Roles of TRPM8 and TRPM3 in sensory transduction

Abstract

Primary afferent neurons are equipped with sensory transduction channels which allow the conversion of physical and chemical stimuli into electrical signals. TRP channels are a heterogeneous superfamily of largely non-selective cation channels, which have been implicated in a myriad of sensory transduction mechanisms from the detection of temperature to the sensation of touch. Many TRP channels are key targets for the study of pain physiology due to their polymodal activation and expression in small diameter, unmyelinated sensory fibres. The aim of my project was to examine the roles of TRP channels in sensory transduction mechanisms. Three results chapters focusing on three different TRP channels are presented.
A novel role for the established cold thermosensor, TRPM8, as a cellular osmosensor was determined. The studies presented establish that TRPM8 is activated by increases in extracellular osmolality and is partially activated at normal physiological osmolalities. Cool temperatures increase the sensitivity of TRPM8 to osmotic stimuli and activation of phospholipase enzymes modulates activation of TRPM8 by hyperosmotic solutions. TRPM8 is expressed within sensory neurons where it functions as the chief detector of increased osmolality in addition to a molecular sensor of cold sensations.
The role of TRPM3 as a candidate heat transduction channel is examined. The findings presented demonstrate that recombinantly expressed TRPM3 channels are heat-sensitive and mice lacking functional TRPM3 channels lose a population of heat-activated neurons and have impaired behavioural responses to noxious heat. Moreover, modulation of TRPM3 by intracellular pathways downstream of G-protein coupled receptor activation has been determined. Activation of TRPM3 in sensory neurons is shown to be robustly inhibited by morphine in a predominantly mu-opioid receptor and Gi dependent mechanism. Additionally the role of TRPM3 in several pain states is examined.
Finally, this thesis reports on the characterisation of a medium-throughput CGRP release assay for examining activation of TRPA1 natively expressed on the central terminals of dorsal root ganglion neurons. Activation of TRPA1 expressed on spinal cord synaptosomes by a selection of agonists evokes a concentration-dependent release of CGRP which is inhibited by TRPA1 antagonists. The VGCC subtypes important for TRPA1 and depolarisation-induced CGRP release are examined.

Date of Award	2015
Original language	English
Awarding Institution	King's College London
Supervisor	Stuart Bevan (Supervisor) & Sarah Flatters (Supervisor)