Cooling from noxious heat to normal skin temperatures excites a subpopulation of cutaneous Aβ-fiber low-threshold mechanoreceptors

Sensing cooling temperatures is achieved by primary afferent endings located in the skin and is essential for the survival of animals. TRPM8 channels, primarily expressed in cutaneous C-fibers, have been established as receptors for cooling temperatures, sensing innocuous cooling from the normal skin temperature near 30°C to 17°C, and noxious cooling below 17°C. A cooling sensation is also felt when skin temperatures are first elevated to higher temperatures, for example, noxious heat, and then cool down to the normal skin temperature near 30°C. It is currently not clear what types of cutaneous afferent fibers are involved in sensing the cooling from a high heat to the normal skin temperature. Cutaneous Aβ-fiber low-threshold mechanoreceptors (Aβ-LTMRs) are primarily involved in the sense of touch and are thought to play no role in cooling sensation. In the present study, we conducted the opto-electrophysiological recordings from the skin-nerve preparations made from the hindpaw glabrous skin of Nav1.8-ChR2 transgenic mice. In these transgenic mice, Nav1.8-ChR2-negative Aβ-fiber mechanoreceptors are primarily Aβ-LTMRs, and Nav1.8-ChR2-positive Aβ-fiber mechanoreceptors are mainly high-threshold mechanoreceptors (Aβ-HTMRs). Neither Aβ-LTMRs nor Aβ-HTMRs responded to temperature rising from 30°C to the noxious heat of 43°C. However, a subpopulation of Aβ-LTMRs, but not Aβ-HTMRs, robustly fires action potential impulses in response to the temperature drop from 43°C to 30°C. This finding reveals for the first time that a subpopulation of Aβ-LTMRs senses the cooling for a temperature drop from noxious heat to normal skin temperature.