Light-activated Ion Channels Photoreceptors of the Scallop Pecten irradians in Solitary

I N T R O D U C T I O N It has been established that gated ion channels underlie the generat ion o f the light response in both vertebrate and invertebrate photoreceptors (Bacigalupo and Lisman, 1983; Matthews, 1987). While in amphibian rods and cones unitary currents Address reprint requests to Dr. Enrico Nasi, Department of Physiology, Boston University School of Medicine, 80 E. Concord St., Boston, MA 02118. j. GEN. PHYSIOL. © The Rockefeller University Press • 0022-1295/92/05/0747/23 $2.00 Volume 99 May 1992 747-769 747 on A uust 9, 2017 jgp.rress.org D ow nladed fom 748 T H E JOURNAL OF GENERAL PHYSIOLOGY • VOLUME 99 • 1992 can only be resolved in the absence of extracellular divalent cations (Haynes, Kay, and Yau, 1986; Zimmerman and Baylor, 1986; Matthews, 1987), the transduction channels of invertebrate visual cells can be examined under physiological conditions. However, rapid progress toward a detailed characterization of their properties has been hindered in part by the technical difficulties involved in obtaining glia-free cells suitable for patch clamp recording from the standard invertebrate eye preparations (Bacigalupo and Lisman, 1983). We have circumvented some of these problems by enzymatically isolating visual cells from the eye of the scallop. One of the remarkable features of its retina is the presence of two separate layers of photoreceptors (Dakin, 1910). The proximal layer is composed of rhabdomeric cells that depolarize in response to light (McReynolds and Gorman, 1970a), while cells in the distal retina possess modified ciliary appendages (Miller, 1958; Barber, Evans, and Land, 1967) and produce hyperpolarizing photoresponses (McReynolds and Gorman, 1970a). In b o t h cases a light-induced increase in membrane conductance has been demonstrated (McReynolds and Gorman, 1970b), but with markedly different ion selectivity (Gorman and McReynolds, 1978). The apparent lack of interneurones and synaptic interconnections (Barber et al., 1967) indicates that these responses are true receptor potentials by primary sensory cells, making this organism particularly interesting for vision research. In this report we examine light-dependent, single-channel currents recorded from cell-attached patches in isolated photoreceptors of the rhabdomeric type. The properties of these channels closely resemble those of the cell's macroscopic light response. Patch clamp measurements in hyperpolarizing ciliary cells (Gomez and Nasi, 1991) will be described in a separate article. Preliminary reports of these results have appeared previously (Nasi and Gomez,