TRP channels as novel players in the pathogenesis and therapy of itch

AbstractItch (pruritus) is a sensory phenomenon characterized by a (usually) negative affective component and the initiation of a special behavioral act, i.e. scratching. Older studies predominantly have interpreted itch as a type of pain. Recent neurophysiological findings, however, have provided compelling evidence that itch (although it indeed has intimate connections to pain) rather needs to be understood as a separate sensory modality. Therefore, a novel pruriceptive system has been proposed, within which itch-inducing peripheral mediators (pruritogens), itch-selective receptors (pruriceptors), sensory afferents and spinal cord neurons, and defined, itch-processing central nervous system regions display complex, layered responses to itch. In this review, we begin with a current overview on the neurophysiology of pruritus, and distinguish it from that of pain. We then focus on the functional characteristics of the large family of transient receptor potential (TRP) channels in skin-coupled sensory mechanisms, including itch and pain. In particular, we argue that – due to their expression patterns, activation mechanisms, regulatory roles, and pharmacological sensitivities – certain thermosensitive TRP channels are key players in pruritus pathogenesis. We close by proposing a novel, TRP-centered concept of pruritus pathogenesis and sketch important future experimental directions towards the therapeutic targeting of TRP channels in the clinical management of itch

Endocannabinoids enhance lipid synthesis and apoptosis of human sebocytes via cannabinoid receptor-2-mediated signaling

We had previously shown that both locally produced endocannabinoids and exocannabinoids, via cannabinoid receptor‐1 (CB1), are powerful inhibitors of human hair growth. To further investigate the role of the cannabinoid system in pilosebaceous unit biology, we have explored in the current study whether and how endocannabinoids have an impact on human sebaceous gland biology, using human SZ95 sebocytes as cell culture model. Here, we provide the first evidence that SZ95 sebocytes express CB2 but not CB1. Also, prototypic endocannabinoids (arachidonoyl ethanolamide/anandamide, 2‐arachidonoyl glycerol) are present in SZ95 sebocytes and dose‐dependently induce lipid production and (chiefly apoptosis‐driven) cell death. Endocannabinoids also up‐regulate the expression of key genes involved in lipid synthesis (e.g., PPAR transcription factors and some of their target genes). These actions are selectively mediated by CB2‐coupled signaling involving the MAPK pathway, as revealed by specific agonists/antagonists and by RNA interference. Because cells with “silenced” CB2 exhibited significantly suppressed basal lipid production, our results collectively suggest that human sebocytes utilize a paracrine‐autocrine, endogenously active, CB2‐mediated endocannabinoid signaling system for positively regulating lipid production and cell death. CB2 antagonists or agonists therefore deserve to be explored in the management of skin disorders characterized by sebaceous gland dysfunctions (e.g., acne vulgaris, seborrhea, dry skin).—Dobrosi, N., Tóth, B. I., Nagy, G., Dózsa, A., Géczy, T., Nagy, L., Zouboulis, C. C., Paus, R., Kovács, L., Bíró, T. Endocannabinoids enhance lipid synthesis and apoptosis of human sebocytes via cannabinoid receptor‐2‐mediated signaling. FASEB J. 22, 3685–3695 (2008