Temporal contrast adaptation in salamander bipolar cells

Abstract

This work investigates how the light responses of salamander bipolar cells adapt to changes in temporal contrast: changes in the depth of the temporal fluctuations in light intensity about the mean. Contrast affected the sensitivity of bipolar cells but not of photoreceptors or horizontal cells, suggesting that adaptation occurred in signal transfer from photoreceptors to bipolars. This suggestion was confirmed by recording from photoreceptor-bipolar pairs and observing a direct dependence of the gain of signal transfer on the contrast of the light input. After an increase in contrast, the onset of adaptation in the bipolar cell had a time constant of 1-2 sec, similar to a fast component of contrast adaptation in the light responses of retinal ganglion cells (Kim and Rieke, 2001). Contrast adaptation was mediated by processes in the dendrites of both on and off bipolars. The functional properties of adaptation differed for the two bipolar types, however, with contrast having a much more pronounced effect on the kinetics of the responses of off cells than on cells.

Fig. 1.

Static-nonlinearity model. A, The transformation between light inputs and cellular response was described using a model consisting of a linear filter followed by a time-independent or static nonlinearity. The filter and static nonlinearity were calculated from recordings of 5–10 min of the response of a cell to a fluctuating light input. B, Linear filter for a current-clamped on bipolar cell stimulated with a 30% contrast light input. The insetshows the response of a cell to a 1 sec light step. C, Static nonlinearity determined by plotting the measured response against the linear prediction formed by convolving the light input with the linear filter in B. Each pointrepresents the average measured current (y-axis) for a particular value of the linear prediction (x-axis). Error bars are SE and are mostly obscured by the data points. D, Short section of the measured and predicted response. Mean light intensity, 20,300 photons μm⁻² sec⁻¹ using 640 nm LED; bandwidth, 0–30 Hz. Holding current, 0 pA. Perforated-patch recording.

Fig. 2.

Cone, horizontal, and bipolar responses to an alternating contrast signal. The stimulus alternated between 30 and 10% contrast every 20 sec (see stimulustrace at bottom). Current-clamp responses to a single cycle of this stimulus are shown in A for a cone, B for a horizontal cell, and C for an off bipolar cell. The time dependence of the amplitude of the response of each cell to the fluctuating contrast stimulus was measured by calculating the time-dependent variance from 15–30 cycles of the stimulus. The fluctuating stimulus was independent in each cycle. The variance is shown in D for the cone,E for the horizontal cell, and F for the bipolar. The variance measured in the cone and horizontal cell showed little time-dependent structure after a change in contrast, whereas the variance measured in the bipolar cell showed transients after increases (at t = 0) and decreases (at t= 20 sec) in contrast. Mean light intensity, 18,800 photons μm⁻² sec⁻¹ for the cone and bipolar cell and 18,200 photons μm⁻²sec⁻¹ for the horizontal cell (all using 640 nm LED). Holding currents: −50 pA for the cone, 0 pA for the horizontal, and 0 pA for the bipolar. Perforated-patch recordings.

Fig. 3.

Contrast adaptation in a current-clamped cone, horizontal cell, and off bipolar cell measured using the static nonlinearity model. Same cells and recording conditions as Figure 2. The contrast was alternated between 10 and 30% every 20 sec. Linear filters and static nonlinearities were calculated from 15–30 cycles of this stimulus, excluding the first 4 sec of record after a change in contrast. Linear filters measured for 10% (thick trace) and 30% (thin trace) contrast stimuli are shown in A for the cone, B for the horizontal cell, and C for the off bipolar cell. Static nonlinearities are shown in D for the cone,E for the horizontal cell, and F for the bipolar cell. In each case, the static nonlinearities at 10 and 30% contrast overlapped and hence did not contribute to contrast adaptation. Linear filters at 10 and 30% contrast were similar in the cone and horizontal cell but differed substantially in the bipolar.

Fig. 4.

Contrast affected gain of signal transfer from photoreceptors to bipolars. The gain of signal transfer was measured by injecting depolarizing current into the photoreceptor and measuring the resulting postsynaptic response in the bipolar cell. This was repeated in the presence and absence of a 25% contrast light input.A, Measurements from a current-clamped cone and voltage-clamped on bipolar cell. The cone current was stepped from −50 to +50 pA for 20 msec, and the average change in cone voltage (bottom) and bipolar current (top) was measured. The voltage change in the cone was essentially identical in the presence (thin trace) and absence (thick trace) of the contrast stimulus. The bipolar response to depolarization of the cone decreased in the presence of the contrast stimulus, indicating a contrast-dependent change in the gain of signal transfer. Mean light intensity, 18,800 photons μm⁻² sec⁻¹ using 640 nm LED; bandwidth, 0–30 Hz. Bipolar holding potential, −60 mV.B, Measurements from a current-clamped rod and voltage-clamped off bipolar cell. The bipolar response to stepping the rod holding current from −50 to +50 pA decreased in the presence of a 25% contrast light stimulus, indicating a change in the gain of signal transfer. Mean light intensity, 22 photons μm⁻² sec⁻¹ using 470 nm LED; bandwidth, 0–10 Hz. Bipolar holding potential, −60 mV. Perforated-patch recordings.

Fig. 5.

Kinetics of onset and offset of contrast adaptation. An off bipolar cell was voltage clamped while the contrast of the light input was switched between 10 and 30% every 10 sec. The time-dependent variance was computed from 21 repetitions of this stimulus. The smooth curves fit to the variance are single exponentials, with time constants of 2.1 sec (fit between 0 and 10 sec) and 6.2 sec (fit between 10 and 20 sec). Mean light intensity, 18,200 photons μm⁻² sec⁻¹ using 640 nm LED; bandwidth, 0–30 Hz. The bipolar holding potential was −60 mV, resulting in a −70 pA average current. Perforated-patch recording.

Fig. 6.

Contrast adaptation differed in on andoff bipolar cells. Contrast adaptation was measured for 10 and 30% contrast light inputs using the static nonlinearity model as in Figures 1 and 3. Linear filters measuring the amplitude and kinetics of the current response of a cell are shown in A for a voltage-clamped off bipolar and in B for a voltage-clamped on bipolar. Insets show responses to 1 sec light steps. The holding potential in both cases was −60 mV, resulting in a current of −60 pA in the offbipolar and −90 pA in the on bipolar. Mean light intensity, 17,800 photons μm⁻²sec⁻¹ in A and 18,200 photons μm⁻² sec⁻¹ inB, both using 640 nm LED. C, Collected measures of the amplitude of the filter at 30% contrast relative to that at 10% contrast for 17 off bipolars and 25on bipolars. Error bars are SEM. D, Collected measures of the time-to-peak of the filter at 30% contrast relative to that at 10% contrast. Perforated-patch recordings.

Fig. 7.

Contrast affected both rod- and cone-mediated responses. A, Linear filters for a voltage-clampedoff bipolar cell for 10 and 30% contrast light inputs using 470 nm light at a mean intensity of 24 photons μm⁻² sec⁻¹ and a bandwidth of 0–10 Hz. The wavelength and low mean intensity favored responses of rods over those of cones. B, Linear filters for the same cell for 10 and 30% contrast light inputs using 640 nm light at a mean intensity of 18,800 photons μm⁻²sec⁻¹ and a bandwidth of 0–30 Hz. These conditions favored the responses of L cones over those of rods. The holding potential was −60 mV, resulting in a current of −40 pA. Perforated-patch recording.

Fig. 8.

Contrast adaptation was similar under current and voltage clamp. A, Linear filters for a voltage-clampedoff bipolar cell for 10 and 30% contrast light inputs. Holding potential was −60 mV, resulting in a current of −90 pA.B, Linear filters for the same cell from current-clamp responses. Holding current was −50 pA, resulting in a voltage of −48 mV. Mean light intensity, 16,100 photons μm⁻²sec⁻¹ using 640 nm LED. C, Collected results on the change in amplitude of the filter from voltage-clamp (y-axis) and current-clamp (x-axis) responses. Each point represents one cell. The line has a slope of 1 and hence is the expectation of contrast adaptation was the same under current and voltage clamp. D, Collected results on the change in time-to-peak of the filter. Perforated-patch recording.

Fig. 9.

Amacrine and horizontal cells contributed little to contrast adaptation. A, Linear filters for a voltage-clamped off bipolar cell superfused in normal Ringer's solution for light inputs of 10 and 30% contrast.B, Linear filters for the same cell as inA superfused in Ringer's solution containing 150 μm picrotoxin and 5 μm strychnine. Holding potential was −60 mV, resulting in a current of −60 pA.C, Collected results on the amplitude of the filter at 30% contrast relative to that at 10% contrast for 13 cells tested in both normal Ringer's solution and Ringer's solution containing picrotoxin and strychnine. Each point represents one cell as in A and B. Theline has a slope of 1 and thus represents the expectation if picrotoxin and strychnine had no effect on contrast adaptation. D, Linear filters for a voltage-clampedoff bipolar in normal Ringer's solution. E, Linear filters for the same cell as in D superfused in Ringer's solution containing 5 μm bicuculline. Holding potential was −60 mV, resulting in a current of −30 pA.F, Collected results on amplitude of the filter at 30% contrast relative to that at 10% contrast for seven cells tested in both normal Ringer's solution and Ringer's solution containing bicuculline. All were perforated-patch recordings. Mean light intensity, 17,200 photons μm⁻²sec⁻¹ using 640 nm LED; bandwidth, 0–30 Hz.

Fig. 10.

High concentrations of Ca²⁺buffers suppress contrast adaptation in off, but noton, bipolars. Contrast adaptation was compared in whole-cell recordings from bipolar cells dialyzed with internal solutions containing either 1 or 10 mm of the Ca²⁺ buffer HEDTA. The total Ca²⁺in the internal solutions was changed along with the Ca²⁺ buffer concentration to keep the free Ca²⁺ constant. A, Linear filters for 10 and 30% contrast light inputs for a voltage-clamped offbipolar dialyzed with 1 mm HEDTA. Holding potential was −60 mV, resulting in a current of −50 pA. B, Filters for a voltage-clamped off bipolar dialyzed with 10 mm HEDTA. Holding potential was −60 mV, resulting in a current of −40 pA. C, Collected results fromoff cells on the effect of contrast on the filter amplitude for perforated-patch recordings (native) and whole-cell recordings from cells dialyzed with 1 and 10 mm HEDTA. All were voltage-clamp recordings. D, Filters for a voltage-clamped on bipolar dialyzed with 1 mmHEDTA. Holding potential was −60 mV, resulting in a current of −70 pA. E, Filters for a voltage-clamped onbipolar dialyzed with 10 mm HEDTA. Holding potential was −60 mV, resulting in a current of −40 pA. F, Collected results from on cells. Mean light intensity, 18,500 photons μm⁻² sec⁻¹ using 640 nm LED; bandwidth, 0–30 Hz.