Serotonin Involvement in the Regulation of Gonadotropin Secretion During Lactation in the Rat

Abstract

A role for serotonin (5-HT) in lactational anestrus was investigated in ovariectomized lactating rats nursing 0, 2, 4, 8, or 12 pups on Days 10-12 of lactation. Hypothalamic 5-HT concentrations did not vary with litter size. On the other hand, suckling of 8 or 12 pups was associated with increased levels of the major metabolite of 5HT, 5-hydroxyindole acetic acid (5-HIAA). Rats suckled for 30 min after a 6-hr period of separation from their six-pup litters on Day 12 of lactation exhibited decreased amounts of 5-HT and increased levels of 5-HIAA compared to nonsuckled controls. These experiments indicate an increased activity of hypothalamic 5-HT-containing neurons following suckling.

Separation from their pups for 12 hr produced a threefold increase in plasma LH and a twofold increase in plasma FSH in ovariectomized, lactating rats. Reinitiation of suckling returned both LH and FSH to preseparation levels by +12 hr. A similar pattern was observed in rats pretreated with ergocornine, a dopamine agonist which blocked prolactin release. This was interpreted to mean that elevated prolactin is not required for suckling-induced declines in gonadotropins. Pretreatment with 5-HT antagonists, metergoline and pizotifen, appeared to interfere partially with the LH decrease which followed reinitiation of suckling in control animals. Moreover, metergoline blocked the suckling-induced decrease in FSH. Thus, the increased activity of 5-HT-containing neurons in the hypothalamus which follows suckling may in part mediate the suppression of gonadotropins that occurs in lactating rats.