A B S T R A C TTight-seal recording was employed to investigate membrane currents in hyperpolarizing ciliary photoreceptors enzymatically isolated from the eyes of the file clam (Lima scabra) and the bay scallop (Pecten irradians). These two organisms are unusual in that their double retinas also possess a layer of depolarizing rhabdomeric cells. Ciliary photoreceptors from Lima have a rounded soma, 15-20 p.m diam, and display a prominent bundle of fine processes up to 30 p.m long. The cell body of scallop cells is similar in size, but the ciliary appendages are modified, forming small spherical structures that protrude from the cell. In both species light stimulation at a voltage near the resting potential gives rise to a graded outward current several hundred pA in amplitude, accompanied by an increase in membrane conductance. The reversal potential of the photocurrent is ---80 mV, and shifts in the positive direction by ~ 39 mV when the concentration of extracellular K is increased from 10 to 50 raM, consistent with the notion that light activates K-selective channels. The light-activated conductance increases with depolarization in the physiological range of membrane voltages (-30 to -70 mV). Such outward rectification is greatly reduced after removal of divalent cations from the superfusate. In Pecten, cellattached recordings were also obtained; in some patches outwardly directed single-channel currents could be activated by light but not by voltage. The unitary conductance of these channels was ~ 26 pS. Solitary ciliary cells also gave evidence of the post stimulus rebound, which is presumably responsible for initiating the "off" discharge of action potentials at the termination of a light stimulus: in patches containing only voltage-dependent channels, light stimulation suppressed depolarization-induced activity, and was followed by a strong burst of openings, directly related to the intensity of the preceding photostimulation.

The hyperpolarizing receptor potential of scallop ciliary photoreceptors is attributable to light-induced opening of K(+)-selective channels. Having previously demonstrated the activation of this K(+) current by cGMP, we examined upstream events in the transduction cascade. GTP-gamma-S produced persistent excitation after a flash, accompanied by decreased sensitivity and acceleration of the photocurrent, whereas GDP-beta-S only inhibited responsiveness, consistent with the involvement of a G-protein. Because G(o) (but not G(t) nor G(q)) recently has been detected in the ciliary retinal layer of a related species, we tested the effects of activators of G(o); mastoparan peptides induced an outward current suppressible by blockers of the light-sensitive conductance such as l-cis-diltiazem. In addition, intracellular dialysis with the A-protomer of pertussis toxin (PTX) depressed the photocurrent. The mechanisms that couple G-protein stimulation to changes in cGMP were investigated. Intracellular IBMX enhanced the photoresponse with little effect on the baseline current, a result that argues against regulation by light of phosphodiesterase activity. LY83583, an inhibitor of guanylate cyclase (GC), exerted a reversible, dose-dependent suppression of the photocurrent. By contrast, ODQ, an antagonist of NO-sensitive GC, and YC-1, an activator of NO-sensitive GC, failed to alter the light response or the holding current; furthermore, the NO synthase inhibitor N-methyl- l-arginine was inert, indicating that the NO signaling pathway is not implicated. Taken together, these results suggest a novel type of phototransduction cascade in which stimulation of a PTX-sensitive G(o) may activate a membrane GC to induce an increase in cGMP and the consequent opening of light-dependent channels.

Xenopus oocytes express a gene encoding bovine rhodopsin as well as its SP6 RNA polymerase-derived transcripts and total retinal mRNA. The opsin produced is in unglycosylated (30 kDa) and two glycosylated (35 kDa and 41 kDa) forms. Incubation of the cells expressing the above proteins with 11-cis-retinal generates rhodopsin, which was purified by iminunoaffinity chromatography. The purified protein shows the expected UV/visible absorption spectrum and characteristic light-dependent activation of the rod outer segment GTP binding protein. Oocytes expressing rhodopsin exhibit light-dependent ionic currents that are detected by voltage-clamp techniques.The process of visual transduction in rod photoreceptor cells begins with photon absorption by rhodopsin, 11-cis to all-trans isomerization of its retinal chromophore, and relaxation of the protein through a series of metastable photointermediates (1). One of the intermediates, metarhodopsin II, is believed to activate a guanine nucleotide binding protein (G protein)-transducin (GT)-and thus initiate an enzymatic cascade that results in hyperpolarization of the rod cell (2). Similar interactions between membrane-bound receptors and G proteins occur in many signal transduction systems (3). Sequence homology between the receptors (4) on the one hand and the G proteins (5) on the other hand suggests that the mechanism of interaction between these two classes of proteins has been conserved. However, the structural basis underlying receptor-mediated G-protein activation remains unclear. The visual system provides a highly attractive model for the study of this interaction.As an approach to rhodopsin structure-function studies, we have expressed a synthetic rhodopsin gene (6) in mammalian cells and have shown that the rhodopsin isolated after addition of 11-cis-retinal to the cells is active in vitro (7,8). Xenopus oocytes offer the possibility of an alternative expression system in which recombinant proteins can be assayed in situ, often by electrophysiological recordings (9). Furthermore, oocytes contain G protein-regulated ionic conductances (9, 10), and coupling to these could form the basis of an assay for rhodopsin. Indeed, a number of cloned receptor genes expressed in oocytes have been assayed by their ability to activate endogenous channels (11)(12)(13). In this report we demonstrate that a bovine opsin gene is expressed by Xenopus oocytes, and the opsin produced is active in vitro. Further, we show that rhodopsin in oocytes produces light-dependent ionic currents that are detected by voltageclamp techniques.

A B S T R A C T Retinas from the scallopPecten irradians were enzymatically dispersed, yielding a large number of isolated photoreceptors suitable for tight-seal recording. Whole-cell voltage clamp measurements demonstrated that the phototransducing machinery remained intact: quantum bumps could be elicited by dim illumination, while brighter flashes produced larger, smooth photocurrents. Single-channel currents specifically activated by light were recorded in cell-attached patches, and were almost exclusively confined to the rhabdomeric region. Their density is sufficiendy high to account for the macroscopic photoresponse. Channel activation is graded with stimulus intensity in a range comparable to that of the whole-cell response, and can be recorded with illumination sufficiently dim to evoke only quantum bumps. Light-dependent channel openings are very brief, on average 1 ms or less at 20-22°C, apparently not because of blockage by extracellular divalent cations. The mean open time does not change substantially with stimulus intensity. In particular, since dwell times are in the millisecond range even with the dimmest lights, the channel closing rate does not appear to be the rate-limiting step for the decay kinetics of discrete waves. The latency of the first opening after light onset is inversely related to light intensity, and the envelope of channel activity resembles the time course of the whole-cell photocurrent. Unitary currents are inward at resting potential, and have a reversal voltage similar to that of the macroscopic light response. Voltage modulates the activity of light-sensitive channels by increasing the opening rate and also by lengthening the mean open times as the patch is depolarized. The unitary conductance of the predominant class of events is ~ 48 pS, but at least one additional category of smaller-amplitude openings was observed. The relative incidence of large and small events does not appear to be related in a simple way to the state of adaptation of the cell.