Distribution of ionotropic glutamate receptor subunit immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet of the hamster

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.

SynGAP is expressed in the murine suprachiasmatic nucleus and regulates circadian-gated locomotor activity and light-entrainment capacity

The suprachiasmatic nucleus (SCN) of the hypothalamus functions as the master circadian clock. The phasing of the SCN oscillator is locked to the daily solar cycle, and an intracellular signaling cassette from the small GTPase Ras to the p44/42 mitogen-activated protein kinase (ERK/MAPK) pathway is central to this entrainment process. Here, we analyzed the expression and function of SynGAP-a GTPase-activating protein that serves as a negative regulator of Ras signaling-within the murine SCN. Using a combination of immunohistochemical and Western blotting approaches, we show that SynGAP is broadly expressed throughout the SCN. In addition, temporal profiling assays revealed that SynGAP expression is regulated over the circadian cycle, with peak expression occurring during the circadian night. Further, time-of-day-gated expression of SynGAP was not observed in clock arrhythmic BMAL1 null mice, indicating that the daily oscillation in SynGAP is driven by the inherent circadian timing mechanism. We also show that SynGAP phosphorylation at serine 1138-an event that has been found to modulate its functional efficacy-is regulated by clock time and is responsive to photic input. Finally, circadian phenotypic analysis of Syngap1 heterozygous mice revealed enhanced locomotor activity, increased sensitivity to light-evoked clock entrainment, and elevated levels of light-evoked MAPK activity, which is consistent with the role of SynGAP as a negative regulator of MAPK signaling. These findings reveal that SynGAP functions as a modulator of SCN clock entrainment, an effect that may contribute to sleep and circadian abnormalities observed in patients with SYNGAP1 gene mutations.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

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

Assessing ethanol's actions in the suprachiasmatic circadian clock using in vivo and in vitro approaches

Research over the past decade has demonstrated substantial interactions between the circadian system and the processes through which alcohol affects behavior and physiology. Here we summarize the results of our collaborative efforts focused on this intersection. Using a combination of in vivo and in vitro approaches, we have shown that ethanol affects many aspects of the mammalian circadian system, both acutely as well as after chronic administration. Conversely, we have shown circadian influences on ethanol consumption. Importantly, we are beginning to delve into the cellular mechanisms associated with these effects. We are also starting to form a picture of the neuroanatomical bases for many of these actions. Finally, we put our current findings into perspective by suggesting new avenues of inquiry for our future efforts.

Acamprosate-responsive brain sites for suppression of ethanol intake and preference

Acamprosate suppresses alcohol intake and craving in recovering alcoholics; however, the central sites of its action are unclear. To approach this question, brain regions responsive to acamprosate were mapped using acamprosate microimplants targeted to brain reward and circadian areas implicated in alcohol dependence. mPer2 mutant mice with nonfunctional mPer2, a circadian clock gene that gates endogenous timekeeping, were included, owing to their high levels of ethanol intake and preference. Male wild-type (WT) and mPer2 mutant mice received free-choice (15%) ethanol/water for 3 wk. The ethanol was withdrawn for 3 wk and then reintroduced to facilitate relapse. Four days before ethanol reintroduction, mice received bilateral blank or acamprosate-containing microimplants releasing ∼50 ng/day into reward [ventral tegmental (VTA), peduculopontine tegmentum (PPT), and nucleus accumbens (NA)] and circadian [intergeniculate leaflet (IGL) and suprachiasmatic nucleus (SCN)] areas. The hippocampus was also targeted. Circadian locomotor activity was measured throughout. Ethanol intake and preference were greater in mPer2 mutants than in wild-type (WT) mice (27 g·kg(-1)·day(-1) vs. 13 g·kg(-1)·day(-1) and 70% vs. 50%, respectively; both, P < 0.05). In WTs, acamprosate in all areas, except hippocampus, suppressed ethanol intake and preference (by 40-60%) during ethanol reintroduction. In mPer2 mutants, acamprosate in the VTA, PPT, and SCN suppressed ethanol intake and preference by 20-30%. These data are evidence that acamprosate's suppression of ethanol intake and preference are manifest through actions within major reward and circadian sites.

Circadian integration of glutamatergic signals by little SAAS in novel suprachiasmatic circuits

Background

Neuropeptides are critical integrative elements within the central circadian clock in the suprachiasmatic nucleus (SCN), where they mediate both cell-to-cell synchronization and phase adjustments that cause light entrainment. Forward peptidomics identified little SAAS, derived from the proSAAS prohormone, among novel SCN peptides, but its role in the SCN is poorly understood.

Methodology/Principal Findings

Little SAAS localization and co-expression with established SCN neuropeptides were evaluated by immunohistochemistry using highly specific antisera and stereological analysis. Functional context was assessed relative to c-FOS induction in light-stimulated animals and on neuronal circadian rhythms in glutamate-stimulated brain slices. We found that little SAAS-expressing neurons comprise the third most abundant neuropeptidergic class (16.4%) with unusual functional circuit contexts. Little SAAS is localized within the densely retinorecipient central SCN of both rat and mouse, but not the retinohypothalamic tract (RHT). Some little SAAS colocalizes with vasoactive intestinal polypeptide (VIP) or gastrin-releasing peptide (GRP), known mediators of light signals, but not arginine vasopressin (AVP). Nearly 50% of little SAAS neurons express c-FOS in response to light exposure in early night. Blockade of signals that relay light information, via NMDA receptors or VIP- and GRP-cognate receptors, has no effect on phase delays of circadian rhythms induced by little SAAS.

Conclusions/Significance

Little SAAS relays signals downstream of light/glutamatergic signaling from eye to SCN, and independent of VIP and GRP action. These findings suggest that little SAAS forms a third SCN neuropeptidergic system, processing light information and activating phase-shifts within novel circuits of the central circadian clock.

Circadian and developmental regulation of N-methyl-d-aspartate-receptor 1 mRNA splice variants and N-methyl-d-aspartate-receptor 3 subunit expression within the rat suprachiasmatic nucleus

The circadian rhythms of mammals are generated by the circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Its intrinsic period is entrained to a 24 h cycle by external cues, mainly by light. Light impinging on the SCN at night causes either advancing or delaying phase shifts of the circadian clock. N-methyl-d-aspartate receptors (NMDAR) are the main glutamate receptors mediating the effect of light on the molecular clockwork in the SCN. They are composed of multiple subunits, each with specific characteristics whose mutual interactions strongly determine properties of the receptor. In the brain, the distribution of NMDAR subunits depends on the region and developmental stage. Here, we report the circadian expression of the NMDAR1 subunit in the adult rat SCN and depict its splice variants that may constitute the functional receptor channel in the SCN. During ontogenesis, expression of two of the NMDAR1 subunit splice variants, as well as the NMDAR3A and 3B subunits, exhibits developmental loss around the time of eye opening. Moreover, we demonstrate the spatial and developmental characteristics of the expression of the truncated splice form of NMDAR1 subunit NR1-E in the brain. Our data suggest that specific properties of the NMDAR subunits we describe within the SCN likely influence the photic transduction pathways mediating the clock entrainment. Furthermore, the developmental changes in NMDAR composition may contribute to the gradual postnatal maturation of the entrainment pathways.

The serotonergic inhibition of slowly bursting cells in the intergeniculate leaflet of the rat

Electrophysiological studies combined with local neurotoxic lesions were conducted on anaesthetized rats in order to determine whether the dorsal raphe nucleus (DRN) inhibits the intergeniculate leaflet (IGL) of the lateral geniculate nucleus by means of innervation by serotonin-containing fibres. In the control animals, electrical stimulation of the DRN induced the long-latency and long-lasting inhibition of the neuronal firing of the IGL cells that are characterized by rhythmic, slow-bursting activity in light conditions. The electrical destruction of the DRN resulted in an increase in the firing rate of the recorded IGL cells, whilst at the same time not affecting the rhythmic, bursting pattern of the activity. In the second group of animals, local neurotoxic lesion of serotonergic fibres was performed by injection of the toxin 5,7-dihydroxytryptamine into the IGL. After 10 days of postoperative recovery, electrophysiological experiments were performed on the toxin-treated rats. In these animals, electrical stimulation as well as electrical lesion of the DRN did not induce any change in the firing of the slowly bursting cells in the 5,7-dihydroxytryptamine-injected IGL. The results obtained provide evidence that inhibition of the IGL slowly bursting cells, by innervation from the dorsal raphe, is mediated by the release of serotonin. Furthermore, the observed serotonergic inhibition of the light-dependent activity of slowly bursting cells can contribute to the neuronal mechanism gating the information that flows through this nucleus to the vestibular, visuomotor, circadian and sleep/arousal systems, with which the IGL is strongly interconnected.

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."

AMPA glutamate receptor subunits in the guinea pig hypothalamus: distribution and colocalization with progesterone receptor

Excitatory amino acids (EAAs), particularly glutamate, have been implicated in the control of luteinizing hormone (LH) secretion through facilitation of gonadotropin-releasing hormone release. The effects of EAAs are mediated by means of ionotropic glutamate receptors, which are divided into N-methyl-D-aspartate (NMDA) and non-NMDA (kainate and AMPA) subtypes. Moreover, ovarian steroids are responsible for inducing the preovulatory surge of LH and are involved in the actions of EAAs on LH release. Progesterone is directly involved in the potentiating effect of ovarian steroids on the stimulating effect of AMPA neurotransmission on gonadotropin secretion. To broaden our understanding of the role of hypothalamic AMPA receptors in the steroid-induced LH surge, we determined the cellular localization of AMPA receptors in the hypothalamus of guinea pigs by using antibodies that recognize the GluR1, GluR2, GluR2/3, or GluR4 subunits, and then we examined the neuroanatomic relationships between these receptors and the progesterone receptor (PR). Different patterns of immunostaining within the preoptic area and hypothalamus were evident with the antibodies to the four subunits with marked contrasts between moderate staining for GluR1, intensely stained structures for GluR2 and GluR2/3, and little specific staining for GluR4. Immunoreactive (IR) neurons were visualized in many regions, including the two regions known to contain a dense population of estradiol-induced PR-IR cells: the preoptic periventricular and ventrolateral hypothalamic nuclei. Approximately 60% of GluR1-IR and 39% of GluR2-IR cells in the preoptic region possessed PR, whereas 46% of GluR1-IR and 54% of GluR2-IR cells in the ventrolateral nucleus expressed PR. These neuroanatomic results suggest that the coordinated actions of progesterone and glutamatergic inputs on mammalian reproductive functions are integrated at the cellular level.

N-Methyl-d-aspartate microinjected into the suprachiasmatic nucleus mimics the phase-shifting effects of light in the diurnal Nile grass rat (Arvicanthis niloticus)

Mammals exhibit circadian rhythms in behavior generated by the suprachiasmatic nucleus (SCN). Exposure to light synchronizes the circadian clock to the environmental light:dark cycle through the release of glutamate into the SCN. In nocturnal animals such as Syrian hamsters, direct application of NMDA to the SCN results in phase shifts similar to those produced by exposure to light. This study was designed to determine if light phase shifts the circadian pacemaker of diurnal Nile grass rats (Arvicanthis niloticus) housed in constant darkness by acting on NMDA-type glutamate receptors in the suprachiasmatic nucleus (SCN). N-Methyl-D-aspartate (NMDA; 0, 1, 10, 50, and 100 mM) was administered through guide cannulae aimed at the SCN at circadian times when light induces phase shifts. Maximal phase delays were attained with 50 mM NMDA, and maximal phase advances were seen after 100 mM NMDA. A phase-response curve (PRC) for NMDA, determined by administering NMDA at each hour over the circadian cycle, resembled the PRC to light in this species. These data support the hypothesis that NMDA-type glutamate receptors play a critical role in mediating the phase shifting effects of light in diurnal, as well as nocturnal, animals. In addition, these data suggest that diurnal grass rats may be less sensitive to the phase shifting properties of NMDA than nocturnal rodents.

Excitatory mechanisms in the suprachiasmatic nucleus: the role of AMPA/KA glutamate receptors

A variety of evidence suggests that the effects of light on the mammalian circadian system are mediated by direct retinal ganglion cell projection to the suprachiasmatic nucleus (SCN). This synaptic connection is glutamatergic and the release of glutamate is detected by both N-methyl-D-asparate (NMDA) and amino-methyl proprionic acid/kainate (AMPA/KA) iontotropic glutamate receptors (GluRs). It is well established that NMDA GluRs play a critical role in mediating the effects of light on the circadian system; however, the role of AMPA/KA GluRs has received less attention. In the present study, we sought to better understand the contribution of AMPA/KA-mediated currents in the circadian system based in the SCN. First, whole cell patch-clamp electrophysiological techniques were utilized to measure spontaneous excitatory postsynaptic currents (sEPSCs) from SCN neurons. These currents were widespread in the SCN and not just restricted to the retino-recipient region. The sEPSC frequency and amplitude did not vary with the daily cycle. Similarly, currents evoked by the exogenous application of AMPA onto SCN neurons were widespread within the SCN and did not exhibit a diurnal rhythm in their magnitude. Fluorometric techniques were utilized to estimate AMPA-induced calcium (Ca(2+)) concentration changes in SCN neurons. The resulting data indicate that AMPA-evoked Ca(2+) transients were widespread in the SCN and that there was a daily rhythm in the magnitude of AMPA-induced Ca(2+) transients that peaked during the night. By itself, blocking AMPA/KA GluRs with a receptor blocker decreased the spontaneous firing of some SCN neurons as well as reduced resting Ca(2+) levels, suggesting tonic glutamatergic excitation. Finally, immunohistochemical techniques were used to describe expression of the AMPA-preferring GluR subunits GluR1 and GluR2/3s within the SCN. Overall, our data suggest that glutamatergic synaptic transmission mediated by AMPA/KA GluRs play an important role throughout the SCN synaptic circuitry.

Circadian regulation of diverse gene products revealed by mRNA expression profiling of synchronized fibroblasts

Genes under a 24-h regulation period may represent drug targets relevant to diseases involving circadian dysfunctions. As a testing model of the circadian clock system, we have used synchronized rat fibroblasts that are known to express at least six genes in a circadian fashion. We have determined the expression patterns of 9957 transcripts every 4 h over a total period of 76 h using high density oligonucleotide microarrays. The spectral analysis of our mRNA profiling data indicated that approximately 2% (85 genes) of all expressed genes followed a robust circadian pattern. We have confirmed the circadian expression of previously known clock or clock-driven genes, and we identified 81 novel circadian genes. The majority of the circadian-regulated gene products are known and are involved in diverse cellular functions. We have classified these circadian genes in seven clusters according to their phase of cycling. Our pathway analysis of the mRNA profiling data strongly suggests a direct link between circadian rhythm and cell cycle.

Distribution of substance P and neurokinin-1 receptor immunoreactivity in the suprachiasmatic nuclei and intergeniculate leaflet of hamster, mouse, and rat

The circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) receives photic information directly via the retinohypothalamic tract (RHT) and indirectly from retinally innervated cells in the thalamic intergeniculate leaflet (IGL) that project to the SCN. Using standard immunohistochemical methods, we examined the presence and distribution of substance P (SP) and the neurokinin-1 receptor (NK-1) in the SCN and IGL of rat and determined whether the patterns of immunostaining generalized to the SCN and IGL of Syrian hamster, Siberian hamster, and mouse. Terminals immunoreactive for SP were sparse within the SCN of Siberian and Syrian hamsters and mouse but were intense in the ventral, retinally innervated portion of the rat SCN. Immunostaining for the NK-1 receptor was mainly absent from the SCN of hamster and mouse. In contrast, a plexus of NK-1-ir cells and processes that was in close proximity to SP-ir terminals was found in the ventral SCN of the rat. Substance P-ir terminals were observed in the IGL of all four species, as were NK-1-ir cells and fibres. Double-labelled IGL sections of hamster or rat revealed SP-ir terminals in close apposition to NK-1-immunostained cells and/or fibres. These data indicate that SP could be a neurotransmitter of the RHT in rat, but not in hamster or in mouse, and they highlight potential species differences in the role of SP within the SCN circadian pacemaker. Such species differences do not appear to exist at the level of the IGL, where SP-ir and NK-1-ir were similar in all species studied.

NMDA-evoked calcium transients and currents in the suprachiasmatic nucleus: gating by the circadian system

A variety of evidence suggests that the effects of light on the mammalian circadian system are mediated by glutamatergic mechanisms and that the N-methyl- D-aspartate (NMDA) receptor plays an important role in this regulation. One of the fundamental features of circadian oscillators is that their response to environmental stimulation varies depending on the phase of the daily cycle when the stimuli are applied. For example, the same light treatment, which can produce phase shifts of the oscillator when applied during subjective night, has no effect when applied during the subjective day in animals held in constant darkness (DD). We examined the hypothesis that the effects of NMDA on neurons in the suprachiasmatic nucleus (SCN) also vary from day to night. Optical techniques were utilized to estimate NMDA-induced calcium (Ca2+) changes in SCN cells. The resulting data indicate that there was a daily rhythm in the magnitude and duration of NMDA-induced Ca2+ transients. The phase of this rhythm was determined by the light-dark cycle to which the rats were exposed with the Ca2+ transients peaking during the night. This rhythm continued when animals were held in DD. gamma-Aminobutyric acid (GABA)ergic mechanisms modulated the NMDA response but were not responsible for the rhythm. Finally, there was a rhythm in NMDA-evoked currents in SCN neurons that also peaked during the night. This study provides the first evidence for a circadian oscillation in NMDA-evoked Ca2+ transients in SCN cells. This rhythm may play an important role in determining the periodic sensitivity of the circadian systems response to light.

N-methyl-D-aspartate receptor subtype 2C is not involved in circadian oscillation or photoic entrainment of the biological clock in mice

Ishida et al. [1994: Neurosci Lett 166: 211-215] reported the circadian change of N-methyl-D-aspartate (NMDA) receptor subtype 2C mRNA and photic induction of this receptor's mRNA in the suprachiasmatic nucleus (SCN). Therefore, we investigated the role of NMDA receptor subtypes in the biological clock using NMDA receptor 2A (NR2A)- or 2C (NR2C)-deficient mice. However, NR2C-/- mice showed normal light-dark (LD)-entrained locomotor activity rhythms and free-running rhythms under constant darkness and also exhibited normal reentrainment to 6-hr LD shifts and phase delays with single light pulses. Thus, present results demonstrated no significant NR2C contribution to circadian oscillation and photic entrainment, even though expression of NR2C mRNA was highly observed in the SCN. On the other hand, the period of the free-running activity rhythm in NR2A-/- mice but not NR2C-/- mice was slightly longer than that in wild-type mice in spite of low expression of NR2A in the SCN. Furthermore, reentrainment to an LD advance in NR2A-/- mice was slower under low-intensity light conditions. Thus, we suggest that NR2A plays a role in determining the behavioral state that affects the circadian rhythm. In order to elucidate the role of NR2A and NR2C in the SCN, we examined NMDA-induced Ca(2+) elevations in the SCN of mutant mice using a Ca(2+) imaging method. A partial reduction in Ca(2+) elevation was observed in both NR2A-/- and NR2C-/- mice when high concentrations (100 or 300 microM) of NMDA were applied. The present results suggest that NR2A plays a weak role in oscillation or entrainment of the biological clock, and that NR2C does not participate in the functions of circadian oscillation and light entrainment.

Endothelial and neuronal nitric oxide synthases are present in the suprachiasmatic nuclei of Syrian hamsters and rats

Nitric oxide (NO) is involved in the transmission of light information to suprachiasmatic nuclei (SCN). By immunocytochemistry, we showed that both neuronal and endothelial NO synthase isoforms (nNOS and eNOS) were present in the SCN of rats and hamsters. nNOS-immunoreactive neurons were located mainly around the SCN with only a few nNOS neurons within the nucleus. By double-label immunocytochemistry, we also found, within the population of SCN glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes, a subpopulation of eNOS-immunoreactive astrocytes. Using Western blot analysis, we detected in SCN protein extracts eNOS and nNOS proteins having the expected 140 and 150 kDa molecular weights, respectively. By in situ hybridization of a 2.4-kb murine eNOS probe, mRNA for eNOS was located in the SCN of rats and hamsters. The transcript was further identified by detection of a RT-PCR product of the predicted size, after amplification of total RNA with primers specific for eNOS. In the SCN and cerebellum, the size of the mRNA for nNOS, detected with a rat probe on Northern blot, was approximately 10.5 kb, corresponding to that previously published. In the same tissues, we found two transcripts, one weakly expressed at approximately 4.0 kb and another more strongly expressed at approximately 2.6 kb, both hybridizing with two non-overlapping murine and rat eNOS probes. These results suggested the existence in the SCN of alternate transcripts for eNOS. We propose that two pathways could link light stimuli and NO release in the SCN: one involving N-methyl-D-aspartate (NMDA) receptors and nNOS in neurons; the other linking alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and eNOS in astrocytes.

Circadian rhythm of AMPA receptor GluR2/3 subunit-immunoreactivity in the suprachiasmatic nuclei of Syrian hamster and effect of a light-dark cycle

In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus are the site of the circadian clock that generates and coordinates many endogenous physiological and behavioral rhythms. SCN are normally entrained to light/dark (LD) cycle by direct retinal afferents using glutamate as neurotransmitter. N-Methyl-d-aspartate (NMDA) and non-NMDA receptors are involved in photic entrainment of SCN. In rodents, the presence of three of the four known 2-amino-3-(3-hydroxy-(-methylisoaxol-4-yl) propanoic acid (AMPA) receptor subunits has been demonstrated by in situ hybridization. This study analyzes the expression of GluR2/3 subunits in SCN of Syrian hamsters maintained under constant darkness (DD) or 12:12 LD cycle. In animals submitted to DD or LD, small immunoreactive neurons were located in the ventral and external latero-ventral parts of the rostral two-thirds of the SCN and along the symmetrical plane. The number of intensely labeled neurons with or without long process(es) were counted at six circadian times (CTs) in three groups of animals maintained in DD and six nycthemeral (zeitgeber time, ZT) times in one group of hamsters submitted to LD. In DD, we observed significantly more GluR2/3 subunit-immunoreactive (GluR2/3-ir) neurons during the subjective day than during the subjective night, with minima at CT 19-CT 23. The LD cycle significantly reduced the number of immunoreactive neurons, lessened the differences between LD phases and depressed immunoreactivity at light transition, i.e., at ZT 11 and ZT 23. This study demonstrates for the first time by immunohistochemistry the existence of a circadian dynamic of the expression of AMPA receptor subunits in SCN of rodents and the effect of the LD cycle on this dynamic.

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.

Glutamatergic induction of CREB phosphorylation and Fos expression in primary cultures of the suprachiasmatic hypothalamus in vitro is mediated by co-ordinate activity of NMDA and non-NMDA receptors

Exposure of Syrian hamsters to light 1 h after lights-off rapidly (10 min) induced nuclear immunoreactivity (-ir) to the phospho-Ser133 form of the Ca2+/cAMP response element (CRE) binding protein (pCREB) in the retinorecipient zone of the suprachiasmatic nuclei (SCN). Light also induced nuclear Fos-ir in the same region of the SCN after 1 h. The glutamatergic N-methyl-D-aspartate (NMDA) receptor blocker MK801 attenuated the photic induction of both factors. To investigate glutamatergic regulation of pCREB and Fos further, tissue blocks and primary cultures of neonatal hamster SCN were examined by Western blotting and immunocytochemistry in vitro. On Western blots of SCN tissue, the pCREB-ir signal at 45 kDa was enhanced by glutamate or a mixture of glutamatergic agonists (NMDA, amino-methyl proprionic acid (AMPA), and Kainate (KA)), whereas total CREB did not change. Glutamate or the mixture of agonists also induced a 56 kDa band identified as Fos protein in SCN tissue. In dissociated cultures of SCN, glutamate caused a rapid (15 min) induction of nuclear pCREB-ir and Fos-ir (after 60 min) exclusively in neurones, both GABA-ir and others. Treatment with NMDA alone had no effect on pCREB-ir. AMPA alone caused a slight increase in pCREB-ir. However, kainate alone or in combination with NMDA and AMPA induced nuclear pCREB-ir equal to that induced by glutamate. The effects of glutamate on pCREB-ir and Fos-ir were blocked by antagonists of both NMDA (MK801) and AMPA/KA (NBQX) receptors. In the absence of extracellular Mg2+, MK801 blocked glutamatergic induction of Fos-ir. However, the AMPA/KA receptor antagonist was no longer effective at blocking glutamatergic induction of either Fos-ir or pCREB-ir, consistent with the model that glutamate regulates gene expression in the SCN by a co-ordinate action through both NMDA and AMPA/KA receptors. Glutamatergic induction of nuclear pCREB-ir in GABA-ir neurones was blocked by KN-62 an inhibitor of Ca2+/Calmodulin (CaM)-dependent kinases, implicating Ca2+-dependent signalling pathways in the glutamatergic regulation of gene expression in the SCN.

Glutamatergic antagonists do not attenuate light-induced fos protein in rat intergeniculate leaflet

Photic information that entrains circadian rhythms is transmitted to the suprachiasmatic nucleus (SCN) from the retina and from the retinorecipient intergeniculate leaflet (IGL). Expression of light-induced Fos protein in SCN neurons is correlated with the effectiveness of such light to induce phase shifts, and is prevented by pretreatment with glutamate receptor antagonists that prevent phase shifts as well. In the present study we demonstrate that treatments with N-methyl-d-aspartate (NMDA) and non-NMDA receptor antagonists prior to light pulses during the subjective night have no effect on light-induced Fos immunoreactivity (Fos-IR) in IGL neurons despite attenuating Fos-IR in the SCN. Transmission of photic information along retinogeniculate and retinohypothalamic pathways appears to be mediated by different mechanisms.

Differential localization of GABA(A) receptor alpha and beta subunits in the hamster retina and relationship with glutamic acid decarboxylase immunoreactivity

In order to determine the cellular localization of GABA(A)alpha and beta subunits in the hamster retina, single and double immunocytochemistry was performed in perfuse-fixed hamster retina using commercially-available antibodies against the two receptor subunits and glutamic acid decarboxylase. Strong GABA(A)beta immunoreactivity was found in two strata of the inner plexiform layer and in perikarya of amacrine cells and bipolar cells in the inner nuclear layer. In contrast, no GABA(A)alpha immunoreactivity was detected. All but a few of the GABA(A)beta-immunoreactive amacrine cells were found not to display glutamic acid decarboxylase immunoreactivity. The present results indicate that there is a differential localization of GABA(A)alpha and beta subunits in different neuronal subpopulations in the hamster retina.