ROSTROVENTROLATERAL MEDULLA NEURONS PREFERENTIALLY PROJECT TO TARGET-SPECIFIED SYMPATHETIC PREGANGLIONIC NEURONS

The rostroventrolateral medulla is a key site for the regulation of va somotor lone. Sympathoexcitatory neurons project from this region to c ontact sympathetic preganglionic neurons located in the intermediolate ral nucleus of the thoracic and lumbar spinal cord. Functional studies show that stimulation of specific sites in the ventral medulla lead t o selective activation of different vascular effecters. The present st udy was designed to determine the anatomical basis for this selectivit y in vasomotor control. Anterograde and retrograde tracing methods wer e utilized to determine if the descending rostral ventrolateral projec tion is topographically organized such that neurons in particular loca tions within the nucleus project preferentially and contact a specific group of sympathetic preganglionic neurons. For this purpose spinally -projecting neurons at 15 sites from three separate rostrocaudal locat ions within the rostroventrolateral medulla in nine rats were anterogr adely labelled with biotin dextran amine. The spinal cord was examined for axon terminals having close apposition to two groups of sympathet ic preganglionic neurons, those projecting to the superior cervical ga nglion and those to the adrenal medulla which were retrogradely labell ed with cholera B chain-conjugated horseradish peroxidase. Areas of cl ose apposition between retrogradely-labelled dendrites, cell bodies an d anterogradely-labelled axons were found. Axons descending from the m ore rostral part of the rostroventrolateral medulla produced the highe st density of close appositions to sympathetic preganglionic neurons i n both target-specific populations. Caudal rostroventrolateral medulla injection sites gave rise to a less dense distribution of axons and t erminals around the spinal sympathetic nuclei. This study has demonstr ated that spinally-projecting neurons in the rostroventrolateral medul la are both topographically and viscerotopically organized. It is sugg ested that such an arrangement provides the means for selective and di fferential control of autonomic effecters and in particular those invo lved in cardiovascular regulation. (C) 1998 IBRO. Published by Elsevie r Science Ltd.