Facilitation of olfactory learning by a modulator of AMPA receptors

The effects of a benzoyl-piperidine drug (BDP) that facilitates AMPA receptor-mediated synaptic responses were tested on the acquisition and retention of long-term memory at dosages that had no detectable effects on a variety of performance measures. BDP-12 produced a dose-dependent suppression of exploratory activity in rats with statistically reliable effects occurring at 50 mg/kg (i.p.). The drug had no effects on balance beam performance at 30 mg/kg but at 45 mg/kg reduced the number of crossings made within a session; it did not, however, affect the time required to perform a traversal. The performance of well-trained rats presented with a familiar pair of odors (correct and incorrect) was not not detectably altered by BDP-12 at 30 mg/kg; however, the number of correct responses made in a five-trial test was reduced at 45 mg/kg. These results indicate that the AMPA receptor modulator at 30 mg/kg has little influence on arousal, motivation, sensori-motor processing, and attention; higher dosages cause a depression of learned and unlearned prepotent responses. The effects of the lower concentration were tested on two-odor discrimination learning in rats that had extensive training on the task. The animals (n = 20) were given three or five acquisition trials with novel odor pairs immediately after an injection of drug or vehicle and then tested 1–3 d later for retention in five unrewarded probe trials. Retention performance was not significantly better than chance 52.6 +/- 4.5% correct) for odors learned on vehicle injection days but was well above chance for odors learned on drug injection days (70.6 +/- 4.2% correct). Within-subject comparisons confirmed the memory enhancing effect of BDP-12 (p < 0.01). Analyses of performance during five training trials indicated that the rats made more correct responses on days on which they were given the drug than on days on which they were injected with vehicle (p < 0.02). Within-subject differences in acquisition were correlated with differences in retention (r = 0.70). There were no evident effects of the drug on response latencies during acquisition. These results suggest that AMPA receptor modulators reduce the amount of training needed for the formation of long-term memory and do so at dosages which have little effect on variables that secondarily influence acquisition. Possible reasons for this selectivity are discussed.