Midbrain pathways for the initiation and maintenance of the nocturnal prolactin surge in pseudopregnant rats.

After stimulation of the uterine cervix of the rat, a nocturnal PRL surge that is responsible for pseudopregnancy is initiated and maintained daily. This study was designed to determine the midbrain pathways for the neural control of the initiation and maintenance of the nocturnal PRL surge in pseudopregnant rats. Cervical stimulation, applied in the evening of proestrus, initiated a nocturnal PRL surge in the early morning of estrus. Neural transections, when placed in the ventromedial or lateral part of the midbrain before cervical stimulation, completely suppressed the nocturnal surge, whereas sham transections or transections in the dorsal part of the midbrain did not eliminate the nocturnal surge. On the other hand, in rats that received sham transections in the evening on day 5 of pseudopregnancy previously induced by cervical stimulation, a nocturnal PRL surge was observed in the early morning on day 6. None of transections in the ventromedial, lateral, or dorsal part of the midbrain placed in the evening on day 5 of pseudopregnancy affected the nocturnal surge on day 6. These results suggest that the initiation of the nocturnal PRL surge by cervical stimulation requires the neural components that are contained in the ventromedial and lateral parts of the midbrain. In contrast, the maintenance of the nocturnal surge possibly occurs independent of neural afferents from the midbrain.