Coordinated gating of TRP-dependent channels in rhabdomeral membranes from Drosophila retinas.

Using a newly developed dissociation procedure, we isolated the specialized rhabdomeral membranes from Drosophila retinal photoreceptors. From these membranes, we have recorded spontaneous active currents in excised patch, voltage-clamp recordings. We observed rapid opening events that closely resembled those ascribed to one class of light-activated channels, TRP. All activity exhibited Ba(2+) permeability, little voltage dependence, and sensitivity to La(3+) block. Mutational analysis indicated that the spontaneous activity present in these membranes was TRP-dependent. Excised patches from wild-type rhabdomeral membranes exhibited a wide range of conductance amplitudes. In addition, large conductance events exhibited many conductance levels in the open state. Block of activity by La(3+) both developed and recovered in a stepwise manner. Our results indicate that TRP-dependent channels have a small unitary conductance and that many channels can be gated coordinately.

Current issues in invertebrate phototransduction. Second messengers and ion conductances.

Investigation of phototransduction in invertebrate photoreceptors has revealed many physiological and biochemical features of fundamental biological importance. Nonetheless, no complete picture of phototransduction has yet emerged. In most known cases, invertebrate phototransduction involves polyphosphoinositide and cyclic GMP (cGMP) intracellular biochemical signaling pathways leading to opening of plasma membrane ion channels. Excitation is Ca(2+)-dependent, as are adaptive feedback processes that regulate sensitivity to light. Transduction takes place in specialized subcellular regions, rich in microvilli and closely apposed to submicrovillar membrane systems. Thus, excitation is a highly localized process. This article focuses on the intracellular biochemical signaling pathways and the ion channels involved in invertebrate phototransduction. The coupling of signaling cascades with channel activation is not understood for any invertebrate species. Although photoreceptors have features that are common to most or all known invertebrate species, each species exhibits unique characteristics. Comparative electrophysiological, biochemical, morphological, and molecular biological approaches to studying phototransduction in these species lead to fundamental insights into cellular signaling. Several current controversies and proposed phototransduction models are evaluated.