Ontogenetic differences in sensitivity to LiCl- and amphetamine-induced taste avoidance in preweanling rats.

When amphetamine is associated with a tastant conditioned stimulus, rats learn to avoid the taste even when employing doses that promote conditioned place preference. One hypothesis raised to account for this effect proposes that taste avoidance induced by amphetamine may be motivated by fear. A sensitive period has been identified in the rat (until postnatal day 10) in which infants learn conditioned appetitive effects to stimuli to which aversions are conditioned after this period. Exogenous administration of corticosterone within this period reverses this effect, generating aversive conditioning. In the present study, we tested conditioning of aversions to amphetamine or LiCl, within and after the sensitive period (Experiments 1 and 2). A third experiment evaluated unconditioned rejection of an aversive quinine solution within the sensitive period. Finally, we tested whether corticosterone administration before conditioning modulates amphetamine-induced taste avoidance. After the sensitive period, infant rats rejected the solution paired with amphetamine or LiCl after 2 conditioning trials, but within the sensitive period, aversions were conditioned only by LiCl and after 4 conditioning trials. Amphetamine-induced taste avoidance was not observed even when corticosterone was administered before conditioning. Additionally, during the sensitive period, a low LiCl dose promoted conditioned taste preference. According to Experiment 3, parameters employed in this study were suitable to yield rejection of aversive solutions within the sensitive period. These results suggest that during the sensitive period, there is a notable resistance to the acquisition of taste avoidance induced by amphetamine. The present experimental framework may represent a useful tool for studying mechanisms underlying taste avoidance and aversion effects.