Effect of frontal hypothalamic deafferentation on photoperiod-induced changes in the secretion of prolactin in the ewe.

The purpose of these experiments was to determine the effect of frontal hypothalamic deafferentation (FHD) between the suprachiasmatic-preoptic region and the mediobasal hypothalamus on diurnal, seasonal, and photoperiod-induced changes in PRL secretion in ewes. In Exp 1, intact cycling ewes were subjected to either FHD or sham FHD. They were kept outdoors, and sequential hourly blood samples were taken for either 24 or 36 h during two breeding seasons and one nonbreeding season. Mean plasma PRL concentrations were higher (P less than 0.01) during the nonbreeding season than during the breeding season in both FHD and sham FHD ewes. Although FHD did not block the seasonal changes, it reduced (P less than 0.05) the 24-h mean concentration and diurnal peak concentration during the nonbreeding season. During the breeding seasons, there was no consistent diurnal pattern of PRL concentrations. In Exp 2, ovariectomized ewes were placed in a photochamber. After adaptation to an 8-h light, 16-h dark (8L:16D) photoperiod, they were subjected to either FHD or sham FHD. Sequential hourly blood samples were collected on the last day of exposure to each of three sequential lighting periods (120 days 8L:16D, 90 days 16L:8D, and 90 days 8L:16D). The 24-h mean plasma PRL concentrations were higher (P less than 0.01) after exposure to 16L:8D than after exposure to 8L:16D in both FHD and sham FHD ewes. FHD did not affect the 24-h mean concentrations. In both groups, there was a distinct diurnal pattern of plasma PRL after exposure to 16L:8D, but not after exposure to 8L:16D. FHD did not alter the diurnal pattern, but significantly (P less than 0.05) reduced the magnitude of the nocturnal elevation under 16L:8D. In summary, FHD reduced, but did not block, seasonal changes in PRL secretion in intact ewes and did not alter photoperiod-induced changes in 24-h patterns and 24-h mean concentrations of plasma PRL in ovariectomized ewes. These results suggest that direct neural pathways between the supra-chiasmatic-preoptic region and the mediobasal hypothalamus play a relatively minor role in mediating photoperiod-induced changes in PRL secretion in sheep.