Descending phenylethanolamine-N-methyltransferase projections to the monkey spinal cord: an immunohistochemical double labeling study.

In the present study, we determined that a population of spinally projecting neurons in the monkey brainstem also contained the enzyme phenylethanolamine-N-methyltransferase (PNMT). Following bilateral placements of horseradish peroxidase (HRP) in the cervical spinal cord, brainstem sections containing retrogradely labeled cells were immunohistochemically stained for PNMT. Single labeled PNMT-positive cells were found in a distinctive pattern in the dorsomedial and ventrolateral medulla. A population of double labeled cells was observed in the latter group only. This population was dispersed among other single labeled HRP and single labeled PNMT neurons. Possible functional roles of descending PNMT cells include involvement in sympathetic control of cardiovascular mechanisms and/or tonic descending inhibition of dorsal horn neurons.