Expression of the CS- and US-pre-exposure effects in the conditioned taste aversion paradigm and their abolition following systemic amphetamine treatment in C57BL6/J mice.

In classical conditioning, pre-exposures to either the to-be-conditioned stimulus (CS) or unconditioned stimulus (US) can retard subsequent conditioning between the CS and US. The present experiment evaluated the expression of these two pre-exposure effects in mice of the C57BL6/J strain, one of the most common background strains for genetically altered mice. We tested whether their expression would be disrupted by amphetamine treatment (2.5 mg/kg, i.p.) in a conditioned taste aversion paradigm with sucrose as the CS and lithium chloride-induced gastric malaise as the US. We found that one pre-exposure (PE) to either the CS or the US reduced aversion to sucrose solution in the controls following conditioning, but no such tendency was evident in the amphetamine-treated mice. The present study represents the first report of amphetamine-induced disruption of the CS-PE effect (ie latent inhibition) in mice, and the first attempt to compare it directly with the US-PE effect in any species. It extended previous reports in rats and humans, suggesting that the sensitivity of latent inhibition to amphetamine is largely comparable across species, thereby lending credence to the use of the latent inhibition effect as a behavioral assay for psychotic-like phenotype in transgenic mice. The parallel observation in the US-PE effect further indicates that its expression, at least in the present conditioned taste aversion paradigm, may also be under similar influence of the dopaminergic system.