A role for GABAA receptors in the modulation of Paramecium swimming behavior

The presence in Paramecium of γ-aminobutyric acid A-type receptors (GABAA) and the capability of the protozoon to synthesize and release the GABA neurotransmitter into the environment have already been demonstrated. This study investigates the involvement of the GABAA complex in the swimming control of the ciliated protozoon. The GABAA receptors were pharmacologically activated by the selective agonist muscimol and the effect on Paramecium primaurelia swimming behavior was analyzed. Paramecium normally swims forward, but the activation of GABAA receptors induced a peculiar response, characterized by alternate periods of whirling and forward swim. This effect was inhibited by the GABAA selective antagonists bicuculline and picrotoxin in a dose-dependent manner. Moreover, the application of benzodiazepines did not enhance the agonist action but only reduced it. Response to muscimol was also suppressed by nimodipine, a selective antagonist of dihydropyridine-sensitive calcium channels. This inhibition suggests that an inflow of calcium ions through L-type channels mediates the muscimol-induced swimming behavior.