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M.E. Estevez, P. Ala–Laurila, R.K. Crouch, M.C. Cornwall; Turning Cones Off: The Role of the 9–Methyl Group of Retinal . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4314.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the molecular mechanisms underlying the rapid rate of flash response recovery in cone photoreceptors following exposure to bright light.
Electrophysiological recordings were made from tiger salamander cone photoreceptors. Flash responses were elicited from cones containing native pigment as well as cones bleached and their pigment regenerated with either 11–cis retinal or 11–cis 9–demethyl retinal. Rates of flash response recovery after bleaching were compared. In addition, changes in the guanylyl cyclase (GC) rates were evaluated by measuring circulating current while cells were jumped into Ringer containing 500µM IBMX, a blocker of phosphodiesterase activity.
Following exposure to dim light, no significant differences were observed between the time courses of current recovery between red cones containing 11–cis retinal and those containing 11–cis 9–methyl retinal. However, following exposure to bright light (>0.3% bleach), red cones containing 11–cis 9–demethyl retinal recovered flash response amplitude and GC rates significantly slower than red cones containing 11–cis retinal. No such prolonged recovery rate was observed in similar experiments on blue or UV cones. This finding is consistent with previously published biochemical data demonstrating that red (but not blue or UV) cone opsins expressed in vitro lacking the 9–methyl group exhibit prolonged activation due to slowed Meta II decay.
Our results suggest that two different mechanisms regulate the recovery of responsiveness in red cones following exposure to light. We propose a model in which, in addition to recovery mechanisms regulated by phosphorylation and arrestin binding, response recovery is also regulated (particularly at high light intensities) by Meta II decay rate. In red cones, recovery depends critically on the 9–methyl group on retinal. These findings provide a partial mechanistic explanation for the high temporal rates at which cones recover sensitivity following exposure to bleaching light.
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