Cold sensing by Na1.8-positive and Na1.8-negative sensory neurons.

The ability to detect environmental cold serves as an important survival tool. The sodium channels Na1.8 and Na1.9, as well as the TRP channel Trpm8, have been shown to contribute to cold sensation in mice. Surprisingly, transcriptional profiling shows that Na1.8/Na1.9 and Trpm8 are expressed in nonoverlapping neuronal populations. Here we have used in vivo GCaMP3 imaging to identify cold-sensing populations of sensory neurons in live mice. We find that ∼80% of neurons responsive to cold down to 1 °C do not express Na1.8, and that the genetic deletion of Na1.8 does not affect the relative number, distribution, or maximal response of cold-sensitive neurons. Furthermore, the deletion of Na1.8 had no observable effect on transient cold-induced (≥5 °C) behaviors in mice, as measured by the cold-plantar, cold-plate (5 and 10 °C), or acetone tests. In contrast, nocifensive-like behavior to extreme cold-plate stimulation (-5 °C) was completely absent in mice lacking Na1.8. Fluorescence-activated cell sorting (FACS) and subsequent microarray analysis of sensory neurons activated at 4 °C identified an enriched repertoire of ion channels, which include the Trp channel Trpm8 and potassium channel Kcnk9, that are potentially required for cold sensing above freezing temperatures in mouse DRG neurons. These data demonstrate the complexity of cold-sensing mechanisms in mouse sensory neurons, revealing a principal role for Na1.8-negative neurons in sensing both innocuous and acute noxious cooling down to 1 °C, while Na1.8-positive neurons are likely responsible for the transduction of prolonged extreme cold temperatures, where tissue damage causes pan-nociceptor activation.