Chronic itch and pain—Similarities and differences

Abstract

Both, pruritus and pain are aversive, but clearly distinct sensations originating in the peripheral and central nervous system. During the last years, many interactions between itch and pain in acute transmission and sensitization processes have been identified. It is common experience that the itch sensation can be reduced by the painful sensations caused by scratching. Vice versa analgesia may reduce this inhibition and thus enhance itch. This phenomenon is particularly relevant to spinally administered mu-opioid receptor agonists, which induce segmental analgesia often combined with segmental pruritus. The peripheral and central sensitization to pain and to itch exhibits striking similarities. Classical inflammatory mediators such as bradykinin have been shown to sensitize nociceptors for both itch and pain. Also regulation of gene expression induced by trophic factors, such as NGF, plays a major role in persistently increased neuronal sensitivity for itch and pain. Finally, itch and pain exhibit corresponding patterns of central sensitization. The knowledge of antagonistic interaction, but also of similar sensitization processes has major implication for antipruritic therapeutic approaches.

Le prurit : des avancées physiopathologiques considérables

“…• Au cours de la sensibilisation centrale, l'excitabilité des neurones du système nerveux central est altérée de telle manière que des stimulus non prurigineux sont perçus comme prurigineux [38]. Ainsi, des stimulus qui déclenchent normalement la douleur peuvent déclencher des démangeaisons lorsqu'ils sont appliqués sur la peau lésée de patients ayant une dermatite atopique [39].…”

Geschlechtsspezifische Unterschiede bei chronischem Pruritus

“…Diese Form des Pruritus entsteht dadurch, dass Nervenfasern, die für die Weiterleitung von Juck-und Schmerzinformationen verantwortlich sind, durch Kompression geschädigt werden oder degenerieren. Heute weiß man, dass es zwar starke Parallelen zwischen der Pathophysiologie von Pruritus und Schmerzen gibt, aber auch deutliche Unterschiede (Ständer/Schmelz 2006).…”

TRP Channels and Pruritus

“…The logical consequence of these data and other observations together led to establish the so called specificity theory which proposes the existence of specialized receptor structures, neural pathways as well as higher centers processing exclusively itch, but not pain. Although recent data (see below) strongly support the specificity theory, it is also clear that the pain and itch sensing systems are intimately related to each other [3,10,11]. Below, we introduce several overlaps of the pruriceptive and nociceptive systems, especially regarding receptors and mediators shared by the two systems.…”

“…Indeed, a growing number of evidence supports the presence of itchspecific sensory fibers which are different from the pain sensing ones [7,8]. Furthermore, it is well known that pain inhibits itch [3,5,[8][9][10][11]; actually, the itch-induced scratching response itself is a motor activity to induce pain and hence to alleviate itch. The logical consequence of these data and other observations together led to establish the so called specificity theory which proposes the existence of specialized receptor structures, neural pathways as well as higher centers processing exclusively itch, but not pain.…”

“…With respect to its aim, the itch-induced scratching (defined as a goaldirected movement against noxious agents that have successfully passed the skin barrier and have already invaded the organism) can be compared to the withdrawal reflex evoked by painful stimuli. However, the appearances of these two motor responses are quite distinct and they represent different strategies of protection [2][3][4]. Due to the intimate relationship of itch and pain sensations (see below), the distinction between the pruri-end nociceptive systems is not a trivial task, and there are different approaches existing even till today [5].…”