Prolactin changes after administration of agonist and antagonist dopaminergic drugs in puerperal women

Prolactin (PRL) is an anterior pituitary hormone which plays a large part in the reproductive function of mammals. Its only well-documented effect in humans is that of initiating and maintaining lactation. Among hypothalamic neurotransmitters regulating the anterior pituitary function, dopamine (DA) is currently considered to correspond to the PRL-inhibiting factor. The central control mechanisms which induce high PRL levels in puerperal women are not well understood. To study DA tonus in puerperium we tested plasma PRL levels in different groups of puerperal subjects (6 per group) after acute administration of direct or indirect DA agonists or placebo: DA, L-dopa (a DA precursor), L-dopa plus carbidopa (a peripheral dopa-decarboxylase inhibitor), nomifensine (a DA-releasing and blocking or reuptake agent) and amphetamine (a DA releaser). The same tests with the same drug doses were performed on groups of healthy volunteers. A consistent reduction in plasma PRL levels after both direct and indirect DA agonist drugs compared to placebo was evident in puerperal and in control women. A different trend was only observed with the use of DA and amphetamine in puerperal subjects, who, unlike controls, failed to show a rebound in plasma PRL levels after the termination of drug infusion. These findings support the view that the inhibitory control of tuberoinfundibular neurons over PRL secretion is maintained in puerperium and changes in the affinity of DA receptors are related to the endocrine milieu which occurs during gestation.