N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4- isoxazoleproprionate (AMPA) glutamate-receptor antagonists have different interactions with the discriminative stimuli of abused drugs.

The effects of the AMPA-receptor antagonists NBQX and GYKI 52466 were compared with those of the NMDA-receptor channel blocker dizocilpine in two drug discrimination tests. In the first, rats were trained to discriminate morphine (2 mg/kg) from saline and in the second, to discriminate ketamine (7 mg/kg) from saline, using a two-lever food reinforced method. NBQX (1-6 mg/kg) did not substitute for either morphine or ketamine, even at a dose which reduced response rates (6 mg/kg). Likewise, the noncompetitive antagonist GYKI 52466 (5 and 10 mg/kg) produced only saline lever responding in the ketamine trained rats. When tested in combination with the training drug, NBQX (4.5 mg/kg) did not alter the morphine generalisation gradient, and similarly, neither NBQX (3 mg/kg) nor GYKI 52466 (5 and 10 mg/kg) interacted with the ketamine cue. In contrast, dizocilpine (0.05 mg/kg) significantly disrupted discrimination of morphine and produced clear drug lever responding (0.0125-0.1 mg/kg) in ketamine trained rats. These results suggest that AMPA-receptor antagonists and non-competitive NMDA-antagonists have different stimulus properties.