Stimulatory effects of bombesin-like peptides on suprachiasmatic neurons in brain slices

Gastrin-releasing peptide modulates fast delayed rectifier potassium current in Per1-expressing SCN neurons

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) drives and maintains 24-h physiological rhythms, the phases of which are set by the local environmental light-dark cycle. Gastrin-releasing peptide (GRP) communicates photic phase setting signals in the SCN by increasing neurophysiological activity of SCN neurons. Here, the ionic basis for persistent GRP-induced changes in neuronal activity was investigated in SCN slice cultures from Per1::GFP reporter mice during the early night. Recordings from Per1 -fluorescent neurons in SCN slices several hours after GRP treatment revealed a significantly greater action potential frequency, a significant increase in voltage-activated outward current at depolarized potentials, and a significant increase in 4-aminopyridine-sensitive fast delayed rectifier (fDR) potassium currents when compared to vehicle-treated slices. In addition, the persistent increase in spike rate following early-night GRP application was blocked in SCN neurons from mice deficient in Kv3 channel proteins. Because fDR currents are regulated by the clock and are elevated in amplitude during the day, the present results support the model that GRP delays the phase of the clock during the early night by prolonging day-like membrane properties of SCN cells. Furthermore, these findings implicate fDR currents in the ionic basis for GRP-mediated entrainment of the primary mammalian circadian pacemaker.

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.

Signaling within the master clock of the brain: localized activation of mitogen-activated protein kinase by gastrin-releasing peptide

The circadian clock located in the mammalian suprachiasmatic nucleus (SCN) exhibits substantial heterogeneity in both its neurochemical and functional organization, with retinal input and oscillatory timekeeping functions segregated to different regions within the nucleus. Although it is clear that photic information must be relayed from directly retinorecipient cells to the population of oscillator cells within the nucleus, the intra-SCN signal (or signals) underlying such communication has yet to be identified. Gastrin-releasing peptide (GRP), which is found within calbindin-containing retinorecipient cells and causes photic-like phase shifts when applied directly to the SCN, is a candidate molecule. Here we examine the effect of GRP on both molecular and behavioral properties of the hamster circadian system. Within 30 min a third ventricle injection of GRP produces an increase in the number of cells expressing the phosphorylated form of extracellular signal-regulated kinases 1/2 (p-ERK1/2), localized in a discrete group of SCN cells that form a cap dorsal to calbindin cells and lateral to vasopressin cells. At 1 h after the peak of p-ERK expression these cap cells express c-fos, Period1, and Period2. Pharmacological blockade of ERK phosphorylation attenuates phase shifts to GRP. These data indicate that GRP is an output signal of retinorecipient SCN cells and activates a small cluster of SCN neurons. This novel cell group likely serves as a relay or integration point for communicating photic phase-resetting information to the rhythmic cells of the SCN. These findings represent a first step in deconstructing the SCN network constituting the brain 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.

Single-unit activity of dorsomedial arcuate neurons and diurnal changes of tuberoinfundibular dopaminergic neuron activity in female rats with neonatal monosodium glutamate treatment

Neonatal monosodium glutamate (MSG)-treated rats were used in this study to answer two questions: (1) whether or not the dopamine-responsive dorsomedial arcuate (dm-ARN) neurons are tuberoinfundibular dopaminergic (TIDA) neurons, and (2) whether or not the remaining TIDA neurons in MSG-treated rats are functioning normally. MSG (4 mg/g b. wt., subcutaneously [s.c.]) or saline was given to neonatal Sprague-Dawley rats on days 1, 3, 5, 7, and 9 after birth. The female rats were ovariectomized at 50 days of age and treated with estrogen for 1 week before they were used between 65-90 days of age. The tyrosine hydroxylase-immunoreactive (TH-ir) neurons located in the dm and ventrolateral (vl) parts of the ARN were significantly reduced in MSG-treated rats, as determined by immunohistochemical method. Some TH-ir cells, however, were visible along the border of the third ventricle. Using single-unit recording in brain slices, we found that dopamine inhibited significantly fewer percentage of dm-ARN neurons in MSG-treated (28.2%, n = 39) than in saline-treated rats (73.3%, n = 15). In contrast, bombesin exhibited similar effects (over 70% excitation) in both groups. Using neurochemical means, neonatal MSG treatment produced significant decreases of both 3,4-dihydroxyphenylacetic acid and dopamine levels, but not their ratios, in the median eminence. Moreover, the diurnal change of TIDA neuronal activity persisted in the MSG-treated rats; so did the estrogen-induced afternoon prolactin surge. All these results indicate that neonatal MSG-treatment reduced the number and altered the location of TIDA and dopamine-responsive dm-ARN neurons. The remaining TIDA neurons seemed to be able to maintain their basal activities and diurnal rhythm.

Direct retinal projections to GRP neurons in the suprachiasmatic nucleus of the rat

The retinal projections to gastrin-releasing peptide (GRP)-expressing neurons in the rat suprachiasmatic nucleus (SCN) were investigated by double immunofluorescence and immunoelectron microscopy. Optic nerve terminals labeled by cholera toxin B subunit (CTb) which was transported from the retinal ganglion cells were intermingled with GRP-immunoreactive cell bodies and processes in the ventrolateral portion of the SCN. Ultrastructural analysis revealed that CTb-immunoreactive retinal terminals made synaptic contacts with GRP-immunoreactive dendritic processes. These results demonstrated that photic information is directly input from the optic nerve to GRP neurons in the SCN and these GRP neurons may be involved in circadian entrainment by light.

Effects of natriuretic peptides and dopamine on single-unit activity of dorsomedial arcuate neurons in rat brain slices

The effects of atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) on single-unit activity of dorsomedial arcuate (DM-ARC) neurons were reported. The modulatory effect of CNP on dopamine's (DA) action was also studied. ANP alone in 0.05-0.5 nmol doses induced 26% inhibition and 14% excitation of 37 DM-ARC neurons; the majority (60%) were not responsive. CNP, however, inhibited 46% and excited 4% of 74 DM-ARC neurons. Dose-dependent inhibitory effects of CNP were also observed. In 71 neurons tested with both CNP and DA, more neurons were inhibited by DA (66%) than those by CNP (46%). About one-third (34%) of them were inhibited by both. Furthermore, in 41 neurons inhibited by DA, more than half (54%) of their responses were potentiated by co-administration of CNP. In conclusion, CNP by itself exhibited a predominantly inhibitory action on DM-ARC neurons; and it also potentiated the inhibitory effect of DA on these neurons.

Differential effects of angiotensin II on the activities of suprachiasmatic neurons in rat brain slices

The effects of angiotensin II (AII) on the firing rates of suprachiasmatic neurons were determined in rat brain slices. AII in pmol ranges stimulated 25% and inhibited another 25% of 52 irregular firing neurons, while it stimulated 23% and inhibited 4% of 30 regular firing neurons. Three "oscillating" neurons whose firing rates oscillated with rather constant amplitudes and periods were recorded. AII induced the occurrence of oscillation in one unit and modulated the oscillation amplitude of the other two. Pretreatment with saralasin, an AII antagonist, effectively blocked (100%) the actions of AII (n = 5). The present findings suggest that AII may act as an important mediator in the suprachiasmatic nucleus and its mode of action may be variable in different neurons.

Push-pull perfusion reveals meal-dependent changes in the release of bombesin-like peptides in the rat paraventricular nucleus

Bombesin (BN)-like peptides have been implicated in the regulation of ingestive behavior. The main objective of the present study was to monitor the dynamics of central BN-like peptide release in relationship to spontaneous meal ingestion and termination. Peptide level fluctuations were determined using in vivo push-pull perfusion of the hypothalamic paraventricular nucleus (PVN) and off PVN sites, combined with ex vivo radioimmunoassay. Analysis across all meals revealed significant differences between preprandial, prandial and postprandial extracellular BN-like immunoreactivity (BLI) at the anterior aspect of the PVN, with about a 3-fold diminution during a meal as compared to before or after a meal. Meal-related fluctuations were not detected at more distal hypothalamic sites or at sites within the caudate nucleus. When the analysis was restricted exclusively to the first meal after dark onset, a similar pattern of change in the interstitial levels of PVN BLI was generally observed; levels being higher preprandially as compared to the prandially (albeit by a smaller magnitude), and the termination of the first meal being accompanied by a robust (about 3-fold) increase in BLI. This is the first demonstration of site specific in vivo release of BN-like peptides in relation to feeding status and it further supports the physiological role of this family of peptides in the regulation of food intake.

Effects of ionophoretically applied bombesin-like peptides on hamster suprachiasmatic nucleus neurons in vitro

Ionophoresis of the smaller bombesin-like peptides (gastrin releasing peptide [GRP]-(18-27), neuromedin B, and bombesin) evoked responses from 30-60% of hamster suprachiasmatic nucleus cells recorded in a hypothalamic slice preparation, depending on the circadian phase. We also demonstrated for the first time that the putative bombesin-like peptide receptor antagonists [D-F5,D-Phe6,D-Ala11]bombesin-(6-13)methyl ester (BIM 26226) and [D-Phe6,Des-Met14]bombesin-(6-14)ethyl amide can be applied ionophoretically to block physiological responses to bombesin-like peptides. Together with earlier findings, these results show that bombesin-like peptides administered by several methods can potently alter the firing rates of hamster suprachiasmatic nucleus neurons in vitro. These results indicate that bombesin-like peptides affect suprachiasmatic nucleus cells and could play a role in modulating suprachiasmatic nucleus-mediated circadian rhythm entrainment.

GRP immunoreactivity shows a day-night difference in the suprachiasmatic nuclear soma and efferent fibers: comparison to VIP immunoreactivity

Day-night variations of gastrin releasing peptide (GRP) in the neuronal somal area of the SCN and the nearby region on the anterior hypothalamic area (AHA) where GRP fibers project, were examined by semiquantitative immunocytochemistry, and compared with those of vasoactive intestinal peptide (VIP). Both in the SCN and AHA, GRP immunoreactivity was higher during the day than at night, although VIP immunoreactivity was higher at night than during the day. These observations suggest that, although they are produced in the same ventrolateral subdivision of the SCN and, in part, coexist in the same neurons [13], the day-night changes of GRP and VIP immunoreactivity are in the opposite direction to each other.

Different types of bombesin receptors on neurons in the dorsal raphe nucleus and the rostral hypothalamus in rat brain slices in vitro

The actions of the peptides bombesin (BN), gastrin releasing peptide (GRP), neuromedin C (NMC), litorin and neuromedin B (NMB) were studied on neurons in slices of rat brain maintained in vitro to determine the BN receptor type present in different brain areas. Intracellular and extracellular recordings were made from hypothalamic neurons on the border of the periventricular nucleus (PVN) and suprachiasmatic nucleus (SCN) and from mesencephalic 5-HT sensitive neurons in the dorsal raphe nucleus. In the region of the brain containing the SCN and PVN, BN and the BN-related peptides excited 31 out of 77 neurons on which they were tested. There was little difference in the potency of the BN-related peptides as excitants of neurons, the EC50 being about 10 nM. The response to the peptides usually lasted between 5 and 15 min with little sign of desensitization. Using NMC, GRP and NMB as agonists, the equilibrium constant for the GRP receptor antagonist [D-Phe6]-BN-(6-13)-ethylamide was approximately 10 nM. The response to the peptides fully recovered on washout of the antagonist. The CCKB/gastrin receptor antagonist CI-988 (1 microM) had no effect on either GRP- or NMC-mediated excitation. In the dorsal nucleus 40 of 75 neurons were sensitive to the BN-related peptides. BN, [Tyr4]-BN, NMB and litorin, were 10-20 times more potent than GRP and NMC. The responses to the BN-related peptides were not blocked by the selective GRP receptor antagonists [D-Phe6]-BN-(6-13)-methylester, [DF5Phe6][D-Ala11]-BN-(6-13)-methylester and [D-Phe6]-BN-(6-13)- ethylamide.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurochemical organization of circadian rhythm in the suprachiasmatic nucleus

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

Stimulatory effects of bombesin-like peptides on hypothalamic arcuate neurons in rat brain slices

The effects of bombesin, gastrin-releasing peptide, neuromedin C, ranatensin, and neuromedin B on hypothalamic arcuate neurons were tested in this study using extracellular single-unit recording in fresh brain tissue slices. Adult ovariectomized Sprague-Dawley rats were used for preparation of brain slices. All bombesin-like peptides in pmol ranges exhibited potent stimulatory effects on the firing of arcuate neurons, i.e., gastrin-releasing peptide stimulated 90.9% (n = 22), bombesin 78.0% (n = 41), neuromedin C 63.2% (n = 19), ranatensin 58.0% (n = 22), and neuromedin B 50.0% (n = 6) of arcuate neurons tested. Pretreatments with either [Leu13-psi(CH2NH)-Leu14]-bombesin or [D-Phe6,Des-Met14]-bombesin6-14 ethylamide, two bombesin antagonists, significantly blocked most of the actions of bombesin-like peptides tested. The present results further support the notion that bombesin-like peptides may play a significant role in the arcuate nucleus.