l-Fenfluramine in Tests of Dominance and Anxiety in the Rat

l-Fenfluramine (1.25 and 2.5 mg/kg) significantly reduced the success of dominant rats competing with untreated middle rank rats for chocolate. In resident rats, l-fenfluramine (2.5 mg/kg) significantly increased the number of submissions, and the time spent submitting, to untreated rats intruding into their home-cage territory; it also significantly reduced the number of kicks directed at, and the time spent kicking, the intruder; and the incidence of, and time spent in, aggressively grooming the intruder. When the intruder rats were treated with l-fenfluramine the only significant change was a decrease in the number of wrestling bouts and the time spent wrestling. Since l-fenfluramine did not change other behaviours in this test (e.g. sniffing the opponent) the decrease in dominance behaviours was probably not secondary to nonspecific sedation. In the social interaction test of anxiety, l-fenfluramine (2.5 and 5 mg/kg) significantly reduced the time spent in active social interaction, and decreased motor activity. Analyses of covariance indicated that these were two independent effects. In the elevated plus-maze, l-fenfluramine (1.25–5 mg/kg) significantly decreased the percent number of entries made onto open arms, and (2.5 and 5 mg/kg) significantly decreased the percent of time spent on the open arms. The total number of arm entries was reduced by all doses (0.625–5 mg/kg). Analysis of covariance indicated that the decrease in percent of time spent on the open arms was secondary to the drop in overall activity. Thus there was no evidence of anxiolytic action in either of these tests, the changes indicating, if anything, anxiogenic effects. Clinically, the racemate has been reported to have antiaggressive and anxiolytic effects. The results of the present study suggest that the former, but not the latter, effect might lie in the action of the l-isomer.