Johanek Lisa M, Meyer Richard A, Hartke Tim, Hobelmann Joseph Greg, Maine David N, LaMotte Robert H, Ringkamp Matthias
Department of Neurosurgery, Johns Hopkins University, Baltimore, Maryland 21287, USA.
J Neurosci. 2007 Jul 11;27(28):7490-7. doi: 10.1523/JNEUROSCI.1249-07.2007.
The neuronal pathways for itch have been characterized mainly based on responses to histamine. Intracutaneous application of histamine produces intense itch and a large area of axon-reflexive vasodilation ("flare") around the application site. Both phenomena are thought to be mediated through neuronal activity in itch-specific, mechanoinsensitive C-fiber afferents (CMi). However, mechanical and electrical stimuli that do not activate CMi fibers can cause the sensation of itch, and itch may occur without flare, suggesting that other neuronal itch pathways exist. Because cutaneous application of spicules from the plant Mucuna pruriens (cowhage) has been anecdotally reported to produce itch without flare, we performed psychophysical experiments to investigate whether the mechanisms underlying cowhage- and histamine-induced itch differ. Although histamine and cowhage produced itch of similar magnitude, the itch to cowhage was not correlated with the itch to histamine; some subjects had intense itch to cowhage and little itch to histamine and visa versa. Laser Doppler measurements of blood flow revealed that histamine led to a large area of vasodilation, whereas cowhage produced vasodilation restricted to the application site. Pretreatment of the skin with an antihistamine blocked the itch produced by histamine but did not prevent cowhage-induced itch. Desensitization of the skin with topical capsaicin abolished cowhage-induced itch but did not significantly alter histamine-induced itch. These findings indicate that cowhage itch is signaled through a population of capsaicin-sensitive afferent nerve fibers that is distinct from CMi fibers mediating histamine-induced itch. Cowhage may be useful to investigate the neural pathway mediating nonhistaminergic itch.
瘙痒的神经通路主要是根据对组胺的反应来确定的。皮内注射组胺会产生强烈瘙痒，并在注射部位周围引起大面积的轴突反射性血管扩张（“风团”）。这两种现象都被认为是通过瘙痒特异性、机械不敏感的C纤维传入神经（CMi）中的神经活动介导的。然而，不激活CMi纤维的机械和电刺激也会引起瘙痒感，而且瘙痒可能在没有风团的情况下出现，这表明存在其他神经瘙痒通路。因为有轶事报道称，涂抹来自植物黎豆（刺毛黧豆）的刺会产生没有风团的瘙痒，所以我们进行了心理物理学实验，以研究黎豆和组胺引起瘙痒的机制是否不同。尽管组胺和黎豆引起的瘙痒程度相似，但对黎豆的瘙痒与对组胺的瘙痒并无关联；一些受试者对黎豆有强烈瘙痒，对组胺则几乎没有瘙痒，反之亦然。激光多普勒血流测量显示，组胺会导致大面积血管扩张，而黎豆引起的血管扩张仅限于涂抹部位。用抗组胺药预处理皮肤可阻断组胺引起的瘙痒，但不能预防黎豆引起的瘙痒。用局部辣椒素使皮肤脱敏可消除黎豆引起的瘙痒，但对组胺引起的瘙痒没有显著影响。这些发现表明，黎豆瘙痒是通过一群对辣椒素敏感的传入神经纤维传导的，这群纤维与介导组胺引起瘙痒的CMi纤维不同。黎豆可能有助于研究介导非组胺能瘙痒的神经通路。
