Microinjection of NMDA into the SCN region mimics the phase shifting effect of light in hamsters

Neural function of Bmal1: an overview

Bmal1 (Brain and muscle arnt-like, or Arntl) is a bHLH/PAS domain transcription factor central to the transcription/translation feedback loop of the biologic clock. Although Bmal1 is well-established as a major regulator of circadian rhythm, a growing number of studies in recent years have shown that dysfunction of Bmal1 underlies a variety of psychiatric, neurodegenerative-like, and endocrine metabolism-related disorders, as well as potential oncogenic roles. In this review, we systematically summarized Bmal1 expression in different brain regions, its neurological functions related or not to circadian rhythm and biological clock, and pathological phenotypes arising from Bmal1 knockout. This review also discusses oscillation and rhythmicity, especially in the suprachiasmatic nucleus, and provides perspective on future progress in Bmal1 research.

Regulation of Excitatory Amino Acid Transmission in the Retina: Studies on Neuroprotection

Excitotoxicity occurs in neurons due to the accumulation of excitatory amino acids such as glutamate in the synaptic and extrasynaptic locations. In the retina, excessive glutamate concentrations trigger a neurotoxic cascade involving several mechanisms, including the elevation of intracellular calcium (Ca²⁺⁾ and the activation of α-amino-3-hydroxy 5-methyl-4-iso-xazole-propionic acid/kainate (AMPA/KA) and N-methyl-d-aspartate (NMDA) receptors leading to retinal degeneration. Both ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs) are present in the mammalian retina. Indeed, due to the abundant expression of GluRs, the mammalian retina is highly susceptible to excitotoxic neurodegeneration. Excitotoxicity has been postulated to present a common downstream mechanism for several stimuli, including hypoglycemia, hypoxia, ischemia, and chronic neurodegenerative diseases. Experimental approaches to the study of neuroprotection in the retina have utilized insults that trigger hypoxia, hypoglycemia, or excitotoxicity. Using these experimental approaches, the neuroprotective potential of GluR agents, including the NMDA receptor modulators (MK801, ifenprodil, memantine); AMPA/KA receptor antagonist (CNQX); Group II and III mGluR agonists (LY354740, quisqualate); and Ca²⁺-channel blockers (diltiazem, lomerizine, verapamil, ω-conotoxin), and others (pituitary adenylate cyclase activating polypeptide, neuropeptide Y, acetylcholine receptor agonists) have been elucidated. In addition to corroborating the exocytotic role of excitatory amino acids in retinal degeneration, these studies affirm that multiple mechanism/s contribute to the prevention of damage caused by excitotoxicity in the retina. Therefore, it is feasible that several pathways are involved in protecting the retina from toxic insults in ocular neurodegenerative conditions such as glaucoma and retinal ischemia. Furthermore, these experimental models are viable tools for evaluating therapeutic candidates in ocular neuropathies.

Astrocyte deletion of Bmal1 alters daily locomotor activity and cognitive functions via GABA signalling

Circadian rhythms are controlled by a network of clock neurons in the central pacemaker, the suprachiasmatic nucleus (SCN). Core clock genes, such as Bmal1, are expressed in SCN neurons and in other brain cells, such as astrocytes. However, the role of astrocytic clock genes in controlling rhythmic behaviour is unknown. Here we show that ablation of Bmal1 in GLAST-positive astrocytes alters circadian locomotor behaviour and cognition in mice. Specifically, deletion of astrocytic Bmal1 has an impact on the neuronal clock through GABA signalling. Importantly, pharmacological modulation of GABAA-receptor signalling completely rescues the behavioural phenotypes. Our results reveal a crucial role of astrocytic Bmal1 for the coordination of neuronal clocks and propose a new cellular target, astrocytes, for neuropharmacology of transient or chronic perturbation of circadian rhythms, where alteration of astrocytic clock genes might contribute to the impairment of the neurobehavioural outputs such as cognition.

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.

Differential Sensitivity to Ethanol-Induced Circadian Rhythm Disruption in Adolescent and Adult Mice

BACKGROUND

Growing evidence supports a central role for the circadian system in alcohol use disorders, but few studies have examined this relationship during adolescence. In mammals, circadian rhythms are regulated by the suprachiasmatic nucleus, a biological clock whose timing is synchronized (reset) to the environment primarily by light (photic) input. Alcohol (ethanol [EtOH]) disrupts circadian timing in part by attenuating photic phase-resetting responses in adult rodents. However, circadian rhythms change throughout life and it is not yet known whether EtOH has similar effects on circadian regulation during adolescence.

METHODS

General circadian locomotor activity was monitored in male C57BL6/J mice beginning in adolescence (P27) or adulthood (P61) in a 12-hour light, 12-hour dark photocycle for ~2 weeks to establish baseline circadian activity measures. On the day of the experiment, mice received an acute injection of EtOH (1.5 g/kg, i.p.) or equal volume saline 15 minutes prior to a 30-minute light pulse at Zeitgeber Time 14 (2 hours into the dark phase) and then were released into constant darkness (DD) for ~2 weeks to assess phase-resetting responses. Control mice of each age-group received injections but no light pulse prior to DD.

RESULTS

While adults showed the expected decrease in photic phase-delays induced by acute EtOH, this effect was absent in adolescent mice. Adolescents also showed baseline differences in circadian rhythmicity compared to adults, including advanced photocycle entrainment, larger photic phase-delays, a shorter free-running (endogenous) circadian period, and greater circadian rhythm amplitude.

CONCLUSIONS

Collectively, our results indicate that adolescent mice are less sensitive to the effect of EtOH on circadian photic phase-resetting and that their daily activity rhythms are markedly different than those of adults.

Adenosine and glutamate in neuroglial interaction: implications for circadian disorders and alcoholism

Recent studies have demonstrated that the function of glia is not restricted to the support of neuronal function. In fact, astrocytes are essential for neuronal activity in the brain and play an important role in the regulation of complex behavior. Astrocytes actively participate in synapse formation and brain information processing by releasing and uptaking glutamate, D-serine, adenosine 5'-triphosphate (ATP), and adenosine. In the central nervous system, adenosine-mediated neuronal activity modulates the actions of other neurotransmitter systems. Adenosinergic fine-tuning of the glutamate system in particular has been shown to regulate circadian rhythmicity and sleep, as well as alcohol-related behavior and drinking. Adenosine gates both photic (light-induced) glutamatergic and nonphotic (alerting) input to the circadian clock located in the suprachiasmatic nucleus of the hypothalamus. Astrocytic, SNARE-mediated ATP release provides the extracellular adenosine that drives homeostatic sleep. Acute ethanol increases extracellular adenosine, which mediates the ataxic and hypnotic/sedative effects of alcohol, while chronic ethanol leads to downregulated adenosine signaling that underlies insomnia, a major predictor of relapse. Studies using mice lacking the equilibrative nucleoside transporter 1 have illuminated how adenosine functions through neuroglial interactions involving glutamate uptake transporter GLT-1 [referred to as excitatory amino acid transporter 2 (EAAT2) in human] and possibly water channel aquaporin 4 to regulate ethanol sensitivity, reward-related motivational processes, and alcohol intake.

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

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

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

A multicellular model for differential regulation of circadian signals in the core and shell regions of the suprachiasmatic nucleus

We developed a multicellular model of the mammalian circadian clock characterized by a high degree of heterogeneity with respect to single cell periodicity and behavior (intrinsic and driven oscillators), neurotransmitter release (VIP, GABA and glutamate synthesis) and spatial organization (core and shell regions), mimicking structural patterns within the suprachiasmatic nucleus (SCN) associated with distinct circadian functions. We simulated the SCN core and shell separately utilizing experimentally derived connectivity schemes for the two subdivisions as observed within the rat SCN. The core was modeled via a small world network characterized by VIP and GABA co-localization, whereas the shell was simulated as a nearest neighbor network promoting local GABAergic connections. To study the function of the axonal plexus extending from the densely innervated ventrolateral region to distal areas across the dorsomedial SCN, directed long range links from the core to the shell were gradually introduced via a probability p(cs) that ranged from 0 to 1. A probability value of 0 excluded core-shell interactions, whereas p(cs)=1 achieved maximal connectivity between the two regions. Our model exhibited a threshold in the number of core-to-shell links required for sufficient cell-to-cell coordination to maintain periodicity and rhythmic behavior across the entire model network (including both shell and core populations) in constant darkness as well as 12:12h light-dark cycles. By contrast, constant light was shown to increase phase synchronization across the shell while core populations remained poorly synchronized, suggesting differential light response across the two SCN compartments. We further simulated increasing percentages of intrinsic oscillators and demonstrated a negative correlation between the number of intrinsic oscillators distributed across the SCN and the ability of the system to produce synchronized signals. Simulations that differed with respect to the placement of intrinsic oscillators supported the hypothesis that improved synchronization is achieved with networks characterized by localized intrinsic oscillators placed exclusively within the shell versus networks containing uniformly distributed intrinsic oscillators in both SCN compartments. This study has successfully reproduced a number of spatiotemporal and behavioral attributes of the SCN, providing a useful computational tool to correlate observed circadian phenotypes with distinct chemoarchitectural properties of spatially localized neural populations.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

An NMDA antagonist inhibits light but not GRP-induced phase shifts when administered after the phase-shifting stimulus

Brief light pulses or microinjection of gastrin-releasing peptide (GRP) into the third ventricle or near the suprachiasmatic nucleus (SCN) induce phase shifts of circadian rhythms during the subjective night. It has previously been reported that these effects are strongly influenced by the activation of N-methyl-d-aspartate (NMDA) receptors and the availability of glutamate. We hypothesized that the photic signaling pathway in the SCN was dependent on glutamate neurotransmission even after the completion of a photic stimulus. Adult male Syrian hamsters equipped with a surgically implanted guide cannula aimed at the SCN region were housed in constant darkness until stable free-running rhythms of wheel-running activity were apparent. Light pulses administered in the early night induced phase delays of circadian rhythms which were attenuated by the co-administration of (+/-)-2-amino-5-phosphonopentanoic acid (AP5), an NMDA antagonist. Microinjection of AP5 also inhibited light-induced shifts, to a lesser extent, immediately after and 15 min after, but not 30 min after the light pulse. A second experiment was designed to test whether AP5 would be able to attenuate GRP-induced shifts 15 min following microinjection of GRP. Phase shifts of animals that received microinjection of AP5 15 min after the administration of GRP were not different from those that received microinjection of GRP and vehicle. These data suggest that glutamate signaling remains necessary for a full photic response in the SCN even after the termination of the photic signal, but that this dependency ends once GRP-dependent signaling is complete.

Stoichiometry of N-methyl-D-aspartate receptors within the suprachiasmatic nucleus

The circadian pacemaker within the suprachiasmatic nucleus (SCN) confers daily rhythms to bodily functions. In nature, the circadian clock will adopt a 24-h period by synchronizing to the solar light/dark cycle. This light entrainment process is mediated, in part, at glutamatergic synapses formed between retinal ganglion afferents and SCN neurons. N-methyl-D-aspartate receptors (NMDARs) located on SCN neurons gate light-induced phase resetting. Despite their importance in circadian physiology, little is known about their functional stoichiometry. We investigated the NR2-subunit composition with whole cell recordings of SCN neurons within the murine hypothalamic brain slice using a combination of subtype-selective NMDAR antagonists and voltage-clamp protocols. We found that extracellular magnesium ([Mg](o)) strongly blocks SCN NMDARs exhibiting affinities and voltage sensitivities associated with NR2A and NR2B subunits. These NMDAR currents were inhibited strongly by NR2B-selective antagonists, Ro 25-6981 (3.5 microM, 55.0 +/- 9.0% block; mean +/- SE) and ifenprodil (10 microM, 55.8 +/- 3.0% block). The current remaining showed decreased [Mg](o) affinities reminiscent of NR2C and NR2D subunits but was highly sensitive to [Zn](o), a potent NR2A blocker, showing a approximately 44.2 +/- 1.1% maximal inhibition at saturating concentrations with an IC(50) of 7.8 +/- 1.1 nM. Considering the selectivity, efficacy, and potency of the drugs used in combination with [Mg](o)-block characteristics of the NMDAR, our data show that both diheteromeric NR2B NMDARs and triheteromeric NR2A NMDARs (paired with an NR2C or NR2D subunits) account for the vast majority of the NMDAR current within the SCN.

Chronic ethanol attenuates circadian photic phase resetting and alters nocturnal activity patterns in the hamster

Acute ethanol (EtOH) administration impairs circadian clock phase resetting, suggesting a mode for the disruptive effect of alcohol abuse on human circadian rhythms. Here, we extend this research by characterizing the chronobiological effects of chronic alcohol consumption. First, daily profiles of EtOH were measured in the suprachiasmatic nucleus (SCN) and subcutaneously using microdialysis in hamsters drinking EtOH. In both cases, EtOH peaked near lights-off and declined throughout the dark-phase to low day-time levels. Drinking bouts preceded EtOH peaks by approximately 20 min. Second, hamsters chronically drinking EtOH received a light pulse during the late dark phase [Zeitgeber time (ZT) 18.5] to induce photic phase advances. Water controls had shifts of 1.2 +/- 0.2 h, whereas those drinking 10% and 20% EtOH had much reduced shifts (0.5 +/- 0.1 and 0.3 +/- 0.1 h, respectively; P < 0.001 vs. controls). Third, incremental decreases in light intensity (270 lux to 0.5 lux) were used to explore chronic EtOH effects on photic entrainment and rhythm stability. Activity onset was unaffected by 20% EtOH at all light intensities. Conversely, the 24-h pattern of activity bouts was disrupted by EtOH under all light intensities. Finally, replacement of chronic EtOH with water was used to examine withdrawal effects. Water controls had photic phase advances of 1.1 +/- 0.3 h, while hamsters deprived of EtOH for 2-3 days showed enhanced shifts (2.1 +/- 0.3 h; P < 0.05 vs. controls). Thus, in chronically drinking hamsters, brain EtOH levels are sufficient to inhibit photic phase resetting and disrupt circadian activity. Chronic EtOH did not impair photic entrainment; however, its replacement with water potentiated photic phase resetting.

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.

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

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

Acute ethanol impairs photic and nonphotic circadian phase resetting in the Syrian hamster

Disrupted circadian rhythmicity is associated with ethanol (EtOH) abuse, yet little is known about how EtOH affects the mammalian circadian clock of the suprachiasmatic nucleus (SCN). Clock timing is regulated by photic and nonphotic inputs to the SCN involving glutamate release from the retinohypothalamic tract and serotonin (5-HT) from the midbrain raphe, respectively. Our recent in vitro studies in the SCN slice revealed that EtOH blocks photic phase-resetting action of glutamate and enhances the nonphotic phase-resetting action of the 5-HT1A,7 agonist, 8-OH-DPAT. To explore the basis of these effects in the whole animal, we used microdialysis to characterize the pharmacokinetics of intraperitoneal injection of EtOH in the hamster SCN extracellular fluid compartment and then studied the effects of such EtOH treatment on photic and serotonergic phase resetting of the circadian locomotor activity rhythm. Peak EtOH levels (approximately 50 mM) from a 2 g/kg injection occurred within 20-40 min with a half-life of approximately 3 h. EtOH treatment dose-dependently attenuated photic phase advances but had no effect on phase delays and, contrary to in vitro findings, markedly attenuated 8-OH-DPAT-induced phase advances. In a complementary experiment using reverse microdialysis to deliver a timed SCN perfusion of EtOH during a phase-advancing light pulse, the phase advances were blocked, similar to systemic EtOH treatment. These results are evidence that acute EtOH significantly affects photic and nonphotic phase-resetting responses critical to circadian clock regulation. Notably, EtOH inhibition of photic signaling is manifest through direct action in the SCN. Such actions could underlie the disruption of circadian rhythmicity associated with alcohol abuse.

Altered photic and non-photic phase shifts in 5-HT(1A) receptor knockout mice

The mammalian circadian clock located in the suprachiasmatic nucleus (SCN) is thought to be modulated by 5-HT. 5-HT is though to inhibit photic phase shifts by inhibiting the release of glutamate from retinal terminals, as well as by decreasing the responsiveness of retinorecipient cells in the SCN. Furthermore, there is also evidence that 5-HT may underlie, in part, non-photic phase shifts of the circadian system. Understanding the mechanism by which 5-HT accomplishes these goals is complicated by the wide variety of 5-HT receptors found in the SCN, the heterogeneous organization of both the circadian clock and the location of 5-HT receptors, and by a lack of sufficiently selective pharmacological agents for the 5-HT receptors of interest. Genetically modified animals engineered to lack a specific 5-HT receptor present an alternative avenue of investigation to understand how 5-HT regulates the circadian system. Here we examine behavioral and molecular responses to both photic and non-photic stimuli in mice lacking the 5-HT(1A) receptor. When compared with wild-type controls, these mice exhibit larger phase advances to a short late-night light pulse and larger delays to long 12 h light pulses that span the whole subjective night. Fos and mPer1 expression in the retinorecipient SCN is significantly attenuated following late-night light pulses in the 5-HT(1A) knockout animals. Finally, non-photic phase shifts to (+/-)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) are lost in the knockout animals, while attenuation of the phase shift to the long light pulse due to rebound activity following a wheel lock is unaffected. These findings suggest that the 5-HT(1A) receptor plays an inhibitory role in behavioral phase shifts, a facilitatory role in light-induced gene expression, a necessary role in phase shifts to 8-OH-DPAT, and is not necessary for activity-induced phase advances that oppose photic phase shifts to long light pulses.

Acute ethanol modulates glutamatergic and serotonergic phase shifts of the mouse circadian clock in vitro

Alcohol abuse is associated with sleep problems, which are often linked to circadian rhythm disturbances. However, there is no information on the direct effects of ethanol on the mammalian circadian clock. Acute ethanol inhibits glutamate signaling, which is the primary mechanism through which light resets the mammalian clock in the suprachiasmatic nucleus (SCN). Glutamate and light also inhibit circadian clock resetting induced by nonphotic signals, including 5-HT. Thus, we investigated the effects of acute ethanol on both glutamatergic and serotoninergic resetting of the mouse SCN clock in vitro. We show that ethanol dose-dependently inhibits glutamate-induced phase shifts and enhances serotonergic phase shifts. The inhibition of glutamate-induced phase shifts is not affected by excess glutamate, glycine or d-serine, but is prevented by excess brain-derived neurotrophic factor (BDNF). BDNF is known to augment glutamate signaling in the SCN and to be necessary for glutamate/light-induced phase shifts. Thus, ethanol may inhibit glutamate-induced clock resetting at least in part by blocking BDNF enhancement of glutamate signaling. Ethanol enhancement of serotonergic phase shifts is mimicked by treatments that suppress glutamate signaling in the SCN, including antagonists of glutamate receptors, BDNF signaling and nitric oxide synthase. The combined effect of ethanol with these treatments is not additive, suggesting they act through a common pathway. Our data indicate further that the interaction between 5-HT and glutamate in the SCN may occur downstream from nitric oxide synthase activation. Thus, acute ethanol disrupts normal circadian clock phase regulation, which could contribute to the physiological and psychological problems associated with alcohol abuse.

PACAP and PDF signaling in the regulation of mammalian and insect circadian rhythms

Endogenous circadian clocks are inherent to all living organisms. They are needed to guarantee successful life since they regulate very important biological processes such as behavior and reproduction. Secretin-like G-protein coupled receptors are very important factors in the signal transduction pathways of circadian clocks. In this review, we will focus on the role of two secretin-like signaling pathways that play an important role in the regulation of the mammalian and the insect clock, respectively: the pituitary adenylate cyclase-activating polypeptide (PACAP) and pigment dispersing factor (PDF) signaling pathways. Both pathways are most likely related although their function in the biological clock differs.

Glutamatergic activity modulates the phase-shifting effects of gastrin-releasing peptide and light

Previous studies have established that microinjection of gastrin-releasing peptide (GRP) into the suprachiasmatic nucleus (SCN) region or third ventricle causes circadian phase shifts similar to those produced by light pulses. Activation of N-methyl-d-aspartate (NMDA) receptors in the SCN region also produces light-like phase shifts. This study was designed to test the effects of (+/-)-2-amino-5-phosphonopentanoic acid (AP5), an NMDA antagonist, and l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), a glutamate reuptake inhibitor, on GRP-induced phase shifts. Adult male Syrian hamsters equipped with a surgically implanted guide cannula aimed at the third ventricle were housed in constant darkness until stable free-running rhythms of wheel-running activity were apparent. Microinjection of GRP into the third ventricle at circadian time (CT)13 induced large phase delays. These GRP-induced phase delays were completely blocked by co-administration of AP5, suggesting that GRP-induced phase delays require concurrent activation of NMDA receptors. Microinjection of AP5 alone did not induce significant phase shifts. A second set of experiments was designed to test whether GRP-induced phase shifts would be enhanced by PDC. Co-administration of PDC and GRP elicited significantly larger phase delays at CT13 than GRP alone. However, administration of PDC alone did not induce a significant phase shift. Finally, when administered just prior to a light pulse, PDC elicited significantly larger phase delays than light pulse plus vehicle controls. These data suggest that the effects of GRP on the circadian clock phase are highly dependent on the level of excitation provided by activated NMDA receptors.

Paradoxical effects of NPY in the suprachiasmatic nucleus

The circadian clock in the suprachiasmatic nucleus (SCN) is synchronized by the 24 h, light : dark cycle, and is reset by photic and non-photic cues. The acute effects of light in the SCN include the increase of mRNA levels of the circadian clock gene Per1 and a dramatic reduction of pineal melatonin. Neuropeptide Y (NPY), which appears to mediate the phase-resetting effects of non-photic stimuli, prevents the ability of light, and stimuli that mimic light, to phase shift the circadian clock when injected into the SCN. The purpose of the present study was to determine if NPY inhibits the ability of light to suppress pineal melatonin. Surprisingly, NPY injected into the SCN of hamsters mimicked the effects of light by suppressing pineal melatonin levels. To confirm that NPY inhibited the effects of light on the induction of Per1 mRNA levels, Per1 mRNA levels in the SCN were measured in these same animals. NPY significantly reduced Per1 mRNA levels induced by the light pulse. The suppression of melatonin by NPY appears to be mediated by the same subtype of NPY receptors in the SCN that mediate the modulation of phase shifts. Injection of Y5 receptor agonists mimicked the effects of NPY on pineal melatonin, while injection of a Y2 agonist did not. Thus, these data are the first to demonstrate the paradoxical effects of NPY within the SCN. NPY mimics the effects of light on pineal melatonin and inhibits the effects of light on the induction of Per1 mRNA.

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.

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."

Circadian Rhythmicity in AVP Secretion and GABAergic Synaptic Transmission in the Rat Suprachiasmatic Nucleus

A variety of physiological and behavioral functions exhibit circadian changes and these circadian rhythms are driven by oscillatory expression of clock genes in the suprachiasmatic nuclei (SCN). It is still unknown how this molecular clockwork is controlled by extracellular neurohormones and neurotransmitters and which membrane receptors undergo circadian modulation. Circadian rhythm can be measured as a secretion of arginine vasopressin (AVP) in organotypic SCN culture for several weeks. Melatonin applied directly to the SCN late in the day induces a phase advance, when applied late at night or at the beginning of the day melatonin causes a phase delay. The time window for phase advance corresponds with the highest level of melatonin receptors in the SCN but the mechanism of melatonin-induced phase delay is unknown. The principal neurotransmitter on SCN synapses is gamma-aminobutyric acid (GABA), which acts at postsynaptic GABA(A) receptors. Spontaneous release of GABA from presynaptic nerve terminals, recorded as miniature inhibitory postsynaptic currents in the presence of TTX, does not change, but zinc sensitivity of exogenous GABA-induced currents varies during the day and night, possibly due to changes in subunit composition of GABA(A) receptors. We conclude that there is daily variation in the postsynaptic, but not presynaptic, function in the SCN.

Roles of PACAP‐Containing Retinal Ganglion Cells in Circadian Timing

The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms in physiology and behavior. The clock-driven rhythms need daily adjustment (entrainment) to be synchronized with the astronomical day of 24 h. The most important stimulus for entrainment of the clock is the light-dark (LD) cycle. In this review functional elements of the light entrainment pathway will be considered with special focus on the neurotransmitter pituitary adenylate cyclase-activating polypeptide (PACAP), which is found exclusively in the monosynaptic neuronal pathway mediating light information to the SCN, the retinohypothalamic tract (RHT). The retinal ganglion cells of the RHT are intrinsically photosensitive due to the expression of melanopsin and seem to constitute a non-image forming photosensitive system in the mammalian eye regulating circadian timing, masking behavior, light-regulated melatonin secretion, and the pupillary light reflex. Evidence from in vitro and in vivo studies and studies of mice lacking PACAP and the specific PACAP receptor (PAC1) indicate that PACAP and glutamate are neurotransmitters in the RHT which in a clock and concentration-dependent manner interact during light entrainment of the clock.

Role of Neuronal Membrane Events in Circadian Rhythm Generation

Circadian clock systems are composed of an input or "entrainment" pathway by which synchronization to the external environment occurs, a pacemaker responsible for generating rhythmicity, and an output or "expression" pathway through which rhythmic signals act to modulate physiology and behavior. The circadian pacemaker contains molecular feedback loops of rhythmically expressed genes and their protein products, which, through interactions, generate a circa 24-h cycle of transcription and translation of clock and clock-controlled genes. Neuronal membrane events appear to play major roles in entrainment of circadian rhythms in mollusks and mammals. In mammals, the suprachiasmatic nuclei of the hypothalamus receive photic information via the retinohypothalamic tract. Retinal signals, mediated by glutamate, induce calcium release and activate a number of intracellular cascades involved in photic gating and phase shifting. Membrane events are also involved in rhythm expression. Calcium and potassium currents influence the electrical output of pacemaker neurons by altering shape and intervals of impulse prepotentials, afterhyperpolarization periods, and interspike intervals, as well as altering membrane potentials and thereby shaping the spontaneous rhythmic spiking patterns. Unlike the involvement of membrane events in circadian entrainment and expression, it is less clear whether electrical activity, postsynaptic events, and transmembrane ion fluxes also are essential elements in rhythm generation. Studies, however, suggest that neuronal membrane activity may indeed play a crucial role in circadian rhythm generation.

Activation of 5-HT2C receptors acutely induces Per gene expression in the rat suprachiasmatic nucleus at night

The suprachiasmatic nucleus (SCN) of the hypothalamus receives dense serotonergic projections from the raphe nuclei and this input has been implicated in the modulation of circadian rhythms. In the present study, we investigated the effect of 5-HT2C receptor activation on various clock genes within the suprachiasmatic nucleus, including Per1 and Per2, which have previously been demonstrated as necessary for phase shifts. Rats were exposed to light (400 lx, 15 min), administered 5-HT2C receptor agonists (+/-)-1-(4-iodo-2,5-dimethoxy-phenyl)-2-aminopropane (DOI) (2 mg/kg) or RO 60-0175 (10 mg/kg) or vehicle 4 or 10 h after dark onset (ZT16 and ZT22). The expression of Per1, Per2, Cry1, Clock, Bmal1, Dec1, Dec2 and c-fos was determined 30 and 120 min after treatment in suprachiasmatic nucleus punches by real time reverse transcription-polymerase chain reaction (RT-PCR). Light exposure induced a 7-fold increase in c-fos expression within 30 min of treatment at both ZT16 and ZT22. Per1 expression was increased 2-fold following light exposure at ZT22, whereas treatment at ZT16 had no significant effect. Per2 expression was significantly induced following light at ZT16, but was not affected at ZT22. RO 60-0175 or DOI administration induced a 5-fold change in c-fos expression at ZT16 and a 3-fold change at ZT22 within 30 min of treatment. The drug increased both Per1 and Per2 expression at ZT16, but had no effect at ZT22. These results provide evidence for 5-HT2C receptors being involved in the modulation of circadian rhythms during early night.

Light-induced phase shift in the Syrian hamster (Mesocricetus auratus) is attenuated by the PACAP receptor antagonist PACAP6-38 or PACAP immunoneutralization

Circadian rhythms generated by the suprachiasmatic nucleus (SCN) are daily adjusted (entrained) by light via the retinohypothalamic tract (RHT). The RHT contains two neurotransmitters, glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP), which are believed to mediate the phase-shifting effects of light on the clock. In the present study we have elucidated the role of PACAP in light-induced phase shifting at early night in hamsters and shown that (i) light-induced phase delay of running-wheel activity was significantly attenuated by a specific PAC1 receptor antagonist (PACAP6-38) or by immunoblockade with a specific anti-PACAP antibody injected intracerebroventricularly before light stimulation; (ii) PACAP administered close to the SCN was able to phase-delay the circadian rhythm of running-wheel activity in a similar way to light; (iii) PACAP was present in the hamster RHT, colocalized with melanopsin, a recently identified opsin which has been suggested to be a circadian photopigment. The findings indicate that PACAP is a neurotransmitter of the RHT mediating photic information to the clock, possibly via melanopsin located exclusively on the PACAP-expressing cells of the RHT.

Effects of commercial antiglaucoma drugs to glutamate-induced [Ca2+)]i increase in cultured neuroblastoma cells

Over releasing of glutamate and cellular calcium influx always results in neuronal death. In the present study, we investigated various commercial antiglaucoma drugs including timolol (0.58 microM to 58 microM), betaxolol (1.62 microM to 162 microM), carteolol (6.8 microM to 680 microM), pilocarpine (4.08 microM to 408 microM), latanoprost (0.01 microM to 1.1 microM), dorzolamide (6.16 microM to 616 microM), brinzolamide (2.6 microM to 260 microM), brimonidine (0.68 microM to 68 microM), dipivefrin (0.28 microM to 28 microM) and preservative benzalkonium chloride on their effects to inhibit glutamate-induced intracellular free Ca(2+) ([Ca(2+)](i)) increase in cultured N1E-115 neuroblastoma cells. These drugs were diluted from original concentrations to 1/100, 1/1000 and 1/10000. The [Ca(2+)](i) mobility was studied after loading with fura-2-AM and analyzed by spectrofluorometry. It was found that betaxolol, dipivefrin and brimonidine have remarkable effects not only to inhibit the glutamate-induced [Ca(2+)](i) increase but also to decrease the basal [Ca(2+)](i). In the case of other drugs, only high concentration of timolol (58 microM) exhibited significant effect to completely prevent glutamate-induced [Ca(2+)](i) increase. Moreover, benzalkonium chloride did not exhibit any inhibitive effect. These results indicate that betaxolol, dipivefrin and brimonidine may have neuroprotective effects to inhibit the glutamate-induced over Ca(2+) influx damage.

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

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

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.

Aging alters light- and PACAP-induced cAMP accumulation in the suprachiasmatic nucleus of female rats

Light-induced release of pituitary adenylate cyclase activating peptide (PACAP) from retinal ganglion cells can modulate the phase-shifting effects of light though a cAMP-mediated mechanism in neurons of the suprachiasmatic nucleus (SCN). Since older animals (12 months or older) show a reduced behavioral and cellular response to light presented during the early portion of the dark phase of the cycle, we hypothesized that aging may alter the ability of PACAP and cAMP to modulate the phase shifting effects of light. In Expt. 1, we examined basal and light-induced cAMP accumulation at zeitgeber time 14 (ZT14 where ZT0 is the time of lights on). Light exposure resulted in a significant increase in cAMP accumulation in SCN tissue collected from young, but not middle-aged animals. The failure to see an increase in cAMP accumulation in the SCN of middle-aged animals may be related to the fact that basal levels of cAMP were elevated in the SCN of these animals at ZT14. In Expt. 2, we used an in vitro slice preparation of the SCN to determine if aging altered the ability of PACAP to stimulate cAMP accumulation in the SCN at ZT14. PACAP stimulated cAMP in the SCN of both young and middle-aged animals. However, PACAP-induced cAMP accumulation was lower in the SCN of middle-aged animals. Based on these results, we conclude that age-related changes in the responsiveness of the SCN to light input are due to: (1) changes in other input pathways capable of modulating cAMP, and (2) decreases in PACAP receptors in SCN neurons.

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

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

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

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

Glutamate blocks serotonergic phase advances of the mammalian circadian pacemaker through AMPA and NMDA receptors

The phase of the mammalian circadian pacemaker, located in the suprachiasmatic nucleus (SCN), is modulated by a variety of stimuli, most notably the environmental light cycle. Light information is perceived by the circadian pacemaker through glutamate that is released from retinal ganglion cell terminals in the SCN. Other prominent modulatory inputs to the SCN include a serotonergic projection from the raphe nuclei and a neuropeptide Y (NPY) input from the intergeniculate leaflet. Light and glutamate phase-shift the SCN pacemaker at night, whereas serotonin (5-HT) and NPY primarily phase-shift the pacemaker during the day. In addition to directly phase-shifting the circadian pacemaker, SCN inputs have been shown to modulate the actions of one another. For example, 5-HT can inhibit the phase-shifting effects of light or glutamate applied to the SCN at night, and NPY and glutamate inhibit phase shifts of one another. In this study, we explored the possibility that glutamate can modulate serotonergic phase shifts during the day. For these experiments, we applied various combinations of 5-HT agonists, glutamate agonists, and electrical stimulation of the optic chiasm to SCN brain slices to determine the effect of these treatments on the rhythm of spontaneous neuronal activity generated by the SCN circadian pacemaker. We found that glutamate agonists and optic chiasm stimulation inhibit serotonergic phase advances and that this inhibition involves both AMPA and NMDA receptors. This inhibition by glutamate may be indirect, because it is blocked by both tetrodotoxin and the GABA(A) antagonist, bicuculline.

The pituitary adenylate cyclase-activating polypeptide modulates glutamatergic calcium signalling: investigations on rat suprachiasmatic nucleus neurons

Circadian rhythms generated by the hypothalamic suprachiasmatic nucleus (SCN) are synchronized with the external light/dark cycle by photic information transmitted directly from the retina via the retinohypothalamic tract (RHT). The RHT contains the neurotransmitters glutamate and pituitary adenylate cyclase-activating polypeptide (PACAP), which code chemically for 'light' or 'darkness' information, respectively. We investigated interactions of PACAP and glutamate by analysing effects on the second messenger calcium in individual SCN neurons using the Fura-2 technique. PACAP did not affect NMDA-mediated calcium increases, but influenced signalling cascades of non-NMDA glutamate receptors, which in turn can regulate NMDA receptors. On the one hand, PACAP amplified/induced glutamate-dependent calcium increases by interacting with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate signalling. This was not related to direct PACAPergic effects on the second messengers cAMP and calcium. On the other hand, PACAP reduced/inhibited calcium increases elicited by glutamate acting on metabotropic receptors. cAMP analogues mimicked this inhibition. Most neurons displaying PACAPergic neuromodulation were immunoreactive for vasoactive intestinal polypeptide, which is a marker for retinorecipient SCN neurons. The observed PACAPergic effects provide a broad range of interactions that allow a fine-tuning of the endogenous clock by the integration of 'light' and 'darkness' information on the level of single SCN neurons.

Neural basis and biological function of masking by light in mammals: suppression of melatonin and locomotor activity

Light influences mammalian circadian rhythms in two different ways: (1) It entrains endogenous oscillators (clocks), which regulate physiology and behavior; and (2) it affects directly and often immediately physiology and behavior (these effects are also referred to as masking). Masking effects of light on pineal melatonin, locomotor activity, and the sleep-wake cycle in mammals and man are reviewed. They seem to represent a universal response in this group. The review reveals that the mechanism of photic inhibition of melatonin is fairly well understood, whereas only little is known about the influence of light on other circadian rhythm outputs, such as locomotor activity.

Dissociation between light-induced phase shift of the circadian rhythm and clock gene expression in mice lacking the pituitary adenylate cyclase activating polypeptide type 1 receptor

The circadian clock located in the suprachiasmatic nucleus (SCN) organizes autonomic and behavioral rhythms into a near 24 hr time that is adjusted daily to the solar cycle via a direct projection from the retina, the retinohypothalamic tract (RHT). This neuronal pathway costores the neurotransmitters PACAP and glutamate, which seem to be important for light-induced resetting of the clock. At the molecular level the clock genes mPer1 and mPer2 are believed to be target for the light signaling to the clock. In this study, we investigated the possible role of PACAP-type 1 receptor signaling in light-induced resetting of the behavioral rhythm and light-induced clock gene expression in the SCN. Light stimulation at early night resulted in larger phase delays in PACAP-type 1 receptor-deficient mice (PAC1(-)/-) compared with wild-type mice accompanied by a marked reduction in light-induced mPer1, mPer2, and c-fos gene expression. Light stimulation at late night induced mPer1 and c-fos gene expression in the SCN to the same levels in both wild type and PAC1(-)/- mice. However, in contrast to the phase advance seen in wild-type mice, PAC1(-)/- mice responded with phase delays after photic stimulation. These data indicate that PAC1 receptor signaling participates in the gating control of photic sensitivity of the clock and suggest that mPer1, mPer2, and c-fos are of less importance for light-induced phase shifts at night.

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.

Correlative association between N-methyl-D-aspartate receptor-mediated expression of period genes in the suprachiasmatic nucleus and phase shifts in behavior with photic entrainment of clock in hamsters

Because the rapid induction of Period (Per) genes is associated with the photic entrainment of the biological clock, we examined whether N-methyl-D-aspartate (NMDA) receptors were involved in the photic induction of Per genes in the hamster suprachiasmatic nucleus (SCN). In situ hybridization observation revealed that light during the early subjective night [circadian time (CT) 13.5] or the late subjective night (CT20) caused an induction of Per1 and Per2 but not Per3 mRNA in the SCN. Photic induction of Per mRNA at CT13.5 was observed especially in the ventrolateral SCN, whereas that at CT20 was more widespread from the ventrolateral to the dorsal SCN. A noncompetitive NMDA receptor antagonist, +MK801, dose-dependently (0. 1-5.0 mg/kg) suppressed only the ventrolateral part of Per1 and Per2 mRNA induction by light at CT13.5 or CT20 in the SCN. The suppressive effects of +MK801 on Per mRNA strongly correlated with the attenuating action of this compound on phase shifts by light at both CT13.5 and CT20. A competitive NMDA receptor antagonist, D-2-amino-5-phosphonovalerate (D-APV), also exhibited inhibitory actions on light (CT20)-induced Per1 and Per2 mRNA expression in the ventrolateral SCN. Furthermore, local injection of NMDA into the SCN resulted in the induction of Per1 and Per2 mRNA in the SCN. Among NMDA receptors, NR2B and NR2C mRNA were expressed in the ventrolateral and dorsal SCN, respectively. These results suggest that the activation of NMDA receptor is a critical step for photic induction of Per1 and Per2 transcripts in the SCN, which are linked to a photic behavioral entrainment.

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.

Daily changes of cytochrome oxidase activity within the suprachiasmatic nucleus of the Syrian hamster

Cytochrome oxidase (CO) activity was studied over a 24-h period in the Syrian hamster suprachiasmatic nucleus (SCN) (site of the biological clock), anterior hypothalamic area (AHA), and motor cortex. The SCN CO activity was highest at the middle of the day (Zeitgeber time (ZT) 05), decreased at the end of the light period (ZT 10) and continued at a low level during the night (ZT 13, 16, 21.5 and 24). AHA and motor cortex showed a similar profile of CO activity and no changes of CO activity were found in animals maintained under darkness (DD). We propose that photic input plays a role in the SCN neuronal activity that modulates metabolic activity on this area.

Neuropeptide Y phase advances the in vitro hamster circadian clock during the subjective day with no effect on phase during the subjective night

The mammalian daily (circadian) clock is located in the suprachiasmatic nuclei of the hypothalamus. Clock function can be detected by the measurement of the circadian change in spontaneous firing rate of suprachiasmatic nuclei cells in a brain slice preparation in vitro. We investigated the effects of neuropeptide Y on this rhythm of firing rate in hamster suprachiasmatic nuclei neurons. Slices were prepared using standard techniques. On the 1st day in vitro, neuropeptide Y (200 ng/200 nL; 47 pmol) was applied as a microdrop to the suprachiasmatic nuclei region at various times. Spontaneous single-unit firing was measured for 6-12 h on the 2nd day in vitro. Peak firing rate in treated slices was compared with that of untreated control slices to measure phase shifts induced by the peptide. Neuropeptide Y induced phase advances of circa-3h when applied during the subjective day (ZT 2-10) but did not significantly alter phase when applied during the subjective night. The phase shifts to neuropeptide Y in the hamster tissue in vitro are similar in phase dependency and magnitude to shifts measured in vivo.Key words: circadian, neuropeptide Y, rhythm, suprachiasmatic.

Role of membrane conductances and protein synthesis in subjective day phase advances of the hamster circadian clock by neuropeptide Y

Neurons of the mammalian circadian pacemaker in the hypothalamic suprachiasmatic nuclei exhibit a rhythm in firing rate that can be reset by neuropeptide Y. We recorded the effects of neuropeptide Y on Na+ and K+ conductances of hamster suprachiasmatic nuclei neurons using whole-cell, perforated-patch and cell-attached patch-clamp recordings, both in dissociated and brain slice preparations. While neuropeptide Y had no effect on voltage-gated Na+ currents, neuropeptide Y activated a leak K+ current. Neuropeptide Y phase advances in the suprachiasmatic nuclei brain slice preparation were blocked by a number of K+ channel blockers (tetraethylammonium chloride, dendrotoxin-I, glybenclamide). However, a K+ ionophore, valinomycin, did not shift the rhythm. The inhibition by tetraethylammonium chloride did not persist in the presence of glutamatergic receptor blockers. We have previously shown that glutamate can oppose neuropeptide Y phase-shifting actions, suggesting that K+ channel inhibition acts by inducing glutamate release. Protein synthesis inhibitors had no effect on clock phase when applied during the subjective day, and had no influence on neuropeptide Y-induced phase shifts. On the other hand, glutamate's ability to inhibit neuropeptide Y shifts was abolished by protein synthesis inhibition. Thus, while neuropeptide Y phase shifts do not require protein synthesis, glutamate blocks neuropeptide Y shifts via increased gene expression during the subjective day, at a time when it does not reset the clock. These results indicate that neuropeptide Y phase shifts via a mechanism that does not involve changes in membrane conductance or protein synthesis.

Involvement of glutamate release in substance P-induced phase delays of suprachiasmatic neuron activity rhythm in vitro

The suprachiasmatic nucleus (SCN) has been identified as a mammalian circadian rhythm clock. Treatment with substance P (SP) at zeitgeber time 13-14 produced phase delays of circadian rhythm in spontaneous neural activity in SCN neurons in vitro. SP-induced phase delays are blocked by treatment with not only SP receptor antagonist, spantide, but N-methyl-D-aspartate receptor antagonist, MK-801. In the biochemical experiment, we demonstrated that SP-induced glutamate release from the SCN slices was observed by the high-performance liquid chromatography assay. The present results suggest that glutamate release may be involved in SP-induced phase delays.

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.