Dopamine produced by tuberoinfundibular dopaminergic (TIDA) neurons inhibits prolactin secretion from the anterior pituitary. Dopamine is considered as the physiological prolactin release-inhibiting hormone. The activity of TIDA neurons decreases during lactation which can be further reduced by the suckling stimulus. We have shown that TIDA neurons of lactating. pregnant and aged female rats, in contrast to cycling female or male rats produce enkephalin. The incidence of colocalization is high; practically each dopaminergic neuron contains enkephalin. Enkephalin is a potent prolactin-releasing factor. The elevated levels of prolactin during pregnancy, lactation and aging is probably due, at least in part, to enkephalin produced by TIDA neurons. In addition, enkephalin may sustain elevated prolactin levels during the nonsuckling periods of lactation and, therefore, be responsible for preparing the mammary gland for milk secretion and ejection. Since enkephalin exhibits anti-gonadotropin activity, it is likely that it participates in the events inhibiting ovulation during pregnancy, lactation and aging. Each animal model whose TIDA neurons produce enkephalin exhibits elevated levels of prolactin and progesterone indicating that these factors are essential for the activation of enkephalin gene. Our experimental approaches (hypophysectomized rats with pituitary grafts under their kidney capsule and treated with sex steroids) indicate that these factors are indeed necessary for enkephalin gene activation. The need for elevated prolactin levels for the phenomenon is also supported by the results from haloperidol-treated intact animals. Haloperidol, a dopamine D2 receptor antagonist, increases prolactin levels and enkephalin """"""""appears"""""""" in TIDA neurons. The necessity of sex steroids, primarily progesterone, in addition to chronically elevated prolactin levels, is indicated by our observations with lactating animals that were bromocryptine treated. Bromocryptine, a 02 receptor agonist, decreases prolactin levels. Although prolactin levels are low, TIDA neurons of these animals still produce enkephalin.
