Effect of central infusions of neuropeptide Y on GnRH/LH axis in ewes during the early anoestrous period.

Neuropeptide Y (NPY) is a putative mediator of many physiological processes in the central nervous system including reproductive functions. In this study we examined the effect of NPY on the GnRH/LH secretory activity in early anoestrous ewes. Crossbreed ewes one week (group 1w, n=7) and six weeks (group 6w, n=7) after the last oestrous cycle were infused with Ringer solution (control) or 50 microg of NPY to the third ventricle for 5 minutes. Blood samples were collected during the days of infusion over six hours at ten minute intervals and then the ewes were slaughtered and their brains fixed in situ. Immunoreactive (ir) GnRH in the median eminence and LH cells in the adenohypophysis were localised by immunohistochemistry technique. Data were generated using computer analysis to determine the optical density for immuno-staining and the proportion of ir LH cells. Plasma LH was determined by radioimmunoassay. It was found that investigated anoestral groups differed only in the optical densities of ir GnRH and ir LH, which were lower in group 6w compared to group 1w (p<0.001). LH concentrations and parameters of pulsatile LH secretion were similar in both anoestral groups. Acute NPY infusions induced in group 1w decreased both in ir GnRH and ir LH optical densities and in area fraction for ir LH cells (p<0.001) whilst it increased in plasma LH concentration (p<0.05) compared to vehicle infused animals. There was no changes in the secretory activity of the GnRH/LH axis following NPY infusion in group 6w. In conclusion, we suggest that NPY may be involved in the central regulation of reproductive function in ewes, however, the sensitivity of the GnRH/LH axis to NPY modulation declines throughout the early anoestrous period.