Involvement of GABAergic mechanisms in the optokinetic nystagmus of the frog.

Frog optokinetic nystagmus (OKN) was studied before and after an intravitreal injection of picrotoxin, a specific non-competitive GABA antagonist. In monocular vision, the OKN displayed a directional asymmetry favouring the Temporal-Nasal (T-N) stimulation. In that case, the nystagmus extinction frequency (NEF) is low, about 2 frames/s. In binocular vision, the OKN is symmetrical with a facilitation of performances compared to monocular vision (NEF = 3 frames/s). In monocular as in binocular vision, an intravitreal injection of picrotoxin (between 1 X 10(-4) and 5 X 10(-3) M) provoked the disappearance of the injected eye OKN and a spectacular facilitation in the performances of the intact eye, with the appearance of a N-T component and the increase of the NEF value reaching 7 or 11.5 frames/s according to the experimental conditions. This contralateral facilitation was no longer observed after the optic nerve of the injected eye had been cut, indicating that such a facilitation can only be explained by alterations of a central process triggered by the visual input. It is concluded that GABAergic mechanisms might be responsible for the inhibition of the N-T component in the frog OKN and might be involved in the control of the power of temporal resolution in this animal.