ALPHA(2)-ADRENOCEPTOR BLOCKADE, PITUITARY-ADRENAL HORMONES, AND AGONISTIC INTERACTIONS IN RATS

The effects of adrenergic activation on aggressiveness and the aggress ion induced endocrine changes were tested in rats. alpha(2) adrenocept or blockers were used for enhancing activation of the adrenergic syste m, and changes in aggressiveness were tested in resident-intruder cont ests. Three experiments were conducted. In experiment 1, saline inject ed rats responded to the presence of an opponent by aggression and the increase in plasma ACTH and corticosterone. Intraperitoneal administr ation of 1 mg/kg CH-38083 (an alpha(2) adrenoceptor antagonist) produc ed a several fold increase in clinch fighting and mutual upright score s, and also further enhanced the plasma ACTH and corticosterone respon se. In experiment 2, the effect of three doses (0.5, 1 and 2 mg/kg) of three different alpha(2) adrenoceptor blockers CH-38083, idazoxan and yohimbine were tested. All the substances increased aggression at 0.5 and 1 mg/kg; at 2 mg/kg the effect of idazoxan and yohimbine disappea red, while with CH-38083 an additional increase was obtained. In yohim bine treated animals the enhancement of aggression was reduced already at 1 mg/kg. In experiment 3, indomethacin, a potent inhibitor of the catecholamine-induced ACTH release completely abolished the effects of the alpha(2) adrenoceptor antagonist CH-38083: the intensity of agoni stic interactions, as well as ACTH and corticosterone plasma concentra tions, returned to control levels. The possible role of catecholamines and the stress hormones in the activation of aggression is discussed.