Control of prolactin release induced by suckling

In the present study, the role of dopamine and TRH in suckling-induced PRL release was investigated. Bupropion, a dopamine reuptake blocker, increased hypophysial stalk dopamine levels and inhibited suckling-induced PRL release. A short period of suckling, thought to induce a transient decrease in hypothalamic dopamine release, led to higher PRL levels following an iv injection of TRH than in rats which had not nursed their young for a short period after 4- to 6-h separation. These results, in combination with previous data, suggest that a decrease in hypothalamic dopamine release is important for suckling-induced PRL release. Increased PRL release may be in part due to an augmented hypothalamic release of TRH. Since serotonergic mechanisms seem involved in TRH release, lactating rats were treated with drugs acting on serotonergic pathways. Parachlorophenylalanine and pizotifen did not alter suckling-induced PRL release. Methysergide, a serotonin receptor blocker, prevented this PRL release when administered ip but not when injected into the lateral brain ventricle. Since methysergide is converted peripherally into metabolite(s) with dopamine agonistic activity, its effect on suckling-induced PRL release may be due to this action, rather than to its action on serotonin receptors. Thus, these data do not indicate that serotonergic mechanisms are important for suckling-induced PRL release. Passive immunization against TRH inhibited suckling-induced PRL release, indicating that TRH is a hypophysiotropic mediator of this PRL release.