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

Abstract

Many primary sensory neurons are polymodal, responding to multiple stimulus modalities (chemical, thermal, or mechanical), yet each modality is recognized differently. While 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+ 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. Lastly, 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 while some limited cold sensitivity remains in Trpm8−/− mice, TRPM8 neurons are required for the breadth of behavioral responses evoked by cold temperatures.

Inflammatory and neuropathic cold allodynia are selectively mediated by the neurotrophic factor receptor GFRα3

“…WT mice show robust cold allodynia 2 d after unilateral injections of the inflammatory agent complete Freund's adjuvant (CFA) (Fig. 1A) (P < 0.01, pre-vs. post-CFA or ipsilateral vs. contralateral), which we and others have previously reported (22)(23)(24). In contrast, Gfrα3 −/− mice show no differences in their hind paw lift latencies between the ipsilateral (inflamed) and the contralateral (control) sides, and there were no differences in their sensitivity compared with the basal, preinflamed state (Fig.…”

“…1B) (P < 0.01, preinjury vs. 7 d postinjury; P < 0.001, ipsilateral vs. contralateral), but cold sensitivity was remarkably unchanged in Gfrα3 −/− mice (ipsilateral vs. contralateral; preinjury vs. 7 d postinjury; P > 0.05). Lastly, one of the major side effects of platin-based chemotherapeutics is cold pain (26), a phenotype that can be modeled in mice given a single systemic injection of oxaliplatin (22,23). As with the previous pain models, the cold allodynia observed in WT mice (P < 0.001, basal vs. 7 d postinjection) was completely absent in mice null for GFRα3 ( Fig.…”

“…Artemin-and NGF-induced sensitization of cold responses is TRPM8-dependent, and multiple studies have shown that TRPM8 plays a role in the development of cold allodynia (8,(22)(23)(24)57). However, inflammatory cold allodynia is also diminished by both an TRPA1 antagonist and reduced TRPA1 transcript expression, and cold hypersensitivity can be induced by TRPA1 agonism in WT but not Trpa1 −/− mice (58, 59).…”

“…Adult WT or GFRα3 −/− mice (a gift from Brian Davis, University of Pittsburgh, Children's Hospital of Pittsburgh, Pittsburgh, PA) of both sexes were used in behavioral assays. Cold, heat, and mechanical sensitivity were assayed as described (19,22).…”

“…Next, to test the role of GFRα3 in pathological cold pain, we examined adult WT and Gfrα3 −/− littermates in classical models of inflammation, nerve injury, and chemotherapeutic-induced neuropathic pain (22,23). WT mice show robust cold allodynia 2 d after unilateral injections of the inflammatory agent complete Freund's adjuvant (CFA) (Fig.…”