Peripheral serotonergic inhibition of suckling.

Systemically administered serotonin, which does not pass the blood-brain barrier, inhibited nipple attachment behavior in 20- and 30-day-old rat pups. Xylamidine, a peripheral serotonin antagonist, attenuated the effects of serotonin, quipazine, and fenfluramine on nipple attachment behavior. Thus, serotonin receptors in the periphery may play an important role in the serotonergic inhibitory mechanism that has been hypothesized as the developing system leading to weaning. However, unlike more general 5-HT antagonists, xylamidine given alone failed to facilitate suckling, suggesting different sites of action for facilitation and inhibition of this infantile behavior.