Effects of naloxone and of intraperitoneal hypertonic saline upon oxytocin release and upon supraoptic neuronal activity.

Urethane anaesthetized male rats were given an i.p. injection of hypertonic saline to increase plasma osmotic pressure. This injection resulted in significantly elevated plasma oxytocin levels and increased discharge activity of putative oxytocin cells in the supraoptic nucleus. Subsequent injection of naloxone (1 mg/kg) i.v. resulted in a similarly large increase in plasma oxytocin, but did not affect the discharge activity of putative oxytocin neurones. The results suggest that, following an i.p. injection of hypertonic saline, endogenous opioids act at the neurosecretory terminals to partially inhibit oxytocin release.