Phasic spike patterning in rat supraoptic neurones in vivo and in vitro

Local kisspeptin excitation of rat oxytocin neurones in late pregnancy

Abstract

The hormone, oxytocin, is synthesised by magnocellular neurones of the supraoptic and paraventricular nuclei and is released from the posterior pituitary gland into the circulation to trigger uterine contractions during parturition. Kisspeptin fibre density increases around the supraoptic nucleus over pregnancy and intracerebroventricular kisspeptin excites oxytocin neurones only in late pregnancy. However, the mechanism of this excitation is unknown. Here, we found that microdialysis administration of kisspeptin into the supraoptic nucleus consistently increased the action potential (spike) firing rate of oxytocin neurones in urethane‐anaesthetised late‐pregnant rats (gestation day 18–21) but not in non‐pregnant rats. Hazard analysis of action potential firing showed that kisspeptin specifically increased the probability of another action potential firing immediately after each action potential (post‐spike excitability) in late‐pregnant rats. Patch‐clamp electrophysiology in hypothalamic slices showed that bath application of kisspeptin did not affect action potential frequency or baseline membrane potential in supraoptic nucleus neurones. Moreover, kisspeptin superfusion did not affect the frequency or amplitude of excitatory postsynaptic currents or inhibitory postsynaptic currents in supraoptic nucleus neurones. Taken together, these studies suggest that kisspeptin directly activates oxytocin neurones in late pregnancy, at least in part, via increased post‐spike excitability.

Key points

Oxytocin secretion is triggered by action potential firing in magnocellular neurones of the hypothalamic supraoptic and paraventricular nuclei to induce uterine contractions during birth.

In late pregnancy, kisspeptin expression increases in rat periventricular nucleus neurones that project to the oxytocin system.

Here, we show that intra‐supraoptic nucleus administration of kisspeptin increases the action potential firing rate of oxytocin neurones in anaesthetised late‐pregnant rats, and that the increased firing rate is associated with increased oxytocin neurone excitability immediately after each action potential.

By contrast, kisspeptin superfusion of hypothalamic slices did not affect the activity of supraoptic nucleus neurones or the strength of local synaptic inputs to supraoptic nucleus neurones.

Hence, kisspeptin might activate oxytocin neurons in late pregnancy by transiently increasing oxytocin neuron excitability after each action potential.

The interplay between glutamatergic circuits and oxytocin neurons in the hypothalamus and its relevance to neurodevelopmental disorders

Oxytocin (OXT) neurons of the hypothalamus are at the center of several physiological functions, including milk ejection, uterus contraction, and maternal and social behavior. In lactating females, OXT neurons show a pattern of burst firing and inter-neuron synchronization during suckling that leads to pulsatile release of surges of OXT into the bloodstream to stimulate milk ejection. This pattern of firing and population synchronization may be facilitated in part by hypothalamic glutamatergic circuits, as has been observed in vitro using brain slices obtained from male rats and neonates. However, it remains unknown how hypothalamic glutamatergic circuits influence OXT cell activity outside the context of lactation. In this review, we summarize the in vivo and in vitro studies that describe the synchronized burst firing pattern of OXT neurons and the implication of hypothalamic glutamate in this pattern of firing. We also make note of the few studies that have traced glutamatergic afferents to the hypothalamic paraventricular and supraoptic nuclei. Finally, we discuss the genetic findings implicating several glutamatergic genes in neurodevelopmental disorders, including autism spectrum disorder, thus underscoring the need for future studies to investigate the impact of these mutations on hypothalamic glutamatergic circuits and the OXT system.

Visualising oxytocin neurone activity in vivo: The key to unlocking central regulation of parturition and lactation

During parturition and lactation, oxytocin neurones in the supraoptic and paraventricular nuclei fire high-frequency bursts of action potentials that are coordinated across the entire population. Each burst generates a large pulse of oxytocin release into the circulation to induce uterine contraction for parturition and mammary duct contraction for milk ejection. Bursts are stimulated by cervical stretch during parturition and by suckling during lactation. However, the mechanisms by which these stimuli are translated into episodic bursts are poorly understood, as are the mechanisms that coordinate bursts across the oxytocin neurone population. An elegant series of experiments conducted in the 1980s and 1990s used serial paired recordings to show that oxytocin neurones do not act as a syncytium during bursts; rather, they start each burst within a few hundred milliseconds of each other but with no distinct "leaders" or "followers". In addition to afferent noradrenergic inputs that relay the systemic stimuli to oxytocin neurones, bursts depend on somato-dendritic oxytocin release within the hypothalamus. Hence, bursts are considered to be an emergent property of oxytocin neurones that is bootstrapped by appropriate afferent stimulation. Although much progress was made using traditional electrophysiological recordings in head-fixed anaesthetised animals, research has effectively stalled in the last few decades. However, the emergence of new technologies to monitor neuronal activity in freely-behaving animals has reinvigorated efforts to understand the biology underpinning burst firing in oxytocin neurones. Here, we report the use of fibre photometry to monitor the dynamics of milk ejection bursts in the oxytocin neurone population of freely-behaving mice. This approach will shed light on the neural mechanisms that control the oxytocin bursts underpinning parturition and lactation.

Phasic spiking in vasopressin neurons: How and Why

The plain title might have been an almost retro sounding grumpy retort, but it has inspired a journey of sorts, and something along the way I hope you won't have come across before. An opinionated exploration of the distinctive phasic spiking patterns of magnocellular vasopressin neurons of the supraoptic and paraventricular nuclei of the hypothalamus. A mostly life essential population of neurons that signal the kidneys to regulate water loss in response to signals that encode plasma volume and osmotic pressure, as well as regulating blood pressure, and possibly metabolism and social behaviour. The viewpoint of a modeller shorn of any explicit maths.

Plasticity in Intrinsic Excitability of Hypothalamic Magnocellular Neurosecretory Neurons in Late-Pregnant and Lactating Rats

Oxytocin and vasopressin secretion from the posterior pituitary gland are required for normal pregnancy and lactation. Oxytocin secretion is relatively low and constant under basal conditions but becomes pulsatile during birth and lactation to stimulate episodic contraction of the uterus for delivery of the fetus and milk ejection during suckling. Vasopressin secretion is maintained in pregnancy and lactation despite reduced osmolality (the principal stimulus for vasopressin secretion) to increase water retention to cope with the cardiovascular demands of pregnancy and lactation. Oxytocin and vasopressin secretion are determined by the action potential (spike) firing of magnocellular neurosecretory neurons of the hypothalamic supraoptic and paraventricular nuclei. In addition to synaptic input activity, spike firing depends on intrinsic excitability conferred by the suite of channels expressed by the neurons. Therefore, we analysed oxytocin and vasopressin neuron activity in anaesthetised non-pregnant, late-pregnant, and lactating rats to test the hypothesis that intrinsic excitability of oxytocin and vasopressin neurons is increased in late pregnancy and lactation to promote oxytocin and vasopressin secretion required for successful pregnancy and lactation. Hazard analysis of spike firing revealed a higher incidence of post-spike hyperexcitability immediately following each spike in oxytocin neurons, but not in vasopressin neurons, in late pregnancy and lactation, which is expected to facilitate high frequency firing during bursts. Despite lower osmolality in late-pregnant and lactating rats, vasopressin neuron activity was not different between non-pregnant, late-pregnant, and lactating rats, and blockade of osmosensitive ΔN-TRPV1 channels inhibited vasopressin neurons to a similar extent in non-pregnant, late-pregnant, and lactating rats. Furthermore, supraoptic nucleus ΔN-TRPV1 mRNA expression was not different between non-pregnant and late-pregnant rats, suggesting that sustained activity of ΔN-TRPV1 channels might maintain vasopressin neuron activity to increase water retention during pregnancy and lactation.

Effects of datumetine on hippocampal NMDAR activity

The usage (abuse) of Datura metel is becoming increasingly worrisome among the Nigerian populace especially among the youth considering its side effects such as hallucination. This work was designed to identify the phytochemicals in datura plant that potentially interact with NMDAR as it affects the electrical and memory activities of the brain. Ligand-protein interaction was assessed using autodock vina to identify phytochemicals that can interact with NMDAR. Datumetine was found to have the best interaction fit with NMDAR at both allosteric and orthosteric binding sites. Furthermore, using electrophysiological, behavioural and western blotting techniques, it was observed that the administration of datumetine positively modulates the NMDAR current by prolonging burst duration and interspike interval, induces seizures in C57BL/6 mice. Acute exposure leads to memory deficit on NOR and Y-maze test while immunoblotting results showed increased expression of GluN1 and CamKIIα while pCamKIIα-T286, CREB and BDNF were downregulated. The results showed that the memory deficit seen in datura intoxication is possibly the effects of datumetine on NMDAR.

Oxytocin, Neural Plasticity, and Social Behavior

Oxytocin regulates parturition, lactation, parental nurturing, and many other social behaviors in both sexes. The circuit mechanisms by which oxytocin modulates social behavior are receiving increasing attention. Here, we review recent studies on oxytocin modulation of neural circuit function and social behavior, largely enabled by new methods of monitoring and manipulating oxytocin or oxytocin receptor neurons in vivo. These studies indicate that oxytocin can enhance the salience of social stimuli and increase signal-to-noise ratios by modulating spiking and synaptic plasticity in the context of circuits and networks. We highlight oxytocin effects on social behavior in nontraditional organisms such as prairie voles and discuss opportunities to enhance the utility of these organisms for studying circuit-level modulation of social behaviors. We then discuss recent insights into oxytocin neuron activity during social interactions. We conclude by discussing some of the major questions and opportunities in the field ahead.

Phasic Neuronal Firing in the Rodent Nucleus of the Solitary Tract ex vivo

Phasic pattern of neuronal activity has been previously described in detail for magnocellular vasopressin neurons in the hypothalamic paraventricular and supraoptic nuclei. This characteristic bistable pattern consists of alternating periods of electrical silence and elevated neuronal firing, implicated in neuropeptide release. Here, with the use of multi-electrode array recordings ex vivo, we aimed to study the firing pattern of neurons in the nucleus of the solitary tract (NTS) - the brainstem hub for homeostatic, cardio-vascular, and metabolic processes. Our recordings from the mouse and rat hindbrain slices reveal the phasic activity pattern to be displayed by a subset of neurons in the dorsomedial NTS subjacent to the area postrema (AP), with the inter-spike interval distribution closely resembling that reported for phasic magnocellular vasopressin cells. Additionally, we provide interspecies comparison, showing higher phasic frequency and firing rate of phasic NTS cells in mice compared to rats. Further, we describe daily changes in their firing rate and pattern, peaking at the middle of the night. Last, we reveal these phasic cells to be sensitive to α ₂ adrenergic receptors activation and to respond to electrical stimulation of the AP. This study provides a comprehensive description of the phasic neuronal activity in the rodent NTS and identifies it as a potential downstream target of the AP noradrenergic system.

Peripheral insulin administration enhances the electrical activity of oxytocin and vasopressin neurones in vivo

Oxytocin neurones are involved in the regulation of energy balance through diverse central and peripheral actions and, in rats, they are potently activated by gavage of sweet substances. Here, we test the hypothesis that this activation is mediated by the central actions of insulin. We show that, in urethane-anaesthetised rats, oxytocin cells in the supraoptic nucleus show prolonged activation after i.v. injections of insulin, and that this response is greater in fasted rats than in non-fasted rats. Vasopressin cells are also activated, although less consistently. We also show that this activation of oxytocin cells is independent of changes in plasma glucose concentration, and is completely blocked by central (i.c.v.) administration of an insulin receptor antagonist. Finally, we replicate the previously published finding that oxytocin cells are activated by gavage of sweetened condensed milk, and show that this response too is completely blocked by central administration of an insulin receptor antagonist. We conclude that the response of oxytocin cells to gavage of sweetened condensed milk is mediated by the central actions of insulin.

Effects of optogenetic stimulation of vasopressinergic retinal afferents on suprachiasmatic neurones

Physiological circadian rhythms are orchestrated by the hypothalamic suprachiasmatic nucleus (SCN). The activity of SCN cells is synchronised by environmental signals, including light information from retinal ganglion cells (RGCs). We recently described a population of vasopressin-expressing RGCs (VP-RGC) that send axonal projections to the SCN. To determine how these VP-RGCs influence the activity of cells in the SCN, we used optogenetic tools to specifically activate their axon terminals within the SCN. Rats were intravitreally injected with a recombinant adeno-associated virus to express the channelrhodopsin-2 and the red fluorescent protein mCherry under the vasopressin promoter (VP-ChR2mCherry). In vitro recordings in acute brain slices showed that approximately 30% of ventrolateral SCN cells responded to optogenetic stimulation with an increase in firing rate that progressively increased during the first 200 seconds of stimulation and which persisted after the end of stimulation. Finally, application of a vasopressin V1A receptor antagonist dampened the response to optogenetic stimulation. Our data suggest that optogenetic stimulation of VP-RGC axons within the SCN influences the activity of SCN cells in a vasopressin-dependent manner.

Activation of the rat hypothalamic supramammillary nucleus by food anticipation, food restriction or ghrelin administration

The circulating orexigenic hormone ghrelin targets many brain areas involved in feeding control and signals via a dedicated receptor, the growth hormone secretagogue receptor 1A. One unexplored target area for ghrelin is the supramammillary nucleus (SuM), a hypothalamic area involved in motivation and reinforcement and also recently linked to metabolic control. Given that ghrelin binds to the SuM, we explored whether SuM cells respond to ghrelin and/or are activated when endogenous ghrelin levels are elevated. We found that peripheral ghrelin injection activates SuM cells in rats, reflected by an increase in the number of cells expressing c-Fos protein in this area, as welll as by the predominantly excitatory response of single SuM cells recorded in in vivo electrophysiological studies. Further c-Fos mapping studies reveal that this area is also activated in rats in situations when circulating ghrelin levels are known to be elevated: in food-restricted rats anticipating the consumption of food and in fed rats anticipating the consumption of an energy-dense food. We also show that intra-SuM injection of ghrelin induces a feeding response in rats suggesting that, if peripheral ghrelin is able to access the SuM, it may have direct effects on this brain region. Collectively, our data demonstrate that the SuM is activated when peripheral ghrelin levels are high, further supporting the emerging role for this brain area in metabolic and feeding control.

Emergent decision-making behaviour and rhythm generation in a computational model of the ventromedial nucleus of the hypothalamus

The ventromedial nucleus of the hypothalamus (VMN) has an important role in diverse behaviours. The common involvement in these of sex steroids, nutritionally-related signals, and emotional inputs from other brain areas, suggests that, at any given time, its output is in one of a discrete number of possible states corresponding to discrete motivational drives. Here we explored how networks of VMN neurons might generate such a decision-making architecture. We began with minimalist assumptions about the intrinsic properties of VMN neurons inferred from electrophysiological recordings of these neurons in rats in vivo, using an integrate-and-fire based model modified to simulate activity-dependent post-spike changes in neuronal excitability. We used a genetic algorithm based method to fit model parameters to the statistical features of spike patterning in each cell. The spike patterns in both recorded cells and model cells were assessed by analysis of interspike interval distributions and of the index of dispersion of firing rate over different binwidths. Simpler patterned cells could be closely matched by single neuron models incorporating a hyperpolarising afterpotential and either a slow afterhyperpolarisation or a depolarising afterpotential, but many others could not. We then constructed network models with the challenge of explaining the more complex patterns. We assumed that neurons of a given type (with heterogeneity introduced by independently random patterns of external input) were mutually interconnected at random by excitatory synaptic connections (with a variable delay and a random chance of failure). Simple network models of one or two cell types were able to explain the more complex patterns. We then explored the information processing features of such networks that might be relevant for a decision-making network. We concluded that rhythm generation (in the slow theta range) and bistability arise as emergent properties of networks of heterogeneous VMN neurons.

Electrophysiological properties of identified oxytocin and vasopressin neurones

To understand the contribution of intrinsic membrane properties to the different in vivo firing patterns of oxytocin (OT) and vasopressin (VP) neurones, in vitro studies are needed, where stable intracellular recordings can be made. Combining immunochemistry for OT and VP and intracellular dye injections allows characterisation of identified OT and VP neurones, and several differences between the two cell types have emerged. These include a greater transient K⁺ current that delays spiking to stimulus onset, and a higher Na⁺ current density leading to greater spike amplitude and a more stable spike threshold, in VP neurones. VP neurones also show a greater incidence of both fast and slow Ca²⁺ -dependent depolarising afterpotentials, the latter of which summate to plateau potentials and contribute to phasic bursting. By contrast, OT neurones exhibit a sustained outwardly rectifying potential (SOR), as well as a consequent depolarising rebound potential, not found in VP neurones. The SOR makes OT neurones more susceptible to spontaneous inhibitory synaptic inputs and correlates with a longer period of spike frequency adaptation in these neurones. Although both types exhibit prominent Ca²⁺ -dependent afterhyperpolarising potentials (AHPs) that limit firing rate and contribute to bursting patterns, Ca²⁺ -dependent AHPs in OT neurones selectively show significant increases during pregnancy and lactation. In OT neurones, but not VP neurones, AHPs are highly dependent on the constitutive presence of the second messenger, phosphatidylinositol 4,5-bisphosphate, which permissively gates N-type channels that contribute the Ca²⁺ during spike trains that activates the AHP. By contrast to the intrinsic properties supporting phasic bursting in VP neurones, the synchronous bursting of OT neurones has only been demonstrated in vitro in cultured hypothalamic explants and is completely dependent on synaptic transmission. Additional differences in Ca²⁺ channel expression between the two neurosecretory terminal types suggests these channels are also critical players in the differential release of OT and VP during repetitive spiking, in addition to their importance to the potentials controlling firing patterns.

A Predictive, Quantitative Model of Spiking Activity and Stimulus-Secretion Coupling in Oxytocin Neurons

Oxytocin neurons of the rat hypothalamus project to the posterior pituitary, where they secrete their products into the bloodstream. The pattern and quantity of that release depends on the afferent inputs to the neurons, on their intrinsic membrane properties, and on nonlinear interactions between spiking activity and exocytosis: A given number of spikes will trigger more secretion when they arrive close together. Here we present a quantitative computational model of oxytocin neurons that can replicate the results of a wide variety of published experiments. The spiking model mimics electrophysiological data of oxytocin cells responding to cholecystokinin (CCK), a peptide produced in the gut after food intake. The secretion model matches results from in vitro experiments on stimulus-secretion coupling in the posterior pituitary. We mimic the plasma clearance of oxytocin with a two-compartment model, replicating the dynamics observed experimentally after infusion and injection of oxytocin. Combining these models allows us to infer, from measurements of oxytocin in plasma, the spiking activity of the oxytocin neurons that produced that secretion. We have tested these inferences with experimental data on oxytocin secretion and spiking activity in response to intravenous injections of CCK. We show how intrinsic mechanisms of the oxytocin neurons determine this relationship: In particular, we show that the presence of an afterhyperpolarization (AHP) in oxytocin neurons dramatically reduces the variability of their spiking activity and even more markedly reduces the variability of oxytocin secretion. The AHP thus acts as a filter, protecting the final product of oxytocin cells from noisy fluctuations.

A switch from GABA inhibition to excitation of vasopressin neurons exacerbates the development angiotensin II-dependent hypertension

Hypothalamic magnocellular neurons secrete vasopressin into the systemic circulation to maintain blood pressure by increasing renal water reabsorption and by vasoconstriction. When blood pressure rises, baroreflex activation normally inhibits vasopressin neurons via activation of GABAergic inputs. However, plasma vasopressin levels are paradoxically elevated in several models of hypertension and in some patients with essential hypertension, despite increased blood pressure. We have previously shown that vasopressin neuron activity is increased early in the development of moderate angiotensin II-dependent hypertension via blunted baroreflex inhibition of vasopressin neurons. Here, we show that antagonism of vasopressin-induced vasoconstriction slows the development of hypertension and that local administration of a GABAA receptor antagonist inhibits vasopressin neurons during, but not before, the onset of hypertension. Taken together, our data suggest that vasopressin exacerbates the increase in blood pressure evident early in the development hypertension and that blunted baroreflex inhibition of vasopressin neurons is underpinned by an excitatory shift in their response to endogenous GABA signalling. This article is protected by copyright. All rights reserved.

Effect of dietary salt intake on epithelial Na+ channels (ENaC) in vasopressin magnocellular neurosecretory neurons in the rat supraoptic nucleus

KEY POINTS

A growing body of evidence suggests that epithelial Na⁺ channels (ENaCs) in the brain play a significant role in the regulation of blood pressure; however, the brain structures that mediate the effect are not well understood. Because vasopressin (VP) neurons play a pivotal role in coordinating neuroendocrine and autonomic responses to maintain cardiovascular homeostasis, a basic understanding of the regulation and activity of ENaC in VP neurons is of great interest. We show that high dietary salt intake caused an increase in the expression and activity of ENaC which resulted in the steady state depolarization of VP neurons. The results help us understand one of the mechanisms underlying how dietary salt intake affects the activity of VP neurons via ENaC activity.

ABSTRACT

All three epithelial Na⁺ channel (ENaC) subunits (α, β and γ) are located in vasopressin (VP) magnocellular neurons in the hypothalamic supraoptic (SON) and paraventricular nuclei. Our previous study demonstrated that ENaC mediates a Na⁺ leak current that affects the steady state membrane potential in VP neurons. In the present study, we evaluated the effect of dietary salt intake on ENaC regulation and activity in VP neurons. High dietary salt intake for 7 days caused an increase in expression of β- and γENaC subunits in the SON and the translocation of αENaC immunoreactivity towards the plasma membrane. Patch clamp experiments on hypothalamic slices showed that the mean amplitude of the putative ENaC currents was significantly greater in VP neurons from animals that were fed a high salt diet compared with controls. The enhanced ENaC current contributed to the more depolarized basal membrane potential observed in VP neurons in the high salt diet group. These findings indicate that high dietary NaCl intake enhances the expression and activity of ENaCs, which augments synaptic drive by depolarizing the basal membrane potential close to the action potential threshold during hormonal demand. However, ENaCs appear to have only a minor role in the regulation of the firing activity of VP neurons in the absence of synaptic inputs as neither the mean intraburst frequency, burst duration, nor interspike interval variability of phasic bursting activity was affected. Moreover, ENaC activity did not affect the initiation, sustention, or termination of the phasic bursting generated in an intrinsic manner without synaptic inputs.

Spike patterning in oxytocin neurons: Capturing physiological behaviour with Hodgkin-Huxley and integrate-and-fire models

Integrate-and-fire (IF) models can provide close matches to the discharge activity of neurons, but do they oversimplify the biophysical properties of the neurons? A single compartment Hodgkin-Huxley (HH) model of the oxytocin neuron has previously been developed, incorporating biophysical measurements of channel properties obtained in vitro. A simpler modified integrate-and-fire model has also been developed, which can match well the characteristic spike patterning of oxytocin neurons as observed in vivo. Here, we extended the HH model to incorporate synaptic input, to enable us to compare spike activity in the model with experimental data obtained in vivo. We refined the HH model parameters to closely match the data, and then matched the same experimental data with a modified IF model, using an evolutionary algorithm to optimise parameter matching. Finally we compared the properties of the modified HH model with those of the IF model to seek an explanation for differences between spike patterning in vitro and in vivo. We show that, with slight modifications, the original HH model, like the IF model, is able to closely match both the interspike interval (ISI) distributions of oxytocin neurons and the observed variability of spike firing rates in vivo and in vitro. This close match of both models to data depends on the presence of a slow activity-dependent hyperpolarisation (AHP); this is represented in both models and the parameters used in the HH model representation match well with optimal parameters of the IF model found by an evolutionary algorithm. The ability of both models to fit data closely also depends on a shorter hyperpolarising after potential (HAP); this is explicitly represented in the IF model, but in the HH model, it emerges from a combination of several components. The critical elements of this combination are identified.

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.

Development of an excitatory kisspeptin projection to the oxytocin system in late pregnancy

KEY POINTS

Oxytocin release from the posterior pituitary gland stimulates uterine contraction during birth but the central mechanisms that activate oxytocin neurones for birth are not well characterized. We found that that kisspeptin fibre density around oxytocin neurones increases in late-pregnant rats. These kisspeptin fibres originated from hypothalamic periventricular nucleus neurones that upregulated kisspeptin expression in late pregnancy. Oxytocin neurones were excited by central kisspeptin administration in late-pregnant rats but not in non-pregnant rats or early- to mid-pregnant rats. Our results reveal the emergence of a new excitatory kisspeptin projection to the oxytocin system in late pregnancy that might contribute to oxytocin neurone activation for birth.

ABSTRACT

The hormone oxytocin promotes uterine contraction during parturition. Oxytocin is synthesized by magnocellular neurones in the hypothalamic supraoptic and paraventricular nuclei and is released into the circulation from the posterior pituitary gland in response to action potential firing. Systemic kisspeptin administration increases oxytocin neurone activity to elevate plasma oxytocin levels. Here, immunohistochemistry revealed that rats on the expected day of parturition (day 21 of gestation) had a higher density of kisspeptin-positive fibres in the perinuclear zone surrounding the supraoptic nucleus (which provides dense glutamatergic and GABAergic innervation to the supraoptic nucleus) than was evident in non-pregnant rats. Retrograde tracing showed the kisspeptin projections to the perinuclear zone originated from the hypothalamic periventricular nucleus. Quantitative RT-PCR showed that kisspeptin receptor mRNA, Kiss1R mRNA, was expressed in the perinuclear zone-supraoptic nucleus and that the relative Kiss1R mRNA expression does not change over the course of pregnancy. Finally, intracerebroventricular administration of kisspeptin increased the firing rate of oxytocin neurones in anaesthetized late-pregnant rats (days 18-21 of gestation) but not in non-pregnant rats, or in early- or mid-pregnant rats. Taken together, these results suggest that kisspeptin expression is upregulated in the periventricular nucleus projection to the perinuclear zone of the supraoptic nucleus towards the end of pregnancy. Hence, this input might activate oxytocin neurones during parturition.

Anti-opioid Effects of RFRP-3 on Magnocellular Neuron Activity in Morphine-naïve and Morphine-treated Female Rats

Neuropeptide FF receptors (NPFFR1 and NPFFR2) have been proposed to possess anti-opioid properties, and be involved in the development of opiate tolerance and dependence. However, there is no evidence to date supporting such opioid effects at the cellular level in vivo. Using in vivo electrophysiological recordings from vasopressin and oxytocin neurons in the supraoptic nucleus, we aimed to determine the effects of NPFFRs on opiate inhibition, tolerance, and dependence at a cellular level. Both vasopressin and oxytocin neurons are acutely inhibited by opioids and develop opiate tolerance. Oxytocin neurons also develop cellular opiate dependence and undergo withdrawal hyperexcitation upon cessation of opiate administration. Here, the classical μ-opioid receptor agonist, morphine robustly inhibited the spontaneous firing rate of vasopressin and oxytocin neurons, and this inhibition was attenuated by pretreatment with the NPFFR1 agonist, RFamide-related peptide-3. In rats infused with morphine for 6 d, vasopressin neurons were unresponsive to morphine, indicating the development of cellular tolerance, but pretreatment with the NPFFR antagonist, GJ14, restored acute morphine inhibition. In morphine-infused rats, RFamide related peptide-3 did not induce withdrawal excitation in oxytocin neurons and GJ14 did not reverse naloxone-precipitated withdrawal excitation. This is the first evidence of anti-opioid effects of the NPFFR system at a cellular level in vivo. Our results suggest that the anti-opioid properties of the NPFFR system reduce morphine sensitivity during tolerance but that it is not involved in dependence.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

The rat suprachiasmatic nucleus: the master clock ticks at 30 Hz

Key points

Light‐responsive neurones in the rat suprachiasmatic nucleus discharge with a harmonic distribution of interspike intervals, whereas unresponsive neurones seldom do.

This harmonic patterning has a fundamental frequency of close to 30 Hz, and is the same in light‐on cells as in light‐off cells, and is unaffected by exposure to light.

Light‐on cells are more active than light‐off cells in both subjective day and subjective night, and both light‐on cells and light‐off cells respond more strongly to changes in light intensity during the subjective night than during the subjective day.

Paired recordings indicate that the discharge of adjacent light‐responsive cells is very tightly synchronized.

The gap junction inhibitor carbenoxolone increases the spontaneous activity of suprachiasmatic nucleus neurones but does not block the harmonic discharge patterning.

Abstract

The suprachiasmatic nucleus (SCN) of the hypothalamus has an essential role in orchestrating circadian rhythms of behaviour and physiology. In the present study, we recorded from single SCN neurons in urethane‐anaesthetized rats, categorized them by the statistical features of their electrical activity and by their responses to light, and examined how activity in the light phase differs from activity in the dark phase. We classified cells as light‐on cells or light‐off cells according to how their firing rate changed in acute response to light, or as non‐responsive cells. In both sets of light‐responsive neurons, responses to light were stronger at subjective night than in subjective day. Neuronal firing patterns were analysed by constructing hazard functions from interspike interval data. For most light‐responsive cells, the hazard functions showed a multimodal distribution, with a harmonic sequence of modes, indicating that spike activity was driven by an oscillatory input with a fundamental frequency of close to 30 Hz; this harmonic pattern was rarely seen in non‐responsive SCN cells. The frequency of the rhythm was the same in light‐on cells as in light‐off cells, was the same in subjective day as at subjective night, and was unaffected by exposure to light. Paired recordings indicated that the discharge of adjacent light‐responsive neurons was very tightly synchronized, consistent with electrical coupling.

Light‐responsive neurones in the rat suprachiasmatic nucleus discharge with a harmonic distribution of interspike intervals, whereas unresponsive neurones seldom do.

This harmonic patterning has a fundamental frequency of close to 30 Hz, and is the same in light‐on cells as in light‐off cells, and is unaffected by exposure to light.

Light‐on cells are more active than light‐off cells in both subjective day and subjective night, and both light‐on cells and light‐off cells respond more strongly to changes in light intensity during the subjective night than during the subjective day.

Paired recordings indicate that the discharge of adjacent light‐responsive cells is very tightly synchronized.

The gap junction inhibitor carbenoxolone increases the spontaneous activity of suprachiasmatic nucleus neurones but does not block the harmonic discharge patterning.

Oxytocin Neurones: Intrinsic Mechanisms Governing the Regularity of Spiking Activity

Oxytocin neurones of the rat supraoptic nucleus are osmoresponsive and, with all other things being equal, they fire at a mean rate that is proportional to the plasma sodium concentration. However, individual spike times are governed by highly stochastic events, namely the random occurrences of excitatory synaptic inputs, the probability of which is increased by increasing extracellular osmotic pressure. Accordingly, interspike intervals (ISIs) are very irregular. In the present study, we show, by statistical analyses of firing patterns in oxytocin neurones, that the mean firing rate as measured in bins of a few seconds is more regular than expected from the variability of ISIs. This is consistent with an intrinsic activity-dependent negative-feedback mechanism. To test this, we compared observed neuronal firing patterns with firing patterns generated by a leaky integrate-and-fire model neurone, modified to exhibit activity-dependent mechanisms known to be present in oxytocin neurones. The presence of a prolonged afterhyperpolarisation (AHP) was critical for the ability to mimic the observed regularisation of mean firing rate, although we also had to add a depolarising afterpotential (DAP; sometimes called an afterdepolarisation) to the model to match the observed ISI distributions. We tested this model by comparing its behaviour with the behaviour of oxytocin neurones exposed to apamin, a blocker of the medium AHP. Good fits indicate that the medium AHP actively contributes to the firing patterns of oxytocin neurones during non-bursting activity, and that oxytocin neurones generally express a DAP, even though this is usually masked by superposition of a larger AHP.

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.

Electrophysiology of Hypothalamic Magnocellular Neurons In vitro: A Rhythmic Drive in Organotypic Cultures and Acute Slices

Hypothalamic neurohormones are released in a pulsatile manner. The mechanisms of this pulsatility remain poorly understood and several hypotheses are available, depending upon the neuroendocrine system considered. Among these systems, hypothalamo-neurohypophyseal magnocellular neurons have been early-considered models, as they typically display an electrical activity consisting of bursts of action potentials that is optimal for the release of boluses of the neurohormones oxytocin and vasopressin. The cellular mechanisms underlying this bursting behavior have been studied in vitro, using either acute slices of the adult hypothalamus, or organotypic cultures of neonatal hypothalamic tissue. We have recently proposed, from experiments in organotypic cultures, that specific central pattern generator networks, upstream of magnocellular neurons, determine their bursting activity. Here, we have tested whether a similar hypothesis can be derived from in vitro experiments in acute slices of the adult hypothalamus. To this aim we have screened our electrophysiological recordings of the magnocellular neurons, previously obtained from acute slices, with an analysis of autocorrelation of action potentials to detect a rhythmic drive as we recently did for organotypic cultures. This confirmed that the bursting behavior of magnocellular neurons is governed by central pattern generator networks whose rhythmic drive, and thus probably integrity, is however less satisfactorily preserved in the acute slices from adult brains.

Asynchronous presynaptic glutamate release enhances neuronal excitability during the post-spike refractory period

KEY POINTS

Many excitatory synapses in the brain release glutamate with both synchronous and asynchronous components. Immediately following an action potential, neurons display a reduced excitability due to the post-spike afterhyperpolarization (AHP). This gives rise to a relative refractory period. When an action potential is evoked by glutamate synaptic input possessing asynchronous release, the delayed glutamate release events act to depolarize the neuron during the AHP and overcome the relative refractory period. These results demonstrate a new role for asynchronous release in regulating post-spike excitability and the relative refractory period in central neurons.

ABSTRACT

Post-spike afterhyperpolarizations (AHPs) functionally inhibit neuronal excitability for tens to hundreds of milliseconds following each action potential. This imposes a relative refractory period during which synaptic excitation is less effective at evoking spikes. Here we asked whether some synapses have mechanisms in place that allow them to overcome the AHP and drive spiking in target cells during this period of reduced excitability. We examined glutamate synapses onto oxytocin and vasopressin neurons in the paraventricular nucleus of the hypothalamus. These synapses can display pronounced asynchronous glutamate release following a single presynaptic spike, with the time course of release being similar to that of the post-spike AHP. To test whether asynchronous release is more effective at overcoming the relative refractory period, we evoked a single action potential with either a brief synchronous depolarization or an asynchronous potential and then assessed excitability at multiple time points following the spike. Neurons receiving asynchronous depolarizing synaptic inputs had a shorter relative refractory period than those receiving synchronous depolarizations. Our data demonstrate that synapses releasing glutamate in an asynchronous and delayed manner are ideally adapted to counter the AHP. By effectively overcoming the relative refractory period, the kinetics of excitatory synaptic input can play an important role in controlling post-spike excitability.

Induction of hypertension blunts baroreflex inhibition of vasopressin neurons in the rat

Vasopressin secretion from the posterior pituitary gland is determined by action potential discharge of hypothalamic magnocellular neurosecretory cells. Vasopressin is a potent vasoconstrictor, but vasopressin levels are paradoxically elevated in some patients with established hypertension. To determine whether vasopressin neurons are excited in hypertension, extracellular single-unit recordings of vasopressin neurons from urethane-anaesthetized Cyp1a1-Ren2 rats with inducible angiotensin-dependent hypertension were made. The basal firing rate of vasopressin neurons was higher in hypertensive Cyp1a1-Ren2 rats than in non-hypertensive Cyp1a1-Ren2 rats. The increase in firing rate was specific to vasopressin neurons because oxytocin neuron firing rate was unaffected by the induction of hypertension. Intravenous injection of the α1-adrenoreceptor agonist, phenylephrine (2.5 μg/kg), transiently increased mean arterial blood pressure to cause a baroreflex-induced inhibition of heart rate and vasopressin neuron firing rate (by 52 ± 9%) in non-hypertensive rats. By contrast, intravenous phenylephrine did not inhibit vasopressin neurons in hypertensive rats, despite a similar increase in mean arterial blood pressure and inhibition of heart rate. Circulating angiotensin II can excite vasopressin neurons via activation of afferent inputs from the subfornical organ. However, the increase in vasopressin neuron firing rate and the loss of inhibition by intravenous phenylephrine were not blocked by intra-subfornical organ infusion of the angiotensin AT1 receptor antagonist, losartan. It can be concluded that increased vasopressin neuron activity at the onset of hypertension is driven, at least in part, by reduced baroreflex inhibition of vasopressin neurons and that this might exacerbate the increase in blood pressure at the onset of hypertension.

High salt intake increases blood pressure via BDNF-mediated downregulation of KCC2 and impaired baroreflex inhibition of vasopressin neurons

The mechanisms by which dietary salt promotes hypertension are unknown. Previous work established that plasma [Na(+)] and osmolality rise in proportion with salt intake and thus promote release of vasopressin (VP) from the neurohypophysis. Although high levels of circulating VP can increase blood pressure, this effect is normally prevented by a potent GABAergic inhibition of VP neurons by aortic baroreceptors. Here we show that chronic high salt intake impairs baroreceptor inhibition of rat VP neurons through a brain-derived neurotrophic factor (BDNF)-dependent activation of TrkB receptors and downregulation of KCC2 expression, which prevents inhibitory GABAergic signaling. We show that high salt intake increases the spontaneous firing rate of VP neurons in vivo and that circulating VP contributes significantly to the elevation of arterial pressure under these conditions. These results provide the first demonstration that dietary salt can affect blood pressure through neurotrophin-induced plasticity in a central homeostatic circuit.

Transient and sustained afterdepolarizations in accessory olfactory bulb mitral cells are mediated by distinct mechanisms that are differentially regulated by neuromodulators

Social interactions between mammalian conspecifics rely heavily on molecular communication via the main and accessory olfactory systems. These two chemosensory systems show high similarity in the organization of information flow along their early stages: social chemical cues are detected by the sensory neurons of the main olfactory epithelium and the vomeronasal organ. These neurons then convey sensory information to the main (MOB) and accessory (AOB) olfactory bulbs, respectively, where they synapse upon mitral cells that project to higher brain areas. Yet, the functional difference between these two chemosensory systems remains unclear. We have previously shown that MOB and AOB mitral cells exhibit very distinct intrinsic biophysical properties leading to different types of information processing. Specifically, we found that unlike MOB mitral cells, AOB neurons display persistent firing responses to strong stimuli. These prolonged responses are mediated by long-lasting calcium-activated non-selective cationic current (Ican). In the current study we further examined the firing characteristics of these cells and their modulation by several neuromodulators. We found that AOB mitral cells display transient depolarizing afterpotentials (DAPs) following moderate firing. These DAPs are not found in MOB mitral cells that show instead robust hyperpolarizing afterpotentials. Unlike Ican, the DAPs of AOB mitral cells are activated by low levels of intracellular calcium and are relatively insensitive to flufenamic acid. Moreover, the cholinergic agonist carbachol exerts opposite effects on the persistent firing and DAPs of AOB mitral cells. We conclude that these phenomena are mediated by distinct biophysical mechanisms that may serve to mediate different types of information processing in the AOB at distinct brain states.

Discharge patterning in rat olfactory bulb mitral cells in vivo

Here we present a detailed statistical analysis of the discharge characteristics of mitral cells of the main olfactory bulb of urethane-anesthetized rats. Neurons were recorded from the mitral cell layer, and antidromically identified by stimuli applied to the lateral olfactory tract. All mitral cells displayed repeated, prolonged bursts of action potentials typically lasting >100 sec and separated by similarly long intervals; about half were completely silent between bursts. No such bursting was observed in nonmitral cells recorded in close proximity to mitral cells. Bursts were asynchronous among even adjacent mitral cells. The intraburst activity of most mitral cells showed strong entrainment to the spontaneous respiratory rhythm; similar entrainment was seen in some, but not all nonmitral cells. All mitral cells displayed a peak of excitability at ~25 msec after spikes, as reflected by a peak in the interspike interval distribution and in the corresponding hazard function. About half also showed a peak at about 6 msec, reflecting the common occurrence of doublet spikes. Nonmitral cells showed no such doublet spikes. Bursts typically increased in intensity over the first 20-30 sec of a burst, during which time doublets were rare or absent. After 20-30 sec (in cells that exhibited doublets), doublets occurred frequently for as long as the burst persisted, in trains of up to 10 doublets. The last doublet was followed by an extended relative refractory period the duration of which was independent of train length. In cells that were excited by application of a particular odor, responsiveness was apparently greater during silent periods between bursts than during bursts. Conversely in cells that were inhibited by a particular odor, responsiveness was only apparent when cells were active. Extensive raw (event timing) data from the cells, together with details of those analyses, are provided as supplementary material, freely available for secondary use by others.

Glial regulation of extrasynaptic NMDA receptor-mediated excitation of supraoptic nucleus neurones during dehydration

Magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) project to the posterior pituitary gland where they release the hormones, vasopressin and oxytocin into the circulation to maintain plasma osmolality. Hormone release is proportionate to SON MNC action potential (spike) firing rate. When activated by ambient extracellular glutamate, extrasynaptic NMDA receptors (eNMDARs) mediate a tonic (persistent) depolarisation to increase the probability of action potential firing. In the present study, in vivo single-unit electrophysiological recordings were made from urethane-anaesthetised female Sprague-Dawley rats to investigate the impact of tonic eNMDAR activation on MNC activity. Water deprivation (for up to 48 h) caused an increase in the firing rate of SON MNCs that was associated with a general increase in post-spike excitability. To determine whether eNMDAR activation contributes to the increased MNC excitability during water deprivation, memantine, which preferentially blocks eNMDARs, was administered locally into the SON by microdialysis. Memantine significantly decreased the firing rate of MNCs recorded from 48-h water-deprived rats but had no effect on MNCs recorded from euhydrated rats. In the presence of the glial glutamate transporter-1 (GLT-1) blocker, dihydrokainate, memantine also reduced the MNC firing rate in euhydrated rats. Taken together, these observations suggest that GLT-1 clears extracellular glutamate to prevent the activation of eNDMARs under basal conditions and that, during dehydration, eNMDAR activation contributes to the increased firing rate of MNCs.

Spike triggered hormone secretion in vasopressin cells; a model investigation of mechanism and heterogeneous population function

Vasopressin neurons generate distinctive phasic patterned spike activity in response to elevated extracellular osmotic pressure. These spikes are generated in the cell body and are conducted down the axon to the axonal terminals where they trigger Ca²⁺ entry and subsequent exocytosis of hormone-containing vesicles and secretion of vasopressin. This mechanism is highly non-linear, subject to both frequency facilitation and fatigue, such that the rate of secretion depends on both the rate and patterning of the spike activity. Here we used computational modelling to investigate this relationship and how it shapes the overall response of the neuronal population. We generated a concise single compartment model of the secretion mechanism, fitted to experimentally observed profiles of facilitation and fatigue, and based on representations of the hypothesised underlying mechanisms. These mechanisms include spike broadening, Ca²⁺ channel inactivation, a Ca²⁺ sensitive K⁺ current, and releasable and reserve pools of vesicles. We coupled the secretion model to an existing integrate-and-fire based spiking model in order to study the secretion response to increasing synaptic input, and compared phasic and non-phasic spiking models to assess the functional value of the phasic spiking pattern. The secretory response of individual phasic cells is very non-linear, but the response of a heterogeneous population of phasic cells shows a much more linear response to increasing input, matching the linear response we observe experimentally, though in this respect, phasic cells have no apparent advantage over non-phasic cells. Another challenge for the cells is maintaining this linear response during chronic stimulation, and we show that the activity-dependent fatigue mechanism has a potentially useful function in helping to maintain secretion despite depletion of stores. Without this mechanism, secretion in response to a steady stimulus declines as the stored content declines.

Vasopressin is a hormone that is secreted from specialised brain cells into the bloodstream; it acts at the kidneys to control water excretion, and thereby help to maintain a stable ‘osmotic pressure’. Specialised cells in the brain sense osmotic pressure, and generate electrical signals which the thousands of vasopressin neurons process and respond to by producing and secreting vasopressin. In response to these signals, vasopressin neurons generate complex “phasic” patterns of electrical activity, and this activity leads to vasopressin secretion in a complex way that depends on both the rate and pattern of this activity. We have now built a computational model that describes both how the vasopressin neurons generate electrical activity and also how that activity leads to secretion. The model, which gives a very close fit to experimental data, allows us to explore the adaptive advantages of particular features of the vasopressin neurons. This analysis reveals the importance of heterogeneity in the properties of vasopressin neurons, and shows how the vasopressin system is optimally designed to maintain a consistent hormonal output in conditions where its stores of releasable hormone are severely depleted.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

In vivo recordings of GnRH neuron firing reveal heterogeneity and dependence upon GABAA receptor signaling

The gonadotropin-releasing hormone (GnRH) neurons are the key cells regulating fertility in all mammalian species. The scattered distribution of these neurons has made investigation of their properties extremely difficult and the key goal of recording their electrical activity in vivo near impossible. The caudal-most extension of the GnRH neuron continuum brings some cells very close to the base of the brain at the level of the anterior hypothalamic area. Taking insight from this, we developed an experimental procedure in anesthetized GnRH-GFP mice that allows the electrical activity of these GnRH neurons to be recorded in vivo. On-cell recordings revealed that the majority of GnRH neurons (86%) were spontaneously active, exhibiting a range of firing patterns, although only a minority (15%) exhibited burst firing. Mean firing frequencies ranged from 0.06 to 3.65 Hz, with the most common interspike interval being ~500 ms. All GnRH neurons tested were activated by AMPA and kisspeptin. Whereas the GABAA receptor agonist muscimol evoked excitatory, inhibitory, or mixed effects on GnRH neuron firing, the GABAA receptor antagonist picrotoxin resulted in a consistent suppression of firing. These observations represent the first electrical recordings of GnRH neurons in vivo. They reveal that GnRH neurons in vivo exhibit considerable heterogeneity in their firing patterns with both similarities and differences to firing in vitro. These variable patterns of firing in vivo are found to be critically dependent upon ongoing GABAA receptor signaling.

Systemic leptin increases the electrical activity of supraoptic nucleus oxytocin neurones in virgin and late pregnant rats

In the rat hypothalamus, fasting attenuates the expression of oxytocin and this can be reversed by exogenous leptin administration. In the present study, we investigated the effects of systemically administered leptin on the electrical activity of magnocellular neurones in the supraoptic nucleus of urethane-anaesthetised rats. In virgin female rats, systemic leptin significantly excited identified oxytocin neurones with no detected effects on the patterning of activity, as reflected by hazard function analyses. The lowest dose that was consistently effective was 100 μg/i.v., and this dose had no significant effect on vasopressin neurones. In virgin rats fasted overnight, the spontaneous firing rate of oxytocin neurones was significantly lower than in unfasted rats, although leptin had a similar excitatory effect as in unfasted rats. In late pregnant rats (days 19-21 of pregnancy), spontaneous firing rates of oxytocin neurones were higher than in virgins, and the initial response to leptin was similar to that in virgin rats, although the increase in activity was more persistent. In fasted pregnant rats, the mean spontaneous firing rate of oxytocin neurones was again lower than in unfasted rats, although leptin had no significant effect even at the higher dose of 1 mg/rat. Thus, fasting reduced the spontaneous firing rates of oxytocin neurones in nonpregnant rats, and this effect could be reversed by the excitatory effects of leptin. Pregnant rats showed some evidence of leptin resistance but only after an overnight fast.

Information coding in vasopressin neurons--the role of asynchronous bistable burst firing

The task of the vasopressin system is homeostasis, a type of process which is fundamental to the brain's regulation of the body, exists in many different systems, and is vital to health and survival. Many illnesses are related to the dysfunction of homeostatic systems, including high blood pressure, obesity and diabetes. Beyond the vasopressin system's own importance, in regulating osmotic pressure, it presents an accessible model where we can learn how the features of homeostatic systems generally relate to their function, and potentially develop treatments. The vasopressin system is an important model system in neuroscience because it presents an accessible system in which to investigate the function and importance of, for example, dendritic release and burst firing, both of which are found in many systems of the brain. We have only recently begun to understand the contribution of dendritic release to neuronal function and information processing. Burst firing has most commonly been associated with rhythm generation; in this system it clearly plays a different role, still to be understood fully.

Phasic firing in vasopressin cells: understanding its functional significance through computational models

Vasopressin neurons, responding to input generated by osmotic pressure, use an intrinsic mechanism to shift from slow irregular firing to a distinct phasic pattern, consisting of long bursts and silences lasting tens of seconds. With increased input, bursts lengthen, eventually shifting to continuous firing. The phasic activity remains asynchronous across the cells and is not reflected in the population output signal. Here we have used a computational vasopressin neuron model to investigate the functional significance of the phasic firing pattern. We generated a concise model of the synaptic input driven spike firing mechanism that gives a close quantitative match to vasopressin neuron spike activity recorded in vivo, tested against endogenous activity and experimental interventions. The integrate-and-fire based model provides a simple physiological explanation of the phasic firing mechanism involving an activity-dependent slow depolarising afterpotential (DAP) generated by a calcium-inactivated potassium leak current. This is modulated by the slower, opposing, action of activity-dependent dendritic dynorphin release, which inactivates the DAP, the opposing effects generating successive periods of bursting and silence. Model cells are not spontaneously active, but fire when perturbed by random perturbations mimicking synaptic input. We constructed one population of such phasic neurons, and another population of similar cells but which lacked the ability to fire phasically. We then studied how these two populations differed in the way that they encoded changes in afferent inputs. By comparison with the non-phasic population, the phasic population responds linearly to increases in tonic synaptic input. Non-phasic cells respond to transient elevations in synaptic input in a way that strongly depends on background activity levels, phasic cells in a way that is independent of background levels, and show a similar strong linearization of the response. These findings show large differences in information coding between the populations, and apparent functional advantages of asynchronous phasic firing.

Vasopressin is a hormone secreted from specialised brain cells into the bloodstream, acting at the kidneys to control water excretion, and thereby help regulate osmotic pressure. This is a cell membrane property determined by the ratio between body salt and water, and its maintenance is essential to the function of all our cells and organs, which depend on a stable fluid volume and extracellular salt concentration. Specialised cells in the brain sense osmotic pressure and generate electrical signals, which the thousands of vasopressin neurons process and respond to by producing and secreting vasopressin. The individual vasopressin cells generate an interesting phasic pattern of electrical activity in response to rises in osmotic pressure – they fire in long bursts, separated by long silences. In our project we're using modelling to simulate this phasic pattern of electrical activity and how it relates to the input signals, trying to understand exactly why vasopressin cells generate this kind of pattern and exactly what advantages it offers to signal processing and the control of vasopressin secretion.

Kisspeptin activation of supraoptic nucleus neurons in vivo

Oxytocin and vasopressin are synthesized by magnocellular neurosecretory cells in the hypothalamic supraoptic and paraventricular nuclei and are released from the posterior pituitary gland into the circulation. Intravenous administration of the ligand for the G protein-coupled receptor 54 receptor, kisspeptin-10, increases plasma oxytocin levels and intracerebroventricular kisspeptin-10 increases vasopressin levels, indicating that kisspeptin might play a role in various physiological functions via stimulation of oxytocin and vasopressin secretion. Because posterior pituitary hormone secretion is dependent on action potential (spike) discharge, we used in vivo extracellular single unit recording to determine the effects of kisspeptin-10 on supraoptic nucleus neurons in urethane-anaesthetized female rats. Intravenous kisspeptin-10 (100 μg) increased the firing rate of oxytocin neurons from 3.7 ± 0.8 to 4.7 ± 0.8 spikes/sec (P = 0.0004), but only a quarter of vasopressin neurons responded to iv kisspeptin-10, showing a short (<3 sec) high-frequency (>15 spikes/sec) burst of firing. By contrast, intracerebroventricular kisspeptin-10 (2 and 40 μg) did not alter oxytocin or vasopressin neuron firing rate. To investigate the pathway involved in the peripheral action of kisspeptin-10, we used i.p. capsaicin to desensitize vagal afferents, which prevented the i.v. kisspeptin-10-induced increase of oxytocin neuron firing rate. This is the first report to show that peripheral, but not central, kisspeptin-10 increases the activity of oxytocin neurons and a proportion of vasopressin neurons and that endogenous kisspeptin regulation of supraoptic nucleus neurons is likely via vagal afferent input, with kisspeptin acting as a hormone rather than as a neuropeptide in this system.

State-dependent changes in astrocyte regulation of extrasynaptic NMDA receptor signalling in neurosecretory neurons

Despite the long-established presence of glutamate NMDA receptors at extrasynaptic sites (eNMDARs), their functional roles remain poorly understood. Factors influencing the concentration and time course of glutamate in the extrasynaptic space, such as the topography of the neuronal–glial microenvironment, as well as glial glutamate transporters, are expected to affect eNMDAR-mediated signalling strength. In this study, we used in vitro and in vivo electrophysiological recordings to assess the properties, functional relevance and modulation of a persistent excitatory current mediated by activation of eNMDARs in hypothalamic supraoptic nucleus (SON) neurons. We found that ambient glutamate of a non-synaptic origin activates eNMDARs to mediate a persistent excitatory current (termed tonic I(NMDA)), which tonically stimulates neuronal activity. Pharmacological blockade of GLT1 astrocyte glutamate transporters, as well as the gliotoxin α-aminodadipic acid, enhanced tonic I(NMDA) and neuronal activity, supporting an astrocyte regulation of tonic I(NMDA) strength. Dehydration, a physiological challenge known to increase SON firing activity and to induce neuroglial remodelling, including reduced neuronal ensheathment by astrocyte processes, resulted in blunted GLT1 efficacy, enhanced tonic I(NMDA) strength, and increased neuronal activity. Taken together, our studies support the view that glial modulation of tonic I(NMDA) activation contributes to regulation of SON neuronal activity, contributing in turn to neuronal homeostatic responses during a physiological challenge.

Reading between the spikes of the hypothalamic neural code

Reading the spike coding of hypothalamic neurones presents a considerable challenge because they exhibit highly irregular firing patterns. Electrophysiologists working in the motor and sensory systems, in which neurones fire more regularly, have devised satisfactory methods to describe the firing of cells, although the statistical assumptions that underlie the methods do not apply to hypothalamic neurones. Measurement of neural activity is nevertheless vital to characterise the activity of neuroendocrine cells. It has thus become necessary to develop methods suitable for the analysis of the highly irregular spike discharge patterns of both spontaneous and stimulus-evoked firing of hypothalamic neurones. We review techniques used to meet this challenge and demonstrate their considerable capacity to address important physiological questions. We also introduce a novel approach for valid statistical estimation of the information conveyed by the response of a single neurone to a periodic stimulus. The approach demonstrated significant diurnal rhythms of synaptic connectivity between hypothalamic nuclei.

Modelling the in vivo spike activity of phasically-firing vasopressin cells

A minimalist model of magnocellular vasopressin neurones was developed to examine the hypothesis that their phasic behaviour is the product of intrinsic voltage- and activity-dependent intracellular mechanisms that create a bistable dynamical system. The model can closely match a range of phasic behaviours recorded in vasopressin cells in vivo, as well as reproduce the three archetypal behaviours of vasopressin cells (continuous firing, sparse sporadic firing and phasic firing) by varying one of the fourteen model parameters. In addition, the mean and standard deviation of burst and silence periods can be matched by varying a further two parameters. In the model, the long-term behaviour (phasic characteristics) of cells is largely independent of the short-term behaviour (interspike intervals).

Glutamatergic inputs contribute to phasic activity in vasopressin neurons

Many neurons in the CNS display rhythmic patterns of activity to optimize excitation-secretion coupling. However, the mechanisms of rhythmogenesis are only partially understood. Magnocellular vasopressin (VP) neurons in the hypothalamus display a phasic activity that consists of alternative bursts of action potentials and silent periods. Previous observations from acute slices of adult hypothalamus suggested that VP cell rhythmicity depends on intrinsic membrane properties. However, such activity in vivo is nonregenerative. Here, we studied the mechanisms of VP neuron rhythmicity in organotypic slice cultures that, unlike acute slices, preserve functional synaptic connections. Comparative analysis of phasic firing of VP neurons in vivo, in acute slices, and in the cultures revealed that, in the latter, the activity was closely related to that observed in vivo. It was synaptically driven, essentially from glutamatergic inputs, and did not rely on intrinsic membrane properties. The glutamatergic synaptic activity was sensitive to osmotic challenges and kappa-opioid receptor activation, physiological stimuli known to affect phasic activity. Together, our data thus strongly suggest that phasic activity in magnocellular VP neurons is controlled by glutamatergic synaptic inputs rather than by intrinsic properties.

Responses to cholecystokinin in the ventromedial nucleus of the rat hypothalamus in vivo

The peptide cholecystokinin (CCK) is a short-term satiety signal released from the gastrointestinal tract during food intake. From the periphery, CCK signalling travels via the vagus nerve to reach the brainstem from which it is relayed higher into the brain. The hypothalamus is a key integrator of appetite-related stimuli and the ventromedial nucleus of the hypothalamus (VMN) is thought to have an important role in the regulation of satiety. We investigated the effect of intravenous injections of CCK on the spontaneous firing activity of single VMN neurons in urethane-anaesthetised rats in vivo. We found that the predominant effect of CCK on the electrical activity in the VMN is inhibitory. We analysed the responses to CCK according to electrophysiologically distinct subpopulations of VMN neurons and found that four of these VMN subpopulations were inhibited by CCK, while five were not significantly affected. Finally, CCK-induced inhibitory response in VMN neurons was not altered by pre-administration of intravenous leptin.

Circulating secretin activates supraoptic nucleus oxytocin and vasopressin neurons via noradrenergic pathways in the rat

Secretin is a 27-amino acid brain-gut peptide from duodenal S-cells. We tested the effects of systemic administration of secretin to simulate its postprandial release on neuroendocrine neurons of the supraoptic nucleus (SON) in urethane-anesthetized female rats. Secretin dose-dependently increased the firing rate of oxytocin neurons, more potently than cholecystokinin, and dose-dependently increased plasma oxytocin concentration. The effect of secretin on SON vasopressin neurons was also predominantly excitatory, in contrast to the inhibitory actions of cholecystokinin. To explore the involvement of noradrenergic inputs in secretin-induced excitation, benoxathian, an alpha1-adrenoceptor antagonist, was infused intracerebroventricularly. Benoxathian intracerebroventricular infusion blocked the excitation by secretin of both oxytocin and vasopressin neurons. To test the role of local noradrenaline release in the SON, benoxathian was microdialyzed onto the SON. The basal firing rate of oxytocin neurons was slightly reduced and the secretin-induced excitation was attenuated during benoxathian microdialysis. Hence, noradrenergic pathways mediate the excitation by systemic secretin of oxytocin neurons via alpha1-adrenoceptors in the SON. As both systemic secretin and oxytocin are involved in regulating gastrointestinal functions and natriuresis, systemically released secretin might act partly through oxytocin.

State-dependent plasticity in vasopressin neurones: dehydration-induced changes in activity patterning

Moderate dehydration impairs concentration and co-ordination, whereas severe dehydration can cause seizures, brain damage or death. To slow the progression of dehydration until body fluids can be replenished by drinking, the increased body fluid osmolality associated with dehydration increases vasopressin (antidiuretic hormone) secretion from the posterior pituitary gland. Increased vasopressin secretion reduces water loss in the urine by promoting water reabsorption in the collecting ducts of the kidney. Vasopressin secretion is largely determined by action potential discharge in vasopressin neurones, and depends on both the rate and pattern of discharge. Vasopressin neurone activity depends on intrinsic and extrinsic mechanisms. We review recent advances in our understanding of the physiological regulation of vasopressin neurone activity patterning and the mechanisms by which this is altered to cope with the increased secretory demands of dehydration.

Performance, properties and plasticity of identified oxytocin and vasopressin neurones in vitro

The neurohypophysial hormones oxytocin (OT) and vasopressin (VP) originate from hypothalamic neurosecretory cells in the paraventricular and supraoptic (SON) nuclei. The firing rate and pattern of action potentials arising from these neurones determine the timing and quantity of peripheral hormone release. We have used immunochemical identification of biocytin-filled SON neurones in hypothalamic slices in vitro to uncover differences between OT and VP neurones in membrane and synaptic properties, firing patterns, and plasticity during pregnancy and lactation. In this review, we summarise some recent findings from this approach: (i) VP neuronal excitability is influenced by slow (sDAP) and fast (fDAP) depolarising afterpotentials that underlie phasic bursting activity. The fDAP may relate to a transient receptor potential (TRP) channel, type melastatin (TRPM4 and/or TRPM5), both of which are immunochemically localised more to VP neurones, and especially, to their dendrites. Both TRPM4 and TRPM5 mRNAs are found in the SON, but single cell reverse transcriptase-polymerisation suggests that TRPM4 might be the more prominent channel. Phasic bursting in VP neurones is little influenced by spontaneous synaptic activity in slices, being shaped largely by intrinsic currents. (ii) The firing pattern of OT neurones ranges from irregular to continuous, with the coefficient of variation determined by randomly distributed, spontaneous GABAergic, inhibitory synaptic currents (sIPSCs). These sIPSCs are four- to five-fold more frequent in OT versus VP neurones, and much more frequent than spontaneous excitatory synaptic currents. (iii) Both cell types express Ca(2+)-dependent afterhyperpolarisations (AHPs), including an apamin-sensitive, medium duration AHP and a slower, apamin-insensitive AHP (sAHP). In OT neurones, both AHPs are enhanced during pregnancy and lactation. During pregnancy, the plasticity of the sAHP is blocked by antagonism of central OT receptors. AHP enhancement is mimicked by exposing slices from day 19 pregnant rats to OT and oestradiol, suggesting that central OT and sex steroids programme this plasticity during pregnancy by direct hypothalamic actions. In conclusion, the differences in VP and OT neuronal function are underlain by differences in both membrane and synaptic properties, and differentially modulated by reproductive state.

Increased sensitivity of monoamine release in the supraoptic nucleus in late pregnancy: region- and stimulus-dependent responses

Oxytocin neurone activation at birth depends upon noradrenaline-mediated signals from the uterus via a brainstem pathway, as well as on factors within the supraoptic nucleus (SON), including oxytocin itself, and the system adapts during pregnancy to optimise the delivery process. We determined whether noradrenaline release in the SON in response to stimuli activating brainstem inputs or antidromically activating magnocellular neurones is enhanced at term pregnancy. Noradrenaline, serotonin and dopamine concentrations were measured in microdialysis samples collected from the dorsal and ventral SON before, during and after either i.v. cholecystokinin (CCK) or neural stalk stimulation in virgin and late pregnant rats. Each stimulus transiently increased noradrenaline and serotonin but not dopamine concentration in the dorsal SON, and responses were increased on days 21 and 22 of pregnancy compared to day 20 pregnant and virgin rats. Neural stalk stimulation induced sensitisation to subsequent stalk stimulation and so the responses in the dorsal SON were doubled; on day 22 of pregnancy, the area under the curve of monoamine concentration was 3.4-fold greater than in virgins, suggesting that adaptations perinatally enhance responsiveness. In conclusion, there are enhanced responses of noradrenaline and serotonin release in the SON that can generate very high, transient extracellular concentrations at term. This may be a consequence of neuroendocrine adaptations in late pregnancy and probably contributes to optimal oxytocin neurone activation during parturition.

Somatostatin actions on rat supraoptic nucleus oxytocin and vasopressin neurones

Magnocellular neurones in the supraoptic nucleus (SON) receive major afferent inputs from the brainstem that have been implicated in the regulation of oxytocin and vasopressin secretion from the posterior pituitary. Notably, at parturition, some neurones that project from the nucleus tractus solitarii (NTS) in the brainstem directly to the SON are activated. Many of these are noradrenergic and regulate oxytocin secretion during parturition, whereas others contain somatostatin and their role is unclear. In the present study, we report that, at parturition, somatostatin mRNA expression in the NTS is significantly increased compared to pregnancy, suggesting an active role for these neurones at that time. Intracerebroventricular somatostatin infusion significantly increased plasma oxytocin secretion in both virgin female and pregnant rats. Intracerebroventricular somatostatin increased SON oxytocin and vasopressin neurone firing-rates, and increased Fos expression in the SON and paraventricular nucleus and in the subfornical organ. Retrodialysis of somatostatin onto the ventrally exposed SON also increased vasopressin neurone firing rate but, unexpectedly, decreased oxytocin neurone firing rate. The experiments indicate that somatostatin neurones in the NTS are activated during parturition but, because the direct effects of somatostatin on oxytocin neurones are inhibitory, this direct pathway does not appear to contribute to enhanced oxytocin release at this time, although indirect somatostatin effects may do so.

An intrinsic vasopressin system in the olfactory bulb is involved in social recognition

Many peptides, when released as chemical messengers within the brain, have powerful influences on complex behaviours. Most strikingly, vasopressin and oxytocin, once thought of as circulating hormones whose actions were confined to peripheral organs, are now known to be released in the brain where they play fundamentally important roles in social behaviours1. In humans, disruptions of these peptide systems have been linked to several neurobehavioural disorders, including Prader-Willi syndrome, affective disorders, and obsessive-compulsive disorder, and polymorphisms of the vasopressin V1a receptor have been linked to autism2,3. Here we report that the rat olfactory bulb contains a large population of interneurones which express vasopressin, that blocking the actions of vasopressin in the olfactory bulb impairs the social recognition abilities of rats, and that vasopressin agonists and antagonists can modulate the processing of information by olfactory bulb neurones. The findings indicate that social information is processed in part by a vasopressin system intrinsic to the olfactory system.