Effects of excitatory amino acid receptor antagonists and agonists on suprachiasmatic nucleus responses to retinohypothalamic tract volleys

Circadian neurogenetics and its implications in neurophysiology, behavior, and chronomedicine

The circadian rhythm affects multiple physiological processes, and disruption of the circadian system can be involved in a range of disease-related pathways. The genetic underpinnings of the circadian rhythm have been well-studied in model organisms. Significant progress has been made in understanding how clock genes affect the physiological functions of the nervous system. In addition, circadian timing is becoming a key factor in improving drug efficacy and reducing drug toxicity. The circadian biology of the target cell determines how the organ responds to the drug at a specific time of day, thus regulating pharmacodynamics. The current review brings together recent advances that have begun to unravel the molecular mechanisms of how the circadian clock affects neurophysiological and behavioral processes associated with human brain diseases. We start with a brief description of how the ubiquitous circadian rhythms are regulated at the genetic, cellular, and neural circuit levels, based on knowledge derived from extensive research on model organisms. We then summarize the latest findings from genetic studies of human brain disorders, focusing on the role of human clock gene variants in these diseases. Lastly, we discuss the impact of common dietary factors and medications on human circadian rhythms and advocate for a broader application of the concept of chronomedicine.

The evaluation of the effect of tafluprost on the intraocular pressure of healthy male guinea pigs under different light-and-darkness regimes

BACKGROUND

Ocular hypertension is one of the most underdiagnosed ocular abnormalities among guinea pigs around the world.

OBJECTIVES

The current study investigates the effect of 0.0015% preservative-free tafluprost ophthalmic solution (Zioptan) on the intraocular pressure of 16 healthy male guinea pigs (Cavia porcellus) under different light/darkness regimes.

METHODS

All guinea pigs received a single drop of tafluprost at 5:30 in the right eye, whereas the contralateral eyes served as control to receive a placebo. Then, the animals were randomly divided into two groups; group A was exposed to light, whereas group B was placed in darkness from 5:30 to 18:00. Rebound tonometry (TonoVet) was instrumented to measure IOP values at 5:30 (baseline), 6:00, 7:00, 8:00, 9:00 and then every 3 h until 18:00.

RESULTS

The maximum IOP reduction associated with tafluprost was observed at 6:00 by -1.4 ± 1.1 mmHg (p-value = 0.026) and -2.5 ± 1.2 mmHg (p-value = 0.011) in group A and B, respectively (repeated measure ANOVA test). There was a significant difference between the mean right and left eye IOP values in both groups at 5:30, 6:00, 7:00 and 8:00 (p-value <0.05), which was greater in amount in group B compared to group A due to the effect of darkness on IOP reduction.

CONCLUSIONS

It is suggested that the variations of IOP in different light/dark conditions be taken into consideration when applying ocular hypotensive agents on guinea pigs' eyes.

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.

PACAP in hypothalamic regulation of sleep and circadian rhythm: importance for headache

The interaction between sleep and primary headaches has gained considerable interest due to their strong, bidirectional, clinical relationship. Several primary headaches demonstrate either a circadian/circannual rhythmicity in attack onset or are directly associated with sleep itself. Migraine and cluster headache both show distinct attack patterns and while the underlying mechanisms of this circadian variation in attack onset remain to be fully explored, recent evidence points to clear physiological, anatomical and genetic points of convergence. The hypothalamus has emerged as a key brain area in several headache disorders including migraine and cluster headache. It is involved in homeostatic regulation, including pain processing and sleep regulation, enabling appropriate physiological responses to diverse stimuli. It is also a key integrator of circadian entrainment to light, in part regulated by pituitary adenylate cyclase-activating peptide (PACAP). With its established role in experimental headache research the peptide has been extensively studied in relation to headache in both humans and animals, however, there are only few studies investigating its effect on sleep in humans. Given its prominent role in circadian entrainment, established in preclinical research, and the ability of exogenous PACAP to trigger attacks experimentally, further research is very much warranted. The current review will focus on the role of the hypothalamus in the regulation of sleep-wake and circadian rhythms and provide suggestions for the future direction of such research, with a particular focus on PACAP.

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.

Calcium Dynamics and Circadian Rhythms in Suprachiasmatic Nucleus Neurons

The hypothalamic suprachiasmatic nucleus (SCN) has a pivotal role in the mammalian circadian clock. SCN neurons generate circadian rhythms in action potential firing frequencies and neurotransmitter release, and the core oscillation is thought to be driven by “clock gene” transcription-translation feedback loops. Cytosolic Ca2+mobilization followed by stimulation of various receptors has been shown to reset the gene transcription cycles in SCN neurons, whereas contribution of steady-state cytosolic Ca2+levels to the rhythm generation is unclear. Recently, circadian rhythms in cytosolic Ca2+levels have been demonstrated in cultured SCN neurons. The circadian Ca2+rhythms are driven by the release of Ca2+from ryanodine-sensitive internal stores and resistant to the blockade of action potentials. These results raise the possibility that gene translation/transcription loops may interact with autonomous Ca2+oscillations in the production of circadian rhythms in SCN neurons.

Impaired circadian photosensitivity in mice lacking glutamate transmission from retinal melanopsin cells

Intrinsically photoreceptive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin and convey retinal light inputs to the circadian system via the retinohypothalamic tract (RHT) projection to the suprachiasmatic nucleus (SCN). The principal neurotransmitter of this projection is glutamate, and ipRGCs use the vesicular glutamate transporter 2 (VGLUT2) to package glutamate into synaptic vesicles. However, these neurons contain other potential neurotransmitters, such as pituitary adenylate cyclase activating polypeptide (PACAP). To test the role of glutamate in mediating ipRGC light inputs into the SCN, we crossed mice in which Cre-recombinase expression is driven by the melanopsin promotor (Opn4(Cre/+)) with mice in which the second exon of VGLUT2 is flanked by loxP sites (VGLUT2(fl/fl)), producing ipRGCs that are unable to package glutamate into synaptic vesicles. Such mice had free-running circadian rhythms that did not entrain to a 12:12 light-dark (12:12 LD) cycle, nor did they show a phase delay after a 45-min light pulse administered at circadian time (CT) 14. A small subset of the mice did appear to entrain to the 12:12 LD cycle with a positive phase angle to lights-off; a similar entrainment pattern could be achieved in free-running mice if they were exposed to a 12:12 LD cycle with light of a greater intensity. Glutamate transmission from the ipRGCs is necessary for normal light entrainment of the SCN at moderate (0.35 W/m(2)) light levels, but residual transmission (possibly by PACAP in ipRGCs or by other RGCs) can weakly entrain animals, particularly at very high (6.53 W/m(2)) light levels, although it may be less effective at suppressing locomotor activity (light masking).

The role of retinal photoreceptors in the regulation of circadian rhythms

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

Circadian regulation of a-type potassium currents in the suprachiasmatic nucleus

In mammals, the precise circadian timing of many biological processes depends on the generation of oscillations in neural activity of pacemaker cells in the suprachiasmatic nucleus (SCN) of the hypothalamus. Understanding the ionic mechanisms underlying these rhythms is an important goal of research in chronobiology. Previous work has shown that SCN neurons express A-type potassium currents (IAs), but little is known about the properties of this current in the SCN. We sought to characterize some of these properties, including the identities of IA channel subunits found in the SCN and the circadian regulation of IA itself. In this study, we were able to detect significant hybridization for Shal-related family members 1 and 2 (Kv4.1 and 4.2) within the SCN. In addition, we used Western blot to show that the Kv4.1 and 4.2 proteins are expressed in SCN tissue. We further show that the magnitude of the IA current exhibits a diurnal rhythm that peaks during the day in the dorsal region of the mouse SCN. This rhythm seems to be driven by a subset of SCN neurons with a larger peak current and a longer decay constant. Importantly, this rhythm in neurons in the dorsal SCN continues in constant darkness, providing an important demonstration of the circadian regulation of an intrinsic voltage-gated current in mammalian cells. We conclude that the anatomical expression, biophysical properties, and pharmacological profiles measured are all consistent with the SCN IA current being generated by Kv4 channels. Additionally, these data suggest a role for IA in the regulation of spontaneous action potential firing during the transitions between day/night and in the integration of synaptic inputs to SCN neurons throughout the daily cycle.

Synaptic circuitry in the retinorecipient layers of the optic tectum of the lamprey (Lampetra fluviatilis). A combined hodological, GABA and glutamate immunocytochemical study

The ultrastructure of the retinorecipient layers of the lamprey optic tectum was analysed using tract tracing techniques combined with GABA and glutamate immunocytochemistry. Two types of neurons were identified; a population of large GABA-immunonegative cells, and a population of smaller, highly GABA-immunoreactive interneurons, some of whose dendrites contain synaptic vesicles (DCSV). Five types of axon terminals were identified and divided into two major categories. The first of these are GABA-immunonegative, highly glutamate-immunoreactive, contain round synaptic vesicles, make asymmetrical synaptic contacts, and can in turn be divided into AT1 and AT2 terminals. The AT1 terminals are those of the retinotectal projection. The origin of the nonretinal AT2 terminals could not be determined. AT1 and AT2 terminals establish synaptic contacts with DCSV, with dendrites of the retinopetal neurons (DRN), and with conventional dendritic (D) profiles. The terminals of the second category are GABA-immunoreactive and can similarly be divided into AT3 and AT4 terminals. The AT3 terminals contain pleiomorphic synaptic vesicles and make symmetrical synaptic contacts for the most part with glutamate-immunoreactive D profiles. The AT4 terminals contain rounded synaptic vesicles and make asymmetrical synaptic contacts with DRN, with DCSV, and with D profiles. A fifth, rarely observed category of terminals (AT5) contain both clear synaptic vesicles and a large number of dense-core vesicles. Synaptic triads involving AT1, AT2 or AT4 terminals are rare. Our findings are compared to these of previous studies of the fine structure and immunochemical properties of the retinorecipient layers of the optic tectum or superior colliculus of Gnathostomes.

Circadian rhythm dysfunction in glaucoma: A hypothesis

The absence of circadian zeitgebers in the social environment causes circadian misalignment, which is often associated with sleep disturbances. Circadian misalignment, defined as a mismatch between the sleep-wake cycle and the timing of the circadian system, can occur either because of inadequate exposure to the light-dark cycle, the most important synchronizer of the circadian system, or reduction in light transmission resulting from ophthalmic diseases (e.g., senile miosis, cataract, diabetic retinopathy, macular degeneration, retinitis pigmentosa, and glaucoma). We propose that glaucoma may be the primary ocular disease that directly compromises photic input to the circadian time-keeping system because of inherent ganglion cell death. Glaucomatous damage to the ganglion cell layer might be particularly harmful to melanopsin. According to histologic and circadian data, a subset of intrinsically photoresponsive retinal ganglion cells, expressing melanopsin and cryptochromes, entrain the endogenous circadian system via transduction of photic input to the thalamus, projecting either to the suprachiasmatic nucleus or the lateral geniculate nucleus. Glaucoma provides a unique opportunity to explore whether in fact light transmission to the circadian system is compromised as a result of ganglion cell loss.

Calcium response to retinohypothalamic tract synaptic transmission in suprachiasmatic nucleus neurons

Glutamate released from retinohypothalamic tract (RHT) synapses with suprachiasmatic nucleus (SCN) neurons induces phase changes in the circadian clock presumably by using Ca2+ as a second messenger. We used electrophysiological and Ca2+ imaging techniques to simultaneously record changes in the membrane potential and intracellular calcium concentration ([Ca2+]i) in SCN neurons after stimulation of the RHT at physiologically relevant frequencies. Stimulation of the RHT sufficient to generate an EPSP did not produce detectable changes in [Ca2+]i, whereas EPSP-induced action potentials evoked an increase in [Ca2+]i, suggesting that the change in postsynaptic somatic [Ca2+]i produced by synaptically activated glutamate receptors was the result of membrane depolarization activating voltage-dependent Ca2+ channels. The magnitude of the Ca2+ response was dependent on the RHT stimulation frequency and duration, and on the SCN neuron action potential frequency. Membrane depolarization-induced changes in [Ca2+]i were larger and decayed more quickly in the dendrites than in the soma and were attenuated by nimodipine, suggesting a compartmentalization of Ca2+ signaling and a contribution of L-type Ca2+ channels. RHT stimulation at frequencies that mimicked the output of light-sensitive retinal ganglion cells (RGCs) evoked [Ca2+]i transients in SCN neurons via membrane depolarization and activation of voltage-dependent Ca2+ channels. These data suggest that for Ca2+ to induce phase advances or delays, light-induced signaling from RGCs must augment the underlying oscillatory somatic [Ca2+]i by evoking postsynaptic action potentials in SCN neurons during a period of slow spontaneous firing such as occurs during nighttime.

Immediate-early genes and the neural bases of photic and non-photic entrainment

The expression of immediate-early genes (IEGs) within the mammalian suprachiasmatic nucleus (SCN) identifies individual light-responsive cells of the circadian system. Cells immunoreactive for products of IEGs form a neurochemically heterogeneous population, of which a few are VIP (vasoactive intestinal peptide)-immunoreactive or GRP (gastrin-releasing peptide)-immunoreactive, although the phenotypes of most of the others have yet to be determined. Dual-labelling experiments with anatomical tracers reveal that only a minority of efferent projection neurons of the SCN are immunoreactive for IEG products, and it is likely that the majority of the immunoreactive cells are interneurons or glia. Photic induction of IEGs is mediated via NMDA (N-methyl-D-aspartate) and non-NMDA glutamatergic receptors, the SCN expressing a topographically specific complement of subtypes of the NMDA receptor. Non-photic cues (arousal) can shift the clock but this is not associated with expression of IEGs, demonstrating that the proteins encoded by IEGs are probably involved in transducing photic cues, rather than shifting the clock per se. Their induction provides an anatomically explicit marker for circadian phase and photic sensitivity and so is useful in analyses of circadian function, for example, in the tau mutant hamster. Non-photic phase shifts are accompanied by adrenocortical activation, confirming the importance of arousal in shifting of the clock. The phase-shifting effect of arousal can be blocked by treatment with the serotonin receptor antagonist ketanserin, suggesting that ascending serotonergic input to the forebrain, possibly directly to the SCN, is an important mediator of entrainment by arousal.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Circadian rhythm of intraocular pressure in cats

OBJECTIVE

To evaluate the rhythm of intraocular pressure (IOP) in healthy domestic cats with no evidence of ocular disease and to analyze the influence of photoperiod, age, gender and ocular diseases on diurnal-nocturnal variations of cat IOP.

ANIMALS

All animals were Domestic Short-haired cats; 30 were without systemic or ocular diseases, classified as follows: 12 male intact adult cats, five intact adult female, five adult spayed female, and eight male cats; the latter were less than 1 year of age. In addition, five adult cats with uveitis and three adult cats with secondary glaucoma were included.

PROCEDURE

IOP was assessed with a Tono-Pen XL at 3-h intervals over a 24-h period in 12 healthy adult male cats kept under a photoperiod of 12-h light/12-h darkness for 2 weeks. Eight animals from the same group were then kept under constant darkness for 48 h, and IOP was measured at 3-h intervals for the following 24 h. In addition, IOP was assessed at 3 p.m. and 9 p.m. in five intact females, five spayed females, and in eight young cats, as well as in five adult cats with uveitis and three glaucomatous cats.

RESULTS

Consistent, daily variations in IOP were observed in animals exposed to a light-dark cycle, with maximal values during the night. In cats exposed to constant darkness, maximal values of IOP were observed at subjective night. Differences of IOP values between 3 p.m. and 9 p.m. (diurnal-nocturnal variations) persisted in intact females, spayed females, and young animals, as well as in uveitic and glaucomatous eyes.

CONCLUSIONS

The present results indicate a daily rhythm of cat IOP, which appears to persist in constant darkness, suggesting some level of endogenous circadian control. In addition, daily variations of cat IOP seem to be independent of gender, age, or ocular diseases (particularly uveitis and glaucoma).

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

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

Tetrodotoxin administration in the suprachiasmatic nucleus prevents NMDA-induced reductions in pineal melatonin without influencing Per1 and Per2 mRNA levels

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus contains a light-entrainable circadian pacemaker. Neurons in the SCN are part of a circuit that conveys light information from retinal efferents to the pineal gland. Light presented during the night acutely increases mRNA levels of the circadian clock genes Per1 and Per2 in the SCN, and acutely suppresses melatonin levels in the pineal gland. The present study investigated whether the ability of light to increase Per1 and Per2 mRNA levels and suppress pineal melatonin levels requires sodium-dependent action potentials in the SCN. Per1 and Per2 mRNA levels in the SCN and pineal melatonin levels were measured in Syrian hamsters injected with tetrodotoxin (TTX) prior to light exposure or injection of N-methyl-D-aspartate (NMDA). TTX inhibited the ability of light to increase Per1 and Per2 mRNA levels and suppress pineal melatonin levels. TTX did not, however, influence the ability of NMDA to increase Per1 and Per2 mRNA levels, though it did inhibit the ability of NMDA to suppress pineal melatonin levels. These results demonstrate that action potentials in the SCN are not necessary for NMDA receptor activation to increase Per1 and Per2 mRNA levels, but are necessary for NMDA receptor activation to decrease pineal melatonin levels. Taken together, these data support the hypothesis that the mechanism through which light information is conveyed to the pacemaker in the SCN is separate from and independent of the mechanism through which light information is conveyed to the SCN cells whose efferents suppress pineal melatonin levels.

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.

Blockade of Glutamatergic Neurotransmission in the Suprachiasmatic Nucleus Prevents Cellular and Behavioural Responses of the Circadian System to Light

The aim of this study was to test the role of glutamatergic neurotransmission in photic entrainment of the circadian oscillator of the suprachiasmatic nuclei (SCN) in the Syrian hamster. The response of the oscillator to a brief pulse of light was assessed using two independent indices, the phase shift of the free-running activity rhythm, and the photically induced expression of the immediate-early gene c-fos within neurons of the SCN. The behavioural and the cellular responses to light were compared in animals which received intracerebroventricular (icv) infusions into the region of the SCN of either a vehicle solution or a solution of gammad-glutamyl-glycine (DGG), a competitive antagonist at both N-methyl-d-aspartate (NMDA) and non-NMDA types of glutamate receptor. Infusions of vehicle or DGG (200 nmol) were given 10 min before presentation of a 15-min light pulse at either circadian time (CT) 14 or CT20 (onset of activity defined as CT12). As anticipated, animals treated with vehicle and light at CT14 exhibited phase delays in the activity rhythm, whereas animals treated at CT20 exhibited phase advances. Central infusion of DGG prior to a light pulse at CT14 blocked the phase-delaying effect of light, and DGG delivered before a light pulse at CT20 markedly attenuated the phase-advancing effect of light. In a separate group of animals, the expression of the immediate-early gene c-fos was assessed by immunocytochemical staining for its protein product Fos. Exposure of vehicle-infused animals to light at CT14 caused extensive expression of c-fos throughout the retinorecipient region of the SCN. However, when the light pulse was preceded by icv fusion of DGG at a dose which would block the phase-shifting response to light, the total number of neurons immunopositive for Fos was significantly reduced ( approximately 50%) and the expression was confined to a restricted area of the dorsolateral SCN. The precise correlation between the effects of glutamatergic blockade upon both the behavioural and the cellular responses of the circadian system to light demonstrates that effective glutamatergic neurotransmission within or adjacent to the SCN is a necessary component of the mechanism which mediates photic entrainment of the circadian clock. The results further demonstrate a pharmacological and anatomical compartmentalization of the retinorecipient zone of the SCN, consistent with the view that retinal afferents to the ventral region employ glutamate as a transmitter, whereas more dorsal input may be dependent upon non-glutamatergic (DGG-insensitive) pathways.

Circadian organization in male mice lacking the gene for endothelial nitric oxide synthase (eNOS-/-)

Circadian (approximately 24 h) rhythms in physiology and behavior are generated by the bilateral suprachiasmatic nucleus (SCN) of the anterior hypothalamus. For these endogenous rhythms to be synchronized with the external environment, light information must be transmitted to pacemaker cells within the SCN. This transmission of light information is accomplished via a direct retino-hypothalamic tract (RHT). Nitric oxide (NO), an endogenous gas that functions as a neurotransmitter, has been implicated as a messenger necessary for photic entrainment. Three isoforms of the enzyme that form NO, NO synthase, have been identified (a) in neurons (nNOS), (b) in the endothelial lining of blood vessels (eNOS), and (c) as an inducible form in macrophages (iNOS). The present study was undertaken to determine the specific role of eNOS in circadian organization and photic entrainment. Wild-type (WT) and eNOS-/- mice were initially entrained to a 14:10 light:dark (LD) cycle. After 3 weeks, the LD cycle was phase advanced. After an additional 3 weeks, animals were held in constant darkness (DD). eNOS-/- animals did not exhibit a deficit in the ability to entrain to the LD cycle, phase-shift locomotor activity, or free-run in constant conditions. Animals held in DD were killed after light exposure during either the subjective day or the subjective night to assess c-fos induction in the SCN. Light exposure during the subjective night increased c-fos protein expression in the SCN of both WT and eNOS-/- mice relative to animals killed after light exposure during the subjective day. Taken together, these findings suggest that endothelial isoform of NOS may not be necessary for photic entrainment in mice.

Enhanced NMDA receptor activity in retinal inputs to the rat suprachiasmatic nucleus during the subjective night

Circadian oscillator mechanisms in the suprachiasmatic nucleus (SCN) can be reset by photic input, which is mediated by glutamatergic afferents originating in the retina. A key question is why light can only induce phase shifts of the biological clock during a restricted period of the circadian cycle, namely the subjective night. One of several possible mechanisms holds that glutamatergic transmission at retinosuprachiasmatic synapses would be altered, in particular the contribution of glutamate receptor subtypes to the postsynaptic response. By studying the contributions of NMDA and non-NMDA glutamate receptors to the retinal input to SCN in whole-cell patch-clamp recordings in acutely prepared slices, we tested the hypothesis that NMDA receptor current evoked by optic nerve activity is potentiated during the subjective night. During the day the NMDA component of the EPSC evoked by optic nerve stimulation was found less frequently and was significantly smaller in magnitude than during the night. In contrast, the non-NMDA component did not show a significant day-night difference. When the magnitude of the NMDA component was normalized to that of the non-NMDA component, the day-night difference was maintained, suggesting a selective potentiation of NMDA receptor conductance. In addition to contributing to electrically evoked EPSCs, the NMDA receptor was found to sustain a small, tonically active inward current during the night phase. No significant tonic contribution by NMDA receptors was detected during the day. These results suggest, first, a dual mode of NMDA receptor function in the SCN and, second, a clock-controlled type of receptor plasticity, which may gate the transfer of photic input to phase-shifting mechanisms operating at the level of molecular autoregulatory feedback loops.

Serotonergic modulation of retinal input to the mouse suprachiasmatic nucleus mediated by 5-HT1B and 5-HT7 receptors

Serotonin (5-HT) and 5-HT receptor agonists can modify the response of the mammalian suprachiasmatic nucleus (SCN) to light. It remains uncertain which 5-HT receptor subtypes mediate these effects. The effects of 5-HT receptor activation on optic nerve-mediated input to SCN neurons were examined using whole-cell patch-clamp recordings in horizontal slices of ventral hypothalamus from the male mouse. The hypothesis that 5-HT reduces the effect of retinohypothalamic tract (RHT) input to the SCN by acting at 5-HT1B receptors was tested first. As previously described in the hamster, a mixed 5-HT(1A/1B) receptor agonist, 1-[3-(trifluoromethyl)phenyl]-piperazine hydrochloride (TFMPP), reduced the amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) evoked by selectively stimulating the optic nerve of wild-type mice. The agonist was negligibly effective in a 5-HT1B receptor knockout mouse, suggesting minimal contribution of 5-HT1A receptors to the TFMPP-induced reduction in the amplitude of the optic nerve-evoked EPSC. We next tested the hypothesis that 5-HT also reduces RHT input to the SCN via activation of 5-HT7 receptors. The mixed 5-HT(1A/7) receptor agonist, R(+)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT), reduced the evoked EPSC amplitude in both wild-type and 5-HT1B receptor knockout mice. This effect of 8-OH-DPAT was minimally attenuated by the selective 5-HT1A receptor antagonist WAY 100635 but was reversibly and significantly reduced in the presence of ritanserin, a mixed 5-HT(2/7) receptor antagonist. Taken together with the authors' previous ultrastructural studies of 5-HT1B receptors in the mouse SCN, these results indicate that in the mouse, 5-HT reduces RHT input to the SCN by acting at 5-HT1B receptors located on RHT terminals. Moreover, activation of 5-HT7 receptors in the mouse SCN, but not 5-HT1A receptors, also results in a reduction in the amplitude of the optic nerve-evoked EPSC. The findings indicate that 5-HT may modulate RHT glutamatergic input to the SCN through 2 or more 5-HT receptors. The likely mechanism of altered RHT glutamatergic input to SCN neurons is an alteration of photic effects on the SCN circadian oscillator.

Circadian variation in the activity of the 5-HT(1B) autoreceptor in the region of the suprachiasmatic nucleus, measured by microdialysis in the conscious freely-moving rat

Intracerebral microdialysis was used to examine the function of the terminal 5-hydroxytryptamine(1B) (5-HT(1B)) autoreceptor in the region of the suprachiasmatic nuclei (SCN) of freely moving conscious rats at six time points or zeitgeber times (ZTs) across the light:dark cycle. Infusion of the 5-HT(1A/1B) agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU24969) (1 microM) via the microdialysis probe produced a decrease in 5-HT output when applied at ZTs 3, 6, 15 and 21 (69.8+/-11.9, 59+/-11.7, 43.9+/-17.2 and 45.7+/-17.0% respectively). At ZTs 9 and 18 RU24969 (1 microm) failed to affect the 5-HT output significantly (28.0+/-11 and 32.8+/-24.6% decrease respectively). The profile of inhibition of 5-HT output following infusion of RU24969 (1 microM) at ZT 6 was unaffected by concurrent infusion of the specific 5-HT(1A) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide trihydrochloride (WAY100635) (1 microM) (52.48+/-17.5% decrease). The data demonstrate a circadian rhythm in the activity of the 5-HT(1B) autoreceptor in the region of the SCN.

The suprachiasmatic nucleus and the circadian time-keeping system revisited

Many physiological and behavioral processes show circadian rhythms which are generated by an internal time-keeping system, the biological clock. In rodents, evidence from a variety of studies has shown the suprachiasmatic nucleus (SCN) to be the site of the master pacemaker controlling circadian rhythms. The clock of the SCN oscillates with a near 24-h period but is entrained to solar day/night rhythm by light. Much progress has been made recently in understanding the mechanisms of the circadian system of the SCN, its inputs for entrainment and its outputs for transfer of the rhythm to the rest of the brain. The present review summarizes these new developments concerning the properties of the SCN and the mechanisms of circadian time-keeping. First, we will summarize data concerning the anatomical and physiological organization of the SCN, including the roles of SCN neuropeptide/neurotransmitter systems, and our current knowledge of SCN input and output pathways. Second, we will discuss SCN transplantation studies and how they have contributed to knowledge of the intrinsic properties of the SCN, communication between the SCN and its targets, and age-related changes in the circadian system. Third, recent findings concerning the genes and molecules involved in the intrinsic pacemaker mechanisms of insect and mammalian clocks will be reviewed. Finally, we will discuss exciting new possibilities concerning the use of viral vector-mediated gene transfer as an approach to investigate mechanisms of circadian time-keeping.

Impaired behavioral suppression by light in metabotropic glutamate receptor subtype 6-deficient mice

The metabotropic glutamate receptor subtype 6 is localized on the dendrites of ON bipolar cells in mammalian retina, and is responsible for synaptic transmission from photoreceptors to ON bipolar cells. We have previously provided electrophysiological evidence that metabotropic glutmate receptor subtype 6-deficient mice have an impairment in the ON visual pathway. In this study, we compared, between metabotropic glutamate receptor subtype 6-deficient (n=9) and wild-type mice (n=7), their daily wheel-running activity in constant dark and light-dark cycle environments. There was no difference in their free-running rhythmicity in a constant dark environment nor in their ability to entrain their active/rest phase to the phase-shifted light-dark cycle environment, indicating that the circadian system in mutant mice was functioning normally. However, the wheel-running activity was suppressed immediately after light onset of the light-dark cycle in wild-type mice (suppressive effect), whereas that of mutant mice was prolonged for several hours in spite of light onset (very weak suppressive effect). The suppression of activity in wild-type mice is a "masking effect" of the endogenous circadian rhythm in response to light stimuli. The results indicate that the failure of mutant mice to suppress their activity upon light onset is not due to abnormality in their circadian system, but to their lack of response to light stimuli. This study clearly demonstrates that the dysfunction of the ON visual pathway in metabotropic glutamate receptor subtype 6-deficient mice impairs their behavioral responsiveness to light and yet preserves their circadian system.

A 5-HT(1B) receptor agonist inhibits light-induced suppression of pineal melatonin production

Serotonin (5-HT) modulates the phase adjusting effects of light on the mammalian circadian clock through the activation of presynaptic 5-HT(1B) receptors located on retinal terminals in the suprachiasmatic nucleus (SCN). The current study was conducted to determine whether activation of 5-HT(1B) receptors also alters photic regulation of nocturnal pineal melatonin production. Systemic administration of the 5-HT(1B) receptor agonist TFMPP attenuated the inhibitory effect of light on pineal melatonin synthesis in a dose-related manner with an apparent ED(50) value of 0.9 mg/kg. The effect of TFMPP on light-induced melatonin suppression was blocked by the 5-HT(1) receptor antagonist, methiothepin, but not by the 5-HT(1A) antagonist, WAY 100,635, consistent with the involvement of 5-HT(1B) receptors. The results are consistent with the interpretation that activation of presynaptic 5-HT(1B) receptors on retinal terminals in the SCN attenuates the effect of light on pineal melatonin production, as well as on circadian phase.

Activation of NMDA receptors in the suprachiasmatic nucleus produces light-like phase shifts of the circadian clock in vivo

Although there is substantial evidence that glutamate mimics the effects of light on the mammalian circadian clock in vitro, it has been reported that microinjection of glutamate into the suprachiasmatic nucleus of the hypothalamus (SCN) region in vivo does not result in a pattern of phase shifts that mimic those caused by light pulses. The present study was designed to test the hypothesis that microinjection of NMDA into the SCN would induce light-like phase shifts of the circadian clock through activation of the NMDA receptor. Hamsters housed in constant darkness received microinjections of NMDA through guide cannulas aimed at the SCN region at various times throughout the circadian cycle. Wheel running was monitored as a measure of circadian phase. Microinjection of NMDA resulted in circadian phase shifts, the size and direction of which were dependent on the time of injection. The resulting phase-response curve closely resembled that of light. The circadian response showed a clear dose-dependence at circadian time (CT) 13.5 but not at CT19. Both phase delays and advances induced by NMDA were blocked by coinjection of the NMDA antagonist 2-amino-5-phosphopentanoic acid but were slightly attenuated by the non-NMDA antagonist 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione disodium. The ability of NMDA to induce phase shifts was not altered by coinjection with tetrodotoxin. These data are consistent with the hypothesis that activation of NMDA receptors is a critical step in the transmission of photic information to the SCN.

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.

Differential effects of glutamatergic blockade on circadian and photic regulation of gene expression in the hamster suprachiasmatic nucleus

Nocturnal light exposure induces immediate-early gene (IEG) expression in the hypothalamic suprachiasmatic nucleus (SCN) and causes phase shifts of activity rhythms in mammals. Some IEGs also show a circadian rhythm of expression in the SCN. While excitatory amino acids (EAAs) are known to be involved in mediating photic regulation of entrainment and gene expression, their involvement in spontaneous rhythms of gene expression has not been studied. We assessed the role of NMDA receptors in the expression of NGFI-A, junB and fosB mRNAs induced by light pulses of different intensities late in the night (Zeitgeber Time [ZT] 18). We also examined the spontaneous expression of junB mRNA near subjective dawn (ZT 0). Both dim (5 lx) and bright (100 lx) light pulses induced similar levels of expression of NGFI-A and junB in the SCN late in the night. fosB mRNA was strongly induced by bright light but was less sensitive to dim light. At ZT 18, dizocilpine (MK-801) (3 mg/kg, i.p. ), a non-competitive NMDA receptor antagonist, almost completely blocked light-evoked expression of IEG mRNAs in the ventral SCN but not in the dorsolateral region at a mid-caudal level using either light intensity. At ZT 0, MK-801 strongly reduced light-evoked expression of junB mRNA in both SCN subdivisions, but inhibited spontaneous expression significantly only in the dorsal region. NMDA receptors appear to play an important role in mediating photic input regulating IEG expression only in the ventral SCN at night. At dawn, however, NMDA receptors are involved in mediating photic effects in both parts of the SCN, as well as being involved in spontaneous activation of junB expression selectively in the dorsal SCN. These findings support the idea that the effects in the dorsolateral SCN of nocturnal light exposure are mediated by different mechanisms than those in other portions of the nucleus.

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.

Coexpression of multiple metabotropic glutamate receptors in axon terminals of single suprachiasmatic nucleus neurons

Glutamate is the primary excitatory transmitter in axons innervating the hypothalamic suprachiasmatic nucleus (SCN) and is responsible for light-induced phase shifts of circadian rhythms generated by the SCN. By using self-innervating single neuron cultures and patch-clamp electrophysiology, we studied metabotropic glutamate receptors (mGluRs) expressed by SCN neurons. The selective agonists for group I (3,5-dihydroxy-phenylglycine), group II ((S)-4-carboxy-3-hydroxyphenylglycine), and group III ((+)-2-amino-4-phosphonobutyric acid) mGluRs all depressed the evoked IPSC in a subset (33%) of single autaptic neurons, suggesting a coexpression of all three groups of mGluRs in the same axon terminals of a single neuron. Other neurons showed a variety of combinations of mGluRs, including an expression of only one group of mGluR (18%) or coexpression of two groups of mGluRs (27%). Some neurons had no response to any of the three agonists (22%). The three mGluR agonists had no effect on postsynaptic gamma-aminobutyric acid (GABA) receptor responses, indicating a presynaptic modulation of GABA release by mGluRs. We conclude that multiple mGluRs that act through different second messenger pathways are coexpressed in single axon terminals of SCN neurons where they modulate the release of GABA presynaptically, usually inhibiting release.

Photic entrainment of circadian rhythms in rodents

Photic entrainment of circadian rhythms occurs as a consequence of daily, light-induced adjustments in the phase and period of the suprachiasmatic nuclei (SCN) circadian clock. Photic information is acquired by a unique population of retinal photoreceptors, processed by a distinct subset of retinal ganglion cells, and conveyed to the SCN through the retinohypothalamic tract (RHT). RHT neurotransmission is mediated by the release of the excitatory amino acid glutamate and appears to require the activation of both NMDA- and non-NMDA-type glutamate receptors, the expression of immediate early genes (IEGs), and the synthesis and release of nitric oxide. In addition, serotonin appears to regulate the response of the SCN circadian clock to light through postsynaptic 5-HT1A or 5-ht7 receptors, as well as presynaptic 5-HT1B heteroreceptors on RHT terminals.

Circadian release of amino acids in the suprachiasmatic nucleus in vitro

Temporal patterns of release of aspartate, glutamate and glycine, which are related to excitatory amino acidergic transmission, were examined in organotypic slice cultures of rat suprachiasmatic nucleus over a 60 h period. Vasopressin release in the same culture was measured simultaneously to compare the temporal pattern with that of the amino acids. Amino acids and vasopressin were measured by high performance liquid chromatography and enzyme immunoassay, respectively. Robust circadian rhythms were detected in release of aspartate, glutamate and glycine. Glycine levels were about 10 times higher than those of aspartate and glutamate in the culture. Vasopressin also showed a clear circadian rhythm and the phase angle difference between each amino acid and AVP was not significantly different. The results indicate that cultured SCN cells release these amino acids and the release is under the control of the circadian pacemaker.

Light regulates Homer mRNA expression in the rat suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus (SCN) of the mammal is the circadian pacemaker responsible for generation of circadian rhythms. Several immediate-early genes are expressed in the SCN by light stimuli which induce phase shifts of animal activity rhythms. In the present study, we investigated whether Homer, a PDZ-like protein which is rapidly induced following synaptic activation, mRNA expression is regulated by light in rat SCN. Homer mRNA expression in the SCN of rat killed at 4 h after onset of the light and dark phases was very low. One hour light stimuli during the subjective night dramatically induced Homer mRNA expression in the ventrolateral portion of the SCN, whereas light stimuli during the subjective light phase did not. This finding implies that Homer may be involved in the photic entrainment of the circadian clock.

Developmentally regulated gene expression of all eight metabotropic glutamate receptors in hypothalamic suprachiasmatic and arcuate nuclei--a PCR analysis

Previous studies have demonstrated the critical role glutamate plays in the hypothalamus, both in the developing and adult brain. The expression of metabotropic glutamate receptor (mGluR) mRNA (mGluR1-8) was studied in the suprachiasmatic (SCN) and arcuate (ARC) nuclei. Using reverse Northern blots and cDNA-PCR, we found that all eight cloned mGluRs were expressed in these brain regions. Most had not previously been detected here. Surprisingly, this included mGluRs that had previously been thought to be restricted to the retina, such as mGluR6. We also detected, cloned, and sequenced a splice variant of mGluR7 (mGluR7b). Developmentally, the age of maximal expression of mGluRs was dependent on the region. For instance, mGluR5 was more strongly expressed in neonatal ARC than in adult, whereas the opposite was true in the SCN. Compared with P10 neonates, mGluR1, R3, R6, R7a, R7b, and R8 showed a greater expression in adult SCN and ARC.

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

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

Enrichment of glutamate-like immunoreactivity in the retinotectal terminals of the viper Vipera aspis: an electron microscope quantitative immunogold study

A post-embedding immunogold study was carried out to estimate the immunoreactivity to glutamate in retinal terminals, P axon terminals and dendrites containing synaptic vesicles in the superficial layers of the optic tectum of Vipera. Retinal terminals, identified following either intraocular injection of tritiated proline, horseradish peroxidase (HRP) or short-term survivals after retinal ablation, were observed to be highly glutamate-immunoreactive. A detailed quantitative analysis showed that about 50% of glutamate immunoreactivity was localized over the synaptic vesicles, 35.8% over mitochondria and 14.2% over the axoplasmic matrix. The close association of immunoreactivity with the synaptic vesicles could indicate that Vipera retino-tectal terminals may use glutamate as their neurotransmitter. P axon terminals and dendrites containing synaptic vesicles, strongly gamma-aminobutyric (GABA)-immunoreactive, were shown to be also moderately glutamate-immunoreactive, but two to three times less than retinal terminals. Moreover, in P axon terminals, the glutamate immunoreactivity was denser over mitochondria than over synaptic vesicles, possibly reflecting the 'metabolic' pool of glutamate, which serves as a precursor in the formation of GABA.

Metabotropic glutamate receptor activation modulates kainate and serotonin calcium response in astrocytes

Although metabotropic glutamate receptor (mGluR) modulation has been studied extensively in neurons, it has not been investigated in astrocytes. We studied modulation of glutamate-evoked calcium rises in primary astrocyte cultures using fura-2 ratiometric digital calcium imaging. Calcium plays a key role as a second messenger system in astrocytes, both in regulation of many subcellular processes and in long distance intercellular signaling. Suprachiasmatic nucleus (SCN) and cortical astrocytes showed striking differences in sensitivity to glutamate and to mGluR agonists, even after several weeks in culture. Kainate-evoked intracellular calcium rises were inhibited by concurrent application of the type I and II mGluR agonists quisqualate (10 micro;M), trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylate (100-500 micro;M), and (2S-1'S-2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) (10 micro;M). Inhibition mediated by L-CCG-I had long-lasting effects (>45 min) in approximately 30% of the SCN astrocytes tested. The inhibition could be mimicked by the L-type calcium channel blocker nimodipine (1 micro;M) as well as by protein kinase C (PKC) activators phorbol 12,13-dibutyrate (10 micro;M) and phorbol 12-myristate 13-acetate (500 nM), and blocked by the PKC inactivator (+/-)-1-(5-isoquinolinesulfonyl)-2-methylpiperazine (200 micro;M), suggesting a mechanism involving PKC modulation of L-type calcium channels. In contrast, mGluRs modulated serotonin (5HT)-evoked calcium rises through a different mechanism. The type III mGluR agonist L-2-amino-4-phosphonobutyrate consistently inhibited 5HT-evoked calcium rises, whereas in a smaller number of cells quisqualate and L-CCG-I showed both inhibitory and additive effects. Unlike the mGluR-kainate interaction, which required a pretreatment with an mGluR agonist and was insensitive to pertussis toxin (PTx), the mGluR modulation of 5HT actions was rapid and was blocked by PTx. These data suggest that glutamate, acting at several metabotropic receptors expressed by astrocytes, could modulate glial activity evoked by neurotransmitters and thereby influence the ongoing modulation of neurons by astrocytes.

Local synaptic release of glutamate from neurons in the rat hypothalamic arcuate nucleus

1. The hypothalamic arcuate nucleus (ARC) contains neuroendocrine neurons that regulate endocrine secretions by releasing substances which control anterior pituitary hormonal release into the portal blood stream. Many neuroactive substances have been identified in the ARC, but the existence of excitatory neurons in the ARC and the identity of an excitatory transmitter have not been investigated physiologically. 2. In the present experiments using whole-cell current- and voltage-clamp recording of neurons from cultures and slices of the ARC, we demonstrate for the first time that some of the neurons in the ARC secrete glutamate as their transmitter. 3. Using microdrop stimulation of presynaptic neurons in ARC slices, we found that local axons from these glutamatergic neurons make local synaptic contact with other neurons in the ARC and that all evoked excitatory postsynaptic potentials could be blocked by the selective ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and D,L-2-amino-5-phosphonovalerate (AP5; 100 microM). To determine the identity of ARC neurons postsynaptic to local glutamatergic neurons, we used antidromic stimulation to reveal that many of these cells were neuroendocrine neurons by virtue of their maintaining axon terminals in the median eminence. 4. In ARC cultures, postsynaptic potentials, both excitatory and inhibitory, were virtually eliminated by the glutamate receptor antagonists AP5 and CNQX, underlining the functional importance of glutamate within this part of the neuroendocrine brain. 5. GABA was secreted by a subset of ARC neurons from local axons. The GABAA receptor antagonist bicuculline released glutamatergic neurons from chronic inhibition mediated by synaptically released GABA, resulting in further depolarization and an increase in the amplitude and frequency of glutamate-mediated excitatory postsynaptic potentials.

Regulation of cAMP response element binding protein (CREB) binding in the mammalian clock pacemaker by light but not a circadian clock

Mammalian circadian rhythms are considered to be regulated by a clock pacemaker located in the suprachiasmatic nuclei (SCN) of the hypothalamus. The molecular mechanism of entrainment and oscillation of circadian rhythm are not well understood but photic induction of immediate-early gene (IEG) expression in the SCN is thought to play a role. Here we show that under 12 h light:12 h dark (LD) condition, the cAMP response element binding protein (CREB) binding to cAMP responsive promoter element (CRE) of NMDAR1/zeta1 promoter region in the SCN is higher during the light than the dark by electro-mobility shift assay (EMSA). When animals are placed in constant dark, CREB DNA binding activity in the SCN is low and does not vary with circadian time when compared with cortex nuclear extract as a control. Most significantly, photic induction of CREB binding activity in the SCN occurs at all circadian times tested, indicating that CREB DNA binding in the SCN is not gated by the endogenous clock. These results implicate the role of CREB in photic neuronal signaling in the SCN and suggest that CREB DNA binding activities may not be regulated by a circadian clock.

Methylcobalamin induces a long-lasting enhancement of the field potential in rat suprachiasmatic nucleus slices

Optic nerve stimulation has been reported to evoke a field potential (FP) in rat suprachiasmatic nucleus (SCN) slices. Methylcobalamin,delta-(5,6-dymethylbenzimidazolyl)-Co-methyl-cobam ide (Me-B12) enhanced this FP and the enhancement lasted more than 1 h after washing out. Maximal enhancement (143.6 +/- 9.8%) was achieved at a concentration of 10 microM. By contrast, cyanocobalamin containing CN- instead of CH3- showed no enhancement of the amplitude in the FP. Me-B12 induced enhancement of FP was strongly blocked by an N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV). These results indicate that CH3- in the Me-B12 is required to modulate the FP amplitude and the NMDA receptor is involved in the long-lasting FP enhancement induced by Me-B12. The present results suggest that Me-B12 modifies the photic entrainment of the circadian clock in the suprachiasmatic nucleus via an activation of NMDA receptors.

Putative excitatory amino acid projections to the suprachiasmatic nucleus in the rat

Retrograde axonal transport of the select neuronal tracer [3H]D-aspartate was used to demonstrate possible sources of excitatory input to the suprachiasmatic nucleus (SCN) in the albino rat. Following injection of [3H]D-aspartate into the SCN, neurons were retrogradely labeled in the infralimbic cortex, the lateral septal nucleus, the paraventricular thalamic nucleus, the medial preoptic area, the ventromedial, dorsomedial and posterior hypothalamic nuclei, the zona incerta, the intergeniculate leaflet and the ventral subiculum. Retinal ganglion cells, which project to the SCN and use glutamate as a neurotransmitter, were not labeled in our [3H]D-aspartate experiments, demonstrating a limitation of this method (i.e., false negatives). Our results show that the [3H]D-aspartate neuronal tracer labels a subset of areas known to project to the SCN, indicating these areas as likely sources of excitatory input to the SCN.

Neuropeptide Y-mediated long-term depression of excitatory activity in suprachiasmatic nucleus neurons

A brief exposure to light can shift the phase of mammalian circadian rhythms by 1 hr or more. Neuropeptide Y (NPY) administration to the hypothalamic suprachiasmatic nucleus, the circadian clock in the brain, also causes a phase shift in circadian rhythms. After a phase shift, the neural clock responds differently to light, suggesting that learning has occurred in neural circuits related to clock function. Thus, certain stimuli can produce effects that last for an extended period, but possible mechanisms of this long-term effect have not been previously examined at the cellular level. Here, we report that NPY caused a long-term depression in both electrical activity and intracellular calcium levels of neurons, as studied with whole-cell patch-clamp recording and Fura-2 digital imaging. In contrast to the immediate (1 sec) recovery after relief from glutamate receptor blockade, a brief single application of NPY (100 nM) depressed cytosolic Ca2+ for > 1 hr. The mechanism of this long-term calcium depression, a form of cellular learning, is dependent on the simultaneous release of glutamate and activation of NPY receptors, because both the extended response to NPY and any aftereffect were blocked by coapplication of glutamate receptor antagonists. Postsynaptic actions of NPY, mediated by both Y1- and Y2-like receptors, were short term and recovered rapidly. The primary site of long-term NPY actions may be on presynaptic glutamatergic axons, because the frequency of miniature excitatory postsynaptic currents in the presence of tetrodotoxin was reduced by transient exposure to NPY in both cultures and slices.

Synaptic input from the retina to the suprachiasmatic nucleus changes with the light-dark cycle in the Syrian hamster

1. Single cell extracellular recordings were made from the suprachiasmatic nucleus (SCN) in urethane-anaesthetized Syrian hamsters at different times of the light-dark cycle. Peristimulus time histograms (PSTHs) were created following stimulation of the optic nerve. 2. Both short-latency (< 50 ms) and long-latency (> 50 ms) excitatory responses were seen. Almost all inhibitory responses had a short latency. 3. A total of 288 SCN neurones were recorded. Taking all types of response together, 55 (36.9%) of the 149 neurones tested in the dark period responded to optic nerve stimulation while only 23 (16.6%) of the 139 neurones tested in the light period responded. The difference between the proportion of all responsive and non-responsive neurones in the dark and light periods was highly significant (P < 0.01, Fisher's exact probability test). The difference in the proportion of excitatory responses was also significant (P < 0.01). 4. During the dark period, the mean spontaneous firing rate (5.00 +/- 0.88 spikes s-1; mean +/- S.E.M., n = 55) of the responsive cells was significantly higher than that of the non-responsive cells (2.65 +/- 0.33 spikes s-1; mean +/- S.E.M., n = 74; P < 0.01; Student's unpaired t test). 5. Injection of APV (20 mM, 2 microliters, I.C.V.; n = 6), an antagonist for the NMDA receptor, or CNQX (10 mM, 2 microliters, I.C.V.; n = 5), an antagonist of the non-NMDA receptor, significantly reduced the responses of all the neurones tested. 6. We conclude that there is daily variation in the firing of SCN neurones in vivo and the variation is restricted to those cells receiving optic nerve inputs. The change in the responsiveness of the SCN to optic nerve stimulation at different times of day suggests that there is a rapidly changing cycle of synaptic function in the SCN. The action of the antagonists suggests that the excitatory retinal projections to the SCN which show this variation are mediated by glutamate and that both NMDA and non-NMDA receptors are involved.

Involvement of 5-HT1A receptor mechanisms in the inhibitory effects of methamphetamine on photic responses in the rodent suprachiasmatic nucleus

We examined the role of serotonin 1A (5-HT1A) receptors in the inhibitory effects of methamphetamine (MA) on photic entrainment to the circadian pacemaker in the suprachiasmatic nucleus (SCN) of rodents. MA inhibited optic nerve stimulation-evoked field potential in the SCN, light-induced Fos expression in the SCN and light-induced phase shift of hamster wheel-running rhythm. NAN-190, a 5-HT1A receptor antagonist, eliminated the inhibitory effects of MA. NAN-190 has also been reported to antagonize alpha 1 adrenergic receptors. However, prazosin, which selectively antagonizes alpha 1 adrenergic receptors, did not affect the inhibitory action of MA on light-induced Fos expression. In addition, parachloroamphetamine, which is known to be a 5-HT releaser, dose-dependently inhibited light-induced phase shift of wheel-running rhythm. These findings suggest that elevation of endogenous 5-HT levels by MA inhibits the photic entraining responses of the circadian pacemaker in the SCN via 5-HT1A receptor stimulation of the 5-HT released by MA.

Adenosine A1-receptor agonist attenuates the light-induced phase shifts and fos expression in vivo and optic nerve stimulation-evoked field potentials in the suprachiasmatic nucleus in vitro

Adenosine is widely accepted to act as an inhibitory neuromodulator in the mammalian central nervous system. In the present study, we examined whether adenosine receptor agonist modifies the photic entraining responses in the rat suprachiasmatic nucleus both in vivo and in vitro. Light (200 lux, 15 min)-induced phase shifts of hamster wheel-running rhythms was attenuated by a systemic administration of A1-adenosine receptor agonist N6-cyclohexyladenosine (N-CHA) in a dose-dependent manner; 0.5 mg/kg N-CHA caused 60% inhibition of light-induced phase shifts. On the other hand, A2-adenosine receptor agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) failed to inhibit light-induced phase shifts. Systemic administration of N-CHA but not of DPMA inhibited light (300 lux, 1 h)-induced Fos expression in the suprachiasmatic nucleus in a dose-dependent manner; 1 mg/kg N-CHA caused 73% inhibition of light-induced Fos expression. Bath application of N-CHA but not of DPMA inhibited optic nerve stimulation-evoked field potentials in rat suprachiasmatic nucleus slices. The present results suggest that activation of adenosine A1-receptor attenuates the photic input through the inhibition of retinohypotalamic pathway to the SCN.

The role of glutamate in the photic regulation of the suprachiasmatic nucleus

Endogenous circadian rhythms govern most aspects of physiology and behaviour in mammals, including body temperature, autonomic and endocrine function, and sleep-wake cycles. Such rhythms are generated by the suprachiasmatic nucleus of the hypothalamus (SCN), but are synchronised to the environmental light-dark cycle by photic cues perceived by the retina and conveyed to the SCN via the retinohypothalamic tract (RHT). This review considers many lines of evidence from diverse experimental approaches indicating that the RHT employs glutamate (or a related excitatory amino acid) as a neurotransmitter. Ultrastructural studies demonstrate the presence of glutamate in presynaptic terminals within the SCN. In situ hybridisation and immunocytochemical studies reveal the presence of several NMDA (NMDAR1, NMDAR2C), non-NMDA (GluR1, GluR2, GluR4) and metabotropic (mGluR1) glutamate receptor subunits in the SCN. Messenger RNA encoding a glutamate transporter protein is also present. In behavioural tests, glutamate antagonists can block the effects of light in phase-shifting circadian rhythms. Such treatments also block the induction of c-fos within SCN cells by light, whereas a glutamate agonist (NMDA) induces c-fos expression. In hypothalamic slice preparations in vitro, electrical stimulation of the optic nerves induces release of glutamate and aspartate, and glutamate antagonists block field potentials in the SCN evoked by stimulation of the optic nerve. Circadian rhythms of electrical activity which persist in vitro are phase shifted by application of glutamate in a manner which mimics the phase shifting effects of light in vivo. This wide range of experimental findings provides strong support for the hypothesis that glutamate is the principal neurotransmitter within the RHT, and thus conveys photic cues to the circadian timing system in the SCN.