Dark noise in the outer segment membrane current of green rod photoreceptors from toad retina.

The outer segment membrane current of single green rods from toad retina was recorded with a suction electrode, and the rate of spontaneous noise events similar to single-photon responses was measured at different temperatures. The activation energy, Gibbs free energy of activation, and entropy of activation for the process producing spontaneous events were similar to the values reported for thermal isomerization of free 11-cis-retinal (Hubbard, 1966), suggesting that thermal isomerization of the chromophore may be the trigger for the spontaneous events. The apparent rate constant for thermal activation of the green rod photopigment was about 4 times higher than in red rods but about 1000 times lower than for free 11-cis-retinal. Thus, both red and green rod opsin appear to stabilize 11-cis-retinal against thermal isomerization, but green rod opsin is somewhat less effective. The speed of the average dim-flash response increased as temperature was raised, as reported previously in both cone and rod photoreceptors. The reciprocal of the time-to-peak of the dim-flash response had an average Q10 of 3.3 between 20 and 30 degrees C. Changing temperature shifted the time scale of the response without altering response wave form, suggesting that all delay stages shaping the light response were approximately equally affected by temperature. At temperatures greater than 25 degrees C, flash responses were sometimes biphasic, i.e. inward dark current first decreased after a flash, then transiently increased beyond the resting dark level.