Role of the medial preoptic area in the neural control of the nocturnal prolactin surge in the rat.

In the present study, the role of the medial preoptic area (MPO) in the neural control of the nocturnal prolactin (PRL) surge was investigated in ovariectomized rats. Cervical stimulation (CS) or bilateral MPO lesions caused a marked nocturnal PRL surge at 0400 h on the fourth day after CS or the lesions in ovariectomized rats in which blood samples were obtained by decapitation. However, operation for indwelling a catheter and serial blood collection completely eliminated the MPO lesion induced nocturnal surge while they did not affect the CS-induced surge. On the other hand, MPO lesions could not induce the nocturnal PRL surge in neonatally androgenized female rats. These results suggest that the MPO not only tonically inhibits the initiation of the nocturnal PRL surge but also has a buffer action on the PRL surge-suppressing action of stress. Furthermore, it may be possible that the failure of CS to initiate and maintain the nocturnal PRL surge in neonatally androgenized rats is not due to the inability of CS to disinhibit the inhibitory action of the MPO, but rather due to the extinction of the circadian rhythm itself of the nocturnal PRL surge in these rats.