Adrenalectomy potentiates the morphine--but not cocaine-induced place preference in rats

The conditioned place preference paradigm is commonly used to study the reinforcing properties of various drugs. In the present study, the effect of adrenalectomy (ADX) on the morphine-induced place preference was examined in rats. Morphine produced a significant preference for the drug-associated place in sham-operated (sham) and ADX rats. In sham rats, only the highest dose of morphine (8 mg/kg, i.p.) produced a significant preference, while in ADX rats, lower doses of morphine (1 and 2 mg/kg, i.p.) produced a significant preference for the drug-associated place. Furthermore, the morphine-induced place preference was blocked by the dopamine D1 antagonist SCH23390 in both sham and ADX rats. On the other hand, the cocaine-induced place preference was not affected by ADX. In the present study, we found that ADX potentiates the reinforcing effect induced by morphine, but not that induced by cocaine, which suggests that the enhancement by ADX may be due to a change in opioid receptors, morphine metabolism and/or some other cause, but not a change in dopamine receptors.