Retinal photoreceptors and visual pigments in Boa constrictor imperator.

The photoreceptors of Boa constrictor, a boid snake of the subfamily Boinae, were examined with scanning electron microscopy and microspectrophotometry. The retina of B. constrictor is duplex but highly dominated by rods, cones comprising 11% of the photoreceptor population. The rather tightly packed rods have relatively long outer segments with proximal ends that are somewhat tapered. There are two morphologically distinct, single cones. The most common cone by far has a large inner segment and a relatively stout outer segment. The second cone, seen only infrequently, has a substantially smaller inner segment and a finer outer segment. The visual pigments of B. constrictor are virtually identical to those of the pythonine boid, Python regius. Three different visual pigments are present, all based on vitamin A(1.) The visual pigment of the rods has a wavelength of peak absorbance (lambda(max)) at 495 +/- 2 nm. The visual pigment of the more common, large cone has a lambda(max) at 549 +/- 1 nm. The small, rare cone contains a visual pigment with lambda(max) at 357 +/- 2 nm, providing the snake with sensitivity in the ultraviolet. We suggest that B. constrictor might employ UV sensitivity to locate conspecifics and/or to improve hunting efficiency. The data indicate that wavelength discrimination above 430 nm would not be possible without some input from the rods.

Low levels of cadmium chloride alter the immunoprecipitation of corneal cadherin-complex proteins.

The effect of cadmium chloride on the immunoprecipitation of cadherin and the associated adherens junctional proteins, alpha- and beta-catenin, was examined in isolated bullfrog (Rana catesbeiana) corneas utilizing Western blot and enhanced chemoluminescent techniques. Application of either 1.0 microM or 75.0 microM CdCl2 to the corneal endothelium for 2 h markedly decreased the immunoprecipitation of cadherins as compared to paired control corneas. Immunoprecipitation of alpha-catenin was increased in response to both doses of CdCl2, while the immunoprecipitation of beta-catenin was little changed by either cadmium dose. There is accumulating evidence that cadmium may increase epithelial paracellular permeability by interfering with cadherin complex activity at intercellular junctions. The present study suggests that inorganic cadmium in low micromolar concentrations may decrease the integrity of the corneal endothelium, at least in part through a similar mechanism involving disruption of junctional cadherin complex function.

An action spectrum for the light-dependent inhibition of swimming behavior in newly hatched white sturgeon, Acipenser transmontanus.

Newly hatched white sturgeon (Acipenser transmontanus) yolk-sac larvae are negatively phototactic, photokinetic and geotactic--seeking cover in the substrate during daylight hours. If cover is unavailable and the light intensity is low, they move into the water column and alternate periods of rapid, upward swimming with periods of quiescence during which they sink. In the dark they swim almost continuously. A flash of light inhibits swimming activity totally for a period dependent on the light intensity. Using this observation, we obtained an action spectrum for swimming inhibition for 30 newly hatched larvae. This action spectrum is best fit by a single visual pigment with absorbance maximum at 537 nm. The lack of red sensitivity fits well with previous work showing that red-sensitive photoreceptor cells are absent in young sturgeon. The possible significance of red insensitivity is discussed from both ecological and practical standpoints.

The photoreceptors and visual pigments of the garter snake (Thamnophis sirtalis): a microspectrophotometric, scanning electron microscopic and immunocytochemical study.

Scanning electron microscopy, immunocytochemistry, and single cell microspectrophotometry were employed to characterize the photoreceptors and visual pigments in the retina of the garter snake, Thamnophis sirtalis. The photoreceptor population was found to be comprised entirely of cones, of which four distinct types were identified. About 45.5% of the photoreceptors are double cones consisting of a large principal member joined near the outer segment with a much smaller accessory member. About 40% of the photoreceptors are large single cones, and about 14.5% are small single cones forming two subtypes. The outer segments of the large single cones and both the principal and accessory members of the doubles contain the same visual pigment, one with peak absorbance near 554 nm. The small single cones contain either a visual pigment with peak absorbance near 482 nm or one with peak absorbance near 360 nm. Two classes of small single cones could be distinguished also by immunocytochemistry and scanning electron microscopy. The small single cones with the 360-nm pigment provide the garter snake with selective sensitivity to light in the near ultraviolet region of the spectrum. This ultraviolet sensitivity might be important in localization of pheromone trails.

Low levels of cadmium chloride damage the corneal endothelium.

The effect of cadmium chloride on the integrity of the endothelium of isolated bullfrog (Rana catesbeiana) corneas was examined by spectrophotometric analysis of corneal uptake of the vital stain Janus green and by scanning electron microscopy (SEM). The uptake of Janus green by the endothelium was dose related between 1.0 and 100.0 microM CdCl2. The effect of cadmium was significantly attenuated by the calcium channel blocker SKF 96365 and was augmented by the calcium ionophore A23187, indicating that cadmium influx through calcium channels is an important determinant of its cellular effect. The effect of cadmium was not altered by changes in the external calcium concentration, indicating that the mechanism does not involve competitive inhibition by calcium. SEM demonstrated significant structural damage to the corneal endothelium exposed to cadmium chloride, including focal disruption and denuding of the apical endothelial membrane.

Low levels of inorganic mercury damage the corneal endothelium.

The effect of inorganic mercury on the integrity of the endothelium of isolated bullfrog (Rana catesbeiana) corneas was examined by spectrophotometric analysis of corneal uptake of the vital stain Janus green, and by both transmission (TEM) and scanning (SEM) electron microscopy. The uptake of Janus green by the endothelium is dose related between 1.0 and 30.0 microM HgCl2. The effect of mercury is not altered by changes in external calcium concentration, nor is it influenced by the calcium ionophore A23187, indicating that inorganic mercury damages the corneal endothelium through a mechanism which does not involve competition with external calcium or interaction with calcium channels. TEM and SEM demonstrate significant ultrastructural damage to the endothelium exposed to inorganic mercury, including cellular swelling, increased vacuolization, focal denuding of Descemet's membrane, and diminished integrity at the intercellular junctions.

Temperature dependency of light adaptation in bullfrog cone photoreceptors.

Isolated, superfused bullfrog retinas were stimulated in such a manner as to allow examination of the dynamics of the shift in the stimulus-response curve that accompanies light adaptation of cone photoreceptors. Observed phenomena were highly temperature dependent with complex Arrhenius relationships, suggesting that the mechanisms responsible for the shift involve multiple enzyme systems.

The effects of barium on the suppression-recovery phenomenon in the aspartate isolated mass receptor response.

The relationship between the suppression-recovery phenomenon and sensitivity changes during light adaptation was investigated in the cone photoreceptors of the isolated, superfused bullfrog retina using barium as a tool. Barium reduced the shift of the stimulus-response function that occurs during light adaptation. Barium also reduced the maximum response amplitude seen after recovery from suppression induced by a flickering light stimulus. The effect of barium on the recovered response amplitude could not be explained by barium's known ability to attenuate the dark adapted cone response, since the magnitude of the depression of the recovered response amplitude was significantly greater at all concentrations than was the magnitude of the depression of the response of the dark adapted cones. Moreover, the thresholds for barium's effect, as well as the shapes of the dose-response curves, were quite different for the two types of responses. The results are consistent with the idea that the recovery from suppression induced by flicker is due to the same change in receptor sensitivity that manifests itself as a shift of the stimulus-response curve during light adaptation.

Sensitivity and response kinetics alter during suppression-recovery in cone photoreceptors.

Threshold and response amplitude of cone photoreceptors were measured during progression from flicker-induced suppression to recovery. Increases in sensitivity closely paralleled increases in amplitude. Recovered responses exhibited faster kinetics than suppressed responses. The idea that recovery from suppression is a manifestation of the shift in the stimulus-response relationship which occurs with light adaptation is supported.

The effect of lead on photoreceptor response amplitude--influence of removing external calcium and bleaching rhodopsin.

The relationship between the effect of lead on the amplitude of the photoreceptor response to light and the ability of the photoreceptor response to be enhanced by a light stimulus was studied in the excised, superfused, bullfrog retina. Photoreceptor responses were isolated by treatment of the retina with sodium aspartate and were recorded with extracellular electrodes. Rod and cone responses were monitored separately by employing a stimulus regimen which took advantage of differing rates of rapid dark adaptation. When the ability of the rod response to undergo enhancement by light was severely reduced by bleaching 80% of the rhodopsin in the retina, the usual depressive effect of 12.5 microM lead was not observed. When light enhancement of the rod response was completely eliminated by removing calcium from the superfusing solution, the effect of 12.5 microM lead was actually to increase rather than decrease rod response amplitude. We conclude from this and previous studies that the effect of lead on the rod photoreceptor is complex, that it can involve both inhibitory and excitatory components, and that the inhibitory effect is exerted on the same mechanism which is responsible for the ability of the rod response to be enhanced by exposure of the retina to light. The cone response never exhibited enhancement by light and was never depressed by exposure of the retina to lead, but the ability of lead to increase cone response amplitude appeared to be greater in the absence of calcium.

Barium removes the ouabain-induced increase in the rod response to light.

The mass receptor potential of the excised, superfused retina of the bullfrog was isolated with aspartate. Rods were selectively stimulated by using very dim flashes of light. In the presence of 0.1 mM ouabain, the amplitude of the receptor response was found first to increase transiently and, subsequently, to decrease progressively. The ouabain-induced transient increase in receptor response was completely eliminated by 0.4 mM barium chloride. However, barium did not affect the rate at which the response decayed in the presence of ouabain. The ability of barium to remove the ouabain-induced transient increase in the amplitude of the receptor response is discussed in terms of reducing the coupling ratio of the postulated electrogenic sodium-potassium pump of rods.

The effect of lead on photoreceptor response amplitude--influence of the light stimulus.

The mass receptor potential of the excised, superfused retina of the bullfrog was studied. Photoreceptor responses were isolated by addition of sodium aspartate to the Ringer solutions. Responses of the cones were monitored independently from responses of the rods by employing a two-flash method of stimulation which took advantage of the very different rates of rapid dark adaptation of rods and of cones. Stimulation with paired flashes of white light at regular intervals caused enhancement of rod response amplitude in that the response grew larger with subsequent flashes until reaching a stable plateau. The degree of enhancement was directly proportional to the amount of light exposure and increased with either increasing stimulus intensity or decreasing stimulus interval. Only the rod response was affected; the cone response was not enhanced by continued stimulation. The effects of 12.5 microM PbCl2 on rod response amplitude were complex. There was a small (less than 10%) but consistent depression of rod response amplitude even when the rods were in the unenhanced state. However, the most striking effect of lead was on the enhanced response, where treatment with 12.5 microM PbCl2 led to a depression of about 33%. When added prior to light stimulation, lead significantly decreased the degree to which the rod response could be enhanced, but never prevented enhancement entirely. Removal of lead resulted in a very large increase in the degree to which the rod response was enhanced by light, even when compared to the first, lead-free control. The cone response was unaffected by 12.5 microM PbCl2.

Dichotomous effects of cadmium on the cone photoreceptor response to light.

The effects of cadmium (1.0 to 400.0 microM CdCl2) on the cone photoreceptor's response to light were studied in the superfused bullfrog retina. The effects of cadmium were complex, characterized by a purely excitatory component at the lower levels and an inhibitory component that became evident with 75.0 microM CdCl2. The maximum increase in cone response amplitude due to the excitatory component was about 60% at 100.0 microM CdCl2. At 400.0 microM CdCl2, the highest concentration examined in this study, cone response amplitude was suppressed by about 35%. Both components were dose-dependent. The excitatory component was reversible only at CdCl2 concentrations below 50.0 microM, and irreversible at levels at and above 50.0 microM. The inhibitory component was always reversible. The slope of the dose-response curve describing the excitatory component (0.60) was about twice that describing the inhibitory component (0.31). The effects of cadmium on cone response amplitude are discussed in terms of the most probable sensitive sites.

The effects of lead and of cadmium on the mass photoreceptor potential: the dose-response relationship.

The effects of micromolar (1.0-60.0 microM) amounts of lead and of cadmium on the light response of photoreceptors were studied using the isolated, perfused bullfrog retina. The effect of lead in depressing rod activity is readily and fully reversible. Lead is very effective in depressing the rod response at concentrations as low as 1.0 microM, but the effect of lead saturates at about 25.0 microM with about 34% depression of the rod response. At the higher concentrations some spontaneous recovery of rod response amplitude is observed shortly after exposure of the retina to lead begins. The cone response is affected by lead only rarely and then only at the higher concentrations. When affected, the cone response is enhanced rather than depressed. The effects of cadmium are generally similar to those of lead. However, saturating concentrations of cadmium depress the rod response to a greater degree than lead and the cadmium effects are not as readily reversible as the effects of lead at the higher concentrations. Cones are much more sensitive to cadmium than they are to lead and 60.0 microM cadmium always enhances cone response amplitude. The results of experiments in which lead and cadmium were both added to the retinal perfusate indicate to us that cadmium and lead both affect the same sensitive site or sites responsible for generating the rod response, but that cadmium affects an additional site that is not sensitive to lead. The depressive effects of lead and of cadmium on the rod response are discussed in terms of the postulated mechanisms of transduction in the vertebrate photoreceptor.

Rapid dark adaptation of bullfrog rods is delayed by barium.

With use of sodium aspartate, the late receptor potential of the excised, perfused bullfrog retina was isolated. By means of a two-flash technique, rapid dark adaptation of rods was monitored. As in cones, barium ions were found to delay the onset of rapid dark adaptation of rods, but the rate of recovery, once begun, was virtually unaffected. The effect of barium on the amplitude-intensity relationship of rods was also determined. Unlike its ability to dramatically increase the amplitude of the receptor potential of rods, barium had no effect on the absolute threshold of rods. We propose that barium ions act on the enzyme system postulated to govern the onset of rapid dark adaptation of rods and suggest that a reduction in the activity of an ATP protein kinase might be the basis for this effect.

The influence of calcium on the aspartate-isolated mass receptor potential of bullfrog cones.

The aspartate-isolated receptor potential was studied in the excised, perfused bullfrog retina. Cones were monitored without interference from rods by employing conditioning and test stimuli in a manner previously described (23,24). Lowering extracellular calcium by switching from a perfusate containing 0.4mM CaCl2 to one having no added calcium resulted in an increase in cone response amplitude. Conversely, elevating extracellular calcium by perfusing with a Ringer containing 0.8mM CaCl2 resulted in a decrease in cone response amplitude. These changes were sustained and fully reversible. In contrast, perfusing the retina with a Ringer solution containing EGTA resulted in a transient increase in cone response amplitude. Decreasing external calcium by simple depletion also shortened the delay prior to onset of rapid dark adaptation of the cones, thereby hastening the entire process of recovery. Increasing external calcium had little effect on rapid dark adaptation. Decreasing external calcium with EGTA led to extremely rapid response recovery, but the effect was not reversible. In no case did EGTA lead to a complete suppression of the response. The results of this study are interpreted as being inconsistent with the view that calcium is the internal transmitter responsible for the generation of the receptor potential in cones. They are consistent with the view that calcium functions to modulate recovery of the cones' ability to generate a response following a stimulus, perhaps by affecting the activity of a cyclic nucleotide.

Heavy metals affect rod, but not cone, photoreceptors.

Low concentrations of lead, mercury, or cadmium depress the amplitude of the rod receptor potential in the perfused bullfrog retina. Responses from the cones were not affected. The data implicate the rods as a lesion site in animals exhibiting scotopic vision deficits as a result of heavy metal poisoning.

Barium delays the onset of rapid dark adaptation in bullfrog cones.

The late receptor potential of the excised, perfused bullfrog retina was isolated with sodium aspartate. By employing a three-flash technique, cone responses were monitored without interference from rods. In cones barium ions were found to delay the onset of rapid dark adaptation, but the rate of recovery, once begun, was unaffected. We propose that barium ions act directly upon the enzyme system postulated to govern the onset of rapid dark adaptation of cones. In addition, barium was found to affect the amplitude of the rod receptor potential differently from that of cones, increasing the former but decreasing the latter. The effect of barium upon photoreceptor potential amplitude is discussed in terms of a reduction in the potassium conductance of the photoreceptors and the mechanisms postulated for photoreceptor excitation and rapid dark adaptation.