Regulation of prolactin secretion during pregnancy and lactation

Prolactin plays major roles in maintaining the corpora lutea of pregnancy and in the synthesis of milk during lactation. The hypothalamic mechanisms involved in these functions have been investigated. Mating leads to a surge of prolactin and programs daily surges during early pregnancy. The expression of Fos-immunoreactivity shows that mating activates several hypothalamic nuclei, particularly the arcuate nucleus and medial preoptic area. In the arcuate nucleus, mating is associated with Fos expression in beta-endorphin neurons, and infusion of naloxone blocks both mating-induced and diurnal prolactin surges. Tyrosine hydroxylase-immunoreactive dopamine neurons appear not to participate in surge generation. However, after day 10 of gestation the secretion of placental lactogens suppresses prolactin secretion via activation of dopamine neurons without involvement of beta-endorphin neurons. Intracerebroventricular implantation of placental lactogen-secreting cells will block pregnancy prolactin surges, increase Fos expression in dopamine neurons, and increase tyrosine hydroxylase activity. During lactation the mechanisms regulating dopamine and beta-endorphin neurons are further modified. In early lactation a prolactin-induced increase in tyrosine hydroxylase activity leads to negative feedback, but this effect is lost by mid-lactation. Overriding this negative feedback is the inhibitory effect that suckling has on dopaminergic activity. This may involve beta-endorphin-mediated inhibition of dopamine neurons, as naloxone causes a marked increase in tyrosine hydroxylase activity and suppression of circulating prolactin. However, removal of tonic dopamine inhibition is not sufficient to account for the high levels of prolactin attained during lactation, and additional releasing factors are probably involved. In situ hybrization histochemistry for the most recent candidate, prolactin-releasing peptide, suggests that this may involve brain stem neurons that co-localize noradrenaline. Thus, prolactin secretion during pregnancy and lactation involve complex interactions of regulatory factors and plasticity of neuronal responsiveness.