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 th e response amplitude-spot radius relationship. 2. In each horizontal c ell, the length constants were calculated for different intensities of the test light stimuli. The effects of dopamine and/or background lig ht on the small-spot/full-field amplitude ratio and on the length cons tants were studied. 3. The receptive field of an L1 horizontal cell co uld not be defined by a single length constant of fixed value. Rather, the length constant changed with the experimental conditions. Two typ es of changes were noted. An instantaneous one, which was expressed in an increase in the length constant when the test flash was made brigh ter, and a slow one that occurred when the eyecup was exposed to dopam ine. 4. Dopamine increased the small-spot/full-field amplitude ratio a nd reduced the length constant for a given full-field-response amplitu de. It did not alter the responsiveness to light of the horizontal cel ls. These effects of dopamine were consistent with its action on the c oupling resistance between adjacent horizontal cells. 5. Continuous ba ckground illumination increased the small-spot/full-field-response amp litude ratio whether studied in normal Ringer or during superfusion wi th dopamine solution. 6. The relationship between the length constants and the relative amplitude of the full-held responses did not change when the level of ambient illumination was raised either during superf usion with normal Ringer solution or during superfusion with dopamine solution. 7. These data indicate that background lights do not alter t he receptive field size of turtle L1 horizontal cells.