Function and Postnatal Changes of Dural Afferent Fibers Expressing TRPM8 Channels

Abstract

BackgroundGenome-wide association studies have identified TRPM8 (transient receptor potential melastatin 8) as one of the susceptibility genes for common migraine. Here, we investigated the postnatal changes of TRPM8-expressing dural afferent fibers as well as the function of dural TRPM8 channels in mice.ResultsFirst, we quantified the density and the number of axonal branches of TRPM8-expressing fibers in the dura of mice expressing farnesylated enhanced green fluorescent protein (EGFPf) from one TRPM8 allele between postnatal day 2 (P2) to adulthood. The number of axonal branches on individual dural EGFP-positive fibers was decreased by 30% between P2 and P11. The density of dural EGFP-positive fibers was subsequently reduced by 50% between P16 and P21. Conversely, the density and the number of branches of axons expressing calcitonin gene-related peptide remained stable in postnatal mouse dura. The density of TRPM8-expressing fibers innervating the mouse cornea epithelium was significantly increased from P2 to adulthood. Next, we tested the function of dural TRPM8 channels in adult mice and found that TRPM8 agonist menthol effectively inhibited the nocifensive behavior evoked by dural application of inflammatory mediators.ConclusionsOur results indicate that the TRPM8-expressing dural afferent fibers undergo cell- and target tissue-specific axonal pruning during postnatal development. Activation of dural TRPM8 channels decreases meningeal irritation-evoked nocifensive behavior in adult mice. This provides a framework to further explore the role of postnatal changes of TRPM8-expressing dural afferents in the pathophysiology of pediatric and adult migraine.Electronic supplementary materialThe online version of this article (doi:10.1186/s12990-015-0043-0) contains supplementary material, which is available to authorized users.

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 …”

Quantitative Analysis of Mouse Dural Afferent Neurons Expressing TRPM8, VGLUT3, and NF200

“…Using FG to label dural afferent neurons in TRPM8 EGFPf/+ mice, we found that only 3%‐4% of dural afferent neurons express TRPM8 channels in adult mice, much lower than the percentage of total TG neurons expressing TRPM8 (10%). This is surprising, given that activation of dural TRPM8 channels significantly modifies headache‐related behaviors in rodent models . It is possible that we may underestimate the abundance of dural afferent neurons expressing TRPM8, as we removed the muscle and periosteal sheath on the skull before tracer application.…”

“…In a previous study, we have measured the abundance of dural afferent neurons that express neuropeptide calcitonin gene‐related peptide (CGRP), transient receptor potential (TRP) channel subfamily V member 1 (TRPV1) and subfamily A member 1 (TRPA1), respectively . However, using fluorescent retrograde tracer DiI (1,1′‐dioctadecyl‐3,3,3′,3′‐tetramethylindocarbocyanine perchlorate), we were not able to label the cell body of dural afferent neurons expressing TRP channel melastatin 8 (TRPM8) in mouse trigeminal ganglion (TG), despite the fact that TRPM8‐expressing fibers are known to be present on mouse dura and dural TRPM8 channels have been shown to regulate headache‐related behaviors in rodent models . This presents a technical hurdle for further investigation of the neurochemical and physiological properties of TRPM8‐expressing dural afferent neurons.…”

“…This presents a technical hurdle for further investigation of the neurochemical and physiological properties of TRPM8‐expressing dural afferent neurons. We speculated that sparse meningeal innervation and lack of extensive axonal branches might limit the likelihood and/or the amount of tracer taken up by individual TRPM8‐expressing dural afferent neurons …”

TRPM8 and Migraine

“…Sparse innervation of the EGFP‐positive, TRPM8‐expressing fibers are observed in some areas of the dura in adult mice . Interestingly, both the density and the number of axonal branches of dural fibers expressing TRPM8 are decreased substantially from postnatal day 2 (P2) to adulthood . The reduction occurs before the onset of puberty (P25) in both male and female mice, and is independent of the expression and/or the activation of TRPM8 channels per se.…”