도파민 유발 가려움증의 분자적 기전연구

Abstract

An itch is an extremely unpleasant skin sensation that elicits the desire or reflex to scratch. People with chronic itching cannot lead a normal life because of psychological disorder, such as depression or sleep deprivation. Recently, it was reported that some Parkinson’s disease patients treated with L-DOPA suffered from an itch feeling. Notably, dopamine D1-like receptors mediate itch sensations, and compound 48/80-induced scratching was suppressed by the injection of dopamine D1-like receptor antagonist. Thus, the ability of endogenous dopamine to produce an itch sensation was assessed. TPRV1 was shown to act as a molecular target of endogenous dopamine-induced itch sensations with in vivo (cheek, nape injection assay, and siRNA knock-down test), in vitro (calcium imaging, western blot, and RT-PCR) and ex-vivo (performed using the saphenous nerve in hairy skin) experiments. Dopamine significantly induced scratching in a dose-dependent manner compared with the vehicle (vehicle; saline 50 µl, 4.00 ± 1.67; dopamine, 10 µg/50 µl, 102.40 ± 9.56; p < 0.001). Next, the effect of dopamine receptor agonists on dopamine-induced scratching was assessed using a cheek injection model. Cheek model mice displayed increased scratching behavior without signs of pain after treatment with dopamine and SKF-38393, a dopamine D1-like receptor (D1 and D5) agonist (SKF-38393, 4 µg / 20 µl, 56.40 ± 9.07, p < 0.001 compared with vehicle, saline 20 µl, 5.60 ± 3.08 and 2-bromo-α-ergocryptine, 4 µg/20 µl, 10.20 ± 3.43; dopamine, 4 µg/20 µl, 60.40 ± 8.01, p < 0.001 compared with vehicle and 2-bromo-α-ergocryptine). Exposure to 2-Bromo-α-ergocryptine, a dopamine D2-like receptor agonist (D2 to D4), did not induce scratching behavior (10 µg/50 µl 10.20 ± 3.43, p > 0.05 compared with the vehicle) or pain behavior (3.60 ± 1.03). Dopamine- and SKF-38393-induced scratching decreased in dopamine D5 receptor knock down mice (52.00 ± 2.19) compared with that in scrambled control mice (111.00 ± 9.30) and dopamine D1 receptor knock down mice (118.80 ± 4.71). The response of dopamine D1 receptor knock down mice was similar to that of the scrambled control. The in vitro and ex-vivo experiments showed that dopamine induced a transient Ca2+ response, and the inward current and discharge rates of single fiber were reduced by a transient receptor potential vanilloid-1 (TRPV1) antagonist (capsazepine). The TRPV1 antagonist reduced the scratching behavior induced by dopamine and the dopamine D1-like agonist (SKF-38343). I confirmed the co-localization of the D5 receptor, gastrin release peptide, and TRPV1 in the dorsal root ganglion neurons using immunohistochemistry. These results show that TRPV1 activation via the D5 receptor mediates the dopamine-induced itch response. Thus, TRPV1 may serve as a new therapeutics target, including for the itching side effect of L-DOPA in the Parkinson’s disease patient.