatment. An additional study investigated the maximum behavioural phase advance inducible by GABA(A) receptor activation.Together, these data indicate that, like other nonphotic stimuli, GABA suppresses Period 1- and Period 2 mRNA in the SCN.

Circadian control during the day and night: Role of neuropeptide Y Y5 receptors in the suprachiasmatic nucleus

Circadian rhythms are reset by light during the night or by nonphotic stimuli during the day. Neuropeptide Y (NPY), which appears to mediate at least some nonphotic phase shifts by its actions in the suprachiasmatic nucleus (SCN), induces phase advances during the day and inhibits light-induced phase advances during the night. In this study, we used a highly selective Y5-like agonist to test whether activation of NPY Y5 receptors is sufficient to mimic NPY during the day and late night in Syrian hamsters. We also tested whether NPY in the early night reduces light-induced phase delays in a dose-dependent manner. Microinjection of a selective Y5 receptor agonist, (Ala(31), Aib(32))-NPY, into the SCN significantly inhibited light-induced phase advances during the late night, but did not induce phase advances during the day. In addition, concentrations of NPY ranging from 0.23 to 23 mM did not attenuate light-induced phase delays in the early night. These results suggest that activation of Y5-like receptors is sufficient to inhibit light-induced phase advances during the late night but is not sufficient to induce phase advances during the day. Furthermore, this study provided no evidence that NPY can inhibit light-induced phase shifts early in the night.

Blockade of GABAA receptors disrupts isoperiodic neuronal oscillations in the intergeniculate leaflet of the rat

The intergeniculate leaflet of the thalamus is, besides the suprachiasmatic nucleus of the hypothalamus, the other important neuronal element of the mammalian biological clock. The extracellularly recorded activity of neurons constituting the intergeniculate leaflet, recorded in vivo, is characterized by distinct, very regular ultradian oscillations. The majority of neurons in the circadian timing system are GABAergic. Many, if not all, neurons of the suprachiasmatic nucleus and intergeniculate leaflet contain GABA. In the present study we examined the effects of the GABA(A) receptor antagonist bicuculline and the chloride channel blocker picrotoxin on isoperiodic neuronal oscillations in the intergeniculate leaflet of rats. We recorded extracellular multiple-unit neuronal activity from the intergeniculate leaflet of anesthetized rats. During the recording of isoperiodic oscillations, bicuculline or picrotoxin were stereotaxically injected at different concentrations into the lateral ventricle of rat brain. In all the experiments, injection of GABA(A) receptor antagonists transiently disrupted the isoperiodic phasic discharge recorded from the intergeniculate leaflet. These data suggest that GABA(A) receptors are involved in the generation of ultradian rhythmical neuronal oscillations in rat intergeniculate leaflet.

Circadian Phase Alteration by GABA and Light Differs in Diurnal and Nocturnal Rodents During the Day

These studies investigated the circadian effects of light and gamma aminobutyric acid-A (GABAA) receptor activation in the suprachiasmatic nucleus (SCN) of the diurnal unstriped Nile grass rat (Arvicanthis niloticus). Microinjection of the GABAA agonist muscimol into the SCN during the day produced phase shifts that were opposite in direction to those previously reported in nocturnal rodents. In addition, light had no significant effect on the magnitude of muscimol-induced phase delays during the daytime. Injection of muscimol during the night, however, significantly inhibited light-induced phase delays and advances in a manner similar to that previously reported in nocturnal rodents. Therefore, the circadian effects of GABAA receptor activation are similar in diurnal and nocturnal species during the night but differ significantly during the day.

Neuropeptide y and n-methyl-d-aspartic acid interact within the suprachiasmatic nuclei to alter circadian phase

Circadian rhythms are reset by exposure to photic stimuli and nonphotic stimuli. Glutamate appears to be the primary neurotransmitter that communicates photic stimuli to the circadian clock located in the suprachiasmatic nucleus. There is also substantial evidence that neuropeptide Y (NPY) mediates the effects of at least some nonphotic stimuli on the circadian clock. The purpose of this study was to investigate how NPY and glutamate receptor activation interact to reset the phase of the circadian clock. Microinjection of the glutamate agonist N-methyl-D-aspartic acid (NMDA) during the subjective day significantly decreased NPY-induced phase advances. During the late subjective night, NMDA induced light-like phase advances, which were significantly reduced by microinjection of NPY. Microinjection of NPY inhibited NMDA-induced phase advances during the late subjective night, even when sodium-dependent action potentials were inhibited by tetrodotoxin. These data support the hypothesis that, during the subjective night, NPY and NMDA act on the same clock cells or on cells that communicate with clock cells by mechanisms not requiring action potentials. Although NPY and NMDA appear to be mutually inhibitory during both the day and the night, the mechanisms of this inhibition appear to be different during the day versus the night.

Novel phase-shifting effects of GABAA receptor activation in the suprachiasmatic nucleus of a diurnal rodent

The vast majority of neurons in the suprachiasmatic nucleus (SCN), the primary circadian pacemaker in mammals, contain the inhibitory neurotransmitter GABA. Most studies investigating the role of GABA in the SCN have been performed using nocturnal rodents. Activation of GABA(A) receptors by microinjection of muscimol into the SCN phase advances the circadian activity rhythm of nocturnal rodents, but only during the subjective day. Nonphotic stimuli that reset the circadian pacemaker of nocturnal rodents also produce phase advances during the subjective day. The role of GABA in the SCN of diurnal animals and how it may differ from nocturnal animals is not known. In the studies described here, the GABA(A) agonist muscimol was microinjected directly into the SCN region of diurnal unstriped Nile grass rats (Arvicanthis niloticus) at various times in their circadian cycle. The results demonstrate that GABA(A) receptor activation produces large phase delays during the subjective day in grass rats. Treatment with TTX did not affect the ability of muscimol to induce phase delays, suggesting that muscimol acts directly on pacemaker cells within the SCN. These data suggest that the circadian pacemakers of nocturnal and diurnal animals respond to the most abundant neurochemical signal found in SCN neurons in opposite ways. These findings are the first to demonstrate a fundamental difference in the functioning of circadian pacemaker cells in diurnal and nocturnal animals.

Neuropeptide Y, GABA and circadian phase shifts to photic stimuli

Circadian rhythms can be phase shifted by photic and non-photic stimuli. The circadian clock, anatomically defined as the suprachiasmatic nucleus (SCN), can be phase delayed by light during the early subjective night and phase advanced during the late subjective night. Non-photic stimuli reset the clock when presented during the subjective day. A possible pathway for the non-photic resetting of the clock is thought to originate from the intergeniculate leaflet, which conveys information to the SCN through the geniculohypothalamic tract and utilizes among others neuropeptide Y (NPY) and GABA as neurotransmitters. Photic and non-photic stimuli have been shown to interact during the early and late subjective night. Microinjections of NPY or muscimol, a GABA(A) receptor agonist, into the region of the SCN can attenuate light-induced phase shifts during the early and late subjective night. The precise mechanism for these interactions is unknown. In the current study we investigate the involvement of a GABAergic mechanism in the interaction between NPY and light during the early and late subjective night. Microinjections of NPY significantly attenuated light-induced phase delays and inhibited phase advances (P<0.05). The administration of bicuculline during light exposure, before NPY microinjection did not alter the ability of NPY to attenuate light-induced phase delays and block photic phase advances. These results indicate that NPY attenuates photic phase shifts via a mechanism independent of GABA(A) receptor activation. Furthermore it is evident that NPY influences circadian clock function via differing cellular pathways over the course of a circadian cycle.

Tetrodotoxin blocks the circadian effects of NMDA during the day but not at night

Light resets the circadian clock in the suprachiasmatic nucleus (SCN) during the night and inhibits the ability of non-photic stimuli, such as injection of muscimol, to reset the clock during the day. Since activation of NMDA receptors appears to mediate the effects of light in the SCN, we investigated whether there are day-night differences in the SCN responses to NMDA. During the night, microinjection of NMDA mimicked the ability of light to induce phase delays, and these effects were not inhibited by blocking Na+-dependent action potentials with tetrodotoxin (TTX). During the day, NMDA mimicked the ability of light to inhibit muscimol-induced phase advances; however, this was inhibited by TTX. These data suggest that Na+-dependent action potentials within the SCN are required for NMDA to mimic the effects of light during the day but not during the night.

Developmental and circadian changes in Ca2+ mobilization mediated by GABAA and NMDA receptors in the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN) develops as the circadian pacemaker during postnatal life. Although both GABAA and NMDA receptors are expressed in the majority of SCN neurons, postnatal development of their functions has not been analysed. Thus, we studied the receptor-mediated Ca2+ responses in mouse hypothalamic slices prepared on postnatal days (P) 6-16. The NMDA-induced Ca2+ flux was prominent in the SCN and maximal Ca2+ responses in Mg2+-free conditions had no day-night variations in P14-16 mice. At P6-7, extracellular Mg2+ reduced the NMDA-induced Ca2+ flux irrespective of the circadian time whereas, after P9-10, Mg2+ produced a larger reduction at night than during the daytime. Muscimol also significantly increased Ca2+ in the developing SCN. Voltage-sensitive Ca2+ channel blockers inhibited the muscimol-induced Ca2+ increase whereas tetrodotoxin had no effect, suggesting that stimulation of postsynaptic GABAA receptors depolarizes SCN neurons to increase Ca2+. Macroscopic imaging analysis demonstrated a developmental reduction in the muscimol-induced Ca2+ increase preferentially in the nighttime group older than P9-10. The day-night variation in the magnitude of the Ca2+ response was due to two cell populations, one of which exhibited an increase and the other a decrease in Ca2+ in response to muscimol. Because the critical developmental stages for exhibiting day-night variations in the receptor-mediated Ca2+ responses overlapped the maturation of firing rhythms in SCN neurons, the Ca2+ signalling may be necessary for or regulated by the mature circadian clock.

Translational and transcriptional inhibitors block serotonergic phase advances of the suprachiasmatic nucleus circadian pacemaker in vitro

The mammalian circadian pacemaker is located in the suprachiasmatic nucleus (SCN). Various inputs modulate pacemaker phase, including the serotonergic (5HTergic) input from the midbrain raphe. 5HT phase-advances the SCN pacemaker when applied during mid subjective day. In vitro studies indicate that 5HT advances the mammalian circadian pacemaker through a process that includes stimulating 5HT7 receptors, activating protein kinase A, and opening K+ channels. How these cytoplasmic and membrane events translate into a shift in the molecular core of the circadian oscillator is not known. To further understand this process, the authors investigated whether 5HTergic phase advances require transcription or translation. Using two reversible translational inhibitors, anisomycin and cycloheximide, the authors show that inhibiting protein synthesis blocks 5HTergic phase shifts. The authors further show that the transcriptional inhibitor 5,6-dichloro-1-beta-ribobenzimidazole also blocks 5HTergic phase shifts. These results are similar to those found previously with respect to 5HTergic modulation of the Aplysia ocular circadian clock, and suggest that 5HT may phase-shift the SCN pacemaker through increasing transcription and translation of specific proteins.

Neuromodulator content of hamster intergeniculate leaflet neurons and their projection to the suprachiasmatic nucleus or visual midbrain

The intergeniculate leaflet (IGL) of the lateral geniculate complex has widespread, bilateral, and reciprocal connections with nuclei in the subcortical visual shell. Its function is poorly understood with respect to its role in visual processing. The most well-known IGL projection, and the only one with a clear function, is the geniculohypothalamic tract (GHT) that terminates in the suprachiasmatic nucleus (SCN), site of the primary circadian clock. The hamster GHT is derived, in part, from IGL neurons containing neuropeptide Y and enkephalin. IGL neurons containing these peptides also project to the pretectal region. The present studies used a combination of immunohistochemical, lesion, and retrograde tracing techniques to study neuron types in the IGL and their projections to hamster SCN and pretectum. Two additional neuromodulators, gamma-aminobutyric acid (GABA) and neurotensin, are shown to be present in IGL neurons. The GABA- and neurotensin-immunoreactive neurons project to the SCN with terminal field patterns very similar to those for neuropeptide Y and enkephalin. IGL neurons of all four types also send projections to the pretectum, but rarely do individual cells project to both the SCN and the pretectum. Nearly all neurotensin is colocalized with neuropeptide Y in IGL neurons, although about half of the neuropeptide Y cells do not contain neurotensin. Otherwise, the extent to which the four neuromodulators are colocalized varies from 6% to 54%. Nearly every SCN neuron appears to contain GABA. In the IGL, the majority of cells studied are not identifiable by GABA immunoreactivity. Putative functions of the various neuromodulator projections from the IGL to pretectum or SCN are discussed.

TTX blocks baclofen-induced phase shifts of the mammalian circadian pacemaker in vitro

The mammalian circadian pacemaker, located in the suprachiasmatic nucleus (SCN), expresses 24-h rhythms when isolated in vitro. The GABA(A) agonist, muscimol, induces phase advances during the mid-subjective day, while the GABA(B) agonist, baclofen, induces both daytime phase advances and nighttime phase delays. Here, we present evidence that tetrodotoxin (TTX) completely blocks baclofen-induced phase shifts in vitro, but does not block in vitro phase advances induced by muscimol. These results suggest that GABA(A), but not GABA(B), receptors are located on SCN pacemaker cells.

Electrical stimulation of the median or dorsal raphe nuclei reduces light-induced FOS protein in the suprachiasmatic nucleus and causes circadian activity rhythm phase shifts

Several pharmacological studies have suggested that the large median raphe serotonergic projection to the circadian clock in the suprachiasmatic nucleus may modulate circadian rhythm phase. The present experiments studied the role of dorsal and median raphe nuclei as regulators of circadian rhythmicity by evaluating the ability of electrical stimulation to shift rhythm phase or to alter photic induction of FOS protein synthesis. Male hamsters implanted with bipolar electrodes in either the median or dorsal raphe nucleus were stimulated during the early subjective night coincident with exposure to a saturating light pulse. About 90 min later, animals were anesthetized, perfused and the brains processed for FOS protein immunoreactivity. As previously demonstrated, light alone induces FOS immunoreactivity in nuclei of suprachiasmatic nucleus neurons. This was significantly attenuated by stimulation of either the median or dorsal raphe nucleus, with the extent of attenuation proportional to the intensity of stimulation. Electrical stimulation without light exposure had no effect on FOS expression. The effect of light on FOS expression in the suprachiasmatic nucleus was not modified by pre-treatment with the 5-HT1/2 serotonin receptor antagonist, metergoline, although it greatly reduced electrical stimulation-induced FOS expression in the hippocampus. In a second experiment, hamsters housed with running wheels in constant light were electrically stimulated in the median or dorsal raphe nucleus 6 h prior to (CT6) or 2 h after (CT14) expected activity onset. Regardless of which raphe nucleus was electrically stimulated, approximately 22 min phase advances were elicited at CT6 and 36 min phase delays were elicited at CT14. Despite the fact that the sole direct projection from the raphe complex to the suprachiasmatic nucleus is from the median nucleus, the present data do not distinguish between the median and dorsal raphe with respect to their impact on circadian rhythm regulation. Instead, two possible roles for each raphe nucleus are demonstrated. One main effect is that both raphe nuclei modulate rhythm phase. The second is an interaction between raphe efferent activity and light which, in the present studies, is demonstrated by the ability of raphe stimulation to modulate the action of light on the circadian system. While serotonin is a likely neurotransmitter mediating one or both effects, alternatives such as GABA, must be considered.

Melatonin inhibits in vitro serotonergic phase shifts of the suprachiasmatic circadian clock

The suprachiasmatic (SCN) circadian pacemaker generates 24 h rhythms of spontaneous neuronal activity when isolated in an acute brain slice preparation. The isolated pacemaker also retains its capacity to be reset, or phase-shifted by exogenous stimuli. For example, serotonin (5-HT) agonists advance the SCN pacemaker when applied during mid subjective day, while neuropeptide Y (NPY) agonists and melatonin advance the pacemaker when applied during late subjective day. Previous work has demonstrated interactions between NPY and 5-HT agonists, such that NPY can block 5-HTergic phase advances, while 5-HT agonists do not prevent NPY-induced advances. Due to a number of similarities in the actions of melatonin and NPY in the SCN, it seemed possible that melatonin and 5-HT might interact in the SCN as well. Therefore, in this study potential interactions between melatonin and 5-HT agonists were explored. Melatonin inhibited phase advances by the 5-HT agonist, (+)DPAT, and this inhibition was decreased by co-application of tetrodotoxin. Conversely, melatonin was unable to block phase advances by the cyclic AMP analog, 8BA-cAMP. Finally, neither 5-HT agonists nor 8BA-AMP were able to block melatonin-induced phase advances. These results demonstrate a clear interaction between melatonin and 5-HT in the SCN, and suggest that melatonin and NPY may play similar roles with respect to modulating the phase of the SCN circadian pacemaker in rats.

Serotonin antagonists do not attenuate activity-induced phase shifts of circadian rhythms in the Syrian hamster

A variety of observations from several rodent species suggest that a serotonin (5-HT) input to the suprachiasmatic nucleus (SCN) circadian pacemaker may play a role in resetting or entrainment of circadian rhythms by non-photic stimuli such as scheduled wheel running. If 5-HT activity within the SCN is necessary for activity-induced phase shifting, then it should be possible to block or attenuate these phase shifts by reducing 5-HT release or by blocking post-synaptic 5-HT receptors. Animals received one of four serotonergic drugs and were then locked in a novel wheel for 3 h during the mid-rest phase, when novelty-induced activity produces maximal phase advance shifts. Drugs tested at several doses were metergoline (5-HT1/2 antagonist; i.p.), (+)-WAY100135 (5-HT1A postsynaptic antagonist, which may also reduce 5-HT release by an agonist effect at 5-HT1A raphe autoreceptors; i.p.), NAN-190 (5-HT1A postsynaptic antagonist, which also reduces 5-HT release via an agonist effect at 5-HT1A raphe autoreceptors; i.p.) and ritanserin (5-HT2/7 antagonist; i.p. and i.c.v.). Mean and maximal phase shifts to running in novel wheels were not significantly affected by any drug at any dose. These results do not support a hypothesis that 5-HT release or activity at 5HT1, 2 and 7 receptors in the SCN is necessary for the production of activity-induced phase shifts in hamsters.

Neuropeptide Y blocks serotonergic phase shifts of the suprachiasmatic circadian clock in vitro

The mammalian circadian pacemaker in the suprachiasmatic nuclei (SCN) can be reset in vitro by various neurochemical stimuli. This study investigated the phase-shifting properties of neuropeptide Y (NPY) and serotonin (5-HT) agonists when applied alone, as well as their combined effects on clock resetting. These neurotransmitters have both been shown to advance the SCN clock in vitro when applied during the daytime. By monitoring the SCN neuronal activity rhythm in vitro, I first confirm that the 5HT1A/5HT7 agonist (+)DPAT maximally advances the SCN clock when applied at zeitgeber time 6 (ZT6). Conversely, NPY only phase advances the neuronal activity rhythm when applied at ZT 10. This effect occurs through stimulation of Y2 receptors. NPY, again acting through Y2 receptors, blocks (+)DPAT-induced phase shifts at ZT 6, while neither (+)DPAT nor 5-HT affect NPY-induced phase shifts at ZT 10. NPY appears to block (+)DPAT-induced phase shifts by preventing increases in cyclic AMP. These data are the first to demonstrate in vitro interactions between daytime resetting stimuli in the rat, and provide critical insights into mechanisms controlling circadian clock phase.