A novel type of cold-sensitive neuron in rat dorsal root ganglia with rapid adaptation to cooling stimuli.

Cold sensing in mammals is heterogeneous and more than one type of receptor molecule is likely to be involved in the transduction process. Most features of innocuous cold receptors have been explained by TRPM8, the cold and menthol receptor, but their fast adaptation to cooling has not yet been reproduced in cellular systems. In this study we have used a newly developed system for applying fast thermal stimuli to dissociated dorsal root ganglia (DRG) neurons from young rats (150-200 g) in primary culture. We describe a novel type of cold-sensitive rat DRG neuron with rapid adaptation to cooling. These cells (4.3% of the total DRG population) do not express either TRPM8 or the other cold-activated TRP channel, TRPA1, and the epithelial sodium channel (ENaC) is not involved in their transduction. Increases in intracellular calcium induced by cooling in rapidly adapting neurons are caused by calcium entry. These neurons express a large and rapidly adapting cold-induced inward current with a time constant of adaptation in the seconds range, and may correspond to the rapidly adapting cold receptors described in vivo.

Two populations of cold-sensitive neurons in rat dorsal root ganglia and their modulation by nerve growth factor.

Cold sensing in mammals is not completely understood, although significant progress has been made recently with the cloning of two cold-activated ion channels, TRPM8 and TRPA1. We have used rat DRG neurons in primary culture and calcium fluorimetry to identify distinct populations of cold-sensitive neurons, which may underlie different functions. Menthol sensitivity clearly separated two classes of cold-responding neurons. One group was menthol-sensitive (MS), was activated at warmer temperatures and responded faster and with a larger increase in intracellular calcium concentration during cooling; the fraction of MS neurons in culture and their cold sensitivity were both increased in the presence of nerve growth factor. Neurons in the menthol-insensitive (MI) group required stronger cooling for activation than MS cells and neither their proportion nor their cold sensitivity were significantly altered by nerve growth factor. The two groups of cold-sensitive neurons also had different pharmacology. A larger fraction of MS cells were capsaicin-sensitive and coexpression of menthol and capsaicin sensitivity was observed in the absence of NGF. MI neurons were not stimulated by the super-cooling agent icilin or by the irritant mustard oil. Taken together these findings support a picture in which TRPM8 is the major player in detecting gentle cooling, while TRPA1 does not seem to be involved in cold sensing by MI neurons, at least in the temperature range between 32 and 12 degrees C.