A cold- and menthol-activated current in rat dorsal root ganglion neurones: properties and role in cold transduction.
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 [Ca(2+)](i) on cooling; 49 % of them were also excited by 0.5 microM 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 Ca(2+) entry; a minor fraction of Ca(2+) 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 Ca(2+) modulated the cold- and menthol-activated current in a similar way to its action on intact cold receptors: lowered [Ca(2+)](o) sensitised the current, while raised [Ca(2+)](o) antagonised the menthol-induced sensitisation. During long cooling pulses the current showed adaptation, which depended on extracellular Ca(2+) and was mediated by a rise in [Ca(2+)](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 microM capsazepine and 100 microM 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.
Reid
Gordon
G
Department of Animal Physiology and Biophysics, Faculty of Biology, University of Bucharest, Splaiul Independenţei 91-95, 76201 Bucharest, Romania. gordon@biologie.kappa.ro
Babes
Alexandru
A
Pluteanu
Florentina
F
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002211:Capsaicin / Q000494:pharmacology; D003080:Cold Temperature; D004594:Electrophysiology; D005456:Fluorescent Dyes; D005727:Ganglia, Spinal / Q000187:drug effects / Q000502:physiology*; D007093:Imidazoles / Q000494:pharmacology; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D008610:Menthol / Q000737:chemistry / Q000494:pharmacology*; D020932:Nerve Growth Factor / Q000494:pharmacology; D009930:Organic Chemicals; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000502:physiology; D013237:Stereoisomerism; D013823:Thermoreceptors / Q000187:drug effects / Q000502:physiology*
D005456:Fluorescent Dyes; D007093:Imidazoles; D007473:Ion Channels; D009930:Organic Chemicals; C071336:calcium green; D008610:Menthol; D020932:Nerve Growth Factor; D012964:Sodium; C063159:1-(2-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenylethyl)-1H-imidazole; D002211:Capsaicin; D002118:Calcium