VISUAL ECOLOGY AND VOLTAGE-GATED ION CHANNELS IN INSECT PHOTORECEPTORS

That particular membrane conductances are selected for expression to e nable the efficient coding of biologically relevant signals is illustr ated by recent work on insect photoreceptors. These studies exploit th e richness of insect vision and the accessibility of insect photorecep tors to cellular analysis in both intact animal and isolated cell prep arations. The distribution of voltage-gated conductances among photore ceptors of different species correlates with visual ecology. Delayed-r ectifier K+ channels are found in the rapidly responding photoreceptor s of fast-flying flies. The conductance's activation range and dynamic s match light-induced signals, and enable a rapid response by reducing the membrane time constant. Slow-moving flies have slowly responding photoreceptors that lack the delayed rectifier, but express an inactiv ating K+ conductance that is metabolically less demanding, Complementi ng these findings, locust photoreceptor membranes are modulated diurna lly. The delayed rectifier is exhibited during the day and the inactiv ating K+ current is exhibited at night. Insect photoreceptors also dem onstrate the amplification of signals by voltage-gated Na+ channels. I n drone-bee photoreceptors, voltage-gated Na+ channels combine with K channels to enhance the small transient signals produced by the image of a queen bee passing over the retina. This subthreshold amplifier o perates most effectively over the range of light intensities at which drones pursue queens,