Two different avian cold-sensitive sensory neurons: Transient receptor potential melastatin 8 (TRPM8)-dependent and -independent activation mechanisms.

Sensing the ambient temperature is an important function for survival in animals. Some TRP channels play important roles as detectors of temperature and irritating chemicals. There are functional differences of TRP channels among species. TRPM8 in mammals is activated by cooling compounds and cold temperature, but less information is available on the functional role of TRPM8 in avian species. Here we investigated the pharmacological properties and thermal sensitivities of chicken TRPM8 (cTRPM8) and cold-sensitive mechanisms in avian sensory neurons. In heterologously expressed cTRPM8, menthol and its derivative, WS-12 elicited [Ca2+]i increases, but icilin did not. In chicken sensory neurons, icilin increased [Ca2+]i, in a TRPA1-dependent manner. Icilin selectively stimulated heterologously expressed chicken TRPA1 (cTRPA1). Similar to mammalian orthologue, cTRPM8 was activated by cold. Both heterologous and endogenous expressed cTRPM8 were sensitive to mammalian TRPM8 antagonists. There are two types of cold-sensitive cells regarding menthol sensitivity in chicken sensory neurons. The temperature threshold of menthol-insensitive neurons was significantly lower than that of menthol-sensitive ones. The population of menthol-insensitive neurons was large in chicken but almost little in mammals. The cold-induced [Ca2+]i increases were not abolished by the external Ca2+ removal or by blockades of PLC-IP3 pathways and ryanodine channels. The cold stimulation failed to evoke [Ca2+]i increases after intracellular Ca2+ store-depletion. These results indicate that cTRPM8 acts as a cold-sensor similar to mammals. It is noteworthy that TRPM8-independent cold-sensitive neurons are abundant in chicken sensory neurons. Our results suggest that most of the cold-induced [Ca2+]i increases are mediated via Ca2+ release from intracellular stores and that these mechanisms may be specific to avian species.

Yamamoto A ，Takahashi K ，Saito S ，Tominaga M ，Ohta T ... -  《-》

Molecular identification and functional characterization of a temperature-sensitive transient receptor potential channel (TRPM8) from canine.

Liu Y ，Lubin ML ，Reitz TL ，Wang Y ，Colburn RW ，Flores CM ，Qin N ... -  《EUROPEAN JOURNAL OF PHARMACOLOGY》

Camphor activates and sensitizes transient receptor potential melastatin 8 (TRPM8) to cooling and icilin.

Selescu T ，Ciobanu AC ，Dobre C ，Reid G ，Babes A ... -  《-》

The roles of iPLA2, TRPM8 and TRPA1 in chemically induced cold hypersensitivity.

Gentry C ，Stoakley N ，Andersson DA ，Bevan S ... -  《Molecular Pain》

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

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.

Memon T ，Chase K ，Leavitt LS ，Olivera BM ，Teichert RW ... -  《-》