Detection of retinal dysfunction induced by HCN channel inhibitors using multistep light stimulus and long-duration light stimulus ERG in rats.

Ivabradine, a hyperpolarization-activated cyclic nucleotide-gated (HCN) channel inhibitor, has been reported to induce photosensitivity-related visual disturbances such as phosphene in humans. Ivabradine-induced visual disturbances are caused by inhibition of HCN channels in the retina, and the mechanisms have been verified using HCN channel knockout mice and electroretinography (ERG). However, in rats, classical ERG using single flash light stimulus with standard analyses of waveform amplitude and latency has not revealed abnormal retinal function after administration of ivabradine. To verify whether retinal dysfunction after ivabradine administration was detectable in rats, we performed ERG using multistep flash light stimulation at the time when plasma concentration of ivabradine was high. Furthermore, the mechanism of the change in the waveform that appeared after the b-wave was investigated. Ivabradine and cilobradine, a selective HCN channel inhibitor, were administered subcutaneously to rats at 4-40 mg/kg as a single dose, and flash or long-duration ERG recordings at each light stimulus luminance were conducted 1.5 h after administration. Plasma and retinal concentrations of both compounds were measured immediately after the ERG recordings. In the flash ERG, prolongation of a- and/or b-wave latencies were detected at each light stimulus, and dose-dependent waveform changes after the b-wave were recorded at the specific light stimulus luminance for both compounds. These ERG changes increased in response to increasing plasma and retinal concentrations for both ivabradine and cilobradine. In the long-duration light stimulus ERG, a change in the waveform of the b-wave trough and attenuation of the c-wave were recorded, suggesting that the feedback control in the photoreceptor cells may be inhibited. This study revealed that the retinal dysfunction by HCN channel inhibitors in rats can be detected by multistep light stimulus ERG. Additionally, we identified that the inhibition of feedback current and the sustained responses in the photoreceptor cells cause the retinal dysfunction of HCN channel inhibitors in rats.