Early receptor current of wild‐type and transducin knockout mice: photosensitivity and light‐induced Ca²⁺ release

Abstract

We have used suction-electrode recording to measure the early receptor current (ERC) from single, isolated mammalian photoreceptors. When a wild-type mouse rod was illuminated with light sufficient to close all the cGMP-gated channels, a succeeding bright laser flash bleaching a large proportion of the visual pigment produced an ERC, which at 37• C consisted primarily of a single component of transient positive current. The amplitude of total charge movement of this component declined exponentially with successive flashes, consistent with the direct proportionality of the ERC to the quantity of pigment bleached. From the constant of exponential decline, it was possible to estimate the in vivo photosensitivity of mouse rhodopsin to be about 6 × 10 −9 µm 2 per molecule. We have also measured the ERC from rods of transducin-knockout mice, for which previous illumination to close the cGMP-gated channels was not required. The ERC of these rods was similar to that of wild-type rods but was followed by a slow component of outward current whose maximum amplitude in some cells approached that of the normal light response. This slow current was blocked by L-cis diltiazem, indicating that it was produced by ion flux through the cyclic nucleotide-gated channels of the outer segment; however, it could not have been produced by the normal transduction cascade, since it was recorded from rods lacking transducin. Since it was depressed by prior incorporation of the Ca 2+ buffer BAPTA, it was probably generated by light-activated Ca 2+ release earlier demonstrated in salamander and zebrafish. Recordings of the ERC from normal and mutant mice may provide a useful tool for the analysis of models of retinal disease, as well as exploration of the molecular origin of light-activated Ca 2+ release.

Melanopsin-Expressing Retinal Ganglion-Cell Photoreceptors: Cellular Diversity and Role in Pattern Vision

“…Surprisingly, the triple knockout animals (like Gnat1 −/− ; Cnga3 −/− and wildtype mice) were able to discriminate between two uniform stimuli differing markedly in luminance. In triple knockout animals, this ability may be traceable to the early receptor potential of rods and cones, a very small, transient hyperpolarization caused by charge movements triggered when light causes conformational changes in a large numbers of photopigments molecules in rod outer segments (Cone, 1967; Woodruff et al, 2004). This mechanism has been proposed to explain the very weak pupillary light reflex in some of the triple knockout animals (Guler et al, 2007; Hattar et al, 2003).…”

Three-dimensional organization of nascent rod outer segment disk membranes

“…However, because the ERP occurs across the plasma membrane, only rhodopsin molecules that are in the plasma membrane can contribute to it. The signal from mammalian rods is too large to be explained without a subset of disks being open to the extracellular space, thus increasing the effective rhodopsin content of the plasma membrane (13). If the nascent disks are open, they could contribute to the ERP; if not, some of the mature rod disks must be exposed to the extracellular space, an observation not reported in any study to date.…”

“…Differences stem from interpretation of whether the nascent disks are open to the extracellular space (9, 10), or closed from it, like the mature disks (11). Whether the nascent disks are open or closed also has important implications for understanding questions of photoreceptor cell biology and electrophysiology, such as how membrane proteins are sorted into the subdomains of the OS (12) and the origin of the early receptor potential (13).…”

Dynamics of mouse rod phototransduction and its sensitivity to variation of key parameters

“…Reported values are 8.9 pA in [38], 9 pA in [73], 10.4 pA in [69], 11.3 pA in [68], 12.1 pA in [36], 12.3 pA in [36, 70, 71], 12.5 pA in [65], 14.7 pA in [37], 17 pA in [67], and 21 pA in [28]. In some contributions [66, 88], the circulating dark current was assumed to be equal to the value of the saturating light response, and estimated, by indirect simulations as 8.2 pA in [34], 8.8 pA in [72], 8.9 pA in [74], 14.1 pA in [66], 10.8 pA in [37] and 15 pA in [36].…”