A neurotoxic lesion of serotonergic neurones using 5,7-dihydroxytryptamine does not disrupt latent inhibition in paradigms sensitive to low doses of amphetamine.

Testing the effects of low doses of d-amphetamine on latent inhibition (LI) in two different conditioning paradigms, passive avoidance and conditioned taste aversion, provided evidence of their pharmacological equivalence. For passive avoidance, LI was expressed by the decreased latency to enter a shock compartment in preexposed rats placed 5 min in the compartment during 3 consecutive days before conditioning. In the conditioned taste aversion paradigm, a group of rats was preexposed to a solution of sucrose also for 3 consecutive days prior to the establishment of an association between sucrose and sickness elicited by an injection of LiCl. On the following day, the preexposed rats drunk more sucrose when allowed to choose between one tube containing water and an other containing sucrose. In both paradigms, 0.25 mg/kg d-amphetamine, injected daily on the 3 preexposure days and on the conditioning day, decreased LI. A dose of 0.5 mg/kg suppressed LI in the passive avoidance paradigm. The effect of a serotonergic lesion induced by i.c.v. injection of 5,7-dihydroxytryptamine (5,7-DHT) was evaluated in the same paradigms. The lesion procedure that lowered hippocampal serotonin and 5 HIAA levels by more than 80% did not affect LI. Taken together, the present results lessens the hypothesis that LI is prone to an opposing influence of the two monoaminergic systems considered in this work.