Two light-dependent conductances in Lima rhabdomeric photoreceptors.

Abstract

Light-dependent membrane currents were recorded from solitary Lima photoreceptors with the whole-cell clamp technique. Light stimulation from a holding voltage near the cell's resting potential evokes a transient inward current graded with light intensity, accompanied by an increase in membrane conductance. While the photocurrent elicited by dim flashes decays smoothly, at higher stimulus intensities two kinetically distinct components become visible. Superfusion with TEA or intracellular perfusion with Cs do not eliminate this phenomenon, indicating that it is not due to the activation of the Ca-sensitive K channels that are present in these cells. The relative amplitude of the late component vs. the early peak of the light response is significantly more pronounced at -60 mV than at -40 mV. At low light intensities the reversal potential of the photocurrent is around 0 mV, but with brighter lights no single reversal potential is found; rather, a biphasic response with an inward and an outward component can be seen within a certain range of membrane voltages. Light adaptation through repetitive stimulation with bright flashes diminishes the amplitude of the early but not the late phase of the photocurrent. These observations can be accounted for by postulating two separate light-dependent conductances with different ionic selectivity, kinetics, and light sensitivity. The light response is also shown to interact with some of the voltagesensitive conductances: activation of the Ca current by a brief conditioning prepulse is capable of attenuating the photocurrent evoked by a subsequent test flash. Thus, Ca channels in these cells may not only shape the photoresponse, but also participate in the process of light adaptation.

Ion permeation through light-activated channels in rhabdomeric photoreceptors. Role of divalent cations.

“…By contrast, in Drosophila, photoreceptors' reversal potential measurements indicate that the light-dependent conductance may select Ca over Na by a factor as large as 40• However, as shown by Hardie and Minke (1992), such selectivity for divalent cations is greatly attenuated in the trp null mutant, indicating that it concerns primarily a specific subpopulation of channels. The presence of multiple light-dependent ionic mechanisms seems relatively widespread across invertebrate species (e.g., Nasi, 1991c;Nasi and Gomez, 1992;Hardie and Minke (1992); Deckert et al, 1992). It is unlikely that, at least in mollusks, the secondary component that appears at high stimulus intensities may exhibit a higher Ca selectivity than the primary one, because its reversal potential is significantly more negative (Nasi, 1991c).…”

“…The presence of multiple light-dependent ionic mechanisms seems relatively widespread across invertebrate species (e.g., Nasi, 1991c;Nasi and Gomez, 1992;Hardie and Minke (1992); Deckert et al, 1992). It is unlikely that, at least in mollusks, the secondary component that appears at high stimulus intensities may exhibit a higher Ca selectivity than the primary one, because its reversal potential is significantly more negative (Nasi, 1991c). The difficulties of examining this additional component in isolation preclude a more detailed analysis of its ion selectivity and conduction properties.…”

“…The residual response, normalized and superimposed on a control photocurrent, is shown in Fig. 2 C; an acceleration of the decay of the current was seen in both cells treated with Tris, an effect reminiscent of the faster kinetics induced by light adaptation and by increasing internal Ca (Brown and Lisman, 1975;Fein and Charlton, 1977;Nasi, 1991c). It is likely that a build-up of cytosolic Ca occurs in Nafree media because of disruption of the activity of the N a / C a exchanger, as it was d o c u m e n t e d in Limulus ions is not necessarily the same as the preferred intracellular substitute: NMDG, which displayed a very low permeability but markedly degraded the light response when applied internally, may be suitable externally, precisely because its influx during the photoresponse could be negligible.…”

“…Macroscopic light-evoked currents recorded under whole-cell voltage clamp (Nasi, 1991c) were low-pass filtered at 500-1,500 Hz with a Bessel four-pole filter. For cell-attached recordings of lightdependent single-channel currents (Nasi and Gomez, 1992), the cutoff frequency ranged from 2 to 5 KHz.…”

“…An optical stimulator was used to deliver flashes confined to a small circular area (~150 Ixm in diameter), as previously described (Nasi and Gomez, 1992). Because near-saturating lights can induce the appearance of a second, slower component of the light response, which in Lima has a more negative reversal voltage (Nasi, 1991c), particular care was taken to use moderate stimulus intensities, at which the photocurrent displays a unique reversal potential. Light intensity was measured with a radiometer (United Detector Technology, Hawthorne, CA) and is expressed in terms of flux of effective 500-nm photons, as determined by an in ~ivo calibration, as described elsewhere (Gomez and Nasi, 1994).…”