Phototransduction by microvillar photoreceptors of invertebrates: mediation of a visual cascade by inositol trisphosphate

The basic anatomy and electrophysiology of the microvillar photoreceptors of invertebrates are reviewed. Excitation and adaptation of the photoreceptor are defined and the need for transmitters to mediate these processes within the cell is discussed. The review then focuses on one internal messenger, intracellular calcium, and describes the light-induced release of calcium from stores within microvillar photoreceptors. A biochemical mechanism for calcium-release by light mediated by inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3 ) is proposed, based on experimental evidence derived chiefly from studies on the photoreceptors of the ventral eye of Limulus polyphemus. Intracellular injection of Ins(1,4,5)P 3 mimics excitation and adaptation of Limulus ventral photoreceptors by light. These actions appear to be mediated through the release by Ins(1,4,5)P 3 of calcium from stores within the photoreceptor. Biochemical evidence for a cascade of reactions that link the absorption of light to the elevation in Ins(1,4,5)P 3 is presented. Rhodopsin activates a GTP-binding protein which, in turn, activates a phospholipase-C so as to elevate levels of Ins(1,4,5)P 3 . This visual cascade is identical to cascades that have been proposed to link the binding of hormones to calcium-release in many other non-photoreceptive cell types. The roles of calcium in excitation and adaptation by light and the possibility of other internal messengers are discussed.