For over a decade, members of the vanilloid subfamily of transient receptor potential channels (the TRPVs) have been known to transduce heat. Now, a distant relative joins them as an additional sensor for noxious heat. Researchers in Belgium and Germany have found that TRPM3, a member of the melastatin family of TRPs, is activated by heating in vitro and mediates in vivo responses including aversion to acute heat and development of inflammatory heat hyperalgesia in mice. The results suggest that TRPM3 could be a novel target for pain treatments. The findings, from Joris Vriens, Katholieke Universiteit, Leuven, Belgium, and colleagues appear in the May 12 issue of Neuron.
Among the heat-sensing TRPs, TRPV1 has been identified previously to respond to noxious heat (>43oC; Caterina et al., 2000; Davis et al., 2000). However, from the beginning it was clear that TRPV1 is not the sole sensor of high heat, since TRPV1 knockout mice retain sensitivity to noxious heat. Researchers have had a hard time pinning down channels responsible for the remaining activity.
This study hits upon an unlikely candidate. Like heat-sensitive TRPV channels, TRPM3 is a voltage-dependent, nonselective cation channel permeable to calcium ions. However, TRPM3 has limited sequence homology to the TRPVs, and has not previously been implicated in heat sensing. TRPM3 is widely expressed in neuronal and nonneuronal tissues, and is potently activated in vitro by the neurosteroid pregnenolone sulfate (PS, see Wagner et al., 2008). Its close relative TRPM8 is a menthol-responsive cold sensor, but little is known about TRPM3’s physiological functions (for a review, see Oberwinkler, 2007).
To start, Vriens and his colleagues used calcium imaging and whole-cell patch-clamp recordings to show that cells transiently expressing TRPM3 responded to heat (40oC), and that dorsal root and trigeminal ganglia (DRG and TG) neurons from TRPM3 knockout mice had impaired heat responses. TRPM3 also proved to mediate nocifensive responses to heat in vivo: TRPM3 knockout mice had delayed responses to noxious heat stimuli (45–58oC) in tail immersion and hot plate assays. Further, the TRPM3 knockouts did not develop heat hyperalgesia after injection of complete Freund’s adjuvant, indicating that TRPM3, like TRPV1, is involved not only in acute heat sensation, but also in sensitization during inflammation.
The investigators used PS and the TRPV1 agonist capsaicin to further characterize TRPM3 distribution in DRG and TG neurons from mice. Vriens found that nearly 60% of the cells responded to PS, which activated mainly small-diameter neurons, similar in size to capsaicin-responsive cells. TRPM3 and TRPV1 appeared often to be present in the same nociceptors, since half of all heat-sensitive DRG neurons responded to both PS and capsaicin. An additional one-third of the cells responded to PS but not capsaicin, whereas only 3% responded to capsaicin but not PS, indicating that TRPM3 is expressed in more heat-sensing neurons than is TRPV1.
The study showed that heat and PS activate TRPM3 synergistically, implying that TRPM3 could integrate thermal stimuli and endogenous steroidal signals. The authors note that plasma PS concentrations are known to increase during childbirth and in certain pathological conditions, but physiological activation of TRPM3 by PS has not been proven.
The search for heat-sensitive channels will not end with TRPM3. When the researchers analyzed neurons from TRPM3 knockout mice and blocked TRPV1 with a selective antagonist, they found that a few cells still responded to high temperature. Thus, Vriens told PRF, “There are definitely other channels also involved in the detection of high temperatures.” He says the group is now working on a TRPM3/TRPV1 double knockout mouse in order to learn more about the remaining heat-sensing activity.
One candidate for the remaining noxious heat sensor is TRPV2 (Caterina et al., 1999; see also Basbaum et al., 2009), but its role is still uncertain, and the heat-sensing behavior of TRPV2 knockout mice has not been reported. TRPV3 and TRPV4 have also been nominated. However, in a study published online on May 17 in Molecular Pain, Michael Caterina and colleagues at Johns Hopkins School of Medicine, Baltimore, Maryland report that TRPV3 and TRPV4 knockout mice retain normal heat responses, suggesting that neither of these channels makes a major contribution to heat sensation in vivo. If not TRPVs, then what? TRPM3 was an unexpected addition; perhaps another surprise is yet in store.