TRPA1 expression levels and excitability brake by K V channels influence cold sensitivity of TRPA1-expressing neurons

Abstract

The molecular sensor of innocuous (painless) cold sensation is well-established to be Transient Receptor Potential cation channel, subfamily M, member 8 (TRPM8). However, the role of Transient Receptor Potential cation channel, subfamily A, member 1 (TRPA1) in noxious (painful) cold sensation has been controversial. We find that TRPA1 channels contribute to the noxious cold sensitivity of mouse somatosensory neurons, independent of TRPM8 channels, and that TRPA1-expressing neurons are largely non-overlapping with TRPM8-expressing neurons in mouse dorsal-root ganglia (DRG). However, relatively few TRPA1-expressing neurons (e.g., responsive to allyl isothiocyanate or AITC, a selective TRPA1 agonist) respond overtly to cold temperature in vitro, unlike TRPM8-expressing neurons, which almost all respond to cold. Using somatosensory neurons from TRPM8 −/− mice and subtype-selective blockers of TRPM8 and TRPA1 channels, we demonstrate that responses to cold temperatures from TRPA1-expressing neurons are mediated by TRPA1 channels. We also identify two factors that affect the cold-sensitivity of TRPA1-expressing neurons: 1) Cold-sensitive AITC-sensitive neurons express relatively more TRPA1 transcripts than cold-insensitive AITC-sensitive neurons and 2) voltage-gated potassium (KV) channels attenuate the cold-sensitivity of some TRPA1-expressing neurons. The combination of these two factors, combined with the relatively weak agonist-like activity of cold temperature on TRPA1 channels, partially explains why few TRPA1-expressing neurons respond to cold. Blocking KV channels also reveals another subclass of noxious cold-sensitive DRG neurons that do not express TRPM8 or TRPA1 channels. Altogether, the results of this study provide novel insights into the cold-sensitivity of different subclasses of somatosensory neurons.

Molecular Prerequisites for Diminished Cold Sensitivity in Ground Squirrels and Hamsters

“…We wondered whether the remaining cold sensitivity in squirrels is dictated by TRPA1, a polymodal ion channel which is expressed in a distinct neuronal population from TRPM8, and which was proposed to contribute to cold responses (del Camino et al 2010, Memon et al 2017). We therefore cloned TRPA1 from squirrel DRG and tested its temperature and chemical sensitivity.…”

“…Consistently, TRPM8-deficient mice show a significant reduction, but not a complete elimination, of cold-evoked responses at the behavioral, cellular and nerve fiber levels (Bautista et al 2007, Dhaka et al 2007, Milenkovic et al 2014). The residual cold responses are attributed to the presence of additional cold-activated ion channels in TRPM8 neurons, as well as non-TRPM8 cold receptors (Knowlton et al 2013, Memon et al 2017, Pogorzala et al 2013, Zimmermann et al 2007, Lolignier et al 2015). We focused on TRPM8-expressing neurons and show that these cells are present in squirrel and hamster DRG at proportions identical to mice, ruling out insufficient number of cold receptors as the cause of the observed behavioral phenotype.…”

“…Squirrels and hamsters remain sensitive to cooling below 10°C, suggesting the presence of a TRPM8-independent mechanism of cold detection. A number of molecules were suggested for this role, including TRPA1 and the voltage-gated sodium channels Na v 1.8 and Na v 1.9 (Lolignier et al 2015, Memon et al 2017, Zimmermann et al 2007). Our data show that squirrel TRPA1 is not activated by temperature within the 10°C–22°C range, arguing against its role in cold detection in squirrels, although we were not able to test its activity at temperatures below 10°C.…”

TRESK background K+ channel deletion selectively uncovers enhanced mechanical and cold sensitivity

“…; Memon et al . ). It is possible that TRESK might exert a similar effect, hence silencing cold responses in normal conditions.…”

“…By contrast, it seems unlikely that enhanced cold sensitivity in TRESK KO animals is related to the observed decrease in TRPA1 activity. K v channels can blunt cold responses by opposing depolarizing stimuli in TRPA1 + and TRPM8 + neurons (Madrid et al 2009;Memon et al 2017). It is possible that TRESK might exert a similar effect, hence silencing cold responses in normal conditions.…”