pituitrin and norbinaltorphimine

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 kappa-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 kappa-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 -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 kappa-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.

Topics: Action Potentials; Animals; Cholecystokinin; Dehydration; Electrophysiology; Enkephalins; Female; Hypernatremia; Hyponatremia; Immunohistochemistry; In Situ Hybridization; Naltrexone; Narcotic Antagonists; Neurons; Oxytocin; Protein Precursors; Rats; Rats, Sprague-Dawley; Receptors, Opioid, kappa; RNA, Messenger; Vasopressins

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.

Topics: Action Potentials; Animals; Electric Stimulation; In Vitro Techniques; Male; Naltrexone; Narcotic Antagonists; Neurons; Rats; Rats, Long-Evans; Receptors, Opioid, kappa; Statistics, Nonparametric; Supraoptic Nucleus; Time Factors; Vasopressins

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.

Topics: Action Potentials; Animals; Antidiuretic Hormone Receptor Antagonists; Dendrites; Dynorphins; Feedback, Physiological; Female; Naltrexone; Narcotic Antagonists; Periodicity; Piperidines; Quinolones; Rats; Rats, Sprague-Dawley; Receptors, Opioid, kappa; Supraoptic Nucleus; Vasopressins

We investigated the influence of endogenous kappa-opioids on the activity of supraoptic neurons in vivo. Administration of the kappa-antagonist nor-binaltorphimine (200 micrograms/kg, i.v.), increased the activity of phasic (vasopressin), but not continuously active (oxytocin), supraoptic neurons by increasing burst duration (by 69 +/- 24%) and decreasing the interburst interval (by 19 +/- 11%). Similarly, retrodialysis of nor-binaltorphimine onto the supraoptic nucleus increased the burst duration (119 +/- 57% increase) of vasopressin cells but did not alter the firing rate of oxytocin cells (4 +/- 8% decrease). Thus, an endogenous kappa-agonist modulates vasopressin cell activity by an action within the supraoptic nucleus. To eliminate kappa-agonist actions within the supraoptic nucleus, we infused the kappa-agonist U50,488H (2.5 micrograms/hr at 0.5 micrograms/hr) into one supraoptic nucleus over 5 d to locally downregulate kappa-receptor function. Such infusions reduced the spontaneous activity of vasopressin but not oxytocin cells and reduced the proportion of cells displaying spontaneous phasic activity from 26% in vehicle-infused nuclei to 3% in U50, 488H-infused nuclei; this treatment also prevented acute inhibition of both vasopressin and oxytocin cells by U50,488H (1000 micrograms/kg, i.v.), confirming functional kappa-receptor downregulation. In U50, 488H-infused supraoptic nuclei, vasopressin cell firing rate was increased by nor-binaltorphimine (100 and 200 micrograms/kg, i.v.) but not to beyond that found in vehicle-treated nuclei, indicating that these cells were not U50,488H-dependent. Thus, normally functioning kappa-opioid mechanisms on vasopressin cells are essential for the expression of phasic firing.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Analgesics, Non-Narcotic; Animals; Cells, Cultured; Dendrites; Down-Regulation; Dynorphins; Electrophysiology; Female; Injections, Intravenous; Membrane Potentials; Microdialysis; Naloxone; Naltrexone; Narcotic Antagonists; Neurons; Oxytocin; Rats; Rats, Sprague-Dawley; Receptors, Opioid, kappa; Supraoptic Nucleus; Vasopressins

Magnocellular neurons synthesize vasopressin (VP) or oxytocin (OT) and release these hormones preferentially from the neural lobe during physiological stimulation. In the rat, VP is secreted preferentially during dehydration and hemorrhage, whereas OT is released without VP by suckling, parturition, stress, and nausea. Vasopressinergic neurons also synthesize and release dynorphin-related peptides--alpha- and beta-neoendorphin, dynorphin A (1-8) or (1-17), dynorphin B--which are agonists selective for kappa opiate receptors in the neural lobe. We proposed that one mechanism for preferential secretion of neurohypophysial hormones is that a dynorphin-related peptide(s) coreleased with VP inhibits selectively OT secretion from magnocellular neurons. We tested this hypothesis in conscious adult male Sprague-Dawley rats which were stimulated by either hypertonic saline administered intraperitoneally (2.5%, 20 ml/kg) or subcutaneously (1 M, 15 ml/kg) or by 24 h of water deprivation. Two approaches were used: (1) dynorphin-related peptides (0.02-20.4 mM) were injected intracerebroventricularly 1 min before decapitating the animal, and (2) the action of endogenous opioid peptides was blocked by injecting subcutaneously or intracerebroventricularly either naloxone or a selective kappa receptor antagonist, Mr 2266 or nor-binaltorphimine. VP and OT were measured by radioimmunoassay. After 24 h of water deprivation, the elevation in plasma [OT] but not [VP] was attenuated (p less than 0.05) by alpha-neoendorphin. Dynorphin A (1-8) also inhibited the release of OT and not VP after intraperitoneal administration of hypertonic saline. Blocking the action of endogenous opioid peptides at kappa receptors with Mr 2266 given peripherally (s.c.) elevated plasma [OT] but not [VP] after stimulation with hypertonic saline administered intraperitoneally or subcutaneously.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Benzomorphans; Dehydration; Dynorphins; Endorphins; Hypertonic Solutions; Injections, Intraventricular; Male; Naloxone; Naltrexone; Narcotic Antagonists; Neurons; Osmolar Concentration; Oxytocin; Peptide Fragments; Pituitary Gland, Posterior; Protein Precursors; Radioimmunoassay; Rats; Rats, Inbred Strains; Receptors, Opioid; Receptors, Opioid, kappa; Vasopressins