K. Asami

The Photoreceptor Molecules in Xenopus Tadpole Tail Fin, in which Melanophores Exist

Abstract Melanophores of the isolated tail fin of the Xenopus tadpole respond to light, resulting in melanin aggregation in the melanophores. Western blot analysis showed that a protein in the Xenopus tail fins, in which photosensitive melanophores exist, had reacted with the antibody against bovine rhodopsin. RT-PCR and nested-PCR using rhodopsin-specific primers showed the expression of rhodopsin mRNA in the tail fins. The amino acid sequences deduced from the PCR products were completely identical to those of rhodopsin. We also detected the mRNA of melanopsin in the tail fin, another opsin originally described in cultured melanophores of Xenopus. These results indicate that these two types of opsin molecules exist in Xenopus tail fin and may take part in the photo-response in melanophores of the Xenopus tadpole.

Preference for background color of the Xenopus laevis tadpole

The background color preferences of three age groups of Xenopus laevis tadpoles (young-stages tadpoles [stages 44-46], premetamorphic tadpoles [stages 54-56], and metamorphic tadpoles [stages 58-60]) were examined. Young tadpoles selected a white background, metamorphic tadpoles preferred a black background, and premetamorphic tadpoles selected a white or black background consistent with the background black or white of the test box on which they previously were conditioned. If premetamorphic tadpoles were conditioned in the checkerboard-pattern (white and black) box or white box which was placed a small black square at the center of the box, they preferred to stay in the white area just after transfer to the test box, then shifted their preference for background color to black strongly. Premetamorphic tadpoles conditioned on a white background lost this preference if kept in the dark for 12 hours. Blinding of premetamorphic tadpoles by severance of the optic nerves resulted in loss of preference for a specific background matching the white or black background to which they had been adapted while sighted. Given a choice 3 days postblinding, they tended to congregate on a white background. Injection of MSH into premetamorphic tadpoles conditioned to white shifted their preference to black. In contrast, injection of melatonin stimulated black adapted tadpoles to select a white background. Young frogs showed a preference for a black background.

Does the low respiratory rate in unfertilized eggs result mainly from depression of the redox reaction catalyzed by flavoproteins? Analysis of the respiratory system by light-induced release of CO-mediated inhibition

In unfertilized eggs of the sea urchin, the quite low respiratory rate is enhanced by tetramethyl‐p‐phenylenediamine (TMPD), phenazine methosulfate (PMS) and sperm and this augmentation is completely inhibited by carbon monoxide (CO). Exposure to light releases eggs from this CO‐mediated inhibition. The action spectra for photoreactivation of CO‐inhibited cytochrome c oxidase in isolated mitochondria and CO‐blocked respiration in TMPD‐treated eggs were found to be similar to the absorption spectrum of CO‐bound cytochrome aa3. In PMS‐treated eggs and fertilized eggs, the maximum photoreactivation of CO‐inhibited respiration occurred at a light fluence rate higher than that for maximum photoreactivation of CO‐inhibited respiration in TMPD‐treated eggs, with peaks at the same wavelengths as those in the absorption spectrum of reduced cytochrome b. A similar phenomenon was seen for NADH cytochrome c reductase in mitochondria. Thus, cytochrome c oxidase and NADH cytochrome c reductase, whose activities are not altered by fertilization, seem to be functional, even in unfertilized eggs. In unfertilized eggs, difference spectra indicated that PMS and sperm augmented cytochrome b reduction and that TMPD accelerated cytochrome c reduction without cytochrome b reduction. Therefore, it is likely that depression of electron transport to cytochrome b, which is augmented by PMS and sperm, is responsible for the low respiratory rate in unfertilized eggs.

LIGHT-SENSITIVE RESPONSE IN MELANOPHORES OF XENOPUS LAEVIS: II. RHO IS INVOLVED IN LIGHT-INDUCED MELANIN AGGREGATION

Melanophores of the isolated tail fin of the Xenopus tadpole aggregate melanin granules in response to light. This aggregation was found to be inhibited by subcutaneous injection of exoenzyme C3 of Clostridium botulinum. A 26 kDa protein in homogenate obtained from the Xenopus tail fin was ADP-ribosylated by exoenzyme C3. This reaction was inhibited effectively by a monoclonal antibody, anti-Rho mab A5. raised against the small GTP-binding protein Rho. The extent of ADP-ribosylation depended on light and guanine nucleotide. Incubation under illumination partly reduced ADP-ribosylation and the reduction was restored by addition of guanine nucleotide during incubation. These findings suggest that Rho is involved in the photo-sensitive melanophore response as a signal transducer linking photo-stimuli to melanin granule translocation with Xenopus melanophores.