Quantitative analysis of spinally projecting adrenaline-synthesising neurons of C1, C2 and C3 groups in rat medulla oblongata.

Spinal projections of the phenylethanolamine N-methyltransferase (PNMT) immunoreactive neurons of the medulla were investigated using a combination of immunohistochemistry and retrograde transport of colloidal gold particles conjugated to cholera toxin B subunit (CTB-gold). The PNMT-containing adrenergic neurons were localised in three groups, the C1 group in the rostral ventrolateral medulla, the C2 group in the nucleus tractus solitarius/dorsal vagal motor complex in the dorsal medulla and the C3 group in the mediodorsal medulla. The C1 group contained 72% of the medullary PNMT-IR neurons, while C2 comprised 13% and C3 15% of the medullary PNMT-IR neuron population. CTB-gold was injected in the area of the intermediolateral cell column in either upper (T2-T4) or lower (T8-T9) thoracic spinal cord and retrogradely labelled cells were found in the areas of the C1, C2 and C3 groups and in other regions of the medulla which did not contain PNMT-IR neurons. After tracer injections bilaterally at levels T2-T4, retrograde labelling suggested that at least 21% of all medullary PNMT-IR neurons projected to these levels. As a proportion of each group, 26% of C1, 9% of C2 and 33% of C3 neurons projected spinally to T2-T4. After tracer injections bilaterally at levels T8-T9, retrograde labelling suggested that at least 17% of all medullary PNMT-IR neurons projected to these levels. As a proportion of each group, 16% of C1, 9% of C2 and 30% of C3 neurons projected spinally to T8-T9. These figures represent minimum numbers since it is impossible to ensure that every neuron has equal access to the tracer. The results demonstrate that contrary to previous belief, the PNMT-IR innervation of the spinal cord derives from PNMT-IR neurons in the dorsal medulla, as well as from the rostral ventrolateral medulla. Indeed 24% of the PNMT-IR neurons terminating at spinal cord levels T2-T4, and 35% of those terminating at levels T8-T9, derive from the dorsal (C2 and C3) medullary cell groups. Since the PNMT-IR projections are directed towards the intermediolateral cell column, it seems likely that all three groups of medullary adrenaline-containing neurons contribute to the regulation of sympathetic outflow.