Naltrindole administration during the preweanling period and manipulation affect adrenocortical reactivity in young rats

The effect of a daily injection of the delta-selective opioid antagonist na ltrindole (1 mg/kg), from birth to postnatal day 19, on basal and post-stre ss corticosterone levels in 25-day old rats of both sexes was investigated. The effects of manipulation were studied by including two control groups, one group received daily injections of saline and a second one was not mani pulated. The stress protocol consisted of a 3 min swimming session in water at 20 degrees C. Corticosterone determinations were performed by radioimmu noassay. Control non-manipulated animals showed a significant increase in c orticosterone levels in response to stress. Manipulation decreased basal ho rmone levels in females and prevented the stress-induced rise in corticoste rone in males. Functional blockade of the delta-receptor during the prewean ling period by the naltrindole treatment inhibited the corticosterone respo nse to stress in females. The results indicate the existence of sex differe nces in the effects of manipulation on hypothalamus-pituitary-adrenal axis activity and the involvement of the delta-opioid receptor in the modulation of the adrenocortical response to stress during the postnatal period. (C) 1999 Elsevier Science B.V. All rights reserved.