Suppressive and Enhancing Effects of 5-HT Agonist, Fluoxetine, and 5-HT Antagonist, Ondansetron, Respectively, on Food Competition in Rats

Background and aim of the work: Reduced brain serotonin (5-HT) activity was noted to be associated with increased aggression in animals, and vice versa. The current study aimed to investigate the effects of 5-HT agonist and antagonist, fluoxetine, and ondansetron, respectively, on the competitive behavior of rats and their brain 5-HT levels. Methods: Forty male albino rats were housed in pairs. All rats were food-deprived 23 hours/day. Based on competition results, all rats were classified into stable and unstable pairs, according to their success or failure in maintaining their social ranking. Stable pairs of rats were divided into 2 halves and 4 treatment groups. In the first half, stable winners and partner losers were treated with fluoxetine (FLX) (10 mg/kg body weight, intraperitoneal, i.p) or ondansetron (OND) (1 mg/kg body weight, i.p), respectively, and vice versa in the second half. Unstable pairs of rats that failed to retain their social ranking formed an additional treatment group. The effect of treatment with fluoxetine (10 mg/kg body weight, i.p) or ondansetron (1 mg/kg body weight, i.p) on inducing a stable social ranking was investigated. At the end of the study, using Elisa, 5-HT concentrations were determined in brains extracted from winners and losers of all treatment groups. Results: Fourteen out of twenty pairs of rats showed a stable hierarchy. FLX treatment of winners produced a significant reduction in their competition scores; whereas OND produced a substantial increase in the competition scores of treated losers. Similarly, FLX depressed the competitive behavior of the previously unstable rats to become stable losers; OND could enhance the competitive behavior of the previously unstable pairs of rats to retain the stable winner position. OND has shown a depressive effect on the serotonin levels in the brains of all OND-treated rats. In contrast, serotonin levels were elevated in the brains of all FLX-treated rats. Conclusion: FLX and OND had depressive and enhancing effects, respectively, on the competitive behavior of rats. The rats’ brain 5-HT levels were inversely related to the competitive behavior of rats.