Enigma of early receptor potential in fly eyes

The early receptor potential (ERP) of fly photoreceptors R1-6 has been recorded in the blowfly Calliphora erythrocephala, and quantitatively analyzed. Photoconversion of the native visual pigment (P) into the meta-state (M) does not induce a measurable ERP. Photoconversion of M into P induces a biphasic ERP, presumably due to the slow thermal decay of intermediate N, recently discovered by Roebroek, Gagné and Stavenga (1989). The experimentally measured ERP's are interpreted with an RC-model which incorporates the time constant of the flash, the time constant of the thermal decay of intermediate N, the electrical properties of the photoreceptor cell and the charge displacements within the visual pigment molecules occurring upon photoconversion. The model satisfactorily describes the ERP for the M to P conversion by assuming that effectively 0.03 electron charges are moved across the cell membrane during conversion of the M to the N state and that 0.04 electron charges are moved in the opposite direction during conversion of N to P. The net 0.01 electron charge movement during conversion of P to M would, according to the simple RC-model, induce a measurable ERP, in conflict with the experimental results. The longstanding enigma of the ERP of fly photoreceptors appears largely but not completely resolved.