Expression of NPY-immunoreactive neurons in the hypothalamus of the cycling ewe

The purpose of the study was to localise neuropeptide Y (NPY) immunoreactive (ir) neurons in the hypothalamus during two phases of the oestrous cycle in the ewe. Hypothalamic tissue was collected from Polish Merino ewes (n=8) in the follicular (15th day) and preovulatory (17th day) phases of the oestrous cycle. NPY-ir neurons were detected in the hypothalamus using immuohistochemistry followed by image analysis; positive staining was expressed as the percentage of stained area and optical density. Two populations of the NPY-positive neurons were detected and evaluated in the infundibular and periventricular nuclei of the hypothalamus. The population of NPY-ir neurons located in the infundibular nucleus exhibited a prominent expression of NPY immunoreactivity in the perikarya and fibres only during the preovulatory phase. Both, percent area and the optical density of NPY immunostaining measured in this area were higher (P < 0.01) in the preovulatory than in the follicular phase. Another population of NPY-ir neurons was localised in the periventricular nucleus and did not show any changes during the two phases of the cycle. The present study suggests that NPY-ir neurons present in the infundibular nucleus can play a role in the preovulatory GnRH discharge from the median eminence.