A Sensory-Labeled Line for Cold: TRPM8-Expressing Sensory Neurons Define the Cellular Basis for Cold, Cold Pain, and Cooling-Mediated Analgesia

Neurobiology Section, Department of Biological Sciences, Neuroscience Graduate Program, Neurobiology Graduate Program, and Molecular and Computational Biology Graduate Program, University of Southern California, Los Angeles, California 90089.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 02/2013; 33(7):2837-48. DOI: 10.1523/JNEUROSCI.1943-12.2013
Source: PubMed


Many primary sensory neurons are polymodal, responding to multiple stimulus modalities (chemical, thermal, or mechanical), yet each modality is recognized differently. Although polymodality implies that stimulus encoding occurs in higher centers, such as the spinal cord or brain, recent sensory neuron ablation studies find that behavioral responses to different modalities require distinct subpopulations, suggesting the existence of modality-specific labeled lines at the level of the sensory afferent. Here we provide evidence that neurons expressing TRPM8, a cold- and menthol-gated channel required for normal cold responses in mammals, represents a labeled line solely for cold sensation. We examined the behavioral significance of conditionally ablating TRPM8-expressing neurons in adult mice, finding that, like animals lacking TRPM8 channels (Trpm8(-/-)), animals depleted of TRPM8 neurons ("ablated") are insensitive to cool to painfully cold temperatures. Ablated animals showed little aversion to noxious cold and did not distinguish between cold and a preferred warm temperature, a phenotype more profound than that of Trpm8(-/-) mice which exhibit only partial cold-avoidance and -preference behaviors. In addition to acute responses, cold pain associated with inflammation and nerve injury was significantly attenuated in ablated and Trpm8(-/-) mice. Moreover, cooling-induced analgesia after nerve injury was abolished in both genotypes. Last, heat, mechanical, and proprioceptive behaviors were normal in ablated mice, demonstrating that TRPM8 neurons are dispensable for other somatosensory modalities. Together, these data show that, although some limited cold sensitivity remains in Trpm8(-/-) mice, TRPM8 neurons are required for the breadth of behavioral responses evoked by cold temperatures.

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Available from: Wendy M Knowlton, Aug 12, 2015
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    • "Mice lacking TRPM8 exhibited decreased sensitivity to cold surfaces that are normally avoided, as assessed by temperature preference and acetone-evoked flinch tests (Bautista et al. 2007; Colburn et al. 2007; Dhaka et al. 2007). Cold avoidance and icilin sensitivity were also reduced or absent in TRPM8 conditionally ablated mice, even though these animals appeared to have normal heat and mechanical pain sensitivity (Knowlton et al. 2013). In agreement with these findings, global knockout mice lacking both TRPM8 and TRPA1 were no different than knockout mice lacking only TRPM8 in their avoidance of innocuous and noxious cold stimuli (Knowlton et al. 2010). "
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    ABSTRACT: Introduction: The present paper summarizes research using animal models to investigate the roles of thermosensitive transient receptor potential (TRP) channels in somatosensory functions including touch, temperature and pain. We present new data assessing the effects of eugenol and carvacrol, agonists of the warmth-sensitive TRPV3, on thermal, mechanical and pain sensitivity in rats. Methods: Thermal sensitivity was assessed using a thermal preference test, which measured the amount of time the animal occupied one of two adjacent thermoelectric plates set at different temperatures. Pain sensitivity was assessed as an increase in latency of hindpaw withdrawal away from a noxious thermal stimulus directed to the plantar hindpaw (Hargreaves test). Mechanical sensitivity was assessed by measuring the force exerted by an electronic von Frey filament pressed against the plantar surface that elicited withdrawal. Results: Topical application of eugenol and carvacrol did not significantly affect thermal preference, although there was a trend toward avoidance of the hotter surface in a 30 vs. 45°C preference test for rats treated with 1 or 10% eugenol and carvacrol. Both eugenol and carvacrol induced a concentration-dependent increase in thermal withdrawal latency (analgesia), with no significant effect on mechanosensitivity. Conclusions: The analgesic effect of eugenol and carvacrol is consistent with previous studies. The tendency for these chemicals to increase the avoidance of warmer temperatures suggests a possible role for TRPV3 in warmth detection, also consistent with previous studies. Additional roles of other thermosensitive TRP channels (TRPM8 TRPV1, TRPV2, TRPV4, TRPM3, TRPM8, TRPA1, TRPC5) in touch, temperature and pain are reviewed.
    Chemosensory Perception 03/2015; 8(2). DOI:10.1007/s12078-015-9176-9 · 1.30 Impact Factor
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    • "In behavioural tests , temperature deficit is evident only at mildly cooling range , and becomes insignificant at temperatures below 10°C . At the same time , obliteration of the neuronal line which gives rise to TRPM8 - positive neurons ( Mishra et al . 2011 ) or targeted ablation of TRPM8 neurons in adult mice ( Knowlton et al . 2013 ; Pogorzala et al . 2013 ) eliminates cold sensitivity in the full testable range . These data strongly suggest that TRPM8 - positive neurons contain an unidentified TRPM8 - independent molecular mechanism responsible for the detection of mildly and severely noxious cold . A growing body of evidence supports the notion that in addition "
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    ABSTRACT: Thermal perception is a fundamental physiological process pertaining to the vast majority of organisms. In vertebrates, environmental temperature is detected by the primary afferents of the somatosensory neurons in the skin, which express a 'choir' of ion channels tuned to detect particular temperatures. Nearly two decades of research have revealed a number of receptor ion channels that mediate the perception of several temperatures ranges, but most still remain molecularly orphaned. Yet even within this well-researched realm, most of our knowledge largely pertains to two closely related species of rodents-mice and rats. While these are standard biomedical research models, mice and rats provide a limited perspective to elucidate the general principles that drive somatosensory evolution. In recent years, significant advances have been made in understanding the molecular mechanism of temperature adaptation in evolutionarily distant vertebrates and in organisms with acute thermal sensitivity. These studies have revealed the remarkable versatility of the somatosensory system and highlighted adaptations at the molecular level, which often include changes in biophysical properties of ion channels from the transient receptor potential family. Exploiting non-standard animal models has the potential to provide unexpected insights into general principles of thermosensation and thermoregulation, unachievable using the rodent model alone. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.
    The Journal of Physiology 11/2014; 593(16). DOI:10.1113/jphysiol.2014.280446 · 5.04 Impact Factor
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    • "CA stimulates TRPA1 + fibres of the thermal nociceptive system while L-menthol sensitizes the principal innocuous cold receptor TRPM8 (Namer et al., 2005). However, administration of L-menthol + CA did not, as hypothesized, further aggravate cold allodynia supporting the notion of a TRPM8 + labelled line for both noxious and innocuous cold perception, as recently suggested by Knowlton et al. (2013). Some uncertainties adhere to this result. "
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    ABSTRACT: Activation of TRPM8 and TRPA1 receptors generates cold and cold pain sensations, respectively, and is presumably important in clinical pain manifestations, such as cold hyperalgesia. This study investigated the interaction between TRPM8 and TRPA1 receptors through stimulation of glabrous human skin (volar forearm) by topical administration of 40% L-menthol and 10% trans-cinnamaldehyde (CA), individually and in combination. Sensory manifestations were assessed in 10 healthy volunteers via a platform of 11 quantitative sensory (thermal and mechanical stimuli) and vasomotor tests (skin temperature, perfusion and axon-reflex-flare) in a double-blinded randomized crossover design. Cold pain threshold was increased (p < 0.01, cold allodynia) by L-menthol alone and L-menthol + CA in combination but unaffected by CA. Mechanical pain threshold was significantly decreased (mechanical hyperalgesia) by all three substances (p < 0.01), with a significant intergroup difference found between CA alone and the less decreased L-menthol + CA (p < 0.05). Application of CA alone and L-menthol + CA in combination showed an increase in skin temperature and perfusion significantly larger than that induced by L-menthol alone (p < 0.05). An axon-reflex-flare was present after CA administration, but was significantly reduced upon addition of L-menthol (p < 0.01). This study elucidates the potential of L-menthol as a counter-irritant to secondary neurogenic inflammation and provides evidence of an intricate interplay between cold receptors TRPA1 and TRPM8, warranting further investigation of the neural coding of cold pain perception.
    European journal of pain (London, England) 10/2014; 18(9). DOI:10.1002/j.1532-2149.2014.494.x · 2.93 Impact Factor
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