Biphasic effects of glycine on synaptic responses of the frog optic tectum in vitro

Electrophysiological recordings from the in vitro amphibian optic tectum were used to study the action of bath-applied glycine (Gly) on synaptic transmission between optic tract fibres and tectal neurones. Gly enhanced synaptic field potentials (ED50 = 0.3 mM) and, when applied in concentrations greater than or equal to 1 mM subsequently blocked them. All these effects of Gly were antagonized by 0.2-1 microM strychnine. Recordings from optic tract afferent fibres indicated that Gly increased the amplitude of small submaximal compound action potentials while depressing those which were near maximal. It is suggested that Gly exerted a novel modulatory effect on synaptic transmission via a combination of pre- and postsynaptic sites of action.