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Effect of frontal hypothalamic deafferentation on duration of breeding season and melatonin secretion in the ewe.

The objectives of this study were to determine if ewes subjected to frontal hypothalamic deafferentation (FHD) during anestrus remained anestrus or began to have estrous cycles, and if melatonin secretion was disrupted by FHD. Ovary-intact ewes in Group 1 were subjected to either FHD (n = 10) or sham FHD (n = 5) in early July 1983. Estrous cycles were monitored by measuring circulating progesterone concentrations from before FHD until September 1985. Group 2 ewes (n = 4) were subjected to FHD in October 1984. In late April 1985, blood samples were taken from all ewes at 1- to 4-h intervals from 1100 h to 0700 h of the following day to monitor diurnal changes of melatonin. Hypothalami were collected for histological evaluation of lesions. All Group 1 ewes (sham FHD and FHD) initiated normal estrous cycles in August and September 1983, and all ceased cycles by mid-February 1984. All sham FHD and 4 FHD ewes remained anestrus until August or September of 1984 and then resumed normal cycles. In contrast, 5 FHD ewes resumed cycles as early as April 1984 and then cycled intermittently or almost continuously. Two Group 2 ewes cycled continuously after FHD and 2 cycled infrequently. FHD ewes that showed prolonged breeding seasons had cuts that damaged the suprachiasmatic nucleus (SCN) and adjacent structures. Mean nocturnal (2000 h-0500 h) melatonin concentrations did not differ (p greater than 0.05) between sham FHD, FHD "normal season," and FHD "continuous cycle" ewes. In summary, damage to the SCN region by FHD during anestrus had no detectable effect on either onset or cessation of the next breeding season but greatly prolonged subsequent breeding seasons. Thus, the environmental signals that both initiated and terminated the 1983 breeding season apparently had been given before FHD was performed in midsummer. Damage to the SCN region during the breeding season caused some ewes to cycle continuously. The effects of FHD apparently were not due to disruption of melatonin secretion. FHD ewes that showed prolonged breeding seasons had normal seasonal changes of plasma prolactin concentrations. This suggests that different neural structures control seasonal patterns of gonadotropin and prolactin secretion.