Does chronic morphine treatment induce a supersensitivity of dopamine receptors in rat brain?

These experiments were carried out to investigate whether chronic morphine treatment increases the sensitivity of dopamine receptors in rat brain. This was studied by considering apomorphine effectiveness in a) inducing stereotyped behaviour and b) decreasing the dopamine turnover in the striata. Acute morphine treatment slightly reduced stereotyped behaviour induced by L-Dopa and by apomorphine. In morphine-withdrawn rats, apomorphine was no more effective than saline in inducing stereotypies (sniffing, licking, gnawing). However, the ability of apomorphine to lower striatal dopamine turnover was increased in morphine-withdrawn rats. Despite these latter observations, chronic morphine treatment probably did not induce a supersensitivity of dopamine receptors in rat brain.