pyrimidinones and Hyperesthesia

The cation channel TRPM8 plays a central role in the somatosensory system, as a key sensor of innocuously cold temperatures and cooling agents. Although increased functional expression of TRPM8 has been implicated in various forms of pathological cold hypersensitivity, little is known about the cellular and molecular mechanisms that determine TRPM8 abundance at the plasma membrane. Here we demonstrate constitutive transport of TRPM8 towards the plasma membrane in atypical, non-acidic transport vesicles that contain lysosomal-associated membrane protein 1 (LAMP1), and provide evidence that vesicle-associated membrane protein 7 (VAMP7) mediates fusion of these vesicles with the plasma membrane. In line herewith, VAMP7-deficient mice exhibit reduced functional expression of TRPM8 in sensory neurons and concomitant deficits in cold avoidance and icilin-induced cold hypersensitivity. Our results uncover a cellular pathway that controls functional plasma membrane incorporation of a temperature-sensitive TRP channel, and thus regulates thermosensitivity in vivo.

Topics: Animals; Calcium; Cell Membrane; Cold Temperature; Female; Ganglia, Spinal; HEK293 Cells; Humans; Hyperesthesia; Lysosomal Membrane Proteins; Mice; Mice, Inbred C57BL; Mice, Knockout; Microscopy, Fluorescence; Patch-Clamp Techniques; Pyrimidinones; R-SNARE Proteins; Reverse Transcriptase Polymerase Chain Reaction; Sensory Receptor Cells; Transport Vesicles; Trigeminal Ganglion; TRPM Cation Channels

Cold allodynia is a poorly understood symptom of neuropathic pain. Two members of the transient receptor potential (TRP) family of proteins, TRPM8 and TRPA1, may contribute to cold somatosensation. The aim of the present study was to investigate the usefulness of icilin as a pharmacological tool to study primary afferent fibre responses to cold stimuli and to determine whether there are differences in the responses of spinal neurones to cooling of peripheral receptive fields in control versus neuropathic rats. The effects of icilin, a TRPM8 and TRPA1 agonist, on intracellular Ca(2+) ([Ca(2+)](i)) responses of small diameter adult dorsal root ganglia (DRG) neurones were determined. Icilin (10 nM-10 microM) produced a concentration-related increase in [Ca(2+)](i) in DRG neurones, which was attenuated by the non-selective TRP channel antagonist ruthenium red (10 microM). In vivo electrophysiology in naïve, sham-operated and SNL rats demonstrated that application of ice to receptive fields evoked firing of wide dynamic range (WDR) neurones, which was significantly greater in SNL rats than naïve and sham-operated rats. Intraplantar injection of icilin did not evoke firing of WDR neurones in naïve, sham-operated or SNL rats but inhibited mechanically-evoked responses of WDR neurones in naïve and sham-operated rats, whilst facilitating mechanically-evoked responses in SNL rats. Icilin increased both innocuous (sham-operated and SNL rats) and noxious (SNL rats) receptive field sizes of WDR neurones. Our data suggests that icilin modulates the mechanosensitivity of dorsal horn neurones. The differing effects of ice and icilin on dorsal horn neurones indicate different mechanisms of action.

Topics: Analysis of Variance; Animals; Ankyrins; Calcium; Calcium Channel Blockers; Calcium Channels; Cells, Cultured; Cold Temperature; Disease Models, Animal; Dose-Response Relationship, Drug; Evoked Potentials; Ganglia, Spinal; Hyperesthesia; Ligation; Male; Mechanoreceptors; Neuralgia; Neurons; Pain Threshold; Pyrimidinones; Rats; Rats, Sprague-Dawley; Spinal Cord; Spinal Nerves; TRPA1 Cation Channel; TRPC Cation Channels; TRPM Cation Channels