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SUMMARY1. The electrical activity of caudate neurones was recorded with intracellular electrodes in halothane anaesthetized cats. Agonists and antagonists of excitatory amino acid receptors were applied by micro-ionophoresis and their effects on membrane-and action potentials and on cortically evoked synaptic potentials evaluated.2. The agonists, L-aspartate (asp), L-glutamate (glu), N-methyl-DL-aspartate (NMA), quinolinate and quisqualate all depolarized the membrane, caused repetitive firing, reduced the apparent amplitude of the cortically evoked excitatory postsynaptic potentials (e.p.s.p.s) and increased the amplitude of the associated inhibitory post-synaptic potential. Two of the agonists, NMA and quinolinate, additionally caused the appearance of up to 500 ms long depolarizations (plateaus) on the falling phase of action potentials. These plateaus were seen in about two-thirds of the cells in this sample while in the other third the excitatory effects of NMA and quinolinate were indistinguishable from those of glu and quisqualate.3. The N-methyl-D-aspartate (NMDA) receptor antagonist D-a-aminoadipate (DAA) reversibly inhibited the effects of NMA and quinolinate but only on those cells where these two agents evoked action potential plateaus while on the same cells the effects of asp, glu and quisqualate were either only weakly antagonized or not affected. On cells not displaying plateaus to NMA or quinolinate none of the effects of the agonists could be antagonized by DAA. DAA applications that completely antagonized the effects of NMA never reduced the amplitudes of cortically evoked e.p.s.p.s.4. Cis-2,3-piperidine dicarboxylate also blocked the effects of NMA and asp at low application currents while at higher currents it enhanced the effects of glu or asp although still retaining its NMA antagonistic activity.5. High-frequency stimulation of the cortico-caudate pathway resulted in longlasting depolarizations and repetitive firing, but plateaus of the type caused by NMA or quinolinate were not seen.

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