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T D Lamb

Publications and source records attributed to T D Lamb.

18 recordsLinked to original sources
PubMed1992

Different channel-gating properties of two classes of cyclic GMP-activated channel in vertebrate photoreceptors.

We report that two types of cGMP-activated channel coexist in the photoreceptor plasma membrane, with the most commonly encountered class appearing broadly similar to the channel reported in previous patch-pipette experiments. However, we find that flickering of this channel between the open and closed states is so rapid that a discrete single-channel conductance cannot unequivocally be resolved; the occurrence of flickering is largely independent of membrane voltage and of the presence of cytoplasmic Ca2+ or Mg2+. In recordings from the inner segment we occasionally find a second class of cGMP-gated channel, with activity resembling that reported for cloned channels. This channel does not flicker, but instead exhibits distinct open-close transitions. Our results suggest that the predominant form of channel in vivo differs significantly from cloned channels, and that its gating properties are not as simple as reported previously.

Ambystoma↗
PubMed1992

G-protein cascades: gain and kinetics.

G-protein cascades provide amplification in a wide variety of biological signal transducers--from hormonal and synaptic systems to the receptor cells of vision and olfaction. Through recent understanding of the molecular mechanisms involved, it is possible to construct a quantitative description of the amplification and speed of response of the cascade. The gain and kinetics can now be described in terms of physical parameters, such as enzyme activities and the densities and lateral diffusion coefficients of the proteins involved.

Animals↗
PubMed1992

A quantitative account of the activation steps involved in phototransduction in amphibian photoreceptors.

1. We have undertaken a theoretical analysis of the steps contributing to the phototransduction cascade in vertebrate photoreceptors. We have explicitly considered only the activation steps, i.e. we have not dealt with the inactivation reactions. 2. From the theoretical analysis we conclude that a single photoisomerization leads to activation of the phosphodiesterase (PDE) with a time course which approximates a delayed ramp; the delay is contributed by several short first-order delay stages. 3. We derive a method for extracting the time course of PDE activation from the measured electrical response, and we apply this method to recordings of the photoresponse from salamander rods. The results confirm the prediction that the time course of PDE activation is a delayed ramp, with slope proportional to light intensity; the initial delay is about 10-20 ms. 4. We derive approximate analytical solutions for the electrical response of the photoreceptor to light, both for bright flashes (isotropic conditions) and for single photons (involving longitudinal diffusion of cyclic GMP in the outer segment). The response to a brief flash is predicted to follow a delayed Gaussian function of time, i.e. after an initial short delay the response should begin rising in proportion to t2. Further, the response-intensity relation is predicted to obey an exponential saturation. 5. These predictions are compared with experiment, and it is shown that the rising phase of the flash response is accurately described over a very wide range of intensities. We conclude that the model provides a comprehensive description of the activation steps of phototransduction at a molecular level.

Animals↗
PubMed1990

Cyclic GMP and calcium: the internal messengers of excitation and adaptation in vertebrate photoreceptors.

The roles of cyclic GMP (cGMP) and calcium (Ca2+) in vertebrate rod phototransduction are reviewed, with the emphasis on developments since the discovery of the cGMP-activated conductance of the rod outer segment. The first hypothesis subjected to critical examination is that cGMP acts as the sole internal messenger of excitation. This hypothesis is evaluated with a formal, quantitative model of the biochemical actions of cGMP. Application of the model shows a remarkable agreement between independent electrophysiological and biochemical measurements of the resting dark amounts of (1) total cGMP (2) free cGMP (3) fraction of open cGMP-activated channels and (4) the rate of cGMP hydrolysis. The second hypothesis examined is that Ca2+ acts as an internal messenger in rod light adaptation. Recent electrophysiological evidence has shown minimization of the normal light-induced reduction of free Ca2+ prevents rods from exhibiting the change in sensitivity and speed characteristic of light adaptation. Physiological effects, formerly attributed to a role of calcium as an excitational messenger are shown to be consistent with a biochemical model in which Ca2+ serves as the cytoplasmic signal in a powerful feedback loop that acts to restore the concentration of cGMP both during and after exposure to light. Residual problems facing the "cGMP cascade theory of phototransduction" are reviewed. Issues are itemized that will have to be resolved quantitatively before it will be possible to develop a fully comprehensive theory of photoreceptor excitation, restoration and adaptation combining the roles of Ca2+ and cGMP.

Adaptation, Physiological↗
PubMed1990

Light adaptation in cone photoreceptors of the salamander: a role for cytoplasmic calcium.

1. Light adaptation has been studied in isolated red-sensitive cone photoreceptors of the salamander, using suction pipette recordings of circulating current. 2. In the presence of background illumination, the response to incremental dim flashes became desensitized according to the Weber-Fechner law. The recovery phase of the flash response was accelerated significantly, although the time-to-peak was reduced only slightly, and for dim backgrounds the rising phase was unaltered. 3. The role of cytoplasmic calcium concentration, Cai2+, in mediating cone adaptation was investigated by minimizing light-induced changes in Cai2+, either by incorporating calcium buffer into the cytoplasm or by exposing the outer segment to low-Ca2+, 0-Na+ solution. Both treatments appeared to slow dramatically or even to eliminate the onset of light adaptation in the cone. 4. When the low-Ca2+, 0-Na+ solution was presented in darkness, responses to subsequent illumination were affected in a characteristic manner: (i) the response-intensity relation was steepened and shifted to lower intensities, (ii) the response to a step of light could be predicted by integration and compression of the flash response, and (iii) the flash sensitivity declined steeply as a function of background intensity. 5. After extended exposure of the cone to bright backgrounds, the sensitivity in darkness failed to return to its original level. The flash response kinetics were faster and more biphasic than for dark-adapted responses or for responses desensitized to a comparable degree by exposure to steady background illumination. 6. The results indicate that, in cones isolated from the pigment epithelium, the primary factor influencing the adaptational state of the cell is the cytoplasmic concentration of free calcium, but that at high intensities the effects of pigment bleaching are likely to be significant.

Action Potentials↗
PubMed1979

The membrane current of single rod outer segments.

1. Outer segments of individual rods in the retina of the toad, Bufo marinus, were drawn into a glass pipette to record the membrane current. 2. Light flashes evoked transient outward currents. The peak response amplitude was related to flash intensity by a Michaelis equation with half-saturating intensity about 1 photon mum-2. 3. The saturating response amplitude ranged up to 27 pA and corresponded closely to complete suppression of the steady inward current present in darkness. 4. For a given cell the saturating response amplitude varied linearly with the length of outer segment within the pipette. This is consistent with a uniform density of light-sensitive channels and negligible gradient of membrane potential along the outer segment. 5. Responses to bright flashes never showed the relaxation from an initial peak seen previously in intracellular voltage recordings, suggesting that the conductance change responsible for the relaxation does not occur in the outer segment. 6. Responses to local illumination of only the recorded outer segment were very similar to those obtained with diffuse light at the same intensity, indicating that peripheral rods made little contribution to the responses. 7. The spectral sensitivity of 'red' rods was consistent with a retinal1-based pigment with lambda max = 498 +/- 2 nm. 8. The kinetics of the response were consistent with four stages of delay affecting action of the internal transmitter. Responses were faster at the basal end of the outer segment than at the distal tip. 9. Background light reduced the sensitivity to a superposed dim test flash and shortened the time course of the response, indicating that adapting light modifies the kinetics and gain of the transduction mechanism within the outer segment. 10. Responses to dim lights exhibited pronounced fluctuations which are attributed in the succeeding paper (Baylor, Lamb & Yau, 1979) to the quantal nature of light.

Animals↗
PubMed1979

Responses of retinal rods to single photons.

1. A suction electrode was used to record the membrane current of single rod outer segments in pieces of toad retina. During dim illumination the membrane current showed pronounced fluctuations. 2. Amplitude histograms of responses to dim flashes of fixed intensity exhibited two discrete peaks, one at 0 pA and one near 1 pA, suggesting that the response was quantized. By setting a criterion amplitude level, flash responses could be classed as 'failures' (no response) or as 'successes' (at least one quantal event). 3. The variation of fraction of successes with flash intensity was consistent with the hypothesis that each quantal electrical event resulted from a single photoisomerization. 4. The quantal event had a mean amplitude of about 1 pA (5% of the standing dark current) and a standard deviation of 0.2 pA. Dispersion in the event amplitude prevented identification of histogram peaks corresponding to two or more photoisomerizations. 5. Individual quantal responses exhibited a smooth shape very similar to that of the average quantal response. This suggests that a single photoisomerization releases many particles of transmitter and that radial diffusion of internal transmitter is not a major source of delay in the light response. 6. The 'quantum efficiency' with which an absorbed photon generated an electrical event was measured as 0.5 +/- 0.1 (S.E. of mean, n = 4). This is slightly lower than the quantum efficiency of photoisomerization obtained previously for rhodopsin in solution. 7. At wavelengths between 420 and 700 nm the quantal event was invariant in size, although the cell's sensitivity varied over a range of 10(5). 8. The power spectrum of the fluctuations in dim steady light was predicted by assuming that a random series of quantal events occurred independently. 9. In brighter light the fluctuations were faster, and the response to an incremental flash was reduced in size and duration. The power spectrum could be predicted by assuming random superposition of events with the shape of the incremental flash response.

Animals↗
PubMed1978

Properties of the membrane current of rod outer segments.

The membrane current of single rod outer segments in pieces of isolated toad retina was recorded with a glass suction electrode. Light evoked a slow net outward photocurrent consisting of a reduction in the steady inward dark current. In very dim light, the photocurrent broke up into discrete shot effects with a rounded shape and an amplitude of about 1 pA. These events were shown to result from photoisomerization of single rhodopsin molecules. The current in darkness showed fluctuations consisting of (a) discrete events apparently resulting from thermal isomerization of rhodopsin molecules, and (b) smaller amplitude shot effects shaped by two of the four rate processes of the light response.

Animals↗
PubMed1977

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↗
PubMed1977

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↗
PubMed1976

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↗
PubMed1976

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↗