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O. P. Gross, M. E. Burns; Light-Dependent Changes in Rod Responses Consistent With Accelerated Rhodopsin Deactivation. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1051.
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In normal mouse rods, rhodopsin deactivation mediated by phosphorylation and arrestin binding is considerably faster than deactivation of transducin (Gt), raising the question of the physiological significance of light-induced increases in outer segment arrestin levels. We have examined the effects of steady light expected to induce arrestin translocation on the amplitude and time course of dim flash responses of mouse rods.
Suction electrodes were used to record flash families and single photon responses from wild type rods (c57B/6; WT) and rods with faster than normal Gt deactivation (RGS9 overexpressing rods; RGS9-ox). RGS9-ox rods recover more rapidly and should therefore follow the time course of rhodopsin activity more closely (Krispel et al., 2006). Recordings were made before and after a sustained dim light stimulus (10 min) sufficient to induce mass movement of arrestin (Strissel et al., 2006). The amplification constant, a measure of the gain of rhodopsin activity, was determined by parabolic fitting of the early rising phase of the light response as previously described (Lamb and Pugh, 1993).
Average dim flash response amplitudes were reduced after a sustained light stimulus in a manner that was subject to a steep threshold near the 3% cumulative rhodopsin bleach level for WT and RGS9-ox rods. This reduction was not accompanied by a decrease in dark current in either WT or RGS9-ox rods, although responses of RGS9-ox rods also showed an accelerated time constant of recovery (τrec). A clear decrease in the amplification constant was observed for an overlapping, though clearly distinct, subset of rods. These subpopulations suggest that at least two cellular processes result from the adaptation protocol.
The decreases in amplitude and τrec are consistent with faster rhodopsin deactivation, which may result from arrestin translocation. These changes, which show a steep light-dependent threshold for small bleaches, are consistent with the observed ~3% bleach threshold for arrestin translocation (Strissel et al., 2006). Finally, the effect on the amplification constant is likely due to a separate mechanism, possibly a decrease in the available pool of membrane-associated Gt.
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