Hypothalamic sites which control the surges of prolactin secretion induced by cervical stimulation

Artificial stimulation of the uterine cervix (CS) of the rat institutes two daily surges of PRL. These surges in turn activate and maintain luteal progesterone secretion for a 13-day interval known as pseudopregnancy. Electrolytic lesions of the medial preoptic area (MPOA) or electrical stimulation of the dorsomedial-ventromedial areas (DMN-VMN) of the hypothalamus also induce pseudopregnancies. This study was designed to determine if these pseudopregnancies are also characterized by the biphasic pattern of PRL secretion. Bilateral electrolytic lesions were placed in the MPOA on the morning of either estrus or the 14th day after ovariectomy. Both experimental models had nocturnal (0500 h) but no diurnal (1900 h) surges of PRL on the second day after placement of the lesion. Sham-lesioned rats had neither surge. When both surges were initiated by CS on proestrus, placement of the lesion on estrus blocked the diurnal but not the nocturnal surge of PRL. These data suggest that the MPOA possesses neurons which are inhibitory to nocturnal surges and stimulatory to diurnal surges of PRL and that CS removes the inhibition over the former and stimulates the latter. Electrical stimulation of the DMN-VMN initiates both surges of PRL in intact or ovariectomized rats. Thus, CS may also act either directly or indirectly w;thin the DMN-VMN to induce both surges of PRL.