PERIPHERAL SEROTONIN IS AN INCOMPLETE SIGNAL FOR ELICITING SATIETY IN SHAM-FEEDING RATS

Peripheral administration of serotonin [5-hydroxytryptamine (5-HT)] to rats equipped with gastric cannulae reduced their 30-min consumption of sweetened milk after overnight deprivation whether the cannulae were closed (real feeding) or open (sham feeding). The anorectic action of 5-HT (1.6, 4.0, and 10.0 mumol/kg, IP) in sham feeding was dose-related, rapid in onset, and persisted during the 30-min testing session. However, 5-HT failed to elicit resting - the terminal behavioral phase of satiety-in sham-feeding rats. Direct comparison of the effects of 4.0 mumol/kg 5-HT under both feeding conditions established that this dose promoted resting only when rats fed with their cannulae closed. The actions of 5-HT on feeding and resting were behaviorally selective because serotonergic treatment did not retard the beginning of feeding, alter sham drinking of water, or reduce investigation by food-deprived rats of a novel object in an open field. Together, the results suggest that 5-HT exerts separate actions to inhibit feeding and accelerate the process of satiation as marked by resting. However, peripheral 5-HT is inadequate as a signal for modulating satiety in the absence of postingestive stimuli.