In Xenopus laevis Daud., the ontogenetic occurrence of two photoreceptor-specific proteins, S-antigen and rod-opsin, was investigated and correlated to the maturation of the neurohormonal effector system involved in melatonin-dependent color-change mechanisms. Tadpoles ranging from stage 12 to 57 (Nieuwkoop and Faber 1956) were fixed in Zamboni's or Bouin's solution. Frozen or paraffin sections of either total heads or dissected brains and eyes were prepared and treated with highly specific antisera against S-antigen and rod-opsin. In the retina, immunoreactive S-antigen and rod-opsin were first demonstrated in a few centrally located photoreceptors at stage 37/38. Photoreceptors of the peripheral (iridical) portions of the retina gradually became immunoreactive during further development. As in the retina, the first S-antigen-immunoreactive photoreceptors in the pineal complex appeared at stage 37/38. At this and all later stages investigated rod-opsin immunoreactivity was restricted to a few dot-like structures resembling developing pineal outer and inner segments. In most animals rod-opsin immunoreactivity was completely absent from the pineal complex. The analysis of retinal proteins with the immunoblotting technique (Western blot) revealed that the S-antigen antibody bound to a 48-kDa protein and the rod-opsin antibody to a 38-kDa protein. The body lightening reaction was determined with the aid of the melanophore index in larvae fixed in light or darkness, respectively. Aggregation of melanophore melanosomes in darkness (the melatonin-dependent primary chromatic response) first occurred at stage 37/38 when melanophores started to differentiate and became pigmented. These results indicate that in Xenopus laevis (i) the molecular mechanisms of photoreception develop simultaneously in retina and pineal complex; (ii) most pineal photoreceptors differ from retinal rods in that they contain immunoreactive S-antigen but essentially no immunoreactive rod-opsin; and (iii) the differentiation of phototransduction processes coincides with the onset of melatonin-dependent photoneuroendocrine regulation of color-change mechanisms.
