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.

The gut metabolite indole-3-propionic acid activates ERK1 to restore social function and hippocampal inhibitory synaptic transmission in a 16p11.2 microdeletion mouse model

BACKGROUND

Microdeletion of the human chromosomal region 16p11.2 (16p11.2+ / - ) is a prevalent genetic factor associated with autism spectrum disorder (ASD) and other neurodevelopmental disorders. However its pathogenic mechanism remains unclear, and effective treatments for 16p11.2+ / -  syndrome are lacking. Emerging evidence suggests that the gut microbiota and its metabolites are inextricably linked to host behavior through the gut-brain axis and are therefore implicated in ASD development. Despite this, the functional roles of microbial metabolites in the context of 16p11.2+ / -  are yet to be elucidated. This study aims to investigate the therapeutic potential of indole-3-propionic acid (IPA), a gut microbiota metabolite, in addressing behavioral and neural deficits associated with 16p11.2+ / - , as well as the underlying molecular mechanisms.

RESULTS

Mice with the 16p11.2+ / -  showed dysbiosis of the gut microbiota and a significant decrease in IPA levels in feces and blood circulation. Further, these mice exhibited significant social and cognitive memory impairments, along with hyperactivation of hippocampal dentate gyrus neurons and reduced inhibitory synaptic transmission in this region. However, oral administration of IPA effectively mitigated the histological and electrophysiological alterations, thereby ameliorating the social and cognitive deficits of the mice. Remarkably, IPA treatment significantly increased the phosphorylation level of ERK1, a protein encoded by the Mapk3 gene in the 16p11.2 region, without affecting the transcription and translation of the Mapk3 gene.

CONCLUSIONS

Our study reveals that 16p11.2+ / -  leads to a decline in gut metabolite IPA levels; however, IPA supplementation notably reverses the behavioral and neural phenotypes of 16p11.2+ / -  mice. These findings provide new insights into the critical role of gut microbial metabolites in ASD pathogenesis and present a promising treatment strategy for social and cognitive memory deficit disorders, such as 16p11.2 microdeletion syndrome. Video Abstract.

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.

Genesis of the Master Circadian Pacemaker in Mice

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central circadian clock of mammals. It is responsible for communicating temporal information to peripheral oscillators via humoral and endocrine signaling, ultimately controlling overt rhythms such as sleep-wake cycles, body temperature, and locomotor activity. Given the heterogeneity and complexity of the SCN, its genesis is tightly regulated by countless intrinsic and extrinsic factors. Here, we provide a brief overview of the development of the SCN, with special emphasis on the murine system.

The Excitatory Effects of GABA within the Suprachiasmatic Nucleus: Regulation of Na-K-2Cl Cotransporters (NKCCs) by Environmental Lighting Conditions

The suprachiasmatic nucleus (SCN) contains a pacemaker that generates circadian rhythms and entrains them with the 24-h light-dark cycle (LD). The SCN is composed of 16,000 to 20,000 heterogeneous neurons in bilaterally paired nuclei. γ-amino butyric acid (GABA) is the primary neurochemical signal within the SCN and plays a key role in regulating circadian function. While GABA is the primary inhibitory neurotransmitter in the brain, there is now evidence that GABA can also exert excitatory effects in the adult brain. Cation chloride cotransporters determine the effects of GABA on chloride equilibrium, thereby determining whether GABA produces hyperpolarizing or depolarizing actions following activation of GABA_A receptors. The activity of Na-K-2Cl cotransporter1 (NKCC1), the most prevalent chloride influx cotransporter isoform in the brain, plays a critical role in determining whether GABA has depolarizing effects. In the present study, we tested the hypothesis that NKCC1 protein expression in the SCN is regulated by environmental lighting and displays daily and circadian changes in the intact circadian system of the Syrian hamster. In hamsters housed in constant light (LL), the overall NKCC1 immunoreactivity (NKCC1-ir) in the SCN was significantly greater than in hamsters housed in LD or constant darkness (DD), although NKCC1 protein levels in the SCN were not different between hamsters housed in LD and DD. In hamsters housed in LD cycles, no differences in NKCC1-ir within the SCN were observed over the 24-h cycle. NKCC1 protein in the SCN was found to vary significantly over the circadian cycle in hamsters housed in free-running conditions. Overall, NKCC1 protein was greater in the ventral SCN than in the dorsal SCN, although no significant differences were observed across lighting conditions or time of day in either subregion. These data support the hypothesis that NKCC1 protein expression can be regulated by environmental lighting and circadian mechanisms within the SCN.

A Symphony of Signals: Intercellular and Intracellular Signaling Mechanisms Underlying Circadian Timekeeping in Mice and Flies

The central pacemakers of circadian timekeeping systems are highly robust yet adaptable, providing the temporal coordination of rhythms in behavior and physiological processes in accordance with the demands imposed by environmental cycles. These features of the central pacemaker are achieved by a multi-oscillator network in which individual cellular oscillators are tightly coupled to the environmental day-night cycle, and to one another via intercellular coupling. In this review, we will summarize the roles of various neurotransmitters and neuropeptides in the regulation of circadian entrainment and synchrony within the mammalian and Drosophila central pacemakers. We will also describe the diverse functions of protein kinases in the relay of input signals to the core oscillator or the direct regulation of the molecular clock machinery.

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.

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.

Collective timekeeping among cells of the master circadian clock

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the master circadian clock that coordinates daily rhythms in behavior and physiology in mammals. Like other hypothalamic nuclei, the SCN displays an impressive array of distinct cell types characterized by differences in neurotransmitter and neuropeptide expression. Individual SCN neurons and glia are able to display self-sustained circadian rhythms in cellular function that are regulated at the molecular level by a 24h transcriptional-translational feedback loop. Remarkably, SCN cells are able to harmonize with one another to sustain coherent rhythms at the tissue level. Mechanisms of cellular communication in the SCN network are not completely understood, but recent progress has provided insight into the functional roles of several SCN signaling factors. This review discusses SCN organization, how intercellular communication is critical for maintaining network function, and the signaling mechanisms that play a role in this process. Despite recent progress, our understanding of SCN circuitry and coupling is far from complete. Further work is needed to map SCN circuitry fully and define the signaling mechanisms that allow for collective timekeeping in the SCN network.

Photic Entrainment Is Altered in the 5-HT1B Receptor Knockout Mouse

The hypothalamic suprachiasmatic nucleus (SCN) is a circadian oscillator that receives glutamatergic afferents from the retina and serotonergic afferents from the midbrain. Activation of presynaptic serotonin 1B (5-HT1B) receptors on retinal terminals in the SCN inhibits retinohypothalamic neurotransmission and light-induced behavioral phase shifts. To assess the role of 5-HT1B receptors in photic entrainment, 5-HT1B receptor knockout (5-HT1B KO) and wild-type (WT) mice were maintained in non-24 h L:D cycles (T cycles). WT mice entrained to T = 21 h and T = 22 h cycles, whereas 5-HT1B KO animals did not. 5-HT1B KO animals did entrain to T = 23 h and T = 26 h cycles, although their phase angle of entrainment was altered compared to WT animals. 5-HT1BKO mice were significantly more phase delayed under T = 23 h conditions and significantly more phase advanced under T = 26 h conditions compared to WT mice. When 5-HT1B KO mice were housed in a T = 23 h short-day photoperiod (9.5L:13.5D), the delayed phase angle of entrainment was more pronounced. Light-induced phase shifts were reduced in 5-HT1B KO mice, consistent with their behavior in T cycles, suggesting an attenuated response to light. Based on previous work, this attenuated response to light might not have been predicted but can be explained by consideration of GABAergic mechanisms within the SCN. Phase-delayed circadian rhythms during the short days of winter are characteristic of patients suffering from seasonal affective disorder, and 5-HT has been implicated in its pathophysiology. The 5-HT1B KO mouse may be useful for investigating the altered entrainment evident during this serious mood disorder.

Enhanced GABAergic inhibition in the premammillary nucleus of photorefractory turkey hens via GABAA receptor upregulation

The premammillary nucleus (PMM) of the turkey mediobasal hypothalamus, where dopamine-melatonin (DA-Mel) neurons are localized, is a site for photoreception and photoperiodic time measurement, which is essential for the initiation of avian reproductive seasonality. In addition, this area could also be responsible for the onset and maintenance of photorefractoriness at the end of the breeding season due to the enhanced inhibitory effect of γ-aminobutyric acid (GABA). GABA is an inhibitory neurotransmitter in the central nervous system which interferes with the photosexual response in the turkey, a seasonally breeding bird. Here, we further characterized the GABAA receptor subunits in the PMM DA-Mel neurons related to reproductive seasonality and the onset of photorefractoriness. GABAA receptor subunits and GABA synthesis enzymes in the PMM of photosensitive and photorefractory turkey hens were identified using real-time qRT-PCR. The upregulation of GABAA receptor α1-3, β2-3, γ1-3, ρ1-3, δ, and θ mRNA expression were observed in the PMM of photorefractory birds when compared to those of photosensitive ones while there is no change observed in the GABA synthesis enzymes, glutamate decarboxylase 1 and 2. Those upregulated GABAA receptor subunits were further examined using immunohistochemical staining and they appeared to be co-localized within the PMM DA-Mel neurons. The upregulation of GABAA receptor subunits observed in the PMM of photorefractory birds coincides with a lack of responsiveness to a light stimulus provided during the photosensitive phase. This is supported by the absence of c-fos induction and TH upregulation in the PMM and a subsequence inhibition of c-fos and GnRH-I expression in the nucleus commissurae pallii. The augmented GABAA receptor subunits expression may mediate an enhancement of inhibitory GABAergic neurotransmission and the subsequent interference with the photosexual response. This could contribute to the state of photorefractoriness and the termination of breeding activities in the turkey, a temperate zone bird.

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.

GABAB receptor-mediated frequency-dependent and circadian changes in synaptic plasticity modulate retinal input to the suprachiasmatic nucleus

Light is the most important environmental signal that entrains the circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN). The retinohypothalamic tract (RHT) was stimulated to simulate the light intensity-dependent discharges of intrinsically photosensitive retinal ganglion cells projecting axons to the hypothalamus. EPSCs were evoked by paired-pulse stimulation or by application of stimulus trains, and recorded from SCN neurons in rat brain slices. Initial release probability (Pr) and synaptic plasticity changes depended on the strength of GABAB receptor (GABABR)-mediated presynaptic inhibition and could be different at the same GABABR agonist concentration. Facilitation caused by frequency-dependent relief of GABABR-mediated inhibition was observed when the initial Pr was decreased to less than 15% of control during strong activation of presynaptic GABAB receptors by (±)baclofen (10 μm), GABA (2 mm) or by GABA uptake inhibitor nipecotic acid (5 mm). In contrast, short-term synaptic depression appeared during baclofen (10 μm) application when initial Pr was greater than 30% of control. Block of 4-aminopyridine-sensitive K(+) currents increased the amplitude and time constant of decay of evoked EPSCs (eEPSCs), and decreased the GABABR-mediated presynaptic inhibition. The GABAB receptor antagonist CGP55845 (3 μm) increased the eEPSCs amplitude 30% throughout the light-dark cycle. During light and dark phases the RHT inputs to 55% and 33% of recorded neurons, respectively, were under GABAB inhibitory control indicating that the tonic inhibition induced by local changes of endogenous GABA concentration contributes to the circadian variation of RHT transmitter release. We conclude that GABABR-mediated presynaptic inhibition decreased with increasing frequency and broadening of presynaptic action potentials, and depended on the sensitivity of RHT terminals to GABABR agonists, and diurnal changes of the extracellular GABA concentration around RHT axon terminals 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.

The role of GABAergic neuron on NMDA- and SP-induced phase delays in the suprachiasmatic nucleus neuronal activity rhythm in vitro

Gamma-aminobutyric acid (GABA), and its biosynthetic enzyme, glutamic decarboxylase, are widely distributed in the suprachiasmatic nucleus (SCN). In the present study, we examined the role of the GABA(A) receptor on in vitro SCN responses to photic-like signals. We found that 100microM GABA(A) receptor antagonist bicuculline partially blocked field potentials evoked by optic nerve stimulation. NMDA- and SP-induced phase shifts of SCN neuronal activity rhythms, were blocked with 10microM bicuculline. Application of 100microM bicuculline alone induced phase advance of SCN neuronal activity rhythm. These results show that NMDA- and SP-induced phase shifts are blocked by bicuculline and suggest GABA has an important role as neurotransmitter in the neuronal network regulating phase shifts of the circadian clock.

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.

Investigating the role of substance P in photic responses of the circadian system: individual and combined actions with gastrin-releasing peptide

The suprachiasmatic nucleus (SCN) contains the master mammalian circadian pacemaker. It is comprised of several phenotypically distinct cell groups, some of which are situated in the weakly rhythmic retinoresponsive ventrolateral region while others are found in the rhythmic, non-retinoresponsive dorsomedial region. The mechanism by which retinorecipient cells convey photic information to the dorsomedial clock cells is unclear. The ventrolateral SCN core contains a variety of cell phenotypes. Two neuropeptides, namely substance P (SP) and gastrin-releasing peptide (GRP) extensively colocalize with calbindin D28K, a marker for SCN cells that are strongly light-responsive. Previous studies have implicated these neuropeptides in photic phase shifting of the circadian system. The present study examines how these peptides interact to regulate photic responses of the circadian system. It was observed that 55.5 +/- 9.1% of SP cells colocalized GRP. SP did not enhance GRP-induced phase shifts in the early-subjective night, while it significantly attenuated GRP-induced phase shifts during the late-subjective night. SP induced significant phase shifts that did not resemble light in the early-subjective night, but was not necessary for light-induced phase shifts and Fos expression at this time. SP induced significant Fos expression only in the late subjective night. SP may not be a necessary component in the pathway(s) involved in photic phase shifting during the early-subjective night, but may modulate phase shifts during the late-subjective night. Distinct biochemical mechanisms that underlie behavioral phase shifts may account for the differences observed in the early- vs. late-subjective night.

Circadian and dark-pulse activation of orexin/hypocretin neurons

Temporal control of brain and behavioral states emerges as a consequence of the interaction between circadian and homeostatic neural circuits. This interaction permits the daily rhythm of sleep and wake, regulated in parallel by circadian cues originating from the suprachiasmatic nuclei (SCN) and arousal-promoting signals arising from the orexin-containing neurons in the tuberal hypothalamus (TH). Intriguingly, the SCN circadian clock can be reset by arousal-promoting stimuli while activation of orexin/hypocretin neurons is believed to be under circadian control, suggesting the existence of a reciprocal relationship. Unfortunately, since orexin neurons are themselves activated by locomotor promoting cues, it is unclear how these two systems interact to regulate behavioral rhythms. Here mice were placed in conditions of constant light, which suppressed locomotor activity, but also revealed a highly pronounced circadian pattern in orexin neuronal activation. Significantly, activation of orexin neurons in the medial and lateral TH occurred prior to the onset of sustained wheel-running activity. Moreover, exposure to a 6 h dark pulse during the subjective day, a stimulus that promotes arousal and phase advances behavioral rhythms, activated neurons in the medial and lateral TH including those containing orexin. Concurrently, this stimulus suppressed SCN activity while activating cells in the median raphe. In contrast, dark pulse exposure during the subjective night did not reset SCN-controlled behavioral rhythms and caused a transient suppression of neuronal activation in the TH. Collectively these results demonstrate, for the first time, pronounced circadian control of orexin neuron activation and implicate recruitment of orexin cells in dark pulse resetting of the SCN circadian clock.

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."

Presynaptic GABABReceptors Regulate Retinohypothalamic Tract Synaptic Transmission by Inhibiting Voltage-Gated Ca2+Channels

Presynaptic GABABreceptor activation inhibits glutamate release from retinohypothalamic tract (RHT) terminals in the suprachiasmatic nucleus (SCN). Voltage-clamp whole cell recordings from rat SCN neurons and optical recordings of Ca2+-sensitive fluorescent probes within RHT terminals were used to examine GABAB-receptor modulation of RHT transmission. Baclofen inhibited evoked excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner equally during the day and night. Blockers of N-, P/Q-, T-, and R-type voltage-dependent Ca2+channels, but not L-type, reduced the EPSC amplitude by 66, 36, 32, and 18% of control, respectively. Joint application of multiple Ca2+channel blockers inhibited the EPSCs less than that predicted, consistent with a model in which multiple Ca2+channels overlap in the regulation of transmitter release. Presynaptic inhibition of EPSCs by baclofen was occluded by ω-conotoxin GVIA (≤72%), mibefradil (≤52%), and ω-agatoxin TK (≤15%), but not by SNX-482 or nimodipine. Baclofen reduced both evoked presynaptic Ca2+influx and resting Ca2+concentration in RHT terminals. Tertiapin did not alter the evoked EPSC and baclofen-induced inhibition, indicating that baclofen does not inhibit glutamate release by activation of Kir3 channels. Neither Ba2+nor high extracellular K+modified the baclofen-induced inhibition. 4-Aminopyridine (4-AP) significantly increased the EPSC amplitude and the charge transfer, and dramatically reduced the baclofen effect. These data indicate that baclofen inhibits glutamate release from RHT terminals by blocking N-, T-, and P/Q-type Ca2+channels, and possibly by activation of 4-AP–sensitive K+channels, but not by inhibition of R- and L-type Ca2+channels or by Kir3 channel activation.

GABA modulates Drosophila circadian clock neurons via GABAB receptors and decreases in calcium

Circadian clocks play vital roles in the control of daily rhythms in physiology and behavior of animals. In Drosophila, analysis of the molecular and behavioral rhythm has shown that the master clock neurons are entrained by sensory inputs and are synchronized with other clock neurons. However, little is known about the neuronal circuits of the Drosophila circadian system and the neurotransmitters that act on the clock neurons. Here, we provide evidence for a new neuronal input pathway to the master clock neurons, s-LN(v)s, in Drosophila that utilizes GABA as a slow inhibitory neurotransmitter. We monitored intracellular calcium levels in dissociated larval s-LN(v)s with the calcium-sensitive dye Fura-2. GABA decreased intracellular calcium in the s-LN(v)s and blocked spontaneous oscillations in calcium levels. The duration of this response was dose-dependent between 1 nM and 100 microM. The response to GABA was blocked by a metabotropic GABA(B) receptor (GABA(B)-R) antagonist, CGP54626, but not by an ionotropic receptor antagonist, picrotoxin. The GABA(B)-R agonist, 3-APMPA, produced a response similar to GABA. An antiserum against one of the Drosophila GABA(B)-Rs (GABA(B)-R2) labeled the dendritic regions of the s-LN(v)s in both adults and larvae, as well as the dissociated s-LN(v)s. We found that some GABAergic processes terminate at the dendrites of the LN(v)s, as revealed by GABA immunostaining and a GABA-specific GAL4 line (GAD1-gal4). Our results suggest that the s-LN(v)s receive slow inhibitory GABAergic inputs that decrease intracellular calcium of these clock neurons and block their calcium cycling. This response is mediated by postsynaptic GABA(B) receptors.

Female pheromones stimulate release of luteinizing hormone and testosterone without altering GnRH mRNA in adult male Syrian hamsters (Mesocricetus auratus)

In many species chemosensory stimuli function as important signals that influence reproductive status. Neurons synthesizing the peptide gonadotropin-releasing hormone (GnRH) are critical mediators of reproductive function via their regulation of the hypothalamic-pituitary-gonadal (HPG) axis, and they are thought to be responsive to chemosensory information. In the present study, we sought to elucidate the effects of female chemosensory stimuli on the HPG axis in sexually naive adult male Syrian hamsters. In Experiment 1, serial blood samples were collected from catheterized male hamsters following exposure to female pheromones in order to characterize the luteinizing hormone (LH) response to this chemosensory stimulus. In Experiment 2, brains and terminal blood samples were collected from animals 0, 60, and 120 min following pheromone exposure. GnRH mRNA was measured in brain tissue sections using in situ hybridization, and plasma concentrations of LH and testosterone were measured using radioimmunoassay. Data from Experiment 1 indicated that female pheromones elicited a rapid rise in plasma LH that peaked at 15 min and returned to baseline 45 min after exposure. In Experiment 2, testosterone was elevated in terminal blood samples obtained 60 min, but not 120 min, after exposure to pheromones. LH levels were unaffected at both of these time points. The chemosensory-induced increases in LH and testosterone release were not accompanied by subsequent changes in GnRH mRNA over the time course studied. These data suggest that while activation of the male HPG axis by female pheromones involves release of GnRH, it does not involve increases in GnRH mRNA 1-2 h after pheromonal stimulation as a mechanism for replenishment of released peptide.

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.

Fos and FRA protein expression in rat precerebellar structures during the Neurolab Space Mission

Changes in gene expression were examined in precerebellar structures during and after space flight. These structures included the inferior olive (IO), the source of climbing fibers, and the lateral reticular nucleus (LRt) and basilar pontine nuclei (PN), sources of mossy fibers. We examined two immediate early gene products with two different time courses of expression: Fos, which persists only for a few (6-8)h after activation and FRA expression, which lasts for longer periods of time, i.e. hours and/or days after activation. Gravity effects on Fos and FRA gene expression were evident in vestibular and visual areas of the IO, including the dorsomedial cell column, the beta subnucleus and the dorsal cap of Kooy of the medial nucleus (which projects to the flocculonodular lobe, i.e. to the vestibular area of the IO involved in the olivary control of the vestibulo-ocular reflex (VOR)). Gene expression also affected the subnuclei A, B, and C and the caudal part of the medial IO. These olivary regions do not receive vestibular afferents, but rather spinal afferents, and are particularly involved in the olivary control of the vestibulospinal reflex (VSR). Changes in Fos expression were also observed in the LRt and the PN. We suggest that sensory substitution, in which signals produced by a subject's own activity replace activity normally provided by macular stimulation, contributes to the recovery of microgravity-related postural and motor deficits. While no consistent increases in FRA expression occurred in vestibular IO regions 24h after launch, consistent increases in FRA expression occurred 24h after landing. We hypothesize that this asymmetrical pattern of gene expression resulted from (i). tonic microgravity experienced after launch counteracting the effects of increased phasic gravitational forces experienced during launch, and (ii). the tonic gravitational field experienced after landing potentiating the effects of increased phasic gravitational forces experienced during landing. The specificity of these results is demonstrated by an absence of direct gravity-related changes in Fos expression in other precerebellar structures such as the external cuneate nucleus, group X, and the dorsal column nuclei that transmit exteroceptive and proprioceptive signals to thalamic nuclei and somatosensory areas of the cerebral cortex. The gravity-related Fos and FRA expression changes in the IO and the LRt seen here are of interest in view of the important role their projections play in adaptive gain changes of the VOR and VSR during sustained visuo-vestibular and neck-vestibular stimulation.

Regulation of inhibitory synaptic transmission by vasoactive intestinal peptide (VIP) in the mouse suprachiasmatic nucleus

Circadian rhythmicity in mammals is generated by a pair of nuclei in the anterior hypothalamus known as the suprachiasmatic nuclei (SCN), whose neurons express a variety of neuropeptides that are thought to play an important role in the circadian timing system. To evaluate the influence of VIP on inhibitory synaptic transmission between SCN neurons, we used whole cell patch-clamp recording in an acute brain slice preparation of mouse SCN. Baseline spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) varied significantly between regions and across phases, with a greater frequency of IPSCs observed in the dorsomedial region during the early night. Bath-applied VIP caused a significant increase in the frequency of spontaneous inhibitory postsynaptic currents (sIPSC) in a reversible and dose-dependent manner with no effect on the mean amplitude or kinetic parameters. The effect of VIP was widespread throughout the SCN and observed in both ventrolateral (VL) and dorsomedial (DM) regions. In the presence of tetrodotoxin, VIP increased the frequency of miniature IPSCs without affecting the mean magnitude or kinetic parameters. The magnitude of the enhancement by VIP was significantly larger during the day than during the night. Pretreatment with the VIP-PACAP receptor antagonist [Ac-Tyr1, D-Phe2]-GHRF 1-29 or the selective VPAC2 receptor antagonist PG 99-465 completely blocked the VIP-induced enhancement. The effect of VIP appears to be mediated by a cAMP/PKA-dependent mechanism as forskolin mimics, while the PKA antagonist H-89 blocks the observed enhancement of GABA currents. Our data suggest that VIP activates presynaptic VPAC2 receptors to regulate inhibitory synaptic transmission within the SCN and that this effect varies from day to night.

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.

In search of the pathways for light-induced pacemaker resetting in the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus is a circadian pacemaker that functions as a clock. Its endogenous period is adjusted to the external 24-h light-dark cycle, primarily by light-induced phase shifts that reset the pacemaker's oscillation. Evidence using a wide variety of neurobiological and molecular genetic tools has elucidated key elements that comprise the visual input pathway for SCN photoentrainment in rodents. Important questions remain regarding the intracellular signals that reset the autoregulatory molecular loop within photoresponsive cells in the SCN's retino-recipient subdivision, as well as the intercellular coupling mechanisms that enable SCN tissue to generate phase shifts of overt behavioral and physiological circadian rhythms such as locomotion and SCN neuronal firing rate. Multiple neurotransmitters, protein kinases, and photoinducible genes add to system complexity, and we still do not fully understand how dawn and dusk light pulses ultimately produce bidirectional, advancing and delaying phase shifts for pacemaker entrainment.

Gene expression in rat vestibular and reticular structures during and after space flight

Space flight produces profound changes of neuronal activity in the mammalian vestibular and reticular systems, affecting postural and motor functions. These changes are compensated over time by plastic alterations in the brain. Immediate early genes (IEGs) are useful indicators of both activity changes and neuronal plasticity. We studied the expression of two IEG protein products [Fos and Fos-related antigens (FRAs)] with different cell persistence times (hours and days, respectively) to identify brainstem vestibular and reticular structures involved in adaptation to microgravity and readaptation to 1 G (gravity) during the NASA Neurolab Mission (STS-90). IEG protein expression in flight animals was compared to that of ground controls using Fisher 344 rats killed 1 and 12 days after launch and 1 and 14 days after landing. An increase in the number of Fos-protein-positive cells in vestibular (especially medial and spinal) regions was observed 1 day after launch and 1 day after landing. Fos-positive cell numbers were no different from controls 12 days after launch or 14 days after landing. No G-related changes in IEG expression were observed in the lateral vestibular nucleus. The pattern of FRA protein expression was generally similar to that of Fos, except at 1 day after landing, when FRA-expressing cells were observed throughout the whole spinal vestibular nucleus, but only in the caudal part of the medial vestibular nucleus. Fos expression was found throughout the entire medial vestibular nucleus at this time. While both Fos and FRA expression patterns may reflect the increased G force experienced during take-off and landing, the Fos pattern may additionally reflect recent rebound episodes of rapid eye movement (REM) sleep following forced wakefulness, especially after landing. Pontine activity sources producing rhythmic discharges of vestibulo-oculomotor neurons during REM sleep could substitute for labyrinthine signals after exposure to microgravity, contributing to activity-related plastic changes leading to G readaptation. Reticular structures exhibited a contrasting pattern of changes in the numbers of Fos- and FRA-positive cells suggestive of a major influence from proprioceptive inputs, and plastic re-weighting of inputs after landing. Asymmetric induction of Fos and FRAs observed in some vestibular nuclei 1 day after landing suggests that activity asymmetries between bilateral otolith organs, their primary labyrinthine afferents, and vestibular nuclei may become unmasked during flight.

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.

Analysis of immunohistochemical label of Fos protein in the suprachiasmatic nucleus: comparison of different methods of quantification

The induction of the proto-oncogene c-fos, and its phosphoprotein product Fos, has been extensively used to study the effects of light on the circadian pacemaker in the suprachiasmatic nucleus (SCN). Experimental approaches to the quantification of Fos induction have mainly been based on immunohistochemistry and subsequent measure of Fos immunoreactivity (IR) in sections of the SCN. In this study, the authors compare several methods of quantification using optical density image analysis or counts of Fos-IR labeled cells. To assess whether optical density measures using image analysis reflect the amount of Fos in brain tissue, the authors developed standards of known concentrations of Fos protein in an agar matrix. The agar standards were sectioned and treated simultaneously with sections of the SCN from animals exposed to different levels of irradiance. Optical density was found to be proportional to the quantity of Fos in the sections, indicating that this measure accurately reflects relative levels of Fos protein induction. Quantification by optical density analysis allows an objective measure in which the various parameters, conditions of illumination, and threshold can be maintained constant throughout the analysis. Counting cells by visual observation is more subjective because threshold values cannot be precisely defined and can vary according to the observer, illumination, degree of label, and other factors. In addition, cell counts involving direct visual observation, automated cell counts, or stereological methods do not take into account the difference in the density of label between cells, thus giving equal weight to lightly or densely stained cells. These measures are more or less weakly correlated with measures of optical density and thus do not accurately reflect the amount of bound Fos protein in the tissue sections. In contrast, labeled surface area as measured by image analysis shows a linear relationship with optical density. The main outcome of this study is that computer-assisted image analysis provides an accurate and rapid method to determine the relative amount of Fos protein in the SCN and the effects of light on intracellular signaling mechanisms involved in the circadian clock.

Immortalized suprachiasmatic nucleus cells express components of multiple circadian regulatory pathways

We undertook an extensive antigenic characterization of the SCN 2.2 cell line in order to further evaluate whether the line expresses components of circadian regulatory pathways common to the hypothalamic suprachiasmatic nucleus (SCN), the central circadian clock in mammals. We found that differentiated SCN 2.2 cultures expressed a broad range of putative clock genes, as well as components of daytime, nighttime, and crepuscular circadian regulatory pathways found within the SCN in vivo. The line also exhibits several antigens that are highly expressed in a circadian pattern and/or differentially localized in the SCN relative to other hypothalamic regions. Expression of a broad complement of circadian regulatory proteins and putative clock genes further support growing evidence in recent reports that the SCN 2.2 cell line is an appropriate model for investigating the regulation of central mammalian pacemaker.

The suprachiasmatic nucleus exhibits diurnal variations in spontaneous excitatory postsynaptic activity

A most prominent feature of neurons in the suprachiasmatic nucleus (SCN) is the circadian rhythm in spontaneous firing frequency. To disclose synaptic mechanisms associated with the rhythmic activity, the spontaneous postsynaptic activity was studied using whole-cell, patch clamp recordings in the ventral region of the SCN in slice preparations from rats. The synaptic events were compared between two time intervals corresponding to the highest and lowest electrical activity within the SCN during subjective daytime and nighttime, respectively. The gamma-aminobutyric acid (GABA)-mediated spontaneous inhibitory activity showed no diurnal variations, but the excitatory activity was markedly higher in frequency, without differences in amplitude, during the subjective day compared to the subjective night. Spontaneous and evoked inhibitory synaptic events were blocked by the GABA(A) receptor antagonist bicuculline. The alpha-amino-hydroxy-5-methylisoxazole-4-propionic acid (AMPA/kainate) receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) blocked most of the excitatory activity. In addition, CNQX reduced the spontaneous inhibitory activity. The N-methyl-D-aspartate antagonist D-2-amino-5-phosphonopentanoic acid reduced the inhibitory activity to a lesser degree, and there was no significant difference in amplitude or frequency of synaptic events in control and Mg2+-free solutions, indicating that the AMPA receptor plays an important role in regulating the inhibitory release of GABA within the SCN. Ipsi- and contralateral stimulation of the SCN consistently evoked excitatory synaptic responses. Inhibitory synaptic responses occurred in some neurons upon increasing stimulus strength. In conclusion, this study shows that there is a substantial influence from spontaneous glutamatergic synapses on the ventral part of the SCN and that these exhibit daily variations in activity. Diurnal fluctuations in spontaneous excitatory postsynaptic activity within this network may contribute to the mechanisms for synchronization of rhythms between individual SCN neurons and may underlie the daily variations in the spontaneous firing frequency of SCN neurons.

Rhythmic variation in gamma-aminobutyric acid(A)-receptor subunit composition in the circadian system and median eminence of Syrian hamsters

Temporal changes in the level of expression of gamma-aminobutyric acid (GABA)(A) receptor subunits alpha2, alpha5, beta1 and beta3 were characterized by Western blot analysis in the hamster suprachiasmatic nuclei, retina and median eminence. A nocturnal maximum in the level of GABA(A) receptor beta1 subunit at midday and midnight (12:00 and 00.00 h) was found in the suprachiasmatic nucleus (SCN), the retina and the median eminence of Syrian hamsters. Alpha2 and beta3 subunit levels peaked during the day in the median eminence. Finally, retinal alpha5 levels were maximal during the night. beta1 temporal changes in the SCN and median eminence, as well as alpha2 variations in the median eminence were maintained under constant dark conditions, suggesting an endogenous control, while the other variations were only observed under light-dark cycle conditions.

Light induces chromatin modification in cells of the mammalian circadian clock

The mammalian circadian clock resides in neurons of the hypothalamic suprachiasmatic nucleus (SCN). Light entrains phase resetting of the clock using the retino-hypothalamic tract, via release of glutamate. Nighttime light exposure causes rapid, transient induction of clock and immediate-early genes implicated in phase-shifting the pacemaker. Here we show that a nighttime light pulse caused phosphorylation of Ser10 in histone H3's tail, in SCN clock cells. The effect of light was specific, and the kinetics of H3 phosphorylation were characteristic of the early response, paralleling c-fos and Per1 induction. Using fos-lacZ transgenic mice, we found that H3 phosphorylation and Fos induction occurRed in the same SCN neurons. Systemic treatment with the GABAB receptor agonist baclofen prevented light-induced c-fos and Per1 expression and H3 phosphorylation, indicating that one signaling pathway governs both events. Our results suggest that dynamic chromatin remodeling in the SCN occurs in response to a physiological stimulus in vivo.

Stimulus frequency affects c-fos expression in the rat visual system

We have characterised the c-fos expression patterns in various centers of the visual pathway of adult rats monocularly stimulated either by continuous or flickering light at different frequencies. Results show different immunocytochemical patterns in all centers studied, the geniculate lateral complex (LGC), superior colliculus (SC) and primary visual cortex (Oc1), depending on the physical characteristics of the stimulus (blinking frequency and light wavelength). After stimulation of the left eye, the ipsilateral pathway presents a substantial density of immunoresponsive cells, which is greater than expected with respect to the number of fibers that project ipsilaterally from the retina to the LGC and the superficial layers of the SC. A surprisingly high positive immunoresponsiveness is obtained in all cases with coherent light stimulation in the red spectrum (634 nm).

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.