Synaptic organization of substance P, glutamate and GABA-immunoreactive boutons on functionally identified neurons in...

In order to determine how nociceptive input conveyed by the C-fibers terminating in superficial laminae of the spinal cord reaches the wide dynamic range (WDR) cells in deeper dorsal horn, which functions as ascending projection pathway, the morphological features of some WDR cells in the deeper dorsal horn of the cat lumbar spinal cord were studied by intracellular injection of horseradish peroxidase and physiological characterization. One of the fully stained neurons with somata in lamina V and dendrites that entered lamina II were examined by electron microscopy. Immunogold staining of ultrathin sections through the labeled proximal dendrites in lamina II revealed that these dendrites received numerous synapses from substance P and glutamate immunoreactive (IR) axons, which were considered originating from C-fibers. In addition, many GABA-IR terminals were found presynaptic to the labeled dendrites. The results, therefore, suggest that the information carried by primary afferent can be sent from the superficial dorsal horn to the deeper laminae through monosynaptic contacts between C-fiber terminals and the long dorsal dendrites of some WDR cells in the deeper laminae, and that GABAergic system is involved in postsynaptic control to modulate the transmission of nociceptive sensory information.