Time-dependent effects of PCPA on social aggression in chicks

We investigated the effects of para-chlorophenylalanine (PCPA), a serotonin (5-HT) antagonist, on social aggression and brain neurochemistry in young domestic chickens (Gallus domesticus). In Experiment 1, the effects of four different doses of PCPA (0, 100, 200, and 400 mg/kg) were examined for 3 days after injection. Immediately after PCPA injection, aggressive pecking was low and then increased over the 3-day test period. PCPA significantly increased 5-HT, 5-hydroxyindoleacetic acid (5-HIAA), and 5-HT turnover. In addition, the frequency of aggression was negatively correlated with levels of 5-HIAA. In Experiment 2, the time-dependent effects of a single 400-mg/kg dose of PCPA were examined for 5 and 7 days after drug exposure. PCPA-treated chicks observed for 5 days after injection had significantly greater frequencies of aggression 4 days following drug exposure and significantly reduced 5-HT levels when measured on the next day. Similarly, chicks observed for 7 days exhibited significantly elevated aggression 5 days after injection, after which their pecking decreased to control levels on days 6 and 7. Coinciding with this behavioral pattern, 5-HT levels from these PCPA-treated chicks when assessed 7 days after drug exposure were the same as those for control birds. We concluded that PCPA increased social aggression in birds, an effect that diminished as brain 5-HT levels recovered over a 1-week period.