The amplitude of the response depends upon the rates of

GPCR activation and deactivation and any inherent nonlinearities imposed by spatial compartmentalization or signal saturation (e.g., Ramanathan et al., 2005). In retinal rod photoreceptors, a single activated GPCR, rhodopsin (R∗), drives the signaling cascade that decreases cGMP and its associated inward cation current in a manner that is highly reproducible from trial to trial (e.g., Rieke and Baylor, 1998). The amplitude of the single-photon response (SPR) is considered a key factor in overcoming intrinsic cellular noise and thus for reliable transmission through the visual pathway (Field Luminespib research buy et al., 2005). There has been much progress in understanding the molecular basis of the amplification and deactivation steps that underlie the rod SPR (reviewed in Burns and Pugh, 2010). In the initial amplifying step, a R∗ activates G proteins at a rate of several hundred per second (Leskov et al., 2000; Heck and Hofmann, 2001) until R∗ is deactivated by phosphorylation by GRK1 and arrestin-1 binding (Kühn and Wilden, 1987). The second major amplifying step arises from cGMP ZD1839 order hydrolysis by the activated G protein-PDE6 enzyme complex (G∗-E∗), whose activity persists until deactivation by GTP hydrolysis catalyzed by

the RGS9 complex (He et al., 1998; Makino et al., 1999; Hu and Wensel, 2002). Although rapid R∗ and G∗-E∗ deactivation are required for normal recovery of the SPR, they are not sufficient; the fall in intracellular calcium that accompanies the SPR Adenylyl cyclase must activate the synthesis of cGMP through guanylate cyclase activating proteins (GCAPs). Abolishing calcium feedback via GCAPs increases the amplitude and slows the recovery of the SPR (Mendez et al., 2001; Burns et al., 2002). In addition, loss of feedback via GCAPs increases the intrinsic cellular noise in a manner that can impair transmission at the rod-to-rod bipolar synapse and behavioral performance at visual threshold (Okawa et al., 2010). Genetic perturbations of R∗

and G∗-E∗ deactivation also produce dramatic changes in the overall time course of the rod photoresponse. Nonetheless, the SPRs of rods with defective deactivation have average peak amplitudes very close to those of wild-type rods. For example, preventing rhodopsin phosphorylation slows the rate of R∗ deactivation 75-fold, from a normal average lifetime of ∼40 ms (Gross and Burns, 2010) to about 3 s (Grk1−/−; Chen et al., 1999), yet the amplitude of the SPR is increased by a factor of only two. Similarly, abolishing expression of the RGS9 complex slows G∗-E∗ deactivation about 10-fold yet has only a subtle effect on the SPR amplitude ( Chen et al., 2000; Krispel et al., 2003; Keresztes et al., 2004).