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Received May 17, 2002
Accepted after revision September 17, 2002
1 Department of Animal Physiology and Biophysics, Faculty of Biology, University of Bucharest, Splaiul Independentei 91-95, 76201 Bucharest, Romania
* To whom correspondence should be addressed. E-mail: gordon{at}biologie.kappa.ro.
Skin temperature is sensed by peripheral thermoreceptors. Using the neuronal soma in primary culture as a model of the receptor terminal, we have investigated the mechanisms of cold transduction in thermoreceptive neurones from rat dorsal root ganglia. Cold-sensitive neurones were pre-selected by screening for an increase in [Ca2+]i on cooling; 49 % of them were also excited by 0.5 µM capsaicin. Action potentials and voltage-gated currents of cold-sensitive neurones were clearly distinct from those of cold-insensitive neurones. All cold-sensitive neurones expressed an inward current activated by cold and sensitised by (-)-menthol, which was absent from cold-insensitive neurones. This current was carried mainly by Na+ ions and caused a depolarisation on cooling accompanied by action potentials, inducing voltage-gated Ca2+ entry; a minor fraction of Ca2+ entry was voltage-independent. Application of (-)-menthol shifted the threshold temperatures of the cold-induced depolarisation and the inward current to the same extent, indicating that the cold- and menthol-activated current normally sets the threshold temperature for depolarisation during cooling. The action of menthol was stereospecific, with the (+)-isomer being a less effective agonist than the (-)-isomer. Extracellular Ca2+ modulated the cold- and menthol-activated current in a similar way to its action on intact cold receptors: lowered [Ca2+]o sensitised the current, while raised [Ca2+]o antagonised the menthol-induced sensitisation. During long cooling pulses the current showed adaptation, which depended on extracellular Ca2+ and was mediated by a rise in [Ca2+]i. This adaptation consisted of a shift in the temperature sensitivity of the channel. In capsaicin-sensitive neurones, capsaicin application caused a profound depression of the cold-activated current. Inclusion of nerve growth factor in the culture medium shifted the threshold of the cold-activated current towards warmer temperatures. The current was blocked by 50 µM capsazepine and 100 µM SKF 96365. We conclude that the cold- and menthol-activated current is the major mechanism responsible for cold-induced depolarisation in DRG neurones, and largely accounts for the known transduction properties of intact cold receptors.
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