Excitatory and inhibitory action of ibotenic acid on frog spinal motoneurones in vitro.

Ibotenic acid (IBO), a conformationally-restricted analogue of the putative excitatory transmitter glutamate, produced biphasic effects on frog spinal motoneurones recorded wih DC-coupled Ag/AgCl electrodes placed on the central end of ventral roots. The initial effect consisted of a motoneuronal depolarization with an increase in excitability tested with low-intensity dorsal root stimulation (the depolarizing activity of IBO was about 6 times greater than that of glutamate). On washout, the depolarizing effect of IBO subsided and complete motoneuronal repolarization occurred; however, motoneuronal responses to glutamate and to trans-synaptic stimulation remained depressed for prolonged periods. This depressant effect was not reproduced following prolonged administrations of depolarizing doses of glutamate but it was similar to that produced by muscimol, an agonist on inhibitory GABA receptors. Unlike the muscimol-evoked depression, the IBO-induced depression was insensitive to bicuculline and was antagonized by local anaesthetics or high Mg2+ solutions. It is suggested that a substantial part of the IBO depressant action is likely due to release of long-acting inhibitory transmitters onto motoneurones.