Identification of PAC1 receptor isoform mRNAs by real-time PCR in rat suprachiasmatic nucleus

The VIP/VPAC1R Pathway Regulates Energy and Glucose Homeostasis by Modulating GLP-1, Glucagon, Leptin and PYY Levels in Mice

Simple Summary

The current study is the first complete characterization of the phenotypic, metabolic, calorimetric, and homeostatic effects of VPAC1R in a null murine model. To evaluate the role of VPAC1R on body phenotype, feeding behavior, glucose/energy homeostasis, metabolic rate and plasma hormones, a long-term study was conducted in VPAC1R^−/− and WT mice. The outcome data document that VPAC1R^−/− mice have altered metabolism and insulin intolerance, with significant increase of feeding bouts, reduction of total energy expenditure and respiratory gases during both the dark and light cycle, together with elevated fasting levels of GLP-1 and PYY, and higher postprandial levels of GLP-1, glucagon leptin and PYY. These findings suggests that VPAC1R controls glucose homeostasis and energy balance by regulating plasma metabolic hormones.

Abstract

Vasoactive Intestinal Peptide binds with high affinity to VPAC1R and VPAC2R, thus regulating key physiologic functions. Previously, we documented in VIP^−/− mice a leaner body phenotype and altered metabolic hormones. Past reports described in VPAC2^−/− mice impaired circadian rhythm, reduced food intake, and altered metabolism. To better define the effects of VPAC1R on body phenotype, energy/glucose homeostasis, and metabolism, we conducted a 12-week study in a VPAC1R null model. Our results reveal that VPAC1^−/− mice experienced significant metabolic alterations during the dark cycle with greater numbers of feeding bouts (p = 0.009), lower Total Energy Expenditure (p = 0.025), VO₂ (p = 0.029), and VCO₂ (p = 0.016); as well as during the light cycle with lower Total Energy Expenditure (p = 0.04), VO₂ (p = 0.044), and VCO₂ (p = 0.029). Furthermore, VPAC1^−/− mice had significantly higher levels of GLP-1 and PYY during fasting, and higher levels of GLP-1, glucagon leptin and PYY during postprandial conditions. In addition, VPAC1^−/− mice had lower levels of glucose at 60′ and 120′, as assessed by insulin tolerance test. In conclusion, this study supports a key role for VPAC1R in the regulation of body glucose/energy homeostasis and metabolism.

Genomic factors underlying sex differences in trauma-related disorders

Post-traumatic stress disorder (PTSD) is a devastating illness with treatment that is effective in only approximately half of the population. This limited rate of response highlights the necessity for research into underlying individual biological mechanisms that mediate development and progression of this disease, allowing for identification of patient-specific treatments. PTSD has clear sex differences in both risk and symptom patterns. Thus, one approach is to characterize trauma-related changes between men and women who exhibit differences in treatment efficacy and response to trauma. Recent technological advances in sequencing have identified several genomic loci and transcriptional changes that are associated with post-trauma symptomatology. However, although the diagnosis of PTSD is more prevalent in women, the genetic factors underlying sex differences remain poorly understood. Here, we review recent work that highlights current understanding and limitations in the field of sex differences in PTSD and related symptomatology.

Current State of Understanding of the Role of PACAP in the Hypothalamo-Hypophyseal Gonadotropin Functions of Mammals

PACAP was discovered 30 years ago in Dr. Akira Arimura's laboratory. In the past three decades since then, it has become evident that this peptide plays numerous crucial roles in mammalian organisms. The most important functions of PACAP are the following: 1. neurotransmitter, 2. neuromodulator, 3. hypophysiotropic hormone, 4. neuroprotector. This paper reviews the accumulated data regarding the distribution of PACAP and its receptors in the mammalian hypothalamus and pituitary gland, the role of PACAP in the gonadotropin hormone secretion of females and males. The review also summarizes the interaction between PACAP, GnRH, and sex steroids as well as hypothalamic peptides including kisspeptin. The possible role of PACAP in reproductive functions through the biological clock is also discussed. Finally, the significance of PACAP in the hypothalamo-hypophysial system is considered and the facts missing, that would help better understand the function of PACAP in this system, are also highlighted.

Stress-related disorders, pituitary adenylate cyclase-activating peptide (PACAP)ergic system, and sex differences

Trauma-related disorders, such as posttraumatic stress disorder (PTSD) are remarkably common and debilitating, and are often characterized by dysregulated threat responses. Across numerous epidemiological studies, females have been found to have an approximately twofold increased risk for PTSD and other stress-related disorders. Understanding the biological mechanisms of this differential risk is of critical importance. Recent data suggest that the pituitary adenylate cyclase-activating polypeptide (PACAP) pathway is a critical regulator of the stress response across species. Moreover, increasing evidence suggests that this pathway is regulated by both stress and estrogen modulation and may provide an important window into understanding mechanisms of sex differences in the stress response. We have recently shown that PACAP and its receptor (PAC1R) are critical mediators of abnormal processes after psychological trauma. Notably, in heavily traumatized human subjects, there appears to be a robust sex-specific association of PACAP blood levels and PAC1R gene variants with fear physiology, PTSD diagnosis, and symptoms, specifically in females. The sex-specific association occurs within a single-nucleotide polymorphism (rs2267735) that resides in a putative estrogen response element involved in PAC1R gene regulation. Complementing these human data, the PAC1R messenger RNA is induced with fear conditioning or estrogen replacement in rodent models. These data suggest that perturbations in the PACAP-PAC1R pathway are regulated by estrogen and are involved in abnormal fear responses underlying PTSD.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

Alternative Splicing of the Pituitary Adenylate Cyclase-Activating Polypeptide Receptor PAC1: Mechanisms of Fine Tuning of Brain Activity

Alternative splicing of the precursor mRNA encoding for the neuropeptide receptor PAC1/ADCYAP1R1 generates multiple protein products that exhibit pleiotropic activities. Recent studies in mammals and zebrafish have implicated some of these splice isoforms in control of both cellular and body homeostasis. Here, we review the regulation of PAC1 splice variants and their underlying signal transduction and physiological processes in the nervous system.

PACAP causes PAC1/VPAC2 receptor mediated hypertension and sympathoexcitation in normal and hypertensive rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an excitatory neuropeptide that plays an important role in hypertension and stress responses. PACAP acts at three G protein-coupled receptors [PACAP type 1 receptor (PAC(1)) and vasoactive intestinal peptide receptor types 1 and 2 (VPAC(1) and VPAC(2))] and is localized to sites involved in cardiovascular control, most significantly the rostral ventrolateral medulla (RVLM). The RVLM is crucial for the tonic and reflex control of efferent sympathetic activity. Increases in sympathetic activity are observed in most types of hypertension and heart failure. PACAP delivered intrathecally also causes massive sympathoexcitation. We aimed to determine the presence and abundance of the three PACAP receptors in the RVLM, the role, in vivo, of PACAP in the RVLM on tonic and reflex cardiovascular control, and the contribution of PACAP to hypertension in the spontaneously hypertensive rat (SHR). Data were obtained using quantitative PCR and microinjection of PACAP and its antagonist, PACAP(6-38), into the RVLM of anesthetized artificially ventilated normotensive rats or SHRs. All three receptors were present in the RVLM. PACAP microinjection into the RVLM caused sustained sympathoexcitation and tachycardia with a transient hypertension but did not affect homeostatic reflexes. The responses were partially mediated through PAC(1)/VPAC(2) receptors since the effect of PACAP was attenuated (∼50%) by PACAP(6-38). PACAP was not tonically active in the RVLM in this preparation because PACAP(6-38) on its own had no inhibitory effect. PACAP has long-lasting cardiovascular effects, but altered PACAP signaling within the RVLM is not a cause of hypertension in the SHR.

Neuropeptide-mediated calcium signaling in the suprachiasmatic nucleus network

Neuroactive peptides and the intracellular calcium concentration ([Ca(2+) ](i) ) play important roles in light-induced modulation of gene expression in the suprachiasmatic nucleus (SCN) neurons that ultimately control behavioral rhythms. Vasoactive intestinal peptide (VIP) and arginine vasopressin (AVP) are expressed rhythmically within populations of SCN neurons. Pituitary adenylate cyclase-activating peptide (PACAP) is released from retinohypothalamic tract (RHT) terminals synapsing on SCN neurons. Nociceptin/orphanin FQ (OFQ) receptors are functionally expressed in the SCN. We examined the role of several neuropeptides on Ca(2+) signaling, simultaneously imaging multiple neurons within the SCN neural network. VIP reduced the [Ca(2+) ](i) in populations of SCN neurons during the day, but had little effect at night. Stimulation of the RHT at frequencies that simulate light input signaling evoked transient [Ca(2+) ](i) elevations that were not altered by VIP. AVP elevated the [Ca(2+) ](i) during both the day and night, PACAP produced variable responses, and OFQ induced a reduction in the [Ca(2+) ](i) similar to VIP. During the day, VIP lowered the [Ca(2+) ](i) to near nighttime levels, while AVP elevated [Ca(2+) ](i) during both the day and night, suggesting that the VIP effects on [Ca(2+) ](i) were dependent, and the AVP effects independent of the action potential firing activity state of the neuron. We hypothesize that VIP and AVP regulate, at least in part, Ca(2+) homeostasis in SCN neurons and may be a major point of regulation for SCN neuronal synchronization.

Pituitary adenylate cyclase-activating polypeptide stimulates glucose production via the hepatic sympathetic innervation in rats

OBJECTIVE

The unraveling of the elaborate brain networks that control glucose metabolism presents one of the current challenges in diabetes research. Within the central nervous system, the hypothalamus is regarded as the key brain area to regulate energy homeostasis. The aim of the present study was to investigate the hypothalamic mechanism involved in the hyperglycemic effects of the neuropeptide pituitary adenylyl cyclase-activating polypeptide (PACAP).

RESEARCH DESIGN AND METHODS

Endogenous glucose production (EGP) was determined during intracerebroventricular infusions of PACAP-38, vasoactive intestinal peptide (VIP), or their receptor agonists. The specificity of their receptors was examined by coinfusions of receptor antagonists. The possible neuronal pathway involved was investigated by 1) local injections in hypothalamic nuclei, 2) retrograde neuronal tracing from the thoracic spinal cord to hypothalamic preautonomic neurons together with Fos immunoreactivity, and 3) specific hepatic sympathetic or parasympathetic denervation to block the autonomic neuronal input to liver.

RESULTS

Intracerebroventricular infusion of PACAP-38 increased EGP to a similar extent as a VIP/PACAP-2 (VPAC2) receptor agonist, and intracerebroventricular administration of VIP had significantly less influence on EGP. The PACAP-38 induced increase of EGP was significantly suppressed by preinfusion of a VPAC2 but not a PAC1 receptor antagonist, as well as by hepatic sympathetic but not parasympathetic denervation. In the hypothalamus, Fos immunoreactivity induced by PACAP-38 was colocalized within autonomic neurons in paraventricular nuclei projecting to preganglionic sympathetic neurons in the spinal cord. Local infusion of PACAP-38 directly into the PVN induced a significant increase of EGP.

CONCLUSIONS

This study demonstrates that PACAP-38 signaling via sympathetic preautonomic neurons located in the paraventricular nucleus is an important component in the hypothalamic control of hepatic glucose production.

The role of PACAP in central cardiorespiratory regulation

Pituitary adenylate cyclase activating polypeptide (PACAP) plays a role in almost every biological process from reproduction to hippocampal function. One area where a role for PACAP is not clearly delineated is central cardiorespiratory regulation. PACAP and its receptors (PAC1, VPAC1 and VPAC2) are present in cardiovascular areas of the ventral medulla and spinal cord and in the periphery. Central administration of PACAP generally increases arterial pressure. Knowledge about the role of PACAP in central cardiovascular regulation is growing, but even less is known about PACAP in central respiratory regulation. No specific data is currently available regarding the presence of PACAP or receptors in key respiratory centers, although it is known that neonatal PACAP knock-out mice die suddenly in a manner similar to sudden infant death syndrome (SIDS). Future studies in mature preparations investigating the role of PACAP in the physiology and integration of central cardiorespiratory reflexes are clearly essential for a full understanding of this important neuropeptide in breathing.

Localization, characterization and function of pituitary adenylate cyclase-activating polypeptide during brain development

Neural development is controlled by region-specific factors that regulate cell proliferation, migration and differentiation. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that exerts a wide range of effects on different cell types in the brain as early as the fetal stage. Here we review current knowledge concerning several aspects of PACAP expression in embryonic and neonatal neural tissue: (i) the distribution of PACAP and PACAP receptors mRNA in the developing brain; (ii) the characteristic generation of neurons, astrocytes and oligodendrocytes in brain areas where the PACAP receptor is expressed and (iii) the role of PACAP as a regulator of neural development, inducing differentiation and proliferation in association with other trophic factors or signal transduction molecules.

Neural adrenergic/cyclic AMP regulation of the immunoglobulin E receptor alpha-subunit expression in the mammalian pinealocyte: a neuroendocrine/immune response link?

The high affinity immunoglobulin E receptor (FcepsilonRI) complex is dedicated to immunoglobulin E-mediated allergic responses. Expression of the FcepsilonRI receptor is thought to be relatively stable and limited to mast cells, basophils, eosinophils, monocytes, Langerhans cells, platelets, and neutrophils. We now report that the FcepsilonRIalpha and FcepsilonRIgamma polypeptides are expressed in the pinealocyte, the melatonin-secreting cell of the pineal gland. Moreover, Fcer1a mRNA levels increased approximately 100-fold at night to levels that were higher than in other tissues examined. Pineal FcepsilonRIalpha protein also increased markedly at night from nearly undetectable daytime levels. Our studies indicate that pineal Fcer1a mRNA levels are controlled by a well described neural pathway that controls pineal function. This pathway includes the master circadian oscillator in the suprachiasmatic nucleus and passes through central and peripheral structures. The circadian expression of FcepsilonRIalpha in the pineal gland is driven by this neural circuit via an adrenergic/cyclic AMP mechanism. Pineal FcepsilonRIalpha and FcepsilonRIgamma may represent a previously unrealized molecular link between the neuroendocrine and immune systems.

Gene deletion reveals roles for annexin A1 in the regulation of lipolysis and IL-6 release in epididymal adipose tissue

In this study, epididymal adipose tissue from male annexin 1 (ANXA1)-null and wild-type control mice were used to explore the potential role of ANXA1 in adipocyte biology. ANXA1 was detected by Western blot analysis in wild-type tissue and localized predominantly to the stromal-vascular compartment. Epididymal fat pad mass was reduced by ANXA1 gene deletion, but adipocyte size was unchanged, suggesting that ANXA1 is required for the maintenance of adipocyte and/or preadipocyte cell number. Epididymal tissue from wild-type mice responded in vitro to noradrenaline and isoprenaline with increased glycerol release, reduced IL-6 release, and increased cAMP accumulation. Qualitatively similar but significantly attenuated responses to the catecholamines were observed in tissue from ANXA1-null mice, an effect that was not associated with changes in beta-adrenoceptor mRNA expression. Lipopolysaccharide (LPS) also stimulated lipolysis in vitro, but its effects were muted by ANXA1 gene deletion. By contrast, LPS failed to influence IL-6 release from wild-type tissue but stimulated the release of the cytokine from tissue from ANXA1-null mice. ANXA1 gene deletion did not affect glucocorticoid receptor expression or the ability of dexamethasone to suppress catecholamine-induced lipolysis. It did, however, augment IL-6 expression and modify the inhibitory effects of glucocorticoids on IL-6 release. Collectively, these studies suggest that ANXA1 supports aspects of adipose tissue mass and alters the sensitivity of epididymal adipose tissue to catecholamines, glucocorticoids, and LPS, thereby modulating lipolysis and IL-6 release.

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.

Expression and function of variants of slob, slowpoke channel binding protein, in Drosophila

Slob binds to and modulates the Drosophila Slowpoke (dSlo) calcium-activated potassium channel and also recruits the ubiquitous signaling protein 14-3-3 to the channel regulatory complex. RT-PCR reveals the presence of multiple slob transcripts in Drosophila heads. The transcripts are predicted to encode proteins that we call Slob51 (kDa), Slob57, Slob65, and Slob71. Slob51 and Slob65 are splice variants that lack a motif important for the binding of 14-3-3. Previous microarray analyses demonstrated the circadian cycling of slob mRNA, and we show by quantitative PCR that more than one transcript cycles in fly heads. Using in situ hybridization, we observe differences in the expression patterns of the different transcripts. Immunohistochemistry on Drosophila heads reveals Slob71/65 protein to be enriched in the lateral neurons, in contrast to Slob57/51 protein, which is expressed most prominently in the pars intercerebralis neurons and dorsal giant interneurons. Using a heterologous expression system, we show that different Slobs bind to different extents to dSlo and 14-3-3. These data reveal an unexpected diversity of the dSlo/Slob/14-3-3 dynamic regulatory complex.

PAC1 receptors mediate pituitary adenylate cyclase-activating polypeptide- and progesterone-facilitated receptivity in female rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts as a feed-forward, paracrine/autocrine factor in the hypothalamic ventromedial nucleus (VMN) for receptivity and sensitizes pituitary hormone release for ovulation. The present study examined receptor(s) and signaling pathway by which PACAP enhances rodent lordosis. PACAP binds to PACAP (PAC1)- and vasoactive intestinal peptide-preferring receptors (VPAC1, VPAC2). Ovariectomized rodents primed with estradiol (EB) were given PACAP or vasoactive intestinal peptide directly onto VMN cells. Only PACAP facilitated receptivity. Pretreatment with VPAC1 and VPAC2 inhibitors blocked both PACAP- and progesterone (P)-induced receptivity. Antisense (AS) oligonucleotides to PAC1 (not VPAC1 or VPAC2) inhibited the behavioral effect of PACAP and P. By real-time RT-PCR, EB, P and EB+P enhanced VMN mRNA expression of PAC1. Within the total PAC1 population, EB and EB+P induced expression of short form PAC1 and PAC1hop2 splice variants. Finally, blocking cAMP/protein kinase A signaling cascade by antagonists to cAMP activity and protein kinase A or by antisense to dopamine- and cAMP-regulated phosphoprotein of 32 kDa blocked the PACAP effect on behavior. Collectively, these findings provide evidence that progesterone receptor-dependent receptivity is, in part, dependent on PAC1 receptors for intracellular VMN signaling and delineate a novel, steroid-dependent mechanism for a feed-forward reinforcement of steroid receptor-dependent reproductive receptivity.

A natural variant type II G protein-coupled receptor for vasoactive intestinal peptide with altered function

The vasoactive intestinal peptide (VIP) and its G protein-coupled receptors VPAC1 and VPAC2 prominently mediate diverse physiological functions in the neural, endocrine, and immune systems. A deletion variant of mouse VPAC2 has been identified in immune cells that lacks amino acids 367-380 at the carboxyl-terminal end of the seventh transmembrane domain. When expressed at equivalent levels in a human Jurkat T cell line, which has very low endogenous expression of human VPAC1 and VPAC2, wild-type and deletion-variant VPAC2 bound the same amount of 125I-VIP with similar affinity. Unlike wild-type VPAC2, however, deletion-variant VPAC2 did not transduce VIP-elicited increases in intracellular concentration of cyclic AMP, chemotaxis, or suppression of generation of interleukin-2. Natural deletion of part of the last transmembrane domain of VPAC2 thus abrogates signaling functions without apparent alterations of expression or ligand binding.

Aberrant gating of photic input to the suprachiasmatic circadian pacemaker of mice lacking the VPAC2 receptor

VIP acting via the VPAC(2) receptor is implicated as a key signaling pathway in the maintenance and resetting of the hypothalamic suprachiasmatic nuclei (SCN) circadian pacemaker; circadian rhythms in SCN clock gene expression and wheel-running behavior are abolished in mice lacking the VPAC(2) receptor (Vipr2(-/-)). Here, using immunohistochemical detection of pERK (phosphorylated extracellular signal-regulated kinases 1/2) and c-FOS, we tested whether the gating of photic input to the SCN is maintained in these apparently arrhythmic Vipr2(-/-) mice. Under light/dark and constant darkness, spontaneous expression of pERK and c-FOS in the wild-type mouse SCN was significantly elevated during subjective day compared with subjective night; no diurnal or circadian variation in pERK or c-FOS was detected in the SCN of Vipr2(-/-) mice. In constant darkness, light pulses given during the subjective night but not the subjective day significantly increased expression of pERK and c-FOS in the wild-type SCN. In contrast, light pulses given during both subjective day and subjective night robustly increased expression of pERK and c-FOS in the Vipr2(-/-) mouse SCN. Although photic stimuli activate intracellular pathways within the SCN of Vipr2(-/-) mice, they do not engage the core clock mechanisms. The absence of photic gating, together with the general lack of overt rhythms in circadian output, strongly suggests that the SCN circadian pacemaker is completely dysfunctional in the Vipr2(-/-) mouse.

Comparison of circadian expression of tryptophan hydroxylase isoform mRNAs in the rat pineal gland using real-time PCR

A second gene encoding a functional tryptophan hydroxylase activity has recently been described (TPH2), which is expressed abundantly in brainstem, the primary site of serotonergic neurons in the CNS. As serotonin (5-HT) has an important role as a precursor of the nocturnal synthesis of the pineal gland hormone, melatonin, it was of interest to determine the relative expression of TPH1 and 2 mRNA in the rat pineal during the light:dark (L:D) cycle using sensitive real-time RT-PCR assays which were developed for each TPH isoform. TPH1 mRNA expression was 105-fold more abundant in rat pineal than TPH2, and showed a significant approximately 4-fold nocturnal increase in expression which may contribute to the previously described nocturnal increase in pineal tryptophan hydroxylase activity. TPH2 expression within the gland showed no significant variation with time of day and was very low (approximately 300 copies/gland) indicating expression in the small proportion of "non-pinealocyte" cells in the gland.

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.

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.