Facilitation in arthropod photoreceptors

Dear Sir, A paradoxical effect of light has been observed in some invertebrate photoreceptors: an increased amplitude of electrophysiological responses to test flashes following illumination by a step or flash of light. This apparent increase in photoreceptor sensitivity is known as facilitation, and is in contrast to the better known case of reduced sensitivity following illumination (light adaptation). Facilitation was first reported in 1954 by Ruck and Jahn for isopod crustaceans. Subsequently, facilitation has been reported for: arthropods of two other classes Facilitation has never been reported for vertebrate photoreceptors, although the effect of light in temporarily reducing sensitivity has been intensively investigated. A paper by Hanani and Hillman (1976) reported verification of facilitation in barnacle lateral photoreceptors, and proposed that facilitation might be understood by processes which are opposite (see below), but related, to those believed to be involved in light adaptation. From the results of experiments in which the conditioning (i.e. facilitation-inducing) light intensities and extracellular calcium concentrations were varied, Hanani and Hiliman explained that facilitation might be due to a decreased intracellular calcium concentration, or to a "decreased activation affinity" of ion channel-blocking sites. I should like to call attention to the fact that the temporary changes found in photo-receptor cells after termination of illumination involve not only sensitivity but also the time course of the electrophysiological response. It is well known, at least for arthropod photoreceptors, that the time scale of responses recorded from light-shaped photore-ceptors is compressed (and the latency of response is decreased) when compared to dark-adapted responses (Fuortes and Hodgkin, 1964; Fein and Charlton, 1975). If processes opposite to those involved in light adaptation were indeed the basis of facilitation, that is decreased intracellular calcium ion concentration or reduced affinities of ion-channel blocking sites, then an expansion of the time scale (i.e. increased response latencies) would be expected together with increased amplitude. Hanani and Hillman did not report latency data in their paper. Ventura and Puglia (1977), however, performed a careful parametric study of facilitation and ant photoreceptors, but their records showed no systematic differences in time scale between facilitated and unfacilitated electrophysiological responses. This finding relates directly to Hanani and Hiilman's membrane-ionic hypothesis. There is good evidence that illumination results in an increased intracellular calcium concentration (Brown and Blinks, 1974), and that this calcium may have a dual effect on the electro-physiological response: there are sensitivity losses (Lisman and Brown, 1975a, b; …