LONG-TERM ADAPTATION OF CA2-DEPENDENT BEHAVIOR IN PARAMECIUM-TETRAURELIA()

Prolonged exposure to KCl has long been recognized to modify swimming behaviour in Paramecium tetraurelia, a phenomenon known as 'adaptation '. In this study, we have investigated behavioural adaptation systemat ically, A 24 h exposure to 30 mmol l(-1) KCl deprived cells of the abi lity to respond behaviourally to two established chemoeffectors. We al so explored the effects of 30 mmol l(-1) KCl on the duration of backwa rd swimming induced by Ba2+ and Mg2+. A brief (60 min) exposure preven ted cells from swimming backwards in response to either cation, but re covery was rapid (<60 min) following a return to control medium. Prolo nged (48 h) exposure caused a more persistent loss of response to Ba2, so that several hours was now required for recovery. Surprisingly, r esponses to Mg2+ reappeared during 6-8 h in KCl, with backward swimmin g durations increasing to more than 300 % of control values after 26 h . Thus, we can distinguish two phases to adaptation. The short-term ph ase is characterized by an inability to respond behaviourally to most stimuli and might be adequately explained in terms of Ca2+ channel ina ctivation and K+-induced shifts in membrane potential. The long-term p hase is characterized by enhanced responses to Mg2+ (and also to Na+), suggesting that a more extensive reprogramming of membrane excitabili ty may occur during chronic Kf-induced depolarization.