We examined immunohistochemically whether the vesicular glutamate transporters (VGluTs), VGluT1 and VGluT2, might be expressed in synaptic terminals of nociceptive primary afferent fibers within laminae I and II of the medullary and spinal dorsal horns of the rat. VGluT1 immunoreactivity (IR) was intense in the inner part of lamina II but weak in lamina I and the outer part of lamina II. VGluT2-IR was most intense in lamina I and the outer part of lamina II. Expression of VGluTs in synaptic terminals was confirmed by dual immunofluorescence histochemistry for VGluTs and synaptophysin. Expression of VGluTs in axon terminals of primary afferent fibers terminating in laminae I and II was also confirmed immunohistochemically after unilateral dorsal rhizotomy. The dual immunofluorescence histochemistry indicated expression of VGluTs in substance P (SP)-containing axon terminals in lamina I and the outer part of lamina II. Electron microscopy confirmed the coexpression of VGluTs and SP in axon terminals within laminae I and II; VGluTs was associated with round synaptic vesicles at the asymmetric synapses. It was further observed that isolectin IB4, a marker for unmyelinated axons, often bound with VGluT2-immunopositive structures but rarely with VGluT1-immunopositive structures in lamina II. Thus, the results indicated in laminae I and II of the medullary and spinal dorsal horns that both VGluT1 and VGluT2 were expressed in axon terminals of primary afferent fibers, including SP-containing nociceptive fibers and that VGluT in unmyelinated primary afferent fibers terminating in lamina II was primarily VGluT2.
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