Extrahypothalamic control of daily surges of prolactin secretion in pseudopregnant rat

The role of the limbic forebrain structures in controlling twice daily surges of prolactin (PRL) induced by cervical stimulation was investigated after acute or chronic deafferentation of the limbic forebrain afferents to the hypothalamus in rats. The preoptic area-roof section (POA-RS), which interrupted the rostral limbic afferents at the dorsal level of the anterior commissure, induced pseudopregnancy (PSP) and initiated the same nocturnal PRL surges as those initiated by the cervical stimulation. Diurnal PRL surges, however, did not occur following this procedure. The nocturnal PRL surge by POA-RS also occurred in ovariectomized rats. Deafferentation between the diagonal band of Broca and the medial preoptic area (F2-cut) initiated PSP in 37 % of the rats and induced an apparent but small nocturnal PRL surge. The rats with POA-RS or F2-cut showed restoration of their regular estrous cyclicities. Cervical stimulation after POA-RS did not affect the initiation of nocturnal PRL surge induced by POA-RS alone. POA-RS after cervical stimulation also did not affect the initiation of nocturnal PRL surge induced by cervical stimulation, though a diurnal PRL surge was initiated in these rats. The cut made just before the diagonal band of Broca after cervical stimulation did not inhibit the occurrence of either surge. Nocturnal and diurnal PRL surges were manifested after cervical stimulation in the rats with chronic POA-RS or F2-cut and their vaginal cyclicities were resumed. These results suggest that the limbic forebrain structures are not indispensable for the initiation of nocturnal PRL surges induced by cervical stimulation but may modify the hypothalamic mechanism(s) initiating a nocturnal PRL surge through the rostral part of the hypothalamus.