GABA and glycine in synaptic glomeruli of the rat spinal dorsal horn

The superficial dorsal horn of rat spinal cord contains two types of synaptic glomerulus, which are centred around the terminals of unmyelinated and myelinated primary afferents respectively. Both types of glomerulus contain GABAergic axons and dendrites, which are thought to originate from local inhibitory interneurons. Some of the dendrites contain synaptic vesicles, and may be presynaptic to the central axon of dendroaxonic synapses. In order to determine whether GABAergic structures in the two types of glomerulus are derived from different populations of interneurons, a quantitative post-embedding immunogold study of GABA- and glycine-immunoreactivity in a rat dorsal horn was performed. In type I glomeruli, all of the peripheral axons and most vesicle-containing dendrites were GABA-immunoreactive, but only one of 32 axons and none of the vesicle-containing dendrites was glycine-immunoreactive. In contrast, most of the peripheral axons and some of the vesicle-containing dendrites in type II glomeruli possessed both GABA- and glycine-immunoreactivity. This strongly suggests that different types of inhibitory interneuron in the dorsal horn are associated with the two types of glomerulus. It is therefore likely that different populations of interneurons mediate presynaptic inhibition of unmyelinated and myelinated primary afferents.