Asymmetrical effect of GABA on the postural orientation in Clione

The marine mollusk Clione limacina, when swimming, normally stabilizes the vertical body orientation by means of the gravitational tail reflexes. Horizontal swimming or swimming along inclined ascending trajectories is observed rarely. Here we report that GABA injection into intact Clione resulted in a change of the stabilized orientation and swimming with a tilt of approximately 45 degrees to the left. The analysis of modifications in the postural network underlying this effect was done with in vitro experiments. The CNS was isolated together with the statocysts. Spike discharges in the axons of two groups of motoneurons responsible for the left and right tail flexion, as well as in the axons of CPB3 interneurons mediating signals from the statocyst receptors to the motoneurons, were recorded extracellularly when the preparation was rotated in space. Normally the tail motoneurons of the left and right groups were activated with the contralateral tilt of the preparation. Under the effect of GABA, the gravitational responses in the right group of motoneurons and in the corresponding interneurons were dramatically reduced while the responses in the left group remained unchanged. The most likely site of the inhibitory GABA action is the interneurons mediating signals from the statocysts to the right group of tail motoneurons. The GABA-induced asymmetry of the left and right gravitational tail reflexes, observed in the in vitro experiments, is consistent with a change of the stabilized orientation caused by GABA in the intact Clione.