Synaptic interaction between GABAergic terminals and substance P receptor-positive neurons in rat spinal superficial laminae

AIM: To identify synaptic relationship between γ-aminobutyric acid (GABA)-containing terminals and substance P receptor-positive neurons in the superficial dorsal horn of rat spinal cord. METHODS: A combination of preembedding immunoperoxidase staining for substance P receptor with postembedding immunogold staining for GABA was employed. RESULTS: Intense substance P receptor-expressing neurons were concentrated in medial half of the lamina I and the outer lamina II of the spinal cord. Under electron microscopy, substance P receptor immunoreactivity was found mainly in perikarya and dendrites, and deposited along neuronal membranes, in Golgi complex, in rough endoplasmic reticulum and on the surface of mitochondria with patch- or granular-like staining. The double-labeling studies revealed that substance P receptor-expressing dendrites received symmetric synaptic contacts from axonal terminals labeled with immunogold particles indicating GABA. GABA also co-localized with substance P receptors in the dendrites. CONCLUSION: The synaptic contact between GABA-containing terminal and substance P receptor-expressing neuron provided important morphological evidence for previous pharmacological studies concerning antinociceptive function of GABA in the spinal cord.