N-methyl-D-aspartate-type glutamate receptors are found in post-synaptic targets of adrenergic terminals in the thoracic spinal cord

Abstract

Adrenergic (C1) neurons in the rostral ventrolateral medulla (RVL) are sympathoexcitatory and project directly to sympathetic preganglionic neurons (SPNs) in the thoracic spinal cord. C1 neurons also contain glutamate which may mediate the excitatory effects of RVL stimulation on SPNs through the N-methyl-D-aspartate (NMDA)-type receptor. Dual-labeling immunocytochemistry, combined with electron microscopy, was used to determine if NMDA receptors are located post-synaptic to adrenergic terminals in the spinal cord. Adrenergic terminals were labeled using an antibody directed against phenylethanolamine-N-methyl transferase (PNMT) and the NMDA receptor was identified using an antibody directed against the R1 subunit of the receptor (NMDAR1). NMDAR1 was found primarily in large and small dendrites and a few perikarya. The presence of NMDAR1 in the dendritic targets of PNMT-containing terminals was quantified in spinal cords sectioned either horizontally or coronally. PNMT-labeled terminals formed asymmetric synapses on dendrites containing immunogold labeling for NMDAR1, but NMDAR1 was more often detected in the targets of PNMT terminals when spinal cords were sectioned horizontally (59%) rather than coronally (28%). This difference in prevalence of NMDAR1 in targets of PNMT terminals is likely due to the preferential orientation of SPN dendrites in the horizontal plane, since longer dendritic shafts were visible in horizontal sections. When NMDAR1 was present in the dendritic targets of many adrenergic terminals, it was usually located at sites distal to the adrenergic input. We conclude that NMDA receptor ligands are likely to modulate the activity of dendritic targets of adrenergic terminals in the spinal cord, but are not closely associated with adrenergic synaptic input.