Central Inhibition by Serotonin of Reflex Release of Oxytocin in Response to Suckling Stimulus in the Rat

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Abstract
The amount of milk obtained by young rats during a 1 hr. nursing period after 8 hr. isolation from the mother was used as an index of oxytocin release. During the isolation period, serotonin or drugs related to serotonin, were given to the mothers. Serotonin, its precursors (tryptophan and 5-hydroxytryptophan), and a monoamine oxidase inhibitor (iproniazid) significantly inhibited milk ejection. The inhibitory effect of these drugs was overcome by injection of oxytocin or by administration of a specific serotonin antagonist (2-bromo-D-lysergic acid diethylamide). Inhibition of milk ejection by tryptophan or 5-hydroxytryptophan was also blocked by injecting [alpha]-methyl-3-4-dihydro-xyphenylalanine, which prevents the conversion of 5-hydroxytryptophan to serotonin. Administration of 5-hydroxyindoleacetic acid, a metabolite of serotonin, did not inhibit milk ejection. These results demonstrate that serotonin alone, but not its precursors nor its metabolite, centrally inhibits the reflex release of oxytocin in response to the suckling stimulus. Serotonin may be involved in the normal reflex release of oxytocin in response to suckling.