Mechanisms of thermal sensitivity in rodent primary afferent neurons innervating the skin

The role of temperature sensations elicited from the skin, include object identification, thermoregulation and the conscious perception of pain. Humans can differentiate at least three distinct cold sensations: innocuous cooling, cold pain and pain evoked by freezing. However, patients with painful neuropathies often suffer from cold allodynia, where normally non-painful cool stimuli begin to induce pain or cold hyperalgesia, a heightened sensitivity to a painful cold stimulus. It is therefore vital to understand the mechanisms by which cold is signalled under normal conditions and to investigate which changes occur under pathological conditions. The thesis will describe sets of electrophysiological recordings carried out from rodent primary afferent neurons using the in vitro skin nerve preparation in an attempt to reveal mechanisms underlying thermal sensitivity. The discovery of thermally sensitive transient receptor potential (TRP) ion channels has given insights into the molecular mechanisms of thermal transduction. These include the heat sensitive TRPV1 ion channel and cold sensitive TRPM8 and TRPA1 ion channels. However, the role of the TRPA1 receptor in cold transduction remains controversial. Cold sensitivity of primary afferents in adult rat was studied. Selective TRP channel agonists capsaicin, menthol, and mustard oil were then applied onto the receptive field of primary afferents to determine the expression pattern of thermosensitive TRP channels. The majority of cold sensitive A and C fibre nociceptors as well as thermoreceptors were sensitive to menthol, indicating that TRPM8 is the transducer of cold on these afferents. The poor correlation of TRPA1 expression and cold sensitivity in nociceptive A and C fibres indicates that TRPA1 is unlikely to play a significant role is detecting noxious cold. TRPV2 is another heat activated ion channel. The sensory phenotype of TRPV2 knock-out mice was studied and compared against TRPV2 wild-type mice in both hairy and glabrous skin. Mice lacking TRPV2 had normal heat sensitive nociceptors and afferents retained mechanical sensitivity. The involvement of potassium (K+) channels in mediating and/or modulating thermosensation has been suggested. Based on these previous findings, the effects of the broad spectrum potassium channel blockers 4-aminopyridine (4- AP) and Tetraethylammonium (TEA) were studied on primary afferents neurons. Application of 4-AP or TEA directly on the receptive fields induced a novel cold sensitivity in a proportion of low threshold mechanoreceptors and increased the cold responses in a proportion of cold sensitive A and C fibre nociceptors. Interestingly 4-AP or TEA had no effect on the cold responses of innocuous cold thermoreceptors. Drug induced cold sensitivity was investigated using the chemotherapeutic agent oxaliplatin, which induces a sensory neuropathy in patients. Following infusion of the drug, patients experience abnormal skin sensations (paresthesias), which are triggered or aggravated by exposures to cold. The receptive properties of afferents were investigated before and after oxaliplatin application to provide an insight into the mechanism by which this abnormal cold sensitivity develops. This study shows for the first time, that oxaliplatin applied directly on the receptive fields induces a novel cold sensitivity in half of previously cold insensitive Aβ mechanoreceptors. Just over a third of Aδ nociceptors also displayed a novel or increased sensitivity to cold after oxaliplatin application. In contrast, receptive properties of C fibres remained unchanged. Overall, the results of the thesis provide evidence that TRPM8 is involved in the transduction of cold stimuli and that potassium and sodium conductances are involved in modulating the final response to a cold stimulus.