Membrane potential affects photocurrent kinetics in salamander rods and cones

Cone-like rectification properties of cGMP-gated channels in transmutated retinal photoreceptors of nocturnal geckoes

Photoreceptors of nocturnal geckoes are scotopic, with rod-shaped outer segments, and sensitivities to light similar to the one of retinal rods from other species of lower vertebrates. However, these cells are not rods, but they originated from cones of ancestral diurnal geckoes with pure-cone retinas, after being forced to adapt to a nocturnal behavior. Several interesting adaptations of these rod-like cones have been studied to date; molecular biology and functional studies confirmed that several proteins of the phototransductive cascade display structural and functional properties that indicate their origin from cones rather than from rods. In this paper, we investigate, with whole cell voltage clamp in the photoreceptor detached outer segment preparation, the voltage rectification properties of cGMP-gated channels in three species, Gekko gecko, Tarentola mauritanica, and Hemidactylus frenatus. We show that the current-voltage properties in the physiological voltage range are reminiscent of the ones of cGMP-gated channels from cones rather than from rods of other cold-blooded vertebrates. The origin and the relevance of the mechanisms investigated are discussed.

Voltage-dependence of ion permeation in cyclic GMP-gated ion channels is optimized for cell function in rod and cone photoreceptors

The kinetics of the photocurrent in both rod and cone retinal photoreceptors are independent of membrane voltage over the physiological range (-30 to -65 mV). This is surprising since the photocurrent time course is regulated by the influx of Ca(2+) through cGMP-gated ion channels (CNG) and the force driving this flux changes with membrane voltage. To understand this paradigm, we measured Pf, the fraction of the cyclic nucleotide-gated current specifically carried by Ca(2+) in intact, isolated photoreceptors. To measure Pf we activated CNG channels by suddenly increasing free 8-Br-cGMP in the cytoplasm of rods or cones loaded with a caged ester of the cyclic nucleotide. Simultaneous with the uncaging flash, we measured the cyclic nucleotide-dependent changes in membrane current and fluorescence of the Ca(2+) binding dye, Fura-2, also loaded into the cells. We determined Pf under physiological solutions at various holding membrane voltages between -65 and -25 mV. Pf is larger in cones than in rods, but in both photoreceptor types its value is independent of membrane voltage over the range tested. This biophysical feature of the CNG channels offers a functional advantage since it insures that the kinetics of the phototransduction current are controlled by light, and not by membrane voltage. To explain our observation, we developed a rate theory model of ion permeation through CNG channels that assumes the existence of two ion binding sites within the permeation pore. To assign values to the kinetic rates in the model, we measured experimental I-V curves in membrane patches of rods and cones over the voltage range -90 to 90 mV in the presence of simple biionic solutions at different concentrations. We optimized the fit between simulated and experimental data. Model simulations describe well experimental photocurrents measured under physiological solutions in intact cones and are consistent with the voltage-independence of Pf, a feature that is optimized for the function of the channel in photoreceptors.

Voltage-dependent gating and block of the cyclic-GMP-dependent current in bovine rod outer segments

The properties of the cyclic-GMP-activated conductance in the plasma membrane of bovine rod outer segments were studied in excised membranes. Multiple-channel and single-channel currents were recorded by the patch-clamp technique in symmetrical NaCl solutions which were free of divalent cations. The current-voltage relationship for the current, recorded when a large population of channels was activated, exhibited outward rectification. Rectification decreased as the concentration of cyclic-GMP was increased, and the concentration of cyclic-GMP required for half maximal activation of the channel decreased with depolarization. At a concentration of 1-3 microM cyclic-GMP, single-channel activity could be observed from these excised patches. The conductance of the open channel was 6 pS and was independent of the membrane potential. These results are consistent with the interpretation that under these conditions, the mechanism responsible for the outward rectification is due to an increase in the probability of an open channel as the membrane is depolarized. The cyclic-GMP-activated current could be blocked by L-cis-diltiazem. Block was voltage and time dependent. The time constant for the onset of block and its steady state level increased with depolarization. The extent of block by diltiazem was not enhanced as the cyclic-GMP concentration was increased, suggesting that the channel is not required to be open for block to occur. Complete block was never attained even for high concentrations of diltiazem. However, the diltiazem-resistant component of the cyclic-GMP-activated current could be blocked by tetracaine.

A derivative of amiloride blocks both the light-regulated and cyclic GMP-regulated conductances in rod photoreceptors

Vertebrate rod photoreceptors in the dark maintain an inward current across the outer segment membrane. The photoresponse results from a light-induced suppression of this dark current. The light-regulated current is not sensitive to either tetrodotoxin or amiloride, potent blockers of Na+ channels. Here, we report that a derivative of amiloride, 3',4'-dichlorobenzamil (DCPA), completely suppresses the dark current and light response recorded from rod photoreceptors. DCPA also blocks a cyclic GMP-activated current in excised patches of rod plasma membrane and a cGMP-induced Ca++ flux from rod disk membranes. These results are consistent with the notion that the Ca++ flux mechanism in the disk membrane and the light-regulated conductance in the plasma membrane are identical. DCPA also inhibits the Na/Ca exchange mechanism in intact rods, but at a 5-10-fold-higher concentration than is required to block the cGMP-activated flux and current. The blocking action of DCPA in 10 nM Ca++ is different from that in 1 mM Ca++, which suggests either that the conductance state of the light-regulated channel may be modified in high and low concentrations of Ca++, or that there may be two ionic channels in the rod outer segment membrane.