A neurotoxic lesion of serotonergic neurones using 5,7-dihydroxytryptaminedoes 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. Fo r passive avoidance, LI was expressed by the decreased latency to enter a s hock compartment in preexposed rats placed 5 min in the compartment during 3 consecutive days before conditioning. In the conditioned taste aversion p aradigm, a group of rats was preexposed to a solution of sucrose also for 3 consecutive days prior to the establishment of an association between sucr ose and sickness elicited by an injection of LiCl, On the following day, th e preexposed rats drunk more sucrose when allowed to choose between one tub e 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 c onditioning day, decreased LI. A dose of 0.5 mg/kg suppressed LI in the pas sive 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 sam e 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 o f the two monoaminergic systems considered in this work. (C) 1999 Elsevier Science B.V. All rights reserved.