Absence of fenfluramine-induced anorexia and reduced c-fos induction in the hypothalamus and central amygdaloid complex of serotonin 1B receptor knock-out mice

Fenfluramine, a serotonin releaser and uptake inhibitor, has been widely prescribed as an appetite suppressant. Despite its popular clinical use, however, the precise neural pathways and specific 5-HT receptors that account for its anorectic effect have yet to be elucidated. To test the hypothesis that stimulation of 5-HT(1B) receptors is required for the anorectic effect of fenfluramine, we assessed food intake in wild-type and 5-HT(1B) knock-out mice. Next, to determine possible brain structures and pathways that may contribute to the 5-HT(1B)-mediated effects of fenfluramine, we studied by immunohistochemistry the induction of the immediate early gene c-fos. Although the effect of fenfluramine on locomotion was indistinguishable between both wild-type and 5-HT(1B). knock-out mice, the anorectic effect of the drug was absent in only the knock-out mice. Furthermore, the induction of c-Fos immunoreactivity found in the paraventricular nucleus of the hypothalamus (PVN) of wildtype mice was substantially reduced in the knock-outs. Induction in the central amygdaloid nucleus (CeA) and in the bed nucleus of the stria terminalis (BNST), although robust in wildtype animals, was completely absent in knock-out animals. The mixed 5-HT(1A/1B) agonist RU24969 was able to mimic both the hypophagia and c-fos induction elicited by fenfluramine in wildtype mice, but not in the 5-HT(1B) knock-out mice: Our results thus demonstrate that stimulation of 5-HT(1B) receptors is required for fenfluramine-induced anorexia and suggest a role for the PVN, CeA, and BNST in mediating this effect.