Enhancement by haloperidol of the locomotor response induced by naloxone in morphine-treated guinea pigs.

Guinea pigs given naloxone hydrochloride, 15 mg/kg subcutaneously (s.c.) 2 h after a single dose of morphine sulphate, 15 mg/kg s.c. exhibit a withdrawal response which is characterized by a marked increase in locomotor activity. In this study the effects of haloperidol on the morphine locomotor withdrawal response of guinea-pigs were investigated. Locomotor activity was measured in a cage fitted with a single infra-red photocell and detector. Haloperidol, at a dose of 1 mg/kg, s.c., given 0.5 h prior to administration of naloxone slightly enhanced the morphine locomotor withdrawal response, and at a dose of 10 mg/kg, markedly enhanced the response. Chronic pretreatment of guinea pigs with haloperidol, 1 mg/kg, daily for 21 days did not alter the locomotor withdrawal response. It remains to be established whether the enhancement by haloperidol of the opiate withdrawal response is a functional reflection of its interaction with dopamine, sigma or other receptors.