Dopamine D2 receptor-mediated modulation of rod-cone coupling in the Xenopus retina

Abstract

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 sinusoidally modulated green lights at temporal frequencies between 1 Hz and 4 Hz. In normal Ringer's solution, 57% of the rods tested could follow red lights that were matched for equal rod absorbance to frequencies >5 Hz, indicating an input from red-sensitive cones. Quinpirole (10 microM), a D2 dopamine agonist, increased rod-cone coupling, whereas spiperone (5 microM), a selective D2 antagonist, completely suppressed it. D1 dopamine ligands were without effect. Neurobiotin that was injected into single rods diffused into neighboring rods and cones in quinpirole-treated retinas but only diffused into rods in spiperone-treated retinas. A subpopulation of rods (ca. 10% total rods) received a very strong cone input, which quickened the kinetics of their responses to red flashes and greatly increased the bandpass of their responses to sinusoidally modulated light. Based on electron microscopic examination, which showed that rod-rod and cone-cone gap junctions are common, whereas rod-cone junctions are relatively rare, we postulate that cone signals 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 of gap-junction size and distribution and estimates of their conductance and open times, provides support for this assumption. The same network would permit rod signals to reach cones.

Fig. 1

Rod responses to flashes and sinusoidally modulated light. Left: Intracellular recordings of rod responses to sinusoidal modulation of 567 nm log quanta (log Q) incident cm⁻² second⁻¹ 11.56 (green; grn) light or 660 nm log Q 13.63 (red) light. Rod types are labeled, and sine wave frequency is indicated below. For type A rods, note the similarity of responses elicited by red or green sinusoids. For type B rods, 8-Hz red response is greater than at 4 Hz. For type C rods, responses to green light are similar to type A rods, but red sinusoids > 2 Hz evoke relatively large responses. Type D rods show the greatest ability to follow red sinusoids. Right: Responses of rod types A and C to 567-nm (upper record of pair) and 667-nm flashes matched in intensity to elicit equal rod plateaux. Stimulus markers are indicated at bottom: type A rod, 300 msec; type C rod, 200 msec. Arrow indicates rod initial transient modified by cone input.

Fig. 2

Rod transients evoked by test flashes. With reference to the sample wave forms shown in Figure 1, right, the fraction of the total voltage (Vmax) corresponding to the initial transient is plotted for rod types A–D. In every case, the stimulus was 100 msec, 660 nm, log Q 13.93. Vertical bars are ± SEM (n = 86).

Fig. 3

a–f: Neurobiotin injections of single rods. Photomicrographs a–d show groups of neurobiotin-stained photoreceptors in retinal wholemount preparations. Up to eight stained receptors are visible. Arrows in c and d indicate cones. Stained photoreceptors are illustrated in e (cone; arrow) and f (rod) and are visible in 2-mm vertical sections. Scale bar = 20 μm.

Fig. 4

Effect of dopamine ligands on rod responses to sinusoids. Rods were classified into subtypes A–D on the basis of their responses to sinusoids, as illustrated in Figure 1. Rods were sampled at random during superfusion with normal Ringer's solution (n = 92) or with Ringer's solution to which 25 μM SKF 38393 (n = 19), 10 μM quinpirole (n = 29), or 5 μM spiperone (n = 10) was added. The graph axes, which are labeled only for normal Ringer's solution, are identical for each of the four graphs. Relative to normal Ringer's solution, quinpirole shifted the distribution toward types B, C, and D, whereas spiperone restricted it to type A. SKF 38393 was without statistically significant effect on the distribution of rod types.

Fig. 5

Flash responses of a type D rod that was stimulated with 100-msec flashes of 567-nm light (open arrow) or 660-nm light (solid arrows). Stimulus markers are indicated at bottom. The profile of the rod after neurobiotin injection is indicated at right (inset). Top to bottom: Stimulus strength increased in 0.4 log unit steps. Note that, for the weakest pair, response to red light is faster than to green stimulus. Middle and bottom records show that, when stimuli are matched to elicit equal rod plateaux, the red flash elicits a larger transient. Scale bar = 20 μm.

Fig. 6

Photoreceptor fins and gap junctions. a: Light microscopic view of a vertical section of Xenopus retina showing apparent contact points between photoreceptors (arrows). b: Tangential section through the photoreceptor inner segments. Both rod and cone profiles show radiating fins. c: Electron microscopic (EM) view of tangential section through photoreceptor inner segments. One pair of fins is indicated by arrows. Note that fins of neighboring photoreceptors do not make contacts. Small, round profiles are villous extensions of Muller glial cells. d: Low-magnification EM view of a gap junction (arrow) between a rod (R) and a cone (C). e: Higher magnification view of a rod-cone gap junction. Scale bars = 10 μm in b (also applies to a), 1 μm in c, 0.5 μm in d, 0.1 μm in e.

Fig. 7

Gap-junctional networks among Xenopus photoreceptors shown in schematic form indicating relative positions of rod and cone photoreceptors in a portion of reconstructed retina viewed in horizontal section. Cones are labeled with c, solid circles indicate are rods. Straight thick bars and thin bars indicate rod-to-rod and cone-to-cone gap junctions, respectively. Double arrowheads indicate rod-cone gap junctions. Irregular boxes delimit two groups of receptors that are redrawn orthogonally at the bottom and that were analyzed for coupling (see text).