Central peptidergic neurons are hyperactive during collateral sprouting and inhibition of activity suppresses sprouting.
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Little is known regarding the effect of chronic changes in neuronal activity on the extent of collateral sprouting by identified CNS neurons. We have investigated the relationship between activity and sprouting in oxytocin (OT) and vasopressin (VP) neurons of the hypothalamic magnocellular neurosecretory system (MNS). Uninjured MNS neurons undergo a robust collateral-sprouting response that restores the axon population of the neural lobe (NL) after a lesion of the contralateral MNS (). Simultaneously, lesioned rats develop chronic urinary hyperosmolality indicative of heightened neurosecretory activity. We therefore tested the hypothesis that sprouting MNS neurons are hyperactive by measuring changes in cell and nuclear diameters, OT and VP mRNA pools, and axonal cytochrome oxidase activity (COX). Each of these measures was significantly elevated during the period of most rapid axonal growth between 1 and 4 weeks after the lesion, confirming that both OT and VP neurons are hyperactive while undergoing collateral sprouting. In a second study the hypothesis that chronic inhibition of neuronal activity would interfere with the sprouting response was tested. Chronic hyponatremia (CH) was induced 3 d before the hypothalamic lesion and sustained for 4 weeks to suppress neurosecretory activity. CH abolished the lesion-induced increases in OT and VP mRNA pools and virtually eliminated measurable COX activity in MNS terminals. Counts of the total number of axon profiles in the NL revealed that CH also prevented axonal sprouting from occurring. These results are consistent with the hypothesis that increased neuronal activity is required for denervation-induced collateral sprouting to occur in the MNS. (+info)
Changes in properties and neurosteroid regulation of GABAergic synapses in the supraoptic nucleus during the mammalian female reproductive cycle.
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1. GABAA receptor-mediated synaptic innervation of oxytocin neurones in the supraoptic nucleus (SON) was analysed in adult female rats going through their first reproductive cycle by recording the spontaneous inhibitory postsynaptic currents (sIPSCs) at six stages of female reproduction. 2. During pregnancy we observed a reduction in the interval between monoquantal sIPSCs. The synaptic current amplitude, current decay and neurosteroid sensitivity of postsynaptic GABAA receptors observed at this stage were not distinguishable from those measured in virgin stage SON. 3. Upon parturition an increase in monoquantal synaptic current decay occurred, whereas potentiation by the progesterone metabolite allopregnanolone (3alpha-OH-DHP) was suppressed. 4. Throughout a substantial part of the lactation period the decay of synaptic currents remained attenuated, whilst the potentiation by 3alpha-OH-DHP remained suppressed. 5. Several weeks after the end of lactation sIPSC intervals, their current decay velocity as well as the potentiation by 3alpha-OH-DHP were restored to pre-pregnancy levels, which is indicative of the cyclical nature of synaptic plasticity in the adult SON. 6. Competitive polymerase chain reaction (PCR) analysis showed that virgin animals expressed alpha1 and alpha2 GABAA receptor subunit mRNA at a relative ratio of 2 : 1 compared with beta-actin. After pregnancy both alpha1 and alpha2 subunit mRNA levels were transiently increased, although at a relative ratio of 1 : 4, in line with the hypothesis that alpha2 plays a large role in postsynaptic receptor functioning. During post-lactation both alpha subunits were downregulated. 7. We propose that synaptic remodelling in the SON during pregnancy includes changes in the putative number of GABA release sites per neurone. At parturition, and during the two consecutive weeks of lactation, a subtype of postsynaptic GABAA receptors was observed, distinct from the one being expressed before and during pregnancy. Synaptic current densities, calculated in order to compare the impact of synaptic inhibition, showed that, in particular, the differences in 3alpha-OH-DHP potentiation of these two distinct GABAA receptor subtypes produce robust shifts in the impact of synaptic inhibition of oxytocin neurones at the different stages of female reproduction. (+info)
Opioidergic modulation of voltage-activated K+ currents in magnocellular neurons of the supraoptic nucleus in rat.
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Opioidergic modulation plays an important role in the control of oxytocin and vasopressin release by magnocellular neurons (MCNs) in the supraoptic and paraventricular nuclei of the hypothalamus. We have used whole cell patch-clamp recording in acute slices of the supraoptic nucleus (SON) of the hypothalamus to study opioidergic modulation of voltage-dependent K+ currents in MCNs that are involved in release activity. The mu-receptor agonist D-Ala2, N-Me-Phe4, Gly5-ol-enkephalin (DAMGO, 2 microM) affected K+ currents in 55% of magnocellular neurons recorded from. In these putative oxytocinergic cells, DAMGO increased the delayed rectifier current (IK(V)) amplitude by approximately 50% without significant effects on its activation kinetics. The transient A current (IA) was enhanced by DAMGO by approximately 36%. Its inactivation kinetic was accelerated slightly while the voltage dependence of steady-state inactivation was shifted by -6 mV to more negative potentials. All DAMGO effects were blocked by the preferential non-kappa-opioid antagonist naloxone (10 microM). The kappa-opioid agonist trans-(+/-)-3, 4-dichloro-N-methyl-N(2-[1-pyrrolidinyl]cyclohexyl)benzeneacetamide (U50,488; 10 microM) strongly suppressed IK(V) by approximately 57% and evoked a 20-mV hyperpolarizing shift and an acceleration of activation in both, DAMGO-sensitive and -insensitive putative vasopressinergic MCNs. U50,488 reduced IA by approximately 29% and tau of inactivation by -20% in DAMGO-sensitive cells. In contrast, in DAMGO-insensitive cells U50,488 increased IA by approximately 23% and strongly accelerated inactivation (tau -44%). The effects of U50,488 were suppressed by the selective kappa-receptor antagonist nor-binaltorphimine (5 microM). We conclude that mu- and kappa-opioidergic inputs decrease and increase excitability of oxytocinergic MCNs, respectively, through modulation of voltage-dependent K+ currents. In vasopressinergic MCNs, kappa-opioidergic inputs differentially modulate these K+ currents. The modulation of K+ currents is assumed to significantly contribute to opioidergic control of hormone release by MCNs within the supraoptic nucleus and from the axon terminals in the neural lobe. (+info)
Central injections of capsaicin cause antidiuresis mediated through neurokinin-1 receptors in rat hypothalamus and vasopressin release.
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Intracerebroventricular injections of capsaicin at 100-500 nmol elicited dose-dependent decreases in urine outflow volume in anesthetized, hydrated rats. The capsaicin (500 nmol)-induced antidiuresis was inhibited by pretreatment with CP96345 (30 nmol, a neurokinin-1-receptor antagonist), but not by that with phenoxybenzamine (20 nmol, an alpha-adrenoceptor antagonist), timolol (100 nmol, a beta-adrenoceptor antagonist) or atropine (300 nmol, a muscarinic antagonist) into the hypothalamic supraoptic nucleus (SON). Intravenous injections of d(CH2)5-D-Tyr(Et)VAVP (50 microg/kg, a vasopressin-receptor antagonist) completely blocked the antidiuresis. In intra-SON microdialysis experiments, acetylcholine concentration in the perfusate of the capsaicin-injected rats was not different from that of the vehicle-injected rats. These findings suggested that capsaicin stimulated substance P release in the SON and caused the antidiuresis as a result of the increased release of vasopressin into the circulation from the neurohypophysis mediated through neurokinin-1 receptors in the SON. (+info)
Differences in the properties of ionotropic glutamate synaptic currents in oxytocin and vasopressin neuroendocrine neurons.
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Oxytocin (OT) and vasopressin (VP) hormone release from neurohypophysial terminals is controlled by the firing pattern of neurosecretory cells located in the hypothalamic supraoptic (SON) and paraventricular nuclei. Although glutamate is a key modulator of the electrical activity of both OT and VP neurons, a differential contribution of AMPA receptors (AMPARs) and NMDA receptors (NMDARs) has been proposed to mediate glutamatergic influences on these neurons. In the present study we examined the distribution and functional properties of synaptic currents mediated by AMPARs and NMDARs in immunoidentified SON neurons. Our results suggest that the properties of AMPA-mediated currents in SON neurons are controlled in a cell type-specific manner. OT neurons displayed AMPA-mediated miniature EPSCs (mEPSCs) with larger amplitude and faster decay kinetics than VP neurons. Furthermore, a peak-scaled nonstationary noise analysis of mEPSCs revealed a larger estimated single-channel conductance of AMPARs expressed in OT neurons. High-frequency summation of AMPA-mediated excitatory postsynaptic potentials was smaller in OT neurons. In both cell types, AMPA-mediated synaptic currents showed inward rectification, which was more pronounced in OT neurons, and displayed Ca2+ permeability. On the other hand, NMDA-mediated mEPSCs of both cell types had similar amplitude and kinetic properties. The cell type-specific expression of functionally different AMPARs can contribute to the adoption of different firing patterns by these neuroendocrine neurons in response to physiological stimuli. (+info)
Distribution of estrogen receptor-beta messenger ribonucleic acid in the male sheep hypothalamus.
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As a first step in determining possible influences of the newly discovered estrogen receptor (ER)-beta on reproduction, we have localized mRNA for ER-beta within the male sheep hypothalamus using in situ hybridization and a rat ER-beta cRNA probe. Highest amounts of hybridization signal were observed in the preoptic area (POA), bed nucleus of the stria terminalis, paraventricular nucleus, and supraoptic nucleus. Relatively moderate amounts of hybridization signal were observed in the retrochiasmatic area (RCH), anterior hypothalamic area, dorsomedial hypothalamus, and lateral hypothalamus. Only a low level of hybridization signal was observed in the ventromedial hypothalamus, suprachiasmatic nucleus, and arcuate nucleus. The presence of ER-beta mRNA in several areas of the male sheep hypothalamus suggests multiple functions for this receptor. The distribution of ER-beta in the ovine hypothalamus was similar to that described for the rat, suggesting a high degree of functional conservation across species. A role for ER-beta in influencing reproduction is suggested by its presence in the POA and RCH, regions of the hypothalamus that control reproduction. (+info)
Nitric oxide via cGMP-dependent mechanisms increases dye coupling and excitability of rat supraoptic nucleus neurons.
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Unlike many neuron populations, supraoptic nucleus (SON) neurons are rich in both nitric oxide synthase (NOS) and the NO receptor-soluble guanylyl cyclase (GC), the activation of which leads to cGMP accumulation. Elevations in cGMP result in increased coupling among SON neurons. We investigated the effect of NO on dye coupling in SONs from male, proestrus virgin female, and lactating rats. In 167 slices 263 SON neurons were recorded; 210 of these neurons were injected intracellularly (one neuron per SON) with Lucifer yellow (LY). The typically minimal coupling seen in virgin females was increased nearly fourfold by the NO precursor, L-arginine, or the NO donor, sodium nitroprusside (SNP). L-Arginine-induced coupling was abolished by a NOS inhibitor. In slices from male and lactating rats who have a higher basal incidence of coupling, SNP increased coupling by approximately twofold over control (p < 0.03). SNP effects were prevented by the NO scavenger hemoglobin (20 microM) and by the selective blocker of NO-activated GC, ODQ (10 microM). These results suggest that NO released from cells within the SON can expand the coupled network of neurons and that this action occurs via cGMP-dependent processes. Because increased coupling is associated with elevated SON neuronal excitability, we also studied the effects of 8-bromo-cGMP on excitability. In both phasically and continuously firing neurons 8-bromo-cGMP (1-2 mM), but not cGMP, produced membrane depolarizations accompanied by membrane conductance increases. Conductance increases remained when depolarizations were eliminated by current-clamping the membrane potential. Thus, NO-induced cGMP increases SON neuronal coupling and excitability. (+info)
V1a- and V2-type vasopressin receptors mediate vasopressin-induced Ca2+ responses in isolated rat supraoptic neurones.
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1. The pharmacological profile of receptors activated by vasopressin (AVP) in freshly dissociated supraoptic magnocellular neurones was investigated using specific V1a- and V2-type AVP receptor agonists and antagonists. 2. In 97 % of AVP-responding neurones (1-3000 nM) V1a or V2 receptor type agonists (F-180 and dDAVP, respectively) elicited dose-dependent [Ca2+]i transients that were suppressed by removal of external Ca2+. 3. The [Ca2+]i response induced by 1 microM F-180 or dDAVP was selectively blocked by 10 nM of V1a and V2 antagonists (SR 49059 and SR 121463A, respectively). The response to V1a agonist was maintained in the presence of the V2 antagonist, and the V2 agonist-induced response persisted in the presence of the V1a antagonist. 4. The [Ca2+]i response induced by 1 microM AVP was partially (61 %) blocked by 10 nM SR 121463A. This blockade was increased by a further 31 % with the addition of 10 nM SR 49059. Similarly, the AVP-induced response was partially (47 %) decreased by SR 49059, and a further inhibition of 33 % was achieved in the presence of SR 121463A. 5. We demonstrate that AVP acts on the magnocellular neurones via two distinct types of AVP receptors that exhibit the pharmacological profiles of V1a and V2 types. However, since V2 receptor mRNA is not expressed in the supraoptic nucleus (SON), and since V1b receptor transcripts are observed in the SON, we propose that the V2 receptor agonist and antagonist act on a 'V2-like' receptor or a new type of AVP receptor that remains to be elucidated. The possibility that V2 ligands act on the V1b receptor cannot be excluded. (+info)