Effects of vasoactive intestinal polypeptide on neurones of the rat suprachiasmatic nuclei in vitro

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.

Timed daily exercise remodels circadian rhythms in mice

Regular exercise is important for physical and mental health. An underexplored and intriguing property of exercise is its actions on the body’s 24 h or circadian rhythms. Molecular clock cells in the brain’s suprachiasmatic nuclei (SCN) use electrical and chemical signals to orchestrate their activity and convey time of day information to the rest of the brain and body. To date, the long-lasting effects of regular physical exercise on SCN clock cell coordination and communication remain unresolved. Utilizing mouse models in which SCN intercellular neuropeptide signaling is impaired as well as those with intact SCN neurochemical signaling, we examined how daily scheduled voluntary exercise (SVE) influenced behavioral rhythms and SCN molecular and neuronal activities. We show that in mice with disrupted neuropeptide signaling, SVE promotes SCN clock cell synchrony and robust 24 h rhythms in behavior. Interestingly, in both intact and neuropeptide signaling deficient animals, SVE reduces SCN neural activity and alters GABAergic signaling. These findings illustrate the potential utility of regular exercise as a long-lasting and effective non-invasive intervention in the elderly or mentally ill where circadian rhythms can be blunted and poorly aligned to the external world.

Using mice with disrupted neuropeptide signaling, Hughes et al. show that daily scheduled voluntary exercise (SVE) promotes suprachiasmatic nuclei (SCN) clock cell synchrony and robust 24 h rhythms in behavior. This study suggests the potential utility of regular exercise as a non-invasive intervention for the elderly or mentally ill, where circadian rhythms can be poorly aligned to the external world.

Dysfunction of VIPR2 leads to myopia in humans and mice

Background

Myopia is the leading cause of refractive errors. As its pathogenesis is poorly understood, we determined if the retinal VIP-VIPR2 signalling pathway axis has a role in controlling signalling output that affects myopia development in mice.

Methods

Association analysis meta-study, single-cell transcriptome, bulk RNA sequencing, pharmacological manipulation and VIPR2 gene knockout studies were used to clarify if changes in the VIP-VIPR2 signalling pathway affect refractive development in mice.

Results

The SNP rs6979985 of the VIPR2 gene was associated with high myopia in a Chinese Han cohort (randomceffect model: p=0.013). After either 1 or 2 days’ form deprivation (FD) retinal VIP mRNA expression was downregulated. Retinal single-cell transcriptome sequencing showed that VIPR2 was expressed mainly by bipolar cells. Furthermore, the cAMP signalling pathway axis was inhibited in some VIPR2+ clusters after 2 days of FD. The selective VIPR2 antagonist PG99-465 induced relative myopia, whereas the selective VIPR2 agonist Ro25-1553 inhibited this response. In Vipr2 knockout (Vipr2-KO) mice, refraction was significantly shifted towards myopia (p<0.05). The amplitudes of the bipolar cell derived b-waves in 7-week-old Vipr2-KO mice were significantly larger than those in their WT littermates (p<0.05).

Conclusions

Loss of VIPR2 function likely compromises bipolar cell function based on presumed changes in signal transduction due to altered signature electrical wave activity output in these mice. As these effects correspond with increases in form deprivation myopia (FDM), the VIP-VIPR2 signalling pathway axis is a viable novel target to control the development of this condition.

Ion Channels Controlling Circadian Rhythms in Suprachiasmatic Nucleus Excitability

Animals synchronize to the environmental day-night cycle by means of an internal circadian clock in the brain. In mammals, this timekeeping mechanism is housed in the suprachiasmatic nucleus (SCN) of the hypothalamus and is entrained by light input from the retina. One output of the SCN is a neural code for circadian time, which arises from the collective activity of neurons within the SCN circuit and comprises two fundamental components: 1) periodic alterations in the spontaneous excitability of individual neurons that result in higher firing rates during the day and lower firing rates at night, and 2) synchronization of these cellular oscillations throughout the SCN. In this review, we summarize current evidence for the identity of ion channels in SCN neurons and the mechanisms by which they set the rhythmic parameters of the time code. During the day, voltage-dependent and independent Na⁺ and Ca²⁺ currents, as well as several K⁺ currents, contribute to increased membrane excitability and therefore higher firing frequency. At night, an increase in different K⁺ currents, including Ca²⁺-activated BK currents, contribute to membrane hyperpolarization and decreased firing. Layered on top of these intrinsically regulated changes in membrane excitability, more than a dozen neuromodulators influence action potential activity and rhythmicity in SCN neurons, facilitating both synchronization and plasticity of the neural code.

Circuit development in the master clock network of mammals

Daily rhythms are generated by the circadian timekeeping system, which is orchestrated by the master circadian clock in the suprachiasmatic nucleus (SCN) of mammals. Circadian timekeeping is endogenous and does not require exposure to external cues during development. Nevertheless, the circadian system is not fully formed at birth in many mammalian species and it is important to understand how SCN development can affect the function of the circadian system in adulthood. The purpose of the current review is to discuss the ontogeny of cellular and circuit function in the SCN, with a focus on work performed in model rodent species (i.e., mouse, rat, and hamster). Particular emphasis is placed on the spatial and temporal patterns of SCN development that may contribute to the function of the master clock during adulthood. Additional work aimed at decoding the mechanisms that guide circadian development is expected to provide a solid foundation upon which to better understand the sources and factors contributing to aberrant maturation of clock function.

Entrainment of Circadian Rhythms Depends on Firing Rates and Neuropeptide Release of VIP SCN Neurons

The mammalian suprachiasmatic nucleus (SCN) functions as a master circadian pacemaker, integrating environmental input to align physiological and behavioral rhythms to local time cues. Approximately 10% of SCN neurons express vasoactive intestinal polypeptide (VIP); however, it is unknown how firing activity of VIP neurons releases VIP to entrain circadian rhythms. To identify physiologically relevant firing patterns, we optically tagged VIP neurons and characterized spontaneous firing over 3 days. VIP neurons had circadian rhythms in firing rate and exhibited two classes of instantaneous firing activity. We next tested whether physiologically relevant firing affected circadian rhythms through VIP release. We found that VIP neuron stimulation with high, but not low, frequencies shifted gene expression rhythms in vitro through VIP signaling. In vivo, high-frequency VIP neuron activation rapidly entrained circadian locomotor rhythms. Thus, increases in VIP neuronal firing frequency release VIP and entrain molecular and behavioral circadian rhythms. VIDEO ABSTRACT.

Distinct Firing Properties of Vasoactive Intestinal Peptide-Expressing Neurons in the Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) regulates daily rhythms in physiology and behavior. Previous studies suggest a critical role for neurons expressing vasoactive intestinal peptide (VIP) in coordinating rhythmicity and synchronization in the SCN. Here we examined the firing properties of VIP-expressing SCN neurons in acute brain slices. Active and passive membrane properties were measured in VIP and in non-VIP neurons during the day and at night. Current-clamp recordings revealed that both VIP and non-VIP neurons were spontaneously active, with higher firing rates during the day than at night. Average firing frequencies, however, were higher in VIP neurons (3.1 ± 0.2 Hz, day and 2.4 ± 0.2 Hz, night) than in non-VIP neurons (1.8 ± 0.2 Hz, day and 0.9 ± 0.2 Hz, night), both day and night. The waveforms of individual action potentials in VIP and non-VIP neurons were also distinct. Action potential durations (APD50) were shorter in VIP neurons (3.6 ± 0.1 ms, day and 2.9 ± 0.1 ms, night) than in non-VIP neurons (4.4 ± 0.3 ms, day and 3.5 ± 0.2 ms, night) throughout the light-dark cycle. In addition, afterhyperpolarization (AHP) amplitudes were larger in VIP neurons (21 ± 0.8 mV, day and 24.9 ± 0.9 mV, night) than in non-VIP neurons (17.2 ± 1.1 mV, day and 20.5 ± 1.2 mV, night) during the day and at night. Furthermore, significant day/night differences were observed in APD50 and AHP amplitudes in both VIP and non-VIP SCN neurons, consistent with rhythmic changes in ionic conductances that contribute to shaping the firing properties of both cell types. The higher day and night firing rates of VIP neurons likely contribute to synchronizing electrical activity in the SCN.

Role of vasoactive intestinal peptide in the light input to the circadian system

The neuropeptide vasoactive intestinal peptide (VIP) is expressed at high levels in a subset of neurons in the ventral region of the suprachiasmatic nucleus (SCN). While VIP is known to be important for the synchronization of the SCN network, the role of VIP in photic regulation of the circadian system has received less attention. In the present study, we found that the light-evoked increase in electrical activity in vivo was unaltered by the loss of VIP. In the absence of VIP, the ventral SCN still exhibited N-methyl-d-aspartate-evoked responses in a brain slice preparation, although the absolute levels of neural activity before and after treatment were significantly reduced. Next, we used calcium imaging techniques to determine if the loss of VIP altered the calcium influx due to retinohypothalamic tract stimulation. The magnitude of the evoked calcium influx was not reduced in the ventral SCN, but did decline in the dorsal SCN regions. We examined the time course of the photic induction of Period1 in the SCN using in situ hybridization in VIP-mutant mice. We found that the initial induction of Period1 was not reduced by the loss of this signaling peptide. However, the sustained increase in Period1 expression (after 30 min) was significantly reduced. Similar results were found by measuring the light induction of cFOS in the SCN. These findings suggest that VIP is critical for longer-term changes within the SCN circuit, but does not play a role in the acute light response.

Rhythmic control of activity and sleep by class B1 GPCRs

Members of the class B1 family of G-protein coupled receptors (GPCRs) whose ligands are neuropeptides have been implicated in regulation of circadian rhythms and sleep in diverse metazoan clades. This review discusses the cellular and molecular mechanisms by which class B1 GPCRs, especially the mammalian VPAC2 receptor and its functional homologue PDFR in Drosophila and C. elegans, regulate arousal and daily rhythms of sleep and wake. There are remarkable parallels in the cellular and molecular roles played by class B1 intercellular signaling pathways in coordinating arousal and circadian timekeeping across multiple cells and tissues in these very different genetic model organisms.

Circadian gating of neuronal functionality: a basis for iterative metaplasticity

Brain plasticity, the ability of the nervous system to encode experience, is a modulatory process leading to long-lasting structural and functional changes. Salient experiences induce plastic changes in neurons of the hippocampus, the basis of memory formation and recall. In the suprachiasmatic nucleus (SCN), the central circadian (~24-h) clock, experience with light at night induces changes in neuronal state, leading to circadian plasticity. The SCN's endogenous ~24-h time-generator comprises a dynamic series of functional states, which gate plastic responses. This restricts light-induced alteration in SCN state-dynamics and outputs to the nighttime. Endogenously generated circadian oscillators coordinate the cyclic states of excitability and intracellular signaling molecules that prime SCN receptivity to plasticity signals, generating nightly windows of susceptibility. We propose that this constitutes a paradigm of ~24-h iterative metaplasticity, the repeated, patterned occurrence of susceptibility to induction of neuronal plasticity. We detail effectors permissive for the cyclic susceptibility to plasticity. We consider similarities of intracellular and membrane mechanisms underlying plasticity in SCN circadian plasticity and in hippocampal long-term potentiation (LTP). The emerging prominence of the hippocampal circadian clock points to iterative metaplasticity in that tissue as well. Exploring these links holds great promise for understanding circadian shaping of synaptic plasticity, learning, and memory.

Vasoactive intestinal peptide produces long-lasting changes in neural activity in the suprachiasmatic nucleus

The neuropeptide vasoactive intestinal peptide (VIP) is expressed at high levels in the neurons of the suprachiasmatic nucleus (SCN). While VIP is known to be important to the input and output pathways from the SCN, the physiological effects of VIP on electrical activity of SCN neurons are not well known. Here the impact of VIP on firing rate of SCN neurons was investigated in mouse slice cultures recorded during the night. The application of VIP produced an increase in electrical activity in SCN slices that lasted several hours after treatment. This is a novel mechanism by which this peptide can produce long-term changes in central nervous system physiology. The increase in action potential frequency was blocked by a VIP receptor antagonist and lost in a VIP receptor knockout mouse. In addition, inhibitors of both the Epac family of cAMP binding proteins and cAMP-dependent protein kinase (PKA) blocked the induction by VIP. The persistent increase in spike rate following VIP application was not seen in SCN neurons from mice deficient in Kv3 channel proteins and was dependent on the clock protein PER1. These findings suggest that VIP regulates the long-term firing rate of SCN neurons through a VIPR2-mediated increase in the cAMP pathway and implicate the fast delayed rectifier (FDR) potassium currents as one of the targets of this regulation.

Spatiotemporal distribution of vasoactive intestinal polypeptide receptor 2 in mouse suprachiasmatic nucleus

Vasoactive intestinal polypeptide (VIP) signaling is critical for circadian rhythms. For example, the expression of VIP and its main receptor, VPAC2R, is necessary for maintaining synchronous daily rhythms among neurons in the suprachiasmatic nucleus (SCN), a master circadian pacemaker in animals. Where and when VPAC2R protein is expressed in the SCN and other brain areas has not been examined. Using immunohistochemistry, we characterized a new antibody and found that VPAC2R was highly enriched in the SCN and detectable at low levels in many brain areas. Within the SCN, VPAC2R was circadian, peaking in the subjective morning, and abundantly expressed from the rostral to caudal margins with more in the dorsomedial than ventrolateral area. VPAC2R was found in nearly all SCN cells including neurons expressing either VIP or vasopressin (AVP). SCN neurons mainly expressed VPAC2R in their somata and dendrites, not axons. Finally, constant light increased VIP and AVP expression, but not VPAC2R. We conclude that the circadian clock, not the ambient light level, regulates VPAC2R protein localization. These results are consistent with VPAC2R playing a role in VIP signaling at all times of day, broadly throughout the brain and in all SCN cells.

Circadian regulation of reproduction: from gamete to offspring

Few challenges are more critical to the survival of a species than reproduction. To ensure reproductive success, myriad aspects of physiology and behaviour need to be tightly orchestrated within the animal, as well as timed appropriately with the external environment. This is accomplished through an endogenous circadian timing system generated at the cellular level through a series of interlocked transcription/translation feedback loops, leading to the overt expression of circadian rhythms. These expression patterns are found throughout the body, and are intimately interwoven with both the timing and function of the reproductive process. In this review we highlight the many aspects of reproductive physiology in which circadian rhythms are known to play a role, including regulation of the estrus cycle, the LH surge and ovulation, the production and maturation of sperm and the timing of insemination and fertilisation. We will also describe roles for circadian rhythms in support of the preimplantation embryo in the oviduct, implantation/placentation, as well as the control of parturition and early postnatal life. There are several key differences in physiology between humans and the model systems used for the study of circadian disruption, and these challenges to interpretation will be discussed as part of this review.

Role of vasoactive intestinal peptide in seasonal encoding by the suprachiasmatic nucleus clock

The neuropeptide vasoactive intestinal peptide (VIP) is critical for the proper functioning of the neural circuit that generates circadian rhythms. Mice lacking VIP show profound deficits in the ability to generate many behavioral and physiological rhythms. To explore how the loss of VIP impacts on the intact circadian system, we carried out in vivo multiunit neural activity (MUA) recordings from the suprachiasmatic nucleus of freely moving VIP knockout (KO) mice. The MUA rhythms were largely unaltered in the VIP KO mice, with no significant differences being seen in the amplitude or phase of the rhythms in light-dark conditions. Robust differences between the genotypes were revealed when the mice were transferred from light-dark to constant darkness conditions. In addition, the ability of the VIP KO mice to encode changes in photoperiod was examined. Strikingly, the behavioral and physiological rhythms of VIP KO mice showed no adaptation to short or long photoperiods. The data indicate that the intact circadian system can compensate for some of the consequences of the loss of VIP, whereas this peptide is indispensable for endogenous encoding of seasonal information.

Rhythm-promoting actions of exercise in mice with deficient neuropeptide signaling

Daily exercise promotes physical health as well as improvements in mental and neural functions. Studies in intact wild-type (WT) rodents have revealed that the brain's suprachiasmatic nuclei (SCN), site of the main circadian pacemaker, are also responsive to scheduled wheel running. It is unclear, however, if and how animals with a dysfunctional circadian pacemaker respond to exercise. Here, we tested whether scheduled voluntary exercise (SVE) in a running wheel for 6 hours per day could promote neural and behavioral rhythmicity in animals whose circadian competence is compromised through genetically targeted loss of vasoactive intestinal polypeptide (VIP(-/-) mice) or its VPAC(2) receptor (Vipr2(-/-) mice). We report that in constant dark (DD), rhythmic VIP(-/-) and Vipr2(-/-) mice show weak free-running rhythms with a period of <23 hours and all wild-type mice are strongly rhythmic with approximately 23.5-hour periodicity. VIP(-/-) and Vipr2(-/-) mice rapidly (<7 days) synchronize to daily SVE, while WT mice take much longer (>35 days). Following 21 to 50 days of SVE, WT mice show small changes in their rhythms, and most Vipr2(-/-) mice now sustain robust near 24-hour behavioral rhythms, whereas very few VIP(-/-) mice do. This study demonstrates that scheduled daily exercise can markedly improve circadian rhythms in behavioral activity and raises the possibility that this noninvasive approach may be useful as an intervention in clinical etiologies in which there are dysfunctions of circadian time keeping.

Physiological responses of the circadian clock to acute light exposure at night

Circadian rhythms in physiological, endocrine and metabolic functioning are controlled by a neural clock located in the suprachiasmatic nucleus (SCN). This structure is endogenously rhythmic and the phase of this rhythm can be reset by light information from the eye. A key feature of the SCN is that while it is a small structure containing on the order of about 20,000 cells, it is amazingly heterogeneous. It is likely that anatomical heterogeneity reflects an underlying functional heterogeneity. In this review, we examine the physiological responses of cells in the SCN to light stimuli that reset the phase of the circadian clock, highlighting where possible the spatial pattern of such responses. Increases in intracellular calcium are an important signal in response to light, and this increase triggers many biochemical cascades that mediate responses to light. Furthermore, only some cells in the SCN are actually endogenously rhythmic, and these cells likely do not receive strong direct input from the retina. Therefore, this review also considers how light information is conveyed from the retinorecipient cells to the endogenously rhythmic cells that track circadian phase. A number of neuropeptides, including vasoactive intestinal polypeptide, gastrin-releasing peptide and substance P, may be particularly important in relaying such signals, but other neurochemicals such as GABA and nitric oxide may participate as well. A thorough understanding of the intracellular and intercellular responses to light, as well as the spatial arrangements of such responses may help identify important pharmacological targets for therapeutic interventions to treat sleep and circadian disorders.

Angiotensin II regulates the activity of mouse suprachiasmatic nuclei neurons

Neuropeptide signaling plays key roles in coordinating cellular activity within the suprachiasmatic nuclei (SCN), site of the master circadian oscillator in mammals. The neuropeptide angiotensin II (ANGII) and its cognate receptor AT1, are both expressed by SCN cells, but unlike other SCN neurochemicals, very little is known about the cellular actions of ANGII within this circadian clock. We used multi-electrode, multiunit, extracellular electrophysiology, coupled with whole-cell voltage and current clamp techniques to investigate the actions of ANGII in mouse SCN slices. ANGII (0.001-10 microM) dose dependently stimulated and inhibited extracellularly recorded neuronal discharge in many SCN neurons ( approximately 60%). Both actions were blocked by pre-treatment with the AT1 receptor antagonist ZD7155 (0.03 microM), while suppressions but not activations were prevented by pre-treatment with the GABA A receptor antagonist bicuculline (20 microM). AT1 receptor blockade itself suppressed discharge in a subset ( approximately 30%) of SCN neurons, and this action was not blocked by bicuculline. In voltage-clamped SCN neurons (-70 mV), AT1 receptor activation dose-dependently enhanced the frequency of action potential-driven, GABA A receptor-mediated currents, but did not alter their responses to exogenously applied GABA. In current-clamped SCN neurons perfused with tetrodotoxin, ANGII induced a membrane depolarization with a concomitant decrease in input resistance. In conclusion we show that AT1 receptor activation by ANGII depolarizes SCN neurons and stimulates action potential firing, leading to increased GABA release in the mouse SCN. Additionally we provide the first evidence that endogenous AT1 receptor signaling tonically regulates the activities of some SCN neurons.

Decreased VIP and VPAC2 receptor expression in the biological clock of the R6/2 Huntington's disease mouse

Huntington's disease (HD) is a fatal genetic neurodegenerative disorder caused by a CAG triplet repeat expansion in the gene encoding the protein huntingtin. The most studied model of HD, the R6/2 transgenic mouse, replicates many features of the disease. In addition to motor, cognitive, and endocrine dysfunctions, these mice exhibit a progressive disruption of circadian rhythms. This is accompanied by an altered expression of the circadian clock genes in the suprachiasmatic nucleus/nuclei (SCN), the principal circadian pacemaker in the brain. The neuropeptide vasoactive intestinal polypeptide (VIP) and its receptor VPAC2 are highly expressed in the SCN, and VIPergic signaling plays an essential role in maintenance of ongoing circadian rhythmicity. We found a marked reduction in both VIP mRNA and VPAC2 receptor mRNA, quantified by RT-PCR, as well as a decrease in VIP immunostaining in the SCN of R6/2 mice. These changes were coupled to a disruption of circadian rhythm. We observed no loss of neurons in the SCN and therefore suggest that the changes in VIP and VPAC2 receptor are due to their decreased expression. In conclusion, we propose that impaired VIPergic signaling is an additional candidate mechanism for disruption of circadian rhythms in R6/2 mice.

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.

Vasoactive intestinal peptide and the mammalian circadian system

In mammals, the circadian oscillators that drive daily behavioral and endocrine rhythms are located in the hypothalamic suprachiasmatic nucleus (SCN). While the SCN is anatomically well-situated to receive and transmit temporal cues to the rest of the brain and periphery, there are many holes in our understanding of how this temporal regulation occurs. Unanswered questions include how cell autonomous circadian oscillations within the SCN remain synchronized to each other as well as communicate temporal information to downstream targets. In recent years, it has become clear that neuropeptides are critically involved in circadian timekeeping. One such neuropeptide, vasoactive intestinal peptide (VIP), defines a cell population within the SCN and is likely used as a signaling molecule by these neurons. Converging lines of evidence suggest that the loss of VIP or its receptor has a major influence on the ability of the SCN neurons to generate circadian oscillations as well as synchronize these cellular oscillations. VIP, acting through the VPAC(2) receptor, exerts these effects in the SCN by activating intracellular signaling pathways and, consequently, modulating synaptic transmission and intrinsic membrane currents. Anatomical evidence suggests that these VIP expressing neurons connect both directly and indirectly to endocrine and other output targets. Striking similarities exist between the role of VIP in mammals and the role of Pigment Dispersing Factor (PDF), a functionally related neuropeptide, in the Drosophila circadian system. Work in both mammals and insects suggests that further research into neuropeptide function is necessary to understand how circadian oscillators work as a coordinated system to impose a temporal structure on physiological processes within the organism.

Lack of the PAC1 receptor alters the circadian expression of VIP mRNA in the suprachiasmatic nucleus of mice

PACAP in the retinohypothalamic tract mediates photic information to the suprachiasmatic nucleus via the PAC1 receptor. The diurnal and circadian VIP mRNA expressions in the suprachiasmatic nucleus of PAC1-/- and wild type mice were quantified. During light/dark cycles identical VIP mRNA rhythms were found while the oscillation pattern differed between the two types of animals during constant darkness. The results show that the circadian VIP mRNA expression is influenced by the absence of PAC1 signalling.

Tóth Z, House SB, Gainer H. Depolarization and neurotransmitter regulation of vasopressin gene expression in the rat suprachiasmatic nucleus in vitro

Vasopressin (VP) transcription in the rat suprachiasmatic nucleus (SCN) in organotypic culture was studied by in situ hybridization histochemistry using an intron-specific VP heteronuclear RNA probe. The circadian peak of VP gene transcription in the SCN in vitro is completely blocked by a 2 h exposure to tetrodotoxin (TTX) in the culture medium, and this TTX inhibition of VP gene transcription is reversed by exposure of the SCN to either forskolin or potassium depolarization. This suggests that an intrinsic, spontaneously active neuronal mechanism in the SCN is responsible for the cAMP- and depolarization-dependent pathways involved in maintaining peak VP gene transcription. In this paper, we evaluate a variety of neurotransmitter candidates, membrane receptors, and signal-transduction cascades that might constitute the mechanisms responsible for the peak of VP gene transcription. We find that vasoactive intestinal peptide (VIP) and a VPAC2 (VIP receptor subtype 2) receptor-specific agonist, Ro-25-1553, are the most effective ligands tested in evoking a cAMP-mitogen-activated protein kinase signal transduction cascade leading to an increase in VP gene transcription in the SCN. In addition, a second independent pathway involving depolarization activating L-type voltage-gated calcium channels and a Ca-dependent kinase pathway [inhibited by KN62 (1-[N,O-bis(5-isoquinolinesulphonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine)] rescues VP gene transcription in the presence of TTX. In the absence of TTX, these independent pathways appear to act in a cooperative manner to generate the circadian peak of VP gene transcription in the SCN.

The molecular gatekeeper Dexras1 sculpts the photic responsiveness of the mammalian circadian clock

The mammalian master clock, located in the suprachiasmatic nucleus (SCN), is exquisitely sensitive to photic timing cues, but the key molecular events that sculpt both the phasing and magnitude of responsiveness are not understood. Here, we show that the Ras-like G-protein Dexras1 is a critical factor in these processes. Dexras1-deficient mice (dexras1-/-) exhibit a restructured nighttime phase response curve and a loss of gating to photic resetting during the day. Dexras1 affects the photic sensitivity by repressing or activating time-of-day-specific signaling pathways that regulate extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK). During the late night, Dexras1 limits the capacity of pituitary adenylate cyclase (PAC) activating peptide (PACAP)/PAC1 to affect ERK/MAPK, and in the early night, light-induced phase delays, which are mediated predominantly by NMDA receptors, are reduced as reported previously. Daytime photic phase advances are mediated by a novel signaling pathway that does not affect the SCN core but rather stimulates ERK/MAPK in the SCN shell and triggers downregulation of clock protein expression.

Disrupted neuronal activity rhythms in the suprachiasmatic nuclei of vasoactive intestinal polypeptide-deficient mice

Vasoactive intestinal polypeptide (VIP), acting via the VPAC(2) receptor, is a key signaling pathway in the suprachiasmatic nuclei (SCN), the master clock controlling daily rhythms in mammals. Most mice lacking functional VPAC(2) receptors are unable to sustain behavioral rhythms and lack detectable SCN electrical rhythms in vitro. Adult mice that do not produce VIP (VIP/PHI(-/-)) exhibit less severe alterations in wheel-running rhythms, but the effects of this deficiency on the amplitude, phasing, or periodicity of their SCN cellular rhythms are unknown. To investigate this, we used suction electrodes to extracellularly record multiple- and single-unit electrical activity in SCN brain slices from mice with varying degrees of VIP deficiency, ranging from wild-type (VIP/PHI(+/+)) to heterozygous (VIP/PHI(+/-)) and VIP/PHI(-/-) animals. We found decreasing proportions of rhythmic cells in SCN slices from VIP/PHI(+/+) ( approximately 91%, n = 23) through VIP/PHI(-/+) ( approximately 71%, n = 28) to VIP/PHI(-/-) mice (62%; n = 37) and a parallel trend toward decreasing amplitude in the remaining rhythmic cells. SCN neurons from VIP/PHI(-/-) mice exhibited a broad range in the period and phasing of electrical rhythms, concordant with the known alterations in their behavioral rhythms. Further, treatment of VIP/PHI(-/-) slices with a VPAC(2) receptor antagonist significantly reduced the proportion of oscillating neurons, suggesting that VPAC(2) receptors still become activated in the SCN of these mice. The results establish that VIP is important for appropriate periodicity and phasing of SCN neuronal rhythms and suggest that residual VPAC(2) receptor signaling promotes rhythmicity in adult VIP/PHI(-/-) mice.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Neurotransmitters of the suprachiasmatic nuclei

There has been extensive research in the recent past looking into the molecular basis and mechanisms of the biological clock, situated in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Neurotransmitters are a very important component of SCN function. Thorough knowledge of neurotransmitters is not only essential for the understanding of the clock but also for the successful manipulation of the clock with experimental chemicals and therapeutical drugs. This article reviews the current knowledge about neurotransmitters in the SCN, including neurotransmitters that have been identified only recently. An attempt was made to describe the neurotransmitters and hormonal/diffusible signals of the SCN efference, which are necessary for the master clock to exert its overt function. The expression of robust circadian rhythms depends on the integrity of the biological clock and on the integration of thousands of individual cellular clocks found in the clock. Neurotransmitters are required at all levels, at the input, in the clock itself, and in its efferent output for the normal function of the clock. The relationship between neurotransmitter function and gene expression is also discussed because clock gene transcription forms the molecular basis of the clock and its working.

Regulation of glutamatergic signalling by PACAP in the mammalian suprachiasmatic nucleus

Background

Previous studies indicate that light information reaches the suprachiasmatic nucleus (SCN) through a subpopulation of retinal ganglion cells that contain both glutamate and pituitary adenylyl cyclase activating peptide (PACAP). While the role of glutamate in this pathway has been well studied, the involvement of PACAP and its receptors are only beginning to be understood. Speculating that PACAP may function to modulate how neurons in the suprachiasmatic nucleus respond to glutamate, we used electrophysiological and calcium imaging tools to examine possible cellular interactions between these co-transmitters.

Results

Exogenous application of PACAP increased both the amplitude and frequency of spontaneous excitatory postsynaptic currents recorded from SCN neurons in a mouse brain slice preparation. PACAP also increased the magnitude of AMPA-evoked currents through a mechanism mediated by PAC1 receptors and the adenylyl cyclase-signalling cascade. This enhancement of excitatory currents was not limited to those evoked by AMPA as the magnitude of NMDA currents were also enhanced by application of PACAP. Furthermore, PACAP enhanced AMPA and NMDA evoked calcium transients while PACAP alone produced very little change in resting calcium in most mouse SCN neurons. Finally, in rat SCN neurons, exogenous PACAP enhanced AMPA evoked currents and calcium transients as well evoked robust calcium transients on its own.

Conclusion

The results reported here show that PACAP is a potent modulator of glutamatergic signalling within the SCN in the early night.

VIP receptors control excitability of suprachiasmatic nuclei neurones

The role of vasoactive intestinal polypeptide (VIP) receptors on excitable properties of neurones in slices acutely prepared from the suprachiasmatic nuclei (SCN) of wild-type (WT) and VPAC(2)-receptor-deficient (Vipr2 ( -/- )) mice was studied under voltage clamp with the use of patch-clamp recording in the whole-cell configuration. The resting membrane potential in Vipr2 ( -/- ) neurones was significantly hyperpolarised as compared to WT cells (-60+/-7 vs -72+/-6 mV, p<0.01). Bath application of 100 nM VIP or the VPAC(2) receptor agonist RO 25-1553 triggered a slow inward current in a subpopulation of WT SCN neurones; the VIP-induced current was not affected by slice incubation with 25 microM of bicuculline but disappeared completely when the cells were dialysed with CsCl-containing/K(+)-free solution. Application of VIP or RO 25-1553 to neurones from Vipr2 ( -/- ) mice did not induce currents in all cells tested. Incubation of WT slices with 100 nM VIP or RO 25-1553 resulted in inhibition of fast tetrodotoxin-sensitive sodium currents and delayed rectifier K(+) currents in most of the cells tested. This effect was completely absent in cells from Vipr2 ( -/- ) mice. We postulate that VIP receptors control excitability of SCN neurones at the postsynaptic level by direct modulation of membrane potential via inhibition of K(+) channels and by tonic inhibition of sodium and potassium voltage-gated currents.

Vasoactive intestinal polypeptide mediates circadian rhythmicity and synchrony in mammalian clock neurons

The mammalian suprachiasmatic nucleus (SCN) is a master circadian pacemaker. It is not known which SCN neurons are autonomous pacemakers or how they synchronize their daily firing rhythms to coordinate circadian behavior. Vasoactive intestinal polypeptide (VIP) and the VIP receptor VPAC(2) (encoded by the gene Vipr2) may mediate rhythms in individual SCN neurons, synchrony between neurons, or both. We found that Vip(-/-) and Vipr2(-/-) mice showed two daily bouts of activity in a skeleton photoperiod and multiple circadian periods in constant darkness. Loss of VIP or VPAC(2) also abolished circadian firing rhythms in approximately half of all SCN neurons and disrupted synchrony between rhythmic neurons. Critically, daily application of a VPAC(2) agonist restored rhythmicity and synchrony to VIP(-/-) SCN neurons, but not to Vipr2(-/-) neurons. We conclude that VIP coordinates daily rhythms in the SCN and behavior by synchronizing a small population of pacemaking neurons and maintaining rhythmicity in a larger subset of neurons.

Transgenic approach reveals expression of the VPAC2 receptor in phenotypically defined neurons in the mouse suprachiasmatic nucleus and in its efferent target sites

Circadian rhythms in mammals depend on the properties of cells in the suprachiasmatic nucleus (SCN). The retino-recipient core of the mouse SCN is characterized by vasoactive intestinal peptide (VIP) neurons. Expression within the SCN of VPAC2, a VIP receptor, is required for circadian rhythmicity. Using transgenic mice with beta-galactosidase as a marker for VPAC2, we have phenotyped VPAC2-expressing cells within the SCN and investigated expression of the VPAC2 marker at sites previously shown to receive VIP-containing SCN efferents. In situ hybridization and immunohistochemistry demonstrated identical distributions for VPAC2 mRNA and beta-galactosidase and coexpression of the two signals in the SCN. Double-label confocal immunofluorescence identified beta-galactosidase in 32% of the VIP and 31% of the calretinin neurons in the SCN core. Of the arginine-vasopressin neurons that characterize the SCN shell, 45% expressed beta-galactosidase. In contrast, this marker was not apparent in astrocytes within the SCN core or shell. Cell bodies containing beta-galactosidase were detected at sites reportedly receiving VIP-containing SCN efferents, including the subparaventricular zone and lateral septum and the anteroventral periventricular, preoptic suprachiasmatic, medial preoptic and paraventricular hypothalamic nuclei. The detection of a marker for VPAC2 expression in the SCN in almost one-third of the VIP and calretinin core neurons and nearly half of the arginine-vasopressin shell neurons and also in cell bodies at sites receiving VIP-immunoreactive projections from the SCN indicates that VPAC2 may contribute to autoregulation and/or coupling within the SCN core and to the control of the SCN shell and sites distal to this nucleus.

Expression of VIP and/or PACAP receptor mRNA in peptide synthesizing cells within the suprachiasmatic nucleus of the rat and in its efferent target sites

The suprachiasmatic nucleus (SCN) contains the predominant circadian pacemaker in mammals. Considerable evidence indicates that VPAC(2) and PAC(1), receptors for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP), play critical roles in maintaining and entraining circadian rhythms. Retinal projections to the rat SCN contain PACAP and terminate mostly in the ventral SCN, the site of VIP neurons. The incidence of VPAC(2) and PAC(1) mRNAs within distinct neuronal populations of the rat SCN has been determined using double-label in situ hybridization. VPAC(2) mRNA was detected in almost all arginine-vasopressin (AVP) neurons of the dorsomedial SCN and in 41% of the VIP neurons; somatostatin (SST) neurons, predominantly in dorsomedial and intermediate regions, showed a decreased incidence (23%). PAC(1) mRNA was present in nearly half of the VIP and SST neurons (45% and 40%, respectively) and in one-third of the AVP neurons (32%). Cells expressing VPAC(2) mRNA also were detected in diencephalic areas that receive VIP-immunoreactive SCN efferents, such as the peri-suprachiasmatic region, lateral subparaventricular zone, parvocellular hypothalamic paraventricular subdivisions, dorsomedial hypothalamic nucleus, and anterior thalamic paraventricular and paratenial nuclei. The extensive distribution of PAC(1) mRNA within the SCN suggests that actions of PACAP are not restricted to the predominantly retinorecipient region. The presence of VPAC(2) mRNA in nearly half the VIP neurons, in almost all the AVP neurons, and at sites receiving VIP-immunoreactive SCN efferents suggests that the SCN VIP neurons are coupled and/or autoregulated and also influence the AVP-containing dorsomedial SCN and distal sites via VPAC(2).

The brain's calendar: neural mechanisms of seasonal timing

The suprachiasmatic nucleus (SCN) of the hypothalamus is the principal component of the mammalian biological clock, the neural timing system that generates and coordinates a broad spectrum of physiological, endocrine and behavioural circadian rhythms. The pacemaker of the SCN oscillates with a near 24 h period and is entrained to the diurnal light-dark cycle. Consistent with its role in circadian timing, investigations in rodents and non-human primates furthermore suggest that the SCN is the locus of the brain's endogenous calendar, enabling organisms to anticipate seasonal environmental changes. The present review focuses on the neuronal organization and dynamic properties of the biological clock and the means by which it is synchronized with the environmental lighting conditions. It is shown that the functional activity of the biological clock is entrained to the seasonal photic cycle and that photoperiod (day length) may act as an effective zeitgeber. Furthermore, new insights are presented, based on electrophysiological and molecular studies, that the mammalian circadian timing system consists of coupled oscillators and that the clock genes of these oscillators may also function as calendar genes. In summary, there are now strong indications that the neuronal changes and adaptations in mammals that occur in response to a seasonally changing environment are driven by an endogenous circadian clock located in the SCN, and that this neural calendar is reset by the seasonal fluctuations in photoperiod.

Vasoactive intestinal polypeptide phase-advances the rat suprachiasmatic nuclei circadian pacemaker in vitro via protein kinase A and mitogen-activated protein kinase

The firing activity of suprachiasmatic nuclei (SCN) neurones in vitro shows a circadian rhythm with the peak in average firing frequency during the middle of the projected day. Vasoactive intestinal polypeptide (VIP), which is abundant in the rat SCN, phase-advances the time of peak in SCN neuronal activity when applied in vitro during the late projected night; an effect most likely mediated via the VPAC(2) receptor. Employing extracellular single-unit electrophysiology, we investigated the influence of protein kinase A (PKA) inhibitor KT5720 or mitogen-activated protein kinase kinase (MAPKK) inhibitor U0126 on phase advances evoked during the late projected night by VIP or Ro 25-1553, a VPAC(2) receptor agonist. Both inhibitors blocked VIP or Ro 25-1553-induced phase advances, indicating that the late-night resetting actions of VIP via the VPAC(2) receptor require PKA and MEK.

Disrupted circadian rhythms in VIP- and PHI-deficient mice

The related neuropeptides vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) are expressed at high levels in the neurons of the suprachiasmatic nucleus (SCN), but their function in the regulation of circadian rhythms is unknown. To study the role of these peptides on the circadian system in vivo, a new mouse model was developed in which both VIP and PHI genes were disrupted by homologous recombination. In a light-dark cycle, these mice exhibited diurnal rhythms in activity which were largely indistinguishable from wild-type controls. In constant darkness, the VIP/PHI-deficient mice exhibited pronounced abnormalities in their circadian system. The activity patterns started approximately 8 h earlier than predicted by the previous light cycle. In addition, lack of VIP/PHI led to a shortened free-running period and a loss of the coherence and precision of the circadian locomotor activity rhythm. In about one-quarter of VIP/PHI mice examined, the wheel-running rhythm became arrhythmic after several weeks in constant darkness. Another striking example of these deficits is seen in the split-activity patterns expressed by the mutant mice when they were exposed to a skeleton photoperiod. In addition, the VIP/PHI-deficient mice exhibited deficits in the response of their circadian system to light. Electrophysiological analysis indicates that VIP enhances inhibitory synaptic transmission within the SCN of wild-type and VIP/PHI-deficient mice. Together, the observations suggest that VIP/PHI peptides are critically involved in both the generation of circadian oscillations as well as the normal synchronization of these rhythms to light.

Regulation of inhibitory synaptic transmission by vasoactive intestinal peptide (VIP) in the mouse suprachiasmatic nucleus

Circadian rhythmicity in mammals is generated by a pair of nuclei in the anterior hypothalamus known as the suprachiasmatic nuclei (SCN), whose neurons express a variety of neuropeptides that are thought to play an important role in the circadian timing system. To evaluate the influence of VIP on inhibitory synaptic transmission between SCN neurons, we used whole cell patch-clamp recording in an acute brain slice preparation of mouse SCN. Baseline spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) varied significantly between regions and across phases, with a greater frequency of IPSCs observed in the dorsomedial region during the early night. Bath-applied VIP caused a significant increase in the frequency of spontaneous inhibitory postsynaptic currents (sIPSC) in a reversible and dose-dependent manner with no effect on the mean amplitude or kinetic parameters. The effect of VIP was widespread throughout the SCN and observed in both ventrolateral (VL) and dorsomedial (DM) regions. In the presence of tetrodotoxin, VIP increased the frequency of miniature IPSCs without affecting the mean magnitude or kinetic parameters. The magnitude of the enhancement by VIP was significantly larger during the day than during the night. Pretreatment with the VIP-PACAP receptor antagonist [Ac-Tyr1, D-Phe2]-GHRF 1-29 or the selective VPAC2 receptor antagonist PG 99-465 completely blocked the VIP-induced enhancement. The effect of VIP appears to be mediated by a cAMP/PKA-dependent mechanism as forskolin mimics, while the PKA antagonist H-89 blocks the observed enhancement of GABA currents. Our data suggest that VIP activates presynaptic VPAC2 receptors to regulate inhibitory synaptic transmission within the SCN and that this effect varies from day to night.

Vasoactive intestinal peptide (VIP) and VIP mRNA decrease in the cerebral cortex of nNOS knock-out(-/-) mice

Although there is much evidence showing that NO regulates the release of VIP in several areas, there is no report about the influence of NO on VIP in the cerebral cortex. We therefore examined changes in VIP expression in the cerebral cortex of nNOS knock-out(-/-) mice using immunohistochemistry and in situ hybridization. The nNOS((-/-)) mice had significantly fewer VIP-immunoreactive neurons than the control mice and the VIP mRNA as well as the VIP-immunoreactivity of the individual neuron was decreased in the nNOS((-/-)) mice. The first demonstration of decrease in VIP expression in the cerebral cortex of nNOS((-/-)) mice may provide useful data for investigating the relation between NO and VIP in the cerebral cortex and the mechanisms of many functions of these two neurotransmitters.

A hVIPR transgene as a novel tool for the analysis of circadian function in the mouse suprachiasmatic nucleus

A mouse bearing a novel transgene encoding the human VPAC2 receptor (hVIPR; Shen et al. (2000) PNAS, 97, 11575-11580) was used to investigate circadian function in the hypothalamic suprachiasmatic nuclei (SCN). Neurons expressing hVPAC2R, detected by a beta-galactosidase (beta-GAL) tag, have a distinct distribution within the SCN, closely matching that of neurophysin (NP) neurons and extending into the region of peptide histidine isoleucine (PHI) cells. In common with NP and PHI cells, neurons expressing hVPAC2R are circadian in nature, as revealed by synchronous rhythmic expression of mPERIOD (mPER) proteins. A population of SCN cells not expressing PHI, NP or hVPAC2R exhibited circadian PER expression antiphasic with the rest of the SCN. Nocturnal light exposure induced mPER1 in the ventral SCN and mPER2 widely across the nucleus. Induction of nuclear mPER2 in hVPAC2R cells confirmed their photic responsiveness. Having established their circadian properties, we tested the utility of SCN neurons expressing the hVIPR transgene as functionally and anatomically explicit markers for SCN tissue grafts. Prenatal SCN tissue from hVIPR transgenic pups survived transplantation into adult CD1 mice, and expressed beta-GAL, PER and PHI. Over a series of studies, hVIPR transgenic SCN grafts restored circadian activity rhythms to 17 of 72 arrhythmic SCN lesioned recipients (23.6%). By using heterozygous hVIPR transgenic grafts on a heterozygous Clock mutant background we confirmed that restored activity rhythms were conferred by the donor tissue. We conclude that the hVIPR transgene is a powerful and flexible tool for examination of circadian function in the mouse SCN.

An essential role for peptidergic signalling in the control of circadian rhythms in the suprachiasmatic nuclei

Two structurally related neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP), colocalized with glutamate in neurones of the retinohypothalamic tract, and vasoactive intestinal peptide (VIP), present in light-responsive cells of the suprachiasmatic nuclei (SCN), appear to play distinct and important roles in the control of mammalian circadian rhythms. Mice deficient in the PACAP-selective PAC1 receptor exhibit altered responsiveness of the SCN clock to light-induced phase-shifts, but display robust circadian patterns of wheel-running behaviour. By contrast, our studies of mice lacking the VPAC2 receptor, which responds to both PACAP and VIP, indicate that this receptor plays a critical role in rhythm generation in the SCN. The predominant factor determining wheel-running activity in VPAC2 receptor null (Vipr2-/-) mice is "masking" by light. Mutant animals re-entrain immediately to advances or delays in the light/dark cycle and do not exhibit robust circadian rhythms of behaviour when in constant darkness. The mice do not exhibit circadian expression of core clock genes (mPer1, mPer2, mCry1), or of the clock-controlled gene arginine vasopressin (AVP), in the SCN. We propose that VIP signalling between SCN neurones provides a paracrine reinforcing signal that is essential for sustained rhythm generation. The presence of VIP signalling in the SCN may explain why SCN neurones are capable of generating long-lasting self-sustained oscillations, whereas rhythmic clock gene expression in other tissues is dependent on periodic reinforcement by neural or hormonal signals.

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.

Neurotensin phase-shifts the firing rate rhythm of neurons in the rat suprachiasmatic nuclei in vitro

The suprachiasmatic nuclei (SCN) of the hypothalamus house the main mammalian circadian pacemaker. Cell bodies in the rat SCN contain the neuropeptide neurotensin (NT), and two NT receptor types, NTS1 and nts2. Because the role of NT in the circadian rhythm processes is unknown, we studied the phase-shifting effects of NT on the firing rate rhythm of rat SCN neurons in vitro. Additionally, the NT receptor antagonists SR142948a and SR48692 were used to try and block any NT-induced phase shifts. To elucidate the second messenger pathway responsible for mediating the phase-resetting actions of NT, we utilized the phospholipase C (PLC) and protein kinase A (PKA) inhibitors U-73122 and KT5720, respectively. Application of NT during the projected day resulted in a large advance in the time of peak in FRR, whereas treatments during the projected night had no effect. Both NT receptor antagonists blocked the NT-induced phase shifts, as did the PLC inhibitor U-73122. The PKA inhibitor KT5720 had no influence on the magnitude of the phase shift caused by NT during the middle of the projected day. These results provide the first evidence that NT may play a role in regulating the rat circadian pacemaker, using NTS1 and nts2 receptors presumably coupled to PLC.