Excitation and desensitization of mouse rod photoreceptors in vivo following bright adapting light

Electroretinographic (ERG) methods were used to determine response properties of mouse rod photoreceptors in vivo following adapting illumination that produced a significant extent of rhodopsin bleaching. Bleaching levels prevailing at ∼10 min and ∼20 min after the adapting exposure were on average 14% and 9%, respectively, based on the analysis of visual cycle retinoids in the eye tissues. Recovery of the rod response to the adapting light was monitored by analysing the ERG a -wave response to a bright probe flash presented at varying times during dark adaptation. A paired-flash procedure, in which the probe flash was presented at defined times after a weak test flash of fixed strength, was used to determine sensitivity of the rod response to the test flash. Recovery of the response to the adapting light was 80% complete at 13.5 ± 3.0 min (mean ± s.d .; n = 7) after adapting light offset. The adapting light caused prolonged desensitization of the weak-flash response derived from paired-flash data. By comparison with results obtained in the absence of the adapting exposure, desensitization determined with a test-probe interval of 80 ms was ∼fourfold after 5 min of dark adaptation and ∼twofold after 20 min. The results indicate, for mouse rods in vivo , that the time scale for recovery of weak-flash sensitivity substantially exceeds that for the recovery of circulating current following significant rhodopsin bleaching. The lingering desensitization may reflect a reduced efficiency of signal transmission in the phototransduction cascade distinct from that due to residual excitation.     

Responses of single rods in the retina of the turtle

1. The responses of rods in the retina of the turtle, Chelydra serpentina, have been studied by intracellular recording.2. The identification of rods as the origin of the recorded responses has been confirmed by marking with Procion Yellow.3. The response to a small spot of light was a hyperpolarization which increased with increasing light intensity. For dim, small diameter stimuli, the shape of the rod response was similar to that of cones but 2x slower and 2x larger in amplitude. The time integral of the rod response to a dim, small diameter flash is, therefore, approximately 4x greater than the integral of the cone response.4. The shape of the rod response depended on the pattern of retinal illumination as well as stimulus intensity. Enlarging the area of illumination increased the peak amplitude and delayed repolarization following a light step. The area of retina which influenced the response was approximately 200 mum in radius.5. It is concluded that for dim light the responses of rods are larger than those of cones because of (i) a greater response to direct illumination and (ii) an enhancement of response by interaction from a large retinal area.     

Oscillations in rod and horizontal cell membrane potential: evidence for feed-back to rods in the vertebrate retina.

1. Rods and horizontal cells were studied with intracellular recordings in the retina of the toad, Bufo marinus; 161 cells were from the eyecup preparation and thirty were from the isolated perfused retina. 2. Of these cells, 39% exhibited either transient or sustained oscillations of membrane potential. Light flashes either evoked transient oscillations or temporarily abolished sustained oscillations. The amplitudes of the oscillations could be as large as 27 mV. The frequency of the oscillations at 25 degrees C was between 1-5 and 3-5 Hz and was strongly dependent on temperature and background illumination. 3. The rod oscillation amplitude and the peak of the horizontal cell light response increased similarly with increasing test flash diameters. They continued to grow for diameters much larger than those which increased the peak of the rod light response. 4. Perfusion of the isolated retina with 2 mM aspartate had only a small effect on the rod light response but it completely eliminated the horizontal cell light response as well as the oscillations recorded in both rods and horizontal cells. 5. It is believed that the oscillations result from a reverberating interaction between rods and neurones post-synaptic to rods. Thus, rods can be both post- as well as presynaptic retinal elements.     

Photoreceptor responses to light in the isolated pineal organ of the trout, Salmo gairdneri

Photoreceptor potentials were recorded intracellularly from the isolated pineal organ of the teleost, Salmo gairdneri, maintained in tissue culture medium for 2-20 h. After electrophysiological characterization the photoreceptor cells were iontophoretically injected with Lucifer Yellow or with horseradish peroxidase for subsequent morphological identification. A brief flash of light elicited a hyperpolarization which was graded with light intensity in the dark-adapted photoreceptor. For dim flashes, the responses were purely monophasic. At higher intensities responses either remained purely monophasic or displayed an initial transient wave which became prominent for supersaturating intensities. The latency of the responses and their rise time decreased with increasing light intensity. Threshold responses showed latencies of about 600 ms, reached a maximum in about 1100 ms and returned to the dark potential in about 5 s. Saturating flashes considerably diminished the latency to 55 ms, the rise time to about 250 ms, but increased the time of recovery from peak to dark potential up to 60 s. Intracellular responses to background illumination exhibited two different response types. One type repolarized immediately, when the background light was extinguished, whereas the other type was characterized by a slow recovery of the dark potential. The spectral sensitivity of all intracellular recorded photoreceptors peaked at lambda max = 520-530 nm.     

Detection of thermal responses of the retina by use of polyvinylidene fluoride multilayer detector

The design and properties of thermal detectors constructed with multiple layers of polyvinylidene fluoride film are described. By use of these detectors, production of heat by the photoreceptors in the dark-adapted bullfrog retina in response to brief pulses of very weak light was examined. In response to brief light pulses delivering an average of 1 photon per retinal rod, the retina was found to produce heat more-or-less abruptly after a latent period of 0.2 to 0.4 s. At this level of pulse intensity, the thermal energy produced by the retina was approximately 1,500,000 times as large as the total radiant energy delivered to the retina for stimulation. It appears possible that a large number of disks in the rod outer segment are activated by absorption of a single photon.     

Comparison of frequency characteristics of photopic and scotopic S-potentials in the carp.

Frequency characteristics of photopic and scotopic L-type S-potentials were studied in the carp retina, using either an impulse or a sinusoidal light input superimposed on a steady background light of various intensities. The photopic type responses were faster than the scotopic responses if their frequency characteristics were compared without background illumination. However, the difference was not absolute. Both types of responses became faster in the presence of background illumination. The stronger the background, the faster was the response. At relatively high background intensities, the response to a flash became diphasic due to the rebound effect which corresponds to a low frequency attenuation in the gain characteristics. The main difference between the two types of responses was found in the phase relation; the scotopic units showed more phase lag indicating an extra delay even under a condition in which the time course of their response was faster than that of the photopic units.     

Electrical responses of rods in the retina of Bufo marinus

1. Intracellular responses to flashes and steps of light have been recorded from the outer segment and the cell body of rods in the retina of the Bufo marinus. The identification of the origin of recorded responses has been confirmed by intracellular marking.2. Responses to flashes delivered in darkness or superimposed on a background were analysed. Responses recorded from outer segments conform to the principle of ;spectral univariance'. The shape of the response is not affected by enlarging the spot diameter from 150 to 1000 mum.3. The membrane potential measured in darkness at the outer segments varied from -15 to -25 mV. Injection of steady hyperpolarizing currents increases the size of the response to light; depolarizing currents reduce the response. The mean value of the input resistance is 97 +/- 30 MOmega in darkness and increases by 20-30% during illumination.4. The responses obtained from the cell body of rods have the same shape, time course and spectral sensitivity of those recorded at the outer segment. Injection of steady current at the cell body produces different effects than at the outer segment: hyperpolarizing currents reduce the amplitude of the response to light; depolarizing currents increase the response.5. The experimental data are fitted according to a model similar to that used to describe the responses of turtle cones (Baylor & Hodgkin, 1974; Baylor, Hodgkin & Lamb, 1974a, b).6. The model reproduces the electrical responses of the rod outer segment to a variety of stimuli: (a) brief flashes and steps of light in dark adapted conditions; (b) bright flashes superimposed on background illuminations; (c) pairs of flashes delivered at different time intervals. Responses to hyperpolarizing steps of current are also reproduced by the model.     

Background adaptation in the rods of the frog's retina.

1. Aspartate-isolated photoresponses of the red rods to flashes and steps of light have been recorded, both in the presence of and without background lights of varying strength. 2. The results are interpreted in terms of a model of rod outer segment adaptation, where the three model parameters correspond to the adaptation processes associated with the transmitter release, the transmitter background concentration and the plasma membrane leakage, respectively. 3. The stimulus-response function deviated somewhat from the Michaelis equation U/Umax=I/(I + IH). During light-adaptation the operating curve, the stimulus-response function plotted in a log-log diagram, retained approximately its shape while moving strongly to the right along the log intensity axis and to a lesser degree downwards (Umax-decrease). 4. The movement of the operating curve was such that the rods approximately obeyed Weber's law. In the cases of flash and step of light stimuli the movement of the operating curve was about the same. 5. When a moderate background light was turned on a large decrease of sensitivity was first observed. During a period 0-5-1 min the sensitivity increased towards the stationary value. After extinguishing the background light the dark sensitivity returned in 0-5-1 min and then a period of hypersensitivity lasting typically 1 min was observed. 6. The experimental results, as interpreted according to the model, indicate that light-adaptation decreases q, the number of transmitter molecules released by one bleached rhodopsin molecule. 7. There is probably an adaptation process also in the rod inner segment, which increases the sensitivty of the rod to transient stimuli.     

Anomalous rectification in horizontal cells.

1. The electrical properties of horizontal cells in the mudpuppy in light and dark were measured with a pair of micropipettes separated by about 1 mum with low coupling resistance so that no bridge circuitry was required. 2. All horizontal cells studied showed significant anomalous rectification: the current-voltage characteristic for about 60 per cent of the cells studied had a slope resistance of about 20-30 M omega at the dark potential level; the slope resistance increased by about 15% for each 10 mV depolarization and decreased by about 15% for each 10 mV hyperpolarization. The remaining 40% of the horizontal cells showed a higher input resistance at corresponding potential levels but had similar rectifying properties. 3. The increase in resistance with depolrization developed with a time course of about 1/2 sec when steady steps of outward current were passed across the membrane, but the time course for resistance decrease with hyperpolarization was much shorter for steady inward current steps. In about half the horizontal cells there was a transient decrease in resistance lasting about 100 msec immediately following the outward current steps superimposed upon the slower sustained resistance increase. 4. The normal 20-30 mV hyperpolarizing light response was associated with little or no change in input resistance. However, if the membrane potential was held at the dark potential level with extrinsic current, thereby eliminating the potential-dependent resistance change, a light-elicited resistance increase of about 10 M omega was measured. 5. The time-dependent change in membrane resistance elicited by polarizing steps of current obscured the reversal potential for the response. However, when the reversal potential was measured at short times following polarization of the membrane, before the time-dependent resistance change developed, it was estimated at between +15 and +50 m V. 6. The results suggest that the horizontal cell response is mediated by a light-elicited resistance increase at the synaptic membrane which is obscured by a potential- and time-dependent resistance decrease at another part of the membrane.     

Receptive fields of cones in the retina of the turtle

1. Intracellular recordings have been made of the responses to light of single cones in the retina of the turtle. The shape of the hyperpolarizing response to a flash depends on the pattern of retinal illumination as well as the stimulus intensity.2. Although changes in the stimulus pattern can produce changes in the effective stimulus intensity, the responses to certain patterns cannot be matched by any adjustment of stimulus intensity.3. The initial portion of responses to large or small stimulating spots is proportional to light intensity; this allows comparison of responses when the amount of light on a cone is kept constant but the light on surrounding cones is changed. For equal light intensity on the cone, the response to a spot 2 or 4 mu in radius is smaller than that to a spot 70 mu in radius.4. Responses to spots 70 and 600 mu in radius coincide over their rising phases and peaks without any adjustment of stimulus intensity. The responses to the larger spot, however, contain a delayed depolarization not present with the smaller spot.5. During steady illumination of a cone with a small central spot, the response to transient illumination superimposed on the same area is greatly reduced. Illumination of cones in the near surround, however, produces a hyperpolarizing response, and illumination of cones in the more distant surround generates a delayed depolarization.6. The results described above suggested that synaptic signals might impinge on cones. This possibility was tested by electrically polarizing one retinal cell while recording from another.7. Currents passed through a cone within 40 mu of another cone can change the membrane potential of the latter. Not all cones within this distance show the interaction, however, and it has never been detected at distances greater than 50 mu.8. Hyperpolarization of a horizontal cell with applied current can produce a depolarization of a cone in the vicinity. During this depolarization, the response of the cone to a flash is reduced in size and altered in shape.9. It is concluded that the response of a cone to light may be modified by synaptic mechanisms which are activated by peripheral illumination.     

Bleached rhodopsin and retinal interaction

1. The human rod retina exhibits lateral inhibition which is demonstrated by the sensitization, i.e. improvement in incremental threshold sensitivity, of a steadily illuminated patch of retina when its surround is also illuminated.

2. Experiments were carried out to test whether the previously reported equivalence between steady illumination (real light) and signals from bleached rhodopsin (dark light) also carries over into lateral interaction.

3. Symmetrical experiments involving real light patches surrounded by dark-light annuli, dark-light patches surrounded by real-light annuli, and dark-light patches surrounded by dark-light annuli showed that dark light is neither susceptible to sensitization by lateral interaction nor capable of exerting it.

     

Comparison of the effects of flickering and steady light on dopamine release and horizontal cell coupling in the mudpuppy retina.

1. The effects of flickering adapting illumination (repetitive flashes) on horizontal cell responses to illumination of the center and surround portions of the receptive field were compared with those of steady adapting illumination in dark-adapted mudpuppy retinas. 2. Exposure to flickering adapting light caused an increase in amplitude of responses to small spots in the receptive-field center and a decrease in the response to a concentric annulus. This is interpreted as due to an increase in coupling resistance between horizontal cells. 3. The uncoupling effect of flickering adapting light was no greater than that of the same quantity of steady adapting light at the same intensity, even when the rate of flickering was varied by a factor of 10. 4. The uncoupling effect of flickering light was blocked by the dopamine antagonists fluphenazine and SCH23390, indicating that it is mediated by dopamine release. 5. The uncoupling effect of flickering light was also blocked in the presence of 2-amino-4-phosphonobutyrate (APB), which prevents light responses of on-center but not off-center bipolar cells, suggesting that flickering light increases dopamine release via the on-pathway. 6. The gamma-aminobutyric acid (GABA) antagonist bicuculline had an uncoupling effect similar to that of adapting illumination. This effect was blocked by dopamine antagonists, indicating that there is tonic GABA-mediated inhibition of dopamine release in mudpuppy retina similar to that previously reported by others in fish and turtle retinas. 7. The uncoupling effect of bicuculline was not reversed by APB. However, APB alone caused an increase in coupling that was rapidly reversed by bicuculline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mesopic background lights enhance dark-adapted cone ERG flash responses in the intact mouse retina: a possible role for gap junctional decoupling

The cone-driven flash responses of mouse electroretinogram (ERG) increase as much as twofold over the course of several minutes during adaptation to a rod-compressing background light. The origins of this phenomenon were investigated in the present work by recording preflash-isolated (M-)cone flash responses ex vivo in darkness and during application of various steady background lights. In this protocol, the cone stimulating flash was preceded by a preflash that maintains rods under saturation (hyperpolarized) to allow selective stimulation of the cones at varying background light levels. The light-induced growth was found to represent true enhancement of cone flash responses with respect to their dark-adapted state. It developed within minutes, and its overall magnitude was a graded function of the background light intensity. The threshold intensity of cone response growth was observed with lights in the low mesopic luminance region, at which rod responses are partly compressed. Maximal effect was reached at intensities sufficient to suppress ～ 90% of the rod responses. Light-induced enhancement of the cone photoresponses was not sensitive to antagonists and agonists of glutamatergic transmission. However, applying gap junction blockers to the dark-adapted retina produced qualitatively similar changes in the cone flash responses as did background light and prevented further growth during subsequent light-adaptation. These results are consistent with the idea that cone ERG photoresponses are suppressed in the dark-adapted mouse retina by gap junctional coupling between rods and cones. This coupling would then be gradually and reversibly removed by mesopic background lights, allowing larger functional range for the cone light responses.     

Spatial interaction in human cone vision

1. The adaptation state of a uniformly illuminated patch of human cone retina was determined by finding the threshold for a small, brief light spot seen flashing in its centre.2. When the illuminated patch of the retina is increased in diameter, the adaptation state is first raised, and beyond a critical background diameter, lowered.3. This is interpreted as a manifestation of excitatory and inhibitory interaction of adaptation stimuli: illumination of retinal regions in the immediate neighbourhood of the area tested acts to raise the adaptation level, and of those further removed acts to lower it.4. The critical area beyond which adapting light produces inhibition is about 5 min of arc in diameter in the eye's object space for foveal observation. For peripheral cone vision it increases much as the minimum angle of resolution.5. The inhibiting action of outlying areas is substantially reduced, or perhaps even eliminated, by lowering the background luminance.6. Surrounding the retinal patch with a pair of juxtaposed narrow concentric black and white rings superimposed on a uniform field, simulating a border, irrespective of diameter, does not influence the threshold of the probing spot. This argues against a possible threshold raising effect of the border of the background.7. The inhibiting action on a patch of cone retina of a surrounding annulus occurs only when the annulus is seen by the same eye and not when it is seen by the other eye: the site of inhibitory interaction is, therefore, retinal.     

Light-evoked changes in [K+]o in proximal portion of light-adapted cat retina.

1. The M-wave is a light-adapted response of proximal retina consisting of phasic negative field potentials at light onset and offset that are spatially tuned for small stimuli. We measured light-dependent changes in extracellular K+ concentration ([K+]o) in proximal retina to investigate the hypothesis that the M-wave originates from Müller cell responses to changes in [K+]o. 2. Extracellular field potentials, and changes in [K+]o evoked in response to circular spots of light flashed on steady backgrounds, were recorded with double-barreled K(+)-sensitive electrodes placed in the retina at different depths. 3. Increases in [K+]o during illumination and at light offset were maximal in proximal retina, with the On [K+]o increase located more proximally than the Off increase. The [K+]o increase during illumination consisted of a phasic and sustained response, whereas the Off [K+]o increase was predominantly phasic. The spatial tuning of the [K+]o increases was similar to the tuning of the field potentials. 4. The Off-field potential was larger than the On potential; it tended to be maximal more distally and was more sharply localized in retinal depth. Stimulus-response characteristics of the field potentials were not altered by intravitreal tetrodotoxin (TTX; 3.8 microM) sufficient to block retinal ganglion cell action potentials. 5. There were no rod contributions to the proximal [K+]o increases and field potentials recorded at the background illuminations used in this study (9.5-11.5 log q.deg-2.s-1). 6. An intravitreal injection of L- or DL-2-amino-4-phosphonobutyric acid (APB; 1 mM) was used to block On-system neuronal responses in proximal retina and isolate Off-system responses. After APB the [K+]o response consisted of a sustained decrease in [K+]o during illumination followed by an overshoot at light offset, while the field potential was a sustained positive response at light onset followed by an initially phasic negative response at light onset followed by an initially phasic negative response at light offset. These responses retained spatial tuning. To isolate the On-system components, the APB-isolated responses were subtracted from the controls. The [K+]o response now consisted of a sustained increase during illumination followed by an undershoot at light offset. The field potential was a sustained negative potential with an initial phasic peak that decayed at Off. Results with kynurenate (KYN; 5 mM) and (+/-)cis-2,3-piperidine (PDA; 5 mM) confirmed the sustained nature of the On component.(ABSTRACT TRUNCATED AT 400 WORDS)     

Circadian controls outweigh acute illumination effects on the activity of extracellular signal-regulated kinase (ERK) in the retina

Extracellular signal-regulated kinase (ERK) participates in numerous cellular functions including circadian-related activities. In the retina, the activity of ERK is under circadian control. However, it is not clear whether acute illumination changes or the circadian clocks in the retina have a larger impact on ERK activity, and the cellular distribution of activated ERK (pERK) as a function of circadian time in cone photoreceptors is not known. Chick embryos were exposed to the light or dark for various lengths of time after 12:12 h light–dark (LD) cycles, or on the second day of constant darkness after LD entrainment. Retinas were excised after various exposure times and relative ERK activity was determined by western immunoblotting. We also performed immunohistochemical and immunocytochemical stainings on circadian entrained retina sections and dissociated retina cells. There is about a fourfold difference in ERK activity between retinas harvested at circadian time (CT) 4 and CT 16, and the internal circadian control of ERK activity in the retina overcomes external light exposure. Also, during the subjective night, pERK was more apparent in the outer segment of cones, while pERK distribution was more uniform throughout the photoreceptors during the subjective day. Our results imply that the activity of retinal ERK is influenced more by circadian oscillators than acute illumination changes. Hence, the circadian oscillators in retina photoreceptors play a major role in the regulation of photoreceptor physiology, which leads to the circadian control of light sensitivity in photoreceptors.     

Ionic mechanism of a quasi-stable depolarization in barnacle photoreceptor following red light.

1. The membrane mechanism of a quasi-stable membrane depolarization (latch-up) that persists in darkness following red light was examined in barnacle photoreceptor with micro-electrode techniques including voltage-clamp and Na+-sensitive micro-electrodes. 2. Current-voltage (I-V) relations of the membrane in darkness following red light (latch-up) and in darkness following termination of latch-up with green light, indicate that latch-up is associated with an increase of membrane conductance. 3. The latch-current (membrane current in darkness following red light minus membrane current in darkness following a gree flash that terminates latch-up) was inward at the resting potential, reversed sign at about +26mV (mean of six cells), and became outward at more positive membrance potentials. 4. Current-voltage relations of the membrane during green light (no latch-up) closely resembled those during latch-up. The light-induced current (LIC) elicited by green ligh (membrane current during the light flash minus membrane current in darkness following the light flash) was inward from the resting potential to +26mV (mean of six cells), then reversed sign and became outward. 5. The latch-current and LIC were both augmented in reduced Ca2+ solutions and decreased as Na-+ was reduced at a fixed Ca2+ concentration. 6. Both LIC and latch-current reversed sign at a more negative membrane potential (increment V equals 14mV) in solutions containing one quarter the normal amount of Na+. 7. The internal Na-+ activity (a-iNa) of a photoreceptor increased from about 10-18 mM upon illumination with long steps of intense red or white illumination. Five minutes in darkness after white light, a-iNa had recovered significantly, whereas a-iNa remained elecated following red illumination. 8. Latch-up seems to be a persistence in darkness of the same membrane mechanism that normally occurs during illumination; i.e. a conductance increase to Na+ ions. Ca2+ ions act primarily to suppress this current. There is evidence for a net Na+ influx during illumination that is sustained in darkness during latch-up.     

Changes in time scale and sensitivity in turtle photoreceptors

1. In turtle cones the steady-state relation between the internal potential and log light intensity was much flatter in the steady state than it was at 30 msec after the beginning of a step of light; this is attributed to a desensitization which develops with a delay of 50-100 msec.

2. When a weak flash was superposed on a steady background light which hyperpolarized the cone by 3-6 mV the amplitude of the linear response to a flash was reduced to 1/e and the time to maximum was shortened from about 110 to 70 msec; the response also became diphasic. With stronger background lights the flash sensitivity continued to fall, but the time to maximum did not become shorter than 40-50 msec and lengthened again with very strong lights.

3. In cones the flash sensitivity S_F was reduced to half its dark value S_F^D by a light intensity of 1/S_F^Dζ where ζ is about 20 sec/V.

4. At low levels of background light, about two-thirds of the change in sensitivity was time-dependent and one-third was attributable to the `instantaneous non-linearity' described in the previous paper.

5. The reduction in time to peak and the decrease in sensitivity produced by a background light which hyperpolarized by about 3 mV was little affected by changing the diameter of the area illuminated from 12 to 800 μm.

6. An experiment with a rod showed that a very weak light which hyperpolarized by only 0·5 mV decreased the linear response to 1/e and shortened the time to maximum from 300 to 180 msec.

7. With weak or moderate flashes the time-dependent desensitization lagged behind the potential by 50-100 msec.

8. The desensitization and shortening of time scale which persisted after a flash or step were associated with an after-hyperpolarization. The relaxation of potential, sensitivity and time scale became slower as the preceding illumination was increased from 10³ to 10¹⁰ photons μm^-2; the increase seemed to occur in steps involving components which relaxed with time constants of the order of 0·1, 1, 10 and 100 sec. A rebound phenomenon was observed after steps longer than 30 sec and with equivalent intensities greater than 10⁵ photons μm^-2 sec^-1.

9. Several of the observations are explained by a hypothesis in which the central assumption is that the particles which block the ionic channels are degraded or removed by an autocatalytic reaction.

     

Cold- and menthol-sensitive C afferents of cat urinary bladder

The high sensitivity of the vertebrate visual system results from amplification inherent in phototransduction in rods and from the amplification of rod signals on their synaptic transfer at the first synapse with 'on' bipolar cells. These cells possess a metabotropic glutamate receptor linked via a cGMP cascade to the control of cGMP-activated channels. In the study presented here, we show that very dim background light, isomerising only one rhodopsin in 1 out of 10 rods per second, potentiates 'on' bipolar cell responses to superimposed flashes. Responses to dim flashes, which were undetectable above the noise in the dark, were boosted above the increased noise level induced by the background. This potentiation could be reproduced by elevating cGMP, which increases with light, or by dialysing the cells with a non-hydrolysable cGMP analogue. Inhibition of tyrosine kinase activity also reproduced the effect and induced a speeding up of the rising phase of the flash response, similar to the action of dim background light. Conversely, inhibition of tyrosine phosphatase activity blocked the potentiation. These results suggest that cGMP promotes tyrosine-site dephosphorylation of 'on' bipolar cell cGMP-activated channels, resulting in a rise in the sensitivity to cGMP, as has recently been demonstrated for rod cGMP-activated channels. This constitutes a positive feedback mechanism such that as cGMP increases with light, the sensitivity of the channels to cGMP increases and boosts the signal above background noise. This mechanism would allow stochastic resonance to occur, facilitating single-photon detection when dark-adapted, and may therefore lead to improved discrimination.