Influence of PCPA, shock level, and home-cage conditions on shock-induced aggression

In a series of experiments, the effect of parachlorophenylalanine (PCPA) on shock-induced fighting was assessed rats raised and maintained under either a 12-hr alternating light-dark cycle (LD) or constant light conditions (LL). PCPA increased shock-induced aggression only in LL groups when testing was accomplished using a 2 mA shock; PCPA resulted in increased aggression in groups from the LD condition only when testing was done at 1 mA. A procedure that used castrated and intact cagemates to manipulate home-cage social experience provided evidence for a role for social experience in determining differences between LL and LD reared rats in shock-induced aggression. However, these data also suggested that home-cage social experience was not a factor in the lighting condition influence on the effect of PCPA on shock-induced aggression. Finally, a separate experiment demonstrated that diurnal rhythms in shock-induced aggression were disrupted by handling and vehicle injection in the control procedures, so the possible role of serotonin in diurnal rhythms of aggression behavior could not be assessed.