Neurons in the area postrema are the only catecholamine-synthesizing cells in the medulla or pons with projections to the rostral ventrolateral medulla (C1-area) in the rabbit.

We have identified, in the rabbit medulla and pons, neurons which project to the C1-region of the rostral ventrolateral medulla. By combining tyrosine hydroxylase immunohistochemistry with retrograde transport of Fluoro-Gold we determined whether any of the retrogradely labelled neurons synthesize catecholamines. The only doubly labelled cells were located in the area postrema. No other group of catecholamine-synthesizing neurons in either the medulla or the pons was found to project to the C1-area of the rostral ventrolateral medulla. Pharmacological agents which lower arterial pressure by stimulating adrenoceptors in the rostral ventrolateral medulla may act on receptors which are not innervated by catecholamine-synthesizing perikarya located outside the C1-region.