This Review highlights selected frontiers in pruritus research and focuses on recently attained insights into the neurophysiological, neuroimmunological, and neuroendocrine mechanisms underlying skin-derived itch (pruritogenic pruritus), which may affect future antipruritic strategies. Special attention is paid to newly identified itch-specific neuronal pathways in the spinothalamic tract that are distinct from pain pathways and to CNS regions that process peripheral pruritogenic stimuli. In addition, the relation between itch and pain is discussed, with emphasis on how the intimate contacts between these closely related yet distinct sensory phenomena may be exploited therapeutically. Furthermore, newly identified or unduly neglected intracutaneous itch mediators (e.g., endovanilloids, proteases, cannabinoids, opioids, neurotrophins, and cytokines) and relevant receptors (e.g., vanilloid receptor channels and proteinase-activated, cannabinoid, opioid, cytokine, and new histamine receptors) are discussed. In summarizing promising new avenues for managing itch more effectively, we advocate therapeutic approaches that strive for the combination of peripherally active antiinflammatory agents with drugs that counteract chronic central itch sensitization.
The study of pruritus in a nutshellItching (pruritus) is perhaps the most common symptom associated with numerous skin diseases and can be a lead symptom of extracutaneous disease (e.g., malignancy, infection, and metabolic disorders) (1, S1). However, despite approximately a century of pruritus research (2, S2, S3), there is no generally accepted therapy for the treatment of itch, and many mysteries, misconceptions, and controversies still haunt this rather neglected, yet clinically important and scientifically fascinating, niche in the life sciences (3, 4, 5).
It is the brain that itches, not the skinPruritus causes the desire to scratch the skin and is experienced as a sensation arising in the skin. However, like all other skin sensations, itch, strictly speaking, is an extracutaneous event - a product of CNS activities. The intense itch we feel after an insect bite, in a patch of atopic eczema, during an episode of food-induced urticaria, or in association with diabetes, uremia, or scabies mite infection (S1) represents a neuronal projection of a centrally formed sensation into defined regions of the integument (localized pruritus) or into large territories of our body surface (generalized pruritus).Interestingly, our individual reception of and emotional response to itch strongly depends on its exact quality: while a tickling sensation usually is experienced as pleasurable, persistent itch is an annoying or even torturous sensation (S4). While one is tempted to interpret this as indicating a distinct molecular and/or structural basis of these different itch qualities, it has proven excruciatingly difficult to identify their molecular, structural, and neurophysiological differences (ref. 1; see below).As pruritus can arise from localized or systemic, peripheral o...