Co-localization of substance P- and phenylethanolamine-N-methyltransferase-like immunoreactivity in neurons of ventrolateral medulla that project to the spinal cord: potential role in control of vasomotor tone.

Both substance P (SP)- and epinephrine-containing neurons in the rostral ventrolateral medulla have been thought to play a role in regulating vasomotor tone. The combination of retrograde transport of a fluorescent dye (Fast Blue) and immunofluorescent staining for SP- and phenylethanolamine-N-methyltransferase (PNMT)-immunoreactivity was used to determine the relationships of these two groups of ventrolateral medullary neurons which project to the spinal cord. The majority of spinally projecting neurons in the rostral ventrolateral medulla contain both PNMT-like and SP-like immunoreactivity. The presence of PNMT-immunoreactive material in a neuron implies that epinephrine is a probable neurotransmitter for such a cell. Earlier work demonstrated that epinephrine and SP have opposite effects on the firing of sympathetic preganglionic neurons. Our results raise the possibility of a novel mechanism of synaptic regulation of the sympathetic preganglionic vasomotor neurons.