Diversity in the Neural Circuitry of Cold Sensing Revealed by Genetic Axonal Labeling of Transient Receptor Potential Melastatin 8 Neurons

Abstract

Sensory nerves detect an extensive array of somatosensory stimuli, including environmental temperatures. Despite activating only a small cohort of sensory neurons, cold temperatures generate a variety of distinct sensations that range from pleasantly cool to painfully aching, prickling, and burning. Psychophysical and functional data show that cold responses are mediated by both C-and A␦-fibers with separate peripheral receptive zones, each of which likely provides one or more of these distinct cold sensations. With this diversity in the neural basis for cold, it is remarkable that the majority of cold responses in vivo are dependent on the cold and menthol receptor transient receptor potential melastatin 8 (TRPM8). TRPM8-null mice are deficient in temperature discrimination, detection of noxious cold temperatures, injury-evoked hypersensitivity to cold, and nocifensive responses to cooling compounds. To determine how TRPM8 plays such a critical yet diverse role in cold signaling, we generated mice expressing a genetically encoded axonal tracer in TRPM8 neurons. Based on tracer expression, we show that TRPM8 neurons bear the neurochemical hallmarks of both C-and A␦-fibers, and presumptive nociceptors and non-nociceptors. More strikingly, TRPM8 axons diffusely innervate the skin and oral cavity, terminating in peripheral zones that contain nerve endings mediating distinct perceptions of innocuous cool, noxious cold, and first-and second-cold pain. These results further demonstrate that the peripheral neural circuitry of cold sensing is cellularly and anatomically complex, yet suggests that cold fibers, caused by the diverse neuronal context of TRPM8 expression, use a single molecular sensor to convey a wide range of cold sensations.

Variable Threshold of Trigeminal Cold-Thermosensitive Neurons Is Determined by a Balance between TRPM8 and Kv1 Potassium Channels

“…In addition, we find that many high-threshold CS neurons also express TRPM8 (i.e., cold responses were sensitized by menthol and inhibited by BCTC) and provide a molecular explanation for their difference in threshold (see below). There is now growing morphological (Takashima et al, 2007), functional (Bautista et al, 2007;Xing et al, 2007), and behavioral evidence that TRPM8 also participates in sensing cold as unpleasant or painful (Green and McAuliffe, 2000;Acosta et al, 2001;Wasner et al, 2004;Namer et al, 2005;Green and Schoen, 2007). In this regard, it is most relevant that deletion of TRPM8 in mice produces a marked reduction in the response of high-threshold cold-sensitive afferent fibers (Bautista et al, 2007), and reduces hyperalgesia to cold (Colburn et al, 2007).…”

“…One model contends that TRPM8 (transient receptor potential cation channel, subfamily M, member 8), a nonselective, calcium-permeable, cation channel that is activated by cooling and menthol (McKemy et al, 2002;Peier et al, 2002), is responsible for innocuous cold sensing, whereas a second TRP channel, TRPA1, is activated by noxious cold (Story et al, 2003;Sawada et al, 2007). However, the activation of TRPA1 by cold is controversial (Bautista et al, 2005;Reid, 2005;Zurborg et al, 2007), and some recent behavioral, structural, and genetic studies support the view that TRPM8 could be the principal determinant of the responsiveness of a thermoreceptor to innocuous and noxious cold (Bautista et al, 2007;Colburn et al, 2007;Dhaka et al, 2007;Green and Schoen, 2007;Takashima et al, 2007). Early functional studies characterizing low-and high-threshold coldsensitive neurons also documented a significant overlap in their properties, in particular, menthol sensitivity (Nealen et al, 2003;Thut et al, 2003).…”

Nociceptors: the sensors of the pain pathway

“…Whether TRPM8-mediated analgesia is dependent on peripheral and/or central sites of action is unknown but may be addressed now that TRPM8-expressing neurons and their peripheral and central fibers can be visualized by GFP expression driven by the TRPM8 promoter (76,77). Importantly, studies in humans and mice reveal species differences in pathways sensing innocuous cold: A-fiber block completely suppresses the cold response in humans (78), yet the majority of TRPM8-expressing fibers responsible for innocuous cold transduction in mice have small diameters (76,77) Noxious cold stimuli activate NSC currents and calcium influx (10,79) and decrease K + channel activity (80) and Na + /K + -ATPase function (65) ( Table 3). The temporal dissociation of the qualities of pain/ache vs. prickle/heat to noxious cold (3°C) suggest underlying differences in transduction mechanisms or information processing (66).…”

DrosophilaPainless Is a Ca2+-Requiring Channel Activated by Noxious Heat

“…TRPV1 expressed in C-fibers is activated by heat (Ͼ43°C) (Caterina et al, 1997), and TRPV1-null mice are defective in sensing noxious heat (Caterina et al, 2000). TRPM8, which senses cold temperatures (Ͻ27°C) (McKemy et al, 2002;Peier et al, 2002), is expressed in nociceptive and nonnociceptive neurons and its loss causes deficiencies in cold sensation (Bautista et al, 2007;Colburn et al, 2007;Dhaka et al, 2007Dhaka et al, , 2008Takashima et al, 2007). TRPA1 is expressed in C-fibers, mostly accompanying TRPV1 , and acts as a sensor for harmful stimuli, although its responsiveness to noxious cold is controversial (Jordt et al, 2004;Bautista et al, 2006;Kwan et al, 2006;Sawada et al, 2007).…”