Victoria Scott

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

Non‐technical summaryu2002 Glutamate is a critical excitatory neurotransmitter in the modulation of hypothalamic neuronal activity and neurosecretion from the posterior pituitary. Still, the precise mechanisms and modalities by which it acts remain to be fully elucidated. We show that in addition to mediating conventional fast synaptic transmission, glutamate persistently activates extrasynaptic NMDA receptors, providing a tonic excitatory drive to hypothalamic neurosecretory neurons. We also show that this tonic excitatory modality is influenced by the neighbouring astrocytes, and is enhanced in dehydrated rats. Knowledge on alternative modalities by which glutamate influences hypothalamic neuronal function increases our understanding of general brain mechanisms regulating neurosecretion and bodily homeostasis.

Kisspeptin Activation of Supraoptic Nucleus Neurons in Vivo

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

Population Dynamics in Vasopressin Cells

Most neurons sense and code change, and when presented with a constant stimulus they adapt, so as to be able to detect a fresh change. However, for some things it is important to know their absolute level; to encode such information, neurons must sustain their response to an unchanging stimulus while remaining able to respond to a change in that stimulus. One system that encodes the absolute level of a stimulus is the vasopressin system, which generates a hormonal signal that is proportional to plasma osmolality. Vasopressin cells sense plasma osmolality and secrete appropriate levels of vasopressin from the neurohypophysis as needed to control water excretion; this requires sustained secretion under basal conditions and the ability to increase (or decrease) secretion should plasma osmolality change. Here we explore the mechanisms that enable vasopressin cells to fulfill this function, and consider how coordination between the cells might distribute the secretory load across the population of vasopressin cells.

Somatodendritic dynorphin release : orchestrating activity patterns of vasopressin neurons

Most neurons in the central nervous system co-express peptides alongside their principal transmitter, yet the function of these peptides is largely unknown. Vasopressin neurons of the hypothalamic supraoptic nucleus and paraventricular nucleus contain among the highest concentrations of dynorphin found in the brain. Dynorphin, an endogenous opioid peptide, is co-localized in the same neurosecretory vesicles as vasopressin and is released alongside vasopressin from the dendrites and axon terminals of vasopressin neurons. We and others have shown that neuropeptide release from the soma and dendrites of vasopressin neurons activates vasopressin receptors and kappa-opioid receptors to cause activity-dependent modulation of vasopressin neuron activity, and that this is essential for activity patterning in vasopressin neurons.

Dehydration-induced modulation of κ-opioid inhibition of vasopressin neurone activity

Dehydration increases vasopressin (antidiuretic hormone) secretion from the posterior pituitary gland to reduce water loss in the urine. Vasopressin secretion is determined by action potential firing in vasopressin neurones, which can exhibit continuous, phasic (alternating periods of activity and silence), or irregular activity. Autocrine κ‐opioid inhibition contributes to the generation of activity patterning of vasopressin neurones under basal conditions and so we used in vivo extracellular single unit recording to test the hypothesis that changes in autocrine κ‐opioid inhibition drive changes in activity patterning of vasopressin neurones during dehydration. Dehydration increased the firing rate of rat vasopressin neurones displaying continuous activity (from 7.1 ± 0.5 to 9.0 ± 0.6 spikes s−1) and phasic activity (from 4.2 ± 0.7 to 7.8 ± 0.9 spikes s−1), but not those displaying irregular activity. The dehydration‐induced increase in phasic activity was via an increase in intraburst firing rate. The selective κ‐opioid receptor antagonist nor‐binaltorphimine increased the firing rate of phasic neurones in non‐dehydrated rats (from 3.4 ± 0.8 to 5.3 ± 0.6 spikes s−1) and dehydrated rats (from 6.4 ± 0.5 to 9.1 ± 1.2 spikes s−1), indicating that κ‐opioid feedback inhibition of phasic bursts is maintained during dehydration. In a separate series of experiments, prodynorphin mRNA expression was increased in vasopressin neurones of hyperosmotic rats, compared to hypo‐osmotic rats. Hence, it appears that dynorphin expression in vasopressin neurones undergoes dynamic changes in proportion to the required secretion of vasopressin so that, even under stimulated conditions, autocrine feedback inhibition of vasopressin neurones prevents over‐excitation.

Multi-factorial somato-dendritic regulation of phasic spike discharge in vasopressin neurons

Classically, neuropeptide release occurs from axon terminals to influence post-synaptic neurons. However, it has become increasingly clear that many neurons in the central nervous system also release neuropeptide from their somata and dendrites. This somato-dendritic neuropeptide release can have many functions, amongst which is feedback modulation of activity. In addition, most central neurons also co-express other neurotransmitters/neuromodulators alongside their principal neurotransmitter, yet the function of these co-expressed factors is largely unknown. With regard to the function of somato-dendritic neuropeptide release, hypothalamic vasopressin neurons are amongst the best understood neurons in the central nervous system. Vasopressin neurons co-express a number of other neuropeptides including apelin, dynorphin and galanin as well as the purine, adenosine triphosphate. In addition to factors co-released during exocytosis, vasopressin neurons also generate nitric oxide. Each of these factors has been demonstrated to influence the activity of vasopressin neurons. For at least some of these factors, modulation of the activity of vasopressin neurons is activity dependent; suggesting that autocrine feedback regulation of activity might be an important role for somato-dendritic release of neuromodulators across the central nervous system.

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.

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

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

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

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

Apamin increases post-spike excitability of supraoptic nucleus neurons in anaesthetized morphine-naïve rats and morphine-dependent rats: consequences for morphine withdrawal excitation.

Supraoptic nucleus (SON) oxytocin neurons develop morphine dependence when chronically exposed to this opiate and undergo excitation when morphine is subsequently withdrawn. Morphine withdrawal excitation is evident as an increased action potential (spike) firing rate and is associated with an increased post-spike excitability that is consistent with the expression of an enhanced post-spike afterdepolarization (ADP) during withdrawal. Here, we administered apamin (which inhibits the medium afterhyperpolarization [mAHP] in vitro and unmasks an ADP) into the SON of urethane-anaesthetized rats to determine its effects on oxytocin neurons in vivo. As predicted, intra-SON apamin administration increased the propensity to fire a spike soon (<100xa0ms) after each spike (post-spike excitability) more in oxytocin neurons recorded from morphine-treated rats than in morphine-naïve rats. However, intra-SON apamin did not alter the overall firing rate of oxytocin neurons recorded from morphine-treated rats or morphine-naïve rats, indicating that an increase in post-spike excitability alone is not sufficient to trigger withdrawal excitation of oxytocin neurons. Nevertheless, bilateral intra-SON apamin infusion increased oxytocin secretion (which depends on firing pattern as well as firing rate) by 90xa0±xa046% in morphine-dependent rats (Pxa0<xa00.01 compared to aCSF). Hence, an increase in post-spike excitability does not appear to drive morphine withdrawal-induced increases in oxytocin neuron firing rate, but does contribute to withdrawal-induced hyper-secretion of oxytocin.