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L H Pinto

Publications and source records attributed to L H Pinto.

82 records · Page 5Linked to original sources
PubMed1974

Light-induced changes in photoreceptor membrane resistance and potential in Gecko retinas. II. Preparations with active lateral interactions.

The time-course of light-induced changes in membrane voltage and resistance were measured in single photoreceptors in eyecup preparations of Gekko gekko. A small circular stimulus directed toward the impaled receptor produced membrane hyperpolarization. Application of a steady annular light to the receptor periphery resulted in diminution of the receptor's response to the stimulus. The effects of illumination of the surrounding receptors were isolated by directing a small, steady desensitizing light to the impaled receptor and then applying a peripheral stimulus. Brief stimuli produced a transient decrease in resistance with rapid onset and offset, a time-course similar to that of the response diminution. For some cells a depolarization that coincided with the resistance decrease was seen. During illumination with prolonged stimuli the resistance decrease was followed by a slow increase. After offset resistance rose transiently above the original value and then returned slowly to its original value. The slow resistance changes were not accompanied by changes in membrane voltage. The response diminution, resistance decrease, and depolarization were not observed in retinas treated with aspartate or hypoxia. It is therefore concluded that these effects are mediated by horizontal cells. The diminution is achieved by shunting the receptor potential and may play a role in field adaptation.

Adaptation, Ocular↗
PubMed1974

Ionic mechanism for the photoreceptor potential of the retina of Bufo marinus.

1. Membrane potentials were recorded from single rods in the isolated retina of Bufo marinus while the ionic composition of the extracellular medium was rapidly changed. Substitution of 2 mM aspartate(-) for Cl(-) produced a prompt depolarization of horizontal cells, but no modification of either resting potential or response to light in receptor cells. This implies that feed-back from horizontal cells to receptor cells was not active.2. During substitution of choline(+) or Li(+) for Na(+), and during isosmotic substitution of sucrose for NaCl, the resting potential of receptor cells became more negative and responses to light were abolished. During exposure to K(+)-free medium, the resting potential became slightly more negative and the responses to light became larger and developed small after-depolarizations. Exposure to [K(+)](out) of four times normal resulted in permanent diminution of response magnitude and permanent change of response waveshape. Removal of Mg(2+), four times normal [Mg(2+)](out) or substitution of methylsulphate(-) for Cl(-) had no effect on resting potential or responses to light. With the exception of the small effects seen with altered [K(+)](out) these results are consistent with the receptor potential being generated by a light-induced decrease of membrane conductance to Na(+).3. Exposure to decreased [Ca(2+)](out) caused both a depolarization of the receptor membrane in the dark and an increase in the magnitude of the maximal response that could be evoked by a test stimulus. The magnitude of the increase in response equalled the magnitude of the depolarization. Exposure to increased [Ca(2+)](out) or steady background light caused both a steady hyperpolarization and a decrease in the magnitude of the maximal response that could be evoked by a test stimulus. For steady hyperpolarizations greater than 3.5 mV, whether caused by elevated [Ca(2+)](out) or steady background light, the decrease in response magnitude exceeded the magnitude of the hyperpolarization. These results imply that externally applied Ca(2+) ions mimic the effects of steady background lights, but the applied Ca(2+) ions must do more than merely decrease membrane conductance to Na(+).

Animals↗
PubMed1972

Adaptation in retinal rods of axolotl: intracellular recordings.

Intracellular recordings of the late receptor potential from rods of isolated axolotl retinas revealed the existence of a dark adaptation mechanism that is independent of rod pigment regeneration. Response amplitude of individual rods was measured as a function of intensity both before and at various times after exposure to bleaching illumination. The rod sensitivity increased by at least 3 to 4 log units during a period of 15 to 25 minutes following the bleach. During this time rod pigment regeneration was either too small to be measured or was nonexistent in our preparation.

Ambystoma↗
PubMed1972

Image quality of the cat eye measured during retinal ganglion cell experiments.

1. The modulation transfer function (MTF) of the dioptrics of fifteen cat eyes was determined. The aerial image, formed by the eye of a standard object (a 0.5-1.0 degrees annulus), was photographed. The transmission of the film negative was measured with a scanning microdensitometer to yield the light distribution within the aerial image. Correcting for the double passage, this experimentally determined light distribution and the known object light distribution were used to obtain the MTF, applying Fourier methods. Each MTF was used to calculate the light distribution within the retinal image of stimuli of various geometry used in experiments on retinal ganglion cells in the same eye.2. When the eye was equipped with an artificial pupil of the same size as that used in the neurophysiological experiments (4.0-4.8 mm diam.) the MTF had fallen to 0.5 at 2.43 c/deg. When the pupil was removed the MTF had fallen to 0.5 at a much lower spatial frequency (1.0 c/deg). This shows that even when one uses an artificial pupil too large to provide optimal image quality there is a vast improvement over using no pupil.3. These image quality measurements were prompted by the need to know the actual stimulus image in experiments on the functional organization of the receptive field, a need exemplified in this paper by a few specific physiological results. The full neurophysiological results appear in the next two papers.

Action Potentials↗
PubMed1972

Properties of the surround response mechanism of cat retinal ganglion cells and centre-surround interaction.

1. The properties of the surround response mechanism of on-centre cells and its interaction with the centre mechanism were studied by recording from single optic tract fibres. In many of the experiments the spatial distribution of the light within the retinal image of the stimuli was measured.2. Pure surround responses of on-centre cells were isolated using a centrally located steady light which selectively desensitized (adapted) the centre mechanism. This permitted a peripheral flashing stimulus whose luminance varied over a range as great as 1.38 log units to elicit surround responses which, for any given cell, were of invariant shape. The rate of decay of the firing frequency of the spike burst at ;off' varied from cell to cell. The general characteristics of such pure surround responses to squarewave stimuli were described. The plot of the magnitude of pure responses against stimulus luminance, at constant background conditions, was curvilinear.3. The pure surround response of two off-centre cells was isolated; it was similar in shape to the pure centre response of on-centre cells.4. Interaction of centre and surround mechanisms of on-centre cells was studied by eliciting a pure central and a pure surround response from the same cell. The electronically obtained algebraic sum of these two pure responses equalled the mixed response of the ganglion cell to simultaneous presentation of the stimuli which evoked the pure responses when presented singly. This is probably best explained by algebraic summation of centre and surround inputs.5. The pure surround response from two cells to a fixed flashing stimulus was attenuated by a steady field adapting light, both when this was superimposed upon the stimulus and when not superimposed. In the latter case, (i) when the spatial separation between the flashing stimulus and the adapting light was at a minimum, less than 10% of the adapting flux fell inside the boundaries of the stimulating flux, and (ii) the response was attenuated also if the adapting light was in the geometric centre of the receptive field. These results indicate that the adaptation pool of the surround mechanism extends to the central portions of the receptive field.6. Nearly half the cells tested did not yield pure surround responses. This was probably due to differences, within the ganglion cell population, (i) of the spatial distribution of the ratio of centre to surround signal sensitivity and (ii) of differences in the ratio of centre to surround adaptivity in the receptive field middle. It was not due to excess adaptive flux falling outside the region of maximal centre mechanism adaptivity, nor due to excess stimulus flux falling inside the region of maximal signal sensitivity.

Action Potentials↗
PubMed1972

Pure central responses from off-centre cells and pure surround responses from on-centre cells.

1. Pure responses generated by the centre response mechanism of off-centre cells or the surround response mechanism of on-centre cells were studied in the cat by recording from single optic tract fibres while applying slow square-waves of light of varying duration but constant luminance.2. The response, whether due to a centre or a surround mechanism, consisted of a rapid decrease in firing frequency at onset of light, followed by a gradual recovery of firing rate while the light was on. The time course of the recovery was exponential. At ;off' there was a sudden increase in firing rate (off-discharge) followed by an exponential, more gradual, decay of impulse frequency. Both the gradual recovery during illumination and the off-discharge were absent for the shortest flashes.3. Perhaps the most significant result was that the behaviour exhibited by responses generated by one only of the response mechanisms, centre or surround, is predictable and much simpler than that exhibited by mixed responses; the magnitude of the off-discharge increased, the latency decreased monotonically with increased flash duration and the magnitude of the off-discharge was linearly related to the level of firing at the moment when the light went off.4. For two off-centre cells gain was studied by recording the response to a brief test flash superimposed upon a slow gain changing square-wave stimulus. Centre mechanism gain decreased promptly upon application of the gain changing light, stayed low while it was on, only to return promptly to its former value when the gain changing light went off.5. A simple model which qualitatively accounts for the results (excluding latency) was suggested. The onset of light initiates both an inhibitory and excitatory process whose sum constitutes the transient input to the cell. The excitatory process grows at a slower rate so that ;excitation wins more and more as the light remains on'. Similarly after ;off' the excitatory process decays slower than the inhibitory one, thus leaving the cell with a transient excess of net-excitation.

Action Potentials↗
PubMed1970

Gallamine triethiodide (flaxedil) and cat retinal ganglion cell responses.

1. Repeated flashes of diffuse light were presented to the cat's eye over long periods of time (hours) while the mixed ganglion cell response was recorded from single axons in the optic tract. This was done for cats receiving gallamine triethiodide intravenously and for cats who did not receive the drug.2. All responses were transformed into instantaneous pulse density tracings. Such tracings from control and gallamine cells were compared to observe possible effects of gallamine: (a) the shape (time course) did not change during gallamine administration; (b) maximum firing frequency, total number of spikes and latency was measured for both the on- and the off-component on the pulse density tracings and plotted versus time. Statistical methods failed to reveal any difference between the manner in which these response features of control cells and of gallamine cells varied with time.

Action Potentials↗
PubMedDate not supplied

Response properties of ganglion cells in the isolated mouse retina.

We have studied the organization of receptive fields of ganglion cells in the isolated mouse retina and have shown that the organization is similar to that of the cat. Based upon responses to circular and annular stimuli, most ganglion cells (90%; N = 83) had receptive fields with concentric center-surround organization, either ON or OFF center. The plot of response amplitude vs. stimulus area for these cells increased to a maximum (corresponding to a diameter of 10.0 +/- 2.8 deg S.E.M.; N = 13) and then decreased for larger stimuli, demonstrating the presence of an antagonistic surround. The dark-adapted sensitivity (205 +/- 43.8 impulses quantum-1 rod-1; mean +/- S.E.M.) did not differ from that measured for the intact preparation. We found a subset of OFF-center cells for which the dark discharge was very regular (mean coefficient of variation = 0.30). Using sinusoidal grating stimuli, we classified ganglion cells as X-like (87%) and Y-like (13%) based on spatial summation properties and the presence of subunit activity in the receptive-field center. We found no difference in the spatial-frequency preference between X-like and Y-like cells in the central retina (high cutoff frequency, 0.20 +/- 0.014 cycle/deg, mean +/- S.E.M.), in contrast to the marked difference between X cells and Y cells in the cat. Thus, ganglion cell receptive fields in the mouse retina resemble those of the cat, although the spatial characteristics of the receptive fields in the central retina are more homogeneous. This homogeneity would simplify the comparison of retinas from normal and mutant strains of the mouse.

Animals↗
PubMedDate not supplied

Identification and localization of K+ channels in the mouse retina.

Voltage-gated potassium channels are differentially expressed in the brain, and recent studies have shown that K+ channels show subcellular localization. We characterized the distribution of five different K+ channels in the mouse retina. Each channel was distributed in a unique pattern in the retina and was localized to specific subcellular domains within a given retinal neuron. Kv1.4 and Kv4.2 were consistently found in axonal and somatodendritic portions, respectively, consistent with previous studies in brain. In contrast, Kv1.2, Kv1.3, and Kv2.1 showed variable subcellular distribution depending upon cellular context. These results suggest that no one K+ channel is distributed over the entire length of the neuron to provide a "housekeeping" level of membrane potential stabilization. Instead, we propose that each K+ channel is associated with a specific subcellular functional module, and each local K+ conductance responds uniquely to local voltage and second messenger signals.

Animals↗