Participation of Serotonin-Containing Neurons in the Suckling-Induced Rise in Plasma Prolactin Levels in Lactating Rats

Abstract

Plasma prolactin levels were measured by radioimmuno-assay in free-moving lactating rats bearing heart cannulas. Litters adjusted to 6 pups were returned to the mothers after a separation of 8–12 h from days 6 to 15 postpartum. Blood samples were taken before and 15, 30, 60 and 120 min after suckling had resumed. Lactating mothers were treated at various intervals before the experiment with P-chlorophenylalanine (PCPA), an inhibitor of serotonin (5-HT) synthesis; some animals were treated in addition with 5-hydroxytryptophan (5-HTP), a direct precursor of 5-HT. Blockade of 5-HT biosynthesis completely inhibited the more than 10-fold increase in prolactin levels observed in untreated controls. Inhibition of the prolactin response lasted more than 48 h after administration of PCPA; complete recovery required 120 h. Treatment with 5-HTP, which antagonizes the depletion of 5-HT caused by PCPA, restored the ability of the animals to release prolactin in response to suckling. The basal prolactin levels observed in lactating animals deprived of pups were not significantly affected by either drug at the time of experimentation. The results are consistent with the hypothesis that serotonin-containing neurons have a facilitatory effect on the hypothalamic mechanisms which trigger prolactin release in response to neural inputs involved in the suckling stimulus.