Restricted Spots Of Light Reveal An Efficacy Gradient Of The Phototransduction Cascade Along The Rod Outer Segment

Rod photoreceptors consist of an outer segment (OS) and an inner segment (IS). Inside the OS a biochemical machinery transforms the absorption of photons into electrical signals (Yoshizawa and Wald, 1963; Baylor et al., 1979; Fung and Strayer, 1980). This biochemical machinery has been treated and thought to be in essence homogenous within OS with only marginal dishomogeneities (Lamb et al., 1981; Schnapf, 1983; Jarvinen and Lamb, 2005). In order to verify this assumption, we have used special optical fibres to deliver highly-localized light stimuli to OS. If a Gaussian beam of light is fed into an apertureless tapered optical fibers (TOF), coated with gold and titanium, the exiting spot of light does not have the usual lobes and is fairly restricted. As the TOF is moved from the OS base towards its tip, the amplitude of both the saturating response and the single-photon response decreased, demonstrating a pronounced reduction in the efficacy of the phototransduction machinery. We propose that this reduction in efficacy reflects a reduction in PDE levels along the OS, associated with discs aging. From measurements of the gradient of the saturating responses elicited by local flashes at different positions, we calculate that the longitudinal diffusion coefficient for cyclic guanosine monophosphate (cGMP) exceeds 500 µm2/s. As a result of this high diffusion coefficient, cyclic nucleotide-gated (CNG) channels near the tip of the outer segment are closed by diffuse flashes of light, despite the very low level of PDE in the discs in that region.