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 photorecept or systems that render the cells capable of avoiding light, On account of t his unique feature, these ciliates exhibit photodispersal as they tend to s wim 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 li ke phototaxis or photokinesis may also contribute to the photodispersal. Th e photophobic response caused by a sudden increase in light intensity (ligh t stimulus) starts with a delayed cessation of ciliary beating that results in the disappearance of the cells forward swimming, then a period of cilia ry reversal (backward movement) followed finally by renewed forward movemen t, often in a new direction. Reversal of ciliary beating during the photoph obic response correlates with the generation of an action potential. The ac tion potential is elicited by a photoreceptor potential, a transient membra ne depolarization produced by the light stimulus. The photoreceptor potenti als in both ciliates are initiated by light absorption in a cellular photor eceptor system based on hypericin-like chromophores - blepharismin in Bleph arisma and stentorin in Stentor. Recent evidence indicates that biochemical processes, which couple the photochemical cycle within the cell pigment wi th photoreceptor potential, may be different in these organisms. In the cas e of Stentor, cyclic GMP is the probable candidate for an internal second m essenger in photosignal transduction. In related Blepharisma cells, however , InsP(3) seems to be responsible for the alterations in membrane potential s and induction of light avoiding response. The data show that lower eukary otic cells may use similar signal transduction pathways as observed in mult icellular organisms. Therefore, on the basis of light dependent events obse rved in Blepharisma and Stenter; it seems appropriate to use protozoan cell s as a model system for multidisciplinary studies of sensory signal transdu ction within single cells.