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Gareth Leng

Publications and source records attributed to Gareth Leng.

At least 19 recordsLinked to original sources
PubMed2006

Endogenous activation of supraoptic nucleus kappa-opioid receptors terminates spontaneous phasic bursts in rat magnocellular neurosecretory cells.

Phasic activity in magnocellular neurosecretory vasopressin cells is characterized by alternating periods of activity (bursts) and silence. During phasic bursts, action potentials (spikes) are superimposed on plateau potentials that are generated by summation of depolarizing after-potentials (DAPs). Burst termination is believed to result from autocrine feedback inhibition of plateau potentials by the kappa-opioid peptide, dynorphin, which is copackaged in vasopressin neurosecretory vesicles and exocytosed from vasopressin cell dendrites during phasic bursts. Here we tested this hypothesis, using intracellular recording in vitro to show that kappa-opioid receptor antagonist administration enhanced plateau potential amplitude to increase postspike excitability during spontaneous phasic activity. The antagonist also increased postburst DAP amplitude in vitro, indicating that endogenous dynorphin probably reduces plateau potential amplitude by inhibiting the DAP mechanism. However, the kappa-opioid receptor antagonist did not affect the slow depolarization that follows burst termination, suggesting that recovery from endogenous kappa-opioid inhibition does not contribute to the slow depolarization. We also show, by extracellular single-unit recording, that that there is a strong random element in the timing of burst initiation and termination in vivo. Administration of a kappa-opioid receptor antagonist eliminated the random element of burst termination but did not alter the timing of burst initiation. We conclude that dendritic dynorphin release terminates phasic bursts by reducing the amplitude of plateau potentials to reduce the probability of spike firing as bursts progress. By contrast, dendritic dynorphin release does not greatly influence the membrane potential between bursts and evidently does not influence the timing of burst initiation.

Action Potentials↗
PubMed2006

Neuronal activation in the hypothalamus and brainstem during feeding in rats.

We trained rats to a regime of scheduled feeding, in which food was available for only 2 hr each day. After 10 days, rats were euthanized at defined times relative to food availability, and their brains were analyzed to map Fos expression in neuronal populations to test the hypothesis that some populations are activated by hunger whereas others are activated by satiety signals. Fos expression accompanied feeding in several hypothalamic and brainstem nuclei. Food ingestion was critical for Fos expression in noradrenergic and non-noradrenergic cells in the nucleus tractus solitarii and area postrema and in the supraoptic nucleus, as well as in melanocortin-containing cells of the arcuate nucleus. However, anticipation of food alone activated other neurons in the arcuate nucleus and in the lateral and ventromedial hypothalamus, including orexin neurons. Thus orexigenic populations are strongly and rapidly activated at the onset of food presentation, followed rapidly by activity in anorexigenic populations when food is ingested.

Animals↗
PubMed2006

Dendritic peptide release and peptide-dependent behaviours.

Neuropeptides that are released from dendrites, such as oxytocin and vasopressin, function as autocrine or paracrine signals at their site of origin, but can also act at distant brain targets to evoke long-lasting changes in behaviour. Oxytocin, for instance, has profound effects on social bonding that are exerted at sites that richly express oxytocin receptors, but which are innervated by few, if any, oxytocin-containing projections. How can a prolonged, diffuse signal have coherent behavioural consequences? The recently demonstrated ability of neuropeptides to prime vesicle stores for activity-dependent release could lead to a temporary functional reorganization of neuronal networks harbouring specific peptide receptors, providing a substrate for long-lasting effects.

Animals↗
PubMed2006

Suppressed oxytocin neuron responses to immune challenge in late pregnant rats: a role for endogenous opioids.

Cytokine challenge (mimicking infection) with systemic interleukin-1beta (IL-1beta) stimulates oxytocin neurons via a noradrenergic brainstem pathway similar to that involved in parturition. As the responses of oxytocin neurons to several stimuli are reduced in late pregnancy, we have investigated whether responses to IL-1beta are also suppressed. In virgin Sprague-Dawley rats, IL-1beta (500 ng/kg i.v.) rapidly increased oxytocin secretion (3.2-fold), via a central action as the firing rate of oxytocin neurons in the supraoptic nucleus was increased. In contrast, IL-1beta had no significant effect on the electrical or secretory activity of oxytocin neurons in late pregnant rats. In pregnancy activation of a central inhibitory opioid mechanism restrains oxytocin neuron responses to various stimuli. Accordingly, we tested the effects of the opioid antagonist, naloxone, on oxytocin neuron responses to IL-1beta in pregnancy. Naloxone (5 mg/kg i.v.) did not affect the oxytocin secretory response to IL-1beta in virgin rats, whereas in late pregnant rats naloxone revealed a greater oxytocin secretory response to IL-1beta (3.5-fold) than in virgin rats. In virgin rats, naloxone decreased oxytocin neuron firing rate after IL-1beta, however, in pregnant rats naloxone increased the firing rate response to IL-1beta to the level seen in virgin rats. Thus, systemic IL-1beta acts centrally to increase oxytocin secretion. In pregnancy this response is suppressed by endogenous opioids, thus preserving neurohypophysial oxytocin stores for parturition and minimizing the risk of preterm labour. The exaggerated oxytocin secretory response to IL-1beta in pregnancy after naloxone reflects increased oxytocin stores and/or increased efficiency of excitation-secretion coupling at the posterior pituitary.

Action Potentials↗
PubMed2005

Presynaptic actions of endocannabinoids mediate alpha-MSH-induced inhibition of oxytocin cells.

We recently showed that central injections of alpha-melanocyte-stimulating hormone (alpha-MSH) inhibits oxytocin cells and reduces peripheral release of oxytocin, but induces oxytocin release from dendrites. Dendritic oxytocin release can be triggered by agents that mobilize intracellular calcium. Oxytocin, like alpha-MSH, mobilizes intracellular calcium stores in oxytocin cells and triggers presynaptic inhibition of afferent inputs that is mediated by cannabinoids. We hypothesized that this mechanism might underlie the inhibitory effects of alpha-MSH. To test this, we recorded extracellularly from identified oxytocin and vasopressin cells in the anesthetized rat supraoptic nucleus (SON). Retrodialysis of a CB1 cannabinoid receptor antagonist to the SON blocked the inhibitory effects of intracerebroventricular injections of alpha-MSH on the spontaneous activity of oxytocin cells. We then monitored synaptically mediated responses of SON cells to stimulation of the organum vasculosum of the lamina terminalis (OVLT); this evoked a mixed response comprising an inhibitory component mediated by GABA and an excitatory component mediated by glutamate, as identified by the effects of bicuculline and 6-cyano-7-nitroquinoxaline-2,3-dione applied to the SON by retrodialysis. Application of CB1 receptor agonists to the SON attenuated the excitatory effects of OVLT stimulation in both oxytocin and vasopressin cells, whereas alpha-MSH attenuated the responses of oxytocin cells only. Thus alpha-MSH can act as a "switch"; it triggers oxytocin release centrally, but at the same time through initiating endocannabinoid production in oxytocin cells inhibits their electrical activity and hence, peripheral secretion.

Animals↗
PubMed2005

Regulation of activity-dependent dendritic vasopressin release from rat supraoptic neurones.

Magnocellular neurones of the hypothalamus release vasopressin and oxytocin from their dendrites and soma. Using a combination of electrophysiology, microdialysis, in vitro explants, and radioimmunoassay we assessed the involvement of intracellular Ca(2+) stores in the regulation of dendritic vasopressin release. Thapsigargin and cyclopiazonic acid, which mobilize Ca(2+) from intracellular stores of the endoplasmic reticulum, evoked vasopressin release from dendrites and somata of magnocellular neurones in the supraoptic nucleus. Thapsigargin also produced a dramatic potentiation of dendritic vasopressin release evoked by osmotic or high potassium stimulation. This effect is long lasting, time dependent, and specific to thapsigargin as caffeine and ryanodine had no effect. Furthermore, antidromic activation of electrical activity in the cell bodies released vasopressin from dendrites only after thapsigargin pretreatment. Thus, exposure to Ca(2+) mobilizers such as thapsigargin or cyclopiazonic acid primes the releasable pool of vasopressin in the dendrites, so that release can subsequently be evoked by electrical and depolarization-dependent activation. Vasopressin itself is effective in inducing dendritic vasopressin release, but it is ineffective in producing priming.

Animals↗
PubMed2005

Regulation of oxytocin secretion.

A baby sucks at a mother's breast for comfort and, of course, for milk. Milk is made in specialized cells of the mammary gland, and for a baby to feed, the milk must be released into a collecting chamber from where it can be extracted by sucking. Milk "let-down" is a reflex response to the suckling and kneading of the nipple--and sometimes in response to the sight, smell, and sound of the baby--and is ultimately affected by the secretion of oxytocin. Oxytocin has many physiological roles, but its only irreplaceable role is to mediate milk let-down: oxytocin-deficient mice cannot feed their young; the pups suckle but no milk is let down, and they will die unless cross-fostered. Most other physiological roles of oxytocin, including its role in parturition, are redundant in the sense that the roles can be assumed by other mechanisms in the absence of oxytocin throughout development and adult life. Nevertheless, physiological function in these roles can be altered or impaired by acute interventions that alter oxytocin secretion or change the actions of oxytocin. Here we focus on the diverse stimuli that regulate oxytocin secretion and on the apparent diversity of the roles for oxytocin.

Animals↗
PubMed2005

Hypothalamic expression of NPY mRNA, vasopressin mRNA and CRF mRNA in response to food restriction and central administration of the orexigenic peptide GHRP-6.

In this study, we examined the effects of restricted feeding and of central administration of an orexigenic ghrelin agonist GHRP-6 on peptide mRNA expression in the hypothalamus. We compared rats fed ad libitum with rats that were allowed food for only 2?h every day, and treated with a continuous chronic i.c.v. infusion of GHRP-6 or vehicle. Ad libitum fed rats exposed to GHRP-6 increased their food intake and body weight over 6 days, but, at the end of this period, neuropeptide Y mRNA expression in the arcuate nucleus was not different to that in control rats. By contrast, expression of neuropeptide Y mRNA in the arcuate nucleus was elevated in food-restricted rats, consistent with the interpretation that increased expression reflects increased hunger. However, neuropeptide Y mRNA expression was no greater in food-restricted rats infused with GHRP-6 than in food-restricted rats infused with vehicle; thus if the drive to eat was stronger in rats infused with GHRP-6, this was not reflected by higher levels of neuropeptide Y mRNA expression. Expression of vasopressin mRNA and corticotrophin releasing factor (CRF) mRNA in the paraventricular nucleus (PVN) was not changed by food restriction. GHRP-6 infusion increased CRF mRNA expression in ad libitum rats only.

Animals↗
PubMed2005

Morphine withdrawal increases intrinsic excitability of oxytocin neurons in morphine-dependent rats.

To determine whether intrinsic mechanisms drive supraoptic nucleus oxytocin neuron excitation during morphine withdrawal, we calculated the probability of action potential (spike) firing with time after each spike for oxytocin neurons in morphine-naive and morphine-dependent rats in vivo and measured changes in intrinsic membrane properties in vitro. The opioid receptor antagonist, naloxone, increased oxytocin neuron post-spike excitability in morphine-dependent rats; this increase was greater for short interspike intervals (<0.1 s). Naloxone had similar, but smaller (P=0.04), effects in oxytocin neurons in morphine-naive rats. The increased post-spike excitability for short interspike intervals was specific to naloxone, because osmotic stimulation increased excitability without potentiating excitability at short interspike intervals. By contrast to oxytocin neurons, neither morphine dependence nor morphine withdrawal increased post-spike excitability in neighbouring vasopressin neurons. To determine whether increased post-spike excitability in oxytocin neurons during morphine withdrawal reflected altered intrinsic membrane properties, we measured the in vitro effects of naloxone on transient outward rectification (TOR) and after-hyperpolarization (AHP), properties mediated by K+ channels and that affect supraoptic nucleus neuron post-spike excitability. Naloxone reduced the TOR and AHP (by 20% and 60%, respectively) in supraoptic nucleus neurons from morphine-dependent, but not morphine-naive, rats. In vivo, spike frequency adaptation (caused by activity-dependent AHP activation) was reduced by naloxone (from 27% to 3%) in vasopressin neurons in morphine-dependent, but not morphine-naive, rats. Thus, multiple K+ channel inhibition increases post-spike excitability for short interspike intervals, contributing to the increased firing of oxytocin neurons during morphine withdrawal.

Action Potentials↗
PubMed2004

Regulation of neuronal nitric oxide synthase mRNA expression in the rat magnocellular neurosecretory system.

We examined the activation of nNOS mRNA expression within the supraoptic and paraventricular nuclei (SON and PVN) of the hypothalamus. In salt-loaded rats nNOS mRNA expression was significantly increased in both nuclei. In rats given i.p. injections of 1.5 M NaCl (4 ml/kg), a small but significant increase in nNOS mRNA expression in the SON and PVN was found 6 h after injection; no change was detected 2 or 4 h after injection. In rats in which hyponatraemia had been induced experimentally, nNOS mRNA was downregulated in the SON, and expression levels were not increased within 4 h after intense acute osmotic stimuli. Finally, neurons of the SON were antidromically-activated by neural stalk stimulation for 2 h. No increase of nNOS mRNA expression in the SON was observed 2 h after stimulation. Thus, increased electrical activity is not directly coupled to rapidly increased expression of nNOS mRNA, and hence acute increases in nNOS mRNA expression are unlikely to play a role in short-term adaptation of the magnocellular system to osmotic stimulation.

Analysis of Variance↗
PubMed2004

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

In vivo, most vasopressin cells of the hypothalamic supraoptic nucleus fire action potentials in a 'phasic' pattern when the systemic osmotic pressure is elevated, while most oxytocin cells fire continuously. The phasic firing pattern is believed to arise as a consequence of intrinsic activity-dependent changes in membrane potential, and these have been extensively studied in vitro. Here we analysed the discharge patterning of supraoptic nucleus neurones in vivo, to infer the characteristics of the post-spike sequence of hyperpolarization and depolarization from the observed spike patterning. We then compared patterning in phasic cells in vivo and in vitro, and we found systematic differences in the interspike interval distributions, and in other statistical parameters that characterized activity patterns within bursts. Analysis of hazard functions (probability of spike initiation as a function of time since the preceding spike) revealed that phasic firing in vitro appears consistent with a regenerative process arising from a relatively slow, late depolarizing afterpotential that approaches or exceeds spike threshold. By contrast, in vivo activity appears to be dominated by stochastic rather than deterministic mechanisms, and appears consistent with a relatively early and fast depolarizing afterpotential that modulates the probability that random synaptic input exceeds spike threshold. Despite superficial similarities in the phasic firing patterns observed in vivo and in vitro, there are thus fundamental differences in the underlying mechanisms.

Action Potentials↗
PubMed2003

Alpha-melanocyte-stimulating hormone stimulates oxytocin release from the dendrites of hypothalamic neurons while inhibiting oxytocin release from their terminals in the neurohypophysis.

The peptides alpha-melanocyte stimulating hormone (alpha-MSH) and oxytocin, when administered centrally, produce similar behavioral effects. alpha-MSH induces Fos expression in supraoptic oxytocin neurons, and alpha-MSH melanocortin-4 receptors (MC4Rs) are highly expressed in the supraoptic nucleus, suggesting that alpha-MSH and oxytocin actions are not independent. Here we investigated the effects of alpha-MSH on the activity of supraoptic neurons. We confirmed that alpha-MSH induces Fos expression in the supraoptic nucleus when injected centrally and demonstrated that alpha-MSH also stimulates Fos expression in the nucleus when applied locally by retrodialysis. Thus alpha-MSH-induced Fos expression is not associated with electrophysiological excitation of supraoptic neurons because central injection of alpha-MSH or selective MC4 receptor agonists inhibited the electrical activity of oxytocin neurons in the supraoptic nucleus recorded in vivo. Consistent with these observations, oxytocin secretion into the bloodstream decreased after central injection of alpha-MSH. However, MC4R ligands induced substantial release of oxytocin from dendrites in isolated supraoptic nuclei. Because dendritic oxytocin release can be triggered by changes in [Ca2+]i, we measured [Ca2+]i responses in isolated supraoptic neurons and found that MC4R ligands induce a transient [Ca2+]i increase in oxytocin neurons. This response was still observed in low extracellular Ca2+ concentration and probably reflects mobilization of [Ca2+]i from intracellular stores rather than entry via voltage-gated channels. Taken together, these results show for the first time that a peptide, here alpha-MSH, can induce differential regulation of dendritic release and systemic secretion of oxytocin, accompanied by dissociation of Fos expression and electrical activity.

Animals↗
PubMed2003

Neurosteroid regulation of oxytocin and vasopressin release from the rat supraoptic nucleus.

In adult rats somato-dendritic release of oxytocin and vasopressin from magnocellular neurones in the supraoptic nucleus of the hypothalamus has important autoregulatory actions on the neuronal electrical activity, and in neonatal rats it plays a role in the development of dendritic arborisation. In the adult, oxytocin effects are modulated by allopregnanolone via an interaction with inhibitory GABAA receptors. This study examined the effects of allopregnanolone, progesterone and 17beta-oestradiol on oxytocin and vasopressin release from intact isolated supraoptic nuclei and from the neurophypophyses in rats of differing ages. In supraoptic nuclei from rats of 3-4 weeks old or less, all three neurosteroids induced oxytocin release from the isolated supraoptic nucleus, but only allopregnanolone induced significant release of vasopressin. Surprisingly, in these very young rats, allopregnanolone-induced oxytocin release was inhibited by GABAA receptor antagonists as well as by an oxytocin receptor antagonist. By contrast, in supraoptic nuclei from adult rats allopregnanolone-induced oxytocin release was much smaller, and was enhanced in the presence of bicuculline. The GABAA receptor agonist muscimol also induced oxytocin release from supraoptic nuclei in young rats, but had no effect in adult rats. Oxytocin cells isolated from young rats showed an increase in [Ca2+]i in response to both allopregnanolone and muscimol. Allopregnanolone had no effect on [Ca2+]i or on the release of oxytocin or vasopressin from neurohypophysial axon terminals in either young or old rats. We conclude that, in very young rats, (i) neurosteroids induce oxytocin release from the supraoptic nucleus by a mechanism that partly depends on the presence of GABA, which in young rats is depolarising to oxytocin cells, and which also partly depends upon endogenous oxytocin, and (ii) the effect of allopregnanolone upon oxytocin release changes with age, as the functional activity of GABAA receptors changes from excitation to inhibition of oxytocin cells.

Aging↗
PubMed2003

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.

Action Potentials↗
PubMed2003

Actions of neuropeptide Y and growth hormone secretagogues in the arcuate nucleus and ventromedial hypothalamic nucleus.

Systemic or central administration of growth-hormone secretagogues (GHS) induces dense Fos expression in the arcuate nucleus but little or no Fos expression in the ventromedial hypothalamic nucleus, although both sites show intense expression of mRNA for the GHS receptor. Here, we recorded the electrical activity of single neurons from the arcuate nucleus and from the ventromedial hypothalamic nucleus in a rat hypothalamic slice preparation, and compared responses of these two populations to GHS. At both sites, the predominant neuronal response to GHS was a long-lasting excitation, indicating that GHS receptors at both sites are functional and similarly coupled to electrical excitation. We also tested neurons at both sites for their responses to neuropeptide Y and to somatostatin; at both sites the predominant response to each of these peptides was inhibitory. The arcuate cells that are activated by GHS include neuropeptide Y cells and growth hormone-releasing hormone cells. It seems possible that neuropeptide Y released in the ventromedial hypothalamus from the terminals of arcuate neurons counteracts the activation of ventromedial hypothalamic neurons by GHS in vivo, or that somatostatin released following liberation of growth hormone may do so.

Animals↗
PubMed2003

Oxytocin released from magnocellular dendrites: a potential modulator of alpha-melanocyte-stimulating hormone behavioral actions?

Alpha-melanocyte-stimulating hormone (alpha-MSH) is implicated in a variety of behavioral processes that are remarkably similar to those behaviors in which centrally acting oxytocin has been implicated. Central oxytocin derives in part from centrally projecting parvocellular neurons of the paraventricular nucleus, but large amounts of oxytocin are also released from dendrites of magnocellular oxytocin neurons in the supraoptic and paraventricular nuclei of the hypothalamus. Oxytocin release from dendrites is semi-independent of electrical activity and can be modulated by peptidergic signals independently of release from nerve terminals. Oxytocin is released from dendrites by stimuli that mobilize intracellular calcium stores, and such stimuli also prime dendritic stores of oxytocin, making them available for subsequent activity-dependent secretion. Evidence exists for efferent projections to the supraoptic nucleus from the arcuate nucleus where alpha-MSH neurons are located, and the supraoptic and paraventricular nuclei show high levels of expression of mRNA for the melanocortin receptor MC4R. These projections may be involved specifically in the regulation of dendritic oxytocin release.

Animals↗
PubMed2002

Intracellular calcium stores regulate activity-dependent neuropeptide release from dendrites.

Information in neurons flows from synapses, through the dendrites and cell body (soma), and, finally, along the axon as spikes of electrical activity that will ultimately release neurotransmitters from the nerve terminals. However, the dendrites of many neurons also have a secretory role, transmitting information back to afferent nerve terminals. In some central nervous system neurons, spikes that originate at the soma can travel along dendrites as well as axons, and may thus elicit secretion from both compartments. Here, we show that in hypothalamic oxytocin neurons, agents that mobilize intracellular Ca(2+) induce oxytocin release from dendrites without increasing the electrical activity of the cell body, and without inducing secretion from the nerve terminals. Conversely, electrical activity in the cell bodies can cause the secretion of oxytocin from nerve terminals with little or no release from the dendrites. Finally, mobilization of intracellular Ca(2+) can also prime the releasable pool of oxytocin in the dendrites. This priming action makes dendritic oxytocin available for release in response to subsequent spike activity. Priming persists for a prolonged period, changing the nature of interactions between oxytocin neurons and their neighbours.

Animals↗