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

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Abstract
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. 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. 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 Ca2+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. 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.