An ultrastructural study of the glycine transporter GLYT2 and its association with glycine in the superficial laminae of the rat spinal dorsal horn.

The glycine transporter GLYT2 is present in axonal boutons throughout the spinal cord, and its laminar distribution matches that of glycine-enriched axons, which are presumed to be glycinergic. In order to determine whether boutons which possess GLYT2 are glycine-enriched, we have carried out pre-embedding immunocytochemistry with antibody raised against GLYT2, and combined this with post-embedding detection of glycine, in the rat. GLYT2 immunoreactivity was present in boutons which formed symmetrical axodendritic, axosomatic or axoaxonic synapses, and was often seen in peripheral axons of type II synaptic glomeruli. One hundred and fifty GLYT2-immunoreactive boutons were analysed quantitatively, and in 142 (94.6%) of these the density of gold particles representing glycine-like immunoreactivity exceeded the background level (over presumed glutamatergic boutons) by at least a factor of two. Within immunoreactive boutons, the GLYT2 reaction product was associated with the plasma membrane, but often appeared as discrete clumps and was generally excluded from the region of the active sites of synapses. These results confirm that GLYT2 is associated with glycine-enriched axonal boutons in the superficial dorsal horn. They also suggest that GLYT2 is unevenly distributed on the plasma membrane of these boutons, and raise the possibility that it may be excluded from synaptic clefts.