The TRPM8 channel forms a complex with the 5-HT1B receptor and phospholipase D that amplifies its reversal of pain hypersensitivity

Abstract

Effective relief from chronic hypersensitive pain states remains an unmet need. Here we report the discovery that the TRPM8 ion channel, co-operating with the 5-HT(1B) receptor (5-HT(1B)R) in a subset of sensory afferents, exerts an influence at the spinal cord level to suppress central hypersensitivity in pain processing throughout the central nervous system. Using cell line models, ex vivo rat neural tissue and in vivo pain models, we assessed functional Ca(2+) fluorometric responses, protein:protein interactions, immuno-localisation and reflex pain behaviours, with pharmacological and molecular interventions. We report 5-HT(1B)R expression in many TRPM8-containing afferents and direct interaction of these proteins in a novel multi-protein signalling complex, which includes phospholipase D1 (PLD1). We provide evidence that the 5-HT(1B)R activates PLD1 to subsequently activate PIP 5-kinase and generate PIP2, an allosteric enhancer of TRPM8, achieving a several-fold increase in potency of TRPM8 activation. The enhanced activation responses of synaptoneurosomes prepared from spinal cord and cortical regions of animals with a chronic inflammatory pain state are inhibited by TRPM8 activators that were applied in vivo topically to the skin, an effect potentiated by co-administered 5-HT(1B)R agonists and attenuated by 5-HT(1B)R antagonists, while 5-HT(1B)R agents alone had no detectable effect. Corresponding results are seen when assessing reflex behaviours in inflammatory and neuropathic pain models. Control experiments with alternative receptor/TRP channel combinations reveal no such synergy. Identification of this novel receptor/effector/channel complex and its impact on nociceptive processing give new insights into possible strategies for enhanced analgesia in chronic pain.

Association Between Overall and Mentholated Cigarette Smoking With Headache in a Nationally Representative Sample

“…This result was confirmed in subsequent studies of large cohorts that also analyzed additional TRPM8 SNPs, finding rs6741751 as the most significant SNP ( P = 9 × 10 −14 ) . The mechanism underlying this association has not been adequately explored, although TRPM8 channels are known to be expressed in trigeminal afferents, and form complexes with 5‐HT 1B receptors with amplified analgesic function . Further, activation of dural TRPM8 has been associated with the development of sumatriptan‐responsive facial allodynia in rats …”

Sustained Morphine Administration Induces TRPM8-Dependent Cold Hyperalgesia

“…MOR could form a molecular complex with TRPM8 to enable the activation of PLD, similarly to other G protein-coupled receptors (GPCRs), 15, 32, 72 as was recently shown for 5HT1b and TRPM8. 89 MOR is critical to OIH 82 and presumed to be also key to morphine induced cold hyperalgesia. MOR and TRPM8 are mainly expressed in small sized DRG neurons.…”

Transient Receptor Potential Cation Channel Subfamily M Member 8 channels mediate the anti‐inflammatory effects of eucalyptol

“…Eucalyptol is an agonist of TRPM8 channels and these channels are essential for at least some of eucalyptol's analgesic actions, as eucalyptol was shown to inhibit acid-induced visceral pain in wild-type mice, but not in TRPM8-deficient mice (Liu et al, 2013b). Analgesia mediated by TRPM8 channels is likely to involve activation of central inhibitory circuits activated by input from TRPM8 channel-expressing peripheral neurons (Proudfoot et al, 2006;Liu et al, 2013b;Vinuela-Fernandez et al, 2014). TRPA1 channels are inhibited by eucalyptol and eucalyptol diminished pain elicited by application of TRPA1 channel agonists to human skin (Liu et al, 2013a;Takaishi et al, 2012;Takaishi et al, 2014).…”