The use of fluorescent indicators such as fura-2 (Grynkiewicz et al., 1985) to measure the cytosolic free calcium activity in retinal rods is complicated by the rods' sensitivity both to the fluorescence and to the light that excites it. By stimulating fluorescence from large numbers of rods in whole, loaded retinas and averaging repeated measurements, however, we have been able to monitor changes in free [Ca2+]i during exposure to nonsaturating lights under physiological conditions. Retinas, isolated from the bullfrog Rana catesbeiana, were loaded with fura-2 by incubation and mounted, receptor-side up, in a perfusion chamber placed on the stage of a specially designed apparatus. A step of light delivered from above, whose wavelength alternated between 340 and 380 nm every 110 msec, excited fluorescence from 24 mm2 of retina and evoked a light response (the aspartate-isolated pIII component of the electroretinogram--ERG). By comparing the fluorescence intensities excited by the 2 wavelengths (corrected for background and dark-noise), the free [Ca2+]i of the rod outer segment was determined. In darkness, the [Ca2+]i of the outer segment was found to be approximately 220 nM. A bright light caused it to fall exponentially to approximately 140 nM, with a time constant of approximately 1.6 sec. The value of [Ca2+]i at the onset of illumination was independent of stimulus intensity over a 2 log-unit range, and in all cases the fall was monotonic. After terminating the illumination, [Ca2+]i rose again to its time-zero value.(ABSTRACT TRUNCATED AT 250 WORDS)

5. By comparison, red-sensitive cones showed increased sensitivity as a function of stimulus size only up to a stimulus diameter of 120 /sm. Their over-all sensitivity was lower than that of rods and proved linear with stimulus diameter rather than with stimulus area.6. Simultaneous recordings were made from rod-cone pairs to determine whether the overshoot, and hence the lobe on the amplitude-intensity function, could result from a cone input to the rod response. The time course of the cone response proved much too rapid to fit the overshoot of the rod response. 7. The spectral sensitivity of the dark-adapted rod response closely followed the difference spectrum of the rod photopigment for wave-lengths > 450 nm. This was true throughout the intensity range of the response, including low intensities where response averaging was necessary.8. At low response amplitudes (-1 mV), about 70 % of the 40 rods tested showed responses to long wave-length stimuli consisting of two components. The smaller shorter latency component was found by its spectral response and its time course to result from excitation of cones. Even at the low stimulus intensities which revealed the short-latency component, our results indicate that cones make no significant contribution to the peak amplitude of the response.9. At higher stimulus intensities the rod component became larger in amplitude and shorter in latency, the cone component becoming relatively so small as to be completely masked. Under these conditions the responses to different spectral stimuli were superimposable, provided that relative intensities were adjusted for equal quantum catches by the rod photopigment. Hence the rod responses were spectrally univariant at all but the lowest stimulus intensities.10 (1975 a, b), our rod quantal sensitivities are consistently higher by a factor of 4-5 and we find no cone contribution to the peak amplitude of the response. Evidence is presented that these differences result from our retinas being more fully dark-adapted. Also, our rod receptive fields had only three-fifths the diameter reported by Schwartz; possible explanations of this difference are suggested. 252INTERACTIONS BETWEEN RODS 12. Procion Yellow injections revealed fine processes extending laterally for up to 35 ,um from the rod's synaptic terminal in the outer plexiform layer. These processes exhibit en passage and terminal swellings. A possible pathway is thus suggested for summative interaction between distant rods.

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 μM), a D2 dopamine agonist, increased rod-cone coupling, whereas spiperone (5 μM), 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 gapjunction 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. Keywords gap junction; photoreceptor; retina; neuromodulation When rod photoreceptors are fully dark-adapted, they respond reliably to the absorbance of a single quantum of light (Yau et al., 1977). Rods continue to function when they are desensitized by background lights (Fain, 1976) or by prior exposure to light, and, in those circumstances, the effective stimuli for rods can be sufficiently bright to also activate cone photoreceptors, allowing for the possibility of interactions between rod signals and cone signals. (Raviola and Gilula, 1973), and functional rod-cone coupling in primate retina has been reported (Schneeweis and Schnapf, 1995).The main experimental question of the present study is whether the conductances of photoreceptor gap junctions are subject to neuromodulation. We focus on the intrinsic retinal neurochemical, dopamine (Haggendal and Malmfors, 1965), which is known to modulate the gap-junctional conductance of retinal second-order neurons (Piccolino et al., 1984) and to augment information flow in cone circuits while suppressing that in rod circuits (Witkovsky and Dearry, 1991). Photoreceptors have dopamine receptors of the D2/D4 subtypes (Cohen et al., 1992;Muresan and Besharse, 1993). In the present study, we found that a D2 dopamine agonist, but not a D1 agonist, increases rod-cone coupling in the Xenopus retina, resulting in altered rod light-evoked response k...

SUMMARY1. The spatial properties of rods, horizontal cells and bipolar cells were studied by intracellular recording in the isolated, perfused retina of the tiger salamander, Ambystoma tigrinum. Low stimulus intensities were used in order to keep cell responses close to, or within, their linear intensity/response range.2. Spatial properties of bipolar cell receptive fields, measured while perfusing with normal Ringer solution, were compared with those measured during exposure to agents that eliminated the bipolar cells' receptive field surround (RFS). In this way, the spatial properties of the receptive field centre (RFC) and those of the RFS could be characterized independently.3. To a good approximation, the contribution to the horizontal cell's response of unit area of its receptive field declined exponentially with distance from the centre of the receptive field. The (apparent) length constant describing this decay was 200 ,um. The one-dimensional length constant of the horizontal cell syncytium was thus 248 ,tm. The variation of response amplitude with the radius of a centred circular stimulus was consistent with this finding.4. This was true also of the RFCs of bipolar cells. The one-dimensional length constant of the RFC of off-centre bipolar cells averaged 124 ,um. That of the RFC of on-centre cells averaged 62 ,tm though values were more variable, the RFCs of some on-centre cells being comparable to those of off-centre cells. These values were independent of the class of photoreceptor driving the bipolar cell.5. The large size of the RFCs of off-centre cells and many on-centre cells cannot be explained by light scatter within the retina or by voltage spread within the rod syncytium. We proposed that off-centre cells are tightly coupled in a syncytium. Oncentre cells, on average, are less tightly coupled.6. The spatial properties of the bipolar cell's RFS were consistent with the notion that the RFS represents a convolution of the horizontal cell's receptive field and the bipolar cell's RFC.7. The spatial properties of bipolar cell receptive fields were reconstructed from the measured properties of their RFCs and the measured properties of horizontal cell receptive fields. Under the conditions of our experiments, the bipolar cell's response MS 7478 W. A. HARE AND W. G. OWEN could be described by a linear difference between a component generated by the RFC and a component generated by the RFS.8. The spatial filtering characteristics of the bipolar cells were calculated from our data. They show that, under the conditions of our experiments, bipolar cell receptive fields act as bandpass spatial filters tuned to an optimum spatial frequency of 0-8 mm-' (average for off-centre cells) and 1-02 mm-1 (average for off-centre cells).

Recent experiments indicate that the dark-adapted vertebrate visual system can count photons with a reliability limited by dark noise in the rod photoreceptors themselves. This suggests that subsequent layers of the retina, responsible for signal processing, add little if any excess noise and extract all the available information. Given the signal and noise characteristics of the photoreceptors, what is the structure of such an optimal processor? We show that optimal estimates of time-varying light intensity can be accomplished by a two-stage filter, and we suggest that the first stage should be identified with the filtering which occurs at the first anatomical stage in retinal signal processing, signal transfer from the rod photoreceptor to the bipolar cell. This leads to parameter-free predictions of the bipolar cell response, which are in excellent agreement with experiments comparing rod and bipolar cell dynamics in the same retina. As far as we know this is the first case in which the computationally significant dynamics of a neuron could be predicted rather than modeled.