Hypothalamic projections of medullary catecholamine neurons in the rabbit: a combined catecholamine fluorescence and HRP transport study.

Injections of horseradish peroxidase into the rabbit hypothalamus, centered on the paraventricular nucleus, result in retrograde transport to A1 and A2 neurons in the medulla oblongata, in a bilateral distribution with ipsilateral predominance. Some HRP labeled cells in both A1 and A2 areas do not exhibit a positive histochemical reaction for catecholamines, but, in the A1 area, nearly all HRP-positive neurons are catecholamine-containing A1 cells. Injection of radioisotope into the A1 area of the rabbit results in terminal labeling within the paraventricular and supraoptic nuclei. This study complements previous work showing that rabbit A1 and A2 cells have minimal projections to the spinal cord.