Influence of opioid peptides on release from vasopressin and oxytocin neurones of the hypothalamoneurohypophyseal system: effects of naloxone in the conscious rat

• Published in: Canadian journal of physiology and pharmacology Vol. 68; no. 5; p. 568
• Main Authors: Cheng, S W, O'Connor, E F, North, W G
• Format: Journal Article
• Language: English
• Published: Canada 01.05.1990
• Subjects: Animals; Chromatography, High Pressure Liquid; Hypothalamo-Hypophyseal System - physiology; Male; Naloxone - pharmacology; Naloxone - urine; Narcotics - pharmacology; Neurons - drug effects; Neurons - metabolism; Neurons - physiology; Osmolar Concentration; Oxytocin - metabolism; Rats; Reaction Time - drug effects; Sodium Chloride - pharmacology; Vasopressins - metabolism
• ISSN: 0008-4212
• DOI: 10.1139/y90-083

We examined the effects of acute and chronic treatments with naloxone on release of vasopressin and oxytocin from the hypothalamoneurohypophyseal system (HNS) in conscious, chronically instrumented Long-Evans rats. Plasma concentrations of vasopressin-associated neurophysin and oxytocin-associated neurophysin were evaluated before and during an intravenous infusion of 18% saline at 100 microL.kg-1 body weight.min-1 for 60 min. Acute treatment with naloxone (2.75 mumol/kg, intravenous) did not measurably alter basal plasma osmolality or vasopressin-associated neurophysin concentration, but it caused a three-fold rise in basal plasma oxytocin-associated neurophysin concentration (16 +/- 2 to 46 +/- 3 fmol/mL, p less than 0.005). Chronic treatment with naloxone (13.75 mumol/day, subcutaneous pellets) increased plasma osmolality (292 +/- 1 to 300 +/- 2 mosmol/kg H2O, p less than 0.01) by day 5, but it had no measurable effects on basal vasopressin- or oxytocin-associated neurophysin concentration. There were also no significant differences in plasma sodium concentration (144.8 +/- 1.1 vs. 142.2 +/- 1.4 mequiv./L) under both conditions. Acute and chronic treatments with naloxone accompanied by salt loading produced a five- and four-fold decrease in the rates that plasma concentration of vasopressin-associated neurophysin changed with plasma osmolality, compared with untreated salt-loaded control rats. For oxytocin secretion from the HNS, both treatments accompanied by salt loading substantially decreased the threshold for changes in relation to plasma osmolality; the rise in plasma concentration of oxytocin-associated neurophysin was similar at all levels of hyperosmotic stimulation.