Non-local interactions between light induced processes in Calliphora photoreceptors

The prolonged depolarizing afterpotential (PDA) is a phenomenon which is tightly linked to visual pigment conversion. In order to determine whether processes underlying PDA induction and depression can spread in space, the PDA was recorded intracellularly in white-eyed Calliphora R1-6 photoreceptors and used to examine interactions between processes induced by activating statistically different photopigment molecules (Figs. 3-6). It was found that a PDA induced by converting some fraction of rhodopsin (R) molecules forward into the metarhodopsin (M) state can be completely depressed by equal or smaller amounts of pigment conversion, backward from metarhodopsin to rhodopsin even when largely different sets of pigment molecules were shifted in the respective directions, in agreement with previous experiments conducted on the barnacle. The characteristics of the afterpotentials obtained following the cessation of strong blue and green light stimuli which did not cause a net pigment conversion was examined (Figs. 7, 8). It was found that these afterpotentials, obtained when no net R to M conversion took place, could not be depressed by an opposite net large M to R pigment conversion. Accordingly we propose to restrict the term PDA to an afterpotential which can be depressed by a net M to R pigment conversion. It is concluded: (a) that some processes underlying PDA induction and depression in Calliphora must interact at a distance which extends at least to the nearest neighboring pigment molecule, and (b) that in Calliphora photoreceptors net pigment conversion is required in order to induce and depress a PDA.