Single unit response of suprachiasmatic neurons to arginine vasopressin (AVP) is mediated by a V1-like receptor in the hamster

Vasopressin Resets the Central Circadian Clock in a Manner Influenced by Sex and Vasoactive Intestinal Polypeptide Signaling

BACKGROUND/AIMS

Circadian rhythms in behavior and physiology are programmed by the suprachiasmatic nucleus (SCN) of the hypothalamus. A subset of SCN neurons produce the neuropeptide arginine vasopressin (AVP), but it remains unclear whether AVP signaling influences the SCN clock directly.

METHODS

Here, we test that AVP signaling acting through V1A and V1B receptors influences molecular rhythms in SCN neurons. V1 receptor agonists were applied ex vivo to PERIOD2::LUCIFERASE SCN slices, allowing for real-time monitoring of changes in molecular clock function.

RESULTS

V1A/B agonists reset the phase of the SCN molecular clock in a time-dependent manner, with larger magnitude responses by the female SCN. Further, we found evidence that both Gαq and Gαs signaling pathways interact with V1A/B-induced SCN resetting, and that this response requires vasoactive intestinal polypeptide (VIP) signaling.

CONCLUSIONS

Collectively, this work indicates that AVP signaling resets SCN molecular rhythms in conjunction with VIP signaling and in a manner influenced by sex. This highlights the utility of studying clock function in both sexes and suggests that signal integration in central clock circuits regulates emergent properties important for the control of daily rhythms in behavior and physiology.

The role of vasopressin in olfactory and visual processing

Neural vasopressin is a potent modulator of behaviour in vertebrates. It acts at both sensory processing regions and within larger regulatory networks to mediate changes in social recognition, affiliation, aggression, communication and other social behaviours. There are multiple populations of vasopressin neurons within the brain, including groups in olfactory and visual processing regions. Some of these vasopressin neurons, such as those in the main and accessory olfactory bulbs, anterior olfactory nucleus, piriform cortex and retina, were recently identified using an enhanced green fluorescent protein-vasopressin (eGFP-VP) transgenic rat. Based on the interconnectivity of vasopressin-producing and sensitive brain areas and in consideration of autocrine, paracrine and neurohormone-like actions associated with somato-dendritic release, we discuss how these different neuronal populations may interact to impact behaviour.

Collective timekeeping among cells of the master circadian clock

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the master circadian clock that coordinates daily rhythms in behavior and physiology in mammals. Like other hypothalamic nuclei, the SCN displays an impressive array of distinct cell types characterized by differences in neurotransmitter and neuropeptide expression. Individual SCN neurons and glia are able to display self-sustained circadian rhythms in cellular function that are regulated at the molecular level by a 24h transcriptional-translational feedback loop. Remarkably, SCN cells are able to harmonize with one another to sustain coherent rhythms at the tissue level. Mechanisms of cellular communication in the SCN network are not completely understood, but recent progress has provided insight into the functional roles of several SCN signaling factors. This review discusses SCN organization, how intercellular communication is critical for maintaining network function, and the signaling mechanisms that play a role in this process. Despite recent progress, our understanding of SCN circuitry and coupling is far from complete. Further work is needed to map SCN circuitry fully and define the signaling mechanisms that allow for collective timekeeping in the SCN network.

Suprachiasmatic vasopressin and the circadian regulation of voluntary locomotor behavior

A role for arginine vasopressin in the circadian regulation of voluntary locomotor behavior (wheel running activity) was investigated in the golden hamster, Mesocricetus auratus. Spontaneous nocturnal running was suppressed in a dose-dependent manner by systemic injections of vasopressin, and also in a concentration-dependent manner by microinjections directly into the hypothalamic suprachiasmatic nucleus. Pre-injections of a vasopressin V1 receptor antagonist into the nucleus reduced the suppression of behavior by vasopressin. Ethogram analyses revealed that peripheral drug injections predominantly increased grooming, flank marking, and sleep-related behaviors. Central injections did not induce sleep, but increased grooming and periods of 'quiet vigilance' (awake but not moving). Nocturnal behavioral profiles following either peripheral or central injections were similar to those shown by untreated animals in the hour prior to the onset of nocturnal wheel running. Site control vasopressin injections into the medial preoptic area or periaqueductal gray increased flank marking and grooming, but had no significant effect on locomotion, suggesting behavioral specificity of a vasopressin target near the suprachiasmatic nucleus. Both peripheral and central administration increased FOS-like immunoreactivity in the retinorecipient core of the suprachiasmatic nucleus. The distribution of FOS-positive cells overlapped the calbindin subregion, but was more extensive, and most calbindin-positive cells did not co-express FOS. We propose a model of temporal behavioral regulation wherein voluntary behavior, such as nocturnal locomotor activity, is inhibited by the activity of neurons in the suprachiasmatic ventrolateral core that project to the posterior hypothalamus and are driven by rhythmic vasopressin input from the dorsomedial shell.

Arginine vasopressin immunoreactivity is decreased in the hypothalamic suprachiasmatic nucleus of subjects with suprasellar tumors

Suprasellar tumors with compression of the optic chiasm are associated with an impaired sleep-wake rhythm. We hypothesized that this reflects a disorder of the biological clock of the human brain, the suprachiasmatic nucleus (SCN), which is located just above the optic chiasm. In order to test this hypothesis, we investigated the expression of two key neuropeptides of the SCN, that is, arginine vasopressin (AVP) and vasoactive intestinal peptide (VIP), as assessed by quantitative immunocytochemistry in post-mortem hypothalamic tissue of patients with a suprasellar tumor inducing permanent visual field defects. Post-mortem hypothalamic tissue of 5 patients with a suprasellar tumor inducing permanent visual field defects (acromegaly n = 2, nonfunctioning macro-adenoma n = 1, macroprolactinoma n = 1, infundibular metastasis of a colorectal adenocarcinoma n = 1) and 15 age- and gender-matched controls was obtained from the Netherlands Brain Bank. Total AVP immunoreactivity in the SCN was lower in patients with a suprasellar tumor than in controls (P = 0.03). By contrast, total VIP immunoreactivity was not different between patients and controls (P = 0.44). Suprasellar tumors leading to permanent visual field defects are associated with reduced AVP, but not VIP immunoreactivity, in the SCN. These findings raise the possibility that selective impairment of the SCN contributes to sleep-wake disturbances in these patients.

The role of kisspeptin and RFamide-related peptide-3 neurones in the circadian-timed preovulatory luteinising hormone surge

Many aspects of female reproduction often require intricate timing, ranging from the temporal regulation of reproductive hormone secretion to the precise timing of sexual behaviour. In particular, in rodents and other species, ovulation is triggered by a surge in pituitary luteinising hormone (LH) secretion that is governed by a complex interaction between circadian signals arising in the hypothalamus and ovarian-derived oestradiol signals acting on multiple brain circuitries. These circadian and hormonal pathways converge to stimulate a precisely-timed surge in gonadotropin-releasing hormone (GnRH) release (i.e. positive-feedback), thereby triggering the preovulatory LH surge. Reflecting its control by afferent circadian signals, the preovulatory LH surge occurs at a specific time of day, typically late afternoon in nocturnal rodents. Although the specific mechanisms mediating the hormonal and circadian regulation of GnRH/LH release have remained poorly understood, recent findings now suggest that oestradiol and circadian signals govern specific reproductive neuropeptide circuits in the hypothalamus, including the newly-identified kisspeptin and RFamide-related peptide (RFRP)-3 neuronal populations. Neurones producing kisspeptin, the protein product of the Kiss1 gene, and RFRP-3 have been shown to provide excitatory and inhibitory input to GnRH neurones, respectively, and are also influenced by sex steroid and circadian signals. In the present review, we integrate classic and recent findings to form a new working model for the neuroendocrine regulation of the circadian-timed preovulatory LH surge in rodents. This model proposes kisspeptin and RFRP-3 neuronal populations as key nodal points for integrating and transducing circadian and hormonal signals to the reproductive axis, thereby governing the precisely-timed LH surge.

Vasopressin and the output of the hypothalamic biological clock

The physiological effects of vasopressin as a peripheral hormone were first reported more than 100 years ago. However, it was not until the first immunocytochemical studies were carried out in the early 1970s, using vasopressin antibodies, and the discovery of an extensive distribution of vasopressin-containing fibres outside the hypothalamus, that a neurotransmitter role for vasopressin could be hypothesised. These studies revealed four additional vasopressin systems next to the classical magnocellular vasopressin system in the paraventricular and supraoptic nuclei: a sexually dimorphic system originating from the bed nucleus of the stria terminalis and the medial amygdala, an autonomic and endocrine system originating from the medial part of the paraventricular nucleus, and the circadian system originating from the hypothalamic suprachiasmatic nuclei (SCN). At about the same time as the discovery of the neurotransmitter function of vasopressin, it also became clear that the SCN contain the main component of the mammalian biological clock system (i.e. the endogenous pacemaker). This review will concentrate on the significance of the vasopressin neurones in the SCN for the functional output of the biological clock that is contained within it. The vasopressin-containing subpopulation is a characteristic feature of the SCN in many species, including humans. The activity of the vasopressin neurones in the SCN shows a pronounced daily variation in its activity that has also been demonstrated in human post-mortem brains. Animal experiments show an important role for SCN-derived vasopressin in the control of neuroendocrine day/night rhythms such as that of the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes. The remarkable correlation between a diminished presence of vasopressin in the SCN and a deterioration of sleep-wake rhythms during ageing and depression make it likely that, also in humans, the vasopressin neurones contribute considerably to the rhythmic output of the SCN.

Vasopressin receptor V1a regulates circadian rhythms of locomotor activity and expression of clock-controlled genes in the suprachiasmatic nuclei

The suprachiasmatic nuclei (SCN) serve as the principal circadian pacemakers that coordinate daily cycles of behavior and physiology for mammals. A network of transcriptional and translational feedback loops underlies the operating molecular mechanism for circadian oscillation within the SCN neurons. It remains unclear how timing information is transmitted from SCN neurons to eventually evoke circadian rhythms. Intercellular communication between the SCN and its target neurons is critical for the generation of coherent circadian rhythms. At the molecular level, neuropeptides encoded by clock-controlled genes have been indicated as important output mediators. Arginine vasopressin (AVP) is the product of one such clock-controlled gene. Previous studies have demonstrated a circadian rhythm of AVP levels in the cerebrospinal fluid and the SCN. The physiological effects of AVP are mediated by three types of AVP receptors, designated as V1a, V1b, and V2. In this study, we report that V1a mRNA levels displayed a circadian rhythm in the SCN, peaking during night hours. The circadian rhythmicity of locomotor activities was significantly reduced in V1a-deficient (V1a(-/-)) mice (50-75% reduction in the power of fast Fourier transformation). However, the light masking and light-induced phase shift effects are intact in V1a(-/-) mice. Whereas the expression of clock core genes was unaltered, the circadian amplitude of prokineticin 2 (PK2) mRNA oscillation was attenuated in the SCN of V1a(-/-) mice ( approximately 50% reduction in the peak levels). In vitro experiments demonstrated that AVP, acting through V1a receptor, was able to enhance the transcriptional activity of the PK2 promoter. These studies thus indicate that AVP-V1a signaling plays an important role in the generation of overt circadian rhythms.

Receptors and neural effects of oxytocin in the rodent hypothalamus and preoptic region

Vasopressin and oxytocin are produced in and secreted from not only hypothalamo-hypophysial neurons which shed their products into the circulation to act as hormones or releasing factors, but also from neurons whose axons form tracts which remain within the central nervous system. Using tritiated or radioiodinated ligands, binding sites for vasopressin and for oxytocin have been detected by in vitro autoradiography. In the rat hypothalamus binding sites for vasopressin are present in the suprachiasmatic, sigmoid and arcuate nuclei, and oxytocin receptors in the area of the ventromedial nucleus. Electrophysiological evidence obtained using single cell recordings in slices suggests that oxytocin-binding sites present in the ventromedial hypothalamus and in the bed nucleus of the stria terminalis mostly represent functional, neuronal receptors. The expression of these receptors (but not of the vasopressin receptors) depends on gonadal steroid hormones, as does that of uterine and mammary gland oxytocin receptors. Modifications of the hormonal status associated with, for example, puberty and lactation cause 'up-regulation' of central and peripheral oxytocin receptors. The central administration of oxytocin facilitates (and the administration of oxytocin agonists inhibits) maternal behaviour and the milk ejection reflex, therefore the hormonal and neural actions of oxytocin appear to be complementary in ensuring the birth and development of the offspring.

Electrophysiology of the suprachiasmatic circadian clock

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

Not only vasopressin, but also the intracellular messenger protein kinase Calpha in the suprachiasmatic nucleus correlates with expression of circadian rhythmicity in voles

The suprachiasmatic nucleus (SCN) is the locus of the main pacemaker for circadian behavioral rhythms. In common voles, variation in circadian behavioral rhythmicity correlates with vasopressin (AVP) immunoreactive cells in the SCN. Here we studied the immunostaining of four AVP linked Ca(2+)-dependent protein kinase C (PKC) isoforms (PKCalpha, PKCbeta1, PKCbeta2, and PKCgamma) at the beginning of the light period, and conclude that PKCalpha is highly expressed in the vole SCN compared to the other isozymes. Voles, characterized as strongly circadian rhythmic showed circadian variation in numbers of PKCalpha immunoreactive SCN neurons, while voles with weak or no circadian rhythmicity did not reveal such a circadian profile. PKCalpha immunoreactivity in acute SCN slices that were treated with a physiological dose of AVP was significantly lowered when compared with control slices. The intracellular messenger PKCalpha may reflect variation in locomotor behavior via the AVP system in the vole SCN. This system could play a key role in the vole SCN by mediating output of its circadian clock.

The mouse VPAC2 receptor confers suprachiasmatic nuclei cellular rhythmicity and responsiveness to vasoactive intestinal polypeptide in vitro

Expression of coherent and rhythmic circadian (approximately 24 h) variation of behaviour, metabolism and other physiological processes in mammals is governed by a dominant biological clock located in the hypothalamic suprachiasmatic nuclei (SCN). Photic entrainment of the SCN circadian clock is mediated, in part, by vasoactive intestinal polypeptide (VIP) acting through the VPAC2 receptor. Here we used mice lacking the VPAC2 receptor (Vipr2-/-) to examine the contribution of this receptor to the electrophysiological actions of VIP on SCN neurons, and to the generation of SCN electrical firing rate rhythms SCN in vitro. Compared with wild-type controls, fewer SCN cells from Vipr2-/- mice responded to VIP and the VPAC2 receptor-selective agonist Ro 25-1553. By contrast, similar proportions of Vipr2-/- and wild-type SCN cells responded to gastrin-releasing peptide, arginine vasopressin or N-methyl-D-aspartate. Moreover, VIP-evoked responses from control SCN neurons were attenuated by the selective VPAC2 receptor antagonist PG 99-465. In firing rate rhythm experiments, the midday peak in activity observed in control SCN cells was lost in Vipr2-/- mice. The loss of electrical activity rhythm in Vipr2-/- mice was mimicked in control SCN slices by chronic treatment with PG 99-465. These results demonstrate that the VPAC2 receptor is necessary for the major part of the electrophysiological actions of VIP on SCN cells in vitro, and is of fundamental importance for the rhythmic and coherent expression of circadian rhythms governed by the SCN clock. These findings suggest a novel role of VPAC2 receptor signalling, and of cell-to-cell communication in general, in the maintenance of core clock function in mammals, impacting on the cellular physiology of SCN neurons.

Differential expression of protein kinase C betaI (PKCbetaI) but not PKCalpha and PKCbetaII in the suprachiasmatic nucleus of selected house mouse lines, and the relationship to arginine-vasopressin

The functional significance of the suprachiasmatic nucleus (SCN) in circadian rhythm control of mammals has been well documented. The role of protein phosphorylation mediated by protein kinase C (PKC), however, is not well known. We report the immunocytochemical localization of three Ca(2+)-dependent PKC isoforms (alpha, betaI, betaII) within the SCN of selected house mouse lines that differ in behavioral circadian rhythm parameters. Optical density measurements revealed that the adult mice selected for low levels of nest-building behavior (small nest-builders) had more than threefold higher PKCbetaI immunostaining in the SCN than the mice selected for high levels of nest-building behavior (big nest-builders). A similar twofold difference between the adult small and big nest-builders was observed for the number of PKCbetaI-containing cells in the SCN. The non-selected control lines were intermediate. Ten-day-old pups revealed similar differences in PKCbetaI immunostaining in the SCN between the small and big nest-builders. PKCalpha and PKCbetaII immunostaining in the SCN was not different among the lines. PKCbetaI immunostaining was not different among the selected lines in the lateroanterior hypothalamic nucleus (LA) and the cornu ammonis field 1 (CA1) of the dorsal hippocampus and confirms the specificity of the difference in PKCbetaI immunostaining in the SCN among the selected lines. The significance of these findings is discussed in the context of differences among the lines in arginine-vasopressin (AVP) and light-induced Fos expression in the SCN, behavioral phase-delay responses to 15-min light pulses in constant darkness, and measures of the strength of the circadian activity rhythm expressed.

Vasopressin- and oxytocin-induced activity in the central nervous system: electrophysiological studies using in-vitro systems

During the last two decades, it has become apparent that vasopressin and oxytocin, in addition to playing a role as peptide hormones, also act as neurotransmitters/neuromodulators. A number of arguments support this notion: (i) vasopressin and oxytocin are synthesized not only in hypothalamo-neurohypophysial cells, but also in other hypothalamic and extrahypothalamic cell bodies, whose axon projects to the limbic system, the brainstem and the spinal cord. (ii) Vasopressin and oxytocin can be shed from central axons as are classical neurotransmitters. (iii) Specific binding sites, i.e. membrane receptors having high affinity for vasopressin and oxytocin are present in the central nervous system. (iv) Vasopressin and oxytocin can alter the firing rate of selected neuronal populations. (v) In-situ injection of vasopressin and oxytocin receptor agonists and antagonists can interfere with behavior or physiological regulations. Morphological studies and electrophysiological recordings have evidenced a close anatomical correlation between the presence of vasopressin and oxytocin receptors in the brain and the neuronal responsiveness to vasopressin or oxytocin. These compounds have been found to affect membrane excitability in neurons located in the limbic system, hypothalamus, circumventricular organs, brainstem, and spinal cord. Sharp electrode intracellular recordings and whole-cell recordings, done in brainstem motoneurons or in spinal cord neurons, have revealed that vasopressin and oxytocin can directly affect neuronal excitability by opening non-specific cationic channels or by closing K(+) channels. These neuropeptides can also influence synaptic transmission, by acting either postsynaptically or upon presynaptic target neurons or axon terminals. Whereas, in cultured neurons, vasopressin and oxytocin appear to mobilize intracellular Ca(++), in brainstem slices, the action of oxytocin is mediated by a second messenger that is distinct from the second messenger activated in peripheral target cells. In this review, we will summarize studies carried out at the cellular level, i.e. we will concentrate on in-vitro approaches. Vasopressin and oxytocin will be treated together. Though acting via distinct receptors in distinct brain areas, these two neuropeptides appear to exert similar effects upon neuronal excitability.

Neuropeptide changes in the suprachiasmatic nucleus in primary hypertension indicate functional impairment of the biological clock

Abnormalities in autonomic activity resulting in disturbances of the diurnal rhythm of many physiologic processes were recently revealed in hypertensive patients. These findings suggest deteriorations in the functioning of the suprachiasmatic nucleus (SCN), which is known to be the biological clock of mammals. To test this hypothesis, we carried out an immunocytochemical study of the SCN of primary hypertension patients who had died due to myocardial infarction or brain hemorrhage, and compared them with those of individuals with a normal blood pressure who had never had any autonomic disturbances and died from myocardial infarction after chest trauma or from hypothermia. We found that the staining for the three main neuronal populations of the SCN; i.e., vasopressin, vasoactive intestinal polypeptide, and neurotensin, reduced by more than 50% in the hypertensives compared with controls. The present data indicate a serious dysregulation of the biological clock in hypertensive patients. Such a disturbance may cause a harmful hemodynamic imbalance with a negative effect on circulation, especially in the morning, when the inactivity-activity balance changes. The difficulty in adjusting from inactivity to activity might be involved in the morning clustering of cardiovascular events.

Electrophysiology and pharmacology of projections from the suprachiasmatic nucleus to the ventromedial preoptic area in rat

Extracellular and whole-cell patch-clamp recordings were made from neurons in the ventromedial preoptic area in rat horizontal brain slices. Responses to single-pulse electrical stimulation of the ipsilateral suprachiasmatic nucleus were characterized using peristimulus time histograms or postsynaptic current recordings, and bath application of neurotransmitter receptor antagonists. Extracellular recordings showed that suprachiasmatic nucleus stimulation (50-150 microA) elicited a short-latency suppression in 35 of 64 neurons (55%), with the majority (29/35, 83%) showing a biphasic response consisting of a short-latency suppression followed by a long-duration activation. In addition, 14 cells (22%) showed activation only, while 15 (23%) were unresponsive. The GABA(A) receptor antagonist bicuculline (5-10 microM) reversibly blocked suppressions evoked by suprachiasmatic nucleus stimulation (20/20 cells). In the majority of these neurons (13/20), bicuculline also unmasked an activation in response to stimulation. Activations elicited by suprachiasmatic nucleus stimulation, either in the presence or absence of bicuculline, were blocked by the non-N-methyl-D-aspartate and N-methyl-D-aspartate glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10/10 cells). 6,7-Dinitroquinoxaline-2,3-dione (10 microM) selectively and reversibly blocked the initial, short-duration (<50 ms) activation, but had no effect on the longer-duration activation. In contrast, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10 microM) appeared to inhibit the long-duration activation selectively without affecting the initial rapid activation. Combined applications of the two ionotropic glutamate receptor antagonists blocked stimulation-induced activations completely. All the pharmacological effects were concentration dependent. Whole-cell patch-clamp recordings showed that suprachiasmatic nucleus stimulation elicited inhibitory postsynaptic currents or a combination of inhibitory and excitatory postsynaptic currents in 25 of 33 neurons tested. The inhibitory postsynaptic currents had short onset latencies (4.9+/-0.3 ms) and a reversal potential of -56.0+/-3.8 mV (n=5), and were reversibly blocked by bicuculline (5-10 microM, 4/4 cells). In the presence of bicuculline (5-10 microM), excitatory postsynaptic currents had short onset latencies (4.7+/-0.5 ms), and had a fast and a slow component. (+/-) 3-(2-Carboxypiperazin-4-yl)-propyl-1-phosphonic acid blocked the slow, but not the fast, component, whereas 6,7-dinitroquinoxaline-2, 3-dione blocked the fast, but not the slow, component (n=7). These results suggest that the projection from the suprachiasmatic nucleus conveys both inhibitory (GABA) and excitatory (glutamate) inputs to the ventromedial preoptic area. GABA(A) receptor and both non-N-methyl-D-aspartate and N-methyl-D-aspartate glutamate receptors mediate these influences. These inputs may be responsible for conveying information related to circadian phase from the suprachiasmatic nucleus to regions of the preoptic area known to be involved in regulation of sleep/waking and other physiological functions.

The role of Clock in the developmental expression of neuropeptides in the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the dominant circadian pacemaker in mammals. To understand better the ontogeny of mouse SCN and the role of the pacemaker in peptide expression, the authors examined the distribution of cells that were immunoreactive for vasopressin (AVP) or vasoactive intestinal polypeptide (VIP) in wild type and Clock mutant mice at two developmental stages. Clock homozygous mice failed to show the dramatic increase in the number of VIP-immunoreactive (VIP-ir) neurons from postnatal day 6 (P6) to P30 that was found in the SCN of wild type mice. The number of AVP-ir neurons was relatively constant in the postnatal SCN but was significantly reduced in Clock/Clock mice. The effects of the Clock mutation varied with position in the SCN for both peptides. Densitometry of immunolabeled brains indicated that the Clock mutation reduced AVP expression specifically in the SCN and not in other brain areas. The SCN did not significantly change shape or size with age or Clock genotype. Taken together, these results indicate that the neonatal mouse SCN has its full complement of cells, some of which are not yet mature in their neuropeptide content. Furthermore, the observation that the Clock mutation appears to act on a subset of AVP and VIP cells suggests heterogeneity within these cell classes in the SCN.

The suprachiasmatic nucleus and the circadian time-keeping system revisited

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

Vasopressin neurotransmission and the control of circadian rhythms in the suprachiasmatic nucleus

Vasopressin (VP) is one of the principal transmitters in the suprachiasmatic nucleus (SCN). Approximately 20% of neurones in the dorsomedial division of the SCN synthesize the peptide and a high proportion of SCN neurones (> 40%) are excited by VP acting through the V1 receptor. This suggests that VP may act as a feedback regulator of electrical activity within the nucleus. Such an intrinsic excitatory signal can be demonstrated by perifusion with a V1 antagonist which reduces spontaneous neural activity. As the synthesis and release of VP occurs in a circadian manner, this leads to a variable feedback excitation which may contribute to the circadian pattern of activity of the neural clock. This role in amplifying rhythmicity is supported by observations that animals deficient in VP show a reduced circadian amplitude of behavioural rhythms (e.g. locomotor and cortical electroencephalographic rhythms). VP expression declines during ageing and although aged animals show no change in the proportion of SCN neurones excited by VP, the rhythm of spontaneous electrical activity shows a progressive decline, consistent with the reduced endogenous excitatory feedback. However, the homozygous Brattleboro rat which lacks any VP expression still maintains rhythms of electrical activity, indicating that VP is not the sole factor generating circadian activity. The generation of this rhythmicity may depend upon the interaction of VP with other transmitter systems, such as the inhibitory transmitters somatostatin and GABA which show a circadian variation in efficacy. In addition to its role in feedback amplification of the endogenous rhythm of electrical activity, VP also functions as part of the efferent signal to the rest of the CNS where it potentially regulates a number of behavioural and physiological rhythms, including the circadian activity of the hypothalamo-pituitary-adrenal axis. Thus, the combined amplification and signalling functions makes VP an important component of the neuronal clock function in mammals.

Distribution of Ca2+-dependent protein kinase C isoforms in the suprachiasmatic nucleus of the diurnal murid rodent, Arvicanthis niloticus

The suprachiasmatic nuclei (SCN) contain the major 'biological clock' in mammals that controls most circadian rhythms expressed by these animals. The functional importance of protein phosphorylation and intracellular Ca2+ in the mammalian circadian pacemaker is becoming increasingly apparent. Here we report the immunocytochemical localization of the four Ca2+-dependent protein kinase C (PKC) isoforms (alpha, betaI, betaII, gamma) within the SCN of the diurnal murid rodent, Arvicanthis niloticus, and the nocturnal golden hamster. In the SCN of A. niloticus, PKCalpha was the most abundant of the four isoforms. Cells containing PKCalpha were homogeneously distributed throughout the SCN. PKCbetaI cells were sparsely distributed in the perimeter of the SCN and were absent in its central area. PKCbetaII and -gamma were not found in the SCN of A. niloticus. In the SCN of the golden hamster, PKCalpha cells were most heavily concentrated in the dorsomedial region, though some were also present laterally and ventrally. The distribution of arginine-vasopressin (AVP) cells in the SCN overlapped with that of PKC in both species. Species differences in the location of the Ca2+-dependent PKC isoforms suggest differences in function such as the relaying of photic or non-photic information to the clock mechanism, or the synchronization of AVP neurons and their subsequent output signals.

Membrane properties and morphology of vasopressin neurons in slices of rat suprachiasmatic nucleus

Vasopressin (VP) neurons in the suprachiasmatic nucleus (SCN) are thought to be closely linked to neural mechanisms for circadian timekeeping. To gain insight into the cellular-physiological principles that govern spike-driven VP release and to examine whether VP cells can be electrophysiologically and morphologically identified by a unique combination of features, we recorded membrane properties by whole cell patch-clamp methods and stained the cells with biocytin. In current-clamp mode, VP neurons recorded during subjective daytime expressed a clear time-dependent inward rectification but no pronounced low-threshold Ca2+ potential after hyperpolarizing current pulses. Their spontaneous firing rate varied between 0.6 and 13.4 Hz and was generally tonic and irregular. Spike afterhyperpolarizations (AHPs) were steeply rising and monophasic. Spikes were preceded by depolarizing ramps mediated by a slow component of Na+ current. Spike trains evoked by depolarizing current pulses displayed frequency adaptation and were usually followed by an AHP lasting 0.5-2.0 s. Spontaneous postsynaptic potentials were present in a majority of cells. Voltage-clamp recordings revealed a Ba2+-sensitive K+ current that exerts a tonic, hyperpolarizing influence on the membrane potential. This set of membrane properties was not significantly different from other cells in the dorsomedial region and is characteristic for cluster I cells, which were described previously and are widely encountered throughout the SCN. None of the cells could be classified as belonging to cluster II or III, which were indeed found mainly outside the dorsomedial region. Morphologically, single VP neurons were characterized by compact, mono- or bipolar dendritic branching patterns and numerous varicosities throughout the dendrites. They generally possessed few axon collaterals, most of which remained inside the boundaries of the SCN but were occasionally seen to project to SCN target areas. In conclusion, VP neurons in the SCN express several active membrane poperties, including time-dependent inward rectification, frequency adaptation in spike trains, monophasic spike AHPs, and Ba2+-sensitive K+ current. VP release is proposed to be governed by tonic and irregular patterns of spontaneous firing. The electrophysiological and cytological properties of VP neurons are representative for a majority of SCN cells and define them as a subset of previously defined cluster I cells.

Concurrent decrease of vasopressin and protein kinase Calpha immunoreactivity during the light phase in the vole suprachiasmatic nucleus

Vasopressin (AVP) is a major neuropeptide in the suprachiasmatic nucleus, the mammalian hypothalamic circadian pacemaker. Protein kinase Calpha is a putatively coupled intracellular messenger. Mean numbers of AVP- and protein kinase Calpha-immunoreactive neurons were determined in the suprachiasmatic nucleus of common voles, entrained to a 12:12 h light-dark (LD) cycle, at the beginning of the light period (zeitgeber time zero) and 6 h later (zeitgeber time six). At zeitgeber time zero, mean numbers of AVP- and protein kinase Calpha- immunoreactive neurons were 2194 and 9897, respectively. Both numbers decreased significantly with about 40% at zeitgeber time six. This concurrent decrease was most pronounced in the dorsomedial aspect of the suprachiasmatic nucleus. These findings are consistent with the findings of a peak of AVP release in rats during the early light phase.

Variation in the expression of the mRNA for protein kinase C isoforms in the rat suprachiasmatic nuclei, caudate putamen and cerebral cortex

Using in situ hybridization, we have examined mRNA expression for five isoforms of protein kinase C (PKC alpha, beta1, beta2, gamma and epsilon) in the rat suprachiasmatic nuclei (SCN) and other central site during the 24 h cycle. The signal for each of these isoforms shows a marked local density within the SCN. In the absence of photic cues, there are changes in the expression of the mRNAs for the four isoforms that are Ca2+-dependent (alpha, beta1, beta2 and gamma), but not for one of the Ca2+-independent PKCs (epsilon). PKC alpha mRNA exhibits a monophasic rhythm of expression in the SCN with a peak at early subjective night, circadian time (CT) 14. In contrast, the mRNAs for PKC beta1, beta2 and gamma show a biphasic rhythm in the SCN with peaks at early subjective day, CT 0, and early subjective night, CT 14. The four Ca2+-dependent isoforms may therefore subserve phase-related functions within the SCN at the onset of subjective night and, in the case of beta1, beta2 and gamma, also at the onset of subjective day. Variation in the mRNAs for PKC beta1 and gamma (but not for alpha, beta2 or epsilon) is also found in the caudate putamen and in the cingulate and parietal cortex; the biphasic pattern of expression for these mRNAs is precisely in phase with that observed in the SCN. The beta1 and gamma isoforms may therefore contribute to temporally regulated functions at sites outside the SCN. The present observations raise the possibility that receptor-mediated regulation of circadian functions is modulated or even gated by circadian changes in intracellular components that participate in distinct signal cascades.

Vasopressin/serotonin interactions in the anterior hypothalamus control aggressive behavior in golden hamsters

Studies in several species of rodents show that arginine vasopressin (AVP) acting through a V1A receptor facilitates offensive aggression, i.e., the initiation of attacks and bites, whereas serotonin (5-HT) acting through a 5-HT1B receptor inhibits aggressive responding. One area of the CNS that seems critical for the organization of aggressive behavior is the basolateral hypothalamus, particularly the anterior hypothalamic region. The present studies examine the neuroanatomical and neurochemical interaction between AVP and 5-HT at the level of the anterior hypothalamus (AH) in the control of offensive aggression in Syrian golden hamsters. First, specific V1A and 5-HT1B binding sites in the AH are shown by in vitro receptor autoradiography. The binding for each neurotransmitter colocalizes with a dense field of immunoreactive AVP and 5-HT fibers and putative terminals. Putative 5-HT synapses on AVP neurons in the area of the AH are identified by double-staining immunocytochemistry and laser scanning confocal microscopy. These morphological data predispose a functional interaction between AVP and 5-HT at the level of the AH. When tested for offensive aggression in a resident/intruder paradigm, resident hamsters treated with fluoxetine, a selective 5-HT reuptake inhibitor, have significantly longer latencies to bite and bite fewer times than vehicle-treated controls. Conversely, AVP microinjections into the AH significantly shorten the latency to bite and increase biting attacks. The action of microinjected AVP to increase offensive aggression is blocked by the pretreatment of hamsters with fluoxetine. These data suggest that 5-HT inhibits fighting, in part, by antagonizing the aggression-promoting action of the AVP system.

Circadian rhythm of neuronal activity in suprachiasmatic nucleus slices from the vasopressin-deficient Brattleboro rat

In vitro extracellular recordings were made from tissue slices of suprachiasmatic nucleus from homozygous Brattleboro rats which are deficient in vasopressin. A high proportion (56%) of neurons were excited by application of exogenous vasopressin, indicating that the V1 receptors expressed by these neurons were functional. Basal activity of these vasopressin-sensitive neurons showed a marked circadian variation (higher during the subjective light phase) while vasopressin-insensitive neurons showed no significant variation, suggesting the presence of the V1 receptor identifies a population of highly circadian neurons. Suprachiasmatic neurons from both homozygous rats and their heterozygous (vasopressin-containing) litter mates displayed a circadian rhythm of spontaneous (basal) activity, with firing rates declining during the subjective dark phase, indicating that the endogenous pacemaker driving the circadian rhythm was not dependent upon the presence of vasopressin. However, the peak of spontaneous activity displayed during the subjective light phase was significantly lower in the vasopressin-deficient animals. These data show that the presence of endogenous vasopressin within the suprachiasmatic nucleus is not necessary for the generation of the circadian pattern of activity. However, vasopressin does function to amplify the rhythm by its excitatory effect during the light phase.

Distribution of AVP and Ca(2+)-dependent PKC-isozymes in the suprachiasmatic nucleus of the mouse and rabbit

The suprachiasmatic nucleus (SCN) is the circadian pacemaker in mammals and contains a network of arginine-vasopressin-immunoreactive (AVP-ir) neurons. AVP-recipient cells contain the V1a class of receptors linked to phosphoinositol turnover and protein kinase C (PKC). The present study describes the localization of AVP and the four Ca(2+)-dependent PKC-isoforms in the mouse and rabbit SCN. An estimate of the numerical density of AVP-ir neurons at the rostral, medial, and caudal level of the SCN revealed that the mouse SCN contains more than twice the number of AVP-ir neurons than the rabbit SCN. Neurons immunostained for AVP or PKC dominated in the dorsomedial and ventrolateral aspects of the mouse SCN, while the central area of the SCN revealed only weakly stained neurons. The rabbit SCN was characterized by a more homogeneous distribution of AVP-ir and PKC-ir neurons. PKC alpha was the most abundantly expressed isozyme in both species, whereas the presence of the other isoforms differed (mouse: PKC alpha > PKC beta I >> PKC beta II > PKC gamma; rabbit: PKC alpha > PKC beta II > or = PKC gamma > PKC beta I). Clear PKC gamma-positive neurons were only observed in the rabbit SCN, while the mouse SCN predominantly contained immunolabeled fiber tracts for this PKC isozyme. Astrocytes immunoreactive for each PKC isoform were frequently encountered in the rabbit SCN, but were absent in mice. Immunofluorescence double labeling showed that numerous AVP-recipient cells in the mouse SCN were immunopositive for PKC alpha, and that nearly all AVP-ir neurons express PKC alpha abundantly. These results substantiate the putative role for PKC alpha in vasopressinergic signal transduction in the SCN. The differential expression in degree and cell type of the Ca(2+)-dependent PKC-isoforms in the mouse and rabbit SCN may be related to the differences observed in circadian timekeeping between the two species.

In vivo measurement of a diurnal variation in vasopressin release in the rat suprachiasmatic nucleus

Diurnal changes in the intranuclear release of vasopressin (VP) and oxytocin (OT) in the suprachiasmatic (SCN), paraventricular (PVN) and supraoptic nuclei (SON) of the rat were studied by means of brain microdialysis. A significant diurnal variation in VP release in the SCN was detected, with the highest levels occurring during midday and a trough around midnight. OT release from the SCN was below detection limit. The release of neither of these neurohypophysial peptides showed diurnal variations within the PVN or SON.

The effects of [Arg8]vasopressin and [Arg8]vasotocin on the firing rate of suprachiasmatic neurons in vitro

The excitatory effect of [Arg8]-vasopressin and its potential contribution to the circadian cycle of electrical activity in the suprachiasmatic nucleus of the rat was investigated using extracellular recordings from hypothalamic slices from virgin female rats. The majority of neurons tested for their responses to vasopressin and [Arg8]-vasotocin displayed coincident, dose-dependent excitation by both peptides, although the relative efficacy varied between neurons, with some showing a highly preferential excitation by vasotocin. Perifusion with the vasopressin receptor antagonist d(CH2)5[Tyr(OEt)2,Val4,Cit8]-vasopressin was able to block the majority of responses to vasopressin or vasotocin (20/25), and similar excitation could be induced by the selective agonist [Phe2,Orn8]-vasotocin, indicating a mainly V1 receptor-mediated effect. Few neurons (3/27; 11%) responded to the oxytocin-specific agonist, [Thr4,Gly7]-oxytocin, suggesting a low occurrence of oxytocin receptors. In addition to blocking the action of exogenous vasopressin, the V1 antagonist caused a reversible suppression of spontaneous basal activity in 7/25 cases, consistent with the presence of an endogenous excitatory vasopressin tone. In agreement with previous reports, the activity of suprachiasmatic nucleus neurons showed a significant correlation between spontaneous activity and the light-dark cycle, with activity decreasing during the subjective dark phase. When neurons were divided on the basis of their response to vasopressin and/or vasotocin, the peptide-sensitive neurons continued to show a strong correlation (r = 0.513, P < 0.01) while the insensitive neurons showed no correlation (r = 0.136, P > 0.05). These data confirm the presence of V1 type receptors in the suprachiasmatic nucleus and also indicate a small number of neurons possessing additional classes of receptor selective for either oxytocin or vasotocin. Contrary to previous reports, they also demonstrate that endogenous vasopressin tonically excites suprachiasmatic nucleus neurons. The fact that vasopressin-sensitive (but not vasopressin-insensitive) neurons show a level of basal activity correlated with time, suggests that this tone may contribute to the circadian cycle of electrical activity in the suprachiasmatic nucleus.

Suppression of suprachiasmatic nucleus neurone activity with a vasopressin receptor antagonist: possible role for endogenous vasopressin in circadian activity cycles in vitro

Neurones of the rat suprachiasmatic nucleus (SCN) were tested with [Arg8]vasopressin (AVP) and the AVP receptor antagonist, [d(CH2)5,d-Tyr(OEt)2,Val4,Cit8]-vasopressin in vitro. 52% of AVP-responsive neurones showed an antagonist-induced decrease in activity, indicative of the presence of an endogenous excitatory tone. The magnitude of this effect declined significantly between subjective light and dark phases, consistent with the possibility that circadian fluctuations in endogenous AVP excitation contribute to the cycle of electrical activity within the SCN. However, similar fluctuations in basal activity between the light and dark phases was observed for both antagonist-sensitive and -insensitive neurones, indicating that endogenous AVP was not the only factor determining the circadian cycle.

Gene expression in suprachiasmatic nucleus and circadian rhythms

The suprachiasmatic nuclei (SCN) contain a circadian system consisting of circadian oscillator (clock) that is normally synchronized by the light/dark cycle (input) and drives circadian rhythms (output) that are intrinsic to the SCN. Gene expression of immediate-early genes, such as c-fos and jun-B, in the ventrolateral SCN is associated with circadian synchronization by light pulses and subjected to circadian control. Vasopressin and somatostatin gene expression shown distinct circadian rhythms intrinsic to the dorsomedial SCN with higher peptide levels occurring during the day. In addition, embryonic SCN grafted into the brain of an SCN-lesioned arrhythmic host define the period of the restored circadian locomotor rhythm. Taken together, these and other findings support the notion that the expression of genes underlying circadian synchronization, oscillation and output takes place within individual SCN neurons. However, no information regarding the nature and number of those neurons as well as the molecular mechanisms of the single cell-circadian oscillator and output is currently available. Therefore, we propose a simple two-neuron model as a framework for critically discussing the molecular genetic strategies to analyze the circadian system in SCN.

Neuropeptide Y microinjected into the suprachiasmatic region phase shifts circadian rhythms in constant darkness

The geniculohypothalamic tract (GHT) is a projection from the intergeniculate leaflet to the suprachiasmatic nucleus (SCN). The GHT exhibits neuropeptide Y (NPY) immunoreactivity and appears to communicate photic information to the SCN. Microinjection of NPY into the SCN has been found to phase shift circadian rhythms of hamsters housed in constant light in a manner similar to the phase shifts produced by pulses of darkness or triazolam injections. In the present study, NPY was injected into the SCN of Syrian hamsters housed in constant darkness and was found to produce phase shifts similar to those seen in hamsters housed in constant light. Microinjections were not followed by wheel running during the subjective day (the time when NPY microinjections are followed by significant phase advances). These data suggest that NPY produces phase shifts by some mechanism other than by inducing wheel running or by inhibiting the response of SCN neurons to light and supports a role for NPY in nonphotic shifting of the circadian clock.

Pineal modulation of the rat caudate-putamen spontaneous neuronal activity: roles of melatonin and vasotocin

The effects of microiontophoretic application of melatonin and melatonin plus vasotocin on spontaneously active neurons of caudate-putamen in sham-operated and pinealectomized rats were studied. Extracellular unit recordings showed that in sham-pinealectomized rats, melatonin ejection primarily produced inhibition of the responsive neurons (74.1%), whereas only 24.9% of the neurons were excited. Iontophoretic ejection of vasotocin or melatonin+vasotocin produced, in both cases, an inhibition of 100% of the responsive neurons. In pinealectomized rats, iontophoretic melatonin ejection produced a similar percentage of inhibition (46.1%) and excitation (53.8%) of the responsive neurons. The simultaneous ejection of melatonin+vasotocin further increased the percentage of inhibition (88.8%) compared with the melatonin only treated group. Moreover, iontophoretic ejection of vasotocin inhibited 100% of the responsive neurons in pinealectomized rats. The actions of melatonin and vasotocin seem to be specific, because their effects are dependent on the amount of these compounds ejected, i.e., the intensity of the ejection current. These results indicate that the pineal compounds melatonin and vasotocin are neuromodulators of spontaneous neuronal activity of the rat caudate-putamen.

Vasopressin induction of long-lasting potentiation of synaptic transmission in the dentate gyrus

Vasopressin receptors are present in both the developing and mature dentate gyrus of the rat brain and are of the V1 vasopressor type. Because vasopressin has been shown to influence memory function when injected into the dentate gyrus, the influence of this peptide on an electrophysiological model of learning and memory using the field excitatory postsynaptic potential (EPSP) of the dentate gyrus was investigated. Results of these studies showed that nanomolar concentrations of [Arg8]-vasopressin induced a prolonged increase in the amplitude and slope of the evoked population response in the presence of 1.5 mM calcium. Moreover, the expression of the vasopressin-induced potentiation of the EPSP persisted following removal of vasopressin from the perfusion medium. The vasopressin-induced sustained increase has been termed long-term vasopressin potentiation (LTVP). The closely related neuropeptide oxytocin had no effect upon the EPSP of the dentate gyrus. Preincubation of hippocampal slices in a selective V1 antagonist blocked the expression of LTVP. The ability of the V1 antagonist to block LTVP demonstrates that the potentiation induced by vasopressin is receptor-specific. In the presence of 2.5 mM calcium, the effect of vasopressin was opposite to that observed in 1.5 mM calcium. Under the conditions of 2.5 calcium, vasopressin induced a prolonged depression in the amplitude and slope of the EPSP. Expression of both potentiation and depression appeared within 5 minutes of application and persisted for the length of the observation, 60 minutes. These experiments demonstrate that vasopressin can induce long-lasting changes in the excitability of dentate gyrus neurons that are both calcium-dependent and receptor-specific.

The role of arginine-vasopressin for pineal melatonin synthesis in the rat: involvement of vasopressinergic receptors

The endogenously synthesized nonapeptide arginine vasopressin (AVP) is thought to be involved in transduction of photic information to the pineal gland. The enhancement of circulating AVP leads to a suppression of the nocturnal melatonin surge the mechanisms of which are unknown so far. We therefore studied the effect of dDAVP, an AVP analog with antidiuretic but without vasopressor activity, on pineal melatonin synthesis in Sprague-Dawley and AVP-deficient Brattleboro rats. The nocturnal intra-arterial application of dDAVP mimicked the inhibitory effect of AVP on the activity of the rate-limiting enzyme for pineal melatonin synthesis, N-acetyltransferase (NAT), in both rat strains. Furthermore, since the pineal is equipped with receptors for VP4-9 (the major proteolytic AVP fragment) only, the influence of this substance on the gland's metabolic activity was investigated in vitro. Neither this peptide nor AVP alone did not affect NAT activity, but either substance potentiated the norepinephrine-induced enhancement of NAT activity. These results reveal that at least two mechanisms mediate the influence of AVP on pineal melatonin synthesis. The AVP-induced pineal inhibition in vivo is probably due to a receptor-mediated effect on pinealopetal signal transduction. This inhibition masks the potentiating effect of AVP on the pineal gland itself which is delayed by the conversion of AVP to VP4-9. The present results support the idea of a modulatory role of AVP and its metabolites in the generation and maintenance of the circadian melatonin rhythm in mammals.

Vasopressin in the brain of the golden hamster: the distribution of vasopressin binding sites and of immunoreactivity to the vasopressin-related glycopeptide

Using in vitro light microscopic autoradiography and immunocytochemistry, the distribution of vasopressin binding sites and that of the vasopressin-related glycopeptide are described in the brain of golden hamster (Mesocricetus auratus). Vasopressin binding sites and immunoreactive axons were observed in the suprachiasmatic nucleus, in the anterior hypothalamus/median preoptic area, in the medial preoptic nucleus, in the bed nucleus of the stria terminalis, in the habenular complex, in the thalamic paraventricular nucleus, and in the nucleus of the solitary tract. In addition we observed binding sites in regions where no immunoreactivity could be evidenced: the lateral septal nucleus, the central amygdaloid nucleus, the subiculum, the dentate gyrus, the anterodorsal and anteroventral thalamic nuclei, the superior colliculus, the vestibular nuclei, and in the prepositus hypoglossal nucleus. In the golden hamster, exogenous vasopressin excites single neurones located in the suprachiasmatic nucleus and induces flank-marking behavior when microinjected into the preoptic area. Our results provide a morphological basis for similar effects exerted by endogenous vasopressin. A comparison of the present data with those previously described in the rat reveals marked species differences in the brain distribution of vasopressin and of its binding sites.

Effects of GABA and anxiolytics on the single unit discharge of suprachiasmatic neurons in rat hypothalamic slices

The effects of gamma-aminobutyric acid (GABA), muscimol, baclofen and the anxiolytics; diazepam (DZP), flurazepam (FZP) and zopiclone on single-unit neural activities in the suprachiasmatic nucleus (SCN) were investigated using the rat hypothalamic slice preparation. Exposure of the slice to GABA 10(-4) M produced inhibitory responses in 65% of the 49 SCN neurons examined. The threshold concentration of GABA ranged from 10(-6) to 10(-4) M. Neurons responsive to GABA were not found to be restricted to a subdivision of the SCN, but were diffusely distributed throughout the nucleus. DZP, FZP and zopiclone produced responses similar to those of GABA. The inhibitory effects of GABA (10(-5) M) were potentiated by coadministration of DZP (10(-5) M). Muscimol and baclofen (10(-7) M to 10(-4) M) also inhibited SCN neuronal activity in a dose-dependent manner. Bicuculline (10(-5) M-10(-4) M) scarcely affected the baclofen-induced inhibition (1/6) but strongly antagonized the effects of muscimol (6/6), GABA (6/8) and DZP (4/5). These results suggest that the receptors mediating the inhibitory effects of GABA and anxiolytics within the SCN may be GABAA and/or GABAB or GABA-BDZ receptor complex, respectively.

Single unit response of neurons within the hamster suprachiasmatic nucleus to GABA and low chloride perfusate during the day and night

Using the in vitro hamster hypothalamic slice preparation, the effects of GABA and 80% chloride (Cl-) reduced medium on the single unit activity of SCN neurons was investigated. GABA 10(-4) M produced inhibitory responses in 55% of the 69 SCN neurons examined. No statistically significant day-night difference was observed in either the percentage of SCN units responding to GABA, or in their threshold response. During the day 80% Cl- reduced medium had an excitatory effect on SCN neurons; however, following the return to normal Cl- concentrations a transient, but significant inhibition was observed. During the night, 80% Cl- reduced medium produced an excitatory response similar to that observed during the day, but no inhibition following return to the medium containing normal Cl- concentrations. Only during the night was Cl- reduced medium found to initiate activity in a dose-dependent manner in some silent cells. No significant day-night difference in response to 80% Cl- reduced medium occurred in neurons of the paraventricular nucleus of the hypothalamus. These results indicate that SCN neurons whose activity is mediated by Cl- may be involved in the control of circadian rhythms.

The role of the hypothalamic paraventricular nuclei for the regulation of pineal melatonin synthesis: new aspects derived from the vasopressin-deficient Brattleboro rat

There is evidence for an involvement of the hypothalamic paraventricular nuclei (PVN) in the regulation of pineal melatonin synthesis in rats. Since electrical stimulation of the PVN or the systemic administration of arginine-vasopressin (AVP) result in a depression of the nocturnal melatonin surge, this neuropeptide appears to be pivotal for the transduction of PVN-efferent, pinealopetal signals. We therefore used an AVP-deficient animal model, the Brattleboro rat, to further investigate the mechanisms responsible for pineal regulation. Anesthetized adult male animals received 2 min of bilateral electrical stimulation of the PVN either during the day or at night. Thirty min later, pineal glands were removed and pineal N-acetyltransferase (NAT) activities and melatonin contents were determined. Stimulation resulted neither during the day nor at night in any significant alterations of pineal NAT activity or melatonin content when compared to control or sham-stimulated animals. These data further support the proposed modulatory role of AVP for the regulation of melatonin synthesis in the Epiphysis cerebri of genetically intact rats.

Day-night variation in prepro vasoactive intestinal peptide/peptide histidine isoleucine mRNA within the rat suprachiasmatic nucleus

Neurons within the suprachiasmatic nuclei of the hypothalamus (SCN) appear to function as a circadian clock that controls the timing of many physiological systems. The SCN contain several chemically distinct neuronal subpopulations, including a large group of interneurons within the ventrolateral SCN that exhibit co-localizable immunoreactivity for both vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI). The purpose of the present study was to determine whether VIP/PHI neurons within the rat SCN exhibit rhythmicity in the cellular levels of the messenger RNA encoding the precursor from which both VIP and PHI are derived. Using both quantitative in situ and solution hybridization prepro-VIP/PHI mRNA levels early in the dark phase were demonstrated to be significantly higher than those 5 h after the onset of the daily light period. Since no statistically reliable (P greater than 0.05) day-night variation was observed in the levels of prepro-VIP/PHI mRNA within cortex, these data suggest that the rhythmicity in prepro-VIP/PHI mRNA is an intrinsic property of VIP/PHI-containing SCN neurons, or rhythmically driven by local synaptic events within the SCN.