Heterogenic components of a fast electrical potential in Drosophila compound eye and their relation to visual pigment photoconversion

The electroretinogram of the dipteran compound eye in response to an intense flash contains an early, diphasic potential that has been termed the M potential. Both phases of the M potential arise from the photostimulation of metarhodopsin. The early, corneal-negative component, the M1, can be recorded intracellularly in the photoreceptors and has properties similar to the classical early receptor potential (ERP). The M1 is resistant to cold, anaesthesia, and anoxia and has no detectable latency. It depends on flash intensity and metarhodopsin fraction in the manner predicted for a closed, two-state pigment system, and its saturation is shown to correspond to the establishment of a photoequilibrium in the visual pigment. On the other hand, the dominant, corneal-positive component, the M2, does not behave like an ERP. It arises, not in the photoreceptors, but deeper in the retina at the level of the lamina, and resembles the on-transient of the electroretinogram in its reversal depth and sensitivity to cooling or CO2. The on-transient, which is present over a much wider range of stimulus intensity than the M potential, has been shown to arise from neurons in the lamina ganglionaris. Visual mutants in which the on-transient is absent or late are also defective in the M2. It is proposed that the M2 and the on-transient arise from the same or similar groups of second-order neurons, and that the M2 is a fast laminar response to the depolarizing M1 in the photoreceptors, just as the on-transient is a fast laminar response to the depolarizing late receptor potential. Unlike the M1, the M2 is not generally proportional to the amount of metarhodopsin photoconverted, and the M2 amplitude is influenced by factors, such as a steady depolarization of the photoreceptor, which do not affect the M1.