Test of the paired-flash electroretinographic method in mice lacking b-waves

ON-OFF Interactions in the Retina: Role of Glycine and GABA

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells and it apparently remains as signals propagate to higher brain visual centers. A fundamental question in visual neuroscience is how these two parallel pathways function: are they independent from each other or do they interact somehow? In the latter case, what kinds of mechanisms are involved and what are the consequences from this cross-talk? This review summarizes current knowledge about the types of interactions between the ON and OFF channels in nonmammalian and mammalian retina. Data concerning the ON-OFF interactions in distal retina revealed by recording of single bipolar cell activity and electroretinographic ON (b-wave) and OFF (d-wave) responses are presented. Special emphasis is put on the ON-OFF interactions in proximal retina and their dependence on the state of light adaptation in mammalian retina. The involvement of the GABAergic and glycinergic systems in the ON-OFF crosstalk is also discussed.

Mouse b-wave mutants

The b-wave is a major component of the electroretinogram that reflects the activity of depolarizing bipolar cells (DBCs). The b-wave is used diagnostically to identify patients with defects in DBC signaling or in transmission from photoreceptors to DBCs. In mouse models, an abnormal b-wave has been used to demonstrate a critical role of a particular protein in the release of glutamate from photoreceptor terminals, in establishing the structure of the photoreceptor-to-DBC synapse, in DBC signal transduction, and also in DBC development, survival, or metabolic support. The purpose of this review is to summarize these models and how they have advanced our understanding of outer retinal function.

The rod-driven a-wave of the dark-adapted mammalian electroretinogram

The a-wave of the electroretinogram (ERG) reflects the response of photoreceptors to light, but what determines the exact waveform of the recorded voltage is not entirely understood. We have now simulated the trans-retinal voltage generated by the photocurrent of dark-adapted mammalian rods, using an electrical model based on the in vitro measurements of Hagins et al. (1970) and Arden (1976) in rat retinas. Our simulations indicate that in addition to the voltage produced by extracellular flow of photocurrent from rod outer to inner segments, a substantial fraction of the recorded a-wave is generated by current that flows in the outer nuclear layer (ONL) to hyperpolarize the rod axon and synaptic terminal. This current includes a transient capacitive component that contributes an initial negative "nose" to the trans-retinal voltage when the stimulus is strong. Recordings in various species of the a-wave, including the peak and initial recovery towards the baseline, are consistent with simulations showing an initial transient primarily related to capacitive currents in the ONL. Existence of these capacitive currents can explain why there is always a substantial residual transient a-wave when post-receptoral responses are pharmacologically inactivated in rodents and nonhuman primates, or severely genetically compromised in humans (e.g. complete congenital stationary night blindness) and nob mice. Our simulations and analysis of ERGs indicate that the timing of the leading edge and peak of dark-adapted a-waves evoked by strong stimuli could be used in a simple way to estimate rod sensitivity.

Arrestin competition influences the kinetics and variability of the single-photon responses of mammalian rod photoreceptors

Reliable signal transduction via G-protein-coupled receptors requires proper receptor inactivation. For example, signals originating from single rhodopsin molecules vary little from one to the next, requiring reproducible inactivation of rhodopsin by phosphorylation and arrestin binding. We determined how reduced concentrations of rhodopsin kinase (GRK1) and/or arrestin1 influenced the kinetics and variability of the single-photon responses of mouse rod photoreceptors. These experiments revealed that arrestin, in addition to its role in quenching the activity of rhodopsin, can tune the kinetics of rhodopsin phosphorylation by competing with GRK1. This competition influenced the variability of the active lifetime of rhodopsin. Biasing the competition in favor of GRK1 revealed that rhodopsin remained active through much of the single-photon response under the conditions of our experiments. This long-lasting rhodopsin activity can explain the characteristic time course of single-photon response variability. Indeed, explaining the late time-to-peak of the variance required an active lifetime of rhodopsin approximately twice that of the G-protein transducin. Competition between arrestins and kinases may be a general means of influencing signals mediated by G-protein-coupled receptors, particularly when activation of a few receptors produces signals of functional importance.

Postreceptoral contributions to the light-adapted ERG of mice lacking b-waves

The purpose of this study was to determine the contributions of postreceptoral neurons to the light-adapted ERG of the Nob mouse, a model for complete-type congenital stationary night blindness (CSNB1) that lacks a b-wave from depolarizing bipolar cells. Ganzfeld ERGs were recorded from anesthetized adult control mice, control mice injected intravitreally with L-2-amino-4-phosphonobutyric acid (Control APB mice) to remove On pathway activity, and Nob mice. ERGs also were recorded after PDA (cis-2,3-piperidine-dicarboxylic acid, 3-5mM) was injected to block transmission to hyperpolarizing (Off) bipolar and horizontal cells, and all third-order neurons. Stimuli were brief (<4ms, 0.4-2.5log sc td s) and long (200ms, 2.5-4.6log sc td) LED flashes (lambda(max)=513nm, on a rod suppressing background (2.6log sc td). Sinusoidal modulation of the LEDs (mean, 2.6log sc td; contrast, 100%; 3-36Hz) was used to study flicker ERGs. Brief-flash ERGs of Nob mice presented as long-lasting negative waves with a positive-going intrusion that started about 50ms after the flash and peaked around 120ms. Control APB mice had similar responses, and in both cases, PDA removed the positive-going intrusion. For long flashes, PDA removed a small, slow "d-wave" after light offset. With sinusoidal stimulation, the fundamental (F1) amplitude of control mice ERG peaked at 8Hz ( approximately 70microV). For Nob mice the peak was approximately 20microV at 6Hz before PDA and approximately 10muV at 3Hz or lower after PDA. F1 responses were present up to 21Hz in control and Nob eyes and 15Hz in Nob eyes after PDA. Between 3 and 6Hz, F1 phase was 170-210 degrees more delayed in Nob than control mice; phase was hardly altered by PDA. With vector analysis, a substantial postreceptoral input to the Nob flicker ERG was revealed. In control mice, the second harmonic (F2) response showed peaks of approximately 10mocrpV at 3Hz and 13Hz. Nob mice showed almost no F2. In summary, in this study it was found that in Nob mice, postreceptoral neurons from the Off pathway make a positive-going contribution to the light-adapted flash ERG, and contribute substantially to sinusoidal flicker ERG.

Recovery of rod photoresponses in ABCR-deficient mice

PURPOSE

The ABCR protein of the rod outer segment is thought to facilitate movement of the all-trans retinal photoproduct of rhodopsin bleaching out of the disc lumen. This study was undertaken to investigate the extent to which ABCR deficiency affects the post-bleach recovery of the rod photoresponse in ABCR-deficient (abcr-/-) mice.

METHODS

Electroretinographic (ERG) a-wave responses were recorded from abcr-/- mice and two control strains. A bright probe flash was used to examine the course of rod recovery after fractional rhodopsin bleaches of approximately 10(-6), approximately 3 x 10(-5), approximately 0.03, and approximately 0.30 to approximately 0.40.

RESULTS

Dark-adapted abcr-/- mice and control animals exhibited similar normalized near-peak amplitudes of the paired-flash-ERG-derived, weak-flash response. Response recovery after approximately 10(-6) bleaching exhibited an average exponential time constant of 319, 171, and 213 ms, respectively, in the abcr-/- and the two control strains. Recovery time constants determined for approximately 3 x 10(-5) bleaching did not differ significantly among strains. However, those determined for the approximately 0.03 bleach indicated significantly faster recovery in abcr-/- mice (2.34 +/- 0.74 minutes) than in the controls (5.36 +/- 2.20 and 5.92 +/- 2.44 minutes). After approximately 0.30 to approximately 0.40 bleaching, the initial recovery in the abcr-/- mice was, on average, faster than in control mice.

CONCLUSIONS

By comparison with control animals, abcr-/- mice exhibit faster rod recovery after a bleach of approximately 0.03. The data suggest that ABCR in normal rods may directly or indirectly prolong all-trans retinal clearance from the disc lumen over a significant bleaching range, and that the essential function of ABCR may be to promote the clearance of residual amounts of all-trans retinal that remain in the discs long after bleaching.

Mouse cone photoresponses obtained with electroretinogram from the isolated retina

We characterize the dark-adapted photoresponses from mouse cones intact in the isolated retina, their virtually natural environment, by isolating pharmacologically the photoreceptor light responses from the electroretinogram (ERG). Due to the different photoresponse kinetics and sensitivity of rods and cones, the cone responses were readily attained by using a rod-saturating preflash. The stimulus wavelength (544 nm) was chosen to selectively stimulate the green sensitive ("M"-)pigment. Obtained responses were monophasic, showing fast kinetics (mean t(p)=51 ms) and low sensitivity (fractional single-photon response ca. 0.23%). The amplification coefficient of cones (4.6 s(-2)) was very close to that of rods (5.6 s(-2)), while the dominant time constant of recovery was clearly smaller for cones (33 ms) than for rods (160 ms).