Ultrastructural evidence for GABA-mediated disinhibitory circuits in the spinal cord of the cat.

The synaptic relationships between gamma-aminobutyric acid (GABA)-immunoreactive and enkephalin-immunoreactive profiles in the cat spinal cord were examined using combined pre-embedding immunoperoxidase and post-embedding immunogold electron microscopic immunocytochemistry. Although colchicine was not used, enkephalin-immunoreactive somata and dendrites were detected in regions associated with nociceptive transmission, including laminae I, II, V and X. In each of these laminae, many GABA-immunoreactive terminals were found presynaptic to enkephalin-immunoreactive cell bodies and dendrites. We propose that disinhibition of opioid-containing neurons may be a common feature of pain-related circuits in the cat spinal cord.