Pretecto-tectal interactions: effects of lesioning and stimulating the pretectum on field potentials in the optic tectum of salamanders in vitro.

Interactions between pretectum and optic tectum of salamanders were analyzed by recording evoked potentials (EPs) in the optic tectum in vitro in response to stimulation of the contralateral optic nerve. Neither lesioning of the pretectum nor ablation of the medulla oblongata including the nucleus isthmi altered the shape of tectal EPs, suggesting that the tectal EP in this preparation reflects activation of tectal circuitry by retinal afferents without a major contribution from non-retinal afferents. To analyze the effect of stimulation of the pretectum on the tectal EP, we stimulated the pretectal area pharmacologically. Amplitudes of tectal EPs decreased rapidly after stimulation of the pretectum and recovered within minutes (glutamate) or hours (kainic acid). The pretectal influence on the tectal EP might act presynaptically on retinal afferents or by modulating the response of inhibitory interneurons.