The dynamic characteristics of the feedback signal from horizontal cells to cones in the goldfish retina

Abstract

1. The dynamic properties of the microcircuitry formed by cones and horizontal cells in the isolated goldfish retina were studied. Cones project to horizontal cells and horizontal cells feed back to cones via a relatively slow negative feedback pathway. 2. The time constant of the feedback signal in cones and of the effect this feedback signal had on the responses of second-order neurons was determined using whole-cell patch clamp and intracellular recording techniques. 3. It was found that the feedback signal in cones had a time constant of around 80 ms, whereas the time constant of the effect this feedback signal had on the second-order neurons ranged from 36 to 116 ms. This range of time constants can be accounted for by the non-linearity of the Ca(2+) current in the cones. In depolarized cones, the feedback-mediated response in second-order neurons had a similar time constant to that of the direct light response of the cone, whereas in hyperpolarized cones, the time constant of the feedback-mediated response in second-order neurons was considerably larger. 4. Further, it was shown that there was no delay in the feedback pathway. This is in contrast to what has been deduced from the response properties of second-order neurons. In one type of horizontal cell, the responses to red light were delayed relative to the responses to green light. This delay in the second-order neurons can be accounted for by the interaction of the direct light response of the medium-wavelength-sensitive cones (M-cones) with the feedback response of the M-cones received from the horizontal cells.

Figure 1

A, response of a MHC to a 550 nm, −1.3 log units, 3000 μm spot of 500 ms duration. The rollback (arrow) in the HC response has a time constant of about 120 ms. B, response of a BHC to a 700 nm, −1.3 log units, 3000 μm spot of 500 ms duration. The time constant of the depolarizing response is about 35 ms. The timing of the stimulus and response scale are shown in the figure.

Figure 5

A, simulated current-voltage relation of the Ca²⁺ current (eqn (2) in the Appendix) in a cone during stimulation with an intense 65 μm white spot (continuous line) and during stimulation with an additional 3000 μm spot (dashed line). Negative feedback from HCs to cones results in a shift of the Ca²⁺ current activation function to more negative potentials (continuous arrow). This shift will induce an increase in the Ca²⁺ current (dashed arrow). By choosing a time constant for the feedback signal of 80 ms, the feedback responses with a time constant ranging from about 30 to about 140 ms could be simulated (B and C).

Figure 2

A, feedback-induced response of an M-cone clamped at −45 mV, which was continuously saturated with a bright white spot of 65 μm and, in addition, stimulated for 500 ms with a 3000 μm white spot of -1.3 log units. The timing of the stimulus and scale of the response are shown in the figure. B, intensity dependence of the feedback response. Mean time constants of the feedback response of 7 cones clamped at −47 mV, which were continuously saturated with an intense white spot of 65 μm and, in addition, stimulated for 500 ms with a 3000 μm 550 nm spot of increasing intensity. C, wavelength dependence of the feedback response. Mean time constants of the feedback responses of 5 cones clamped at −47 mV, which were continuously saturated with an intense white spot of 65 μm and, in addition, stimulated for 500 ms with a 3000 μm spot of various wavelengths at an intensity of −1.3 log units.

Figure 3

A, feedback responses of a voltage-clamped M-cone, which was continuously saturated with an intense white bright spot of 65 μm and, in addition, stimulated for 500 ms with a 3000 μm 550 nm spot. The clamp potentials of the cone are given to the left of the figure. An exponential function was fitted through these responses and the mean time constants are plotted as a function of holding potential in B.

Figure 4

Light responses of a BHC to full-field light stimuli of 600, 650 and 700 nm at three intensities. The dashed lines are exponential functions fitted through the depolarizing phase of the response. The time constant of this part of the response increased with intensity (see text).

Figure 6

A, light responses of a BHC to stimuli of three intensities and six wavelengths. Short and middle wavelength stimuli evoked hyperpolarizing responses, whereas long wavelength stimuli induced depolarizing responses. The timing of the stimuli and scale of the responses are shown in the figure. B, light responses of a BHC to 600, 650 and 700 nm full-field light stimuli compared to 500 nm wavelength-induced light responses of the same BHC. The responses are scaled and inverted such that the light responses overlap the 500 nm light responses (grey traces). The timing of the stimulus is indicated in the figure. C, redrawing of the light onset responses shown in B on an expanded time scale.

Figure 7

The normalized onset phase of the direct voltage light response of a cone to a 65 μm spot (black trace) and of the feedback response (current) of the same cone clamped at −45 mV (grey trace). This figure illustrates that the onset of the feedback response is not delayed relative to the onset of the direct light response.

Figure 8

A, simulation of the feedback response in an M-cone for long wavelength stimuli. The time constant of the feedback signal in the cone is 80 ms and the time constant of the direct light response is 30 ms. The continuous trace is the change in Ca²⁺ current when the M-cone does not respond due to direct light stimulation (i.e. deep red light). The dashed traces are the changes in Ca²⁺ current when the M-cone responses have the amplitude indicated on the right of the figure. B, the onset of the change in Ca²⁺ current on an expanded time scale.