Differential responsiveness to fluoxetine during puberty.

In male golden hamsters (Mesocricetus auratus), attack frequency decreases during puberty. As serotonin inhibits offensive responses in adult hamsters, it is hypothesized that the serotonin system becomes upregulated in the hypothalamus during puberty. This hypothesis was tested through acute treatment with fluoxetine, a serotonin reuptake inhibitor, as well as through analysis of serotonin innervation in specific brain areas. In adults, fluoxetine treatment inhibited aggressive behavior. In juveniles, high doses of fluoxetine only reduced offensive responses (i.e., frequency and repetition of attacks), whereas low doses enhanced them. Juveniles also showed a dose-specific maturation of attack targets. In addition, the density of serotonin innervation of the hypothalamus was 20% higher in adult hamsters compared with juveniles. On the basis of these data, it is proposed that the developing serotonergic system shapes the development of offensive behaviors in male golden hamsters.