Plasma beta-endorphin, adrenocorticotropin and corticosterone changes in response to acute and prolonged morphine administration in rats

ABSTRACT

The influence of morphine on the functional activity of the pituitary-adrenocortical system in the rate was studied by investigating its effect on the secretion of beta-endorphin [beta-END] and adrenocorticotropin [ACTH] by the pituitary gland and corticosterone by the adrenal gland. The acute administration of morphine [5 mg / 100 g i.p.] produces a characteristic that increases in plasma beta-END, ACTH and corticosterone after 5, 15 and 25 min, whereas decreases in beta-END, ACTH and corticosterone levels sere seen after 90 and 120 min. Saline[i.p.] produced decreases of beta-END and ACTH levels, 90 and 120 min after injection and an increase in corticosterone after 5, 15 and 25 min. It appears that, in the rat, the response of pituitary-adrenal exis to acute morphine administration could change depending upon the time of sampling. Additionally, prolonged morphine [0.5 mg/100 g i.p. daily] administration for a period of 7 days has little effect on plasma level beta-END, ACTH and corticcosterone. This may indicate that drug tolerance might have developed. Conversely, repeated daily doses of morphine [2 mg/100 g i.p.] for 7 days cause a significant lowering of plasma beta-END and ACTH contents, while non-significant decreases in plasma corticosterone level was recorded. These data suggest that the effect of morphine is dose related. Overall, the present results are consistent with an increased release of pro-opiomelanocortin - derived peptides after acute morphine treatment for a short-term, and with a decreased releae of these peptide in prolonged treatment