GABAergic synapses on mu-opioid receptor-expressing neurons in the superficial dorsal horn: An electron microscope study in the cat spinal cord

A double-immunocytochemical electron microscope study was performed in the cat to examine whether GABAergic axons might be in synaptic contact with spinal neurons expressing mu-opioid receptor (MOR) in laminae I and II of the spinal dorsal horn at the lumbar cord segments. Structures showing MOR-like immunoreactivity (-LI) and those showing GABA-LI were labeled, respectively, with diaminobenzidine/peroxidase-reaction products and immunogold particles. Approximately one-third of dendritic profiles with MOR-LI in laminae I and II were postsynaptic to axon terminals with GABA-LI; about one-fourth of somatic profiles with MOR-LI were also postsynaptic to axon terminals with GABA-LI. The results suggest that activation of MOR on postsynaptic neurons may modulate effects which are induced by GABA released from presynaptic neurons. (C) 1997 Elsevier Science Ireland Ltd.