The bovine median eminence was dissected into eight different subdivisions: rostral, nterior internal, anterior external, middle externalmedial, middle external lateral, middle internal medial, middle internal lateral, and caudal. Thyrotropin-releasing hormone (TRH) was found in the highest concentrations in the middle external medial and lateral subdivisions; luteinizing hormone-releasing hormone (LHRH) was concentrated in the middle external lateral and anterior internal subdivisions. Among the various neurotransmitters and enzymes assayed, only dopamine and choline acetyltransferase were present in highest concentrations in the same subdivisions of the bovine median eminence found to be rich in TRH and LHRH. The distributions of norepinephrine, dopamine-) μ-hydroxylase, serotonin, tryptophan hydroxylase, phenylethanolamine-N-methyltransferase, glutamic acid decarboxylase, and histamine appeared to correlate poorly with the major distributions of TRH and LHRH. These findings suggest that at the level of the median eminence, central neuroendocrine regulation of TRH and LHRH release may involve an interaction only with dopamine and acetylcholine.
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