Effects of topically applied excitatory amino acids on evoked potentials and single cell activity in rat cerebral cortex.

The effect of topically applied excitatory amino acids and related compounds on the somatosensory evoked potential and single cell responses to iontophoretically applied compounds have been studied in the neocortex of urethane anaesthetised rats. N-methyl-aspartate (NMDLA), DL-homocysteic acid (DLH), kainic acid, quinolinic acid and carbachol produced a depression of evoked potential amplitude. L-glutamate and quisqualic acid had no effect. Tetrodotoxin and 2-amino-5-phosphonovaleric acid prevented the effects of NMDLA, but bicuculline and theophylline derivatives did not. There was an apparent desensitisation of the depression of evoked potentials following topical application of NMDLA or DLH, the potentials recovering to control levels during the amino acid application. A second application of amino acid was then ineffective unless applied about 1 h later. Single cell responses to iontophoretically applied NMDLA behaved similarly. There was an apparent cross-desensitisation between NMDLA, DLH and quinolinic acids. The results may indicate a desensitising receptor for amino acids on superficial neocortical neurones.