VOLUNTARY ORAL MORPHINE SELF-ADMINISTRATION IN RATS - EFFECT OF HALOPERIDOL OR ONDANSETRON

Rats were exposed to increasing concentrations of morphine hydrochlori de (up to 0.4 mg/ml) in 5% w/v sucrose solution as their sole source o f drinking water. Physical dependence was established as determined by the precipitation of withdrawal behaviour following administration of 1 mg/kg IP naloxone hydrochloride on day 23. The choice between eithe r a 5% w/v sucrose solution or a 5% w/v sucrose solution containing 0. 4 mg/ml morphine hydrochloride 4 days following withdrawal resulted in rats being categorized into two groups based on their respective cons umption of the morphine-containing solution. The amount of morphine so lution voluntarily consumed by approximately half the rats was suffici ently high as to lead to a relapse into physical dependence to morphin e. The high preference for morphine shown by these rats could not be a ttributed to the taste of the morphine solution. Naive rats or rats ex posed to a 5% w/v sucrose solution for 23 days failed to consume signi ficant quantities of the morphine-containing solution when provided wi th a choice. The administration of either an IM slow-release formulati on of 70.5 mg/kg haloperidol decanoate (=50 mg/kg haloperidol) or 10 m u g/kg IP ondansetron hydrochloride daily did not alter morphine inges tion in the high morphine-preferring rats.