Bicuculline increases and muscimol reduces the phase-delaying effects of light and VIP/PHI/GRP in the suprachiasmatic region

Age-, region-, and day/night-related variation of the chloride reversal potential in the rat suprachiasmatic nucleus

The master control of mammalian circadian rhythms is the suprachiasmatic nucleus (SCN), which is formed by the ventral and dorsal regions. In SCN neurons, GABA has an important function and even excitatory actions in adulthood. However, the physiological role of this neurotransmitter in the developing SCN is unknown. Here, we recorded GABAergic postsynaptic currents (in the perforated-patch configuration using gramicidin) to determine the chloride reversal potential (ECl) and also assessed the immunological expression of the Na-K-Cl cotransporter 1 (NKCC1) at early ages of the rat (postnatal days (P) 3 to 25), during the day and night, in the two SCN regions. We detected that ECl greatly varied with age and depending on the SCN region and time of day. Broadly speaking, ECl was more hyperpolarized with age, except for the oldest age studied (P20-25) in both day and night in the ventral SCN, where it was less negative. Likewise, ECl was more hyperpolarized in the dorsal SCN both during the day and at night; while ECl was more negative at night both in the ventral and the dorsal SCN. Moreover, the total NKCC1 fluorescent expression was higher during the day than at night. These results imply that NKCC1 regulates the circadian and developmental fluctuations in the [Cl-]i to fine-tune ECl, which is crucial for either excitatory or inhibitory GABAergic actions to occur in the SCN.

Melanopsin-mediated optical entrainment regulates circadian rhythms in vertebrates

Melanopsin (OPN4) is a light-sensitive protein that plays a vital role in the regulation of circadian rhythms and other nonvisual functions. Current research on OPN4 has focused on mammals; more evidence is needed from non-mammalian vertebrates to fully assess the significance of the non-visual photosensitization of OPN4 for circadian rhythm regulation. There are species differences in the regulatory mechanisms of OPN4 for vertebrate circadian rhythms, which may be due to the differences in the cutting variants, tissue localization, and photosensitive activation pathway of OPN4. We here summarize the distribution of OPN4 in mammals, birds, and teleost fish, and the classical excitation mode for the non-visual photosensitive function of OPN4 in mammals is discussed. In addition, the role of OPN4-expressing cells in regulating circadian rhythm in different vertebrates is highlighted, and the potential rhythmic regulatory effects of various neuropeptides or neurotransmitters expressed in mammalian OPN4-expressing ganglion cells are summarized among them.

Systematic review of drugs that modify the circadian system's phase-shifting responses to light exposure

We searched PubMed for primary research quantifying drug modification of light-induced circadian phase-shifting in rodents. This search, conducted for work published between 1960 and 2018, yielded a total of 146 papers reporting results from 901 studies. Relevant articles were those with any extractable data on phase resetting in wildtype (non-trait selected) rodents administered a drug, alongside a vehicle/control group, near or at the time of exposure. Most circadian pharmacology experiments were done using drugs thought to act directly on either the brain's central pacemaker, the suprachiasmatic nucleus (SCN), the SCN's primary relay, the retinohypothalamic tract, secondary pathways originating from the medial/dorsal raphe nuclei and intergeniculate leaflet, or the brain's sleep-arousal centers. While the neurotransmitter systems underlying these circuits were of particular interest, including those involving glutamate, gamma-aminobutyric acid, serotonin, and acetylcholine, other signaling modalities have also been assessed, including agonists and antagonists of receptors linked to dopamine, histamine, endocannabinoids, adenosine, opioids, and second-messenger pathways downstream of glutamate receptor activation. In an effort to identify drugs that unduly influence circadian responses to light, we quantified the net effects of each drug class by ratioing the size of the phase-shift observed after administration to that observed with vehicle in a given experiment. This allowed us to organize data across the literature, compare the relative efficacy of one mechanism versus another, and clarify which drugs might best suppress or potentiate phase resetting. Aggregation of the available data in this manner suggested that several candidates might be clinically relevant as auxiliary treatments to suppress ectopic light responses during shiftwork or amplify the circadian effects of timed bright light therapy. Future empirical research will be necessary to validate these possibilities.

Long-Term Imaging Reveals a Circadian Rhythm of Intracellular Chloride in Neurons of the Suprachiasmatic Nucleus

Both inhibitory and excitatory GABA transmission exist in the mature suprachiasmatic nucleus (SCN), the master pacemaker of circadian physiology. Whether GABA is inhibitory or excitatory depends on the intracellular chloride concentration ([Cl^-]_i). Here, using the genetically encoded ratiometric probe Cl-Sensor, we investigated [Cl^-]_i in AVP and VIP-expressing SCN neurons for several days in culture. The chloride ratio (R_Cl) demonstrated circadian rhythmicity in AVP + neurons and VIP + neurons, but was not detected in GFAP + astrocytes. R_Cl peaked between ZT 7 and ZT 8 in both AVP + and VIP + neurons. R_Cl rhythmicity was not dependent on the activity of several transmembrane chloride carriers, action potential generation, or the L-type voltage-gated calcium channels, but was sensitive to GABA antagonists. We conclude that [Cl^-]_i is under circadian regulation in both AVP + and VIP + neurons.

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.

Intracellular Chloride Regulation in AVP+ and VIP+ Neurons of the Suprachiasmatic Nucleus

Several reports have described excitatory GABA transmission in the suprachiasmatic nucleus (SCN), the master pacemaker of circadian physiology. However, there is disagreement regarding the prevalence, timing, and neuronal location of excitatory GABA transmission in the SCN. Whether GABA is inhibitory or excitatory depends, in part, on the intracellular concentration of chloride ([Cl-]i). Here, using ratiometric Cl- imaging, we have investigated intracellular chloride regulation in AVP and VIP-expressing SCN neurons and found evidence suggesting that [Cl-]i is higher during the day than during the night in both AVP+ and VIP+ neurons. We then investigated the contribution of the cation chloride cotransporters to setting [Cl-]i in these SCN neurons and found that the chloride uptake transporter NKCC1 contributes to [Cl-]i regulation in SCN neurons, but that the KCCs are the primary regulators of [Cl-]i in SCN neurons. Interestingly, we observed that [Cl-]i is differentially regulated between AVP+ and VIP+ neurons-a low concentration of the loop diuretic bumetanide had differential effects on AVP+ and VIP+ neurons, while blocking the KCCs with VU0240551 had a larger effect on VIP+ neurons compared to AVP+ neurons.

Temporal Regulation of GABAA Receptor Subunit Expression: Role in Synaptic and Extrasynaptic Communication in the Suprachiasmatic Nucleus

Recent molecular studies suggest that the expression levels of δ and γ2 GABA_A receptor (GABA_AR) subunits regulate the balance between synaptic and extrasynaptic GABA neurotransmission in multiple brain regions. We investigated the expression of GABA_Aδ and GABA_Aγ2 and the functional significance of a change in balance between these subunits in a robust local GABA network contained within the suprachiasmatic nucleus of the hypothalamus (SCN). Muscimol, which can activate both synaptic and extrasynaptic GABA_ARs, injected into the SCN during the day phase advanced the circadian pacemaker, whereas injection of the extrasynaptic GABA_A superagonist 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP) had no effect on circadian phase. In contrast, injection of either THIP or muscimol during the night was sufficient to block the phase shifting effects of light. Gene expression analysis of the whole SCN revealed different temporal patterns in GABA_Aδ and GABA_Aγ2 mRNA expression. When examined across all subregions of the SCN, quantitative immunohistochemical analysis found no significant variations in GABA_Aδ protein immunoreactivity (IR) but did find significant variations in GABA_Aγ2 protein-IR in hamsters housed in either LD cycles or in constant darkness. Remarkably, significant interactions in the ratio of GABA_Aδ:GABA_Aγ2 subunits between lighting condition and circadian phase occurred only within one highly discrete anatomical area of the SCN; a region that functions as the input for lighting information from the retina. Taken together, these data support the hypothesis that the balance between synaptic and extrasynaptic GABA_ARs determines the functional response to GABA, and that this balance is differentially regulated in a region-specific manner.

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.

Phase shifts to light are altered by antagonists to neuropeptide receptors

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) is a heterogeneous structure. Two key populations of cells that receive retinal input and are believed to participate in circadian responses to light are cells that contain vasoactive intestinal polypeptide (VIP) and gastrin-releasing peptide (GRP). VIP acts primarily through the VPAC2 receptor, while GRP works primarily through the BB2 receptor. Both VIP and GRP phase shift the circadian clock in a manner similar to light when applied to the SCN, both in vivo and in vitro, indicating that they are sufficient to elicit photic-like phase shifts. However, it is not known if they are necessary signals for light to elicit phase shifts. Here we test the hypothesis that GRP and VIP are necessary signaling components for the photic phase shifting of the hamster circadian clock by examining two antagonists for each of these neuropeptides. The BB2 antagonist PD176252 had no effect on light-induced delays on its own, while the BB2 antagonist RC-3095 had the unexpected effect of significantly potentiating both phase delays and advances. Neither of the VIP antagonists ([d-p-Cl-Phe6, Leu17]-VIP, or PG99-465) altered phase shifting responses to light on their own. When the BB2 antagonist PD176252 and the VPAC2 antagonist PG99-465 were delivered together to the SCN, phase delays were significantly attenuated. These results indicate that photic phase shifting requires participation of either VIP or GRP; phase shifts to light are only impaired when signalling in both pathways are inhibited. Additionally, the unexpected potentiation of light-induced phase shifts by RC-3095 should be investigated further for potential chronobiotic applications.

Localization and expression of GABA transporters in the suprachiasmatic nucleus

GABA is a principal neurotransmitter in the suprachiasmatic hypothalamic nucleus (SCN), the master circadian clock. Despite the importance of GABA and GABA uptake for functioning of the circadian pacemaker, the localization and expression of GABA transporters (GATs) in the SCN has not been investigated. The present studies used Western blot analysis, immunohistochemistry and electron microscopy to demonstrate the presence of GABA transporter 1 (GAT1) and GAT3 in the SCN. By using light microscopy, GAT1 and GAT3 were co-localized throughout the SCN, but were not expressed in the perikarya of arginine vasopressin- or vasoactive intestinal peptide-immunoreactive (-ir) neurons of adult rats, nor in the neuronal processes labelled with the neurofilament heavy chain. Using electron microscopy, GAT1- and GAT3-ir was found in glial processes surrounding unlabelled neuronal perikarya, axons, dendrites, and enveloped symmetric and asymmetric axo-dendritic synapses. Glial fibrillary acidic protein-ir astrocytes grown in cell culture were immunopositive for GAT1 and GAT3 and both GATs could be observed in the same glial cell. These data demonstrate that synapses in the SCN function as 'tripartite' synapses consisting of presynaptic axon terminals, postsynaptic membranes and astrocytes that contain GABA transporters. This model suggests that astrocytes expressing both GATs may regulate the extracellular GABA, and thereby modulate the activity of neuronal networks in the SCN.

Sustained activation of GABAA receptors in the suprachiasmatic nucleus mediates light-induced phase delays of the circadian clock: a novel function of ionotropic receptors

The suprachiasmatic nucleus (SCN) contains a circadian clock that generates endogenous rhythmicity and entrains that rhythmicity with the day-night cycle. The neurochemical events that transduce photic input within the SCN and mediate entrainment by resetting the molecular clock have yet to be defined. Because GABA is contained in nearly all SCN neurons we tested the hypothesis that GABA serves as this signal in studies employing Syrian hamsters (Mesocricetus auratus). Activation of GABAA receptors was found to be necessary and sufficient for light to induce phase delays of the clock. Remarkably, the sustained activation of GABAA receptors for more than three consecutive hours was necessary to phase-delay the clock. The duration of GABAA receptor activation required to induce phase delays would not have been predicted by either the prevalent theory of circadian entrainment or by expectations regarding the duration of ionotropic receptor activation necessary to produce functional responses. Taken together, these data identify a novel neurochemical mechanism essential for phase-delaying the 'master' circadian clock within the SCN as well as identifying an unprecedented action of an amino acid neurotransmitter involving the sustained activation of ionotropic receptors.

ESR1 and ESR2 differentially regulate daily and circadian activity rhythms in female mice

Estrogenic signaling shapes and modifies daily and circadian rhythms, the disruption of which has been implicated in psychiatric, neurologic, cardiovascular, and metabolic disease, among others. However, the activational mechanisms contributing to these effects remain poorly characterized. To determine the activational impact of estrogen on daily behavior patterns and differentiate between the contributions of the estrogen receptors ESR1 and ESR2, ovariectomized adult female mice were administered estradiol, the ESR1 agonist propylpyrazole triol, the ESR2 agonist diarylpropionitrile, or cholesterol (control). Animals were singly housed with running wheels in a 12-hour light, 12-hour dark cycle or total darkness. Estradiol increased total activity and amplitude, consolidated activity to the dark phase, delayed the time of peak activity (acrophase of wheel running), advanced the time of activity onset, and shortened the free running period (τ), but did not alter the duration of activity (α). Importantly, activation of ESR1 or ESR2 differentially impacted daily and circadian rhythms. ESR1 stimulation increased total wheel running and amplitude and reduced the proportion of activity in the light vs the dark. Conversely, ESR2 activation modified the distribution of activity across the day, delayed acrophase of wheel running, and advanced the time of activity onset. Interestingly, τ was shortened by estradiol or either estrogen receptor agonist. Finally, estradiol-treated animals administered a light pulse in the early subjective night, but no other time, had an attenuated response compared with controls. This decreased phase response was mirrored by animals treated with diarylpropionitrile, but not propylpyrazole triol. To conclude, estradiol has strong activational effects on the temporal patterning and expression of daily and circadian behavior, and these effects are due to distinct mechanisms elicited by ESR1 and ESR2 activation.

Site-specific effects of gastrin-releasing peptide in the suprachiasmatic nucleus

The effects of gastrin-releasing peptide (GRP) on the circadian clock in the suprachiasmatic nucleus (SCN) are dependent on the activation of N-methyl-d-aspartate (NMDA) receptors in the SCN. In this study, the interaction between GRP, glutamate and serotonin in the regulation of circadian phase in Syrian hamsters was evaluated. Microinjection of GRP into the third ventricle induced c-fos and p-ERK expression throughout the SCN. Coadministration of an NMDA antagonist or 8-hydroxy-2-di-n-propylamino-tetralin [a serotonin (5-HT)1A,7 agonist, DPAT] with GRP limited c-fos expression in the SCN to a region dorsal to GRP cell bodies. Similar to the effects of NMDA antagonists, DPAT attenuated GRP-induced phase shifts in the early night, suggesting that the actions of serotonin on the photic phase shifting mechanism occur downstream from retinorecipient cells. c-fos and p-ERK immunoreactivity in the supraoptic (SON) and paraventricular hypothalamic nuclei also increased following ventricular microinjection of GRP. Because of this finding, a second set of experiments was designed to test a potential role for the SON in the regulation of clock function. Syrian hamsters were given microinjections of GRP into the peri-SON during the early night. GRP-induced c-fos activity in the SCN was similar to that following ventricular administration of GRP. GRP or bicuculline (a γ-aminobutyric acidA antagonist) administered near the SON during the early night elicited phase delays of circadian activity rhythms. These data suggest that GRP-induced phase-resetting is dependent on levels of glutamatergic and serotonergic neurotransmission in the SCN and implicate activity in the SON as a potential regulator of photic signaling in the SCN.

Physiological responses of the circadian clock to acute light exposure at night

Circadian rhythms in physiological, endocrine and metabolic functioning are controlled by a neural clock located in the suprachiasmatic nucleus (SCN). This structure is endogenously rhythmic and the phase of this rhythm can be reset by light information from the eye. A key feature of the SCN is that while it is a small structure containing on the order of about 20,000 cells, it is amazingly heterogeneous. It is likely that anatomical heterogeneity reflects an underlying functional heterogeneity. In this review, we examine the physiological responses of cells in the SCN to light stimuli that reset the phase of the circadian clock, highlighting where possible the spatial pattern of such responses. Increases in intracellular calcium are an important signal in response to light, and this increase triggers many biochemical cascades that mediate responses to light. Furthermore, only some cells in the SCN are actually endogenously rhythmic, and these cells likely do not receive strong direct input from the retina. Therefore, this review also considers how light information is conveyed from the retinorecipient cells to the endogenously rhythmic cells that track circadian phase. A number of neuropeptides, including vasoactive intestinal polypeptide, gastrin-releasing peptide and substance P, may be particularly important in relaying such signals, but other neurochemicals such as GABA and nitric oxide may participate as well. A thorough understanding of the intracellular and intercellular responses to light, as well as the spatial arrangements of such responses may help identify important pharmacological targets for therapeutic interventions to treat sleep and circadian disorders.

Ethanol modulates mammalian circadian clock phase resetting through extrasynaptic GABA receptor activation

Ethanol modulates the actions of multiple neurotransmitter systems, including GABA. However, its enhancing effects on GABA signaling typically are seen only at high concentrations. In contrast, although GABA is a prominent neurotransmitter in the circadian clock of the suprachiasmatic nucleus (SCN), we see ethanol modulation of clock phase resetting at low concentrations (<50 mM). A possible explanation is that ethanol enhances GABAergic signaling in the SCN through activating GABA(A) receptors that contain the delta subunit (GABA(Adelta) receptors), which are sensitive to low ethanol concentrations. Therefore, we investigated whether ethanol acts on GABA(Adelta) receptors in the SCN. Here we show that acute application of the GABA(Adelta) receptor antagonist, RO15-4513, to mouse hypothalamic slices containing the SCN prevents ethanol inhibition of nighttime glutamate-induced (photic-like) phase delays of the circadian clock. Diazepam, which enhances activity of GABA(A) receptors containing the gamma subunit (GABA(Agamma) receptors), does not modulate these phase shifts. Moreover, we find that RO15-4513 prevents ethanol enhancement of daytime serotonergic (non-photic) phase advances of the circadian clock. Furthermore, diazepam phase-advances the SCN circadian clock when applied alone in the daytime, while ethanol has no effect by itself at that time. These data support the hypothesis that ethanol acts on GABA(Adelta) receptors in the SCN to modulate photic and non-photic circadian clock phase resetting. They also reveal distinct modulatory roles of different GABA(A) receptor subtypes in circadian clock phase regulation.

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.

Interactions of GABA A receptor activation and light on period mRNA expression in the suprachiasmatic nucleus

Activation of gamma-aminobutyric acid (GABA) A receptors in the suprachiasmatic nucleus (SCN) resets the circadian clock during the day and inhibits the ability of light to reset the clock at night. Light in turn acts during the day to inhibit the phase-resetting effects of GABA. Some evidence suggests that Period mRNA changes in the SCN are responsible for these interactions between light and GABA. Here, the hypothesis that light and the GABA A receptor interact by altering the expression of Period 1 and/or Period 2 mRNA in the SCN is tested. The GABA A agonist muscimol was injected near the SCN just prior to a light pulse, during the mid-subjective day and the early and late subjective night. Changes in Period 1 and Period 2 mRNA were measured in the SCN by in situ hybridization. Light-induced Period 1 mRNA was inhibited by GABA A receptor activation in the early and late subjective night, while Period 2 mRNA was only inhibited during the late night. During the subjective day, light had no effect on the ability of muscimol to suppress Period 1 mRNA hybridization signal. Thus, light and GABA A receptor activation inhibit each other's ability to induce behavioral phase shifts throughout the subjective day and night. However, only in the late night are these behavioral effects correlated with changes in Period gene expression. Together, our data support the hypothesis that the interacting effects of light and GABA are the result of the opposing actions of these stimuli on Period mRNA, but only during the subjective night.

Modulation of photic response by the metabotropic glutamate receptor agonist t-ACPD

Glutamate is the primary excitatory transmitter in the hypothalamus. It conveys photic information to the suprachiasmatic nucleus of the hypothalamus, thereby entraining the circadian clock to environmental light cycles. While ionotropic glutamate receptors have been implicated in the transduction of photic information in suprachiasmatic nucleus cells, there is evidence that metabotropic glutamate receptors play a significant modulatory role. We investigated the effects of the metabotropic glutamate agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) on light-evoked phase responses in Syrian hamsters at three phase points: circadian time 6, a time when light has no effect on the circadian timing system; circadian time 13.5, when light evokes the maximum phase delay; circadian time 19, the maximum phase advance. We found that ACPD significantly increased the light-evoked phase shift at circadian time 13.5, and had no effect at other phase points tested. These data support a role for metabotropic glutamate receptors in the circadian photic signal transduction system.

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.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

A GABAergic Mechanism Is Necessary for Coupling Dissociable Ventral and Dorsal Regional Oscillators within the Circadian Clock

BACKGROUND

Circadian rhythms in mammalian behavior, physiology, and biochemistry are controlled by the central clock of the suprachiasmatic nucleus (SCN). The clock is synchronized to environmental light-dark cycles via the retino-hypothalamic tract, which terminates predominantly in the ventral SCN of the rat. In order to understand synchronization of the clock to the external light-dark cycle, we performed ex vivo recordings of spontaneous impulse activity in SCN slices of the rat.

RESULTS

We observed bimodal patterns of spontaneous impulse activity in the dorsal and ventral SCN after a 6 hr delay of the light schedule. Bisection of the SCN slice revealed a separate fast-resetting oscillator in the ventral SCN and a distinct slow-resetting oscillator in the dorsal SCN. Continuous application of the GABA(A) antagonist bicuculline yielded similar results as cut slices. Short application of bicuculline at different phases of the circadian cycle increased the electrical discharge rate in the ventral SCN but, unexpectedly, decreased activity in the dorsal SCN.

CONCLUSIONS

GABA transmits phase information between the ventral and dorsal SCN oscillators. GABA can act excitatory in the dorsal SCN and inhibits neurons in the ventral SCN. We hypothesize that this difference results in asymmetrical interregional coupling within the SCN, with a stronger phase-shifting effect of the ventral on the dorsal SCN than vice versa. A model is proposed that focuses on this asymmetry and on the role of GABA in phase regulation.

Nociceptin/orphanin FQ (N/OFQ) inhibits excitatory and inhibitory synaptic signaling in the suprachiasmatic nucleus (SCN)

Environmental synchronization of the endogenous mammalian circadian rhythm involves glutamatergic and GABAergic neurotransmission within the hypothalamic suprachiasmatic nucleus (SCN). The neuropeptide nociceptin/orphanin FQ (N/OFQ) inhibits light-induced phase shifts, evokes K(+)-currents and reduces the intracellular Ca(2+) concentration in SCN neurons. Since these effects are consistent with a modulatory role for N/OFQ on synaptic transmission in the SCN, we examined the effects of N/OFQ on evoked and spontaneous excitatory and inhibitory currents in the SCN. N/OFQ produced a consistent concentration-dependent inhibition of glutamate-mediated excitatory postsynaptic currents (EPSC) evoked by optic nerve stimulation. N/OFQ did not alter the amplitude of currents induced by application of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-d-aspartate (NMDA) nor the amplitude of miniature EPSC (mEPSC) consistent with a lack of N/OFQ effect on postsynaptic AMPA or NMDA receptors. N/OFQ significantly reduced the mEPSC frequency. The inhibitory actions of N/OFQ were blocked by omega-conotoxin GVIA, an N-type Ca(2+)channel antagonist and partially blocked by omega-agatoxin TK, a P/Q type Ca(2+) channel blocker. These data indicate that N/OFQ reduces evoked EPSC, in part, by inhibiting the activity of N- and P/Q-type Ca(2+) channels. In addition, N/OFQ produced a consistent reduction in baseline Ca(2+) levels in presynaptic retinohypothalamic tract terminals. N/OFQ also inhibited evoked GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSC) in a concentration dependent manner. However, N/OFQ had no effect on currents activated by muscimol application or on the amplitude of miniature IPSC (mIPSC) and significantly reduced the mIPSC frequency consistent with an inhibition of GABA release downstream from Ca(2+) entry. Finally, N/OFQ inhibited the paired-pulse depression observed in SCN GABAergic synapses consistent with a presynaptic mechanism of action. Together these results suggest a widespread modulatory role for N/OFQ on the synaptic transmission in the SCN.

GABA(B) receptor activation in the suprachiasmatic nucleus of diurnal and nocturnal rodents

Diurnal (day-active) and nocturnal (night-active) animals have very different daily activity patterns. We recently demonstrated that the suprachiasmatic nucleus (SCN) responds to GABAergic stimulation differently in diurnal and nocturnal animals. Specifically, GABAA receptor activation with muscimol during the subjective day causes phase delays in diurnal grass rats while producing phase advances in nocturnal hamsters. The aim of the following experiments was to determine if diurnal and nocturnal animals differ in their response to GABAB receptor activation in the SCN. Baclofen, a GABAB receptor agonist, was microinjected into the SCN region of grass rats or hamsters under free-running conditions and phase alterations were analyzed. Changes in phase were not detected after baclofen treatment during the subjective day in either grass rats or hamsters. During the night, however, GABAB receptor activation significantly decreased the ability of light to induce phase delays in grass rats. Taken together with previous data from our laboratory, these results demonstrate that, in both hamsters and grass rats, GABAB receptor activation in the SCN significantly affects circadian phase during the night, but not during the day.

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, 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.

Presynaptic and postsynaptic GABAA receptors in rat suprachiasmatic nucleus

The mammalian suprachiasmatic nucleus (SCN), the brain's circadian clock, is composed mainly of GABAergic neurons, that are interconnected via synapses with GABA(A) receptors. Here we report on the subcellular localization of these receptors in the SCN, as revealed by an extensively characterized antibody to the alpha 3 subunit of GABA(A) receptors in conjunction with pre- and postembedding electron microscopic immunocytochemistry. GABA(A) receptor immunoreactivity was observed in neuronal perikarya, dendritic processes and axonal terminals. In perikarya and proximal dendrites, GABA(A) receptor immunoreactivity was expressed mainly in endoplasmic reticulum and Golgi complexes, while in the distal part of dendrites, immunoreaction product was associated with postsynaptic plasma membrane. Many GABAergic axonal terminals, as revealed by postembedding immunogold labeling, displayed GABA(A) receptor immunoreactivity, associated mainly with the extrasynaptic portion of their plasma membrane. The function of these receptors was studied in hypothalamic slices using whole-cell patch-clamp recording of the responses to minimal stimulation of an area dorsal to the SCN. Analysis of the evoked inhibitory postsynaptic currents showed that either bath or local application of 100 microM of GABA decreased GABAergic transmission, manifested as a two-fold increase in failure rate. This presynaptic effect, which was detected in the presence of the glutamate receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione and the selective GABA(B) receptor blocker CGP55845A, appears to be mediated via activation of GABA(A) receptors. Our results thus show that GABA(A) receptors are widely distributed in the SCN and may subserve both pre- and postsynaptic roles in controlling the mammalian circadian clock.

GABA interacts with photic signaling in the suprachiasmatic nucleus to regulate circadian phase shifts

Circadian rhythms of physiology and behavior in mammals are driven by a circadian pacemaker located in the suprachiasmatic nucleus of the hypothalamus. The majority of neurons in the suprachiasmatic nucleus are GABAergic, and activation of GABA receptors in the suprachiasmatic nucleus can induce phase shifts of the circadian pacemaker both in vivo and in vitro. GABA also modulates the phase shifts induced by light in vivo, and photic information is thought to be conveyed to the suprachiasmatic nucleus by glutamate. In the present study, we examined the interactions between GABA receptor agonists, glutamate agonists, and light in hamsters in vivo. The GABA(A) receptor agonist muscimol and the GABA(B) receptor agonist baclofen were microinjected into the suprachiasmatic nucleus at circadian time 13.5 (early subjective night), followed immediately by a microinjection of N-methyl-D-aspartate (NMDA). Both muscimol and baclofen significantly reduced the phase shifting effects of NMDA. Further, coadministration of tetrodotoxin with baclofen did not alter the inhibition of NMDA by baclofen, suggesting a postsynaptic mechanism for the inhibition of NMDA-induced phase shifts by baclofen. Finally, the phase shifting effects of microinjection of muscimol into the suprachiasmatic nucleus during the subjective day were blocked by a subsequent light pulse. These data suggest that GABA regulates the phase of the circadian clock through both pre- and postsynaptic mechanisms.

Adenosine A1 receptors regulate the response of the hamster circadian clock to light

Circadian rhythms are synchronized to the environmental light-dark cycle by daily, light-induced adjustments in the phase of a biological clock located in the suprachiasmatic nucleus. Ambient light alters the phase of the clock via a direct, glutamatergic projection from retinal ganglion cells. We investigated the hypothesis that adenosine A1 receptors modulate the phase adjusting effect of light on the circadian clock. Systemic administration of the selective adenosine A1 receptor agonist, N6-cyclohexyladenosine (CHA), significantly (p<0.05) attenuated light-induced phase delays and advances of the circadian activity rhythm. Selective agonists for the adenosine A2A and adenosine A3 receptors were without effect. The inhibitory effect of CHA on light-induced phase advances was dose-dependent (0.025-1.0 mg/kg, ED(50)=0.3 mg/kg), and this effect was blocked in a dose-dependent (0.005-1.0 mg/kg) manner by the adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Injection of CHA (10 microM) into the region of the suprachiasmatic nucleus significantly attenuated light-induced phase advances, and this effect was also blocked by DPCPX (100 microM). The results suggest that adenosine A1 receptors located in the region of the suprachiasmatic nucleus regulate the response of the circadian clock to the phase-adjusting effects of light.

Immunohistochemical localization of the neuropeptide Y Y1 receptor in rat central nervous system

The diverse effects of neuropeptide Y (NPY) are mediated through interaction with G-protein coupled receptors. Pharmacological analysis suggests the Y1 receptor mediates several of NPY's central and peripheral actions. We sought to determine the distribution of Y1 protein throughout the rat central nervous system by means of indirect immunofluorescence using the tyramide signal amplification method and a novel, amino terminally-directed Y1 antisera. This antisera was verified as specific for Y1 by solution-phase competition ELISA, Western blot and in situ blocking experiments. High concentrations of Y1 immunoreactivity were found in the claustrum, piriform cortex (superficial layer), arcuate hypothalamic nucleus, interpeduncular nucleus, paratrigeminal nucleus, and lamina II of the spinal trigeminal nucleus and entire spinal cord. Moderate levels of Y1 immunoreactivity were found the in the main olfactory bulb, dorsomedial part of suprachiasmatic nucleus, paraventricular hypothalamic nucleus, ventral nucleus of lateral lemniscus, pontine nuclei, mesencephalic trigeminal nucleus, external cuneate nucleus, area postrema, and nucleus tractus solitarius. Low levels of Y1 immunostaining were distributed widely throughout layers II-III of the cerebral cortex (i.e., orbital, cingulate, frontal, parietal, insular, and temporal regions), nucleus accumbens core, amygdalohippocampal and amygdalopiriform areas, dentate gyrus, CA1 and CA2 fields of hippocampus, principal and oral divisions of the spinal trigeminal nucleus, islands of Calleja and presubiculum. These findings are discussed with reference to previously reported receptor autoradiography, immunohistochemistry and mRNA analyses to further support the role of Y1 in NPY-mediated biology.

Glutamic acid decarboxylase mRNA in the suprachiasmatic nucleus of rats housed in constant darkness

This study demonstrates that the levels of the mRNAs encoding the two isoforms of glutamic acid decarboxylase (GAD) (i.e., GAD65 and GAD67) do not differ over the circadian activity cycle in the suprachiasmatic nucleus (SCN) of rats housed in constant darkness. These data indicate that the rhythmic expression of GAD56 mRNA previously observed in animals housed in a light:dark cycle [K.L. Huhman, A.C. Hennessey, H.E. Albers, Rhythms of glutamic acid decarboxylase mRNA in the suprachiasmatic nucleus, J. Biol. Rhythms 11 (1996) 311-316.] is the result of the activity of retinal afferents.

GABA release from suprachiasmatic nucleus terminals is necessary for the light-induced inhibition of nocturnal melatonin release in the rat

The daily rhythm of melatonin production in the mammalian pineal is driven by the endogenous circadian pacemaker in the suprachiasmatic nuclei. The major release period of melatonin is closely linked to the dark phase of the 24-h day/night cycle. Environmental light will affect melatonin release in two ways: (i) it entrains the rhythm of the circadian oscillator; and (ii) it causes an acute suppression of nocturnal melatonin release. These two effects of light are both mediated by the suprachiasmatic nucleus and enable the pineal gland to convey information about day length to the reproductive system through changes in melatonin levels. Glutamate is currently believed to be the major transmitter in the retinal ganglion cell fibers reaching the suprachiasmatic nucleus. At present no information is available, however, about the transmitter(s) implicated in the further propagation, i.e. from the suprachiasmatic nucleus onwards, of the light information. In the present study we provide evidence that the endogenous release of GABA from suprachiasmatic nucleus terminals is implicated in the further transmission of light information to the pineal gland. Bilateral administration of the GABA-antagonist bicuculline to hypothalamic target areas of the suprachiasmatic nucleus completely prevents the inhibitory effect of nocturnal light on melatonin secretion and the present study thus documents that retina-mediated photic activation of suprachiasmatic nucleus neurons induces the release of GABA from efferent suprachiasmatic nucleus nerve terminals, resulting in an inhibition of melatonin release by the pineal gland. Together with our previous (electro)physiological data these results identify GABA as an important mediator of rapid synaptic transmission of suprachiasmatic nucleus output to its target areas.

Metabotropic glutamate receptor modulation of glutamate responses in the suprachiasmatic nucleus

Glutamate is the primary excitatory transmitter in the suprachiasmatic nucleus (SCN). Ionotropic glutamate receptors (iGluRs) mediate transduction of light information from the retina to the SCN, an important circadian clock phase shifting pathway. Metabotropic glutamate receptors (mGluRs) may play a significant modulatory role. mGluR modulation of SCN responses to glutamate was investigated with fura-2 calcium imaging in SCN explant cultures. SCN neurons showed reproducible calcium responses to glutamate, kainate, and N-methyl-D-aspartate (NMDA). Although the type I/II mGluR agonists L-CCG-I and t-ACPD did not evoke calcium responses, they did inhibit kainate- and NMDA-evoked calcium rises. This interaction was insensitive to pertussis toxin. Protein kinase A (PKA) activation by 8-bromo-cAMP significantly reduced iGluR inhibition by mGluR agonists. The inhibitory effect of mGluRs was enhanced by activating protein kinase C (PKC) and significantly reduced in the presence of the PKC inhibitor H7. Previous reports show that L-type calcium channels can be modulated by PKC and PKA. In SCN cells, about one-half of the calcium rise evoked by kainate or NMDA was blocked by the L-type calcium channel antagonist nimodipine. Calcium rises evoked by K+ were used to test whether mGluR inhibition of iGluR calcium rises involved calcium channel modulation. These calcium rises were primarily attributable to activation of voltage-activated calcium channels. PKC activation inhibited K+-evoked calcium rises, but PKC inhibition did not affect L-CCG-I inhibition of these rises. In contrast, 8Br-cAMP had no effect alone but blocked L-CCG-I inhibition. Taken together, these results suggest that activation of mGluRs, likely type II, modulates glutamate-evoked calcium responses in SCN neurons. mGluR inhibition of iGluR calcium rises can be differentially influenced by PKC or PKA activation. Regulation of glutamate-mediated calcium influx could occur at L-type calcium channels, K+ channels, or at GluRs. It is proposed that mGluRs may be important regulators of glutamate responsivity in the circadian system.

GABAB receptor stimulation phase-shifts the mammalian circadian clock in vitro

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) generates 24-h rhythms in vitro. Here we show that the GABAB agonist baclofen resets the SCN pacemaker in vitro in a phase-dependent manner: advances were induced at zeitgeber time (ZT) 6, and delays were induced at ZT 22. Both effects were blocked the GABAB antagonist, 2-hydroxysaclofen, while the GABAA antagonist, bicuculline was ineffective. Thus, the SCN pacemaker is sensitive to resetting by GABAB stimulation.

The SCN is comprised of a population of coupled oscillators

The circadian clock in the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus exhibits two necessary properties: (1) a mechanism for the generation of autonomous circadian rhythms in individual pacemaker cells, and (2) a means to synchronize the autonomous pacemaker cells. A variety of potential components of the endogenous pacemaker, including ion channels, second messengers, transcriptional factors, and the protein targets of kinases and transcription factors are reviewed. Similarly, reverse transmitter transport, extracellular ion fluxes, small membrane-diffusible molecules, glial regulation, and neural adhesion molecules are considered as possible synchronizing factors. Provisional criteria are suggested for empirical distinction of endogenous pacemaker versus synchronizing mechanisms.

The rhythmic GABAergic system

GABA is the major inhibitory neurotransmitter in the mammalian brain, and has been implicated in the regulation of a variety of behavioral functions, including biological rhythms. The focus of this minireview is the rhythmic variation of the central GABAergic system, comprising fluctuations of GABA levels and turnover, GABA receptor affinity and postsynaptic activity on the chloride ionophore in rodent's brain. Neurochemical rhythms correlated with diurnal and circadian changes in several behaviors associated with the GABA(A) receptor, e.g., anxiolysis-related behavior. GABA is considered to be the principal neurotransmitter of the mammalian circadian system, being present in the suprachiasmatic nuclei and the intergeniculate leaflet. Pharmacological manipulations of GABA(A) receptors phase shift circadian rhythms and alter circadian responses to light. Administration of putative modulators of GABA function, like melatonin or neuroactive steroids, affects the timing of biological rhythms. Therefore, not only does the GABAergic system exhibit strong diurnal and circadian variations, but it also serves as one of the key modulators of the circadian apparatus.

Provisional quantitative trait loci (QTL) for the Aschoff effect in RI mice

Mice of the CXB recombinant inbred (RI) panel were phenotyped for period of locomotor activity in continuous dark (tau) and in continuous 10-lux light (tauLL). There were significant differences in the effect of light on period, delta tau (tauLL-tau), among CXB RI strains and their progenitors. By comparing strain means for delta tau in the CXB RI strains with typed genetic loci using a product moment correlation, it was possible to hypothesize quantitative trait loci (QTL) important to the genetic variance in the effect of constant low-level light on circadian period. Some of the candidate genes linked to statistically associated markers are neuropharmacologically interesting. Provisional QTL for delta tau were found on proximal Chromosome 8 and mid Chromosome 11 in regions near QTL identified in a similar analysis of the BXD RI panel. This provides additional evidence for the importance of loci on Chromosomes 8 and 11.

Peptide expression in GABAergic neurons in rat suprachiasmatic nucleus in comparison with other forebrain structures: a double labeling in situ hybridization study

We investigated the characteristics of GABAergic neurons in the rat suprachiasmatic nucleus (SCN) in normal untreated rats by examination of co-expressed peptides. We adopted double labeling in situ hybridization using a digoxigenin-labeled glutamic acid decarboxylase (GAD) riboprobe and 35S-labeled peptide riboprobes. GAD mRNA-positive neurons were distributed throughout the SCN from the rostal to the caudal pole. In the dorsomedial part of the SCN, most GAD mRNA-positive neurons co-expressed arginine vasopressin mRNA. In the ventrolateral part of the SCN, about two thirds of GAD mRNA-positive neurons co-expressed vasoactive intestinal peptide (VIP) mRNA. Co-expression of GAD and somatostatin mRNA was observed in virtually all neurons of the intermediate part of the SCN. In contrast, these peptidergic traits were poorly expressed in hypothalamic GABAergic neurons outside the SCN. Vasopressin mRNA-positive cells in the supraoptic nucleus did not express GAD mRNA, and co-expression of somatostatin mRNA and GAD mRNA was rare in the periventricular hypothalamic nucleus. Similarly, the VIP mRNA co-expression ratio of GABAergic neurons in the cerebral cortex was far lower than that in the SCN.

Neuropeptide Y blocks light-induced phase advances but not delays of the circadian activity rhythm in hamsters

In mammals, the suprachiasmatic nuclei (SCN) are the anatomical site of localization of the light-entrainable circadian clock responsible for the generation of daily rhythms in physiology and behavior. In addition to direct retinohypothalamic innervation, the SCN receive a prominent projection of fibers from the intergeniculate leaflet (IGL) of the thalamus, the geniculohypothalamic tract (GHT), some of which contain the neurotransmitter, neuropeptide Y (NPY). Since the GHT has been suggested to play a role in the modulation of photic entrainment of the SCN circadian clock in rodents, we investigated the effects of local administration of NPY into the region of the SCN on light-induced phase shifts of the free-running activity rhythm in hamsters. Injection of 60 nmol of NPY into the SCN region 10 min prior to light exposure at circadian time 19 completely blocked light-induced phase advances. Similar treatment at circadian time 14 had no significant effect on the magnitude of light-induced phase delays. Injection of NPY at either time point without light exposure did not alter circadian phase. The findings support a modulatory role for NPY in the photic entrainment of the SCN circadian clock.

Regulation of circadian photic responses by nitric oxide

A role for nitric oxide in circadian responses to light has been indicated in previous studies. To determine the specific function of NO-, the authors manipulated NO- and nitric oxide synthase (NOS) activity prior to light pulses that would normally induce phase shifts. The NOS inhibitor, L-NAME, selectively attenuated phase advances of locomotor rhythms and had no effect on phase delays. The NO- donor, SNAP, potentiated both photic responses, and phase delays were larger than the maximum responses that could be obtained with light alone. The date suggest a model in which NO- participates in the adaptation of the system to environmental lighting conditions by regulating in a phase-dependent manner responsiveness to light.

GABA(A) and GABA(B) agonists and antagonists alter the phase-shifting effects of light when microinjected into the suprachiasmatic region

GABAergic drugs have profound effects on the regulation of circadian rhythms. The present study evaluated the effects of microinjections of GABAergic drugs into the suprachiasmatic region in hamsters on phase shifts induced by light and by microinjection of a cocktail containing vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI) and gastrin-releasing peptide (GRP). The phase-advancing effects of light at circadian time (CT) 19 were significantly reduced by microinjection of GABA(A) or GABA(B) agonists into the SCN, but were not altered by microinjection of GABA(A) or GABA(B) antagonists. Microinjection of a GABA(B) agonist also reduced the phase-delaying effects of light at CT 13.5-14 while a GABA(B) antagonist increased the phase delays caused by light. Neither GABA(B) drug altered the phase delays produced by microinjection of a peptide cocktail containing VIP, PHI, GRP. These data indicate that changes in GABA(A) or GABA(B) activity within the SCN can alter the phase-shifting effects of light on circadian rhythms and support a role for GABA in gating photic input to the circadian clock.

Rhythms of glutamic acid decarboxylase mRNA in the suprachiasmatic nucleus

The purpose of the present study was to determine whether there is a rhythm in glutamic acid decarboxylase (GAD) message in the suprachiasmatic nucleus (SCN) of rats housed in a light:dark cycle. The mRNAs encoding two isoforms of GAD (i.e., GAD65 and GAD67) were examined using in situ hybridization histochemistry. Computerized image analysis of film autoradiographs revealed that GAD65 mRNA was significantly higher in the light than it was in the dark. GAD67 mRNA levels were lower overall and did not decrease significantly in the dark. Following emulsion autoradiography, silver grain counts over individual SCN cells indicated that GAD65 mRNA was highest in the dorsomedial hypothalamus during the light. These data suggest that GAD mRNA varies rhythmically in the SCN and that mRNA levels are regulated differently within SCN subdivisions during the light:dark cycle.