ELECTROPHYSIOLOGICAL EVIDENCE FOR THE EXISTENCE OF NMDA AND NON-NMDA RECEPTORS ON RAT VENTRAL TEGMENTAL DOPAMINE NEURONS

In vitro extracellular single-unit recordings from rat midbrain slices were used to assess the effects of excitatory amino acid agonists on the activity of A10 dopamine neurons. N-methyl-D-aspartic acid (NMDA), kainic acid (KA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) elicited dose-dependent increases in firing rates. The relative potencies for the 3 compounds was AMPA > KA > NMDA. None of the excitations was accompanied by burst firing, but frequently periods of nonrecordable activity occurred following pronounced stimulation. Concurrent application of the excitatory amino acid antagonist CGS 19755 (cis-4-phosphonomethyl-2-piperidine carboxylate) selectively blocked the excitations elicited by NMDA but not by KA or AMPA. Likewise the selective non-NMDA antagonist NBQX [2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxalinel blocked only the excitatory effects of AMPA and KA but not those elicited by NMDA. NBQX appeared to be less potent at antagonizing KA than AMPA. These results suggest that mesolimbic-mesocortical dopamine neurons possess both NMDA and non-NMDA receptors, and possibly distinct AMPA and KA recognition sites.