Regional Brain Content of Luteinizing Hormone- Releasing Hormone in Sheep during the Estrous Cycle, Seasonal Anestrus, and after Ovariectomy*

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Abstract
Twenty-three sheep were killed during seasonal anestrus or on different days of the estrous cycle. The stalkmedian eminence (SME), hypothalamic tissue (HP) overlying the SME, and preoptic-suprachiasmatic tissue (PO-SC) were assayed for LHRH using RIA. LHRH contents of the SME during anestrus and days 3–8 of the estrous cycle were 35 ± 3 and 33 ± 5 n.g (χ ± SEM), respectively. Higher levels (59 ± 8 ng) were detected in proestrous-estrous ewes (days 1, 16, and 17), and intermediate contents were measured in ewes on days 10–l14. LHRH content of the HP (11 ± 1 ng) was not different during the reproductive stages examined, although it was correlated (r = 0.65) with that in the SME. In the PO-SC area, LHRH increased from 1.3 ± 0.2 ng on days 3–8 to 2.6 ± 0.4 ng by days 10–14, but by days 1, 16, and 17 it had fallen to 0.9 ± 0.2 ng. LHRH content of the PO-SC area of anestrous ewes was 2.1 ± 0.4 ng. Anterior pituitary content of LH was greater on days 1, 10–14, 16, and 17 than on days 3–8 of the estrous cycle or during anestrus. In another experiment, 12 ovariectomized (ovx) and 12 intact ewes were exposed to the natural photoperiod of the breeding season, while 12 other ovx sheep were exposed to 18 h of light/day for 10 weeks. Six ovx ewes in each group were treated daily with im estradiol benzoate (EB; 138 µg) or oil. Ovariectomy resulted in a 47% decrease in the LHRH content of the SME, while that in the HP and PO-SC area remained unchanged. Ten-week exposure to 18 h of light/day did not alter the levels of LHRH in the SME, HP, or PO-SC area. Likewise, EB did not affect the LHRH contents of these tissues. LHRH contents of the HP and SME were correlated (r = 0.42). These data demonstrate that the contents of LHRH in the SME and LH in the AP are low during anestrus and the early stages of the estrous cycle and that they both increase near the time of the preovulatory release of LH. LHRH in the PO-SC region undergoes changes in a pattern unlike those of LHRH in the SME. Ovariectomy resulted in a reduced content of LHRH in the SME which was not restored by treatment with EB.