MODULATION OF TRANSDUCTION GAIN IN LIGHT ADAPTATION OF RETINAL RODS

The effect of light adaptation on the period of photocurrent saturatio n induced by a bright stimulating flash was examined in rod photorecep tors of the larval-stage tiger salamander (Ambystoma tigrinum). Using suction electrodes, photocurrent responses to brief flashes were recor ded from single, isolated rods in the presence and absence of steady b ackground illumination. Background light decreased the saturation peri od (T) measured at fixed flash intensity (fixed I-f) and in this respe ct light-adapted the saturating response. Effects of the background on responses to weak (i.e. subsaturating) and bright flashes were compar ed with changes in a parameter, psi = e(-Delta T/tau R), where Delta T is the decrease in saturation period, and where tau(R) is the slope of the line that relates T and In I in a given state of adaptation. D ark- and light-adapted responses to flash intensities I-f(D) and I-f(L ) respectively, exhibited similar absolute peak photocurrent and falli ng-phase kinetics when I-f(D) and I-f(L) satisfied the relation, I-f(D ) = psi(I-f(L) + I-b tau(R)), where I-b is the background intensity. It is argued that psi approximates the relative PDE/R* gain of transd uction, i.e. the relative peak level of activated cGMP phosphodiestera se (PDE) produced by a given, small amount of photoactivated visual p igment (R). Interpreted on this view, the results imply that light ad aptation derives largely from a decrease in PDE/R* gain, rather than from the stimulation. of guanylate cyclase activity. The data are cons istent with the possibility that modulation of the lifetime of PDE un derlies the background dependence of psi.