Reduced outward K+ conductances generate depolarizing after-potentials in rat supraoptic nucleus neurones

Endoplasmic Reticulum and Mitochondrial Calcium Handling Dynamically Shape Slow Afterhyperpolarizations in Vasopressin Magnocellular Neurons

Many neurons including vasopressin (VP) magnocellular neurosecretory cells (MNCs) of the hypothalamic supraoptic nucleus (SON) generate afterhyperpolarizations (AHPs) during spiking to slow firing, a phenomenon known as spike frequency adaptation. The AHP is underlain by Ca2+-activated K+ currents, and while slow component (sAHP) features are well described, its mechanism remains poorly understood. Previous work demonstrated that Ca2+ influx through N-type Ca2+ channels is a primary source of sAHP activation in SON oxytocin neurons, but no obvious channel coupling was described for VP neurons. Given this, we tested the possibility of an intracellular source of sAHP activation, namely, the Ca2+-handling organelles endoplasmic reticulum (ER) and mitochondria in male and female Wistar rats. We demonstrate that ER Ca2+ depletion greatly inhibits sAHPs without a corresponding decrease in Ca2+ signal. Caffeine sensitized AHP activation by Ca2+ In contrast to ER, disabling mitochondria with CCCP or blocking mitochondria Ca2+ uniporters (MCUs) enhanced sAHP amplitude and duration, implicating mitochondria as a vital buffer for sAHP-activating Ca2+ Block of mitochondria Na+-dependent Ca2+ release via triphenylphosphonium (TPP+) failed to affect sAHPs, indicating that mitochondria Ca2+ does not contribute to sAHP activation. Together, our results suggests that ER Ca2+-induced Ca2+ release activates sAHPs and mitochondria shape the spatiotemporal trajectory of the sAHP via Ca2+ buffering in VP neurons. Overall, this implicates organelle Ca2+, and specifically ER-mitochondria-associated membrane contacts, as an important site of Ca2+ microdomain activity that regulates sAHP signaling pathways. Thus, this site plays a major role in influencing VP firing activity and systemic hormonal release.

Central Kisspeptin Does Not Affect ERK1/2 or p38 Phosphorylation in Oxytocin Neurons of Late-Pregnant Rats

Oxytocin is secreted by hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) oxytocin neurons to induce uterine contractions during parturition. Increased activation of oxytocin neurons at parturition involves a network of afferent inputs that increase oxytocin neuron excitability. Kisspeptin fibre density increases around oxytocin neurons during pregnancy, and central kisspeptin administration excites oxytocin neurons only in late pregnancy. Kisspeptin signals via extracellular regulated kinase 1/2 (ERK1/2) and p38. Therefore, to determine whether kisspeptin excites oxytocin neurons via ERK1/2-p38 signalling in late-pregnant rats, we performed immunohistochemistry for phosphorylated ERK1/2 (pERK1/2) and phosphorylated p38 (p-p38) in oxytocin neurons of non-pregnant and late-pregnant rats. Intracerebroventricular (ICV) kisspeptin administration (2 µg) did not affect pERK1/2 or p-p38 expression in SON and PVN oxytocin neurons of non-pregnant or late-pregnant rats. Furthermore, ICV kisspeptin did not affect pERK1/2 or p-p38 expression in brain areas with major projections to the SON and PVN: the nucleus tractus solitarius, rostral ventrolateral medulla, locus coeruleus, dorsal raphe nucleus, organum vasculosum of the lamina terminalis, median preoptic nucleus, subfornical organ, anteroventral periventricular nucleus, periventricular nucleus and arcuate nucleus. Hence, kisspeptin-induced excitation of oxytocin neurons in late pregnancy does not appear to involve ERK1/2 or p38 activation in oxytocin neurons or their afferent inputs.

Mechanism and function of phasic firing in vasopressin-releasing magnocellular neurosecretory cells

Magnocellular neurosecretory cells that release vasopressin (MNC^VP ) from axon terminals in the neurohypophysis display a unique pattern of action potential firing termed phasic firing. Under basal conditions, only a small proportion of MNC^VP display spontaneous phasic firing. However, acute and chronic conditions that stimulate vasopressin release, such as hemorrhage and dehydration, greatly enhance the number of MNC^VP that fire phasically. Phasic firing optimizes VP neurosecretion at axon terminals by allowing action potential broadening to promote calcium-dependent frequency-facilitation, at the same time as preventing the secretory fatigue caused by spike inactivation that occurs during prolonged continuous stimulation. This review provides an update on our mechanistic understanding of these processes and highlights important gaps in our knowledge that must be addressed in future experiments.

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.

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.

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.

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.

Mechanisms underlying prorenin actions on hypothalamic neurons implicated in cardiometabolic control

Background

Hypertension and obesity are highly interrelated diseases, being critical components of the metabolic syndrome. Despite the growing prevalence of this syndrome in the world population, efficient therapies are still missing. Thus, identification of novel targets and therapies are warranted. An enhanced activity of the hypothalamic renin-angiotensin system (RAS), including the recently discovered prorenin (PR) and its receptor (PRR), has been implicated as a common mechanism underlying aberrant sympatho-humoral activation that contributes to both metabolic and cardiovascular dysregulation in the metabolic syndrome. Still, the identification of precise neuronal targets, cellular mechanisms and signaling pathways underlying PR/PRR actions in cardiovascular- and metabolic related hypothalamic nuclei remain unknown.

Methods and results

Using a multidisciplinary approach including patch-clamp electrophysiology, live calcium imaging and immunohistochemistry, we aimed to elucidate cellular mechanisms underlying PR/PRR actions within the hypothalamic supraoptic (SON) and paraventricular nucleus (PVN), key brain areas previously involved in cardiometabolic regulation. We show for the first time that PRR is expressed in magnocellular neurosecretory cells (MNCs), and to a lesser extent, in presympathetic PVN neurons (PVN_PS). Moreover, we show that while PRR activation efficiently stimulates the firing activity of both MNCs and PVN_PS neurons, these effects involved AngII-independent and AngII-dependent mechanisms, respectively. In both cases however, PR excitatory effects involved an increase in intracellular Ca²⁺ levels and a Ca²⁺-dependent inhibition of a voltage-gated K⁺ current.

Conclusions

We identified novel neuronal targets and cellular mechanisms underlying PR/PRR actions in critical hypothalamic neurons involved in cardiometabolic regulation. This fundamental mechanistic information regarding central PR/PRR actions is essential for the development of novel RAS-based therapeutic targets for the treatment of cardiometabolic disorders in obesity and hypertension.

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PRR is expressed in SON and PVN neurosecretory and presympathetic neurons.

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PRR activation stimulates firing activity of SON and PVN neurons.

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PR/PRR effects on neurosecretory neurons are AngII-independent.

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PR/PRR effects on presympathetic neurons are AngII-dependent.

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PR inhibits a voltage-gated K⁺ current in a Ca²⁺-dependent manner.

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.

Regulation of Neuronal Activity in Hypothalamic Vasopressin Neurons

Vasopressin is a peptide hormone secreted from the posterior pituitary gland in response to various physiological and/or pathological stimuli, including changes in body fluid volume and osmolality and stress exposure. Vasopressin secretion is controlled by the electrical activity of the vasopressinergic magnocellular neurosecretory cells located in the hypothalamic supraoptic nucleus and paraventricular nucleus. Vasopressin release can occur somatodendritically in the hypothalamus or at the level of pituitary axon terminals. The electrical activity of the vasopressin neurons assumes specific patterns of electrical discharge that are under the control of several factors, including the intrinsic properties of the neuronal membrane and synaptic and hormonal inputs. It is increasingly clear that glial cells perform critical signaling functions that contribute to signal transmission in neural circuits. Astrocytes contribute to neuronal signaling by regulating synaptic and extrasynaptic neurotransmission, as well as by mediating bidirectional neuronal-glial transmission. We recently discovered a novel form of neuronal-glial signaling that exploits the full spatial domain of astrocytes to transmit dendritic retrograde signals from vasopressin neurons to distal upstream neuronal targets. This retrograde trans-neuronal-glial transmission allows the vasopressin neurons to regulate their synaptic inputs by controlling upstream presynaptic neuron firing, thus providing a powerful means of controlling hormonal output.

Hypertonicity increases NO production to modulate the firing rate of magnocellular neurons of the supraoptic nucleus of rats

Increases in plasma osmolality enhance nitric oxide (NO) levels in magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) and modulate the secretion of both vasopressin (VP) and oxytocin (OT). In this paper, we describe the effects of hypertonicity on the electrical properties of MNCs by focusing on the nitrergic modulation of their activity in this condition. Membrane potentials were measured using the patch clamp technique, in the presence of both glutamatergic and GABAergic neurotransmission blockers, in coronal brain slices of male Wistar rats. The recordings were first made under a control condition (295 mosm/kg H2O), then in the presence of a hypertonic stimulus (330 mosm/kg H2O) and, finally, with a hypertonic stimulus plus 500 μM L-Arginine or 100 μM N-nitro-L-Arginine methyl ester hydrochloride (L-NAME). Hypertonicity per se increased the firing frequency of the neurons. L-Arginine prevented the increase in fire frequency induced by hypertonic stimulus, and L-NAME (inhibitor of nitric oxide synthase) induced an additional increase in frequency when applied together with the hypertonic solution. Moreover, L-Arginine hyperpolarizes the resting potential and decreases the peak value of the after-hyperpolarization; both effects were blocked by L-NAME and hypertonicity and/or L-NAME reduced the time constant of the rising phase of the after-depolarization. These results demonstrate that an intrinsic nitrergic system is part of the mechanisms controlling the excitability of MNCs of the SON when the internal fluid homeostasis is disturbed.

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.

A neuronal population in hypothalamus that dramatically resists acute ischemic injury compared to neocortex

Pyramidal neurons (PyNs) of the cortex are highly susceptible to acute stroke damage, yet "lower" brain regions like hypothalamus and brain stem better survive global ischemia. Here we show for the first time that a "lower" neuron population intrinsically resists acute strokelike injury. In rat brain slices deprived of oxygen and glucose (OGD), we imaged anoxic depolarization (AD) as it propagated through neocortex or hypothalamus. AD, the initial electrophysiological event of stroke, is a front of depolarization that drains residual energy in compromised gray matter. The extent of AD reliably determines ensuing cortical damage, but do all CNS neurons generate a robust AD? During 10 min of OGD, PyNs depolarize without functional recovery. In contrast, magnocellular neuroendocrine cells (MNCs) in hypothalamus under identical stress generate a weak and delayed AD, resist complete depolarization, and rapidly repolarize when oxygen and glucose are restored. They recover their membrane potential, input resistance, and spike amplitude and can survive multiple OGD exposures. Two-photon microscopy in slices derived from a fluorescent mouse line confirms this protection, revealing PyN swelling and dendritic beading after OGD, whereas MNCs are not injured. Exposure to the Na(+)-K(+)-ATPase inhibitor ouabain (100 μM) induces AD similar to OGD in both cell types. Moreover, elevated extracellular K(+) concentration ([K(+)](o)) evokes spreading depression (SD), a milder version of AD, in PyNs but not MNCs. Therefore overriding the pump by OGD, ouabain, or elevated [K(+)](o) evokes a propagating depolarization in higher gray matter but not in MNCs. We suggest that variation in Na(+)-K(+)-ATPase pump efficiency during ischemia injury determines whether a neuronal type succumbs to or resists stroke.

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).

Identified GnRH neuron electrophysiology: a decade of study

Over the past decade, the existence of transgenic mouse models in which reporter genes are expressed under the control of the gonadotropin-releasing hormone (GnRH) promoter has made possible the electrophysiological study of these cells. Here, we review the intrinsic and synaptic properties of these cells that have been revealed by these approaches, with a particular regard to burst generation. Advances in our understanding of neuromodulation of GnRH neurons and synchronization of this network are also discussed.

Flufenamic acid modulates multiple currents in gonadotropin-releasing hormone neurons

Reproduction in mammals is dependent upon the appropriate neurosecretion of gonadotropin-releasing hormone (GnRH), yet the endogenous generation of activity underlying GnRH secretion remains poorly understood. We have demonstrated that the depolarizing afterpotential (DAP), which modulates bursting activity, is reduced in isolated GnRH neurons from aged animals. Calcium-activated non-specific cation (CAN) channels contribute to the DAP in other vertebrate neurosecretory cells. We used the CAN channel blocker flufenamic acid (FFA) to examine the contribution of CAN channels to the DAP in GnRH neurons during aging. Recordings were performed on isolated fluorescent GnRH neurons from young, middle-aged and aged female mice. Flufenamic acid inhibited spontaneous activity, but significantly increased the DAP in neurons from young and middle-aged animals. Apamin did not significantly potentiate the DAP, but did reduce the effects of FFA, suggesting that the increased DAP is partially due to blockade of apamin-sensitive SK channels. Flufenamic acid increased the current underlying the DAP (I(ADP)) and decreased the preceding fast outward current (I(OUT)) at all ages. These current responses were not affected by apamin, but TEA evoked similar changes. Thus, a potassium current, likely mediated through BK channels, contributes to the fast AHP and appears to offset the DAP; this current is sensitive to FFA, but insensitive to age. The effect of FFA on the DAP, but not I(ADP), is diminished in aged animals, possibly reflecting an age-related modulation of the apamin-sensitive SK channel. Future studies will examine the expression of SK channels during the aging process in GnRH neurons.

Calcium dynamics in gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neurons represent the key output cells of the neuronal network controlling fertility. Intracellular calcium ion concentration ([Ca(2+)](i)) is likely to be a key signaling tool used by GnRH neurons to regulate and co-ordinate multiple cell processes. This review examines the dynamics and control of [Ca(2+)](i) in GT1 cells, embryonic GnRH neurons in the nasal placode culture, and adult GnRH neurons in the acute brain slice preparation. GnRH neurons at all stages of development display spontaneous [Ca(2+)](i) transients driven, primarily, by their burst firing. However, the intracellular mechanisms generating [Ca(2+)](i) transients, and the control of [Ca(2+)](i) by neurotransmitters, varies markedly across the different developmental stages. The functional roles of [Ca(2+)](i) transients are beginning to be unraveled with one key action being that of regulating the dynamics of GnRH neuron burst firing.

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.

Electrophysiological identification of the functional presynaptic nerve terminals on an isolated single vasopressin neurone of the rat supraoptic nucleus

Release of arginine vasopressin (AVP) and oxytocin from magnocellular neurosecretory cells (MNCs) of the supraoptic nucleus (SON) is under the control of glutamate-dependent excitation and GABA-dependent inhibition. The possible role of the synaptic terminals attached to SON neurones has been investigated using whole-cell patch-clamp recording in in vitro rat brain slice preparations. Recent evidence has provided new insights into the repercussions of glial environment modifications on the physiology of MNCs at the synaptic level in the SON. In the present study, excitatory glutamatergic and inhibitory GABAergic synaptic inputs were recorded from an isolated single SON neurone cultured for 12 h, using the whole-cell patch clamp technique. Neurones expressed an AVP-enhanced green fluorescent protein (eGFP) fusion gene in MNCs. In addition, native synaptic terminals attached to a dissociated AVP-eGFP neurone were visualised with synaptic vesicle markers. These results suggest that the function of presynaptic nerve terminals may be evaluated directly in a single AVP-eGFP neurone. These preparations would be helpful in future studies aiming to electrophysiologically distinguish between the functions of synaptic terminals and glial modifications in the SON neurones.

The adaptive brain: Glenn Hatton and the supraoptic nucleus

In December 2009, Glenn Hatton died, and neuroendocrinology lost a pioneer who had done much to forge our present understanding of the hypothalamus and whose productivity had not faded with the passing years. Glenn, an expert in both functional morphology and electrophysiology, was driven by a will to understand the significance of his observations in the context of the living, behaving organism. He also had the wit to generate bold and challenging hypotheses, the wherewithal to expose them to critical and elegant experimental testing, and a way with words that gave his papers and lectures clarity and eloquence. The hypothalamo-neurohypophysial system offered a host of opportunities for understanding how physiological functions are fulfilled by the electrical activity of neurones, how neuronal behaviour changes with changing physiological states, and how morphological changes contribute to the physiological response. In the vision that Glenn developed over 35 years, the neuroendocrine brain is as dynamic in structure as it is adaptable in function. Its adaptability is reflected not only by mere synaptic plasticity, but also by changes in neuronal morphology and in the morphology of the glial cells. Astrocytes, in Glenn's view, were intimate partners of the neurones, partners with an essential role in adaptation to changing physiological demands.

An osmosensitive voltage-gated K+ current in rat supraoptic neurons

The magnocellular neurosecretory cells of the hypothalamus (MNCs) regulate their electrical behaviour as a function of external osmolality through changes in the activity of osmosensitive ion channels. We now present evidence that the MNCs express an osmosensitive voltage-gated K(+) current (the OKC). Whole-cell patch-clamp experiments on acutely isolated MNCs were used to show that increases in the external osmolality from 295 to 325 mosmol/kg cause an increase in a slow, tetraethylammonium-insensitive outward current. The equilibrium potential for this current is close to the predicted E(K) in two different concentrations of external K(+). The OKC is sensitive to block by Ba(2+) (0.3 mm), and by the M-type K(+) current blockers linopirdine (150 microm) and XE991 (5 microm), and to enhancement by retigabine (10 microm), which increases opening of M-type K(+) channels. The OKC is suppressed by muscarine (30 microm) and is decreased by the L-type Ca(2+) channel blocker nifedipine (10 microm), but not by apamin (100 nm), which blocks SK-type Ca(2+)-dependent K(+) currents. Reverse transcriptase-polymerase chain reaction and immunocytochemical data suggest that MNCs express several members of the K(V)7 (KCNQ) family of K(+) channels, including K(V)7.2, 7.3, 7.4 and 7.5. Extracellular recordings of individual MNCs in a hypothalamic explant preparation demonstrated that an XE991- and retigabine-sensitive current contribute to the regulation of MNC firing. Our data suggest that the MNCs express an osmosensitive K(+) current that could contribute to the regulation of MNC firing by external osmolality and that could be mediated by K(V)7/M-type K(+) channels.

Activation of mGluR5 induces spike afterdepolarization and enhanced excitability in medium spiny neurons of the nucleus accumbens by modulating persistent Na+ currents

The involvement of metabotropic glutamate receptors type 5 (mGluR5) in drug-induced behaviours is well-established but limited information is available on their functional roles in addiction-relevant brain areas like the nucleus accumbens (NAc). This study demonstrates that pharmacological and synaptic activation of mGluR5 increases the spike discharge of medium spiny neurons (MSNs) in the NAc. This effect was associated with the appearance of a slow afterdepolarization (ADP) which, in voltage-clamp experiments, was recorded as a slowly inactivating inward current. Pharmacological studies showed that ADP was elicited by mGluR5 stimulation via G-protein-dependent activation of phospholipase C and elevation of intracellular Ca(2+) levels. Both ADP and spike aftercurrents were significantly inhibited by the Na(+) channel-blocker, tetrodotoxin (TTX). Moreover, the selective blockade of persistent Na(+) currents (I(NaP)), achieved by NAc slice pre-incubation with 20 nm TTX or 10 \#956;m riluzole, significantly reduced the ADP amplitude, indicating that this type of Na(+) current is responsible for the mGluR5-dependent ADP. mGluR5 activation also produced significant increases in I(NaP), and the pharmacological blockade of this current prevented the mGluR5-induced enhancement of spike discharge. Collectively, these data suggest that mGluR5 activation upregulates I(NaP) in MSNs of the NAc, thereby inducing an ADP that results in enhanced MSN excitability. Activation of mGluR5 will significantly alter spike firing in MSNs in vivo, and this effect could be an important mechanism by which these receptors mediate certain aspects of drug-induced behaviours.

Calcium-dependent fast depolarizing afterpotentials in vasopressin neurons in the rat supraoptic nucleus

Oxytocin (OT) and vasopressin (VP) synthesizing magnocellular cells (MNCs) in the supraoptic nucleus (SON) display distinct firing patterns during the physiological demands for these hormones. Depolarizing afterpotentials (DAPs) in these neurons are involved in controlling phasic bursting in VP neurons. Our whole cell recordings demonstrated a Cs(+)-resistant fast DAP (fDAP; decay tau = approximately 200 ms), which has not been previously reported, in addition to the well-known Cs(+)-sensitive slower DAP (sDAP; decay tau = approximately 2 s). Immunoidentification of recorded neurons revealed that all VP neurons, but only 20% of OT neurons, expressed the fDAP. The activation of the fDAP required influx of Ca(2+) through voltage-gated Ca(2+) channels as it was strongly suppressed in Ca(2+)-free extracellular solution or by bath application of Cd(2+). Additionally, the current underlying the fDAP (I(fDAP)) is a Ca(2+)-activated current rather than a Ca(2+) current per se as it was abolished by strongly buffering intracellular Ca(2+) with BAPTA. The I-V relationship of the I(fDAP) was linear at potentials less than -60 mV but showed pronounced outward rectification near -50 mV. I(fDAP) is sensitive to changes in extracellular Na(+) and K(+) but not Cl(-). A blocker of Ca(2+)-activated nonselective cation (CAN) currents, flufenamic acid, blocked the fDAP, suggesting the involvement of a CAN current in the generation of fDAP in VP neurons. We speculate that the two DAPs have different roles in generating after burst discharges and could play important roles in determining the distinct firing properties of VP neurons in the SON neurons.

Functional role of A-type potassium currents in rat presympathetic PVN neurones

Despite the fact that paraventricular nucleus (PVN) neurones innervating the rostral ventrolateral medulla (RVLM) play important roles in the control of sympathetic function both in physiological and pathological conditions, the precise mechanisms controlling their activity are still incompletely understood. In the present study, we evaluated whether the transient outward potassium current I(A) is expressed in PVN-RVLM neurones, characterized its biophysical and pharmacological properties, and determined its role in shaping action potentials and firing discharge in these neurones. Patch-clamp recordings obtained from retrogradely labelled, PVN-RVLM neurones indicate that a 4-AP sensitive, TEA insensitive current, with biophysical properties consistent with I(A), is present in these neurones. Pharmacological blockade of I(A) depolarized resting V(m) and prolonged Na(+) action potential duration, by increasing its width and by slowing down its decay time course. Interestingly, blockade of I(A) either increased or decreased the firing activity of PVN-RVLM neurones, supporting the presence of subsets of PVN-RVLM neurones differentially modulated by I(A). In all cases, the effects of I(A) on firing activity were prevented by a broad spectrum Ca(2+) channel blocker. Immunohistochemical studies suggest that I(A) in PVN-RVLM neurons is mediated by Kv1.4 and/or Kv4.3 channel subunits. Overall, our results demonstrate the presence of I(A) in PVN-RVLM neurones, which actively modulates their action potential waveform and firing activity. These studies support I(A) as an important intrinsic mechanism controlling neuronal excitability in this central presympathetic neuronal population.

Somato-dendritic mechanisms underlying the electrophysiological properties of hypothalamic magnocellular neuroendocrine cells: a multicompartmental model study

Magnocellular neuroendocrine cells (MNCs) of the hypothalamus synthesize the neurohormones vasopressin and oxytocin, which are released into the blood and exert a wide spectrum of actions, including the regulation of cardiovascular and reproductive functions. Vasopressin- and oxytocin-secreting neurons have similar morphological structure and electrophysiological characteristics. A realistic multicompartmental model of a MNC with a bipolar branching structure was developed and calibrated based on morphological and in vitro electrophysiological data in order to explore the roles of ion currents and intracellular calcium dynamics in the intrinsic electrical MNC properties. The model was used to determine the likely distributions of ion conductances in morphologically distinct parts of the MNCs: soma, primary dendrites and secondary dendrites. While reproducing the general electrophysiological features of MNCs, the model demonstrates that the differential spatial distributions of ion channels influence the functional expression of MNC properties, and reveals the potential importance of dendritic conductances in these properties.

Dehydration increases L-type Ca(2+) current in rat supraoptic neurons

The magnocellular neurosecretory cells of the hypothalamus (MNCs) regulate water balance by releasing vasopressin (VP) and oxytocin (OT) as a function of plasma osmolality. Release is determined largely by the rate and pattern of MNC firing, but sustained increases in osmolality also produce structural adaptations, such as cellular hypertrophy, that may be necessary for maintaining high levels of neuropeptide release. Since increases in Ca(2+) current could enhance exocytotic secretion, influence MNC firing patterns, and activate gene transcription and translation, we tested whether Ca(2+) currents in MNCs acutely isolated from the supraoptic nucleus (SON) of the hypothalamus are altered by 16-24 h of water deprivation. A comparison of whole-cell patch-clamp recordings demonstrated that dehydration causes a significant increase in the amplitude of current sensitive to the L-type Ca(2+) channel blocker nifedipine (from -56 +/- 6 to -99 +/- 10 pA; P < 0.001) with no apparent change in other components of Ca(2+) current. Post-recording immunocytochemical identification showed that this increase in current occurred in both OT- and VP-releasing MNCs. Radioligand binding studies of tissue from the SON showed there is also an increase in the density of binding sites for an L-type Ca(2+) channel ligand (from 51.5 +/- 4.8 to 68.1 +/- 4.1 fmol (mg protein)(-1); P < 0.05), suggesting that there was an increase in the number of L-type channels on the plasma membrane of the MNCs or some other cell type in the SON. There were no changes in the measured number of binding sites for an N-type Ca(2+) channel ligand. Dehydration was not associated with changes in the levels of mRNA coding for Ca(2+) channel alpha(1) subunits. These data are consistent with the hypothesis that a selective increase of L-type Ca(2+) current may contribute to the adaptation that occurs in the MNCs during dehydration.

Physiologic regulation of a tetrodotoxin-sensitive sodium influx that mediates a slow afterdepolarization potential in gonadotropin-releasing hormone neurons: possible implications for the central regulation of fertility

The brain controls fertility through release of gonadotropin-releasing hormone (GnRH), but the mechanisms underlying action potential patterning and GnRH release are not understood. We investigated whether GnRH neurons exhibit afterdepolarizing potentials (ADPs) and whether these are modified by reproductive state. Whole-cell current-clamp recordings of GnRH neurons in brain slices from ovariectomized mice revealed a slow ADP (sADP) after action potentials generated by brief current injection. Generating two or four spikes enhanced sADP amplitude and duration. sADP amplitude was not affected by blocking selected neurotransmitter/neuromodulator receptors, delayed-rectifier potassium channels, calcium-dependent cation channels, or hyperpolarization-activated cation channels but was halved by the calcium channel blocker cadmium and abolished by tetrodotoxin. Cadmium also reduced peak latency. Intrinsic mechanisms underlying the sADP were investigated using voltage-clamp protocols simulating action potential waveforms. A single action potential produced an inward current, which increased after double and quadruple stimulation. Cadmium did not affect current amplitude but reduced peak latency. Pretreatment with blockers of calcium-activated potassium currents (I(KCa)) reproduced this shift and blocked subsequent cadmium-induced changes, suggesting cadmium changes latency indirectly by blocking I(KCa). Tetrodotoxin abolished the inward current, suggesting that it is carried by sodium. In contrast, I(KCa) blockers increased the inward current, indicating that I(KCa) may oppose generation of the sADP. Strong sADPs were suprathreshold, generating repetitive spontaneous firing. I(ADP), sADP, and excitability were enhanced by in vivo estradiol, which triggers a preovulatory surge of GnRH release. Physiological feedback modification of this inward current and resulting sADP may modulate action potential firing and subsequent GnRH release.

Blanes Cells Mediate Persistent Feedforward Inhibition onto Granule Cells in the Olfactory Bulb

Inhibitory local circuits in the olfactory bulb play a critical role in determining the firing patterns of output neurons. However, little is known about the circuitry in the major plexiform layers of the olfactory bulb that regulate this output. Here we report the first electrophysiological recordings from Blanes cells, large stellate-shaped interneurons located in the granule cell layer. We find that Blanes cells are GABAergic and generate large I(CAN)-mediated afterdepolarizations following bursts of action potentials. Using paired two-photon guided intracellular recordings, we show that Blanes cells have a presumptive axon and monosynaptically inhibit granule cells. Sensory axon stimulation evokes barrages of EPSPs in Blanes cells that trigger long epochs of persistent spiking; this firing mode was reset by hyperpolarizing membrane potential steps. Persistent firing in Blanes cells may represent a novel mechanism for encoding short-term olfactory information through modulation of tonic inhibitory synaptic input onto bulbar neurons.

A novel osmosensitive voltage gated cation current in rat supraoptic neurones

The magnocellular neurosecretory cells of the hypothalamus (MNCs) regulate water balance by releasing vasopressin and oxytocin as a function of plasma osmolality. Release is determined largely by the rate and pattern of action potentials generated in the MNC somata. Changes in firing are mediated in part by a stretch-inactivated non-selective cation current that causes the cells to depolarize when increased osmolality leads to cell shrinkage. We have obtained evidence for a new current that may regulate MNC firing during changes in external osmolality, using whole-cell patch clamp of acutely isolated rat MNC somata. In internal and external solutions lacking K+, with high concentrations of TEA, and with Na+ as the only likely permeant cation, the current appears as a slow inward current during depolarizations and yields a large tail current upon return to the holding potential of -80 mV. Approximately 60% of the MNCs tested (79 out of 134 cells) displayed a large increase in tail current density (from 5.2+/-0.9 to 10.5+/-1.4 pA pF-1; P<0.001) following an increase in external osmolality from 295 to 325 mosmol kg-1. The current is activated by depolarization to potentials above -60 mV and does not appear to depend on changes in internal Ca2+. The current is carried by Na+ under these conditions, but is blocked by Cs+ and Ba2+ and by internal K+, which suggests that the current could be a K+ current under physiological conditions. This current could play an important role in regulating the response of MNCs to osmolality.

Enhancement of calcium-dependent afterpotentials in oxytocin neurons of the rat supraoptic nucleus during lactation

The firing pattern of oxytocin (OT) hormone synthesizing neurons changes dramatically immediately before each milk ejection, when a brief burst of action potentials is discharged. OT neurons possess intrinsic currents that would modulate this burst. Our previous studies showed the amplitude of the Ca2+ -dependent afterhyperpolarization (AHP) following spike trains is significantly larger during lactation. In the present study we sought to determine which component of the AHP is enhanced, and whether the enhancement could be related to changes in whole-cell Ca2+ current or the Ca2+ transient in identified OT or vasopressin (VP) neurons during lactation. We confirmed, with whole-cell current-clamp recordings, our previous finding from sharp electrodes that the size of the AHP following spike trains increased in OT, but not VP neurons during lactation. We then determined that an apamin-sensitive medium-duration AHP (mAHP) and an apamin-insensitive slow AHP (sAHP) were specifically increased in OT neurons. Simultaneous Ca2+ imaging revealed that the peak change in somatic [Ca2+]i was not altered in either cell type, but the slow decay of the Ca2+ transient was faster in both cell types during lactation. In voltage clamp, the whole-cell, Ca2+ current was slightly larger during lactation in OT cells only, but current density was unchanged when corrected for somatic hypertrophy. The currents, ImAHP and IsAHP, also were increased in OT neurons only, but only the apamin-sensitive ImAHP showed an increase in current density after adjusting for somatic hypertrophy. These findings suggest a specific modulation (e.g. increased number) of the small-conductance Ca2+ -dependent K+ (SK) channels, or their interaction with Ca2+, underlies the increased mAHP/ImAHP during lactation. This larger mAHP may be necessary to limit the explosive bursts during milk ejection.

Muscarinic receptor modulation of slow afterhyperpolarization and phasic firing in rat supraoptic nucleus neurons

A slow posttrain afterhyperpolarization (sAHP) was studied in rat magnocellular neurosecretory cells (MNCs) in vitro. The sAHP was isolated from other afterpotentials by blocking the depolarizing afterpotential (DAP) with Cs(+) and the medium afterhyperpolarization (mAHP) with apamin. The sAHP amplitude increased logarithmically with activity ( approximately 3 mV per e-fold increase in number of impulses) and, when firing stopped, decayed exponentially with a time constant of 2 sec. The sAHP was associated with increased membrane conductance, and its amplitude varied linearly with voltage, reversing at the K(+) equilibrium potential. The sAHP was blocked by Cd(2+) but not by charybdotoxin or iberiotoxin, blockers of intermediate- and big-conductance-type Ca(2+)-dependent K(+) (K(Ca)) channels. The sAHP was reversibly inhibited by muscarine, an effect antagonized by atropine, indicating involvement of muscarinic cholinergic receptors. Muscarine did not affect Ca(2+)-dependent features of action potentials, DAPs, or the mAHP in MNCs, indicating selective modulation of K(Ca) channels causing the sAHP. Muscarinic inhibition of the sAHP enhanced plateau potentials and increased the mean firing rate and duration of afterdischarges that followed spike trains evoked from voltages near threshold. Similarly, the frequency and duration of the spontaneous phasic bursts that characterize physiologically activated vasopressin-releasing MNCs were enhanced by muscarine. MNCs thus express apamin- and voltage-insensitive K(Ca) channels that mediate an sAHP. The activity dependence and kinetics of the sAHP cause it to mask DAPs in a manner that attenuates the amplitude of plateau potentials. Muscarinic inhibition of the sAHP provides an effective mechanism for promoting phasic firing in MNCs.

Burst initiation and termination in phasic vasopressin cells of the rat supraoptic nucleus: a combined mathematical, electrical, and calcium fluorescence study

Vasopressin secreting neurons of the rat hypothalamus discharge lengthy, repeating bursts of action potentials in response to physiological stress. Although many electrical currents and calcium-dependent processes have been isolated and analyzed in these cells, their interactions are less well fathomed. In particular, the mechanism of how each burst is triggered, sustained, and terminated is poorly understood. We present a mathematical model for the bursting mechanism, and we support our model with new simultaneous electrical recording and calcium imaging data. We show that bursts can be initiated by spike-dependent calcium influx, and we propose that the resulting elevation of bulk calcium inhibits a persistent potassium current. This inhibition depolarizes the cell above threshold and so triggers regenerative spiking and further calcium influx. We present imaging data to show that bulk calcium reaches a plateau within the first few seconds of the burst, and our model indicates that this plateau occurs when calcium influx is balanced by efflux and uptake into stores. We conjecture that the burst is terminated by a slow, progressive desensitization to calcium of the potassium leak current. Finally, we propose that the opioid dynorphin, which is known to be secreted from the somatodendritic region and has been shown previously to regulate burst length and phasic activity in these cells, is the autocrine messenger for this desensitization.

Temporal dissociation of the feedback effects of dendritically co-released peptides on rhythmogenesis in vasopressin cells

Vasopressin neurones fire action potentials in a rhythmic 'phasic' pattern, characterised by alternating periods of activity and silence. Vasopressin and dynorphin are co-packaged in neurosecretory vesicles that are exocytosed from vasopressin cell dendrites and terminals and both have been implicated in the generation of phasic activity patterning through autoregulatory mechanisms. Here, identified supraoptic nucleus vasopressin cells exhibiting spontaneous phasic activity were recorded from urethane-anaesthetised rats administered the V1 vasopressin receptor antagonist, OPC 21268, or the kappa-opioid receptor antagonist, nor-binaltorphimine. OPC 21268 elevated firing rate throughout each burst whereas nor-binaltorphimine excitation emerged over the course of each burst, indicating a progressive activation of kappa-opioid receptor mechanisms during bursts. To determine whether changes in post-spike excitability could account for these effects, we plotted the probability of action potential firing with time after the preceding action potential (hazard function) and found that, similarly to firing rate, this too was elevated by OPC 21268 throughout each burst whilst the excitatory effects of nor-binaltorphimine progressively increased over the course of each burst. Thus, the temporal organisation of the feedback effects of these co-released peptides is different, with vasopressin effectively causing an immediate reduction in overall excitability whilst dynorphin causes a progressive decrease in post-spike excitability over the course of each burst.

Background and tandem-pore potassium channels in magnocellular neurosecretory cells of the rat supraoptic nucleus

Magnocellular neurosecretory cells (MNCs) were isolated from the supraoptic nucleus of rat hypothalamus, and properties of K(+) channels that may regulate the resting membrane potential and the excitability of MNCs were studied. MNCs showed large transient outward currents, typical of vasopressin- and oxytocin-releasing neurons. K(+) channels in MNCs were identified by recording K(+) channels that were open at rest in cell-attached and inside-out patches in symmetrical 150 mM KCl. Eight different K(+) channels were identified and could be distinguished unambiguously by their single-channel kinetics and voltage-dependent rectification. Two K(+) channels could be considered functional correlates of TASK-1 and TASK-3, as judged by their single-channel kinetics and high sensitivity to pH(o). Three K(+) channels showed properties similar to TREK-type tandem-pore K(+) channels (TREK-1, TREK-2 and a novel TREK), as judged by their activation by membrane stretch, intracellular acidosis and arachidonic acid. One K(+) channel was activated by application of pressure, arachidonic acid and alkaline pH(i), and showed single-channel kinetics indistinguishable from those of TRAAK. One K(+) channel showed strong inward rectification and single-channel conductance similar to those of a classical inward rectifier, IRK3. Finally, a K(+) channel whose cloned counterpart has not yet been identified was highly sensitive to extracellular pH near the physiological range similar to those of TASK channels, and was the most active among all K(+) channels. Our results show that in MNCs at rest, eight different types of K(+) channels can be found and six of them belong to the tandem-pore K(+) channel family. Various physiological and pathophysiological conditions may modulate these K(+) channels and regulate the excitability of MNCs.

Changes in the active membrane properties of rat supraoptic neurones during pregnancy and lactation

To better understand the plasticity of intrinsic membrane properties of supraoptic magnocellular neuroendocrine cells associated with reproductive function, intracellular recordings were performed in oxytocin (OT) and vasopressin (VP) neurones from virgin, late pregnant (E19-22), and lactating (8-12 days of lactation) rats in vitro, using hypothalamic explants. OT neurones from virgin rats displayed a narrower spike width than neurones from pregnant and lactating rats, characterized by faster rise and decay times. Spike width changes in VP neurones were not as prominent as those observed in OT neurones. In OT neurones, the amplitude and the decay of the afterhyperpolarization following spike trains was significantly larger and faster, respectively, in pregnant and lactating rats compared to virgin rats. These properties did not change during pregnancy and lactation in VP neurones. The incidence of the depolarizing afterpotential following spikes significantly increased from approximately 20% in virgin rats to 40-50% during pregnancy and lactation in OT neurones, but was stable (80-90%) across states in VP neurones. Repetitive firing properties (frequency adaptation, the first interspike interval frequency and frequency-current (F-I) relationship) were altered during pregnancy and lactation in OT neurones, but not VP neurones. The increased incidence of depolarizing afterpotentials in OT neurones enhances excitability, while the increased afterhyperpolarization results in suppression of firing rate. Thus, the changes may favour the short bursting activity seen in OT neurones during lactation. These results confirmed reproductive state-dependent changes in intrinsic membrane properties of OT neurones during lactation, and suggest these changes are in place during late pregnancy. This argues that the plasticity in the electrical properties in OT neurones associated with lactation is not instigated by suckling.

Flufenamic acid blocks depolarizing afterpotentials and phasic firing in rat supraoptic neurones

Depolarizing afterpotentials (DAPs) that follow action potentials in magnocellular neurosecretory cells (MNCs) are thought to underlie the generation of phasic firing, a pattern that optimizes vasopressin release from the neurohypophysis. Previous work has suggested that the DAP may result from the Ca(2+)-dependent reduction of a resting K(+) conductance. Here we examined the effects of flufenamic acid (FFA), a blocker of Ca(2+)-dependent non-selective cation (CAN) channels, on DAPs and phasic firing using intracellular recordings from supraoptic MNCs in superfused explants of rat hypothalamus. Application of FFA, but not solvent (0.1 % DMSO), reversibly inhibited (IC(50) = 13.8 microM; R = 0.97) DAPs and phasic firing with a similar time course, but had no significant effects (P > 0.05) on membrane potential, spike threshold and input resistance, nor on the frequency and amplitude of spontaneous synaptic potentials. Moreover, FFA did not affect (P > 0.05) the amplitude, duration, undershoot, or frequency-dependent broadening of action potentials elicited during the spike trains used to evoke DAPs. These findings suggest that FFA inhibits the DAP by directly blocking the channels responsible for its production, rather than by interfering with Ca(2+) influx. They also support a role for DAPs in the generation of phasic firing in MNCs. Finally, the absence of a depolarization and increased membrane resistance upon application of FFA suggests that the DAP in MNCs may not be due to the inhibition of resting K(+) current, but to the activation of CAN channels.

Plasticity in the electrophysiological properties of oxytocin neurons

In mammals, the neurohypophysial hormone oxytocin (OT) is released into the bloodstream during labor and lactation to promote uterine contraction and milk ejection, respectively. Electrophysiological studies have established that OT neurons fire in brief, synchronized bursts during this release. During pregnancy and lactation, the intrinsic membrane and synaptic properties of OT, and to a lesser extent vasopressin (VP) neurons, are altered as a part of the adaptation to these specialized states. During lactation OT neurons specifically exhibit an enhanced rebound depolarization which could assist in instigating bursts and an increased gating of firing frequency which is correlated with an enhanced Ca(2+)-dependent after hyperpolarization. Spike broadening occurs in both VP and OT neurons, but in OT neurons this and other changes are present during late pregnancy, suggesting involvement of steroidal hormones in programming neuronal adaptations. Excitatory and inhibitory synaptic activity also are altered by reproductive state. There is a doubling of glutamatergic activity specific to OT neurons which is consistent with an increase in terminal numbers, but this is accompanied by an increase in paired-pulse facilitation, suggesting an increase in the probability of glutamate release during lactation as well. Together with profound changes in both pre- and postsynaptic GABAergic synaptic activity, these data suggest that neurosecretory, and particularly OT neuronal, properties are state-dependent. These modifications may adjust the responsiveness of these neurons to afferent stimulation during periods of increased hormone demand and thereby enhance stimulus-secretion coupling.

Ionic basis of the caesium-induced depolarisation in rat supraoptic nucleus neurones

1. The effects of external Cs(+) on magnocellular neurosecretory cells were studied during intracellular recordings from 93 supraoptic nucleus neurones in superfused explants of rat hypothalamus. 2. Bath application of 3-5 mM Cs(+) provoked reversible membrane depolarisation and increased firing rate in all of the neurones tested. Voltage-current analysis revealed an increase in membrane resistance between -120 and -55 mV. The increase in resistance was greater below -85 mV than at more positive potentials. 3. Voltage-clamp analysis showed that external Cs(+) blocked the hyperpolarisation-activated inward current, I(H). Under current clamp, application of ZD 7288, a selective blocker of I(H), caused an increase in membrane resistance at voltages < or = -65 mV. Voltage-current analysis further revealed that blockade of I(H) caused hyperpolarisation when the initial voltage was < -60 mV but had no effect at more positive values. 4. Current- and voltage-clamp analysis of the effects of Cs(+) in the presence of ZD 7288, or ZD 7288 and tetraethyl ammonium (TEA), revealed an increase in membrane resistance throughout the range of voltages tested (-120 to -45 mV). The current blocked by Cs(+) in the absence of I(H) was essentially voltage independent and reversed at -100 mV. The reversal potential shifted by +22.7 mV when external [K(+)] was increased from 3 to 9 mM. We conclude that, in addition to blocking I(H), external Cs(+) blocks a leakage K(+) current that contributes significantly to the resting potential of rat magnocellular neurosecretory cells.

A subthreshold persistent sodium current mediates bursting in rat subfornical organ neurones

It is widely accepted that while release of amino acid neurotransmitters occurs with relatively high fidelity, peptidergic synapses require clustered bursts of action potentials for optimal transmitter release. Here we describe for the first time the occurrence and mechanisms of bursting by neurones in the subfornical organ (SFO), cells that utilize the peptide angiotensin II (ANG) in neurotransmission in autonomic pathways. In current clamp recording of isolated SFO neurones in vitro, 53 % (n = 74) showed either spontaneous or evoked burst-like discharge patterns. Bursts typically appeared as shifts in bistable membrane potential, with action potentials superimposed on a depolarizing afterpotential (DAP). Similarly, in vivo single unit recordings of identified SFO neurones showed that 9 of 15 neurones fired in bursts. The pattern of bursting, as well as duration of evoked DAPs was strongly dependent upon membrane potential, suggesting that the DAP contributes to burst generation. Based on our previous observation of calcium-sensing receptor (CaR)-activated bursts, we investigated the effects of NPS R-467, an allosteric agonist of the CaR, on evoked DAPs. NPS R-467 (1 microM) potentiated DAP duration throughout the voltage range tested. We observed a dependence of evoked DAPs upon Na+ channels, as shown by sensitivity to tetrodotoxin (0.5 microM) or reduction of external [Na+] from 140 to 40 mM. The duration of DAPs suggested that a persistent Na+ current mediates these events. Voltage-clamp analysis revealed the presence of a subthreshold sodium current, INaP. Pharmacological blockade of INaP with 100 microM lidocaine reduced the duration of evoked DAPs, and inhibited bursting in SFO neurones. Facilitation of INaP with 10 nM anemone toxin (ATX) increased DAP duration and led to marked excitation of bursting cells. These data indicate that INaP is the main current underlying bursting in SFO neurones. Our observations of receptor-mediated facilitation of bursting by SFO neurones represents an intriguing mechanism through which the release of the peptide neurotransmitter ANG may be regulated.

Inositol 1,4,5-trisphosphate-sensitive Ca2+ stores in rat supraoptic neurons: involvement in histamine-induced enhancement of depolarizing afterpotentials

Histamine, a putative neuromodulator and neurotransmitter, can depolarize supraoptic neurons and enhance depolarizing afterpotentials that play a key role in determining the excitability of these neurons. This study investigated intracellular signal transduction involved in histamine-induced enhancement of depolarizing afterpotentials utilizing immunohistochemical and electrophysiological methods. Abundant inositol 1,4,5-trisphosphate receptor-related immunostaining was seen in all parts of the supraoptic nucleus, mainly within somata and proximal processes of the magnocellular neurons, but also in astrocytes of the ventral glial lamina. In supraoptic neurons displaying depolarizing afterpotentials, three brief depolarizations evoked a slow inward current. Bath application of histamine (1-2.5 microM) reversibly enhanced this slow inward current in almost all supraoptic neurons tested. Amplitudes and durations of the slow inward current were increased by 68.1% and 22.8%, respectively. Pretreatment of cells with a histamine receptor (subtype 1) antagonist (pyrilamine) or inhibitors of phospholipase C activation (neomycin or U73122) prevented histamine-induced enhancement of the slow inward current. When electrodes containing heparin, an inositol 1,4,5-trisphosphate receptor blocker, were used for recording, histamine had no effect on the slow inward current. Heparin, however, failed to abolish norepinephrine-induced enhancement of the slow inward current. After H7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine], an inhibitor of protein kinase C, was infused into supraoptic neurons via the electrodes, histamine-induced enhancement of the slow inward current was also blocked. These results indicate the presence of, and functional roles for, inositol 1,4,5-trisphosphate receptor-sensitive Ca2+ stores in supraoptic neurons. Following activation of histamine receptors (subtype 1) and phospholipase C, Ca2+ mobilization from internal stores participates in mediating histamine-induced enhancement of depolarizing afterpotentials.

Phenotypic and state-dependent expression of the electrical and morphological properties of oxytocin and vasopressin neurones

Oxytocin and vasopressin secreting neurones of the hypothalamic supraoptic nucleus share many membrane characteristics and a roughly similar morphology. However, these two neurone types differ in the relative expression of some intrinsic and synaptic currents, and in the extent of their respective dendritic arbors. Spike depolarizing afterpotentials are present in both types, but more frequently give rise to prolonged burst discharges in vasopressin neurones. Oxytocin, but not vasopressin neurones, are characterized by a depolarization-activated, sustained outward rectifier which turns on near spike threshold, and which can produce prolonged spike frequency adaptation. When this sustained current is deactivated by small hyperpolarizing pulses, a rebound depolarization sufficient to evoke short spike trains follows the offset of these pulses. Both oxytocin and vasopressin neurones exhibit a transient outward rectification underlain by an Ia-type current. This transient rectifier delays spiking to depolarizing stimuli from a relatively hyperpolarized baseline, and is more prominent in vasopressin neurones. As a result, oxytocin neurones may be more reactive to depolarizing inputs. Both cell types receive glutamatergic, excitatory synaptic inputs and both possess R,S- alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor subtypes. The AMPA receptor channel on both cell types is characterized by a relatively high calcium permeability and voltage-dependent rectification, characteristic of a diminished presence of the GluR2 AMPA subunit. However, AMPA-mediated synaptic transients are larger, and decay faster, in oxytocin compared with vasopressin neurones, suggesting a potential difference for synaptic integration. The characteristics of NMDA-mediated synaptic transients are similar in oxytocin and vasopressin neurones, but some data suggest NMDA receptors may be less involved in the glutamatergic activation of oxytocin neurones. In both cell types, synaptic release of glutamate often coactivates AMPA and NMDA receptors. The dendritic morphology of oxytocin and vasopressin neurones in female rats differs from one another and exhibits considerable plasticity as a function of endocrine state. In virgin rats, oxytocin neurones have more dendritic branches and a greater total dendritic length compared with lactation, when the arbor is much less extensive. A complementary change occurs in vasopressin dendrites, which are more extensive during lactation. This reorganization suggests that oxytocin neurones may be more electronically compact during lactation. In addition, such dramatic shifts in overall dendritic length imply that significant gains and losses in either the total number of synapses, or in synaptic density, are incurred by both cell types as a function of reproductive state.

Components of after-hyperpolarization in magnocellular neurones of the rat supraoptic nucleus in vitro

1. The pharmacological sensitivity of hyperpolarizing components of spike train after-potentials was examined in sixty-one magnocellular neurones of the rat supraoptic nucleus using intracellular recording techniques in a brain slice preparation. 2. In 26 % of all neurones a slow after-hyperpolarization (AHP) was observed in addition to a fast AHP. In 31 % of all neurones a depolarizing after-potential (DAP) was observed. 3. The fast AHP was blocked by apamin whereas the slow AHP was blocked by charybdotoxin (ChTX). The DAP was enhanced by ChTX or a DAP was unmasked if not present during the control period. 4. Low concentrations of TEA (0.15-1.5 mM) induced effects on the slow AHP and the DAP essentially resembling those of ChTX. The same was true for the effects of CoCl2 (1 mM). 5. Spike train after-potentials were not affected by either iberiotoxin (IbTX), a selective high-conductance potassium (BK) channel antagonist, or margatoxin (MgTX), a Kv1.3 alpha-subunit antagonist. 6. Kv1.3 alpha-subunit immunohistochemistry revealed that these units are not expressed in the somato-dendritic region of supraoptic neurones. 7. The effects of ChTX, IbTX, MgTX, TEA, CoCl2 and CdCl2 on spike train after-potentials are interpreted in terms of an induction of the slow AHP by the activation of calcium-dependent potassium channels of intermediate single channel conductance (IK channels). 8. The results suggest that at least the majority of supraoptic magnocellular neurones share the capability of generating both a slow AHP and a DAP. The slow AHP may act to control the expression of the DAP, thus regulating the excitability of magnocellular neurones. The interaction of the slow AHP and the DAP may be important for the control of phasic discharge.

Caesium blocks depolarizing after-potentials and phasic firing in rat supraoptic neurones

1. The effects of Cs+ on the action potential, post-train after-hyperpolarization (AHP), Ca2+-dependent post-spike depolarizing after-potential (DAP) and phasic firing were examined during intracellular recordings from magnocellular neurosecretory cells (MNCs) in superfused rat hypothalamic explants. 2. Extracellular Cs+ reversibly inhibited (IC50, approximately 1 mM) DAPs, and associated after-discharges, that followed brief spike trains in each of sixteen cells tested. Although bath application of Cs+ also provoked a small reversible depolarization, inhibition of the DAP was retained when membrane voltage was kept constant by current injection. 3. Application of Cs+ had no significant effects on spike duration (n = 8), frequency-dependent spike broadening (n = 8), spike hyperpolarizing after-potentials (n = 14), or the amplitude of the isolated AHP (n = 7). Caesium-evoked inhibition of the DAP, therefore, does not result from diminished spike-evoked Ca2+ influx, and may reflect direct blockade of the conductance underlying the DAP. 4. Inhibition of the DAP was associated with an enhancement of the amplitude and duration of the AHP, indicating that the currents underlying the AHP and the DAP overlap in time following a train of action potentials, and that the relative magnitude of these currents is an important factor in determining the shape and time course of post-train after-potentials. 5. Bath application of Cs+ reversibly abolished phasic firing in each of seven cells tested. This effect was reversible and persisted at all subthreshold voltages tested. These results indicate that the current underlying the DAP is necessary for the genesis of plateau potentials and phasic firing in MNCs.