Repeated fluoxetine administration during adolescence stimulates aggressive behavior and alters serotonin and vasopressin neural development in hamsters.

Fluoxetine is the only selective serotonin reuptake inhibitor registered for the treatment of major depressive disorder in pediatric populations, despite reports that it is disproportionately associated with an array of adverse side effects that include agitation, hostility, and overt acts of pathological aggression and violence in youth. This study examined the effects of repeated adolescent fluoxetine administration on offensive aggression and the development of the serotonin (5HT) and vasopressin (AVP) neural systems modulating this behavior using pubertal Syrian hamsters (Mesocricetus auratus) as an adolescent-animal model. Adolescent hamsters administered fluoxetine were tested for offensive aggression using the resident/intruder test, sacrificed the following day, and, using immunohistochemistry, examined for 5HT and AVP afferent innervation/development to areas of the brain implicated in aggression control. Repeated exposure to a low dose (0.3 mg/kg/day) of fluoxetine during adolescence increased nearly all measures of offensive aggression (i.e., upright offensive attacks, lateral attacks, flank/rump bites, pursuits, flank marks), whereas measures of social interest (i.e., olfactory investigation, contact time), comfort (i.e., grooming), and locomotion (i.e., contact time, cage climbing) remained constant. Fluoxetine exposure also increased 5HT and AVP afferent development to brain areas implicated in aggressive behavior, most notably the latero-anterior hypothalamus (LAH)-an area of convergence for developmental and neuroplastic changes correlated with offensive aggression in hamsters. These data indicate that repeated administration of clinically relevant doses of fluoxetine during adolescent development directly stimulates aggressive behavior and alters LAH 5HT and AVP development, yet only alterations in AVP afferent development within the LAH correlate with the fluoxetine-induced aggressive behavioral phenotype.