The vanilloid receptor (VR1) protein functions both as a receptor for capsaicin and a transducer of noxious thermal stimuli. To determine the expression and targetting of this protein, we have generated antisera against both the amino and carboxy termini of VR1. Within the dorsal root and trigeminal ganglia of rats, VR1-immunoreactivity (VR1-ir) was restricted to small and medium sized neurons. VR1-ir was transported into both the central and peripheral processes of these primary afferent neurons, as evidenced by: (i) the presence of VR1-ir in nerve fibres and terminals in lamina I and lamina II of the superficial dorsal horn, and the association of VR1-ir with small diameter nerve fibres in the skin and cornea; (ii) the reduction of VR1-ir in the spinal cord after dorsal rhizotomy; and (iii) the accumulation of VR1-ir proximal to sciatic nerve ligation. At the ultrastructural level, VR1-ir was associated with plasma membranes of neuronal perikarya in dorsal root ganglia and nerve terminals in the dorsal horn. VR1-ir was also seen in nerve fibres and terminals in the spinal trigeminal nucleus and nucleus of the solitary tract. Within a large proportion of dorsal root ganglion neurons and the terminals of their axons, VR1-ir was colocalized with staining for the P2X3 purinoceptor, and with binding sites for the lectin IB4. Surprisingly, VR1-ir did not coexist substantially in nerve fibres and terminals that contain substance P and calcitonin gene-related peptide, suggesting complex mechanisms for the release of these neuropeptides in response to capsaicin application.

The cloned vanilloid receptor VR1 has attracted recent attention as a molecular integrator of painful stimuli on primary sensory neurons. The existence of vanilloid-sensitive neurons in the brain is, however, controversial. In this study, we have used an antibody and a complementary RNA probe to explore the distribution of neurons that express VR1 in rat and in certain areas of human brain. In the rat, we observed VR1-expressing neurons throughout the whole neuroaxis, including all cortical areas (in layers 3 and 5), several members of the limbic system (e.g., hippocampus, central amygdala, and both medial and lateral habenula), striatum, hypothalamus, centromedian and paraventricular thalamic nuclei, substantia nigra, reticular formation, locus coeruleus, cerebellum, and inferior olive. VR1-immunopositive cells also were found in the third and fifth layers of human parietal cortex. Reverse transcription-PCR performed with rat VR1-specific primers verified the expression of VR1 mRNA in cortex, hippocampus, and hypothalamus. In the central nervous system, neonatal capsaicin treatment depleted VR1 mRNA from the spinal nucleus of the trigeminal nerve, but not from other areas such as the inferior olive. The finding that VR1 is expressed not only in primary sensory neurons but also in several brain nuclei is of great importance in that it places VRs in a much broader perspective than pain perception. VRs in the brain (and putative endogenous vanilloids) may be involved in the control of emotions, learning, and satiety, just to name a few exciting possibilities.