The messenger RNA (mRNA) that encodes .alpha. subunit of the guanosine triphosphate-binding protein transducin (T.alpha.) and T.alpha. immunoreactivity were localized and measured in the rat retina during the light-dark cycle with in situ hybridization and immunohistochemistry. Both T.alpha. mRNA and T.alpha. immunoreactivity were observed only in photoreceptors. Within the photoreceptor T.alpha. mRNA was present primarily in the inner segments and to a lesser extent in the outer nuclear layer at all times during the day and night. However, the distribution of T.alpha. immunoreactivity varied profoundly with the light-dark cycle; during the day, T.alpha. immunoreactivity was highest in the inner segments, and at night the outer segments were more immunoreactive. The amounts of T.alpha. mRNA and T.alpha. immunoreactivity also depended on the light-dark cycle. Levels of T.alpha. mRNA were high immediately before and after lights on; levels were low for the rest of the light-dark cycle. During the day, T.alpha. immunoreactivity increased in the inner segments following the increase in T.alpha. mRNA. After the lights were turned off, T.alpha. immunoreactivity decreased in the inner segments and increased in the outer segments. Thus, it appears that T.alpha. is synthesized in the inner segments after a morning increase in T.alpha. mRNA. Newly synthesized T.alpha. remains in the inner segments until it is transported to the outer segments at night, where it may be involved in the increase in the sensitivity of photoreceptor rods at night.
