Effects of acute treatment with 8-OH-DPAT and fluoxetine on aggressive behaviour in male song sparrows (Melospiza melodia morphna)

The role of serotonin in modulating male aggressive behaviour was investigated in male song sparrows, Melospiza melodia morphna , using two different serotonergic drugs, fluoxetine and 8-OH-DPAT. Fluoxetine is a selective serotonin reuptake inhibitor of the neuronal reuptake pump increasing synaptic concentrations of serotonin, and 8-OH-DPAT is a specific serotonin (5-HT1A ) receptor agonist. The serotonergic control of aggression in passerines has not been previously investigated. We examined these behaviours within a controlled setting using a laboratory simulated territorial intrusion, with a hierarchical scale to quantify male-male aggressive behaviour. Utilizing this scale, we quantified the extent of male aggressive behaviour in two experiments. In experiment 1, song sparrows were given 100 mul, s.c. injections of either fluoxetine (10 mg/kg) or 8-OH-DPAT (1 mg/kg). Experiment 2 was a dose-response study using three doses of 8-OH-DPAT (0.1, 1 and 10 mg/kg). In both studies, aggressive behaviour was measured 1 h after injection for 10 min in response to the presence of a novel male decoy combined with playback of conspecific song. Both drugs significantly reduced male aggressive behaviour, and 8-OH-DPAT did so in a dose-dependent manner. The effect of the two drugs upon general activity was also measured using infra-red perch hop detectors. Activity levels were not effected by either fluoxetine or 8-OH-DPAT at all of the respective doses, indicating that the reduction in aggressive behaviour was specific. These results demonstrate that, in a passerine species, the serotonergic system negatively regulates male-male aggressive behaviour. These results further demonstrate that aggression can be effectively studied in a laboratory setting and natural aggressive responses can be elicited using this method.