VASOPRESSIN AND SEROTONIN INTERACTIONS IN THE CONTROL OF AGONISTIC BEHAVIOR

In hamsters, dominant/subordinate relationships are initially determin ed by overt aggression, but subsequently communicated by flank marking , an arginine vasopressin (AVP)-dependent behavior, Once a relationshi p is established, dominant males will flank mark at a higher frequency than their subordinate partners. Flank marking displayed during socia l encounters can be turned ''on or off'' by microinjection of AVP or A VP-receptor antagonist within the anterior hypothalamus (AH). For inst ance, microinjecting dominant hamsters with AVP-receptor antagonist bl ocks their flank marking and provokes an immediate induction of flank marking by subordinate animals. The central effects of AVP have been e xtended to include a role in offensive aggression. Microinjection of A VP-receptor antagonist into the AH inhibits the aggression of a reside nt hamster toward an intruder and diminishes aggression between hamste rs placed into a neutral arena. Microinjection of AVP into the ventrol ateral hypothalamus (VLH) facilitates offensive aggression of a reside nt toward an intruder. As AVP receptors in the VLH are testosterone-de pendent, it is possible that the reduction of aggression observed in c astrated hamsters is due to a loss of AVP responsiveness in the VLH. R ecent work has focused on the notion that serotonin (5-HT) antagonizes AVP activity in the CNS. The AH and VLH have a high density of 5-HT t erminals and binding sites. Indeed, there appear to be 5-HT synapses o n AVP neurons in the AH. Microinjection of 5-HT into the AH inhibits A VP-induced flank marking while IP injection of fluoxetine a serotonin reuptake inhibitor inhibits AVP-induced offensive aggression in the VL H. It is possible that serotonin interacts with AVP to modulate offens ive aggression.