title: Influence of opioid peptides on release from vasopressin and oxytocin neurones of the hypothalamoneurohypophyseal system: effects of naloxone in the conscious rat.
abstract: 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.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Physiology, Dartmouth Medical School, Hanover, NH 03756.
authors.forename: S W
authors.initials: SW
authors.lastname: Cheng
authors.forename: E F
authors.initials: EF
authors.lastname: O'Connor
authors.forename: W G
authors.initials: WG
authors.lastname: North
mesh_terms: D000818:Animals; D002851:Chromatography, High Pressure Liquid; D007030:Hypothalamo-Hypophyseal System / Q000502:physiology*; D008297:Male; D009270:Naloxone / Q000494:pharmacology* / Q000652:urine; D009294:Narcotics / Q000494:pharmacology*; D009474:Neurons / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D009994:Osmolar Concentration; D010121:Oxytocin / Q000378:metabolism*; D051381:Rats; D011930:Reaction Time / Q000187:drug effects; D012965:Sodium Chloride / Q000494:pharmacology; D014667:Vasopressins / Q000378:metabolism*
chemical_list: D009294:Narcotics; D014667:Vasopressins; D009270:Naloxone; D012965:Sodium Chloride; D010121:Oxytocin