Expression and function of adenosine receptors in the chinchilla cochlea

Previous studies indicate the presence of adenosine receptors in the cochlear tissues obtained from different animals. This study was initiated to determine the subtypes of adenosine receptor (AR) present in the chinchilla cochlea and to assess their function. Radioligand binding studies demonstrate the presence of both the A(1)AR and A(3)AR in membranes prepared from the cochlea, using the radioligands [H-3]DPCPX and [I-125]APNEA. Estimates of the number (B-max) of A(1)AR and A(1)AR plus A(3)AR by saturation curves were 118 +/- 13 and 417 +/- 120 fmol/mg, respectively, with the respective equilibrium dissociation constants (K-d) averaging 2.7 +/- 0.2 and 26.3 +/- 13.8 nM. No significant number of A(2a)AR were detected using [H-3]CGS21680. The nonhydrolyzable adenosine analog R-phenylisopropyladenosine (R-PIA, 1 mu M) elicited a small but significant degree of inhibition of forskolin-stimulated adenylyl cyclase activity (10.4 +/- 2.5%) in cochlear membrane preparations, which was insensitive to blockade by theophylline (100 mu M). Furthermore, R-PIA elicited an increase in inositol 1,4,5-trisphosphate production in dissociated cell preparations obtained from the cochlea. No significant effect of R-PIA was observed on auditory measures such as auditory brainstem evoked response, cochlear action potential and endocochlear potential following round window application. However, round window application of R-PIA elicited significant increases in the activities of antioxidant enzymes such as superoxide dismutase and glutathione peroxidase and significantly reduced the levels of malondialdehyde, a marker of lipid peroxidation. These results suggest a potential cytoprotective role of adenosine in the cochlea against oxidative damage.