CHEMOSENSORY ADAPTATION TO LYSOZYME AND GTP INVOLVES INDEPENDENTLY REGULATED RECEPTORS IN TETRAHYMENA-THERMOPHILA

Chemosensory adaptation is seen in Tetrahymena thermophila following p rolonged exposure (ten minutes) to micromolar concentrations of the ch emorepellents lysozyme or guanosine triphosphate (GTP). Since these ce lls initially show repeated backward swimming episodes (avoidance reac tions) in these repellents, behavioral adaptation is seen as a decreas e in this repellent-induced behavior. The time course of this behavior al adaptation is paralleled by decreases in the extents of surface bin ding of either [P-32]GTP or [H-3]lysozyme in vivo. Scatchard plot anal yses of repellent binding in adapted cells suggests the behavioral ada ptation is due to a dramatic decrease in the number of surface binding sites, as represented by decreased Bmax values. The estimated K-D val ues for nonadapted cells are 6.6 mu M and 8.4 mu M for lysozyme and GT P binding, respectively. Behavioral adaptation and decreased surface r eceptor binding are specific for each repellent. The GTP adapted cells (20 mu M for ten minutes) still respond behaviorally to 50 mu M lysoz yme and bind [H-3]lysozyme normally. Lysozyme adapted cells (50 mu M f or ten minutes) still bind [P-32]GTP and respond behaviorally to GTP. All the behavioral and binding changes seen are also reversible (deada ptation). Neomycin was shown to be a competitive inhibitor of [H-3]lys ozyme binding and lysozyme-induced avoidance reactions, but it had no effect on either [P-32]GTP binding or GTP-induced avoidance reactions. These results are consistent with the hypothesis that there are two s eparate repellent receptors, one for GTP and the other for lysozyme, t hat are independently downregulated during adaptation to cause specifi c receptor desensitization and consequent behavioral adaptation.