Physiological effects of chronic and acute application of N-methyl-D-aspartate and 5-amino-phosphonovaleric acid to the optic tectum of Rana pipiens frogs.

Visually elicited activity contributes to the formation of orderly connections in the optic tectum of frogs. Glutamate receptors of the N-methyl-D-aspartate class participate in this process. Blocking those receptors interferes with activity-dependent refinement of maps in normal frogs and of ocular dominance bands in surgically produced animals with three eyes. Chronic application of N-methyl-D-aspartate sharpens the bands. The possibility that 5-amino-phosphonovaleric acid depresses tectal responsiveness was motivation for studying the effects of 5-amino-phosphonovaleric acid and N-methyl-D-aspartate applied both chronically and acutely. We evaluated tectal responsiveness to visual input by presenting flashes of light to one eye and recording responses in the ipsilateral tectal lobe. This method reveals the output of the tectal cells contralateral to the stimulated eye. These cells project via the nucleus isthmi to the opposite tectal lobe. We also mapped the receptive field dimensions of the crossed isthmotectal axons. Our results show that acute topical application of 500 microM or 1 mM N-methyl-D-aspartate dramatically increases spontaneous activity, while 100 microM N-methyl-D-aspartate causes little change. Chronic treatment with N-methyl-D-aspartate at a low dose (estimated to be in the micromolar range) shown to influence retinotectal mapping, reduces response latencies but produces no statistically significant changes in tectal cell firing rates or receptive field size. Acute application of 5-amino-phosphonovaleric acid produces complex results: 10 microM produces no changes in firing, 100 microM 5-amino-phosphonovaleric acid decreases firing, and doses of 500-100 microM increase the firing.(ABSTRACT TRUNCATED AT 250 WORDS)