Effect of subchronic and chronic exposure to 5-hydroxytryptophan (5-HTP) on the aggressive behavior induced by food competition in undernourished dominant and submissive pigeons ( Columba livia)

Abstract

The acute administration of 5-HTP was reported to block in undernourished dominant pigeons the aggressive attacks induced in a submissive partner by food competition. In the present study, undernourished pigeons with previously consolidated dominance were submitted to subchronic and chronic 5-HTP treatment. Adult males ( n=28) were kept at 80% of their body weight by a restricted diet. These were divided in pairs made of a previously ranked dominant subject (total time spent in an aggression higher than 200 s/20 min) and a submissive one of similar body weight (time spent in aggression between 90 and 150 s/20 min). The same pairs were exposed to a daily 20 min interaction during each experiment in an observation chamber bearing a central feeder. The time spent in aggressive behavior, feeder control behavior and eating behavior was recorded. Intratest body weight gain was also recorded. In Experiment 1, 8 pairs of pigeons were exposed to a daily trialfor 4 successive days (pretreatment-scores). The dominant subjects were then injected subcutaneously, 30 min. before trials, with 7.5 mg/kg 5-HTP from day 5 to day 8 (Treatment scores). The Recovery scores were obtained through a 4-trial post-treatment schedule. In Experiment 2 different pigeons were used. The pretreatment and recovery scores were obtained according to a 16-trial schedule (16 days). Both 4-day (subchronic) and 16-day (chronic) 5-HTP treatments attenuated aggression by the dominant subjects and reduced their intra-test body weight gain but did not decrease dominance for feeder control. The recovery scores of total aggression in subchronic experiments returned to pretreatment scores. In chronic experiments, instead, the recovery scores of aggression remained lower than pretreatment scores, whereas body-weight-gain scores came back to pretreatment values. This suggests that dominant subjects submitted to chronic 5-HTP might have learned to maintain dominance and feeder control in a virtual absence of aggressive behavior.