SHORT-TERM EFFECTS OF DOPAMINE ON PHOTORECEPTORS, LUMINOSITY-HORIZONTAL AND CHROMATICITY-HORIZONTAL CELLS IN THE TURTLE RETINA

The effects of dopamine on luminosity-type horizontal cells have been documented in different vertebrate retinas, both in vivo and in vitro. Some of these effects may reflect direct action of dopamine onto thes e cells, but indirect effects mediated by presynaptic neurons cannot b e ruled out. Furthermore, direct effects of dopamine on horizontal cel ls may affect other, postsynaptic neurons in the outer plexiform layer . To test these possibilities, we studied the effects of dopamine on p hotoreceptors and all types of horizontal cells in the turtle (Pseudem ys scripta elegans) retina. Receptive-field properties, responsiveness to light, and time course of light responses were monitored with intr acellular recordings. Dopamine at a concentration of 40 mu M exerted e ffects with two different time courses. ''Short-term'' effects were fu lly developed after 3 min of dopamine application and reversed within 30 min of washout of the drug. ''Long-term'' effects were fully develo ped after about 7-10 min and could not be washed out during the course of our experiments. Only the ''short-term'' effects were studied in d etail in this paper. These were expressed in a reduction of the recept ive-field size of all types of horizontal cells studied; L1 and L2 lum inosity types as well as Red/Green and Yellow/Blue chromaticity types. The L1 horizontal cells did not exhibit signs of reduced responsivene ss to light under dopamine, while in the L2 cells and the two types of chromaticity cells responsiveness decreased. None of the rods, long-w avelength-sensitive, or medium-wavelength-sensitive cones exhibited an y apparent reduction in their receptive-field sizes or responsiveness to light. The present results suggest that the ''short-term'' effects of dopamine are not mediated by photoreceptors and are probably due to direct action of dopamine on horizontal cells.