The research aims at elucidating the role of vanilloid receptors (VR) in the mediation of nociception in the dorsal horn of mammals. Immunocytochemistry of the VR1 receptor has shown that this is expressed mainly in small cells of dorsal root ganglia and in afferents to the superficial laminae of the dorsal horn. VR1-immunopositive terminals predominate in lamina I and are present in lamina II. In this lamina, however, most of the staining for VR1 is in dendrites postsynaptic to primary afferents. It was also established that VR1 terminals do not contain substance P (SP). Thus, two main classes of afferents may mediate nociception and the question arises whether they contact the same neurons. This will be established by studying, with confocal microscopy and electron microscopy, whether VR1- and SP-immunopositive terminals contact the same neuron expressing neurokinin receptors (NK1) in lamina I. It is predicted that, if VR1-positive afferents mediate nociception, these neurons will project to the parabrachial nucleus, a major destination for ascending nociceptive pathways in rodents. The research will also establish what glutamate receptor subunits are present at VR1-positive synapses in superficial laminae. In particular, the absence of NMDA receptor would signify a clearly different role for these synapses from that of afferents that release SP. Also this part of the research will require simultaneous multiple labeling at the light and electron microscopic level.
The third aim of the research is to test whether postsynaptic VR1 is related to activity in C fibers, independently from its origin, i.e. from intrinsic neurons or via transsynaptic traffic. The total depletion of VR1 in the dorsal horn after dorsal rhizotomy suggests an activity dependency of postsynaptic VR1 and this will be tested by conduction block of dorsal roots and, conversely, by selective activation of C fibers with cutaneous application of mustard oil.
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