A single exposure to morphine induces long-lasting behavioural and neurochemical sensitization in rats

Repeated exposure to drugs of abuse causes persistent behavioural sensitiza tion and associated adaptations in striatal neurotransmission, which is tho ught to play an important role in certain aspects of drug addiction. Remark ably, even a single exposure to psychostimulant drugs such as amphetamine o r cocaine can be sufficient to elicit long-lasting sensitization. The prese nt study was designed to evaluate whether long-lasting behavioural and neur ochemical sensitization can also be evoked by a single exposure to morphine , an opiate drug of abuse. Rats were pretreated once with morphine (2, 10 o r 30 mg/kg). Three weeks later, the locomotor effects of morphine and amphe tamine, as well as the electrically evoked release of [H-3]dopamine and [C- 14]acetylcholine from slices of nucleus accumbens and caudate-putamen, was assessed. In morphine-p retreated rats, the psychomotor effects of morphine and amphetamine were sensitized. In addition, the electrically evoked rele ase of [H-3]dopamine and [C-14]acetylcholine was augmented in slices of nuc leus accumbens and caudate-putamen from morphine-p retreated animals. Altho ugh the sensitization of the locomotor effect of morphine was less profound than previously observed after repeated intermittent morphine treatment, t he enduring behavioural and neurochemical consequences of a single and repe ated intermittent morphine treatment appear to be highly comparable. We the refore conclude that a single exposure to morphine induces longlasting beha vioural sensitization and associated neuroadaptations.