Effect of morphine on hypothalamic catecholamine and serotonin level in relation to the stress-induced pituitary-adrenocortical activation in the rat.

The relationship between the hypothalamic catecholamine and serotonin level as well as the activation of the pituitary-adrenal axis was investigated after administration or morphine (MO) in the rat. Five mg/kg b. wt. of MO induced a significant increase in norepinephrine and a 78%, but insignificant, increase in dopamine level of the hypothalamus within 60 min without changing corticosterone secretion. Electric footshock, in addition to elevating hypothalamic norepinephrine and dopamine levels, significantly increased the pituitary-adrenocortical response in the MO pretreated rats. Five mg/kg b. wt. of MO, or electric footshock alone did not influence the hypothalamic serotonin level within 60 min, but the hypothalamic serotonin level decreased significantly in the MO pretreated, electrically shocked animals. We conclude, that 1) low dose of MO may induce changes of the hypothalamic catecholamine levels without influencing pituitary-adrenocortical activation. 2) enhanced hypothalamic catecholamines by MO did not prevent increasing pituitary-adrenocortical response elicited by stress. It appears, that the hypothalamic catecholaminergic mechanism which may inhibit ACTH release during stimulation does not function in the MO treated rats.