Rostroventrolateral medulla neurons preferentially project to target-specified sympathetic preganglionic neurons.

The rostroventrolateral medulla is a key site for the regulation of vasomotor tone. Sympatho-excitatory neurons project from this region to contact sympathetic preganglionic neurons located in the intermediolateral nucleus of the thoracic and lumbat spinal cord. Functional studies show that stimulation of specific sites in the ventral medulla lead to selective activation of different vascular effectors. The present study was designed to determine the anatomical basis for this selectivity in vasomotor control. Anterograde and retrograde tracing methods were utilized to determine if the descending rostral ventrolateral projection is topographically organized such that neurons in particular locations 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 locations within the rostroventrolateral medulla in nine rats were anterogradely labelled with biotin dextran amine. The spinal cord was examined for axon terminals having close apposition to two groups of sympathetic preganglionic neurons, those projecting to the superior cervical ganglion and those to the adrenal medulla which were retrogradely labelled with cholera B chain-conjugated horseradish peroxidase. Areas of close apposition between retrogradely-labelled dendrites, cell bodies and anterogradely-labelled axons were found. Axons descending from the more rostral part of the rostroventrolateral medulla produced the highest density of close appositions to sympathetic preganglionic neurons in both target-specific populations. Caudal rostroventrolateral medulla injection sites gave rise to a less dense distribution of axons and terminals around the spinal sympathetic nuclei. This study has demonstrated that spinally-projecting neurons in the rostroventrolateral medulla are both topographically and viscerotopically organized. It is suggested that such an arrangement provides the means for selective and differential control of autonomic effectors and in particular those involved in cardiovascular regulation.