Receptive-field size of L1 horizontal cells in the turtle retina: effects of dopamine and background light.

1. The receptive-field size of turtle L1 horizontal cells was assessed qualitatively from the small-spot/full-field-response amplitude ratio. For quantitative evaluation, the length constant was derived from the response amplitude-spot radius relationship. 2. In each horizontal cell, the length constants were calculated for different intensities of the test light stimuli. The effects of dopamine and/or background light on the small-spot/full-field amplitude ratio and on the length constants were studied. 3. The receptive field of an L1 horizontal cell could not be defined by a single length constant of fixed value. Rather, the length constant changed with the experimental conditions. Two types of changes were noted. An instantaneous one, which was expressed in an increase in the length constant when the test flash was made brighter, and a slow one that occurred when the eyecup was exposed to dopamine. 4. Dopamine increased the small-spot/full-field amplitude ratio and reduced the length constant for a given full-field-response amplitude. It did not alter the responsiveness to light of the horizontal cells. These effects of dopamine were consistent with its action on the coupling resistance between adjacent horizontal cells. 5. Continuous background illumination increased the small-spot/full-field-response amplitude ratio whether studied in normal Ringer or during superfusion with dopamine solution. 6. The relationship between the length constants and the relative amplitude of the full-field responses did not change when the level of ambient illumination was raised either during superfusion with normal Ringer solution or during superfusion with dopamine solution. 7. These data indicate that background lights do not alter the receptive field size of turtle L1 horizontal cells.