N-methyl-D-aspartate receptor participates in neuronal transmission of photic information through the retinohypothalamic tract

Traumatic brain injury-induced disruption of the circadian clock

Disturbances in the circadian rhythm have been reported in patients following traumatic brain injury (TBI). However, the rhythmic expression of circadian genes in peripheral blood leukocytes (PBL) following TBI has not yet been studied. The messenger ribonucleic acid (mRNA) expression of period 1 (Per1), Per2, Per3, cryptochrome 1 (Cry1), Cry2, brain and muscle aryl hydrocarbon receptor nuclear translocator-like 1 (Bmal1), and circadian locomotor output cycles kaput (Clock) was quantified in PBLs from sham-operated rats and rats with acute subdural hematoma (ASDH) over a 48-h period. The rectal temperature of the animals was measured every 4 h over 2 days. The mesor, rhythm, amplitude, and acrophase were estimated using cosinor analysis. Cosinor analysis revealed that Per2, Cry1, and Bmal1 mRNAs were rhythmically expressed in the PBLs of sham-operated rats. In contrast, fluctuations in rhythmic expression were not observed following ASDH. The rectal temperature of sham-operated rats also exhibited rhythmicity. ASDH rats had a disrupted rectal temperature rhythm, a diminished amplitude, and an acrophase shift. TBI with ASDH results in dysregulated expression of some circadian genes and changes in body temperature rhythm. Further research is required to understand the pathophysiology of altered circadian networks following TBI. KEY MESSAGES: First to investigate the mRNA expression of circadian genes in PBLs of ASDH rats. ASDH rats had disrupted rhythmicity of Per2, Cry1, and Bmal1 mRNA expression. Cosinor analysis showed that ASDH rats had a disrupted rectal temperature rhythm.

N-methyl-D-aspartate receptor regulates the circadian clock in megakaryocytic cells and impacts cell proliferation through BMAL1

Peripheral circadian clocks control cell proliferation and survival, but little is known about their role and regulation in megakaryocytic cells. N-methyl-D-aspartate receptor (NMDAR) regulates the central clock in the brain. The purpose of this study was to determine whether NMDAR regulates the megakaryocytic cell clock and whether the megakaryocytic clock regulates cell proliferation and cell death. We found that both the Meg-01 megakaryocytic cell line and native murine megakaryocytes expressed circadian clock genes. Megakaryocyte-directed deletion of Grin1 in mice caused significant disruption of the circadian rhythm pathway at the transcriptional level and increased expression of BMAL1 at the protein level. Similarly, both pharmacological (MK-801) and genetic (GRIN-/-) inhibition of NMDAR in Meg-01 cells in vitro resulted in widespread changes in clock gene expression including increased expression of BMAL1, the core clock transcription factor. BMAL1 overexpression reduced Meg-01 cell proliferation and altered the time-dependent expression of the cell cycle regulators MYC and WEE1, whereas BMAL1 knockdown led to increased cell death in Meg-01-GRIN1-/- cells. Our results demonstrate that NMDAR regulates the circadian clock in megakaryocytic cells and that the circadian clock component BMAL1 contributes to the control of Meg-01 cell proliferation and survival.

IL-1β induces changes in expression of core circadian clock components PER2 and BMAL1 in primary human chondrocytes through the NMDA receptor/CREB and NF-κB signalling pathways

The circadian clock is a specialised cell signalling circuit present in almost all cells. It controls the timing of key cell activities such as proliferation and differentiation. In osteoarthritis, expression of two components of the circadian clock, BMAL1 and PER2 is altered in chondrocytes and this change has been causally linked with the increase in proliferation and altered chondrocyte differentiation in disease. IL-1β, an inflammatory cytokine abundant in OA joints, has previously been shown to induce changes in BMAL1 and PER2 expression in chondrocytes. The purpose of this study is to identify the mechanism involved. We found IL-1β treatment of primary human chondrocytes led to activation of NMDA receptors as evidenced by an increase in phosphorylation of GluN1 and an increase in intracellular calcium which was blocked by the NMDAR antagonist MK801. Levels of phosphorylated CREB were also elevated in IL-1β treated cells and this effect was blocked by co-treatment of cells with IL-1β and the NMDAR antagonist MK-801. Knockdown of CREB or inhibition of CREB activity prevented the IL-1β induced increase in PER2 expression in chondrocytes but had no effect on BMAL1. Phosphorylated p65 levels were elevated in IL-1β treated chondrocytes indicating increased NF-κB activation. Inhibition of NF-κB activity prevented the IL-1β induced reduction in BMAL1 expression and partially mitigated the IL-1β induced increase in PER2 expression in chondrocytes. These data indicate that the NMDAR/CREB and NF-κB signalling pathways regulate the core circadian clock components PER2 and BMAL1 in chondrocytes. Given that changes in expression of these clock components have been observed in a wide range of diseases, these findings may be broadly relevant for understanding the mechanism leading to circadian clock changes in pathology.

Desflurane anesthesia shifts the circadian rhythm phase depending on the time of day of anesthesia

Desflurane is one of the most frequently used inhalational anesthetics in clinical practice. A circadian rhythm phase-shift after general anesthesia with sevoflurane or isoflurane has been reported in mice, but few studies have reported this effect with desflurane. In the present study, we examined the rest/activity rhythm of mice by counting the number of running wheel rotations, and we found that desflurane anesthesia caused a phase shift in the circadian rhythm that was dependent on the time of day of anesthesia. We also found that desflurane anesthesia altered the relative mRNA expression of four major clock genes (Per2, Bmal, Clock, and Cry1) in the suprachiasmatic nucleus (SCN). These results are important for elucidating the effects of desflurane on the SCN, which is the master clock for the mammalian circadian rhythm. Further studies on the relationship between anesthesia and circadian rhythm may lead to the prevention and treatment of postoperative complications related to circadian rhythms.

Effects of Different Light Sources on Neural Activity of the Paraventricular Nucleus in the Hypothalamus

Background and Objectives: Physical function is influenced by light irradiation, and interest in the influence of light irradiation on health is high. Light signals are transmitted from the retina to the suprachiasmatic nucleus (SCN) via the retinal hypothalamic tract as non-image vision. Additionally, the SCN projects a nerve to the paraventricular nucleus (PVN) which acts as a stress center. This study examined the influences of three different light sources on neural activity in the PVN region using two different color temperatures. Materials and Methods: Experiments were conducted using twenty-eight Institute of Cancer Research (ICR) mice (10 week old males). Three light sources were used: (1) organic light-emitting diode (OLED) lighting, (2) LED lighting, and (3) fluorescent lighting. We examined the effects of light irradiation from the three light sources using two different color temperatures (2800 K and 4000 K). Perfusion was done 60 min after light irradiation, and then the brain was removed from the mouse for an immunohistochemistry analysis. c-Fos was immunohistochemically visualized as a marker of neural activity in the PVN region. Results: The number of c-Fos-positive cells was found to be significantly lower under OLED lighting and LED lighting conditions than under fluorescent lighting at a color temperature of 2800 K, and significantly lower under OLED lighting than LED lighting conditions at a color temperature of 4000 K. Conclusions: This study reveals that different light sources and color temperatures alter the neural activity of the PVN region. These results suggest that differences in the light source or color temperature may affect the stress response.

Multilevel Interactions of Stress and Circadian System: Implications for Traumatic Stress

The dramatic fluctuations in energy demands by the rhythmic succession of night and day on our planet has prompted a geophysical evolutionary need for biological temporal organization across phylogeny. The intrinsic circadian timing system (CS) represents a highly conserved and sophisticated internal "clock," adjusted to the 24-h rotation period of the earth, enabling a nyctohemeral coordination of numerous physiologic processes, from gene expression to behavior. The human CS is tightly and bidirectionally interconnected to the stress system (SS). Both systems are fundamental for survival and regulate each other's activity in order to prepare the organism for the anticipated cyclic challenges. Thereby, the understanding of the temporal relationship between stressors and stress responses is critical for the comprehension of the molecular basis of physiology and pathogenesis of disease. A critical loss of the harmonious timed order at different organizational levels may affect the fundamental properties of neuroendocrine, immune, and autonomic systems, leading to a breakdown of biobehavioral adaptative mechanisms with increased stress sensitivity and vulnerability. In this review, following an overview of the functional components of the SS and CS, we present their multilevel interactions and discuss how traumatic stress can alter the interplay between the two systems. Circadian dysregulation after traumatic stress exposure may represent a core feature of trauma-related disorders mediating enduring neurobiological correlates of trauma through maladaptive stress regulation. Understanding the mechanisms susceptible to circadian dysregulation and their role in stress-related disorders could provide new insights into disease mechanisms, advancing psychochronobiological treatment possibilities and preventive strategies in stress-exposed populations.

Altered N-methyl D-aspartate receptor subunit expression causes changes to the circadian clock and cell phenotype in osteoarthritic chondrocytes

The chondrocyte circadian clock is altered in osteoarthritis. This change is implicated in the disease-associated changes in chondrocyte phenotype and cartilage loss. Why the clock is changed is unknown. N-methyl-D-aspartate receptors (NMDAR) are critical for regulating the hypothalamic clock. Chondrocytes also express NMDAR and the type of NMDAR subunits expressed changes in osteoarthritis.

OBJECTIVE

To determine if NMDAR regulate the chondrocyte clock and phenotype.

DESIGN

Chondrocytes isolated from macroscopically-normal (MN) and osteoarthritic human cartilage were treated with NMDAR antagonists or transfected with GRIN2A or GRIN2B-targetting siRNA. H5 chondrocytes were transfected with GluN2B-expression plasmids. Clock genes and chondrocyte phenotypic markers were measured by RT-qPCR.

RESULTS

PER2 amplitude was higher and BMAL1 amplitude lower in osteoarthritic compared to MN chondrocytes. In osteoarthritic chondrocytes, NMDAR inhibition restored PER2 and BMAL1 expression to levels similar to MN chondrocytes, and resulted in reduced MMP13 and COL10A1. Paradoxically, NMDAR inhibition in MN chondrocytes resulted in increased PER2, decreased BMAL1 and increased MMP13 and COL10A1. Osteoarthritic, but not MN chondrocytes expressed GluN2B NMDAR subunits. GluN2B knockdown in osteoarthritic chondrocytes restored expression of circadian clock components and phenotypic markers to levels similar to MN chondrocytes. Ectopic expression of GluN2B resulted in reduced BMAL1, increased PER2 and altered SOX9, RUNX2 and MMP13 expression. Knockdown of PER2 mitigated the effects of GluN2B on SOX9 and MMP13.

CONCLUSIONS

NMDAR regulate the chondrocyte clock and phenotype suggesting NMDAR may also regulate clocks in other peripheral tissues. GluN2B expression in osteoarthritis may contribute to pathology by altering the chondrocyte clock.

Precision Light for the Treatment of Psychiatric Disorders

Circadian timekeeping can be reset by brief flashes of light using stimulation protocols thousands of times shorter than those previously assumed to be necessary for traditional phototherapy. These observations point to a future where flexible architectures of nanosecond-, microsecond-, and millisecond-scale light pulses are compiled to reprogram the brain's internal clock when it has been altered by psychiatric illness or advanced age. In the current review, we present a chronology of seminal experiments that established the synchronizing influence of light on the human circadian system and the efficacy of prolonged bright-light exposure for reducing symptoms associated with seasonal affective disorder. We conclude with a discussion of the different ways that precision flashes could be parlayed during sleep to effect neuroadaptive changes in brain function. This article is a contribution to a special issue on Circadian Rhythms in Regulation of Brain Processes and Role in Psychiatric Disorders curated by editors Shimon Amir, Karen Gamble, Oliver Stork, and Harry Pantazopoulos.

The potential physiological crosstalk and interrelationship between two sovereign endogenous amines, melatonin and homocysteine

The antioxidant melatonin and the non-proteinogenic excitotoxic amino acid homocysteine (Hcy) are very distinct but related reciprocally to each other in their mode of action. The elevated Hcy level has been implicated in several disease pathologies ranging from cardio- and cerebro-vascular diseases to neurodegeneration owing largely to its free radical generating potency. Interestingly, melatonin administration potentially normalizes the elevated Hcy level, thereby protecting the cells from the undesired Hcy-induced excitotoxicity and cell death. However, the exact mechanism and between them remain obscure. Through literature survey we have found an indistinct but a vital link between melatonin and Hcy i.e., the existence of reciprocal regulation between them, and this aspect has been thoroughly described herein. In this review, we focus on all the possibilities of co-regulation of melatonin and Hcy at the level of their production and metabolism both in basal and in pathological conditions, and appraised the potential of melatonin in ameliorating homocysteinemia-induced cellular stresses. Also, we have summarized the differential mode of action of melatonin and Hcy on health and disease states.

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 melatonin in glaucoma: implications concerning pathophysiological relevance and therapeutic potential

Glaucoma is a frequent ophthalmologic condition leading to chronic progressive optic neuropathy, which can result in visual impairment and blindness. In addition, glaucoma is associated with a dysregulation of circadian rhythms, as well as with a high incidence of sleep disorders, depression, and anxiety. However, because of their high comorbidity in older age, these conditions have not received much scientific attention and are often undertreated. In the current paper, we review the available literature on the role of melatonergic mechanisms in glaucoma, regulation of circadian rhythms, and depression. The literature is presented as a narrative review, providing an overview on the most important and clinically relevant publications. Recently, there has been evidence for a progressive loss of intrinsically photosensitive retinal ganglion cells (ipRGC) because of oxidative stress in glaucoma. As ipRGC are responsible for the photic transduction to the circadian system and subsequent melatonin secretion, and melatonin is involved in the pathophysiology of circadian desynchronization, sleep disorder, and depression, an impairment of photo-dependent melatonergic signaling may be a common pathway connecting glaucoma with these comorbidities. This fact, as well as the proven retinal neuroprotective role of melatonin, suggests that melatonergic drugs provide a potentially promising treatment strategy supplementing the management of intraocular pressure by pharmacological and surgical measures. Additionally, multidisciplinary treatment focusing on depression and normalization of circadian rhythms might be beneficial for glaucoma patients. Furthermore, glaucoma might be a useful model for studying the pathophysiological interactions between the melatonergic, circadian, and mood systems.

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.

Transduction of light in the suprachiasmatic nucleus: evidence for two different neurochemical cascades regulating the levels of Per1 mRNA and pineal melatonin

The suprachiasmatic nucleus (SCN) contains a circadian clock and regulates melatonin synthesis in the pineal gland. Light exposure during the subjective night acutely increases the mRNA levels of the Period (Per)1 gene in the SCN and acutely suppresses melatonin levels in the pineal gland. Activation of N-methyl-D-aspartate (NMDA) receptors in the SCN has been demonstrated to phase-shift the circadian clock in a manner similar to light. We tested the hypothesis that activation of excitatory amino acid (EAA) receptors in the SCN mediates the acute effects of light on Per1 mRNA levels and pineal melatonin. NMDA, injected into the SCN of Syrian hamsters during the night, acutely suppressed melatonin levels in the pineal gland. Both the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP5) and the alpha-amino-3-hydroxy-5-methylisoxazoleproprionic acid (AMPA)/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) inhibited the light-induced increase of Per1 mRNA levels in the SCN. In the same animals, however, these antagonists had no effect on the ability of light to suppress pineal melatonin. These results support the hypothesis that EAA receptor activation in the SCN is necessary for the acute effects of light on Per1 mRNA levels. They also indicate that NMDA receptor activation in the SCN is sufficient but may not be necessary for the acute effects of light on pineal melatonin. These data suggest that there may be at least two different neurochemical cascades that transduce the effects of light in the SCN

Cardiovascular control by the suprachiasmatic nucleus: neural and neuroendocrine mechanisms in human and rat

The risk for cardiovascular incidents is highest in the early morning, which seems partially due to endogenous factors. Endogenous circadian rhythms in mammalian physiology and behavior are regulated by the suprachiasmatic nucleus (SCN). Recently, anatomical evidence has been provided that SCN functioning is disturbed in patients with essential hypertension. Here we review neural and neuroendocrine mechanisms by which the SCN regulates the cardiovascular system. First, we discuss evidence for an endogenous circadian rhythm in cardiac activity, both in humans and rats, which is abolished after SCN lesioning in rats. The immediate impact of retinal light exposure at night on SCN-output to the cardiovascular system, which signals 'day' in both diurnal (human) and nocturnal (rat) mammals with opposite effects on physiology, is discussed. Furthermore, we discuss the impact of melatonin treatment on the SCN and its potential medical relevance in patients with essential hypertension. Finally, we argue that regional differentiation of the SCN and autonomous nervous system is required to explain the multitude of circadian rhythms. Insights into the mechanisms by which the SCN affects the cardiovascular system may provide new strategies for the treatment of disease conditions known to coincide with circadian rhythm disturbances, as is presented for essential hypertension.

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.

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.

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.

Melatonin sees the light: blocking GABA-ergic transmission in the paraventricular nucleus induces daytime secretion of melatonin

Despite a pronounced inhibitory effect of light on pineal melatonin synthesis, usually the daily melatonin rhythm is not a passive response to the surrounding world. In mammals, and almost every other vertebrate species studied so far, the melatonin rhythm is coupled to an endogenous pacemaker, i.e. a circadian clock. In mammals the principal circadian pacemaker is located in the suprachiasmatic nuclei (SCN), a bilateral cluster of neurons in the anterior hypothalamus. In the present paper we show in the rat that bilateral abolition of gamma-aminobutyric acid (GABA), but not vasopressin, neurotransmission in an SCN target area, i.e. the paraventricular nucleus of the hypothalamus, during (subjective) daytime results in increased pineal melatonin levels. The fact that complete removal of the SCN results in a pronounced increase of daytime pineal mRNA levels for arylalkylamine N-acetyltransferase (AA-NAT), i.e. the rate-limiting enzyme of melatonin synthesis, further substantiates the existence of a major inhibitory SCN output controlling the circadian melatonin rhythm.

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.

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

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

Anatomical and functional demonstration of a multisynaptic suprachiasmatic nucleus adrenal (cortex) pathway

In view of mounting evidence that the suprachiasmatic nucleus (SCN) is directly involved in the setting of sensitivity of the adrenal cortex to ACTH, the present study investigated possible anatomical and functional connections between SCN and adrenal. Transneuronal virus tracing from the adrenal revealed first order labelling in neurons in the intermedio-lateral column of the spinal cord that were shown to receive an input from oxytocin fibres and subsequently second-order labelling in neurons of the autonomic division of the paraventricular nucleus. The latter neurons were shown to receive an input from vasopressin or vasoactive intestinal peptide (VIP) containing SCN efferents. The true character of this SCN input to second-order neurons was also demonstrated by the fact that third-order labelling was present within the SCN, vasopressin or VIP neurons. The functional presence of the SCN-adrenal connection was demonstrated by a light-induced fast decrease in plasma corticosterone that could not be attributed to a decrease in ACTH. Using intact and SCN-lesioned animals, the immediate decrease in plasma corticosterone was only observed in intact animals and only at the beginning of the dark period. This fast decrease of corticosterone was accompanied by constant basal levels of blood adrenaline and noradrenaline, and is proposed to be due to a direct inhibition of the neuronal output to the adrenal cortex by light-mediated activation of SCN neurons. As a consequence, it is proposed that the SCN utilizes neuronal pathways to spread its time of the day message, not only to the pineal, but also to other organs, including the adrenal, utilizing the autonomic nervous system.

Effect of the menstrual cycle stage on the melatonin suppression by dim white light

Patients with bipolar disorder have been shown to have a supersensitive melatonin suppression to dim white light (200 and 500 lux) compared to normal healthy subjects. Previous studies suggest menstrual cycle dependent changes in the melatonin rhythm, but it is not known if the melatonin sensitivity to light changes during the menstrual cycle. The present study investigated the melatonin suppression to dim white light (200 lux) in different stages of the menstrual cycle. No significant differences in the percent suppression of melatonin were found across the stages of the menstrual cycle (p = .97). Our findings suggest that the menstrual cycle hormonal changes do not affect the melatonin sensitivity to dim light in healthy controls.

A role for serotonin in the circadian system revealed by the distribution of serotonin transporter and light-induced Fos immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet

Components of the circadian system, the suprachiasmatic nucleus and the intergeniculate leaflet receive serotonin input from the raphe nuclei. Manipulations of serotonin neurotransmission disrupt cellular, electrophysiological, and behavioural responses of the circadian system to light, suggesting that serotonin plays a modulatory role in photic regulation of circadian rhythms. To study the relation between serotonin afferents and light-activated cells in the suprachiasmatic nucleus and intergeniculate leaflet, we used immunostaining for the serotonin transporter and for the transcription factor, Fos. Serotonin transporter, a plasma membrane protein located on serotonin neurons, regulates the amount of serotonin available for neurotransmission by re-accumulating released serotonin into presynaptic neurons; expression of Fos in the suprachiasmatic nucleus identifies light-activated cells involved in photic resetting of circadian clock phase. In the suprachiasmatic nucleus, immunostaining for serotonin transporter revealed a dense plexus of fibres concentrated primarily in the ventrolateral region. In the intergeniculate leaflet, serotonin transporter immunostaining identified vertically-oriented columns of fibres. Serotonin transporter immunostaining was abolished by pretreatment with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Exposure to light for 30 min during the dark phase of the light cycle induced Fos expression in the ventrolateral suprachiasmatic nucleus and intergeniculate leaflet regions. In both structures the Fos-expressing cells were encircled by serotonin transporter-immunoreactive fibres often in close apposition to these cells. These results support the idea that serotonin activity plays a modulatory role in processing of photic information within the circadian system.

Quipazine and light have similar effects on c-fos induction in the rat suprachiasmatic nucleus

The effects of the serotonin agonist, quipazine, on the induction of c-fos in the suprachiasmatic nucleus of the rat was examined at different times of the 24 h cycle. Quipazine administered at night induced Fos production in a dose dependent manner (1, 3, 10, 30 mumol/kg) in the ventrolateral portion of the suprachiasmatic nucleus at ZT18. Administration of the highest dose at other times resulted in c-fos induction at ZT15 but not at other times of the day or subjective day examined (CT6 and ZT12). When compared to the effects of light pulses (2 lux/1 min), quipazine only caused c-fos induction at times when light caused induction. Our results support a role of serotonergic pathways in the transmission or modulation of photic information from the retina to the suprachiasmatic nucleus of the rat.

A blocker of nitric oxide synthase, NG-nitro-L-arginine methyl ester, attenuates light-induced Fos protein expression in rat suprachiasmatic nucleus

Nitric oxide (NO) serves as a messenger molecule in some of the neuronal systems that use glutamate as a transmitter. Because glutamate mediates the transmission of photic signals from retinal ganglion cell axons to the suprachiasmatic nucleus (SCN) circadian pacemaker, and because pharmacological treatments which block NO production by NO synthase (NOS) inhibit light-induced pacemaker phase-resetting, it has been proposed that NO is involved in circadian light signaling in the SCN. In the present study we investigated this hypothesis by assessing in rats the effect of treatment with the NOS blocker, NG-nitro-L-arginine methyl ester (L-NAME), on light-induced expression of the transcription factor Fos, a cellular marker of light signaling in the SCN. We found that systemic administration of L-NAME (100 mg/kg) but not of the inactive analog, D-NAME, significantly attenuates light-induced expression of Fos immunoreactivity in the SCN.

Direct vasoactive intestinal polypeptide-containing projection from the suprachiasmatic nucleus to spinal projecting hypothalamic paraventricular neurons

In mammals, photoperiodic information is conveyed from the retina to the pineal through a polysynaptic pathway, which includes the suprachiasmatic nucleus (SCN), the paraventricular nucleus of the hypothalamus (PVN), the spinal preganglionic neurons and, finally, the superior cervical ganglion. Precise data on the site in the PVN or which SCN transmitters are involved in the transmission of information in this pathway is lacking. In the present experiment we investigated whether SCN efferents containing vasoactive intestinal polypeptide (VIP) innervate PVN neurons that project to the spinal cord. A combination of retrograde tracing and immunocytochemistry with the aid of a confocal laser scanning microscope allowed us to assess possible interaction of SCN efferents and spinal cord projecting neurons in the PVN. Approximately 30% of identified autonomic projecting neurons in the dorsal PVN and 40% in the ventral PVN received VIP innervation mainly on their dendrites. These results provide further evidence for the involvement of SCN-derived VIP in the transmission of circadian information to the pineal.

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.

Presence of neuronal nitric oxide synthase in the suprachiasmatic nuclei of mouse and rat

Nitric oxide has recently been identified as a major neural regulator. It is synthesized by the enzyme nitric oxide synthase. Whilst considerable functional evidence has pointed to an involvement of nitric oxide in circadian regulation, all previous morphological studies have failed to demonstrate the presence of nitric oxide synthase in the mammalian suprachiasmatic nucleus. By use of an antibody directed against whole recombinant rat neuronal nitric oxide synthase we have identified the presence of immunoreactivity for this enzyme in the suprachiasmatic nucleus of mouse and rat, to provide the first detailed report of this enzyme in the suprachiasmatic nucleus for any species of mammal. Immunoreactivity for neuronal nitric oxide synthase was found in neurons throughout the suprachiasmatic nucleus of mice and no difference could be detected between the ventrolateral and dorsomedial parts of the nucleus in terms of the optical density of the immunostaining. Very small, rounded neuronal cell bodies were immunopositive. Electron microscopy revealed that these neurons had relatively large nuclei and scant cytoplasm containing relatively few organelles, which sometimes included some 100 nm dense-cored peptidergic vesicles. Only about 5% of such neurons were not detectably immunoreactive. By contrast, in the rat suprachiasmatic nucleus, a much smaller number of neurons were immunopositive and these cells were aggregated in the ventrolateral part of the nucleus. The immunoreactive neurons were bipolar cells with scanty cytoplasm. Electron microscopy revealed diffuse immunoreactivity in the cytosol, but not within any organelles. In the surrounding neuropil immunoreactive dendrites and axons mingled with much larger numbers of immunonegative processes, but immunoreactive boutons were only identified just outside the dorsal margin of the nucleus. Astrocytes, oligodendrocytes and endothelial cells in the suprachiasmatic nucleus were immunonegative. NADPH-diaphorase activity was not detectable in the suprachiasmatic nucleus of either mouse or rat. This morphological evidence for nitric oxide synthase-immunoreactive cells in the suprachiasmatic nucleus supports the existing functional evidence for an involvement of nitric oxide in the transmission of light-induced signals to the suprachiasmatic nucleus in mammals.

Effect of NMDA receptor blockade on melatonin and activity rhythm responses to a light pulse in rats

The possible role of the excitatory amino acids as mediators of the acute suppression and subsequent delay by light of pineal melatonin production was studied in rats using the NMDA receptor antagonist MK-801. Saline or MK-801 in doses up to 3 mg/kg (IP), was administered 15 min before a 15-min light pulse (200 lx), 4 h after dark onset, and the excretion of 6-sulphatoxymelatonin (aMT.6S) determined. Under these conditions saline injected/light exposed animals exhibited an acute, total but transient suppression of urinary aMT.6S excretion and a delay in the onset of aMT.6S the following night of 1.5 +/- 0.2 h. MK-801 failed to block either the acute or phase delaying effect of light (onset delayed by 2.2 +/- 0.4 h). Pretreatment with MK-801 (3 mg/kg) failed to block the effects of shorter, less intense light pulses 15 min before the pulse (e.g., 1 min/2 lx; onset delayed by 2.0 +/- 0.4 h following saline, 1.5 +/- 0.1 h following MK-801) or 60 min before a short duration low intensity pulse. In other experiments MK-801 (1 and 3 mg/kg) failed to affect aMT.6S excretion when injected in the dark at the time of lights out or 4 h after dark onset. NMDA (10 and 30 mg/kg) injection at the time of lights out or 4 h after darkness did not mimic the effects of a light pulse by decreasing aMT.6S excretion or causing a delay in the onset of excretion the following night. Finally MK-801 (3 mg/kg) injected 4 h after dark failed to block the phase delaying effects of a 15 min light pulse (200 lx) on running activity in rats. These results do not support the hypothesis that excitatory amino acids in the retino-hypothalamic tract acting on the NMDA receptor subtype and terminating in the suprachiasmatic nucleus mediate the photic influences upon rat pineal melatonin and activity rhythms.

Distribution of NADPH-diaphorase staining and light-induced Fos expression in the rat suprachiasmatic nucleus region supports a role for nitric oxide in the circadian system

Nitric oxide serves as a messenger molecule in some neuronal systems that use glutamate as a transmitter and it has been shown that glutamate mediates the transmission of photic signals by retinal ganglion cell axons terminating in the hypothalamic suprachiasmatic nucleus, site of the circadian pacemaker in rodents. Recent experiments have demonstrated that pharmacological treatments which block nitric oxide synthesis by nitric oxide synthase prevent glutamate-induced phase shifts of the cell firing rhythm in suprachiasmatic nucleus slice preparation in vitro; similar treatments were found to inhibit light transmission to the suprachiasmatic nucleus as well as light-induced phase shifts in activity rhythms in vivo, implicating nitric oxide in circadian light signalling in vivo. There is limited information, however, about the presence and function of nitric oxide synthase-containing neurons within retinorecipient regions of the rodent suprachiasmatic nucleus. In the present study we used NADPH-diaphorase histochemistry and immunostaining for the nuclear phosphoprotein Fos to assess the co-distribution of nitric oxide synthase-containing neurons and light-responsive cells in the rat suprachiasmatic nucleus region. A strong convergence between NADPH-diaphorase-stained cell bodies and fibres and cells that expressed Fos in response to photic stimulation was noted in the anterior periventricular nucleus, suprachiasmatic preoptic nucleus, retrochiasmatic area, the inter-suprachiasmatic nucleus region, and the dorsal aspect of the optic chiasm, below the suprachiasmatic nucleus. A similar convergence between NADPH-diaphorase-stained fibres and Fos-immunoreactive cells was noted inside the suprachiasmatic nucleus, but the number of NADPH-diaphorase-stained elements found in this region was substantially low compared with that found in retinorecipient regions bordering the nucleus. In many cases both inside and outside the suprachiasmatic nucleus, the Fos-immunoreactive cells appeared to make direct contact with NADPH-diaphorase-stained cells or fibres, but no co-localization of Fos immunoreactivity and NADPH-diaphorase histochemical activity within individual cells was detected. Extensive co-distribution of NADPH-diaphorase-stained cells and fibres and cells that express Fos in response to photic stimulation in the suprachiasmatic nucleus region is in line with the hypothesis that nitric oxide participates in the mechanism mediating circadian light signalling in the suprachiasmatic nucleus. However, lack of co-localization of the two markers to individual cells rules out the possibility that retinorecipient cells in the suprachiasmatic region synthesize and release nitric oxide when photically-activated. Instead, the results support the possibility that photic stimulation triggers nitric oxide synthesis in nitric oxide synthase-containing neurons located near the photically-activated cells.

Nitric oxide synthase in the hypothalamic suprachiasmatic nucleus of rat: evidence from histochemistry, immunohistochemistry and western blot; and colocalization with VIP

Nitric oxide (NO) is a neuroactive substance of high potency. Physiological results revealed the involvement of NO in circadian regulation of rats. Since neuronal structures containing NO-synthase (NOS) were previously not found in the circadian oscillator, the hypothalamic suprachiasmatic nucleus (SCN), in this species but are present in the hamster, we investigated the distribution of NO-producing structures in the rat SCN by Western blot analysis, immunohistochemistry of NOS, and by histochemistry (NADPH-diaphorase (NADPH-d) activity of NOS). Western blot analysis of SCN homogenates from rat (and, for comparison, hamster) showed a NOS-like immunoreactive (-LI) protein band of apparent molecular mass of 150 kDa, consistent with the neuronal NOS molecule. In the rat SCN, perikarya exhibiting NADPH-d staining of NOS-LI with a complete overlapping of both were found. Double-immunofluorescence experiments revealed that NOS cells are a subgroup of the neuronal SCN population that is characterized by immunoreactivity to vasoactive intestinal polypeptide. These data provide evidence for the existence of neuronal nitric oxide synthase in the rat SCN and may explain the involvement of NO in the mediation of photic information.

Red-light-induced suppression of melatonin synthesis is mediated by N-methyl-D-aspartate receptor activation in retinally normal and retinally degenerate rats

Pineal gland N-acetyltransferase (NAT) activity and pineal and serum levels of melatonin declined linearly in albino rats acutely exposed to different intensities of red light (600 nm or higher; low, 140 microW/cm2; moderate, 690 microW/cm2; high, 1200 microW/cm2) during the middle of the night. The high intensity red light was as effective as white light (780 microW/cm2) in suppressing NAT activity and pineal and circulating melatonin. Red-light-inhibited nighttime NAT activity and suppressed nocturnal melatonin levels in both retinally degenerate and normal rats. Pretreatment with the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (10 mg/kg intraperitoneally) completely prevented the red-light-induced inhibition of nighttime melatonin synthesis. Magnesium chloride (300 mg/kg intraperitoneally) reduced the inhibitory effects of low and moderate intensities of red light but was ineffective when high red-light intensity was used. However, both agents failed to antagonize the suppression of nighttime melatonin synthesis elicted by the exposure to white light. Since retinally degenerate and retinally normal animals respond in the same way to both red-light and pharmacological intervention with the NMDA receptor blocker MK-801, the findings indicate that the activation of central hypothalamic NMDA receptors might mediate the photic inhibition of nocturnal melatonin synthesis in the pineal gland elicited by the exposure to red light at night. Red-light-induced suppression of nocturnal melatonin synthesis possibly can be used to investigate the biochemical mechanisms by which light entrains melatonin synthesis and to study the pharmacological and physiological effects of endogenous and synthetic agents that antagonize the NMDA receptor response.

Effects of N-methyl-D-aspartate (NMDA) on seasonal cycles of reproduction, body weight and pelage colour in the male Siberian hamster

Siberian hamsters (Phodopus sungorus) transferred from stimulatory photoperiods (long days: LD) to inhibitory photoperiods (short days: SD) undergo testicular regression within 8 weeks. This reproductive response to photoperiod was blocked by systemic daily treatment with the glutamatergic agonist N-methyl-D-aspartate (NMDA: 20 mg/kg BW, sc). This powerful effect of NMDA demonstrates the potential for endogenous glutamate to regulate reproductive function. The overall aim of the subsequent studies was to investigate the site and mechanism of action of this glutamatergic agonist in order to identify potential mechanisms through which endogenous glutamate might act. To investigate whether the effect of systemic NMDA was via an effect on the circadian timing system, alterations in gonadal regression and recrudescence, seasonal coat changes (pelage) and body weight (BW) were examined. It would be predicted that long-term cycles of all these seasonal parameters would be affected if the action of NMDA were to perturb the transduction of photoperiodic information. Daily treatments with NMDA, which initially maintained reproductive function in hamsters exposed to SD, did not influence the time course of subsequent testicular recrudescence, nor did they influence long-term cycles of pelage and BW. Moreover, treatment with NMDA induced a dose-dependent increase in serum concentrations of LH within 15 min of systemic injection. These data are consistent with the hypothesis that systemic NMDA exerts it reproductive effects not via an action on the circadian system, but via an action on secretion of GnRH. To investigate potential central sites of action of glutamate, induction of the immediate early gene c-fos, an acute marker of cellular response, was evaluated immunocytochemically (ICC) in brain areas after treatment with NMDA. Although dual-label ICC studies revealed that NMDA did not induce c-fos within GnRH neurons, NMDA did induce c-fos in many cells in the region of the organum vasculosum of the lamina terminalis (OVLT), an area containing a large number of GnRH perikarya, and in the arcuate nucleus, a region close to GnRH secretory terminals in the median eminence. The lack of c-fos induction of GnRH cells argues against a direct effect of NMDA on GnRH neurons. Thus, we examined immunocytochemically the distribution of the common NMDAR1 glutamate receptor subunit to evaluate further the potential sites of glutamatergic action. As expected, NMDAR1-ir was widespread in perikarya throughout the brain, including the region of the OVLT and the arcuate nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitric oxide-synthesizing neurons in the hamster suprachiasmatic nucleus: a combined NOS- and NADPH- staining and retinohypothalamic tract tracing study

Neuronal nitric oxide (NO), thought to be a neuroactive substance of high potency, is produced by the enzyme nitric oxide synthase (NOS) which has been demonstrated to additionally exhibit a so-called NADPH-diaphorase (NADPH-d) activity. Since physiological results pointed to the involvement of NO in circadian regulation, and morphological descriptions are not available, we sought to study the distribution of NO-producing cells in the hypothalamic suprachiasmatic nucleus (SCN) in Djungarian hamsters (Phodopus sungorus) by means of histochemistry and immunohistochemistry (IHC). In the SCN, NADPH-d stained perikarya of varying intensity and number were found predominantly in the ventrolateral subdivision. Diaphorase staining combined with the IHC demonstration of NOS revealed a complete overlapping of both. The combination of NADPH-d staining with the demonstration of the retinohypothalamic tract using the anterograde neuronal transport of cholera toxin B (CTB) following intraocular injection showed CTB terminals accumulating at NADPH-d cell bodies mainly in the ventrolateral region of the SCN. These data provide morphological evidence for the involvement of nitric oxide in the mediation of photic stimulation of the circadian oscillator located in the SCN.

Circadian variations of amino acid content of suprachiasmatic nucleus in rats

The suprachiasmatic nucleus (SCN) has been known as an oscillator of circadian rhythm and recently some reports suggested that excitatory amino acids were functioning in this nucleus. The purpose of this study is to measure the physiological amino acid content of this nucleus, to estimate the circadian variation in amino acids and the effect of light on amino acid content in the SCN. The content of aspartate, asparagine, glycine and serine tended to be lower in the light phase compared to the dark, but was not significant. As to the light effect on amino acid content of the SCN, only aspartate decreased when a light pulse was applied. These results suggest that aspartate is involved in the transmission of photic information within the SCN.

Neurochemical organization of circadian rhythm in the suprachiasmatic nucleus

The circadian rhythm in mammals is under control of the pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus. This tiny nucleus contains a number of neurochemicals, including peptides, amines and amino acids. Heterogeneous distribution of these neurochemicals defines the substructures of the SCN. In the present review, functional significance of such neurochemical heterogeneity in the SCN is discussed in the light of circadian patterns of the concentrations of these neurochemicals in the SCN and their effects on SCN neurons in in vitro slice preparation. In particular, the hypothesis that the dorsomedial SCN is involved in maintaining the circadian rhythm, while the ventrolateral SCN is involved in adjusting the phase of the rhythm, is critically discussed. These considerations suggest that distinct sub-components of the SCN as marked by neurochemicals, interact with each other and this organizational architecture could be the basis of the proper operation of the circadian time keeping system in this nucleus.

c-fos mRNA in the suprachiasmatic nuclei in vitro shows a circadian rhythm and responds to a serotonergic agonist

The mammalian suprachiasmatic nuclei (SCN) contain a circadian clock that produces approximately 24 h rhythms of physiology and behavior even during constant dark. Under such conditions, light stimuli applied during the subjective night induce phase shifts of circadian rhythms and increase immediate early gene expression (c-fos) in the SCN. In vitro preparations of the SCN continue to show circadian rhythms of metabolic rate and neuronal firing rates, which can be phase shifted by non-photic stimuli. This study was designed to investigate whether the SCN display a rhythm of c-fos mRNA levels in vitro and whether quipazine, which phase-shifts the SCN circadian clock, induces c-fos expression in vitro. Levels of c-fos mRNA were found to be significantly higher in the subjective day than subjective night in the SCN in vitro. This rhythm parallels other in vivo and in vitro rhythms in SCN metabolic and neuronal activity and is consistent with previous in vivo work showing higher daytime levels of Fos-like immunoreactivity in animals maintained under constant dark conditions. Quipazine treatment during the subjective day (which phase-advances the circadian rhythm of neuronal firing in the SCN) decreased c-fos mRNA levels in the dorsomedial but not ventrolateral SCN, but quipazine did not affect c-fos levels when administered at night. This effect is consistent with serotonergic agonists inhibiting SCN neuronal activity and is the first evidence that a non-photic phase-shifting stimulus alters c-fos in the SCN at a phase-appropriate time.

The effects of electrical stimulation of the optic nerves and anterior optic chiasm on the circadian activity rhythm of the Syrian hamster: involvement of excitatory amino acids

The circadian pacemaker of the suprachiasmatic nuclei (SCN) is entrained to the environmental light-dark cycle via the retinohypothalamic tract (RHT). It is unknown whether light activates or suppresses firing of the retinal ganglion cells which mediate photic entrainment. We therefore electrically stimulated the optic nerves and the anterior optic chiasm of hamsters with free-running activity rhythms in continuous darkness. These electrical stimulations are thought to induce a release of neurotransmitter at the RHT terminals. Electrical stimulation mimicked the phase dependent shifts induced by light pulses. The phase shifts were significantly larger than the shifts induced by sham stimulation in the same animals or by electrical stimulation in animals with an electrode outside the optic nerves and chiasm. Our results indicate that the retinal ganglion cells which project to the SCN are activated by light. Intraperitoneal administration of MK-801, a non-competitive antagonist of the NMDA-receptor, attenuated the phase delays induced by electrical stimulation in the early subjective night. This suggests that an excitatory amino acid mediates the effects of light upon the circadian pacemaker.

NMDA receptor binding in rodent suprachiasmatic nucleus

NMDA receptors are thought to mediate effects of light on circadian rhythms and on immediate-early gene expression in the suprachiasmatic nucleus (SCN), the primary circadian pacemaker in mammals. The present study characterized NMDA receptors in autoradiographs of SCN incubated with the NMDA antagonist [3H]MK-801. In both rat and hamster, [3H]MK-801 binding did not delineate the SCN and was fairly uniformly distributed across the SCN region. Binding levels were unaffected by circadian time, light vs. dark conditions, or enucleation. Scatchard analyses revealed species differences in both receptor number and affinity in the SCN. The [3H]MK-801 binding sites characterized in this study could mediate the NMDA antagonist-sensitive effects of light on the SCN and circadian rhythms.

The circadian visual system

The retina transduces photic stimuli and transmits that information centrally for further processing. This review emphasizes the fact that the nervous system components governing circadian rhythmicity constitute a specialized subdivision of the vertebrate visual system. The brain houses different targets for retinal efferents parcellated according circadian or non-circadian function. Although the suprachiasmatic nucleus (SCN), being the site of the master circadian clock, is necessary for the generation of circadian rhythmicity, precise phase regulation of any rhythm is subject to modulation by SCN-afferent processes. Photic information necessary for entrainment arrives at the SCN via the retinohypothalamic tract. The geniculohypothalamic tract, originating in the intergeniculate leaflet (IGL), provides a secondary route by which photic information can reach the SCN. It also projects extensively to the contralateral IGL and receives reciprocal input from the SCN region. An interaction between the circadian and non-circadian visual systems may exist through connections of the superior colliculus with ventrolateral geniculate leaflet (VLG) and IGL. The SCN, IGL, VLG and superior colliculus are all innervated by serotonin-containing fibers. The following observations are likely to have an impact beyond the rhythm field itself: certain transneuronal tracers label only the circadian visual system; c-fos protein synthesis is induced in the circadian, but not non-circadian, visual system by a phasically active stimulus; blockade of SCN action potentials is unable to alter circadian rhythmicity; transplantation of dispersed fetal SCN cells to arrhythmic adults restores circadian periodicity, but not phase response to light; and the IGL is actually a very extensive part of the lateral geniculate complex.

Distribution of N-methyl D-aspartate (NMDA) receptor mRNAs in the rat suprachiasmatic nucleus

Photic entrainment of the circadian oscillator located in the hypothalamic suprachiasmatic nucleus (SCN) is considered to be mediated at least partly by release of glutamate from the retinal presynaptic nerve terminals acting via a NMDA receptor. Several NMDA receptor subtypes have been cloned and expressed in model systems. The NMDA-R1 subtype is essential for the function of the NMDA receptor, and the multiple NMDA-R2(-A, -B, or -C) subunits potentiate and differentiate the function of the NMDA receptor by forming different heteromeric configurations with NMDA-R1. The aim of this study was to use in situ hybridization histochemistry with oligonucleotide sequences (42-48-mer) labeled with 35S to detect whether NMDA receptor mRNA is present in the rat SCN, and if so, to characterize which receptor subtypes occur. In order to identify the precise location of NMDA receptor mRNAs within the SCN, sections were dipped in emulsion and cellular resolution was achieved. The hybridization revealed a high abundance of NMDA-R1 mRNA in the SCN as well as in many other forebrain areas. The NMDA-R1 expressing cells were distributed throughout the SCN. NMDA-R2A and NMDA-R2B mRNAs were found in the hippocampus, but not in the SCN. In contrast, NMDA-R2C mRNA was found in relative high amounts in the rat SCN, but not in other hypothalamic areas. In dipped sections, it was evident that the localization of NMDA-2RC was mostly confined to the dorsomedial part of the SCN. Thus, the rat SCN contains a specific combination of NMDA receptor mRNA subtypes not found in other forebrain structures. These observations are consistent with the hypothesis that glutamate mediates the effect of light on entrainment of the circadian oscillator.

Effects of excitatory amino acid receptor agonists and antagonists on the secretion of melatonin, luteinizing hormone and prolactin in the ram

To assess the role of excitatory amino acids (EAA) as neurotransmitters in the transmission of light information from the retina to the pineal gland, we have determined whether the systemic injection of EAA agonists in Soay rams will mimic the suppressive effect of light on the secretion of melatonin, and whether pretreatment of rams with EAA antagonists will block this effect. In addition, the efficacy of the drugs in affecting neuroendocrine systems was investigated by measuring the changes in the secretion of luteinizing hormone (LH) and prolactin. Injections fo the EAA receptor agonist, NMDA (N-methyl-D,L-aspartate: 4.0 mg/kg iv), and the non-NMDA type EAA receptor agonist, AMPA (DL-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid: 0.2 mg/kg iv) given at night to rams exposed to long days (16 h light: 8 h darkness), caused no change in the blood plasma concentrations of melatonin. The treatments induced an acute increase in the concentrations of LH, and NMDA, but not AMPA, caused a sustained increase in the concentrations of prolactin. Injections of the specific NMDA-type receptor antagonist, CGP (CGP 37849: 1.0 mg/kg iv) and the non-NMDA-type receptor antagonist, DNQX (6,7 Dinitroquinoxaline-2,3-dione: 0.5 mg/kg iv), given prior to a 1-h light period at night, in rams under long days, caused no change in the light-induced decrease in blood plasma concentrations of melatonin. The drug treatments had no effect on the plasma concentrations of LH, but CGP, and not DNQX, stimulated an acute increase in the plasma concentrations of prolactin. These results provide support for the hypothesis that EAA mechanisms operate in the hypothalamus to regulate the release of peptides and catecholamines which control the secretion of LH and prolactin from the pituitary gland; different sub-types of EAA receptors are involved in the control of the two pituitary hormones. The failure of the treatments to affect the secretion of melatonin may indicate that EAA receptor activation is not involved in the photic relay to the pineal gland, or may merely reflect the inability of the drugs to penetrate into the retina/SCN/pineal neural circuits to produce a response.

Photoperiod regulates the LH response to central glutamatergic stimulation in the male Syrian hamster

This study investigated central glutamatergic function in relation to photoperiodically-induced changes in the secretion of luteinizing hormone (LH). The experimental approach was to compare the central effects of glutamate agonists on LH secretion in reproductively active hamsters kept in long days (LD) with those in photoinhibited hamsters kept in short days (SD) for 6 weeks and having regressed testes. Agonists were delivered via a cannula into the III ventricle of freely moving hamsters, and blood samples collected 10 to 15 min after the start of the infusion. A high dose (3.0 nmole) of N-methyl-D-L-aspartate (NMDA) induced significant (P<0.01) increases in serum concentrations of LH in hamsters in both photoperiods, though the NMDA-induced increase relative to endogenous LH concentrations was greater in SD than in LD. However, a lower dose of NMDA (0.3 nmole revealed a difference in sensitivity. This dose significantly increased serum LH (P<0.05) in hamsters in SD but had no effect in those in LD. The seasonal difference in response to NMDA was compared with the response to an equimolar dose of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a non-NMDA agonist. This dose of AMPA (0.3 nmole) induced a two-fold increase (P<0.05) in serum concentrations of LH in hamsters in both photoperiods, relative to vehicle-treated controls. In a third experiment the dose-response effects of central AMPA on LH secretion were examined more closely. The sensitivity of LH secretion to stimulation with AMPA did not differ between SD- and LD-housed hamsters. Thus the photoperiod-related difference in sensitivity to stimulation with glutamate agonists is specific for NMDA receptor-mediated activation, rather than a passive reflection of differences in the capacity to secrete GnRH/LH in SD and LD photoperiods. To investigate the site of action of NMDA, the expression of the c-fos immediate-early gene, as assessed by immunocytochemistry for its protein product Fos, was used as a marker of neuronal activation, because previous studies in rodents indicate that a high proportion of GnRH neurons express c-fos at the time of the mid-cycle LH surge. NMDA induced widespread expression of c-fos in many periventricular regions including the medial preoptic area (POA) and ventromedial hypothalamic nucleus. However, dual ICC revealed that in neither photoperiod was Fos present in GnRH-positive neurons 1 h after infusion of 3 nmole of NMDA, despite the increases in LH secretion induced by the infusion. AMPA injected icv at doses which released LH did not enhance expression of c-fos in the hypothalamus. Thus, in the male, enhanced expression of c-fos cannot be detected in GnRH neurons at the time of increased secretion of this hormone induced by glutamate agonists. In conclusion, these results show that both NMDA and non-NMDA glutamatergic pathways potentially regulated LH secretion in the Syrian hamster. The increased sensitivity to NMDA but unaltered sensitivity to AMPA in photoinhibited hamsters in SD is consistent with the view that changes in photoperiod might induce specific alterations in NMDA-mediated pathways that ultimately regulate GnRH neurosecretory activity.

Processing of photic information within the intergeniculate leaflet of the lateral geniculate body: assessed by neuropeptide Y immunoreactivity in the suprachiasmatic nucleus of rats

Entrainment of the circadian pacemaker in the suprachiasmatic nucleus is accomplished by two neural pathways, the retinohypothalamic and geniculohypothalamic tracts. The geniculohypothalamic tract, which originates from the intergeniculate leaflet and a portion of the ventral lateral geniculate nucleus, is composed of fibers immunoreactive to neuropeptide Y. To assess the processing of photic information by the geniculohypothalamic tract, neuropeptide Y immunoreactivity in the suprachiasmatic nucleus of rats kept under various external lighting conditions was determined by enzyme immunoassay of micropunched tissues. Neuropeptide Y levels in the suprachiasmatic nucleus steadily increased when rats were exposed to continuous light and reached a peak in 2 h before returning to basal level. The amount of increase did not depend on intensity and duration of light exposure. A light pulse as short as 5 min elicited a similar rise in neuropeptide Y, indicating that the response is due to the sudden transition from dark to light. This response, however, was only observed when the dark to light transition occurred at circadian time 0 (subjective dawn) of the pacemaker. A light pulse at circadian time 0, which effectively induces the increase in neuropeptide Y level, does not significantly shift the phase of the circadian rhythm. This observation indicates that the photic pathway utilizing neuropeptide Y may be functional only when the endogenous circadian rhythm is synchronized to external light and dark cycles. Administration of an excitatory amino acid antagonist (MK-801) blocked the increase of neuropeptide Y by light, while an agonist (N-methyl-D-aspartate) induced similar facilitatory effects to that of light on the neuropeptide Y level in the rat suprachiasmatic nucleus. These results suggest that the geniculohypothalamic tract processes photic information so as to facilitate distinction of the transition between light and darkness that occurs either at subjective dawn or dusk.

Classical acetylcholine receptors do not play a direct role in neuronal transmission of photic information in the suprachiasmatic nucleus in rats

Acetylcholine is said to be involved in the neuronal transmission of photic information in the suprachiasmatic nucleus (SCN). The purpose of this study was to specify what subtypes of acetylcholine receptors play a major role using in vivo system and to examine the interaction of acetylcholine and excitatory amino acid receptors. To evaluate the effect of a selective agonist or antagonist for the nicotinic or muscarinic receptor on the neuronal transmission in the SCNT, N-acetyltransferase (NAT) activity in the pineal gland was measured after microinjection at this site. In the case of pretreatment with an antagonist, light stimulation was given after 20 min. Carbamilcholine chloride (carbachol) mimicked and only alpha-bungarotoxin (alpha-BTX) blocked the light effect; however, more selective agonists or antagonists were not effective. As for the interaction of these two cholinergic agents with an agonist or antagonist for N-methyl-D-aspartate (NMDA) receptor, alpha-BTX or D-2-amino-5-phosphonovalerate (D-APV) significantly blocked the suppressive effect of NMDA or carbachol, respectively. These data suggest that classical acetylcholine receptors do not play a direct role in neuronal transmission of photic information in the SCN in rats.