CHEMOSENSORY ADAPTATION IN PARAMECIUM INVOLVES CHANGES IN BOTH REPELLENT BINDING AND THE CONSEQUENT RECEPTOR POTENTIALS

Two different chemorepellents, GTP and lysozyme, both produce transien t depolarizing somatic receptor potentials and backward swimming (at m icromolar concentrations) in Paramecium. Behavioral adaptation occurs after 10 min in either repellent and the cells regain normal forward s wimming. This is very specific for each repellent. Cells that have beh aviorally adapted to 0.1 mu M lysozyme (for 10 min) show forward swimm ing in lysozyme, a decreased amplitude of lysozyme-induced depolarizat ions and a 10-fold decrease in the estimated number of surface binding sites for H-3-lysozyme (by Scatchard analysis). All of these changes are reversible after 10 min in the absence of lysozyme. The lysozyme-a dapted cells have normal responses to other depolarizing stimuli such as 8 mM Ba++, 40 mM K+, 10 mM Na+ and 10.0 mu M GTP and to the hyperpo larizing chemoattractant, 5 mM acetate. Their surface binding of P-32- GTP was also unaffected. GTP-adapted cells show dramatic decreases in P-32-GTP surface binding sites and in the frequency of GTP-induced dep olarizations but no changes in H-3-lysozyme binding or lysozyme-induce d backward swimming. These results suggest that lysozyme and GTP have separate high affinity surface receptors on the somatic membrane that are specifically down regulated during chemosensory adaptation. (C) 19 97 Elsevier Science Inc.