Protozoa as model system for studies of sensory light transduction:: Photophobic response in the ciliate Stentor and Blepharisma

Stentor coeruleus and the related Blepharisma japonicum possess photoreceptor systems that render the cells capable of avoiding light, On account of this unique feature, these ciliates exhibit photodispersal as they tend to swim away from a bright illumination and accumulate in shady or dark areas. The observed photobehaviour is largely the result of a step-up photophobic response displayed by both ciliates, although other behavioral reactions like phototaxis or photokinesis may also contribute to the photodispersal. The photophobic response caused by a sudden increase in light intensity (light stimulus) starts with a delayed cessation of ciliary beating that results in the disappearance of the cells forward swimming, then a period of ciliary reversal (backward movement) followed finally by renewed forward movement, often in a new direction. Reversal of ciliary beating during the photophobic response correlates with the generation of an action potential. The action potential is elicited by a photoreceptor potential, a transient membrane depolarization produced by the light stimulus. The photoreceptor potentials in both ciliates are initiated by light absorption in a cellular photoreceptor system based on hypericin-like chromophores - blepharismin in Blepharisma and stentorin in Stentor. Recent evidence indicates that biochemical processes, which couple the photochemical cycle within the cell pigment with photoreceptor potential, may be different in these organisms. In the case of Stentor, cyclic GMP is the probable candidate for an internal second messenger in photosignal transduction. In related Blepharisma cells, however, InsP(3) seems to be responsible for the alterations in membrane potentials and induction of light avoiding response. The data show that lower eukaryotic cells may use similar signal transduction pathways as observed in multicellular organisms. Therefore, on the basis of light dependent events observed in Blepharisma and Stenter; it seems appropriate to use protozoan cells as a model system for multidisciplinary studies of sensory signal transduction within single cells.