TESTOSTERONE FACILITATES AGGRESSION BY MODULATING VASOPRESSIN RECEPTORS IN THE HYPOTHALAMUS

In many species, testosterone treatment facilitates offensive aggressi on tested in resident-intruder models. As the mechanisms of action of testosterone remain unclear, we hypothesized that testosterone interac ts with neurotransmitter systems involved in the regulation of offensi ve aggression. We tested this hypothesis with the vasopressinergic sys tem in golden hamsters in three separate experiments. First, we compar ed the density of V-1 vasopressin (AVP) receptor binding between castr ated animals treated with testosterone and their untreated controls. T he most noticeable difference was found within the ventrolateral hypot halamus (VLH), a site involved in the control of aggression in several species of mammals. Within this area, V-1 AVP receptor binding disapp eared after castration, while being maintained by testosterone-treatme nt. Second, we tested behavioral effects of AVP within the VLH. Microi njections of AVP (100 nl, 1 or 100 mu M) within the VLH accelerated th e onset of offensive aggression in testosterone-treated animals. Howev er, AVP-injected animals did not bite more than their vehicle-injected controls. Third, microinjections of AVP failed to activate offensive aggression in animals deprived of testosterone. As AVP receptors appea red to overlay previously described distributions of androgen and estr ogen receptors in golden hamsters, we propose that testosterone facili tates the onset of offensive aggression, at least partly, through an a ctivation of AVP receptors within the VLH.