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

Bicuculline increases and muscimol reduces the phase-delaying effects of light and VIP/PHI/GRP in the suprachiasmatic region

The present study investigated the effects of gamma-amino butyric acid (GABA)A-active drugs on the ability of light or coadministration of vasoactive intestinal peptide (VIP), peptide histidine isoleucine (PHI), and gastrin-releasing peptide (GRP) to phase delay hamster activity rhythms. Microinjection of the GABAA agonist, muscimol, significantly (p < .01) reduced the phase-delaying effect of light administered at circadian time (CT) 13.5. By contrast, microinjection of the GABAA antagonist, bicuculline, significantly (p < .01) increased the phase-delaying effect of light administered at CT 13.5. Microinjection of muscimol or bicuculline into the suprachiasmatic nucleus (SCN) produced little or no effect on circadian phase when no light pulses were provided. Coadministration of muscimol with VIP/PHI/GRP significantly (p < .01) reduced the phase-delaying effect of VIP/PHI/GRP, whereas administration of bicuculline with VIP/PHI/GRP significantly (p < .05) increased the phase-delaying effect of these peptides. These data indicate that changes in GABAA activity within the SCN can modulate the phase-delaying effects of light and VIP/PHI/GRP during the early portion of subjective night.

Methamphetamine modifies the photic entraining responses in the rodent suprachiasmatic nucleus via serotonin release

We examined whether methamphetamine modifies the photic entraining responses in the rat suprachiasmatic nucleus. Optic nerve stimulation increased vasoactive intestinal polypeptide release from rat suprachiasmatic nucleus slices, and methamphetamine inhibited this increase in a concentration-dependent manner. Optic nerve stimulation has been reported to evoke field potentials in rat suprachiasmatic nucleus slices. Methamphetamine attenuated this field potential, and maximal inhibition (75.5%) was achieved at a concentration of 100 microM. Systemic administration of methamphetamine (1-5 mg/kg) inhibited light (300 lux, 1h)-induced Fos expression in the suprachiasmatic nucleus; methamphetamine at a dose of 5 mg/kg, i.p. caused 40% inhibition of light-induced Fos expression. We examined whether the inhibitory effect of methamphetamine on photic entraining responses mediates serotonin release from the suprachiasmatic nucleus. High-performance liquid chromatographic analysis revealed that methamphetamine application increased serotonin release from rat suprachiasmatic nucleus slices in a concentration-dependent manner, but did not affect noradrenaline release. In addition, reduction of serotonin content attenuated the effect of methamphetamine on field potential induced by optic nerve stimulation in vitro and also light-induced phase advances of wheel running activity rhythm in vivo. The present results support the idea that methamphetamine produces an inhibitory effect on photic entrainment in the suprachiasmatic nucleus via serotonin release.

N-acetylaspartylglutamate immunoreactivity in human retina

The acidic dipeptide N-acetylaspartylglutamate (NAAG), which satisfies many of the criteria for a neurotransmitter, was identified immunohistochemically within two human retinae. We observed NAAG immunoreactivity in retinal ganglion cells, their dendrites in the inner plexiform layer, and their axons in the optic nerve fiber layer. The vast majority of ganglion cells were stained, including displaced ganglion cells, ganglion cells of different sizes, and those whose dendrites arborized in the inner and outer sublaminae of the inner plexiform layer, that is, presumed On- and Off- cells. The sizes of labeled and unlabeled cells in the ganglion cell layer, as measured in counterstained material, suggest that the unlabeled cells consist primarily or only of displaced amacrine cells. We also saw immunoreactivity in small cells along the inner margin of the inner nuclear layer, presumably amacrine cells, and in small cells with little cytoplasm in the inner plexiform and ganglion cell layers, presumably displaced amacrine cells. These results are consistent with a role for NAAG in the transmission of visual information from the retina to the rest of the brain. Further, they are similar to those reported previously in rat, cat and monkey, thus demonstrating the relevance of previous studies to humans.

Substance P receptor regulates the photic induction of Fos-like protein in the suprachiasmatic nucleus of Syrian hamsters

Substance P (SP) is a candidate neurotransmitter or neuromodulator for conveying light information from the retina to the hypothalamic suprachiasmatic nucleus (SCN) where a circadian oscillator(s) is located in mammals. Immediate early gene c-fos has been demonstrated to be induced in the SCN with a brief light exposure at the subjective night, and suggested to play an important role in the photic entrainment of the oscillator. To clarify the possibility of an involvement of the SP receptor in the photic-induction of c-fos in the SCN, we examined effects of a SP receptor antagonist, spantide, on the light-induced Fos-like protein immunoreactivity (Fos-lir) in the SCN of Syrian hamster. The light-induced Fos-lir was inhibited with the pretreatment of spantide in a dose-related manner and in an anatomically distinctive way. The higher dose of spantide (8 nmol) blocked light-induced Fos-lir substantially in the rostral and central areas of the SCN, and in the dorsal portion of the caudal SCN. However, it blocked Fos-lir only slightly in the ventral portion of the caudal SCN. These results suggest that the SP is involved in conveying light information to induce Fos protein in the hamster SCN, and that different neurotransmitter systems are involved in the light-induced Fos-lir in the different portions of hamster SCN.

The intrinsic optical signal evoked by chiasm stimulation in the rat suprachiasmatic nuclei exhibits GABAergic day-night variation

Infrared light transmittance imaging was used in rat hypothalamic slices to record an intrinsic optical signal (IOS) of the cell ensemble in the suprachiasmatic nuclei (SCN), the locus of the endogenous circadian clock. Upon optic chiasm stimulation, a transient IOS was observed in an area conforming to the known retinohypothalamic tract innervation in the ventral SCN. An increase in extracellular Mg2+ concentration to 10 mM reduced th IOS, suggesting that the elicited IOS is dependent on synaptic transmission. D-2-amino-5-phosphonopentanoic acid and muscimol suppressed the elicited IOS, indicating that NMDA and GABAA receptor-mediated mechanisms were involved in cell ensemble activity reflected in the IOS. The extracellularly recorded spiking of SCN neurons located outside the IOS area remained largely unaffected by the afferent stimulus. Neurons located within the IOS area responded with a depressed electrical discharge, manifesting an inverse relationship between single-unit discharge and the optical measure. The influence of the endogenous circadian rhythm on the elicited IOS was assessed by carrying out daytime-dependent concentration-response experiments. NMDA and non-NMDA receptor specific compounds did not exhibit significant day-night differences, whereas GABA-specific ligands showed a significant day-night variation in activity. The competitive GABAA receptor antagonist bicuculline enhanced the IOS exclusively in the daytime SCN. 5alpha-Pregnane-3alpha,21-diol-20-one (allotetrahydrodeoxycorticosterone), a neuroactive steroid that potentiates GABAergic inhibition, suppressed the IOS in the night-time SCN more than in the daytime SCN. This suggests that in the rat the level of extracellular GABA is higher in night-time SCN compared to the daytime SCN.

Effects of aging on light-induced phase-shifting of circadian behavioral rhythms, fos expression and CREB phosphorylation in the hamster suprachiasmatic nucleus

Aging is associated with a variety of alterations in circadian rhythms, including changes in the response to environmental stimuli. The underlying causes for these age-related changes in the circadian system remain unknown. Recent studies have demonstrated that light induces the expression of Fos and phosphorylation of the cyclic-AMP response element-binding protein in the rodent suprachiasmatic nuclei, the location of a master circadian pacemaker in mammals, suggesting that these transcription factors may mediate the effects of light on the circadian clock. The purpose of this study was to determine the effects of aging upon light-induced phase-shifting of circadian locomotor activity rhythms, Fos protein expression and cyclic-AMP response element-binding protein phosphorylation in the suprachiasmatic nuclei. Young (three to four months) and old (18-22 months) male golden hamsters free-running in constant darkness were exposed to 5-min monochromatic light pulses of different irradiance levels, at circadian time 19, after which either steady-state phase shifts of locomotor activity rhythms were measured, or else immunocytochemistry for Fos or for phospho-cyclic-AMP response element-binding protein was performed. Old hamsters were approximately 20 times less sensitive to the phase-shifting effects of light on the activity rhythm, and the photic irradiance threshold for Fos-like immunoreactivity induction in the suprachiasmatic nuclei was elevated when compared to young animals. Aging was also associated with a deficit in cyclic-AMP response element-binding protein phosphorylation by light. These data indicate that there are dramatic changes in light-activated molecular responses in the suprachiasmatic nuclei of old hamsters, and suggest that these molecular changes may underlie age-related changes in the effects of light on the circadian clock system.

Chronobiotics--drugs that shift rhythms

A chronobiotic is defined and levels of action within the mammalian circadian pacemaker system, such as the retina, retinohypothalamic tract, geniculohypothalamic tract, suprachiasmatic nuclei, output and feedback systems are identified. Classes of drug that include the indoleamines, cholinergic agents, peptides, and benzodiazepines, which might act as chronobiotics within these levels, are evaluated. Particular emphasis is placed on the indole, melatonin (MLT). The clinical circumstances for use of chronobiotics in sleep disturbances of the circadian kind, such as jet lag, shift work, delayed sleep-phase syndrome, advanced sleep-phase syndrome, irregular and non-24-hr sleep-wake cycles, are described under reorganized headings of disorders of entrainment, partial entrainment, and desynchronization. Specific attention is given to the blind and the aged. Both human and animal studies suggest that MLT has powerful chronobiotic properties. MLT shows considerable promise as a prophylactic and therapeutic alternative or supplement to the use of natural and artificial bright light for resetting the circadian pacemaker. Throughout this discussion, the hypnotic and hypothermic versus the chronobiotic actions of MLT are raised. Finally, problems in the design of delivery systems for MLT are discussed.

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.

Circadian variation of EAAC1 glutamate transporter messenger RNA in the rat suprachiasmatic nuclei

Using in situ hybridization, we examined temporal changes of the EAAC1 glutamate transporter mRNA within the suprachiasmatic nuclei (SCN) of rats in constant darkness. Film autoradiographs showed that the SCN and supraoptic nuclei (SON) contained a marked density of hybridization signal. Analysis of silver grains per cell in emulsion-dipped sections indicated that cellular expression of EAAC1 mRNA in the SCN was elevated during the latter part of the subjective night and at the beginning of the subjective day, with a peak at circadian time 23.1 as determined by cosinor analysis. The times at which EAAC1 mRNA is highest correspond to the time points at which extracellular glutamate, a neurotransmitter that putatively mediates photic entrainment, has been reported to be low within the SCN. The presence of EAAC1 mRNA in the SCN and SON may partially explain the resistance of these nuclei to glutamate receptor-mediated excitotoxins; furthermore, the raised level preceding subjective dawn in the SCN may ensure sub-toxic levels of extracellular glutamate at the onset of photic stimulation during the LD cycle. In contrast, cellular expression of EAAC1 mRNA in the cingulate cortex and reticular thalamus remained constant at all time points studied. These results suggest that there is circadian control of the EAAC1 mRNA by the clock intrinsic to the SCN.

Circadian changes in long-term potentiation of rat suprachiasmatic field potentials elicited by optic nerve stimulation in vitro

Optic nerve stimulation caused a postsynaptic field potential in the rat suprachiasmatic nucleus (SCN) of hypothalamic slices. In the present experiment, we demonstrated whether tetanic stimulation of optic nerve can produce a long-term potentiation (LTP) in the SCN postsynaptic field potential. The amplitude of SCN field potential was higher in the subjective day animals than that in the subjective night animals. Tetanic stimulation of optic nerve (100 Hz, 1 s) at subjective daytime (projected zeitgeber time: ZT 0-8) produced a LTP in this field potential, although the onset of LTP was slow. When tetanic stimulation was applied at ZT4, the percent increase of amplitude was 116.6% immediately after, 159.8% 30 min after and 215.4% 120 min after tetanic stimulation, whereas tetanic stimulation of optic nerve at subjective night-time caused a weak LTP in the SCN. Although tetanic stimulation of Schaffer collaterals induced a LTP formation in the CA1 region of rat hippocampal slices, there were no obvious circadian changes in this LTP formation. The present results demonstrated that excitatory influence on the SCN caused a synaptic plasticity such as LTP. Although the physiological meaning of this LTP is uncertain at present, LTP may be related to adaptation mechanism to photic stimulation.

Morphological diversity and glutamate immunoreactivity of retinal terminals in the suprachiasmatic nucleus of the cat

Although the cat visual system has been the subject of intensive investigation, little attention has been given to the morphological features of ganglion cell projections to the suprachiasmatic nucleus. The present study has utilized anterograde transport of horseradish peroxidase and wheat germ agglutinin-conjugated horseradish peroxidase to label ganglion cell terminals in the cat suprachiasmatic nucleus. Visualization of the reaction product was facilitated through the use of gold-substituted silver intensification. Ganglion cell terminals were found to be morphologically diverse, making both asymmetric and symmetric contacts with postsynaptic processes. Synaptic vesicles were either scattered or densely packed, sometimes forming paracrystalline arrays. In contrast to other retinorecipient areas in which ganglion cell terminals have been characterized by the presence of lightly staining mitochondria, many of the retinal terminals in the suprachiasmatic nucleus were seen to contain darkly stained mitochondria. Postembedding antiglutamate immunocytochemistry was used to evaluate the level of endogenous glutamate in these ganglion cell terminals. Although morphologically diverse, all of the retinal terminals in the suprachiasmatic nucleus were glutamate positive, consistent with the postulated role of glutamate as the neurotransmitter of retinal ganglion cells.

Tryptophan loading modulates light-induced responses in the mammalian circadian system

Enhanced endogenous serotonergic activity, stimulated by L-tryptophan (TRYPT) loading, was found to have a substantial impact on neurochemical and behavioral aspects of the circadian response to light in the male Syrian hamster. An intraperitoneal (i.p.) injection of 150 mg/kg TRYPT significantly stimulated serotonin (5-HT) release in the suprachiasmatic nuclear (SCN) region, as reflected by a 205 +/- 30% maximal increase in the extracellular concentration of 5-HT assessed using microdialysis. Administration of TRYPT 1 h before exposure to a light pulse (30 min, 40 lux) delivered during late subjective night dose-dependently suppressed the number of SCN cells expressing light-induced Fos-like immunoreactivity (Fos-LI; maximal suppression @200 mg/kg was 77 +/- 4%, p < 0.001). This action of TRYPT was attenuated by pretreatment with the 5-HT1a antagonist, NAN-190, and was abolished by the 5-HT2/5-HT7 antagonist, ritanserin, or the nonselective 5-HT antagonist, metergoline (all 10 mg/kg). These antagonists alone had no effect on light-induced Fos. In a second experiment, pretreatment of free-running hamsters housed under constant darkness with 150 mg/kg TRYPT 45-60 min prior to light exposure (10 min, 20 lux) during late subjective night (CT 19) significantly attenuated the light-induced phase advances of the circadian activity rhythm (66 +/- 7 min vs. 100 +/- 6 min for vehicle controls; p < 0.001). The same dose of TRYPT given 1 h before lights-on for 5 consecutive days in hamsters maintained under 14L:10D altered the phase angle of entrainment such that activity onsets were delayed by 36 +/- 8 min relative to controls (p < 0.05). The same dose of TRYPT administered during late subjective night also suppressed the extracellular concentration of glutamate in the SCN region assessed using microdialysis (55 +/- 8% suppression; p < 0.05 vs. baseline). These results support the hypothesis that the ascending serotonergic projection to the SCN modulates photic entrainment processes within the circadian oscillator.

GABAA, GABAC, and NMDA receptor subunit expression in the suprachiasmatic nucleus and other brain regions

Identification of the neurotransmitter receptor subtypes within the suprachiasmatic nuclei (SCN) will further understanding of the mechanism of the biological clock and may provide targets to manipulate circadian rhythms pharmacologically. We have focused on the ionotropic GABA and glutamate receptors because these appear to account for the majority of synaptic communication in the SCN. Of the 15 genes known to code for GABA receptor subunits in mammals we have examined the expression of 12 in the SCN, neglecting only the alpha 6, gamma 3, and rho 2 subunits. Among glutamate receptors, we have focused on the five known genes coding for the NMDA receptor subunits, and two subunits which help comprise the kainate-selective receptors. Expression was characterized by Northern analysis with RNA purified from a large number of mouse SCN and compared to expression in the remaining hypothalamus, cortex and cerebellum. This approach provided a uniform source of RNA to generate many replicate blots, each of which was probed repeatedly. The most abundant GABA receptor subunit mRNAs in the SCN were alpha 2, alpha 5, beta 1, beta 3, gamma 1 and gamma 2. The rho 1 (rho 1) subunit, which produces GABAC pharmacology, was expressed primarily in the retina in three different species and was not detectable in the mouse SCN despite a common embryological origin with the retina. For several GABA subunits we detected additional mRNA species not previously described. High expression of both genes coding for glutamic acid decarboxylase (GAD65 and GAD67) was also found in the SCN. Among the NMDA receptor subunits, NR1 was most highly expressed in the SCN followed in order of abundance by NR2B, NR2A, NR2C and NR2D. In addition, both GluR5 and GluR6 show clear expression in the SCN, with GluR5 being the most SCN specific. This approach provides a simple measure of receptor subtype expression, complements in situ hybridization studies, and may suggest novel isoforms of known subunits.

Physiological mechanisms regulating photic induction of Fos-like protein in hamster suprachiasmatic nucleus

Immediate early genes including c-fos are selectively induced in cells of the hamster suprachiasmatic nucleus (SCN) by nocturnal light stimulation, suggesting that the Fos protein may play a role in the photic entrainment of circadian rhythms. To examine the physiological regulation of the induction of c-fos in the SCN, we studied the effects of antagonists of excitatory amino acids (EAA) receptors on photic induction of Fos-like immunoreactivity (Fos-lir) in the hamster SCN. We also examined the effects of electrical stimulation of the intergeniculate leaflet (IGL) to see whether neural input from IGL to SCN is involved in the induction of Fos protein in SCN cells. The results indicate that for most SCN cells EAA receptors mediate photic input involved in Fos induction but that another mechanism affects cells in restricted area of the caudal SCN. The neurochemical mechanisms and pathways by which these cells are activated by light remain undetermined.

Metabotropic glutamate receptor mGluR1 distribution and ultrastructural localization in hypothalamus

The metabotropic glutamate receptor mGluR1 is a G-protein-coupled glutamate receptor whose activation induces phosphotidylinositol hydrolysis and increases diacylglycerol and cytoplasmic calcium. By using affinity-purified antisera against a partial amino acid sequence of mGluR1 alpha, deduced from the nucleotide sequence of the cloned gene, the heterogeneous expression of this glutamate receptor was studied immunocytochemically with light and electron microscopy in the rat hypothalamus. Immunoreactivity was restricted to cell bodies and dendrites throughout many regions of the adult hypothalamus, including the preoptic area, anterior hypothalamus, suprachiasmatic nucleus, dorsomedial hypothalamus, and periventricular region. Strong immunolabeling was found in the lateral hypothalamus where immunoreactivity could be detected as early as embryonic day 18. Intense immunoreactivity was also found in the medial mammillary nuclei. In contrast to the strong labeling in many other regions, the neuroendocrine neurons of the arcuate, supraoptic, and paraventricular nuclei showed relatively little staining in adults. With light microscopy, immunoperoxidase labeling was found distributed in patches on the cytoplasmic side of the plasma membrane of immunoreactive neurons. When the same tissue was examined ultrastructurally, the patches were not restricted to synaptic specializations but were also found distributed on perikaryal and dendritic membranes sometimes associated with synapses and sometimes not. Some immunoreactive membranes showed no immunolabeling at the synaptic junction. When the tissue was strongly stained, labeling could be found in the cytoplasm of immunoreactive cells. No immunostaining was found on axons or presynaptic boutons. Together with other evidence showing a widespread expression of many different subtypes of both ionotropic and metabotropic receptors, these data support the hypothesis that glutamate may regulate hypothalamic cellular activity with a number of physiologically different mechanisms, and these mechanisms include second-messenger systems activated by G proteins.

An ultrastructural study of somal appositions in the suprachiasmatic nucleus and anterior hypothalamus of the rat

Somatic appositions of glial and neuronal elements in the suprachiasmatic nucleus (SCN) and the anterior hypothalamus (AH) were evaluated. Neuronal somas in the SCN exhibited more coverage by astrocytes than did those in the AH. Conversely, somas in the AH showed more coverage by neuronal elements than those in the SCN. All measurements of somal appositions were independent of circadian influences.

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.

Glutamate shifts the phase of the circadian neuronal firing rhythm in the rat suprachiasmatic nucleus in vitro

The effect of glutamate (GLU) on the phase of the circadian neuronal firing rhythm of the rat suprachiasmatic nucleus (SCN) was examined in vitro. GLU application in late subjective night produces phase-advances whereas GLU applied in early subjective night produces either phase-delays or a bimodal peak in the firing rhythm that appears to be a combination of phase-advances and phase-delays. Optic chiasm (OC) stimulation, or exposure of animals to light just prior to sacrifice, produces phase-delays in early subjective night, and phase-advances in late subjective night. None of GLU, light and OC stimulation produces significant phase-changes during subjective day. The results indicate that GLU is the neurotransmitter of the retinohypothalamic tract.

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.

In situ hybridization of antisense mRNA oligonucleotides for AMPA, NMDA and metabotropic glutamate receptor subtypes in the rat suprachiasmatic nucleus at different phases of the circadian cycle

Eleven oligonucleotides directed against mRNA for AMPA, NMDA and metabotropic glutamate receptor subtypes were hybridized to rat coronal brain sections containing the suprachiasmatic nucleus (SCN). These oligonucleotides were hybridized to tissue samples collected at midday and midnight phases of the circadian cycle. Glutamate receptor mRNA for the AMPA subunits GluR1, GluR2 and GluR4, and the NMDA receptor subtype NMDAR1, were heavily expressed in the SCN and surrounding areas. The mRNA for the metabotropic glutamate subunit mGluR1 was only lightly expressed in the SCN. In contrast, mRNA for NMDAR2A, NMDAR2B, NMDAR2C and GluR3 was not detected in the SCN. The mRNA found to be expressed in the rat SCN was similar in samples collected at midday and midnight, suggesting no circadian variation in endogenous SCN glutamate receptors at these two times of the light-dark cycle.

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.

Circadian expression of NMDA receptor mRNAs, epsilon 3 and zeta 1, in the suprachiasmatic nucleus of rat brain

The presence of the N-methyl-D-aspartate (NMDA) receptor channel subunit epsilon 3 and zeta 1 mRNAs in the rat suprachiasmatic nucleus (SCN) was detected by sensitive in situ hybridization. The daily fluctuations in the epsilon 3 and zeta 1 subunit mRNAs in their abundance were found in the SCN to be high during the day and lower during the night under 12 h light:12 h dark conditions (LD 12:12). Under constant darkness for 15 days, both the epsilon 3 and the zeta 1 mRNA levels in the SCN remained cyclic. Furthermore, after exposure of rats to light, the epsilon 3 and zeta 1 subunit mRNAs increased during the subjective night, but not during the subjective day. These results implicate the involvement of the epsilon 3 and zeta 1 subunits in neuronal signaling in the SCN and suggest that these subunits of the NMDA receptor channel are regulated by light and a circadian clock.

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.

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.

Cellular communication in the circadian clock, the suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus functions as the circadian clock in the mammalian brain. Communication between the cells of the suprachiasmatic nucleus is likely to be responsible for the generation and accuracy of this biological clock. Communication between many cells of the brain is mediated by action potentials that pass down the axon and cause release of neurotransmitters at the neuronal synaptic junction. Additional mechanisms of cellular communication appear to operate in the suprachiasmatic nucleus. Several lines of evidence point to multiple modes of cellular communication: these include the continuing operation of the clock after Na(+)-mediated action potentials have been blocked, the orchestrated metabolic rhythms of suprachiasmatic nucleus cells prior to synaptogenesis, the entrainment of fetal to maternal rhythms, and the rapid recovery of function after suprachiasmatic nucleus transplants into arrhythmic rodents. Possible alternative means of intercellular communication in the suprachiasmatic nucleus are examined, including calcium spikes in presynaptic dendrites, ephaptic interaction, paracrine communication, glial mediation, and gap junctions. This paper identifies and examines some of the unanswered questions related to intercellular communication of suprachiasmatic nucleus cells.

Glutamate receptor immunoreactivity in the rat suprachiasmatic nucleus

Antibodies selective for the glutamate receptor subunits GluR1 and GluR2/3 were used to localize glutamate receptor immunoreactivity in the rat suprachiasmatic nuclei (SCN). These antisera identified two distinct cell populations within the SCN. Cells immunoreactive to GluR2/3 antiserum were located predominantly in the ventral SCN while antiserum selective for GluR1 stained a population located along the dorsal and lateral borders of the nucleus. There were no apparent day-night differences in GluR immunoreactivity observed in the SCN.

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.

Membrane properties of rat suprachiasmatic nucleus neurons receiving optic nerve input

1. The electrophysiological properties of suprachiasmatic nucleus (SCN) neurons (n = 33) receiving optic nerve input were studied with intracellular recordings in rat hypothalamic slices maintained in vitro. Our major goal was to provide baseline data concerning the intrinsic membrane properties of these neurons and to test the hypothesis that the neurons are homogeneous electrophysiologically. 2. Action potentials were short in duration and followed by a pronounced hyperpolarizing after-potential. Spike amplitude (58.2 +/- 1.1 mV, mean +/- S.E.M.; measured from threshold), spike duration (0.83 +/- 0.03 ms; measured at half amplitude) and hyperpolarizing after-potential amplitude (23.9 +/- 1.0 mV; measured from threshold) appeared unimodally distributed and did not co-vary. 3. Intracellular injection of depolarizing current pulses evoked spike trains, and spike inactivation, spike broadening and frequency accommodation were always present. An after-hyperpolarization followed the spike train in all but one neuron. 4. Membrane time constant ranged from 7.5 to 21.1 ms (11.4 +/- 0.7 ms, n = 27), and its distribution appeared to be unimodal with the peak at approximately 10 ms. Input resistance ranged from 105 to 626 M omega (301 +/- 23 M omega, n = 33); the distribution also appeared unimodal with its peak at approximately 250 M omega. 5. A subpopulation (16 of 33, 48%) of the neurons exhibited slight (6-29%) time-dependent inward rectification in their voltage responses to hyperpolarizing current injection. Of the neurons lacking the time-dependent rectification, some (n = 5) exhibited time-independent inward rectification of 6-20% and others showed no (or < 3%) such rectification. The degree of inward rectification was correlated with neuronal excitability (r = 0.60, P < 0.002; assessed by measuring the steepness of the primary slope of the frequency-current plot) and with the spontaneous firing rate (r = 0.49, P < 0.007). Furthermore, the neurons with > 6% inward rectification (neurons with time-dependent and -independent rectification were combined) were more excitable (362 +/- 43 Hz/nA (n = 15) vs. 221 +/- 37 Hz/nA (n = 9), P < 0.05) and had a higher spontaneous firing rate (11.1 +/- 1.9 Hz (n = 19) vs. 3.9 +/- 1.5 Hz (n = 11), P < 0.02) than the neurons with no or negligible (i.e. < 3%) inward rectification. The two groups, however, were not significantly different in membrane time constant and input resistance. 6. When adequately hyperpolarized, twelve of seventeen (71%) neurons generated small low-threshold spike (LTS) potentials in response to depolarizing current pulses.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal synchronization without calcium-dependent synaptic transmission in the hypothalamus

A critical question in understanding the mammalian brain is how populations of neurons become synchronized. This is particularly important for the neurons and neuroendocrine cells of the hypothalamus, which are activated synchronously to control endocrine glands and the autonomic nervous system. It is widely accepted that communication between neurons of the adult mammalian brain is mediated primarily by Ca(2+)-dependent synaptic transmission. Here we report that synchronous neuronal activity can occur in the hypothalamic suprachiasmatic nucleus without active Ca(2+)-dependent synaptic transmission. Simultaneous extracellular recordings of neuronal activity in the suprachiasmatic nucleus, which contains the mammalian biological clock, confirmed a circadian rhythm of synchronized activity in hypothalamic slices. Ca(2+)-free medium, which blocks chemical synaptic transmission and increases membrane excitability, produced periodic and synchronized bursts of action potentials in a large population of suprachiasmatic nucleus neurons with diverse firing patterns. N-Methyl-D-aspartic acid, non-N-methyl-D-aspartic acid, and gamma-aminobutyric acid type A receptor antagonists had no effect on burst synchrony. Whole-cell patch-clamp recordings confirmed that the Ca(2+)-free solution blocked evoked postsynaptic potentials and that the mixture of antagonists blocked the remaining spontaneous postsynaptic potentials. Therefore, mechanisms other than Ca(2+)-dependent synaptic transmission can synchronize neurons in the mammalian hypothalamus and may be important wherever neuronal networks are synchronized.

Glutamate-like immunoreactivity in retinal terminals of the mouse suprachiasmatic nucleus

With a view to identifying the neurotransmitter content of retinal terminals within the mouse suprachiasmatic nucleus, a highly specific antiserum to glutaraldehyde-coupled glutamate was used in a postembedding immunogold procedure at the ultrastructural level. Retinal terminals were identified by cholera toxin--horseradish peroxidase transported anterogradely from the retina and reacted with tetramethyl benzidine/tungstate/H2O2, or by their characteristically pale and distended mitochondria with irregular cristae. Controls included model ultrathin sections containing high concentrations of various amino acids. Alternate serial sections were labelled with anti-glutamate and anti-gamma-aminobutyric acid (GABA). Data were analysed by computer-assisted image analysis. Density of glutamate labelling (gold particles per micron2) on whole retinal terminals was > 3 times higher than that on postsynaptic dendrites, and > 5 times higher than that on miscellaneous non-retinal non-glutamatergic terminals in the suprachiasmatic nucleus. The overall density of gold particles over retinal terminals was approximately 3 times higher than that over GABAergic terminals, in which glutamate-like immunoreactivity was mainly mitochondrial. Labelling of vesicles in retinal terminals was almost 5 times greater than the apparent labelling of vesicles in GABAergic terminals, underscoring the location of transmitter glutamate within synaptic vesicles in retinal terminals. In the retino-recipient region of the suprachiasmatic nucleus there was also a small population of non-retinal glutamatergic terminals. Their overall immunoreactivity was similar to or exceeded that of retinal terminals, but morphological features clearly distinguished between these two types of glutamate-containing terminals. The present results indicate that the vast majority of retinal terminals may use glutamate as a transmitter, in keeping with electrophysiological and neuropharmacological data from other sources. The possibility of cotransmitters within retinal terminals, suggested by the presence of dense-core vesicles among the glutamate-containing synaptic vesicles, has still to be addressed.

The effects of glutamate on membrane potential and discharge rate of suprachiasmatic neurons

The suprachiasmatic nucleus (SCN) is a major pacemaker for circadian rhythms in mammals. Photic entrainment of the circadian pacemaker is mediated by the retinohypothalamic tract (RHT). Most likely, excitatory amino acids function as neurotransmitters in this pathway. We have now investigated the effect of glutamate on the membrane potential of cultured SCN cells of the rat with the aid of the patch clamp technique. It was found that 1 mM glutamate depolarizes the cells by about +44 mV. In spontaneously active neurons, the glutamate induced depolarization caused either an increase in discharge or a depolarization block. We then investigated the effect of 1 mM glutamate on SCN discharge in the acutely prepared hypothalamic slice of the hamster. In most cells glutamate induced an increase in discharge whilst in a few cells discharge was suppressed. Both series of experiments indicate that glutamate in the used dosage was effective and its effect reversible. The data are discussed with respect to the failure of 1 mM glutamate injections to mimic the effect of light on the circadian activity rhythm of the hamster.

The relation between light-induced discharge in the suprachiasmatic nucleus and phase shifts of hamster circadian rhythms

The role of neurophysiological activation of suprachiasmatic nucleus (SCN) cells in phase shifting the circadian pacemaker of the hamster was investigated in a combined behavioural and electrophysiological study. An electrophysiological study examined the relation between the pattern of light presentation and the induced discharge rate in the SCN. Behavioural experiments examined the relation between the pattern of light presentation and the magnitude of phase shift induced. The combination of these results provides an indirect assay of the relations between induced neural discharge in the SCN and phase shifts of the circadian activity rhythm. The data indicate that the magnitude of phase shifts is monotonically, but not linearly, related to photically induced changes in discharge rate.

Effect of substance P on circadian rhythms of firing activity and the 2-deoxyglucose uptake in the rat suprachiasmatic nucleus in vitro

The suprachiasmatic nuclei (SCN) have been identified as a pacemaker for many circadian rhythms in mammals. Although substance P (SP) fibers from retina are found to terminate the SCN, the physiological role of this peptide is uncertain. The 2-deoxyglucose (2-DG) uptake and firing activity in the SCN show a robust circadian change. SP causes an increase in 2-DG uptake by SCN during the subjective night but not during subjective day. SP-induced increase in 2-DG uptake is blocked by co-treatment with the SP receptor antagonist, spantide. Treatment with SP produces phase shifts of circadian rhythm in spontaneous neural activity in SCN neurons with a phase-response curve that is similar to the effect of light pulses to animals under constant darkness. SP-induced phase change is also blocked by pretreatment with spantide. SP-induced increase in 2-DG uptake and phase changes in firing activity occur only during subjective night, at circadian times when photic phase shifting of activity occurs. The present results suggest that SP may be an important transmitter for conveying environmental light-dark information from retina to the SCN.

NMDA as well as non-NMDA receptor antagonists can prevent the phase-shifting effects of light on the circadian system of the golden hamster

The present experiments were designed to evaluate whether the intraventricular administration of excitatory amino acid (EAA) receptor antagonists would prevent light-induced phase shifts of the circadian rhythm of wheel-running activity in the hamster. Administration of the non-N-methyl-D-aspartate (non-NMDA) antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) blocked light-induced phase advances and delays. Similarly, administration of the competitive NMDA receptor antagonist, 3(2-carboxypiperazin-4-yl)-propyl-l-phosphonic acid (CPP), prevented light-induced phase advances and delays. Neither drug by itself caused any consistent effect on the phase of the rhythm. These data provide further evidence that EAA receptors mediate the effects of light on the circadian system, and suggest that both NMDA and non-NMDA receptor types may be involved.

Effects of inhibitory and excitatory drugs on the metabolic rhythm of the hamster suprachiasmatic nucleus in vitro

In order to elucidate the role of excitatory and inhibitory transmitters within the suprachiasmatic nucleus (SCN) in the circadian change of 2-deoxyglucose (2-DG) uptake in this nucleus, the effects of 8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT), muscimol, flurazepam, pentobarbital and glutamate on uptake of 2-DG by hamster SCN were examined in hypothalamic slice preparations. 2-DG uptake in the SCN was high during the subjective day and low during the subjective night. The high uptake of 2-DG in the SCN during the daytime was inhibited by the superfusion of 8-OH-DPAT, muscimol, flurazepam and pentobarbital in a dose-dependent manner, but the low uptake of 2-DG during the night was unaffected. The low uptake during the night was significantly increased by treatment with glutamate, whereas 2-DG uptake during the day was unaffected. In contrast to the above results, 20 mM KCl and 1 microM tetrodotoxin increased and decreased 2-DG uptake during both the day and night, respectively. The present results strongly suggest that agonists of 5-HT1A receptors and GABAA-benzodiazepine-barbiturate complex receptors regulate the function of the SCN through their inhibitory action on 2-DG uptake during the day, and that glutamate also regulates SCN function through it stimulatory action on 2-DG uptake during the night.

NMDA and non-NMDA receptor antagonists inhibit photic induction of Fos protein in the hamster suprachiasmatic nucleus

We previously reported that systemic treatment with a noncompetitive antagonist affecting the NMDA subtype of excitatory amino acid (EAA) receptor, MK-801, inhibits photic induction of Fos-like immunoreactivity (Fos-lir) in the hamster suprachiasmatic nucleus (SCN). Because MK-801 blocks the Ca2+ channel associated with the NMDA receptor, it may also influence the activity of other transmitters acting through Ca2+ channels. To assess the specificity of these effects, we compared the effects on photic induction of Fos-lir of several treatments: central injection of a competitive NMDA antagonist, CPP; central injection of a non-NMDA antagonist, DNQX; and systemic injection of the non-competitive NMDA antagonist, ketamine. Fos-lir was induced in SCN cells of vehicle-injected hamsters exposed to a light pulse 4-5 h after dark onset. Pretreatment with CPP (greater than 2 nmoles) or ketamine (greater than 100 mg/kg) caused a dose-related inhibition of photic induction of Fos-lir in portions of the SCN. These treatments reduced Fos-lir mainly in the rostral SCN and ventrolateral, but not dorsolateral, portions of the caudal SCN. Pretreatment with DNQX (greater than 20 nmoles) also inhibited photic induction of Fos-lir in the same regions of the SCN. These results indicate that photic induction of Fos protein in a portion of the hamster SCN is regulated by both NMDA and non-NMDA types of EAA receptor.

Excitatory effect of and kainate receptor on the 2-deoxyglucose uptake in the rat suprachiasmatic nucleus in vitro

The suprachiasmatic nuclei (SCN) have been identified as a pacemaker for many circadian rhythms in mammals. The previous findings indicate the excitatory amino acid (EAA) receptors play an important role in the transmission of light information from the retina to the circadian clocks. The 2-deoxyglucose (2DG) uptake shows a robust circadian change; high uptake during subjective day and low uptake subjective night. To determine whether EAA agonists regulate 2DG uptake in the SCN, we have measured 2DG uptake in the rat SCN in vitro. We report that, during the subjective night, glutamate, N-methyl-D-aspartate (NMDA) and kainic acid (KA) cause an increase in 2DG uptake and that NMDA- or KA-induced increase of 2DG uptake is blocked by co-treatments with competitive and non-competitive NMDA or KA receptor antagonists. EAA receptor agonist-induced increase in 2DG uptake occurs only during subjective night, at circadian times when photic phase shifting of activity occurs. Taken together, those data suggest that EAA may be an important transmitter of light information from retina to the SCN.

Intracellular electrophysiological study of suprachiasmatic nucleus neurons in rodents: excitatory synaptic mechanisms

1. To study the synaptic mechanisms of excitatory transmission in the suprachiasmatic nucleus (SCN), we assessed the effects of excitatory amino acid receptor antagonists on excitatory postsynaptic potentials (EPSPs) recorded from SCN neurons in horizontal and parasagittal hypothalamic slice preparations from rats and guinea-pigs. The EPSPs were evoked by electrical stimulation of either optic nerve or a site near the SCN. 2. When evoked at membrane potentials between -60 and -100 mV, the EPSPs from optic nerve stimulation were conventional in shape; they rose to the peak quickly (6.2 +/- 0.5 ms, mean +/- S.E.M.; n = 45) and decayed gradually over 50-250 ms. When evoked at membrane potentials between -20 and -55 mV after blockade of outward K+ currents and fast Na+ spikes by intracellular injection of Cs+ and QX-314 (n = 5 neurons), a slow depolarizing potential emerged near the fast peak of the EPSP. This slow potential, unlike the fast peak, was not linearly related to membrane potential. 3. An antagonist for kainate- and quisqualate-type excitatory amino acid receptors, 6,7-dinitroquinoxaline-2,3-dione (DNQX 1-10 microM), depressed in a concentration-dependent and reversible manner the EPSPs evoked by optic nerve stimulation at membrane potentials between -60 and -100 mV (n = 9). The effects of DNQX were not associated with any significant changes in the baseline input resistance or membrane potential of the postsynaptic neurons. The selective N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5, 50-100 microM), did not affect significantly and consistently the EPSPs evoked at these membrane potentials (n = 7). On the other hand, AP5 (50 microM) blocked or depressed the slow depolarizing component of the EPSPs evoked at membrane potentials between -20 and -55 mV (n = 4). No significant changes in baseline input resistance or membrane potential accompanied the effects of AP5. 4. Stimulation of a site lateral or dorsocaudal to the SCN evoked EPSPs distinct from those evoked by optic nerve stimulation. Again, DNQX (0.3-10 microM) depressed the EPSPs evoked at membrane potentials between -60 and -100 mV (n = 4) whereas AP5 (50 microM) had no effect (n = 5). When evoked at less negative membrane potentials (i.e. -20 to -55 mV) after intracellular injection of Cs+ and QX-314, the EPSPs had a slow depolarizing potential, similar to the EPSPs from optic nerve stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of N-methyl-d-aspartate-type glutamatergic neurotransmission in the photic induction of immediate-early gene expression in the suprachiasmatic nuclei of the Syrian hamster

Abstract This study investigated the role of N-methyl-D-aspartate (NMDA)-type glutamatergic neurotransmission in mediating the photic induction of immediate-early gene expression in the Suprachiasmatic nucleus (SCN) of the Syrian hamster. Activation of c-fos, c-jun and egr-1 was assessed by immunocytochemical detection of their protein products. To characterize the circadian basis to the inductive effects of light, hamsters were allowed to free-run in constant dim red light and received a 1 h light pulse at different circadian phases relative to activity onset (defined as CT 12). In control animals which did not receive light pulses, c-fos and egr-1 expression was absent or restricted to a small area of the dorsolateral region of the SCN, and expression of c-jun could not be detected in the SCN. In hamsters killed after presentation of a light pulse at either CT 14 or CT 20, there was a marked increase in c-fos and egr-1 immunoreactivities throughout the ventrolateral division of the SCN. In contrast, light pulses given at CT4 or CT 8 failed to activate immediate-early gene expression. Light pulses did not induce c-jun immunoreactivity at any circadian phase tested. Staining for c-fos was maximal 1 h after the start of the light pulse and had started to decline by 2 h. At this later time, c-jun expression was still undetectable. To compare the distribution of retinal afferents with that of c-fos induction, hamsters held on a light schedule of 16 h light: 8 h dark received an intraocular injection of cholera toxin-horseradish peroxidase conjugate 3 days before exposure to a 1 h light pulse given 2 h after lights off. Comparison of adjacent sections processed for c-fos immunoreactivity or for cholera toxin-horseradish peroxidase revealed that light-induced c-fos expression was precisely restricted to retinal terminal fields in the SCN. Light pulses also induced c-fos expression in the retinoreceptive ventral lateral geniculate nucleus and intergeniculate leaflet but not in the retinal fields of the dorsal lateral geniculate nucleus, indicating that the expression of cfos in response to light is spatially specific. The aim of the subsequent experiments was to investigate the role of NMDA-type glutamatergic neurotransmission in mediating the effects of light on c-fos expression in the SCN. To determine whether NMDA had the potential to activate c-fos expression in the SCN, hamsters were infused with 2.5 nmol NMDA or vehicle via an intracerebroventricular (icv) cannula positioned adjacent to the nuclei. In contrast to the effects of light, icv NMDA activated c-fos expression at both CT8 and CT 14. The distribution of immunoreactivity was more widespread than that observed after light, extending throughout the SCN and adjacent hypothalamus. To test whether NMDA receptors had a physiological role in the photic response, hamsters were treated systemically with the non-competitive NMDA antagonist MK801 (dose range 0.6 to 6.0 mg/kg body wt, ip) or vehicle prior to exposure to a 1 h light pulse given at CT 14 or CT 20. Expression of c-fos was still detectable in the dorsolateral SCN but MK801 blocked expression in the ventral portion of the retinoreceptive zone of the SCN. MK801 (10 or 100 nmol) delivered centrally (icv) also prevented light-induced c-fos expression in the ventral region of the SCN bordering the optic chiasm, though staining again persisted in the dorsolateral region. The induction of c-fos by icv NMDA, and the partial blockade of light-induced c-fos expression by the antagonist MK801, are consistent with the hypothesis that glutamate mediates the effects of light on SCN activity. However, the persistent photic induction of c-fos expression in a subfield of retinal afferents following treatment with MK801 suggests that other, non-NMDA-type mechanisms may contribute to photic entrainment.

N-methyl-D-aspartate, quisqualate and kainate receptors are all involved in transmission of photic stimulation in the suprachiasmatic nucleus in rats

In order to clarify the neuronal transmission mechanism of photic stimulation in the suprachiasmatic nucleus (SCN), the effects of agonists and antagonists for excitatory amino acid receptors on N-acetyltransferase (NAT) activity in the pineal gland were observed following the microinjection of drugs into both sides of the nuclei. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, and kainate (which are selective agonists for three different subtypes, i.e. NMDA, quisqualate and kainate receptors, respectively) significantly decreased NAT activity similarly to the suppressive effect of light. Moreover, compared with a control group, all the groups pretreated with a selective competitive antagonist for NMDA receptor (D-2-amino-5-phosphonovalerate or 3-((+-)-2-carboxypiperazine-4-yl)-propyl-1-phosphonate), or a selective non-competitive antagonist for non-NMDA receptors (Joro spider toxin-3 or 1-naphthylacetyl spermine) partially blocked the suppressive effect of photic stimulation on NAT activity. These results suggest that NMDA, quisqualate and kainate receptors are all involved in mediating photic stimulation in the SCN.

NMDA receptor antagonists block the effects of light on circadian behavior in the mouse

We report here the results of experiments designed to evaluate whether NMDA receptors mediate the phase shifting effects of light on the circadian rhythm of wheel-running activity in mice. Intraperitoneal administration of either the non-competitive NMDA receptor antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]cyclohepten-5,10-imine maleate (MK-801), or the competitive NMDA receptor antagonist, 3(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) blocked light-induced phase advances and delays. Neither drug, by itself, caused any consistent effect on the phase of the rhythm. Furthermore, there was no significant difference between the effects of MK-801 on light-induced phase shifts in a retinally degenerate and retinally normal strain of C57 mouse. These data, coupled with previous findings, indicate that excitatory amino acid receptors play an important role in the transmission of light information from the retina to the circadian system.

Aspartate injections into the suprachiasmatic region of the Syrian hamster do not mimic the effects of light on the circadian activity rhythm

The excitatory amino acids glutamate and aspartate are thought to be involved in the photic entrainment of the circadian pacemaker of the suprachiasmatic nuclei (SCN). When applied to the SCN region glutamate imitates the effects of dark pulses on the circadian activity rhythm rather than those of light. We have now injected aspartate into the SCN region of Syrian hamsters. These injections mimicked the effects of dark pulses as well, in so far as slight advances of the activity rhythm were obtained during the subjective day. However, the mean phase shift was not significantly different from the shift obtained with control injections. It is concluded that (1) aspartate has little or no effect on the phase of the circadian activity rhythm and (2) none of the putative transmitters of the photic afferents of the SCN produces the effects of light upon injection.

Lesions of the suprachiasmatic nucleus disrupt circadian locomotor rhythms in the mouse

Entrained and free-running rhythms of wheel-running activity were recorded in male BALB/cByJ mice with electrolytic lesions of the suprachiasmatic nucleus (SCN), site of a circadian pacemaker in mammals. Complete ablation of the nucleus abolished the circadian locomotor rhythm; in some cases, wheel-running was synchronized by a light-dark cycle, but the phase relationship of this activity to the cycle was often aberrant. Unilateral lesions or those missing the SCN did not eliminate rhythmicity.

Effect of optic nerve transection on N-acetylaspartylglutamate immunoreactivity in the primary and accessory optic projection systems in the rat

Evidence has been presented in recent years that support the hypothesis that N-acetylaspartylglutamate (NAAG) may be involved in synaptic transmission in the optic tract of mammals. Using a modified fixation protocol, we have determined the detailed distribution of NAAG immunoreactivity (NAAG-IR) in retinal ganglion cells and optic projections of the rat. Following optic nerve transection, dramatic losses of NAAG-IR were observed in the neuropil of all retinal target zones including the lateral geniculate nucleus, superior colliculus, nucleus of the optic tract, the dorsal and medial terminal nuclei and suprachiasmatic nucleus. Brain regions were microdissected and NAAG levels measured by a radioimmunoassay (RIA) (IC50: NAAG = 2.5 nM, NAA = 100 microM; smallest detectable amount = 1-2 pg/assay). Large decreases (50-60%) in NAAG levels were detected in the lateral geniculate, superior colliculus and suprachiasmatic nucleus. Moderate losses (25-45%) were noted in the pretectal nucleus and the nucleus of the optic tract. Smaller changes (15-20%) were detected in the paraventricular nucleus and the pretectal area. These results are consistent with a synaptic communication role for NAAG in the visual system.

Differential effects of indolepyruvic acid and 5-hydroxytryptophan on indole metabolism in the pineal gland of the rat during the light-dark cycle

The effect of two serotonin precursors, 5-hydroxytryptophan (5-OH-TRP) and indolepyruvic acid (IPA), a tryptophan ketoanalogue, on rat pineal indole metabolism during the light-dark cycle was investigated. 5-OH-TRP drastically increased the production of 5-hydroxyindoleacetic acid at a dose of only 10 mg/kg, whereas 50-100 mg/kg was needed to reach higher serotonin levels. It had no effect on the pathway leading to the production of N-acetylserotonin and melatonin. IPA, on the other hand, led to a marked dose-related increase in tryptophan, 5-OH-TRP, serotonin and 5-OH-indoleacetic acid, and was also active on N-acetylserotonin and melatonin synthesis in both phases. The different behaviour of these two substances with regard to melatonin synthesis was also confirmed by their effects on N-acetyltransferase, since IPA increased, whereas 5-OH-TRP decreased its activity. These data suggest that an increase in serotonin does not necessarily lead to an increase in melatonin, and that IPA may in fact induce this effect by altering the activity on N-acetyltransferase, which is regarded as a key enzyme in pineal hormone synthesis.

N-acetylaspartylglutamate: a transmitter candidate for the retinohypothalamic tract

The retinohypothalamic tract is the neural pathway mediating the photic entrainment of circadian rhythms in mammals. Important targets for these retinal fibers are the suprachiasmatic nuclei (SCN) of the hypothalamus, which are thought to be primary sites for the biological clock. The neurotransmitters that operate in this projection system have not yet been determined. Immunohistochemistry and radioimmunoassay performed with affinity-purified antibodies to N-acetylaspartylglutamate (NAAG) demonstrate that this neuron-specific dipeptide, which may act as an excitatory neurotransmitter, is localized extensively in the retinohypothalamic tract and its target zones, including the SCN. Optic nerve transections resulted in significant reductions in NAAG immunoreactivity in the optic chiasm and SCN. Analysis of NAAG concentrations in micropunches of SCN, by means of radioimmunoassay, showed approximately 50% reductions in NAAG levels. These results suggest that this peptide may act as one of the neurotransmitters involved in retinohypothalamic communication and circadian rhythm entrainment.

Light regulates expression of a Fos-related protein in rat suprachiasmatic nuclei

Mammalian circadian rhythmicity is endogenously generated by a pacemaker in the suprachiasmatic nuclei and precisely entrained to the 24-hr day/night cycle by periodic environmental light cues. We show that light alters the immunoreactive levels of a transcriptional regulatory protein, Fos, in the suprachiasmatic nuclei of albino rats. Photic regulation of Fos immunoreactivity does not occur in other retino-recipient brain areas except for the intergeniculate leaflet, which appears to be involved in mediating some of the complex effects of light on expressed circadian rhythms. Our results point to a promising new functional marker for the cellular effects of light and suggest that the expression of Fos or a related nuclear protein may be part of the mechanism for photic entrainment of the circadian clock to environmental light/dark cycles.

Do NMDA receptors mediate the effects of light on circadian behavior?

We report here the results of experiments designed to evaluate whether a specific NMDA receptor antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]cyclohepten-5,10-imine maleate (MK-801), blocks the phase shifting effects of light on the circadian rhythm of wheel-running activity in golden hamsters. Intraperitoneal administration of (+)-MK-801 produced a dose-dependent blockade of both light-induced phase advances and delays. The effect was stereoselective and treatment with related compounds, phenylcyclidine and ketamine, also blocked light-induced phase shifts. MK-801, by itself, did not cause any consistent effect on the phase of the rhythm. These data, coupled with previous findings, indicate that excitatory amino acid receptors play an important role in the transmission of light information from the retina to the circadian system.

Carbachol phase shifts the circadian rhythm of locomotor activity in the Djungarian hamster

The effects of light on the circadian system may involve the neurotransmitter acetylcholine (ACh). The purpose of the present study was to determine whether carbachol, a cholinergic agonist, mimics the phase-shifting effects of light on the circadian rhythm of locomotor activity (CRLA) in the Djungarian hamster. Phase response curves (PRCs) were measured for blind animals injected intracerebroventricularly with either vehicle or carbachol at different circadian times. Carbachol produced significant phase delays in the CRLA when administered at circadian time (CT) 12-16 and small, but significant phase advances when given at CT 8. No consistent phase shifts were observed in response to carbachol injections at other circadian times. Phase shifts produced by carbachol injections at CT 8 and 12 were similar for both sighted and blind animals. Light pulses to sighted hamsters produced phase advances at CT 20 and phase delays at CT 12. Both the carbachol PRC and the PRC for light pulses in the Djungarian hamster have phase delay regions in the early subjective night. However, these two PRCs have different phase advance regions. These results indicate that carbachol mimics some of the effects of light on the circadian system in the Djungarian hamster and support the hypothesis that cholinergic mechanisms may play a role in mediating some of the phase-shifting effects of light on the circadian clock of mammals.