During the past year, we have continued to identify the hypophysiotropic neurons, i.e., those which have connections with portal capillaries of the median eminence; and are, therefore, directly involved in the regulation of the secretion of the anterior pituitary. To identify these neurons, we conducted retrograde labeling studies in combination with peptide immunocytochemistry. Fluoro-Gold was used as retrograde tracer, and the endogenous peptides were detected by fluorescence immunocytochemistry. Our observations indicate that the majority of thyrotropin-releasing hormone (TRH). enkephalin and corticotropin- releasing hormone (CRH) neurons are located in the paraventricular nucleus, while neurotensin, galanin and dopamine neurons are found in the arcuate nucleus. However, not each peptidergic neuron in these well- defined nuclei is hypophysiotropic. A small percentage (5-30~) of these neurons does not project to the median eminence but contact other neurons in the brain where the hypophysiotropic factors function as neurotransmitters or neuromodulators. The retrograde label Fluoro-Gold provides an electron-dense substance in the cytoplasm providing a suitable technique to study the afferentation of hypophysiotropic neurons at the electron microscopic level.
