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At least 1,423 records · Page 79Linked to original sources
PubMed1999

Divalent cation interactions with light-dependent K channels. Kinetics of voltage-dependent block and requirement for an open pore.

The light-dependent K conductance of hyperpolarizing Pecten photoreceptors exhibits a pronounced outward rectification that is eliminated by removal of extracellular divalent cations. The voltage-dependent block by Ca(2+) and Mg(2+) that underlies such nonlinearity was investigated. Both divalents reduce the photocurrent amplitude, the potency being significantly higher for Ca(2+) than Mg(2+) (K(1/2) approximately 16 and 61 mM, respectively, at V(m) = -30 mV). Neither cation is measurably permeant. Manipulating the concentration of permeant K ions affects the blockade, suggesting that the mechanism entails occlusion of the permeation pathway. The voltage dependency of Ca(2+) block is consistent with a single binding site located at an electrical distance of delta approximately 0.6 from the outside. Resolution of light-dependent single-channel currents under physiological conditions indicates that blockade must be slow, which prompted the use of perturbation/relaxation methods to analyze its kinetics. Voltage steps during illumination produce a distinct relaxation in the photocurrent (tau = 5-20 ms) that disappears on removal of Ca(2+) and Mg(2+) and thus reflects enhancement or relief of blockade, depending on the polarity of the stimulus. The equilibration kinetics are significantly faster with Ca(2+) than with Mg(2+), suggesting that the process is dominated by the "on" rate, perhaps because of a step requiring dehydration of the blocking ion to access the binding site. Complementary strategies were adopted to investigate the interaction between blockade and channel gating: the photocurrent decay accelerates with hyperpolarization, but the effect requires extracellular divalents. Moreover, conditioning voltage steps terminated immediately before light stimulation failed to affect the photocurrent. These observations suggest that equilibration of block at different voltages requires an open pore. Inducing channels to close during a conditioning hyperpolarization resulted in a slight delay in the rising phase of a subsequent light response; this effect can be interpreted as closure of the channel with a divalent ion trapped inside.

Adaptation, Ocular↗
PubMedDate not supplied

[Possibilities of computer-assisted processing of the results of an examination of the visual system].

The authors present the first results of examining the vision system with the use of an automated programmed complex realized in the 'C' program language in the IBM and Dos operation system, that helped present in a chart the distribution of the ocular light sensitivity and time of the sensomotor reaction in every site of the visual field of 40-50 degrees. The results evidence that computer-aided methods may be effectively used to assess the light sensitivity of the eye, contrast sensitivity, space summation, rod and cone interactions, the topography of color photoreceptors, and the function of the posterior section of the eye in abnormalities of the retina and the optic nerve, as well as to monitor visual function rehabilitation after treatment.

Adaptation, Ocular↗
PubMed1981

Microtubules and 10 nm-filaments in the retinal pigment epithelium during the diurnal light-dark-cycle.

Microtubules and 10 nm-filaments appear to be involved in the functions of the retinal pigment epithelium (RPE). The presence of microtubules in the RPE of light-adapted eyes, but not in dark-adapted eyes, suggests that they may be involved primarily in organelle movement. On the other hand, the random and constant presence of 10 nm-filaments within the basal portion of the PE implies a cytoskeletal role for these filaments.

Adaptation, Ocular↗
PubMed2002

Nitric oxide controls the light adaptive chromatic difference in receptive field size of H1 horizontal cell network in carp retina.

In carp retina, the receptive field size of the H1-type horizontal cell (HC) network is known to be chromatically selective, as electrophysiological signals are generated by short-wavelength (SW) light stimuli, which spread much less than those for long-wavelength (LW). We have shown previously that the signalling mechanism underlying this chromatic difference operates only in the light-adapted retina and that it involves cGMP as an intermediary messenger. In the present study, the possible role of nitric oxide (NO) as such a control mechanism was investigated. Application of a NO donor (SNP or SNOG) to dark-adapted retinae produced a chromatic difference in the receptive field size, such as in the light-adapted state. This effect was due mainly to a reduction in the spread of signals generated by SW stimuli; LW signalling spread was not altered. No such effect was observed in light-adapted retinae where a chromatic difference in receptive field size was already present. On the other hand, application of the NO 'scavenger' haemoglobin to light-adapted retinae suppressed the chromatic difference. These results are consistent with NO being a light-adaptive retinal neuromodulator involved in the generation of the chromatic difference in H1 cell receptive field size. These results are discussed in the context of two different hypotheses.

Action Potentials↗
PubMed1996

Left and right visual field advantages are a function of scotopic and photopic retinal adaptation, respectively, in simple reaction time to near-threshold targets.

Modulation of stimulus luminance in a tachistoscopic face discrimination task has been found to significantly invert visual hemifield advantage in reaction time (RT) (Sergent, 1982a, Sergent, 1982b). However, there is no more physiological rationale for that than for a similar effect, say, of retinal adaptation, and it is even conceivable that the latter may have confounded the former in past experiments. The experiments reported here were therefore designed to tease out the relative contributions of stimulus luminance and of background illumination (i.e., retinal adaptation) in a simple RT task. Two equally difficult conditions of dim targets were set up, one with light-adapted subjects and one with dark-adapted subjects. Similarly, two equally difficult conditions of bright targets were set up with light and dark-adapted subjects. It was found that dim targets (near detection threshold) yielded a significant right visual field RT advantage in light-adapted subjects and that dim targets (equally near detection threshold) yielded a significant left visual field RT advantage in dark-adapted subjects. Future experiments will determine whether cone-mediated RT to detection is left hemisphere dominant and whether rod-mediated RT to detection is right hemisphere dominant.

Adaptation, Ocular↗
PubMed1996

Pyroglutamyl peptidase I levels and their left-right distribution in the rat retina and hypothalamus are influenced by light-dark conditions.

To evaluate the effects of light and darkness on pyroglutamyl peptidase I activity (pGluPI) and its left-right distribution, pGluPI was measured bilaterally in the retina and hypothalamus under selected light-dark schedules. Rats under a 12 h light-dark cycle were divided into four experimental groups. After the end of the 12 h dark period, the animals were kept two additional hours in darkness (group 1), or light (group 2). After the end of the 12 h light period, the animals were kept two additional hours in darkness (group 3), or light (group 4). Experiments were done in light or darkness depending on the 2 h period. In the retina, a previous 12 h light period led to higher values of enzyme activity than dark periods. Left-right predominance, however, depended on the previous 2 h period: the light period led to left predominance, whereas right predominance was found after the 2 h dark period. In the hypothalamus, a left predominance was found only in group 3. These results demonstrate that environmental light conditions influence pGluPI activity in the rat retina and hypothalamus.

Adaptation, Ocular↗
PubMed1998

Interocular effect of actin depolymerization on spinule formation in teleost retina.

Teleost retinas adapted to light show numerous spinules invaginated in the cone pedicles whereas darkness induces a reduction in the number of spinules. Horizontal cells show nematosomes whose size decreases as the number of spinules increases. We have investigated the involvement of actin filaments in spinule formation, by using cytochalasin D through intraocular injection into an eye. The ultrastructural analysis reveals that cytochalasin D impairs spinule formation and nematosome-size reduction in both treated and contralateral untreated retinas.

Actins↗