DOPAMINE D2 RECEPTOR-MEDIATED MODULATION OF ROD-CONE COUPLING IN THE XENOPUS RETINA

We studied the responses of rod photoreceptors that were elicited with light flashes or sinusoidally modulated Light by using intracellular recording. Dark-adapted Xenopus rod photoreceptors responded to sinuso idally modulated green lights at temporal frequencies between 1 Hz and 4 Hz. In normal Ringer's solution, 57% of the rods tested could follo w red Lights that were matched for equal rod absorbance to frequencies >5 Hz, indicating an input from red-sensitive cones. Quinpirole (10 m u M), a D2 dopamine agonist, increased rod-cone coupling, whereas spip erone (5 mu M), a selective D2 antagonist, completely suppressed it. D 1 dopamine ligands were without effect. Neurobiotin that was injected into single rods diffused into neighboring rods and cones in quinpirol e-treated retinas but only diffused into rods in spiperone-treated ret inas. A subpopulation of rods (ca. 10% total rods) received a very str ong cone input, which quickened the kinetics of their responses to red flashes and greatly increased the bandpass of their responses to sinu soidally modulated Light. Based on electron microscopic examination, w hich showed that rod-rod and cone-cone gap junctions are common, where as rod-cone junctions are relatively rare, we postulate that cone sign als enter the rod network through a minority of rods with strong cone connections, from which the cone signal is further distributed in the rod network. A semiquantitative model of coupling, based on measures o f gap-junction size and distribution and estimates of their conductanc e and open times, provides support for this assumption. The same netwo rk would permit rod signals to reach cones. J. Comp. Neurol. 398:529-5 38, 1998. (C) 1998 Wiley-Liss, Inc.