Concanavalin A reduces habituation in the tectum of the frog

In the tectum of Rana pipiens, responses to repeated flashes of light to the ipsilateral eye display considerable habituation. We have employed the plant lectin concanavalin A (Con A), which can diminish desensitization of glutamate receptors in vitro, in order to examine whether desensitization of glutamate receptors contributes to this habituation. The ipsilateral eye's projection reaches each tectal lobe indirectly, being relayed from the opposite tectal lobe via the tecto-isthmo-tectal projection. One of the sites along this pathway at which habituation may take place is the retinotectal synapse, where glutamate is a putative transmitter. Pretreatment of one lobe of the tectum with Con A significantly diminished the habituation of responses recorded in the other tectal lobe to light offset, with less of an effect on responses to light onset. These data suggest that OFF habituation may reflect glutamate receptor desensitization at the retinotectal synapse. In contrast, recordings from retinotectal terminals indicate that a primary site of habituation of ON responses is within the retina.