Effect of intracerebroventricular impromidine on pituitary-adrenocortical response to stress in rats

In the rats subjected to a mild stress of immobilization impromidine, and H2-receptor agonist, given 60 min prior to the stress, intensified the stress-induced increase in hypophyseal-adrenocortical response, evaluated indirectly through the corticosterone concentration in the blood serum. Impromidine was far more potent but only about half as efficient as histamine, 4-methyl histamine (4-MH) and dimaprit. The effect of impromidine was abolished by pretreatment of the rats with cimetidine. The alpha-adrenergic blockers phenoxybenzamine, phentolamine and yohimbine, almost totally antagonized the corticosterone response to impromidine in stressed rats. Propranolol, a beta-adrenergic blocker, abolished the corticosterone response to impromidine but did not antagonize the response to 4-MH and dimaprit. The effect of impromidine was not modified by i.c.v. pretreatment of the rats with atropine. The results obtained show that impromidine is far more potent but less efficient than histamine and the previously known selective H2-receptor agonists in inducing the pituitary-adrenocortical response in stressed rats. These results also suggest that impromidine may release norepinephrine but not interact with cholinergic receptors while stimulating the corticosterone response in stressed rats.