Chromophore switch from 11-cis-dehydroretinal (A2) to 11-cis-retinal (A1) decreases dark noise in salamander red rods

摘要

Dark noise, light-induced noise and responses to brief flashes of light were recorded inthe membrane current of isolated rods from larval tiger salamander retina before and afterbleaching most of the native visual pigment, which mainly has the 11-cis-3,4-dehydroretinal(A2) chromophore, and regenerating with the 11-cis-retinal (A1) chromophore in the sameisolated rods. The purpose was to test the hypothesis that blue-shifting the pigment by switchingfrom A2 to A1 will decrease the rate of spontaneous thermal activations and thus intrinsiclight-like noise in the rod. Complete recordings were obtained in five cells (21◦C). Based onthe wavelength of maximum absorbance, λmax,A1 =502 nm and λmax,A2 =528 nm, the averageA2 : A1 ratio determined from rod spectral sensitivities and absorbances was ∼0.74 : 0.26 inthe native state and ∼0.09 : 0.91 in the final state. In the native (A2) state, the single-quantumresponse (SQR) had an amplitude of 0.41±0.03 pA and an integration time of 3.16±0.15 s(mean±S.E.M.). The low-frequency branch of the dark noise power spectrum was consistentwith discrete SQR-like events occurring at a rate of 0.238±0.026 rod−1 s−1. The correspondingvalues in the final state were 0.57±0.07 pA (SQR amplitude), 3.47±0.26 s (SQR integrationtime), and 0.030±0.006 rod−1 s−1 (rate of dark events). Thus the rate of dark events per rodand the fraction of A2 pigment both changed by ca 8-fold between the native and final states,indicating that the dark events originated mainly in A2 molecules even in the final state. Byextrapolating the linear relation between event rates and A2 fraction to 0% A2 (100% A1) and100% A2 (0% A1), we estimated that the A1 pigment is at least 36 times more stable than the A2pigment. The noise component attributed to discrete dark events accounted for 73% of the totaldark current variance in the native (A2) state and 46% in the final state. The power spectrum ofthe remaining ‘continuous’ noise component did not differ between the two states. The smallerand faster SQR in the native (A2) state is consistent with the idea that the rod behaves as iflight-adapted by dark events that occur at a rate of nearly one per integration time. Both thedecreased level of dark noise and the increased SQR amplitude must significantly improve thereliability of photon detection in dim light in the presence of the A1 chromophore compared tothe native (A2) state in salamander rods.