Oxytocin neurone autoexcitation during morphine withdrawal in anaesthetized rats

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Abstract
WE investigated whether release of oxytocin into the supraoptic nucleus is involved in morphine-withdrawal excitation of oxytocin neurones. Retrodialysis of naloxone into the supraoptic nucleus of morphinedependent rats increased intranuclear oxytocin release by 56.5 p ±12.7% whereas no change was seen in vehicletreated dependent rats. In another experiment, in morphine-dependent rats given intravenous (i.v.) naloxone, intracerebroventricular (i.c.v.) oxytocin receptor antagonist injection reduced the increase of plasma oxytocin concentration (to 28-fold) compared with i.c.v. vehicle (62-fold increase). Finally, the increase in oxytocin neurone firing rate following morphinewithdrawal in the presence of i.c.v. oxytocin antagonist infusion was 28% of the steady state firing rate (15–20 min later) and this was lower (p < 0.05) than the percentage increase in i.c.v. vehicle-infused rats (89%). Thus, central endogenous oxytocin may be involved in withdrawal excitation of oxytocin neurones.