The cyclic GMP-gated channel responds to changes in free intracellular cGMP, and as a result, it plays a central role in the phototransduction process in rod and cone photoreceptor cells. Recent biochemical, immunochemical, and molecular biology studies indicate that this channel consists of a complex of two distinct subunits and one or more associated proteins. Primary structural analysis indicates that the a and (3 subunits contain a cGMP-binding domain, an even number of membrane-spanning segments, a voltage sensor motif and a pore region. The latter two features are found in voltage-gated channels and suggest that these two classes of channels have evolved from the same ancestral channel protein. A working model for the membrane topography of the channel subunits is proposed based on immunogold labeling studies and sequence analysis. Recent studies also indicate that calmodulin binds to the 240 kDa protein of the channel complex and modulates the sensitivity of the channel for cGMP in a Ca2+-dependent manner. The molecular properties of the channel complex and the possible role of Ca2+-calmodulin modulation of the channel during photoactivation and photorecovery are discussed in relation to the current mechanism of phototransduction in photoreceptor cells

Synaptic transmission from photoreceptors to depolarizing bipolar cells is mediated by the APB glutamate receptor. This receptor apparently is coupled to a G-protein which activates cGMP-phosphodiesterase to modulate cGMP levels and thus a cGMP-gated cation channel. We attempted to localize this system immunocytochemically using antibodies to various components of the rod phototransduction cascade, including Gt (transducin), phosphodiesterase, the cGMP-gated channel, and arrestin. All of these antibodies reacted strongly with rods, but none reacted with bipolar cells. Antibodies to a different G-protein, Go, reacted strongly with rod bipolar cells of three mammalian species (which are depolarizing and APB-sensitive). Also stained were subpopulations of cone bipolar cells but not the major depolarizing type in cat (b1). Go antibody also stained certain salamander bipolar cells. Thus, across a wide range of species, Go is present in retinal bipolar cells, and at least some of these are depolarizing and APB-sensitive.

Effects of ATP on the activity of cGMP-gated channels from carp cone photoreceptors were studied. In 29% of the patches examined (N = 45), ATP (1 mM) enhanced a current evoked by cGMP (20 μM, up to about 100%), in 33%, ATP suppressed it by up to about 90%, and in the remaining 38%, ATP had no effect. ATP showed similar effects on a current evoked by 8-bromoguanosine 3′,5′-cyclic monophosphate (2 μM, enhancing in 42% of the patches, suppressing in 25%, no effect in 33%, N = 12), suggesting that the effects were not through modulation of the phosphodiesterase. Both of the effects, enhancement and suppression, were produced by a change in apparent affinity for cGMP, since (1) the maximum current evoked by cGMP of the saturating concentration (≥1 mM) was not affected, and (2) the A1/2 value decreased by approximately 45% (N = 2) or increased by approximately 25% (N = 2). A lower pH (approximately 6) facilitated the enhancing effect. ATP-γ-S (1 mM) showed a suppressing effect in 80% of the patches and no effect in 20% of the patches (N = 10). However, ATP-γ-S did not show an enhancing effect. Thus, ATP had two opposite effects through different mechanisms on the apparent sensitivity of the channel to cGMP; increasing and decreasing.

Certain ganglion cells in the mammalian retina are known to express a cGMP-gated cation channel. We found that a cGMP-gated current modulates spike responses of the ganglion cells in mammalian retinal slice preparation. In such cells under current clamp, bath application of the membrane-permeant cGMP analog (8-bromo-cGMP, 8-p-chlorophenylthio-cGMP) or a nitric oxide donor (sodium nitroprusside, S-nitroso-N-acetyl-penicillamine) depolarized the membrane potential by 5–15 mV, and reduced the amount of current needed to evoke action potentials. Similar effects were observed when the membrane potential was simply depolarized by steady current. The responses to cGMP are unaffected by inhibitors of cGMP-dependent protein kinase and Ca2+/calmodulin-dependent protein kinase. The response to cGMP persisted in Ca2+-free bath solution with Ca2+ buffers in the pipette. Under voltage clamp, cGMP analogs did not affect the response kinetics of voltage-gated currents. We conclude that cGMP modulates ganglion cell spiking simply by depolarizing the membrane potential via the inward current through the cGMP-gated channel. Modulation of this channel via the long-range NO-synthase amacrine cell may contribute to control of contrast gain by peripheral mechanisms.

The present study has examined the spatial and temporal expression patterns of various proteins associated with the structure and function of mature photoreceptor outer segments in the developing ferret's retina using immunocytochemistry and RT-PCR. One set of proteins, including rod opsin, arrestin, and recoverin, was detected progressively in photoreceptors as they became postmitotic, being expressed well before the differentiation of outer segments. A second set of proteins, including β- and γ-transducin, cGMP-phosphodiesterase, phosducin, rhodopsin kinase, rod cGMP-gated cation channel protein, and peripherin, displayed a contrasting temporal onset and pattern of spatial emergence. These latter proteins first became detectable either shortly before or coincident with outer segment formation, and were expressed simultaneously in both older and younger photoreceptor cells. A third set, the short wavelength-sensitive (SWS) and medium wavelength-sensitive (MWS) cone opsin proteins, was the last to be detected, but materialized in a spatio-temporal pattern reminiscent of the neurogenetic gradient of the cones. These different spatial and temporal patterns indicate that cellular maturation must play a primary role in regulating the onset of expression of some of these proteins, while extrinsic signals must act to coordinate the expression of other proteins across photoreceptors of different ages.