Pain and itch: insights into the neural circuits of aversive somatosensation in health and disease.

Although pain and itch are distinct sensations, most noxious chemicals are not very specific to one sensation over the other, and recent discoveries are revealing that Trp channels function as transducers for both. A key difference between these sensations is that itch is initiated by irritation of the skin, whereas pain can be elicited from almost anywhere in the body; thus, itch may be encoded by the selective activation of specific subsets of neurons that are tuned to detect harmful stimuli at the surface and have specialized central connectivity that is specific to itch. Within the spinal cord, cross-modal inhibition between pain and itch may help sharpen the distinction between these sensations. Moreover, this idea that somatosensory modalities inhibit one another may be generalizable to other somatosensory subtypes, such as cold and hot. Importantly, just as there are inhibitory circuits in the dorsal horn that mediate cross-inhibition between modalities, it appears that there are also excitatory connections that can be unmasked upon injury or in disease, leading to abnormally elevated pain states such as allodynia. We are now beginning to understand some of this dorsal horn circuitry, and these discoveries are proving to be relevant for pathological conditions of chronic pain and itch.