Investigating the Ca2+-dependent and Ca2+-independent mechanisms for mammalian cone light adaptation

Abstract

Vision is mediated by two types of photoreceptors: rods, enabling vision in dim light; and cones, which function in bright light. Despite many similarities in the components of their respective phototransduction cascades, rods and cones have distinct sensitivity, response kinetics, and adaptation capacity. Cones are less sensitive and have faster responses than rods. In addition, cones can function over a wide range of light conditions whereas rods saturate in moderately bright light. Calcium plays an important role in regulating phototransduction and light adaptation of rods and cones. Notably, the two dominant Ca²⁺-feedbacks in rods and cones are driven by the identical calcium-binding proteins: guanylyl cyclase activating proteins 1 and 2 (GCAPs), which upregulate the production of cGMP; and recoverin, which regulates the inactivation of visual pigment. Thus, the mechanisms producing the difference in adaptation capacity between rods and cones have remained poorly understood. Using GCAPs/recoverin-deficient mice, we show that mammalian cones possess another Ca²⁺-dependent mechanism promoting light adaptation. Surprisingly, we also find that, unlike in mouse rods, a unique Ca²⁺-independent mechanism contributes to cone light adaptation. Our findings point to two novel adaptation mechanisms in mouse cones that likely contribute to the great adaptation capacity of cones over rods.

Figure 1

GCAPs/recoverin-deficient cones are more sensitive and have slower photoresponse kinetics. Responses of control (A) and GCAPs/recoverin-deficient (B) cones to flashes of 505 nm light from 400 to 460,000 and 220 to 180,000 photons μm⁻² in control Gnat1^−/− and GCAPs^−/− Rv^−/− Gnat1^−/− mice, respectively. (C) Response amplitudes (mean ± SEM) plotted as a function of flash intensity (in photons μm⁻²) for control Gnat1^−/− (black squares, n = 4 mice) and GCAPs^−/− Rv^−/− Gnat1^−/− (blue squares, n = 4 mice) mice. The smooth lines plot Eq. (1) with I_1/2 = 5,600 photons μm⁻² (black) and 3,000 photons μm⁻² (blue) for control and GCAPs/recoverin-deficient cones, respectively.

Figure 2

GCAPs/recoverin-deficient cones can adapt to background light. Responses of control (A) and GCAPs/recoverin-deficient (B) cones to steps of 505 nm light (indicated by green bars) from 2,620 up to 407,300 photons μm⁻² s⁻¹ (numbers on the right indicate the background light intensity, identical for A and B). A flash of light was delivered at 4.5 s after the step onset (arrow) to probe the sensitivity of cones (S_F) during different backgrounds. (C) Normalized sensitivity (S_F /S_F,D, where S_F,D is the sensitivity in darkness, (mean ± SEM) plotted as a function of background light intensity in photons μm⁻² s⁻¹ for control cones (black squares, n = 4 mice) and GCAPs/recoverin-deficient cones (blue squares, n = 3 mice). The smooth traces plot Eq. (2) with I₀ = 39,600 μm⁻² s⁻¹ and n = 1.0 for control cones (black), and with I₀ = 10,200 μm⁻² s⁻¹ and n = 1.4 for GCAPs/recoverin-deficient cones (blue). The blue dashed trace plots Eq. (3) calculated from the data measured from dark-adapted GCAPs/recoverin-deficient cones.

Figure 3

Low Ca²⁺ exposure causes large transient increase of r_max and deceleration of flash response kinetics in control mouse cones. (A) Normalized maximal cone response amplitudes (r_max, mean ± SEM, n = 4 mice) to a saturating bright test flash plotted as a function of time after exposing the retina to low Ca²⁺ medium. Amplitudes have been normalized to r_max just before the low Ca²⁺ exposure. (B) Averaged normalized responses (mean, n = 4 mice) of control cones to a dim test flash producing a response with amplitude < 20% of r_max just before the low Ca²⁺ exposure (black) and about 10 min after the switch to low Ca²⁺ solution (red).

Figure 4

Low Ca²⁺ exposure causes moderate and stable increase of r_max in GCAPs/recoverin-deficient cones. (A) Normalized maximal cone response amplitudes (r_max, mean ± SEM, n = 3 mice) to a saturating bright test flash plotted as a function of time after exposing the retina to low Ca²⁺ medium. Amplitudes have been normalized to r_max just before the low Ca²⁺ exposure. (B) Averaged normalized responses (mean, n = 3 mice) of GCAPs/recoverin-deficient cones to a dim test flash producing a response with amplitude < 20% of r_max just before the low Ca²⁺ exposure (black) and about 10 min after the switch to low Ca²⁺ solution (red).

Figure 5

Ca²⁺-dependent and Ca²⁺-independent light adaptation mechanisms contribute to the light adaptation capacity of cones lacking both GCAPs and recoverin. (A) A response to a step of light (I = 17,100 photons μm⁻² s⁻¹) superimposed with a test flash (arrow) recorded from GCAPs/recoverin-deficient cones in normal (blue) and low (red) Ca²⁺ (same retina). The test flash strength was 1,600 and 570 photons μm⁻² in normal and low Ca²⁺, respectively. (B) Sensitivity of cones (S_F) normalized to the sensitivity in darkness (S_F,D) plotted as function of background light intensity (I) for GCAPs/recoverin-deficient cones in normal (blue squares) and low (red squares) Ca²⁺. Smooth lines plot Eq. (2) with I₀ = 10,200 photons μm⁻² s⁻¹ and n = 1.4 in normal Ca²⁺ (blue) and with I₀ = 27,000 photons μm⁻² s⁻¹ and n = 1.7 in low Ca²⁺ (red). The dashed red and green traces plot Eqs (3) and (4), respectively, with parameter values calculated from dark-adapted responses of these cones in low Ca²⁺. Sensitivity data (mean ± SEM) in (B) is from 3 GCAPs^−/− Rv^−/− Gnat1^−/− mice for which we used identical background light intensities. The theoretical traces plot the mean values for the same 3 GCAPs^−/− Rv^−/− Gnat1^−/− mice. For comprehensive statistical analysis, see Table 1 and text.