Vanilloid receptor VR1-positive primary afferents are glutamatergic and contact spinal neurons that co-express neurokinin receptor NK1 and glutamate receptors

The vanilloid receptor VR1 (TRPV1) is a temperature- and capsaicin-sensitive cation channel expressed by a class of primary afferents involved in nociception. To confirm the hypothesis that VR1-positive primary afferents are glutamatergic and contact spinal neurons that express the main classes of ionotropic glutamate receptors, we performed multiple immunofluorescent staining for VR1 and the glutamate transporter VGLUT2 (a specific marker for glutamatergic transmission) or AMPA and NMDA receptor subunits. VR1-positive cells in the dorsal root ganglion and boutons of their central afferent fibers in the dorsal horn expressed VGLUT2, and the latter contacted AMPA- or NMDA receptor-positive perikarya. Based on our previous observations of preferential targeting of VR1-positive primary afferents to spinal neurons that express the neurokinin receptor NK1 (Hwang et al., 2003), we further quantified the frequency of termination of VR1-positive afferents onto NK1-positive neurons co-expressing glutamate receptors. A larger fraction of NK1/NMDA receptors-positive than NK1/AMPA receptors-positive sites were contacted by VR1-positive boutons. We conclude that VR1-positive primary afferents in the rat use glutamate as neurotransmitter and contact postsynaptic sites that co-express NK1 and ionotropic glutamate receptors.