Chemosensory adaptation in paramecium involves changes in both repellent binding and the consequent receptor potentials

Two different chemorepellents, GTP and lysozyme, both produce transient depolarizing somatic receptor potentials and backward swimming (at micromolar concentrations) in Paramecium. Behavioral adaptation occurs after 10 min in either repellent and the cells regain normal forward swimming. This is very specific for each repellent. Cells that have behaviorally adapted to 0.1 μM lysozyme (for 10 min) show forward swimming in lysozyme, a decreased amplitude of lysozyme-induced depolarizations and a 10-fold decrease in the estimated number of surface binding sites for ³H-lysozyme (by Scatchard analysis). All of these changes are reversible after 10 min in the absence of lysozyme. The lysozyme-adapted cells have normal responses to other depolarizing stimuli such as 8 mM Ba⁺⁺, 40 mM K⁺, 10 mM Na⁺ and 10.0 μM GTP and to the hyperpolarizing chemoattractant, 5 mM acetate. Their surface binding of ³²P-GTP was also unaffected. GTP-adapted cells show dramatic decreases in ³²P-GTP surface binding sites and in the frequency of GTP- induced depolarizations but no changes in ³H-lysozyme binding or lysozyme- induced 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.