Effect of Melanotan-II on Brain Fos Immunoreactivity and Oxytocin Neuronal Activity and Secretion in Rats.

Melanocortins stimulate the central oxytocin systems that are involved in regulating social behaviours. Alterations in central oxytocin have been linked to neurological disorders such as autism, and melanocortins have been proposed for therapeutic treatment. In the present study, we investigated how systemic administration of melanotan-II (MT-II), a melanocortin agonist, affects oxytocin neuronal activity and secretion in rats. The results obtained show that i.v., but not intranasal, administration of MT-II markedly induced Fos expression in magnocellular neurones of the supraoptic (SON) and paraventricular nuclei (PVN) of the hypothalamus, and this response was attenuated by prior i.c.v. administration of the melanocortin antagonist, SHU-9119. Electrophysiological recordings from identified magnocellular neurones of the SON showed that i.v. administration of MT-II increased the firing rate in oxytocin neurones but did not trigger somatodendritic oxytocin release within the SON as measured by microdialysis. Our data suggest that, after i.v., but not intranasal, administration of MT-II, the activity of magnocellular neurones of the SON is increased. Because previous studies showed that SON oxytocin neurones are inhibited in response to direct application of melanocortin agonists, the actions of i.v. MT-II are likely to be mediated at least partly indirectly, possibly by activation of inputs from the caudal brainstem, where MT-II also increased Fos expression.

Measuring Oxytocin and Vasopressin: Bioassays, Immunoassays and Random Numbers.

In this review, we consider the ways in which vasopressin and oxytocin have been measured since their first discovery. Two different ways of measuring oxytocin in widespread use currently give values in human plasma that differ by two orders of magnitude, and the values measured by these two methods in the same samples show no correlation. The notion that we should accept this seems absurd. Either one (or both) methods is not measuring oxytocin, or, by 'oxytocin', the scientists that use these different methods mean something very different. If these communities are to talk to each other, it is important to validate one method and invalidate the other, or else to establish exactly what each community understands by 'oxytocin'. A similar issue concerns vasopressin: again, different ways of measuring vasopressin give values in human plasma that differ by two orders of magnitude, and it appears that the same explanation for discrepant oxytocin measurements applies to discrepant vasopressin measurements. The first assays for oxytocin and vasopressin measured biological activity directly. When immunoassays were introduced, they encountered problems: high molecular weight factors in raw plasma interfered with the binding of antibodies to the hormones, leading to high and erroneous readings. When these interfering factors were removed by extraction of plasma samples, immunoassays gave measurements consistent with bioassays, with measures of turnover and with the sensitivity of target tissues to exogenous hormone. However, many recent papers use an enzyme-linked immunoassay to measure plasma levels without extracting the samples. Like the first radioimmunassays of unextracted plasma, this generates impossibly high and wholly erroneous measurements.

A Direct Neurokinin B Projection from the Arcuate Nucleus Regulates Magnocellular Vasopressin Cells of the Supraoptic Nucleus.

Central administration of neurokinin B (NKB) agonists stimulates immediate early gene expression in the hypothalamus and increases the secretion of vasopressin from the posterior pituitary through a mechanism that depends on the activation of neurokinin receptor 3 receptors (NK3R). The present study reports that, in the rat, immunoreactivity for NK3R is expressed in magnocellular vasopressin and oxytocin neurones in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus, and that NKB immunoreactivity is expressed in fibres in close juxtaposition with vasopressin neurones at both of these sites. Retrograde tracing in the rat shows that some NKB-expressing neurones in the arcuate nucleus project to the SON and, in mice, using an anterograde tracing approach, it is found that kisspeptin-expressing neurones of the arcuate nucleus, which are known to co-express NKB, project to the SON and PVN. Finally, i.c.v. injection of the NK3R agonist senktide is shown to potently increase the electrical activity of vasopressin neurones in the SON in vivo with no significant effect detected on oxytocin neurones. The results suggest that NKB-containing neurones in the arcuate nucleus regulate the secretion of vasopressin from magnocellular neurones in rodents, and the possible significance of this is discussed.

An implementation of a spike-response model with escape noise using an avalanche diode.

This paper introduces a novel probabilistic spike-response model through the combination of avalanche diode-generated Poisson distributed noise, and a standard exponential decay-based spike-response curve. The noise source, which is derived from a 0.35-µm single-photon avalanche diode (kept in the dark), was tested experimentally to verify its characteristics, before being combined with a field-programmable gate-array implementation of a spike-response model. This simple model was then analyzed, and shown to reproduce seven of eight behaviors recorded during an extensive study of the ventral medial hypothalamic (VMH) region of the brain. It is thought that many of the cell types found within the VMH are fed from a tonic noise synaptic input, where the patterns generated are a product of their spike response and not their interconnection. This paper shows how this tonic noise source can be modelled, and due to the independent nature of the noise sources, provides an avenue for the exploration of networks of noise-fueled neurons, which play a significant role in pattern generation within the brain.

Central release of oxytocin and the ventromedial hypothalamus.

Recent studies on the regulation of social behaviours by neuropeptides indicate that it is the distribution of peptide receptor expression in particular brain areas that determines the specificity of peptide actions; and that, accordingly, peptides can evoke specific behaviours when administered centrally without temporal or spatial selectivity of administration. The release of neuropeptides at synaptic sites appears irrelevant, and in the brain, some peptides are released mainly from dendrites rather than from nerve endings. Dendritic peptide release can be long lasting, semi-independent of electrical activity, and allows the diffusion of peptides to distant targets. The peptide oxytocin regulates many behaviours; in particular, it inhibits food intake. Centrally, oxytocin is released in large amounts by the dendrites of hypothalamic magnocellular neurons. This mini-review considers the possible involvement of dendritically released oxytocin in the regulation of food intake by its actions on the ventromedial hypothalamus.

Bistability with hysteresis in the activity of vasopressin cells.

Magnocellular vasopressin neurones generate distinctive 'phasic' patterns of electrical activity during which periods of spiking activity (bursts) alternate with periods of no spikes or occasional spikes. The mechanisms of burst termination in vivo are still not clearly understood. We recorded from single phasic vasopressin cells in vivo and here we show that burst terminations in some phasic cells is preceded by transient increases in activity, consistent with bursts ending as a result of activity-dependent inhibition. We show that extrinsically imposed increases in activity, evoked by brief stimulation of the organum vasculosum of the lamina terminalis, can either trigger bursts if given when a cell is silent, or stop bursts if given when a cell is active. Thus, the magnocellular vasopressin system is a population of independent bistable oscillators. The population as a whole is insensitive to transient changes in input level, whether these are excitatory or inhibitory. The vasopressin cell population thus acts like a 'low-pass filter'; although brief large changes in input rate have little overall effect, the population responds very effectively to small, sustained changes in input rate by evolving a pattern of discharge activity that efficiently maintains secretion. We note that these filtering characteristics are the opposite of the filtering characteristics that are typically associated with neurones.

alpha-Melanocyte-stimulating hormone and oxytocin: a peptide signalling cascade in the hypothalamus.

alpha-Melanocyte-stimulating hormone (alpha-MSH) and oxytocin share remarkable similarities of effects on behaviour in rats; in particular, they both inhibit feeding behaviour and stimulate sexual behaviour. Recently, we showed that alpha-MSH interacts with the magnocellular oxytocin system in the supraoptic nucleus; alpha-MSH induces the release of oxytocin from the dendrites of magnocellular neurones but it inhibits the secretion of oxytocin from their nerve terminals in the posterior pituitary. This effect of alpha-MSH on supraoptic nucleus oxytocin neurones is remarkable for two reasons. First, it illustrates the capacity of magnocellular neurones to differentially regulate peptide release from dendrites and axons and, second, it emphasises the putative role of magnocellular neurones as a major source of central oxytocin release, and as a likely substrate of some oxytocin-mediated behaviours. The ability of peptides to differentially control secretion from different compartments of their targets indicates one way by which peptide signals might have a particularly significant effect on neuronal circuitry. This suggests a possible explanation for the striking way in which some peptides can influence specific, complex behaviours.

Intracellular calcium increase and somatodendritic vasopressin release by vasopressin receptor agonists in the rat supraoptic nucleus: involvement of multiple intracellular transduction signals.

Vasopressin neurones of the supraoptic nucleus are autoregulated by vasopressin released from their soma and dendrites. Vasopressin binds to specific autoreceptors to trigger an influx of Ca(2+), and this response involves both phospholipase C (PLC) and adenylate cyclase (AC) pathways that, in the periphery, are activated by V(1) (V(1a) and V(1b))- and V(2)-type receptors. To investigate the pathways involved in the [Ca(2+)](i) response, [Ca(2+)](i) measurements were made on freshly dissociated neurones using Fura-2 microspectrofluorimetry, and vasopressin release was measured from isolated supraoptic nuclei. The [Ca(2+)](i) increase and vasopressin release induced by the V(1a) agonist were strongly inhibited by a PLC blocker, an IP(3) receptor antagonist, and a PKC blocker. An AC inhibitor did not affect the V(1a) response, while PKA inhibitors significantly reduced the V(1a)-induced [Ca(2+)](i) and release responses. The [Ca(2+)](i) increase and vasopressin release elicited by the V(2) agonist were attenuated not only by AC pathway blockers, but also by PLC inhibitors. Surprisingly, the V(1b) agonist showed no [Ca(2+)](i) or vasopressin release response. In conclusion, the V(1a) agonist activates both PLC and AC pathway, confirming the functional expression of a V(1a) vasopressin receptor on vasopressin neurones. The V(2) agonist activation of both PLC and AC pathways could result from an action on the PLC-linked unknown receptor, and/or the AC-linked dual angiotensin II-vasopressin receptor.

Taurine selectively modulates the secretory activity of vasopressin neurons in conscious rats.

Previous experiments have shown that a 10-min forced swimming session triggers the release of vasopressin from somata and dendrites, but not axon terminals, of neurons of the hypothalamic-neurohypophysial system. To further investigate regulatory mechanisms underlying this dissociated release, we forced male Wistar rats to swim in warm (20 degrees C) water and monitored release of the potentially inhibitory amino acids gamma amino butyric acid (GABA) and taurine into the hypothalamic supraoptic nucleus using microdialysis. Forced swimming caused a significant increase in the release of taurine (up to 350%; P < 0.05 vs. prestress release), but not GABA. To reveal the physiological significance of centrally released taurine, the specific taurine antagonist 6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide was administered into the supraoptic nucleus via retrodialysis. Administration of this antagonist caused a significant increase in the release of vasopressin within the supraoptic nucleus and into the blood both under basal conditions and during stress (up to 800%; P < 0.05 vs. basal values), without affecting hypothalamic or plasma oxytocin. Local administration of the GABA(A) receptor antagonist bicuculline, in contrast, failed to influence vasopressin secretion at either time point. In a separate series of in vivo electrophysiological experiments, administration of the same dosage of the taurine antagonist into the supraoptic nucleus via microdialysis resulted in an increased electrical activity of identified vasopressinergic, but not oxytocinergic, neurons. Taken together our data demonstrate that taurine is released within the supraoptic nucleus during physical/emotional stress. Furthermore, at the level of the supraoptic nucleus, taurine inhibits not only the electrical activity of vasopressin neurons but also acts as an inhibitor of both central and peripheral vasopressin secretion during different physiological states.

Intracellular calcium signalling in magnocellular neurones of the rat supraoptic nucleus: understanding the autoregulatory mechanisms.

Oxytocin and vasopressin, released at the soma and dendrites of neurones, bind to specific autoreceptors and induce an increase in [Ca2+]i. In oxytocin cells, the increase results from a mobilisation of Ca2+ from intracellular stores, whereas in vasopressin cells, it results mainly from an influx of Ca2+ through voltage-dependent channels. The response to vasopressin is coupled to phospholipase C and adenylyl-cyclase pathways which are activated by V1 (V1a and V1b)- and V2-type receptors respectively. Measurements of [Ca2+]i in response to V1a and V2 agonists and antagonists suggest the functional expression of these two types of receptors in vasopressin neurones. The intracellular mechanisms involved are similar to those observed for the action of the pituitary adenylyl-cyclase-activating peptide (PACAP). Isolated vasopressin neurones exhibit spontaneous [Ca2+]i oscillations and these are synchronised with phasic bursts of electrical activity. Vasopressin modulates these spontaneous [Ca2+]i oscillations in a manner that depends on the initial state of the neurone, and such varied effects of vasopressin may be related to those observed on the electrical activity of vasopressin neurones in vivo.