PubMed Health⌕ Search

Biomedical subjects

L Cervetto

Publications and source records attributed to L Cervetto.

At least 37 records · Page 2Linked to original sources
PubMed1985

Mechanisms of light adaptation in toad rods.

A steady illumination equivalent to about 1000 or more photoisomerizations per s per rod completely abolishes the dark current and induces a total desensitization of toad rods. Under these conditions, however, both current and sensitivity recover from the initial suppression and reach a new steady level within about 30 s. Both extent and time course of recovery depend on the intensity of the conditioning light. The ability of rods to respond to light stimuli under light saturating conditions is enhanced by procedures aimed at increasing the sodium gradient across their surface membrane. The results suggest that rods can also contribute significantly to vision at photopic levels of ambient light. The mechanisms underlying this adaptational property of rods are discussed.

Acclimatization↗
PubMed1983

The effect of phosphodiesterase inhibitors on the electrical activity of toad rods.

The membrane potential of toad rods was recorded during addition of small amounts of phosphodiesterase inhibitors to the extracellular medium. Separate application of 3-isobutyl-1-methylxanthine (IBMX), caffeine, theophylline, papaverine and RO 20-1724 slowed down the time course of rod photo-response to dim flashes of light. These changes were associated with a two to six-fold increase in the amplitude of photoresponse. The effects on kinetics may be described simply by an expansion of the photoresponse time scale. When the drug concentration was raised above a certain level, the rods showed supralinear behaviour whereby doubling of the intensity of a dim flash could increase the response more than two-fold. Under similar conditions rods also showed light sensitization whereby responses to dim flashes were enhanced in the presence of dim backgrounds. Taking the drug concentration that induced a two-fold increase in the time-to-peak, IBMX was found the most effective compound, followed by papaverine, RO 20-1724, theophylline and caffeine with relative effectivities 1, 1/2, 1/7, 1/40 and 1/100. Sensitivity, kinetics and supralinear behaviour may be restored to normal by steady background illumination while still in the presence of IBMX. However the intensity of the steady light, needed to restore the sensitivity to control levels, is not sufficient to accelerate the kinetics back to control values. In the presence of 50 microM-IBMX a dim steady background of light enhanced the response to dim flashes. When the intensity of the light background was increased rods were desensitized and the supralinear behaviour disappeared. The antagonism between the effects of IBMX and the effects of background illumination on the kinetics of photoresponse suggests that phosphodiesterase activity controls the time course of light response in vertebrate rods.

1-Methyl-3-isobutylxanthine↗
PubMed1983

Ionic movements through light-sensitive channels of toad rods.

Electrical photoresponses of rods in the isolated toad retina were recorded during ionic manipulations of the Na+-free extracellular medium. In the presence of a concentration of external Ca2+ above 10(-5) M, voltage photoresponses were observed only in the presence of external Na+ or Li+. When external Ca2+ was reduced below 10(-6) M, voltage photoresponses of normal polarity could be detected even in the absence of Na+ or Li+, but in the presence of external Mg2+. In the presence of normal extracellular Ca2+ hyperpolarizing photoresponses were observed even in the absence of Na+ or Li+, provided small amounts of phosphodiesterase inhibitors (IBMX, RO 20-1724, papaverine, caffeine, theophylline) were added to the perfusate. Responses obtained in low-Na+ IBMX solutions required the presence of millimolar amounts of a variety of divalent cations, among which Mn2+ and Ba2+ were the most effective. When the concentration of both external Ca2+ and Mg2+ was reduced to micromolar amounts, depolarizing photoresponses were observed. In these conditions measurements with radioactive tracers showed a light-modulated efflux of 42K+ or 86Rb+. The light-modulated 42K+ or 86Rb+ efflux was halved by 2 X 6 mM-external K+ and was completely blocked when K+ was raised above 10 mM. These results show that ionic movements through light-sensitive channels are controlled by Ca2+ and Mg2+ and possibly also be the intracellular level of cyclic nucleotides. Moreover, the movement of ions through the light-sensitive channel, does not obey the independence principle.

1-Methyl-3-isobutylxanthine↗
PubMed1983

Mechanism of action of the sodium pump in vertebrate photoreceptors.

Membrane potential and photoresponse of rods in the isolated toad retina was recorded while changing the ionic composition of the extracellular medium. In the presence of 10 mM external Cs+ the bright flash response consisted of an initial fast component of about 35 mV followed by a much slower component as large as 50 mV. The slow component of the photoresponse was blocked by micromolar amounts of both ouabain and strophanthidin. The effects induced by the latter were almost completely irreversible. The effects induced by changing the external concentrations of Na+, K+ and Ca2+ were analysed assuming that the amplitude of the slow component of voltage photoresponse reflects the activity of Na+--K+ pump. It is shown that external Na+, K+ and Ca2+ have the same effect on the amplitude of the slow component as on the Na+-K+ pump activity of other tissues.

Animals↗
PubMed1982

Antagonism between steady light and phosphodiesterase inhibitors on the kinetics of rod photoresponses.

Treatment of toad rods with phosphodiesterase inhibitors (3-isobutyl-1-methylxanthine, caffeine, theophylline, papaverine, and RO 20-1724) modifies the properties of the intracellular voltage responses to dim flashes of light. 3-Isobutyl-1-methylxanthine at 1-20 microM causes an increase in flash sensitivity and a slowing down of the kinetics of the photoresponses. When the drug concentration is greater than 20 microM, rods also show supralinear behavior, whereby doubling the intensity of a dim flash may increase the response by greater than 2-fold. Sensitivity, kinetics, and supralinear behavior can be restored to normal by steady background illumination while still in the presence of 3-isobutyl-1-methylxanthine. However, the intensity of the steady light needed to restore the sensitivity to control levels is not sufficient to accelerate the kinetics back to control values. The antagonism between the effects of 3-isobutyl-1-methylxanthine and the effects of background illumination is explained by assuming that: (i) the length of time to peak voltage responses to dim flashes of light is inversely proportional to the rate of a chemical reaction; (ii) the rate of this reaction is controlled by an enzyme that is inhibited competitively by 3-isobutyl-1-methylxanthine with a Ki of 3 x 10(-6) M; and (iii) the concentration of a cofactor of this reaction increases proportionally with the intensity of the background illumination.

1-Methyl-3-isobutylxanthine↗
PubMed1982

Processing of visual signals in vertebrate photoreceptors.

Photoreceptors of the vertebrate retina hyperpolarize in response to illumination. The conductance changes in the plasma membrane associated with the electrical response are the final step of chain of events initiated by light absorption at the outer segment of the visual cell. The mechanism whereby the free energy of photons in converted into neural information is largely unknown. Present knowledge is consistent with the idea that an internal transmitter is modulated by light and modifies the ionic permeability of the plasma membrane. As to the identity of the internal messenger two candidates have been proposed: Ca2+ and cyclic GMP respectively. Increasing evidence suggests that both substances may be involved in the process of phototransduction. The electrical response of photoreceptors does not simply reflect the light absorbed by the cell: complex interactions occurring between adjacent photoreceptors and between photoreceptors and second order neurons cooperate with the initial process in determining the final shape of the receptor message. Recurrent interactions involve particularly cones: their membrane potential can be modified at least by three distinct mechanisms; i) by light absorption at their outer segment; ii) by light absorption at the outer segment of neighbouring cones, and iii) by potential changes occurring in horizontal cells.

Adaptation, Physiological↗
PubMed1981

Rod photoresponses in the absence of external sodium in retinae treated with phosphodiesterase inhibitors.

Exposure of isolated toad retinae to phosphodiesterase inhibitors, induced changes in the ionic permeability of rod cells. Under similar conditions intracellularly recorded light responses were observed also in the absence of external Na+. Hyperpolarizing photoresponses in Na+-free media required the presence of divalent cations among which Mg2+, Mn2+ and Ba2+ were the most effective.

1-Methyl-3-isobutylxanthine↗
PubMed1981

The sodium current underlying the responses of toad rods to light.

1. Intracellular responses were recorded from single rods in the retina of the toads Bufo bufo and Bufo marinus during exposure to solutions in which sodium was replaced by equimolar amounts of choline. 2. Upon moderate reduction (80 and 50 mM) of the external sodium the size of responses to bright flashes decreased as a consequence of both an increase in the resting potential and a decrease of the membrane potential at the peak, while the level of the plateau remained fairly constant. 3. Upon reduction of the external sodium to 22 mM or less, rods hyperpolarized to about the plateau level and failed to respond to illumination. Under these circumstances, membrane depolarization induced by an increased external potassium did not restore the cell responsiveness. Addition of 2-5 mM caesium hyperpolarized the membrane and partially restored the photoresponse. 4. Complete replacements of external sodium with potassium depolarized the rod by 40 +/- 10 mV, and no voltage responses to light could be detected. 5. In the presence of caesium, a nearly complete blockage of the photoresponses was obtained when the external sodium was 5 mM or less. Further reductions of the external sodium did not invert the photoresponses. Application of caesium when the external sodium was nominally zero induced a transient hyperpolarization followed by a slow decay. 6. During exposure to steady illumination, the dependence of the plateau level on the external sodium slowly increased. 7. These results indicate that the ionic current which is directly modulated by the light depends primarily on the external sodium. They suggest also that the current associated with the voltage- and time-dependent process responsible for the sag from peak to plateau of the response to a bright flash of light may have multiple components.

Animals↗
PubMed1981

Diurnal changes in the pigeon electroretinogram.

The spectral sensitivity of the pigeon Electroretinogram was determined at different times during the 24-hour cycle. These measures show that diurnal changes occur in the relative sensitivity of the electroretino-graphic responses to light stimuli of different wavelength. Diurnal variations occur despite the absence of rhythmic photoperiodic stimulation. It is suggested that a spontaneous shift occurs in the functional dominance of cones and rods with a period near to 24 hours.

Animals↗
PubMed1980

Effects of changing external potassium and chloride concentrations on the photoresponses of Bufo bufo rods.

1. Intracellular responses to light were recorded from Bufo bufo rods in different ionic media. 2. The exposure of the retina to high external [K+] depolarized the rod and modified the time course of the photoresponse. The prominent initial transient of rod responses to bright flashes was drastically reduced in 5 mM-external [K+] and completely disappeared in 26 mM. In high external [K+] the kinetics of responses to dim flashes were considerably slower than in control conditions. 3. When external [Cl-] was changed from 120.6 to 10.6 mM the resting membrane potential decreased and the size of photoresponses increased. Changes in the kinetics similar to those described in high external [K+] were also observed. 4. In many cases exposure of the retina to low external [Cl-] induced oscillations of the resting membrane potential that sometimes became sustained. This instability of the membrane completely disappeared upon restoring to normal conditions. 5. The present results may be explained by assuming the existence of a voltage- and time-dependent conductance active near the dark level of membrane potential. This hypothesis can be represented by an equivalent electrical circuit that includes an inductance (Detwiler, Hodgkin & McNaughton, 1980).

Animals↗
PubMed1978

Mechanisms of generation of signals in vertebrate photoreceptors.

The electrical responses of rods are analyzed in different ionic environments. It is shown that the dark level of the membrane potential is predominantly determined by a sodium current, while the peak of responses to bright light is controlled by the concentration of external potassium. The sag from the peak to the plateau of photoresponses seems to be generated by different ionic mechanism. The effects produced by substituting the external calcium with EGTA are also analyzed. It is suggested that calcium plays a role in different mechanisms of generation of electrical responses.

Animals↗
PubMed1977

Effects of applied currents on turtle cones in darkness and during the photoresponse.

1. The voltage changes, produced by pulses of current of various intensities and polarities into turtle cones, were examined both in darkness and during the photoresponse. 2. Depolarizing pulses of current produced voltage deflexions which, during the photoresponse, were increased to a greater extent than those produced by pulses of current of the same intensity and opposite polarity. 3. Voltage-current relationships were measured both in darkness and during illumination. During illumination they display a non-linear behaviour, the curve being steeper for positive than for negative currents. 4. These non-linearities are greatly attenuated following exposure of the retina to a solution containing 5 mM-CoCl2 which is known to block synaptic transmission. The effects of CoCl2 are readily reversible when the retina is returned to normal conditions. 5. By comparing the effects produced by pulses of current injected into individual cones with those observed in horizontal cells following stimulation with a radial current, it is observed that the non-linear behaviour of the cone membrane during the photoresponse can be, at least in part, accounted for by events occurring in post-synaptic elements (i.e. horizontal cells).

Animals↗
PubMed1977

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.

Animals↗
PubMed1974

Synaptic transmission between photoreceptors and horizontal cells in the turtle retina.

Low calcium, high magnesium, and cobalt hyperpolarize the horizontal cell membrane and suppress the response to light, but only partially affect the response of receptor cells. These observations are consistent with the interpretation that a depolarizing transmitter is released by photoreceptors in darkness. The hyperpolarizing response to light of the horizontal cells would then result from a reduction in the amount of transmitter released.

Action Potentials↗