Molecular Mechanisms of Itch and Pain: An Overview

Itch and pain are two distinct sensory modalities elicited by noxious and pruritic stimuli, respectively. The cellular and electrophysiological characteristics of itch and pain are very similar; thus discerning the mechanical difference between these two sensory modalities has long been elusive. Recent advances in this field have revealed various neuronal receptors responsible for itch and pain signal transduction and transmission. These studies have identified a distinct sub-population of sensory neurons responsible for itch signal transduction. In addition, itch and pain information conveyed to the spinal cord are processed and regulated by distinct spinal cord neurons. Although both itch and pain senses are required for our daily lives, chronic and hyper-sensation of itch and pain results in a debilitating disease state. Studies have now begun to elucidate mechanisms for the sensitization of pain and itch and also to unravel counter-regulatory mechanisms between itch and pain at the spinal cord level. Interestingly, toll-like receptors (TLRs), an innate immune receptor, has been implicated in the central sensitization of both pain and itch. In this review, we will briefly provide an overview of the molecular mechanisms of itch and pain. Additionally, we will discuss the recently uncovered role of TLRs in itch and pain sensation.