Arginine-vasotocin influence on aggressive behavior and dominance in rainbow trout.

The mammalian neuropeptide arginine-vasopressin (AVP) and its non-mammalian homologue arginine-vasotocin (AVT) are involved in social behavior including aggression in vertebrates. Juvenile rainbow trout (Onchorhynchus mykiss) are highly territorial and we tested the effect of two different doses each of AVT and Manning compound, an AVP V(1A) receptor antagonist, on aggressive behavior and the outcome of staged fights for social dominance in size matched pairs. Each pair consisted of one fish receiving AVT or Manning compound through an intracerebroventricular (icv) injection and one fish receiving a similar icv injection of saline. In addition, plasma cortisol and brain serotonergic activity were monitored. Fish receiving AVT at a dose of 200 ng became subordinate whereas AVT at a dose of 20 ng had no effect on the outcome of fights for social dominance. Further, Manning compound had no effect on the outcome of fights for dominance but at a dose of 200 ng it had an effect on fight duration. Fish receiving 200 ng Manning compound and losing dyadic fights appeared to take longer to become subordinate. Neither AVT nor Manning compound had any effects on plasma cortisol levels or brainstem serotonergic activity. The apparent inhibiting effect of AVT on aggression could be centrally mediated.