Functional properties of twin and single cones

Phototransduction and adaptation in rods, single cones, and twin cones of the striped bass retina: a comparative study

We investigated the attributes of transduction and light-adaptation in rods, single cones, and twin cones isolated from the retina of striped bass (Morone saxatilis). Outer-segment membrane currents were measured with suction electrodes under voltage clamp provided by tight-seal electrodes applied to the cell's inner segment. Brief flashes of light transiently reduced the outer-segment current with kinetics and sensitivity characteristic of each receptor type. In all cells, the responses to dim lights increased linearly with light intensity. The amplitude-intensity relation for rods and single cones were well described by an exponential saturation function, while for twin cones it was best described by a Michaelis-Menten function. At the wavelength of maximum absorbance, the average intensity necessary to half-saturate the peak photocurrent in dark-adapted rods was 28 photons/microns 2 and in single cones it was 238 photons/microns 2. Among twin cones, the common type (88% of all twins recorded) half-saturated at an average of 1454 photons/microns 2, while the fast type reached half-saturation at an average of 9402 photons/microns 2. The action spectrum of the photocurrent in the three receptor types was well fit by a nomogram that describes the absorption spectrum of a vitamin A2-based photopigment. The wavelength of maximum absorbance for rods was 528 nm, for single cones it was 542 nm and for twin cones it was 605 nm. Both members of the twin pair contained the same photopigment and they were electrically coupled. Under voltage clamp, the response to dim flashes of light in both single and twin cones was biphasic. The initial peak was followed by a smaller amplitude undershoot. Single cones reached peak in 86 ms and common twins in 50 ms. Background light desensitized the flash sensitivity in all photoreceptor types, but was most effective in rods and least effective in fast twins. In the steady state, the desensitizing effect of a background intensity, Ib, at the respective optimum wavelength for each cell was well described by the Weber-Fechner law (1/(1+Ib/Ibo)), where Ibo was, on average (in units of photons/microns 2/s), 1.45 for rods, 1.81 x 10(3) for single cones, 4.56 x 10(3) for common twins, and 6.79 x 10(4) for fast twins.