Involvement of alpha-adrenergic receptors in the amnestic and anti-amnestic action of ACTH, beta-endorphin and epinephrine.

The post-training administration of ACTH1-24 (0.2 microgram/kg), human beta-endorphin (1.0 microgram/kg) or epinephrine HC1 (5.0 micrograms/kg), intraperitoneally, caused retrograde amnesia for a step-down inhibitory avoidance task in rats, and their pre-testing administration reversed this effect. The concomitant administration of the alpha 2-adrenergic receptor blocker, yohimbine HC1 (2.0 mg/kg), antagonized both the post-training amnestic and the pre-testing anti-amnestic effects of the three substances. The anti-amnestic effect of epinephrine, but not that of ACTH or beta-endorphin, was also antagonized by the alpha 1-adrenergic receptor blocker, prazosin HC1 (2.0 mg/kg). These findings suggest that alpha 2-adrenergic receptors are involved both in the amnestic and in the anti-amnestic effect of ACTH, beta-endorphin and epinephrine at the doses used, and that, in the case of the anti-amnestic effect of epinephrine, alpha 1 receptors also are involved. It seems likely that memory regulation by post-training and pre-testing ACTH and beta-endorphin requires the concomitant activity of alpha 2-adrenergic mechanisms, either central or peripheral.