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Effect of Season on Neuropeptide Y and Galanin within the Hypothalamus of the Ewe in Relation to Plasma Luteinizing Hormone Concentrations and the Breeding Season: An Immunohistochemical Analysis

Within the hypothalamus, neurones that express neuropeptide Y (NPY) and galanin have been implicated in the regulation of gonadotropin‐releasing hormone (GnRH) and gonadotropin secretion. We aimed to determine the extent to which the expression of these two neuronal systems is linked to the seasonal reproductive cycle, and the effect of chronic oestrogen treatment. Immunohistochemical analysis was used to examine changes between the breeding season and anestrus in ovariectomized (OVX) ewes with or without oestrogen treatment (s.c. implants for 2 weeks). Serial blood sampling established plasma luteinizing hormone (LH) profiles, and the ewes were subsequently killed and the brains perfused for immunohistochemistry. In OVX ewes, the amplitude of LH pulses was greater in the nonbreeding season than in the breeding season. Oestrogen treatment caused a marked reduction in plasma LH concentrations during anestrus, but not in the breeding season. The number of cells in the arcuate nucleus/median eminence region (ARC‐ME) that stained for NPY was lower in ewes killed in anestrus (September) than in ewes killed in the breeding season (May), but there was no seasonal change in the number of galanin‐stained cells. Within season, oestrogen treatment did not affect NPY‐ or galanin‐cell number. There was no effect of season or oestrogen on the area of varicose fibres/terminals for either peptide in the ARC‐ME, but galanin immunostaining was more intense during the breeding season. We conclude that the amount of NPY in cell bodies of the ARC‐ME is lower in ewes in the nonbreeding season; this could reflect a steroid‐independent effect of photoperiod. We also conclude that the long‐term negative‐feedback effect of oestrogen on GnRH/LH secretion does not appear to be mediated by NPY‐ or galanin‐containing neurones in the ewe.