IDENTIFICATION AND DISTRIBUTION OF NEUROENDOCRINE GONADOTROPIN-RELEASING-HORMONE NEURONS IN THE EWE

The final common pathway controlling reproductive function in vertebra tes is the GnRH neuron and its projection to the median eminence (ME), site of peptide release into the pituitary portal system. GnRH neuron s are widely distributed; therefore we sought to test the hypothesis t hat those projecting to the ME are located in specific regions. We use d as a model the sheep, a species in which a great deal of information regarding the physiology of GnRH secretion is known. To identify cell s projecting to the ME (i.e., neuroendocrine neurons), ewes (n = 10) r eceived injections into the ME of neuronal tract-tracing compounds: ch olera toxin-beta subunit (CT-beta) or one of two fluorescent compounds (rhodamine isothiocyanate or fluorescein-conjugated dextran). Forty-e ight h later, animals were perfused intracranially and their brains we re processed for immunocytochemical localization of GnRH and CT-beta u sing a dual-immunofluorescent procedure or by single-label immunofluor escent visualization of GnRH combined with direct visualization of flu orescent tracers. Small, well-circumscribed injections into the ME wer e made successfully in 6 of 10 animals, and these overlapped the locat ion of GnRH terminals and fibers. Neuroendocrine GnRH neurons (those G nRH neurons containing retrogradely transported tracer) were identifie d throughout their previously reported range: within the diagonal band of the Broca/medial septal region, medial preoptic area (MPOA), anter ior hypothalamic area, and medial basal hypothalamus. Although the abs olute number of neuroendocrine GnRH neurons varied by region, the perc entage of the total GnRH population within each of these areas that wa s retrogradely labeled did not differ (p > 0.05). Injections placed un ilaterally within the ME labeled a similar proportion of GnRH cells bo th ipsilateral and contralateral to the injection site in all areas ex cept the MPOA, where ipsilaterally labeled cells were approximately tw ice as numerous as those labeled contralaterally. Injections-that miss ed the ME and were placed either into the third ventricle or into the arcuate nucleus labeled only 0.5% and 4-11% of GnRH neurons, respectiv ely. These results do not support the hypothesis that in the ewe, GnRH neurons projecting to the ME are localized to specific regions. Thus, we postulate that GnRH release into the hypophyseal portal system ref lects the output of GnRH neurons located in multiple areas.