Swimming behavior regulation by GABA B receptors in Paramecium

Abstract

In Paramecium, internal Ca 2+ concentration increase coupled to membrane depolarization induces a reversal in the direction of ciliary beating and, consequently, a reversal in swimming direction. The ciliary reversal (CR) duration is correlated to Ca 2+ influx, and the addition of drugs that block the Ca 2+ current leads to a reduction in the backward swimming duration. In this study we have examined the possible function of GABA B receptors in P. primaurelia swimming control. The presence of GABA B immunoanalogue in Paramecium was evidenced using SDS-PAGE, Western blotting, and confocal laser scanning microscopy. By applying the specific GABA B receptor agonist baclofen, a dose-dependent inhibition of the membrane depolarization-induced CR duration was observed. This inhibition was antagonized by phaclofen, persisted when K + channel blockers were applied, and disappeared after treatment with nifedipine and verapamil. Moreover, the action of baclofen on depolarization-induced CR was suppressed by treatment with pertussis toxin. Therefore, these experiments suggest that baclofen modulates CR by a G protein (G 0 or G 1) mediated inhibition of dihydropyridine-sensible calcium channels. Finally, synthesis and release of GABA in the environment by Paramecium have been demonstrated by HPLC. Possible correlations between GABA B receptor activation and the regulation of intracellular Ca 2+ levels are discussed.