Noradrenergic modulation of itch transmission in the spinal cord

Scabies itch: an update on neuroimmune interactions and novel targets

Frequently described as 'the worst itch' one can ever experience scabies itch is the hallmark of Sarcoptes scabiei mite infestation. Notably, the itchiness often persists for weeks despite scabicides therapy. The mechanism of scabies itch is not yet fully understood, and effective treatment modalities are still missing which can severely affect the quality of life. The aim of this review is to provide an overview of the scope of itch in scabies and highlight candidate mechanisms underlying this itch. We herein discuss scabies itch, with a focus on the nature, candidate underlying mechanisms and treatment options. We also synthesize this information with current understanding of the mechanisms contributing to non-histaminergic itch in other conditions. Itch is a major problem in scabies and can lead to grave consequences. We provide the latest insights on host-mite interaction, secondary microbial infection and neural sensitization with special emphasis on keratinocytes and mast cells to better understand the mechanism of itch in scabies. Also, the most relevant current modalities remaining under investigation that possess promising perspectives for scabies itch (i.e. protease-activated receptor-2 (PAR-2) inhibitor, Mas-related G protein-coupled receptor X2 (MRGPRX2) antagonist) are discussed. Greater understanding of these diverse mechanisms may provide a rational basis for the development of improved and targeted approaches to control itch in individuals with scabies.

Central circuit mechanisms of itch

Itch, in particular chronic forms, has been widely recognized as an important clinical problem, but much less is known about the mechanisms of itch in comparison with other sensory modalities such as pain. Recently, considerable progress has been made in dissecting the circuit mechanisms of itch at both the spinal and supraspinal levels. Major components of the spinal neural circuit underlying both chemical and mechanical itch have now been identified, along with the circuits relaying ascending transmission and the descending modulation of itch. In this review, we summarize the progress in elucidating the neural circuit mechanism of itch at spinal and supraspinal levels.

Itch: From mechanism to (novel) therapeutic approaches

Itch is a common sensory experience that is prevalent in patients with inflammatory skin diseases, as well as in those with systemic and neuropathic conditions. In patients with these conditions, itch is often severe and significantly affects quality of life. Itch is encoded by 2 major neuronal pathways: histaminergic (in acute itch) and nonhistaminergic (in chronic itch). In the majority of cases, crosstalk existing between keratinocytes, the immune system, and nonhistaminergic sensory nerves is responsible for the pathophysiology of chronic itch. This review provides an overview of the current understanding of the molecular, neural, and immune mechanisms of itch: beginning in the skin, proceeding to the spinal cord, and eventually ascending to the brain, where itch is processed. A growing understanding of the mechanisms of chronic itch is expanding, as is our pipeline of more targeted topical and systemic therapies. Our therapeutic armamentarium for treating chronic itch has expanded in the last 5 years, with developments of topical and systemic treatments targeting the neural and immune systems.

Exacerbating factors of itch in atopic dermatitis

Atopic dermatitis (AD) displays different clinical symptoms, progress, and response to treatment during early infancy and after childhood. After the childhood period, itch appears first, followed by formation of well-circumscribed plaque or polymorphous dermatoses at the same site. When accompanied with dermatitis and dry skin, treatment of skin lesions should be prioritized. When itch appears first, disease history, such as causes and time of appearance of itch should be obtained by history taking. In many cases, itch increases in the evening when the sympathetic nerve activity decreased. Treatment is provided considering that hypersensitivity to various external stimulations can cause itch. Heat and sweating are thought to especially exacerbate itch. Factors causing itch, such as cytokines and chemical messengers, also induce itch mainly by stimulating the nerve. Scratching further aggravates dermatitis. Skin hypersensibility, where other non-itch senses, such as pain and heat, are felt as itch, sometimes occurs in AD. Abnormal elongation of the sensory nerve into the epidermis, as well as sensitizing of the peripheral/central nerve, are possible causes of hypersensitivity, leading to itch. To control itch induced by environmental factors such as heat, treatment for dermatitis is given priority. In the background of itch exacerbated by sweating, attention should be given to the negative impact of sweat on skin homeostasis due to 1) leaving excess sweat on the skin, and 2) heat retention due to insufficient sweating. Excess sweat on the skin should be properly wiped off, and dermatitis should be controlled so that appropriate amount of sweat can be produced. Not only stimulation from the skin surface, but also visual and auditory stimulation can induce new itch. This "contagious itch" can be notably observed in patients with AD. This article reviews and introduces causes of aggravation of itch and information regarding how to cope with such causes.