GONADOTROPIN-RELEASING-HORMONE SECRETION INTO 3RD VENTRICLE CEREBROSPINAL-FLUID OF CATTLE - CORRESPONDENCE WITH THE TONIC AND SURGE RELEASEOF LUTEINIZING-HORMONE AND ITS TONIC INHIBITION BY SUCKLING AND NEUROPEPTIDE-Y

Objectives of the current studies were to characterize the pattern of GnRH secretion in the cerebrospinal fluid of the bovine third ventricl e, determine its correspondence with the tonic and surge release of LH in ovariectomized cows, and examine the dynamics of GnRH pulse genera tor activity in response to known modulators of LH release (suckling; neuropeptide Y [NPY]). In ovariectomized cows, both tonic release patt erns and estradiol-induced surges of GnRH and LH were highly correlate d (0.95; p < 0.01). Collectively, LH pulses at the baseline began coin cident with (84%) or within one sampling point after (100%) the onset of a GnRH pulse, and all estradiol-induced LH surges were accompanied by corresponding surges of GnRH. A 500-mu g dose of NPY caused immedia te cessation of LH pulses and lowered (p < 0.001) plasma concentration s of LH for at least 4 h. This corresponded with declines (p < 0.05) i n both GnRH pulse amplitude and frequency, but GnRH pulses were comple tely inhibited for only 1.5-3 h. In intact, anestrous cows, GnRH poise frequency did not differ before and 48-54 h after weaning on Day 18 p ostpartum, but concentrations of GnRH (p < 0.05) and amplitudes of GnR H pulses (4 of 7 cows) increased in association with weaning and heigh tened secretion of LH. We conclude that the study of GnRH secretory dy namics in third-ventricle CSF provides a reasonable approach for exami ning the activity and regulation of the hypothalamic pulse generator i n adult cattle. However, data generated using this approach must be in terpreted in their broadest context. Although strong neurally mediated inhibitors of LH pulsatility (suckling; NPY) had robust effects on on e or more GnRH secretory characteristics in CSF, only high doses of NP Y briefly abolished GnRH pulses. This implies that the GnRH signal rec eived at the hypophyseal portal vessels under these conditions may dif fer quantitatively or qualitatively from those in CSF, and theoretical ly would be undetectable or below a biologically effective threshold w hen LH pulses are absent.