Interneuronal Inputs to Sympathetic Preganglionic Neurons: Evidence from Transected Spinal Cord

Abstract

Sympathetic preganglionic neurons (SPN) in the spinal cord are important for controlling the heart, blood vessels and release of catecholamines from the adrenal medulla. The activity of SPN is predominantly controlled by nerve pathways originating in the brain; but SPN also receive information from interneurons, which lie near them in the spinal cord and relay peripheral sensory or other central nervous input. Very little is known about the chemical phenotypes of the interneurons that synapse on SPN. Substance P (SP)- and enkephalin (ENK)-immunoreactive axons persist after spinal cord injury and small, neuropeptide Y (NPY)-expressing neurons occur in the spinal cord laminae containing interneurons antecedent to SPN. We therefore examined choline acetyltransferase (ChAT)-immunoreactive SPN caudal to a complete transection of the upper thoracic cord done 7 days earlier for evidence of synapses by nerve fibers containing these 3 neuropeptides. Seven days was chosen because virtually all input to SPN from rostral to the transection has degenerated by this time, leaving only input from intraspinal neurons. We found that all three types of nerve fibers formed synapses on the cell bodies and dendrites of ChAT-positive SPN. These results indicate that spinal interneurons containing SP, ENK and NPY provide synaptic input to SPN. Whether these neuropeptides occur in the same or different populations of interneurons and whether their synapses occur on barosensitive SPN remain to be determined.