Effect of Frontal Hypothalamic Deafferentation on Luteinizing Hormone Secretion and Seasonal Breeding in the Ewe12

These experiments were designed to study the effects of disruption of the neural connections between the anterior and mediobasal hypothalamus on luteinizing hormone (LH) secretion, ovulation, and seasonal breeding in the sheep. In October 1979, 10 intact cycling Suffolk ewes were subjected to frontal hypothalamic deafferentation (FHD) and 5 to sham FHD. Blood samples, for progesterone determination, were taken once every 3 days for 21 days during 2 midbreeding seasons (December 1979 and 1980) and 1 midanestrous season (late June 1980). Then all ewes were ovariectomized. After 30 days, blood samples were taken at 20-mm intervals for 3 h to determine the patterns of basal LH secretion. Then 50μg of estradiol benzoate (EB) was injected intramuscularly into each ewe and blood samples were taken at 3-h intervals for 27 h to determine if the EB-induced LH surge was normal. The ewes then were sacrificed. The location of the FHD was verified histologically and the ewes were grouped, by definition, as sham FHD (N=5), complete FHD (N.5), and incomplete FHD (N=5). After ovariectomy, basal LH concentrations from either complete FHD ewes (8.2 ± 0.77 ng/ml) or incomplete FHD ewes (10.5 ± 0.84 ng/ml) did not differ from sham FHD ewes (9.7 ± 0.98 ng/ml). The patterns of LH secretion did not differ among the 3 groups. In contrast, the EB-induced LH surges of all 5 complete FHD ewes clearly were reduced in comparison to those of sham ewes (peak LH ranging from 25 to 60 ng/ml versus greater than 100 ng/ml, respectively). Blood progesterone concentrations showed that all 15 ewes cycled during both breeding seasons. Neither sham FHD nor incomplete FHD ewes cycled during the nonbreeding season, whereas 3 of 5 complete FHD ewes cycled. These finding suggest that: 1) FHD did not prevent expression of estrous cycles, 2) FHD clearly reduced EB-induced LH surges, 3) input from the anterior to the mediobasal hypothalamus is required for a normal LH surge but not for ovulation, and 4) disruption of input from the anterior to the mediobasal hypothalamus disrupts seasonal breeding in sheep.