Phototransduction mechanism in depolarizing photoreceptors of lizard parietal eye

Purpose. The photoreceptors i n the 1i zard pari étal eye resemble rods and cones in morphology but, unlike the latter, depolarize to light (Solessio & Engbretson, Nature 364, 442, 1993). Using excised-patch recording, we have recently found a cGMP-gated, nonselective cation channel present at high density on the outer segment of the parietal eye photoreceptor, suggesti ng a roi e in phototransduction (Fi nn, Solessio and Yau, Biophys. Soc. Abstr., 1997). In this study, we sought to obtain further evidence on this question. Methods. Single isolated photoreceptors were obtained under room light by triturating a parietal eye treated successively with collagenase, pronase and trypsin. Whole-cell or perforated-patch recordings were made in the voltage-clamp mode from these cells in darkness after 10 min of dark adaptation. Results. Infusing 1 mM cCMP or 500 nM 8-bromo-cCMP from the whole-cell pipette induced a sustained inward current up to 200 pA. A light flash or an external puff of 2 mM IBMX (a phosphodiesterase inhibitor) likewise induced a transient inward current from a photoreceptor under perforated-patch recording; the currents induced by light and IBMX did not summate when the two stimuli were applied simultaneously, suggesting that the same current was activated by both. A puff of 5 mM L-cis-diltiazem inhibited both the light- and cGMP-induced currents. Conclusion. The depolarizing light response of the parietal eye photoreceptor appears to result from a rise in cGMP concentration in the outer segment and the consequent opening of cGMP-gated nonselective cation channels. This cGMP rise can be due to a 1 iaht-activated auanvlate cvclase activity or a light-inhibited phosphodiesterase activity.