SOCIAL ISOLATION-INDUCED ENHANCEMENT OF THE PSYCHOMOTOR EFFECTS OF MORPHINE DEPENDS ON CORTICOSTERONE SECRETION

Short-term social isolation has been shown to increase individual reac tivity to addictive drugs, although the biological factors involved in this effect are largely unknown. In this study, we investigated the i nfluence of corticosterone secretion on the effects of social isolatio n on the response to opioids. The effects of social isolation on morph ine-induced locomotor activity were compared in: (i) animals with an i ntact hypothalamo-pituitary-adrenal (HPA) axis; (ii) animals in which stress-induced corticosterone secretion was blocked by adrenalectomy. The animals in the latter group were implanted with subcutaneous corti costerone pellets (50 mg), which slowly release corticosterone, produc ing stable plasma levels within the physiological range. Social isolat ion increased the locomotor response to morphine (2 mg/kg s.c.) in ani mals with an intact HPA axis, but not in animals in which corticostero ne secretion was blocked. These results suggest that corticosterone se cretion is required for the expression of the enhanced locomotor respo nse to opioids induced by isolation. Since an enhanced locomotor react ivity to addictive drugs has been found to be frequently associated wi th an enhanced vulnerability to drug self-administration, these findin gs suggest a role for glucocorticoids in the vulnerability to the rein forcing effects of opioids.