Control of the estradiol-induced prolactin surge by the suprachiasmatic nucleus

In the present study we investigated how the suprachiasmatic nucleus (SCN) controls the E-2-induced PRL surge in female rats. First, the role of vasop ressin (VP), a SCN transmitter present in medial preoptic area (MPO) projec tions and rhythmically released by SCN neurons, as a circadian signal for t he E-2-induced PRL surge was investigated. Using a reverse microdialysis te chnique, VP was administered in the MPO during the PRL surge, resulting in a suppression of the surge. VP administration before the surge did not affe ct PRL secretion. Also, administration of a Via receptor antagonist before the surge was ineffective. Second, lesions of the SCN were made that result ed in constant basal PRL levels, suggesting that with removal of the SCN a stimulatory factor for PRL secretion disappeared. Indeed, the PRL secretory response to blockade of pituitary dopamine receptors was significantly red uced in SCN-lesioned animals. These data suggest that the afternoon decreas e of VP release in the MPO by SCN terminals enables the PRL surge to occur, and may thus be a circadian signal for the PRL surge. Simultaneously the S CN is involved in the regulation of the secretory capacity of the pituitary , possibly via specific PRL-releasing factors.