FAST-ACTING COMPRESSIVE AND FACILITATORY NONLINEARITIES IN LIGHT-ADAPTED FLY PHOTORECEPTORS

Light-adapted fly photoreceptor cells were stimulated with brief posit ive and negative contrast flashes (contrast = Delta I/I, I = intensity ). Membrane potential responses to a wide range of flash intensities w ere well-fitted by a static nonlinearity followed by a compartmental m odel represented by a gamma function. However, the agreement improved if one parameter of the gamma function, n, varied quadratically with i nput light intensity. Response amplitude and time to peak were estimat ed from the fitted parameters. Response amplitude varied approximately linearly with contrast but showed nonlinear compression with the larg est negative flashes. Reducing the background light level by 3 decades or hyperpolarizing the cell electrically produced stronger nonlinear compression with both contrast polarities. This is probably due to fas t voltage-activated K+ channels. Responses to double flashes with vary ing time separations were well-fitted by summed gamma functions, allow ing separation of the individual flash responses. There was no detecta ble time-dependent interaction between paired positive flashes at all separations. However, the response to two negative flashes was greater than the linear prediction at short separations, and this facilitator y nonlinearity decayed with a time constant of about 1 msec. The facil itation is probably related to resonant behavior in light-adapted phot oreceptors and may be due to an IP3-induced intracellular Ca2+ release .