The pretectal cholinergic system is involved through two opposite ways in frog monocular OKN asymmetry

Frog monocular horizontal optokinetic nystagmus (OKN) has been studied by coil recordings, before and after unilateral microinjection of cholinergic drugs into the pretectum. The recorded eye was either contralateral or ipsilateral to the injected structure. Before injection, monocular OKN displayed a directional asymmetry, reacting only to stimulations in the temporonasal (T-N) direction. The intrapretectal administration of a cholinergic muscarinic agonist (oxotremorine), as well as that of a nicotinic antagonist (D-tubocurarine), abolished the monocular OKN asymmetry, inducing the appearance of the naso-temporal (N-T) component; the difference between the slow phase velocity gain of both components was no longer significant. These data suggest that acetylcholine (ACh), at the level of the pretectum, acts in opposite ways through muscarinic and nicotinic binding sites; monocular OKN asymmetry could result, at least partially, from a facilitating nicotinic effect and an inhibitory muscarinic effect. Possible interactions with other transmitter systems are discussed.