Light-induced increase in discrete waves in the dark in Limulus ventral photoreceptors

Abstract

Illumination of a variety of invertebrate photoreceptors with dim light results in a response that is composed of discrete waves (often referred to as 'quantum bumps') 6, t6,17,,~1,24,z6. These discrete waves are believed to result when a single visual pigment molecule is activated by the absorption of a quantum of light 12. Recently, lightinduced discrete fluctuations in membrane current, presumably produced by the absorption of single photons, have also been observed in vertebrate rods zS. Dodge et al. 7 have proposed that the response of Limulus photoreceptors is produced by a summation of these discrete waves, and they have suggested that a change in discrete wave size is the primary mechanism of lightand dark-adaptation. Thus, it seems that the mechanism of discrete wave production is fundamental to the mechanism of phototransduction. In Limulus photoreceptors discrete waves have been found to occur spontaneously in the dark 1,26,27. We report here that, in the dark, following bright illumination, there is a dramatic increase in the membrane noise of kimulus ventral photoreceptors, and that this increase in noise is the result of an increase in the occurrence of discrete waves. The methods of dissecting, viewing, stimulating and recording from the photoreceptor have been described previouslyS,9, it. The intensity of the unattenuated beam of light used as the stimulus was found to be equivalent to 1.5 × 1015 520 nm photons-cm -2 sec I (see ref. 9). Light intensities are given as log10 l/I0, where I0 is the intensity of the unattenuated beam. Fig. 1 is an example of our basic finding, in the dark, preceding stimulation by a bright light, there is a low frequency of discrete waves. The cell responds to the light (log 1 -0.0, 10 sec duration), with a depolarization followed in the dark by an afterhyperpolarization. The afterhyperpolarization appears to be due to an electrogenic sodium pump that is activated by an influx of sodium during the light responseL In the dark, following the exposure to the light, there is a dramatic increase in the membrane noise. As time progresses, following the light exposure, the membrane noise can be