In vivo patch-clamp analysis of the thermal response in VR-1 knockout mouse

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Recently a capsaicin receptor named VR-1 has been cloned and further clarified to respond to noxious heat and proton, and turned out to be one of the TRP family. However, the physiological significance of the receptor in in vivo has not been elucidated yet. To clarify the function of the receptor in in vivo condition, we need an in vivo patch-clamp recording method which enables us to evaluate receptor's function under the physiological conditions. Whole cell patch-clamp recordings were made from substantia gelatinosa (SG) neurons that received predominantly C afferent fibers in the mouse spinal cord in vivo. Unexpectedly, none of SG neurons tested responded to noxious heat, in spite of the densest termination of C afferents from the skin. Anatomical data has reported that a subpopulation of deep dorsal horn neurons extends their dendrites to the SG and receives inputs from C afferent fibers. Thus, we next made recordings from deep dorsal horn neurons and analyzed a heat response. As expected, about 20 % of neurons responded to noxious heat stimulation with increasing a barrage of fast EPSCs. However, no slow synaptic current was observed in all neurons tested, suggesting that the heat sensation is transmitted by a release of glutamate onto the dendrites of deep dorsal horn neurons. We further confirmed whether SG neurons received C afferent inputs expressing TRPV1 receptors by application of capsaicin during the recording from SG neurons in the spinal cord slices. Application of capsaicin elicited a barrage of mEPSCs, indicating that all SG neurons received C afferent inputs which were endowed the TRPV1 receptor. This clear discrepancy is required to be resolved.