Zinc ions inhibit agonist binding by chelating histidine residues of A1 adenosine receptors

Abstract

Cisplatin is a commonly used antineoplastic agent that causes ototoxicity through the formation of reactive oxygen species (ROS). Previous studies have shown that cisplatin causes an upregulation of A1 adenosine receptor (A1AR) in the cochlea, and that application of the adenosine agonist, R-phenylisopropyladenosine (R-PIA), to the round window (RW) results in significant increases in cochlear glutathione peroxidase and superoxide dismutase. These data suggest that adenosine receptors (ARs) are an important part of the cytoprotective system of the cochlea in response to oxidative stress. The purpose of the current study was to investigate the effect of various adenosine agonists on cisplatin ototoxicity using RW application. Auditory brainstem response (ABR) thresholds were recorded in anesthetized chinchillas at 1, 2, 4, 8 and 16 kHz. The auditory bullae were surgically opened, and 1 mM R-PIA, 10 μM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)/R-PIA (1 mM) cocktail, 100 μM 2-chloro-N-cyclopentyladenosine (CCPA), 2-[4-(2-p-carboxy-ethyl)phenylamino]-5′-N-ethylcarboxamidoadenosine (CGS) or vehicle were applied to the RW. After 90 min, the remaining solution was removed and cisplatin was applied to the RW. The bullae were closed and the animals recovered for 72 h, after which, follow-up ABRs were performed. Cochleae were harvested for scanning electron microscopy (SEM) and for lipid peroxides. Pre-administration of the A1AR agonists R-PIA or CCPA significantly reduced cisplatin-induced threshold changes at all but the highest test frequency. In addition, A1AR agonists protected against cisplatin-induced hair cell damage and significantly reduced cisplatin-induced lipid peroxidation. Co-administration of the A1AR antagonist, DPCPX, completely reversed the protective effects of R-PIA. In contrast, pretreatment with CGS-21680, an A2A adenosine receptor (A2AAR) agonist, significantly increased cisplatin-induced threshold changes. Our findings are consistent with the notion that the A1AR contributes significantly to cytoprotection in the cochlea, and thereby protects against hearing loss.