Comparison of intracellular calcium signals evoked by heat and capsaicin in cultured rat dorsal root ganglion neurons and in a cell line expressing the rat vanilloid receptor, VR1

The cloning of the receptor for capsaicin, vanilloid receptor 1, has shown it to be non-selective cation channel with a high calcium permeability whic h can be opened by noxious heat as well as capsaicin. Here we compare the c alcium signals produced by native and recombinant capsaicin receptors when activated by either heat or capsaicin by imaging intracellular calcium leve ls ([Ca2+](i)) in rat dorsal root ganglion neurons and Chinese hamster ovar y cells transfected with the rat vanilloid receptor, vanilloid receptor 1. Vanilloid receptor 1 transfected cells and a subset of dorsal root ganglion neurons responded to both capsaicin and to heating to 50 degreesC with rap id, substantial and reversible rises in [Ca2+](i). All except one of the do rsal root ganglion neurons responsive to capsaicin also showed sensitivity to heat, and most, but not all, heat-sensitive neurons also responded to ca psaicin. Both capsaicin and heat responses were dependent on the presence o f extracellular Ca2+. Non-transfected Chinese hamster ovary cells and non-r esponsive dorsal root ganglion neurons showed only small rises in [Ca2+](i) in response to heat which did not depend on the presence of external Ca2+. Responsive dorsal root ganglion neurons and vanilloid receptor 1 transfect ed cells showed a clear temperature threshold, above which [Ca2+](i) increa sed rapidly. This was estimated to be 42.6 +/- 0.3 degreesC for vanilloid r eceptor 1 transfected cells and 42.0 +/- 0.6 degreesC for dorsal root gangl ion neurons. The competitive capsaicin antagonist capsazepine (10 muM) abol ished [Ca2+](i) increases stimulated by capsaicin in both dorsal root gangl ion neurons and vanilloid receptor 1 transfected cells. However, responses to heat of a similar magnitude in the same cells were inhibited by only 37% by capsazepine (10 muM). In vanilloid receptor 1 transfected cells, Ruthen ium Red (10 muM) blocked responses to both capsaicin and heat. These results demonstrate that imaging of [Ca2+](i) can identify dorsal roo t ganglion neurons which are responsive to both heat and capsaicin. They sh ow that heat and capsaicin responses mediated by native and recombinant cap saicin receptors are similar with respect to the characteristics and pharma cology examined, suggesting that expression of recombinant vanilloid recept or 1 in cell lines accurately reproduces the properties of the native recep tor. (C) 2001 IBRO, Published by Elsevier Science Ltd. All rights reserved.