Immunocytochemical delineation of thyrotrophic area: origin of thyrotropin-releasing hormone in the median eminence.

To identify the specific thyrotropin-releasing hormone (TRH)-containing neurons projecting to the median eminence (ME), a retrograde tracing method with horseradish peroxidase (HRP) was combined with immunocytochemical staining for TRH. Three days after HRP injection restricted to the ME, several TRH-positive neuronal perikarya were found to contain HRP. Such double-stained cells were exclusively distributed in the anterior parts of the periventricular nucleus and the most medial parts of the paraventricular nucleus. Few double-stained cells were observed in other parts of the brain examined. The present observations appear to demonstrate that the specific TRH neurons projecting to the ME are located along the border of the third ventricle, anterior to the ME.