Immunocytochemical evidence for substance P and serotonin input to medullary bulbospinal adrenergic neurons

Using two-color immunoperoxidase staining combined with the retrograde transport of horseradish peroxidase injected into the rostral thoracic spinal cord, substance P-immunoreactive (SPI) and serotonin-immunoreactive (5HTI) varicosities have been observed in contiguity with medullary bulbospinal phenylethanolamine N-methyl transferase-immunoreactive (PNMTI) neurons of the C1, C2, and C3 cell groups. Since PNMTI terminals in the spinal cord are concentrated among sympathetic preganglionic neurons (SPGN) in the intermediolateral cell column, the close anatomical associations shown in the present study indicate that substance P- and serotonin-containing pathways in the medulla likely affect activity of SPGN via adrenergic bulbospinal neurons.