Neural Processing of Pain and Itch

Abstract Pain is defined as “An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage,” while itch can be defined as “an unpleasant sensation that evokes the desire to scratch.” These sensations are normally elicited by noxious or pruritic stimuli that excite peripheral sensory neurons connected to spinal circuits and ascending pathways involved in sensory discrimination, emotional aversiveness, and respective motor responses. Specialized molecular receptors expressed by cutaneous nerve endings transduce stimuli into action potentials conducted by C- and Aδ-fiber nociceptors and pruriceptors into the outer lamina of the dorsal horn of the spinal cord. Here, neurons selectively activated by nociceptors, or by convergent input from nociceptors, pruriceptors, and often mechanoreceptors, transmit signals to ascending spinothalamic and spinoparabrachial pathways. The spinal circuitry for itch requires interneurons expressing gastrin-releasing peptide and its receptor, while spinal pain circuitry involves other excitatory neuropeptides; both itch and pain are transmitted by ascending pathways that express the receptor for substance P. Spinal itch- and pain-transmitting circuitry is segmentally modulated by inhibitory interneurons expressing dynorphin, GABA, and glycine, which mediate the antinociceptive and antipruritic effects of noxious counterstimulation. Spinal circuits are also under descending modulation from the brainstem rostral ventromedial medulla. Opioids like morphine inhibit spinal pain-transmitting circuits segmentally and via descending inhibitory pathways, while having the opposite effect on itch. The supraspinal targets of ascending pain and itch pathways exhibit extensive overlap and include the somatosensory thalamus, parabrachial nucleus, amygdala, periaqueductal gray, and somatosensory, anterior cingulate, insular, and supplementary motor cortical areas. Following tissue injury, enhanced pain is evoked near the injury (primary hyperalgesia) due to release of inflammatory mediators that sensitize nociceptors. Within a larger surrounding area of secondary hyperalgesia, innocuous mechanical stimuli elicit pain (allodynia) due to central sensitization of pain pathways. Pruriceptors can also become sensitized in pathophysiological conditions, such as dermatitis. Under chronic itch conditions, low-threshold tactile stimulation can elicit itch (alloknesis), presumably due to central sensitization of itch pathways, although this has not been extensively studied. There is considerable overlap in pain- and itch-signaling pathways and it remains unclear how these sensations are discriminated. Specificity theory states that itch and pain are separate sensations with their own distinct pathways (“labeled lines”). Selectivity theory is similar but incorporates the observation that pruriceptive neurons are also excited by algogenic stimuli that inhibit spinal itch transmission. In contrast, intensity theory states that itch is signaled by low firing rates, and pain by high firing rates, in a common sensory pathway. Finally, the spatial contrast theory proposes that itch is elicited by focal activation of a few nociceptors while activation of more nociceptors over a larger area elicits pain. There is evidence supporting each theory, and it remains to be determined how the nervous system distinguishes between pain and itch.