Abstract

Behavioral effects of the selective 2-amino-3-(5-methyl-3-hydroxyisoxazol-4- yl)propanoic acid (AMPA) antagonist LY293558, along with its racemate (LY215490) and opposing enantiomer (LY293559) were evaluated in pigeons. When responding was maintained under a multiple fixed ratio 50 responses, fixed interval 5 minute (FRFI) schedule of food presentation, LY215490 completely antagonized the rate suppression induced by AMPA (10 mg/kg) and by the AMPA analog, 2-amino-3-hydroxy-5-tert-butyl-4-isoxazolepropionic acid (ATPA; 40 mg/kg) at 1.25 and 2.5 mg/kg, respectively. In contrast, LY215490, up to 10 mg/kg, was unable to antagonize the rate suppression induced by N-methyl-D-aspartic acid. LY293558, at 0.32 mg/kg, completely blocked the rate suppression produced by AMPA in both components of the multiple schedule. Similarly, LY293558, at 0.64 mg/kg, blocked the rate suppression induced by ATPA in both components. In contrast, the opposing enantiomer, LY293559, up to 10 mg/kg, was without effect on rate suppression produced by AMPA in this model. In additional studies, behavior was maintained under a schedule in which responding was maintained by food presentation in the presence of one key color and in the presence of a second key color, responding was maintained by food and simultaneously suppressed by electric shock ("punished responding"). LY215490 significantly increased punished responding at 10 and 30 mg/kg, whereas unpunished responding was unaffected until 56 mg/kg depressed it. LY293558 significantly increased punished responding at 3 mg/kg without having an effect on unpunished responding. LY293559, on the other hand, was unable to significantly increase punished responding at doses up to 175 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)