A study of the visual pigments of the clawed toad

Use of a translucent refuge for Xenopus tropicalis with the aim of improving welfare

Xenopus tropicalis is an increasingly important animal model in a variety of biological research fields. In many countries legislation exists to promote and increase welfare wherever possible, including the ability to view animals during daily husbandry with minimal stress to the animal. X. tropicalis ( n = 16) refuge use was investigated; it was found that the animals significantly preferred black opaque overhead cover to open-ended pipes or closed-off ceramic plants pots in refuge choice experiments. This experiment was repeated by replacing the opaque black overhead cover with red filters. A significant preference for overhead cover was seen for the red translucent cover compared with other available refuges, suggesting that X. tropicalis may adopt translucent refuges due to their visual inabilities. The inability of frogs to see certain wavelengths of light may allow staff to view them whilst simultaneously providing the refuge of choice.

Circadian Modulation of Temporal Properties of the Rod Pathway in LarvalXenopus

Circadian clocks are integral components of visual systems. They help adjust an animal's vision to diurnal changes in ambient illumination. To understand how circadian clocks may adapt visual sensitivity, we investigated the spatial and temporal properties of optomotor responses of young Xenopus laevis tadpoles (Nieuwkoop and Faber, developmental stage 48) using a modified 2-alternative preferential-viewing method. We maintained animals in constant darkness and measured temporal sensitivity during their subjective day and night. We found that their behavioral responses can be explained in terms of 2 mechanisms with different temporal properties. The more sensitive mechanism operates at low temporal frequencies and intermediate wavelengths (λmax= 520 nm), properties consistent with rod signals. Threshold for this mechanism is approximately 0.04 photoisomerizations rod−1s−1, consistent with single-photon detection. A less-sensitive mechanism responds to higher temporal frequencies (cutoff = 12 Hz) and has broad spectral sensitivity (370–720 nm), consistent with multiple classes of cone signals. This cone mechanism does not change, but the cutoff frequency of the more sensitive rod mechanism shifts from 0.35 Hz at night to 1.1 Hz during the subjective day, thereby enhancing the animal's sensitivity to dim rapidly changing stimuli. This day–night shift in rod temporal cutoff frequency cycles in complete darkness, characteristic of an endogenous circadian rhythm. The temporal properties of the behaviorally measured rod mechanism correspond closely with those of the electrophysiologically measured retinal response, indicating that the rod signals are modulated at the level of the outer retina.

Photoreceptor classes and transmission at the photoreceptor synapse in the retina of the clawed frog, Xenopus laevis

The photoreceptor population in Xenopus consists of a green-sensitive rod (lambda(max) = 523 nm), a blue-sensitive rod (lambda(max) = 445 nm) and three classes of cone. The largest cone is red-sensitive (lambda(max) = 611 nm). The intermediate cone is presumed to be blue-sensitive based on physiological criteria, whereas the miniature cone may be UV-sensitive. Horizontal cells (HC) are of two sorts: axon-bearing and axonless. The axon-bearing HC is of the luminosity type and probably contacts all types of photoreceptor. The axonless HC is of the chromaticity type and contacts only intermediate (blue) cones and at least one type of rod. During development dendrites of HCs and bipolar neurons penetrate photoreceptor bases. A progressive maturation of HC and bipolar synapses with rods and cones occurs between tadpoles stages 37/8 and 46. Neighboring rods and cones are joined by gap junctions. During this same period, the outer segments are laid down and photopigments synthesized. A linear relation was found between the quantum capturing ability of the rod and its absolute threshold. Mature rods of the Xenopus retina release glutamate in a calcium-dependent manner. Glutamate release was found to be a linear function of calcium influx through L-type calcium channels. Both types of HC possess ionotropic glutamate receptors of the AMPA subtype.

Formation of visual pigment chromophores during the development of Xenopus laevis

Retinoids in the eyes of Xenopus laevis at several developmental stages, were analyzed by high-performance liquid chromatography (HPLC). At stage 37/38, larval eyes contained mainly all-trans isomers of retinal, 3-dehydroretinal, retinyl ester and 3-dehydroretinyl ester. Ratios of all-trans 3-dehydroretinal to retinal and of all-trans 3-dehydroretinyl ester to retinyl ester were almost 1 at the stage. With the advance of development, the amounts of all-trans retinal and 3-dehydroretinal decreased; however, those of all-trans retinyl ester and 3-dehydroretinyl ester increased. The chromophores of visual pigments, 11-cis retinal and 3-dehydroretinal, were detected at stage 40 (total; 0.2 pmol/eye) and their amounts increased after that stage. The ratio of 11-cis 3-dehydroretinal to retinal was almost 1 at stages 40-42. The ratio became larger after stage 43 and was almost 19 at stage 46. The ratio of all-trans 3-dehydroretinyl ester to retinyl ester, also, increased after stage 42 and reached 11 at stage 46. The mechanism of 11-cis formation during development is discussed in relation to retinoid conversions in the eyes.

Two populations of rod photoreceptors in the retina of Xenopus laevis identified with 3H-fucose autoradiography

The retina of Xenopus laevis contains two populations of rod photoreceptors that differ in their utilization of L-fucose. Following intraperitoneal injections with 3H-fucose, all rod and cone photoreceptors incorporate the label. One day after labeling, much of the radioactivity is associated with the photoreceptor outer segments. In rods, a band of radioactivity is initially located at the base of the outer segment. As the time interval between injection and recovery is extended, the band of radioactivity is progressively displaced toward the outer segment tip. When the autoradiography exposure times are reduced so that cone and most rod outer segments no longer appear labeled, a minor population of rod photoreceptors can be distinguished which remains heavily labeled. The outer segment of the principal rod is 29.8 +/- 0.6 micrometers long and 6.4 +/- 0.6 micrometers in diameter, whereas the outer segment of the minor rod is 19.7 +/- 3.4 micrometers long and 4.5 +/- 0.6 micrometers in diameter, the latter accounting for approximately 2-3% of the total rod photoreceptor population. The rate of 3H-band displacement is 2-fold greater in the minor rod outer segment than the renewal rate of the outer segment in the principal rod. The similarity in relative cell density and size of the minor rod suggests that this photoreceptor corresponds to the blue-sensitive rod of Xenopus recently described by Witkovsky et al. (1981a, b), Vision Res. 21, 863-867, 875-883).

Intracellular recording from identified photoreceptors and horizontal cells of the Xenopus retina

Intracellular recordings were made from rods, cones and horizontal cells of the Xenopus retina. The cells under study were identified by injection of the fluorescent dye, Lucifer yellow. Rod spectral sensitivity peaked near 524 nm, that of cones near 612 nm whereas horizontal cells reflected input from both these classes of photoreceptors. No intracellular recordings were made from blue-sensitive rods (lambda max = 445 nm) nor did this rod appear to provide an input to the horizontal cell. Under dark-adapted conditions, horizontal cells had a slow waveform, a Vmax less than or equal to 18 mV and were driven by 524 nm rods only. When light-adapted, horizontal cell responses were fast, Vmax was 30-40 mV and the responses reflected only 612 nm cone input. In the mesopic state rod and cone inputs to the horizontal cell interacted non-linearly: weak green backgrounds greatly enhanced the response to a superimposed red flash compared to the red flash response on a dark field. The length constant of the horizontal cell exceeded its dendritic arbor by 2-15 fold. All of the stained horizontal cells, however, possessed a long slender axon without a terminal but which emitted periodic short branches that appeared to contact receptors.

The spectral absorption of the photopigment of Xenopus laevis measured in single rods

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