The variability of basal action potential firing is positively correlated with bursting in hypothalamic oxytocin neurones

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.

60 YEARS OF NEUROENDOCRINOLOGY: The posterior pituitary, from Geoffrey Harris to our present understanding

Geoffrey Harris pioneered our understanding of the posterior pituitary, mainly with experiments that involved the electrical stimulation of the supraoptico-hypophysial tract. In the present essay, we explain how his observations included clues to the pulsatile nature of the oxytocin signal - clues that were followed up by subsequent workers, including his students and their students. These studies ultimately led to our present understanding of the milk-ejection reflex and of the role of oxytocin in parturition. Discoveries of wide significance followed, including: the recognition of the importance of pulsatile hormone secretion; the recognition of the importance of stimulus-secretion coupling mechanisms in interpreting the patterned electrical activity of neurons; the physiological importance of peptide release in the brain; the recognition that peptide release comes substantially from dendrites and can be regulated independently of nerve terminal secretion; and the importance of dynamic morphological changes to neuronal function in the hypothalamus. All of these discoveries followed from the drive to understand the milk-ejection reflex. We also reflect on Harris's observations on vasopressin secretion, on the effects of stress, and on oxytocin secretion during sexual activity.

Beyond the GnRH axis: kisspeptin regulation of the oxytocin system in pregnancy and lactation

Circulating oxytocin is critical for normal birth and lactation. Oxytocin is synthesised by hypothalamic supraoptic and paraventricular neurons and is released from the posterior pituitary gland into the circulation. Oxytocin secretion depends on action potentials initiated at the cell body, and we have shown that intravenous (IV) administration of kisspeptin-10 transiently increases the firing rate of supraoptic nucleus oxytocin neurons in anaesthetised, non-pregnant, pregnant and lactating rats. This peripheral effect is likely via vagal afferent input, because disruption of vagal afferents prevented the excitation. In our initial studies, intracerebroventricular (icv) administration of kisspeptin-10 did not alter the firing rate of oxytocin neurons in non-pregnant rats. Remarkably, we have now gathered unpublished observations showing that icv kisspeptin-10 transiently excites oxytocin neurons in late pregnancy and during lactation, suggesting that a central kisspeptin excitation of oxytocin neurons emerges at the end of pregnancy, when increased oxytocin secretion is required for delivery of the fetus and for milk let-down after delivery.

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.

Spontaneous ryanodine-receptor-dependent Ca2+-activated K+ currents and hyperpolarizations in rat medial preoptic neurons

The aim of the present study was to clarify the identity of slow spontaneous currents, the underlying mechanism and possible role for impulse generation in neurons of the rat medial preoptic nucleus (MPN). Acutely dissociated neurons were studied with the perforated patch-clamp technique. Spontaneous outward currents, at a frequency of approximately 0.5 Hz and with a decay time constant of approximately 200 ms, were frequently detected in neurons when voltage-clamped between approximately -70 and -30 mV. The dependence on extracellular K(+) concentration was consistent with K(+) as the main charge carrier. We concluded that the main characteristics were similar to those of spontaneous miniature outward currents (SMOCs), previously reported mainly for muscle fibers and peripheral nerve. From the dependence on voltage and from a pharmacological analysis, we concluded that the currents were carried through small-conductance Ca(2+)-activated (SK) channels, of the SK3 subtype. From experiments with ryanodine, xestospongin C, and caffeine, we concluded that the spontaneous currents were triggered by Ca(2+) release from intracellular stores via ryanodine receptor channels. An apparent voltage dependence was explained by masking of the spontaneous currents as a consequence of steady SK-channel activation at membrane potentials > -30 mV. Under current-clamp conditions, corresponding transient hyperpolarizations occasionally exceeded 10 mV in amplitude and reduced the frequency of spontaneous impulses. In conclusion, MPN neurons display spontaneous hyperpolarizations triggered by Ca(2+) release via ryanodine receptors and SK3-channel activation. Thus such events may affect impulse firing of MPN neurons.

Emergent synchronous bursting of oxytocin neuronal network

When young suckle, they are rewarded intermittently with a let-down of milk that results from reflex secretion of the hormone oxytocin; without oxytocin, newly born young will die unless they are fostered. Oxytocin is made by magnocellular hypothalamic neurons, and is secreted from their nerve endings in the pituitary in response to action potentials (spikes) that are generated in the cell bodies and which are propagated down their axons to the nerve endings. Normally, oxytocin cells discharge asynchronously at 1–3 spikes/s, but during suckling, every 5 min or so, each discharges a brief, intense burst of spikes that release a pulse of oxytocin into the circulation. This reflex was the first, and is perhaps the best, example of a physiological role for peptide-mediated communication within the brain: it is coordinated by the release of oxytocin from the dendrites of oxytocin cells; it can be facilitated by injection of tiny amounts of oxytocin into the hypothalamus, and it can be blocked by injection of tiny amounts of oxytocin antagonist. Here we show how synchronized bursting can arise in a neuronal network model that incorporates basic observations of the physiology of oxytocin cells. In our model, bursting is an emergent behaviour of a complex system, involving both positive and negative feedbacks, between many sparsely connected cells. The oxytocin cells are regulated by independent afferent inputs, but they interact by local release of oxytocin and endocannabinoids. Oxytocin released from the dendrites of these cells has a positive-feedback effect, while endocannabinoids have an inhibitory effect by suppressing the afferent input to the cells.

When young suckle, they are rewarded intermittently with a let-down of milk that results from reflex secretion of the hormone oxytocin. Oxytocin is a neuropeptide made by specialised neurons in the hypothalamus, and is secreted from nerve endings in the pituitary gland. During suckling, every 5 min or so, each of these neurons discharges a brief, intense burst of action potentials; these are propagated down the axons, and release a pulse of oxytocin into the circulation. Here, we have built a computational model to understand how these bursts arise and how they are synchronized. In our model, bursting is an emergent behaviour of a complex system, involving both positive and negative feedbacks, between many, sparsely connected cells. The oxytocin cells are regulated by independent afferent inputs, but they interact by local release of oxytocin and endocannabinoids. Oxytocin released from the dendrites of these cells has a positive-feedback effect, while endocannabinoids have an inhibitory effect by suppressing the afferent input to the cells. Many neurons make peptides that act as messengers within the brain, and many of these are also released from dendrites, so this model may reflect a common pattern-generating mechanism in the brain.

Oxytocin-induced postinhibitory rebound firing facilitates bursting activity in oxytocin neurons

During parturition and lactation, neurosecretory oxytocin (OT) neurons in the hypothalamus achieve pulsatile hormone secretion by coordinated bursts of firing that occur throughout the neuronal population. This activity is partly controlled by somatodendritic release of OT, which facilitates the onset and recurrence of synchronized bursting. To further investigate the cellular mechanisms underlying the control exerted by OT on the activity of its own neurons, we studied the effects of the peptide on membrane potential and synaptic activity in OT neurons in hypothalamic organotypic slice cultures. Bath application of low concentrations of OT (<100 nM) facilitated GABA(A) receptor-mediated inhibitory transmission through a presynaptic mechanism without affecting membrane potential and excitatory glutamatergic synaptic activity. The facilitatory action of OT on GABAergic transmission was dose-dependent, starting at 25 nM and disappearing at concentrations >100 nM. As shown previously, higher concentrations of OT (>500 nM) had the opposite effect, inhibiting GABA(A) receptors via a postsynaptic mechanism. Surprisingly, OT-mediated facilitation of GABAergic transmission promoted action potential firing in 40% of the neurons. Each action potential occurred at the end of the repolarizing phase of an inhibitory potential. Pharmacological dissection revealed that this firing involved the activation of low-threshold activated calcium channels. Detailed statistical analysis showed that OT-mediated firing upregulated bursting activity in OT neurons. It is thus likely to optimize OT secretion and, as a consequence, facilitate delivery and milk ejection in mammals.

Differences in spike train variability in rat vasopressin and oxytocin neurons and their relationship to synaptic activity

The firing pattern of magnocellular neurosecretory neurons is intimately related to hormone release, but the relative contribution of synaptic versus intrinsic factors to the temporal dispersion of spikes is unknown. In the present study, we examined the firing patterns of vasopressin (VP) and oxytocin (OT) supraoptic neurons in coronal slices from virgin female rats, with and without blockade of inhibitory and excitatory synaptic currents. Inhibitory postsynaptic currents (IPSCs) were twice as prevalent as their excitatory counterparts (EPSCs), and both were more prevalent in OT compared with VP neurons. Oxytocin neurons fired more slowly and irregularly than VP neurons near threshold. Blockade of Cl- currents (including tonic and synaptic currents) with picrotoxin reduced interspike interval (ISI) variability of continuously firing OT and VP neurons without altering input resistance or firing rate. Blockade of EPSCs did not affect firing pattern. Phasic bursting neurons (putative VP neurons) were inconsistently affected by broad synaptic blockade, suggesting that intrinsic factors may dominate the ISI distribution during this mode in the slice. Specific blockade of synaptic IPSCs with gabazine also reduced ISI variability, but only in OT neurons. In all cases, the effect of inhibitory blockade on firing pattern was independent of any consistent change in input resistance or firing rate. Since the great majority of IPSCs are randomly distributed, miniature events (mIPSCs) in the coronal slice, these findings imply that even mIPSCs can impart irregularity to the firing pattern of OT neurons in particular, and could be important in regulating spike patterning in vivo. For example, the increased firing variability that precedes bursting in OT neurons during lactation could be related to significant changes in synaptic activity.

Activity-dependent feedback modulation of spike patterning of supraoptic nucleus neurons by endogenous adenosine

Neuropeptide secretion from the dendrites of hypothalamic magnocellular supraoptic nucleus (SON) neurons contributes to the regulation of neuronal activity patterning, which ultimately determines their peptide output from axon terminals in the posterior pituitary gland. SON dendrites also secrete a number of other neuromodulators, including ATP. ATP degrades to adenosine in the extracellular space to complement transported adenosine acting on pre- and postsynaptic SON A1 receptors to reduce neuronal excitability, measured in vitro. To assess adenosine control of electrical activity in vivo, we made extracellular single-unit recordings of the electrical activity of SON neurons in anesthetized male rats. Microdialysis application (retrodialysis) of the A1 receptor antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT) increased phasic vasopressin cell intraburst firing rates progressively over the first 5 s by 4.5 +/- 1.6 Hz (P < 0.05), and increased burst duration by 293 +/- 64% (P < 0.05). Hazard function plots were generated from interval interspike histograms and revealed that these effects were associated with increased postspike excitability. In contrast, CPT had no effect on the firing rates and hazard function plot profiles of continuously active vasopressin and oxytocin cells. However, CPT significantly increased clustering of spikes, as quantified by the index of dispersion, in oxytocin cells and continuously active vasopressin cells (by 267 +/- 113% and 462 +/- 67%, respectively, P < 0.05). Indeed, in 4 of 5 continuously active vasopressin cells, CPT induced a pseudophasic activity pattern. Together, these results indicate that endogenous adenosine is involved in the local control of SON cell activity in vivo.

Oxytocin neurones are recruited into co-ordinated fluctuations of firing before bursting in the rat

Hypothalamic oxytocin neurones have dual physiological functions with associated characteristic activity patterns: a homeostatic osmoregulatory role involving continuous low frequency firing at a relatively constant rate, and roles associated with reproduction involving periodic, brief, synchronised, high frequency bursts of spikes. Apparently the same neurones maintain both roles during reproduction, when both activity patterns occur simultaneously, although sometimes factors linked to the homeostatic response predominate and prevent bursting. With the object of understanding how oxytocin neuronal networks manage both roles during lactation, we analysed basal activity between bursts in simultaneously recorded neurones to reveal potentially adaptive changes in network behaviour. Negative autocorrelation on a time scale of 0.5-2 s occurs in basal activity between bursts but also in non-bursting oxytocin neurones, and can therefore be associated with the system's homeostatic role. Although the system responds to the pups suckling by the induction of bursting, there are also increasing fluctuations in firing that are positively correlated in some simultaneously recorded neurones during basal activity between bursts. A few seconds before bursts, cross-correlation strengthens, irregularity of firing increases, and serial correlation (autocorrelation) weakens, all substantially. After pharmacological treatments known to facilitate bursting, cross-correlation and irregularity of firing increase and autocorrelation weakens, and the reverse occurs in conditions that delay bursting (hyperosmotic stress and pharmacological interventions). Our analyses suggest heterogeneity in the population of oxytocin neurones during lactation; the range including 'leader neurones' that readily display co-ordinated fluctuations in firing in response to suckling and escape from negative autocorrelation just before bursts, and 'follower neurones' that fire at a relatively constant rate in no apparent relationship to others, except when recruited late to bursting, probably in response to massive stimulation from already bursting neurones. The steep increases in correlation a few seconds before bursts reflect an accelerating process of recruitment of follower neurones to co-ordinated fluctuations, leading to the phase transition that constitutes the critical stage of burst generation.

Milk ejection burst-like electrical activity evoked in supraoptic oxytocin neurons in slices from lactating rats

To examine the mechanisms underlying milk-ejection bursts of oxytocin (OT) neurons during suckling, both in vivo and in vitro studies were performed on supraoptic OT neurons from lactating rats. The bursts were first recorded extracellularly in anesthetized rats. Burst-related electrical parameters were essentially the same as previous reports except for a trend toward transient decreases in basal firing rates immediately preceding the burst. From putative OT neurons in slices with extracellular recordings, bursts that closely mimicked the in vivo bursts were elicited by phenylephrine, an alpha1-adrenoceptor agonist, in a low-Ca(2+) medium. Moreover, in whole cell patch-clamp recordings, the in vitro bursts were recorded from immunocytochemically identified OT neurons. After a transient decrease in the basal firing rate, the in vitro bursts started with a sudden increase in the firing rate, quickly reaching a peak level, then gradually decaying, and ended with a postburst inhibition. A brief depolarization of the membrane potential and an increase in membrane conductance appeared after the onset of the burst. Spikes during a burst were characterized by a significant increase in the duration and decrease in the amplitude around the peak rate firing. These bursts were significantly different from short-lasting burst firing of vasopressin neurons in membrane potential changes, time to reach peak firing rate, spike amplitude and duration during peak rate firing. Our extensive analysis of these results suggests that the in vitro burst is a useful model for further study of mechanisms underlying milk-ejection bursts of OT neurons in vivo.

Glutamatergic input governs periodicity and synchronization of bursting activity in oxytocin neurons in hypothalamic organotypic cultures

During suckling, oxytocin (OT) neurons display a bursting electrical activity, consisting of a brief burst of action potentials which is synchronized throughout the OT neuron population and which periodically occurs just before each milk ejection in the lactating rat. To investigate the basis of such synchronization, we performed simultaneous intracellular recordings from pairs of OT neurons identified retrospectively by intracellular fluorescent labelling and immunocytochemistry in organotypic slice cultures derived from postnatal rat hypothalamus. A spontaneous bursting activity was recorded in 65% of OT neurons; the remaining showed only a slow, irregular activity. Application of OT triggered bursts in nonbursting neurons and accelerated bursting activity in spontaneously bursting cells. These cultures included rare vasopressinergic neurons showing no bursting activity and no reaction to OT. Bursts occurred simultaneously in all pairs of bursting OT neurons but, as in vivo, there were differences in burst onset, amplitude and duration. Coordination of firing was not due to electrotonic coupling because depolarizing one neuron in a pair had no effect on the membrane potential of its partner and halothane and proprionate did not desynchronize activity. On the other hand, bursting activity was superimposed on volleys of excitatory postsynaptic potentials (EPSPs) which occurred simultaneously in pairs of neurons. EPSPs, and consequently action potentials, were reversibly blocked by the non-NMDA glutamatergic receptor antagonist CNQX. Taken together, these data, obtained from organotypic cultures, strongly suggest that a local hypothalamic network governs synchronization of bursting firing in OT neurons through synchronous afferent volleys of EPSPs originating from intrahypothalamic glutamatergic inputs.

The magnocellular oxytocin system, the fount of maternity: adaptations in pregnancy

Oxytocin secretion from the posterior pituitary gland is increased during parturition, stimulated by the uterine contractions that forcefully expel the fetuses. Since oxytocin stimulates further contractions of the uterus, which is exquisitely sensitive to oxytocin at the end of pregnancy, a positive feedback loop is activated. The neural pathway that drives oxytocin neurons via a brainstem relay has been partially characterised, and involves A2 noradrenergic cells in the brainstem. Until close to term the responsiveness of oxytocin neurons is restrained by neuroactive steroid metabolites of progesterone that potentiate GABA inhibitory mechanisms. As parturition approaches, and this inhibition fades as progesterone secretion collapses, a central opioid inhibitory mechanism is activated that restrains the excitation of oxytocin cells by brainstem inputs. This opioid restraint is the predominant damper of oxytocin cells before and during parturition, limiting stimulation by extraneous stimuli, and perhaps facilitating optimal spacing of births and economical use of the store of oxytocin accumulated during pregnancy. During parturition, oxytocin cells increase their basal activity, and hence oxytocin secretion increases. In addition, the oxytocin cells discharge a burst of action potentials as each fetus passes through the birth canal. Each burst causes the secretion of a pulse of oxytocin, which sharply increases uterine tone; these bursts depend upon auto-stimulation by oxytocin released from the dendrites of the magnocellular neurons in the supraoptic and paraventricular nuclei. With the exception of the opioid mechanism that emerges to restrain oxytocin cell responsiveness, the behavior of oxytocin cells and their inputs in pregnancy and parturition is explicable from the effects of hormones of pregnancy (relaxin, estrogen, progesterone) on pre-existing mechanisms, leading through relative quiescence at term inter alia to net increase in oxytocin storage, and reduced auto-inhibition by nitric oxide generation. Cyto-architectonic changes in parturition, involving evident retraction of glial processes between oxytocin cells so they get closer together, are probably a response to oxytocin neuron activation rather than being essential for their patterns of firing in parturition.

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.