S182 CRITICAL APPRAISAL OF THE CONDITIONED PLACE PREFERENCE PARADIGM AS READOUT FOR SPONTANEOUS PAIN IN STREPTOZOTOCIN-INDUCED DIABETIC RATS

Transient receptor potential ankyrin 1 (TRPA1) ion channel in the pathophysiology of peripheral diabetic neuropathy

Background

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel permeable to calcium that is expressed on pain-mediating primary afferent nerve fibers. Here we review recent experimental evidence supporting the hypothesis that activation of the TRPA1 channel by reactive compounds generated in diabetes mellitus, such as 4-hydroxynonenal and methylglyoxal, exerts an important role in the pathophysiology of peripheral diabetic neuropathy (PDN). The hypothesis includes development of the early diabetic pain hypersensitivity and the later loss of cutaneous nerve endings of pain fibers and their dysfunction, which are hallmarks of peripheral diabetic neuropathy (PDN).

Methods

The evidence for a role of the TRPA1 channel in PDN consists of in vitro patch clamp and calcium imaging data and assessments of pain behavior, axon reflex measurements, and immunohistochemical analyses of cutaneous innervation in an experimental animal model of diabetes. The experiments were combined with blocking the TRPA1 channel with selective antagonists Chembridge-5861528 or A-967079.

Results

In vitro studies indicate that under physiological concentration of Ca2+, methylglyoxal and 4-hydroxynonenal produce sustained activation of the TRPA1 channel and sustained inflow of calcium. In vivo studies indicate that diabetic pain hypersensitivity is maintained by the TRPA1 channel as indicated by the antihypersensitivity effect induced by acute blocking of the TRPA1 channel. Moreover, TRPA1 channel is involved in the development of diabetic hypersensitivity as indicated by prevention of the development of pain hypersensitivity in diabetic animals treated daily with Chembridge-5861528. The diabetes-induced loss of substance P-like cutaneous innervation and that of the TRPA1 channel-mediated cutaneous axon reflex function during the later phase of diabetes were also prevented or delayed by prolonged blocking of the TRPA1 channel. No motor impairment or other obvious side-effects were observed following block of the TRPA1 channel.

Conclusions

Together the in vitro and in vivo results indicate that reactive compounds generated in diabetes exert, through action on the TRPA1 channel, an important role in the pathophysiology of PDN. Sustained activation of the TRPA1 channel is a plausible mechanism that contributes to the early diabetic pain hypersensitivity and the later loss of cutaneous pain fiber endings and their dysfunction with prolonged diabetes.

Implications

Blocking the TRPA1 channel with a selective antagonist provides a promising disease-modifying treatment for PDN, with only minor, if any, side-effects.