Neuropathic Itch

Abstract

Neurologic insults as varied as inflammation, stroke, and fibromyalgia elicit neuropathic pain and itch. Noxious sensation results when aberrantly increased afferent signaling reaches percept-forming cortical neurons and can occur due to increased sensory signaling, decreased inhibitory signaling, or a combination of both processes. To treat these symptoms, detailed knowledge of sensory transmission, from innervated end organ to cortex, is required. Molecular, genetic, and behavioral dissection of itch in animals and patients has improved understanding of the receptors, cells, and circuits involved. In this review, we will discuss neuropathic itch with a focus on the itch-specific circuit.

Satellite Glial Cells and Neurons in Trigeminal Ganglia Are Altered in an Itch Model in Mice

“…The question of how pain and itch are encoded in sensory ganglia has not been resolved. One theory is the “labeled line” which proposes that itch- and pain-specific sensory neurons respond exclusively to their respective stimuli, each constituting a dedicated pathway [ 10 ]. However, alternative explanations, such as intensity and neuropeptide coding, have also gained some support [ 10 , 11 , 12 ].…”

“…One theory is the “labeled line” which proposes that itch- and pain-specific sensory neurons respond exclusively to their respective stimuli, each constituting a dedicated pathway [ 10 ]. However, alternative explanations, such as intensity and neuropeptide coding, have also gained some support [ 10 , 11 , 12 ]. A recent study suggested that coding of itch signals depended on the firing pattern of primary sensory neurons, which indicated that early processing in sensory ganglia was a major factor in pruritus [ 55 ].…”

“…Histamine has long been recognized as a common pruritogen and it activates a subgroup of sensory neurons via H1 receptors [ 3 , 5 ]. A major progress in itch research took place with the identification of a family of receptors called Mrgprs, which mediate responses to a number of non-histaminergic pruritogens [ 8 , 9 , 10 ]. Itch receptors (H1, Mrgprs, and other non-H1 receptors) are all found in a variety of pain sensitive neurons.…”

“…Itch receptors (H1, Mrgprs, and other non-H1 receptors) are all found in a variety of pain sensitive neurons. It has been reported that pruriceptors are a subgroup of nociceptors and there is some overlap between pain and itch pathways [ 1 , 3 , 10 , 11 , 12 ]. It has been found that many pain-sensing sensory neurons can also transmit itch [ 11 ].…”

“…Pain and itch are distinct sensations, although itch is usually accompanied by a weak sensation of pain [ 13 ]. A number of theories have been advanced to explain itch signaling, but this field remains controversial [ 10 , 11 , 12 ].…”