Unilateral pretectal microinjections of SR 95,531, a GABA A antagonist: effects on directional asymmetry of frog monocular OKN.

Monocular eye movements have been studied in frogs using the search coil technique before and after unilateral microinjection of SR 95,531, a GABA A antagonist, into the pretectal nuclei contralateral to the open eye. Before injection, monocular, horizontal optokinetic nystagmus (OKN) in frogs, as in other lower vertebrates, displays a directional asymmetry: the stimulation in the T-N (temporo-nasal) direction is more efficient in evoking OKN than is stimulation in the N-T (naso-temporal) direction. The N-T component is almost absent and displays only slow phases of very low speed. Unilateral SR 95,531 microinjection into the pretectum reversibly decreased the directional asymmetry of monocular horizontal OKN, by strongly increasing the N-T component slow phase velocity while the T-N slow phase velocity remained unchanged. These data show that SR 95,531 injected into the pretectum contralateral to the open eye reversibly decreased the inhibition upon the N-T component of monocular horizontal OKN, which suggests that a pretectal GABAergic system is involved in the directional asymmetry of monocular horizontal OKN in frogs.