Light Adaptation in Pecten Hyperpolarizing Photoreceptors

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Abstract
The ability of scallop hyperpolarizing photoreceptors to respond without attenuation to repetitive flashes, together with their low light sensitivity, lack of resolvable quantum bumps and fast photoresponse kinetics, had prompted the suggestion that these cells may be constitutively in a state akin to light adaptation. We here demonstrate that their photocurrent displays all manifestations of sensory adaptation: (a) The response amplitude to a test flash is decreased in a graded way by background or conditioning lights. This attenuation of the response develops with a time constant of 200–800 ms, inversely related to background intensity. (b) Adapting stimuli shift the stimulus-response curve and reduce the size of the saturating photocurrent. (c) The fall kinetics of the photoresponse are accelerated by light adaptation, and the roll-off of the modulation transfer function is displaced to higher frequencies. This light-induced desensitization exhibits a rapid recovery, on the order of a few seconds. Based on the notion that Ca mediates light adaptation in other cells, we examined the consequences of manipulating this ion. Removal of external Ca reversibly increased the photocurrent amplitude, without affecting light sensitivity, photoresponse kinetics, or susceptibility to background adaptation; the effect, therefore, concerns ion permeation, rather than the regulation of the visual response. Intracellular dialysis with 10 mM BAPTA did not reduce the peak-to-plateau decay of the photocurrent elicited by prolonged light steps, not the background-induced compression of the response amplitude range and the acceleration of its kinetics. Conversely, high levels of buffered free [Ca]i (10 μM) only marginally shifted the sensitivity curve (Δσ = 0.3 log) and spared all manifestations of light adaptation. These results indicate that hyperpolarizing invertebrate photoreceptors adapt to light, but the underlying mechanisms must utilize pathways that are largely independent of changes in cytosolic Ca. The results are discussed in terms of aspects of commonalty to other ciliary sensory receptor cells.