Light-induced fluctuations in membrane current of single toad rod outer segments.
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Animals↗
Biomedical subjects
T D Lamb
Publications and source records attributed to T D Lamb.
Closure of ionic channels in turtle cones.
Recent observations from turtle cones on the kinetics of the light response and the variance and power spectrum of spontaneous noise are discussed in terms of the behaviour of membrane ionic channels.
Animals↗
Analysis of electrical noise in turtle cones.
1. Properties of the light-sensitive voltage noise in cones in the retina of the turtle, Pseudemys scripta elegans, have been studied by intracellular recording.2. Suppression of the noise by light was a function of the hyperpolarizing response of a cone but not of the size or pattern of illumination.3. Power density spectra of the noise were fitted in many cones by the product of two Lorentzians with characteristic time constants tau(1) and tau(2) averaging 40 and 7 msec respectively. The spectra of some cells were peaked and could be fitted by a resonance curve.4. Spectra in dim light exhibited decreased low frequency power. They could often be fitted by a product of two Lorentzians using the same value of tau(2) as used in darkness but decreasing tau(1) and the zero frequency asymptote. An e-fold reduction in tau(1) occurred with lights which hyperpolarized by 4-7 mV.5. Injection of hyperpolarizing currents of about 0.1-0.2 nA into weakly coupled cones reduced the noise, and also reduced the sensitivity to dim flashes.6. The variance-voltage relation during steady illumination of different intensities differed from cone to cone. Dim lights increased the noise in some cells and decreased it in others, but moderately bright lights which gave steady responses of more than about one third maximal reduced the noise in all cells.7. When the cell was transiently depolarized during the differentiated component following steady illumination, the noise was less than it was after prolonged darkness.8. In the after-effect of bright light, the time course of recovery of noise was the same as that of flash sensitivity and voltage. The noise was reduced e-fold for hyperpolarizations averaging 3 mV while for sensitivity this reduction occurred for 1.3 mV. For a given hyperpolarization the noise was lower during the after-effect than during steady dim illumination.9. When a series of dim flashes was delivered to a cone, no significant increase in variance over the dark noise was detected during the photo-response. This implies that each photoisomerization evokes no more than about 1.5 muV at the peak of the response in a coupled cone, corresponding to about 50 muV in an isolated cone.10. The elementary shot events underlying the noise are about 100 muV in amplitude in an isolated cone, have a characteristic time constant of 16-60 msec and reflect unit conductance fluctuations of about 16 pS (S, Siemen identical with Omega(-1)).11. It is concluded that the noise source is internal to the cones. We postulate that the noise arises from opening and closing of the light-sensitive ionic channels in the outer segment, and that in darkness there is a residual concentration of the blocking substance which on average closes up to about one third of the channels. It seems likely that the unit event involves a considerable number of blocking molecules and ionic channels.
Action Potentials↗
Spatial properties of horizontal cell responses in the turtle retina.
1. Intracellular recordings were made from horizontal cells in the retina of the turtle Pseudemys scripta elegans. Spatial properties of the responses were determined using brief flashes of monochromatic light. 2. For a light stimulus in the form of a long narrow slit the peak response decayed approximately exponentially with displacement from the centred position. 3. With variation in the area of a centred circular patch, the peak response increased in a graded manner with stimulus area but was not proportional to area. 4. The model of electrical coupling in the horizontal cell layer proposed by Naka & Rushton (1976) was applied to the results. For the case of dim illumination a simplification is applicable, and the voltage distribution for circular and slit-shaped patches of light can be expressed in terms of two unknowns: the voltage resulting from diffuse illumination and a characteristic 'length constant'. 5. The measured variation of response amplitude was well described by the theory. Measured length constants were distributed from less than 100 mum to greater than 1 mm, and in a given cell the values determined by the slit displacement method and the area variation method were in reasonable agreement. 6. It is concluded that with dim illumination the model provides an accurate description of the voltage spread in the cells. Deviations were found to occur at higher intensities and possible reasons are discussed. 7. The implications of the model on the measurement of resistance changes during illunination are discussed.
Action Potentials↗
The relation between intercellular coupling and electrical noise in turtle photoreceptors.
1. Intracellular recordings from cones and rods in the retina of the turtle, Pseudemys scripta elegans, revealed that in darkness the cell voltage fluctuated spontaneously about its mean level. The fluctuations were reduced during bright steady illmination of the cell often to a level close to that obtained with the electrode outside the cell where the noise did not change significantly during illumination. 2. The magnitude of the intrinsic dark noise (voltage variance in darkness minus voltage variance in strong light) varied widely from cell to cell. In the noisiest cones it was about 0-4 mV2 while in quiet cones it was often as low as 0-01 mV2. The noise appeared radom and could be fitted by a Gaussian probability density function. 3. The spread of voltage in the network of coupled photoreceptors was estimated by measuring the spatial profile of the response to a brief flash of constant intensity moved across the retina. For a light stimulus in the form of a long narrow slit, the peak flash response usually decayed exponentially with displacement from the centred position. 4. For maximum responses less than about 5 mV in cones, the length constant of exponential decay, lambda, varied from less than 10 mum to greater than 35 mum, and the values obtained in opposite directions were often unequal. Background illumination did not significantly change lambda. In cells with extremely narrow spatial profiles, an exponential fit to the decay could not be made reliably. 5. Occasionally the spatial profiles had definite secondary peaks. In the most pronounced examples in a red-sensitive cone and in a rod the maxima were separated by about 20 and 50 mum respectively; for each, one peak was approximately as sharp as the optical stimulator while the second was broader. 6. Cones with short length constants displayed high dark noise while cones with long length constants were relatively quiet. 7. Three models of electrical coupling between cells were investigated: one based on a distributed network, one on a discrete square grid arrangement, and one on a discrete hexagonal array. Each model predicts a strong dependence of both noise and input resistance on length constant, and for tightly coupled cells each predicts that voltage variance is proportional to lambda-2. 8. The measured relationship between voltage variance and lambda in a large sample of cones was well described by both discrete models when the average cell spacing was taken to be approximately 15 mum. 9...
Animals↗
Power spectral measurements of noise in the turtle tetina [proceedings].
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Animals↗
Spontaneous voltage fluctuations in retinal cones and bipolar cells.
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Animals↗
The electrical response of turtle cones to flashes and steps of light.
1. The linear response of turtle cones to weak flashes or steps of light was usually well fitted by equations based on a chain of six or seven reactions with time constants varying over about a 6-fold range.2. The temperature coefficient (Q(10)) of the reciprocal of the time to peak of the response to a flash was 1.8 (15-25 degrees C), corresponding to an activation energy of 10 kcal/mole.3. Electrical measurements with one internal electrode and a balancing circuit gave the following results on red-sensitive cones of high resistance: resistance across cell surface in dark 50-170 MOmega; time constant in dark 4-6.5 msec. The effect of a bright light was to increase the resistance and time constant by 10-30%.4. If the cell time constant, resting potential and maximum hyperpolarization are known, the fraction of ionic channels blocked by light at any instant can be calculated from the hyperpolarization and its rate of change. At times less than 50 msec the shape of this relation is consistent with the idea that the concentration of a blocking molecule which varies linearly with light intensity is in equilibrium with the fraction of ionic channels blocked.5. The rising phase of the response to flashes and steps of light covering a 10(5)-fold range of intensities is well fitted by a theory in which the essential assumptions are that (i) light starts a linear chain of reactions leading to the production of a substance which blocks ionic channels in the outer segment, (ii) an equilibrium between the blocking molecules and unblocked channels is established rapidly, and (iii) the electrical properties of the cell can be represented by a simple circuit with a time constant in the dark of about 6 msec.6. Deviations from the simple theory which occur after 50 msec are attributed partly to a time-dependent desensitization mechanism and partly to a change in saturation potential resulting from a voltage-dependent change in conductance.7. The existence of several components in the relaxation of the potential to its resting level can be explained by supposing that the ;substance' which blocks light sensitive ionic channels is inactivated in a series of steps.
Animals↗
Reconstruction of the electrical responses of turtle cones to flashes and steps of light.
1. Theoretical equations which predict the electrical response of turtle cones to a wide range of light stimuli are developed from the experiments described in previous papers.2. The central points in the theory are that (a) light starts a chain of reactions leading to the production of a substance which blocks ionic channels in the outer segment, (b) an equilibrium between blocking molecules and open channels is rapidly established, (c) the blocking molecules are removed or inactivated by a chain of reactions, the first of which is autocatalytic, (d) in addition to the conductance which decreases with light there is also a conductance which increases with a delay when the cone is hyperpolarized.3. Parameters in the theory were deduced by approximate equations from the experiments described in the previous papers.4. There was good agreement between the properties of real and model cones in the following cases: (a) the response to 10 msec flashes and 0.7 sec steps of light calculated to give between 20 and 5 x 10(7) photoisomerizations per cone at times extending to about 2 sec; (b) the complicated changes in the response to a test flash that occur when it is superposed on background lights of increasing intensity; (c) the after-hyperpolarization and period of reduced sensitivity following a strong flash.5. The main defect of the theory is that the effect of background light in shortening the time to maximum of the response to a flash was more pronounced in a real cone than in the model.
Animals↗
Toward a unified model of vertebrate rod phototransduction.
Recently, we introduced a phototransduction model that was able to account for the reproducibility of vertebrate rod single-photon responses (SPRs) (Hamer et al., 2003). The model was able to reproduce SPR statistics by means of stochastic activation and inactivation of rhodopsin (R*), transducin (G alpha ), and phosphodiesterase (PDE). The features needed to capture the SPR statistics were (1) multiple steps of R* inactivation by means of multiple phosphorylations (followed by arrestin capping) and (2) phosphorylation dependence of the affinity between R* and the three molecules competing to bind with R* (G alpha, arrestin, and rhodopsin kinase). The model was also able to account for several other rod response features in the dim-flash regime, including SPRs obtained from rods in which various elements of the cascade have been genetically disabled or disrupted. However, the model was not tested under high light-level conditions. We sought to evaluate the extent to which the multiple phosphorylation model could simultaneously account for single-photon response behavior, as well as responses to high light levels causing complete response saturation and/or significant light adaptation (LA). To date no single model, with one set of parameters, has been able to do this. Dim-flash responses and statistics were simulated using a hybrid stochastic/deterministic model and Monte-Carlo methods as in Hamer et al. (2003). A dark-adapted flash series, and stimulus paradigms from the literature eliciting various degrees of light adaptation (LA), were simulated using a full differential equation version of the model that included the addition of Ca2+-feedback onto rhodopsin kinase via recoverin. With this model, using a single set of parameters, we attempted to account for (1) SPR waveforms and statistics (as in Hamer et al., 2003); (2) a full dark-adapted flash-response series, from dim flash to saturating, bright flash levels, from a toad rod; (3) steady-state LA responses, including LA circulating current (as in Koutalos et al., 1995) and LA flash sensitivity measured in rods from four species; (4) step responses from newt rods ( Forti et al., 1989) over a large dynamic range; (5) dynamic LA responses, such as the step-flash paradigm of Fain et al. (1989), and the two-flash paradigm of Murnick and Lamb (1996); and (6) the salient response features from four knockout rod preparations. The model was able to meet this stringent test, accounting for almost all the salient qualitative, and many quantitative features, of the responses across this broad array of stimulus conditions, including SPR reproducibility. The model promises to be useful in testing hypotheses regarding both normal and abnormal photoreceptor function, and is a good starting point for development of a full-range model of cone phototransduction. Informative limitations of the model are also discussed.
Adaptation, Ocular↗
Effects on the photoresponse of calcium buffers and cyclic GMP incorporated into the cytoplasm of retinal rods.
It is generally accepted that the light response in retinal rods involves a reduction of ionic permeability (predominantly to Na+) in the plasma membrane of the outer segment and that this is mediated by an internal messenger which diffuses between the disk and plasma membranes. There is controversy, however, over the identity of the diffusible substance; two alternative schemes have received widespread support (for review see refs 1,2). According to the 'calcium hypothesis', light stimulates the release into the cytoplasm of calcium, leading to the blockage of channels which are normally open in darkness, whereas based on the 'cyclic nucleotide hypothesis', cyclic GMP causes the opening of channels in the dark, but is hydrolysed by a light-activated phosphodiesterase. We report here effects of introducing calcium buffers and cyclic GMP into the rod cytoplasm by means of a patch pipette, which seem to be inconsistent with the calcium hypothesis.
3',5'-Cyclic-GMP Phosphodiesterases↗