Reversal of fenfluramine and fluoxetine anorexia by 8-OH-DPAT is attenuated following raphe injection of 5,7-dihydroxytryptamine.

Drugs that enhance serotonergic neurotransmission reduce food intake by directly or indirectly activating serotonergic receptors. In contrast drugs that inhibit serotonergic neurotransmission such as the 5-HT1A agonist 8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT) stimulate food intake. The present study examined the effects of 8-OH-DPAT on the feeding suppressant action of the indirect 5-HT agonists fenfluramine (FEN; 0.63-2.5 mg/kg) and fluoxetine (FLU; 2.5-10 mg/kg), as well as the 5-HT1B/2C agonist 1-(3-trifluoromethylphenyl)piperazine (TFMPP; 0.5-2 mg/kg). 8-OH-DPAT (62.5-250 microg/kg) was administered 5 min prior to FEN, FLU or TFMPP, injected 30 min before food access. While FEN, FLU and TFMPP dose-dependently reduced 2 h food intake, 8-OH-DPAT stimulated eating behavior. 8-OH-DPAT (62.5-250 microg/kg) pretreatment reversed the anorectic action of FEN (1.25 mg/kg) and FLU (5 mg/kg) but not TFMPP (1 mg/kg). Separate groups of rats were injected with 5,7-dihydroxytryptamine (5,7-DHT; 3 microg free base) into both the dorsal and median raphe, which resulted in extensive 5-HT depletion in hypothalamus (80%), striatum and hippocampus (90%). In both 5, 7-DHT and vehicle-injected rats, FEN (1.25 mg/kg) and FLU (5 mg/kg) suppressed feeding. In 5,7-DHT treated rats, however, the ability of 8-OH-DPAT (125 microg/kg) to block FEN and FLU induced anorexia was attenuated. That is, 8-OH-DPAT pretreatment did not reverse the feeding inhibitory effects of either FEN or FLU. Further, the ability of FEN and FLU to suppress food intake was not altered by the 5,7-DHT lesion. These findings suggest that the reversal of FEN and FLU anorexia by 8-OH-DPAT is partially dependent on the integrity of brain 5-HT systems since their disruption compromises the ability of this 5-HT1A agonist to antagonize the feeding suppressant action of either FEN or FLU. However, the ability of treatments which impair 5-HT neurotransmission to reverse FEN and FLU induced suppression of food intake may depend upon whether this impairment is acute and reversible (8-OH-DPAT), or chronic and irreversible (5,7-DHT).