Dynamic expression of the TRPM subgroup of ion channels in developing mouse sensory neurons.

Despite the significance of transient receptor potential (TRP) channels in sensory physiology, little is known of the expression and developmental regulation of the TRPM (melastatin) subgroup in sensory neurons. In order to find out if the eight TRPM subgroup members (TRPM1-TRPM8) have a possible role in the sensory nervous system, we characterized the developmental regulation of their expression in mouse dorsal root ganglion (DRG) from embryonic (E) day 12 to adulthood. Transcripts for all channels except for TRPM1 were detected in lumbar and thoracic DRG and in nodose ganglion (NG) with distinguishable expression patterns from E12 until adult. For most channels, the expression increased from E14 to adult with the exception of TRPM5, which displayed transient high levels during embryonic and early postnatal stages. Cellular localization of TRPM8 mRNA was found only in a limited subset of very small diameter neurons distinct in size from other populations. These neurons did not bind isolectin B4 (IB4) and expressed neither the neuropeptide calcitonin gene-related peptide (CGRP) nor neurofilament (NF)200. This suggests that TRPM8(+) thermoreceptive sensory neurons fall into a separate group of very small sized neurons distinct from peptidergic and IB4(+) subtypes of sensory neurons. Our results, showing the expression and dynamic regulation of TRPM channels during development, indicate that many TRPM subfamily members could participate during nervous system development and in the adult by determining distinct physiological properties of sensory neurons.

Differential regulation of TRP channels in a rat model of neuropathic pain.

Neuropathic pain is a chronic disease resulting from dysfunction of the nervous system often due to peripheral nerve injury. Hypersensitivity to sensory stimuli (mechanical, thermal or chemical) is a common source of pain in patients and ion channels involved in detecting these stimuli are possible candidates for inducing and/or maintaining the pain. Transient receptor potential (TRP) channels expressed on nociceptors respond to different sensory stimuli and a few of them have been studied previously in the models of neuropathic pain. Using real-time PCR for quantification of all known TRP channels we identified several TRP channels, which have not been associated with nociception or neuropathic pain before, to be expressed in the DRG and to be differentially regulated after spared nerve injury (SNI). Of all TRP channel members, TRPML3 showed the most dramatic change in animals exhibiting neuropathic pain behaviour compared to control animals. In situ hybridisation showed a widespread increase of expression in neurons of small, medium and large cell sizes, indicating expression in multiple subtypes. Co-localisation of TRPML3 with CGRP, NF200 and IB4 staining confirmed a broad subtype distribution. Expression studies during development showed that TRPML3 is an embryonic channel that is induced upon nerve injury in three different nerve injury models investigated. Thus, the current results link for the first time a re-expression of TRPML3 with the development of neuropathic pain conditions. In addition, decreased mRNA levels after SNI were seen for TRPM6, TRPM8, TRPV1, TRPA1, TRPC3, TRPC4 and TRPC5.

Down regulation of TRPC1 by shRNA reduces mechanosensitivity in mouse dorsal root ganglion neurons in vitro.

Mechanosensitivity is a crucial but poorly understood property of the sensory nervous system. Transient receptor potential (TRP) channels, which have been found to be responsible for the detection of other sensory stimuli such as temperature and pungent chemicals, have been suggested to also recognize stretch or pressure to cell membranes. TRPC1 is one candidate from studies in oocytes but evidence in native sensory neurons has been lacking. Therefore, we have measured an increase in intracellular Ca(2+) levels upon mechanical activation of native mouse dorsal root ganglion (DRG) neurons in culture using hypoosmolar buffer. Our results show that down regulation of TRPC1 with short hairpin RNA results in a 65% reduction of neurons with stretch activated responses. These results implicate a direct or indirect involvement of TRPC1 in the mechanosensitivity of DRG neurons.