INCREASED EXTRACELLULAR DOPAMINE IN THE NUCLEUS-ACCUMBENS OF THE RAT DURING ASSOCIATIVE LEARNING OF NEUTRAL STIMULI

Brain microdialysis was used to study changes in dopamine in the nucle us accumbens and the dorsal striatum during associative learning betwe en two neutral stimuli, flashing light and lone, presented on a paired schedule during stage 1 of a sensory preconditioning paradigm. The to ne was subsequently paired with mild footshock using standard aversive conditioning procedures and the Formation of a conditioned associatio n between the hashing light and the tone in stage 1 was assessed by me asuring the ability of the flashing light to elicit the same condition ed response as the tone when presented at test. The first experiment u sed behavioural monitoring only, to establish stimulus parameters for subsequent microdialysis experiments. Animals receiving paired present ation of the light and lone in stage 1 showed a conditioned suppressio n of licking to the light as well as to the tone, indicating that asso ciative learning between the flashing light and the lone had occurred during stage, whilst in a separate group of animals given the same sti muli over the same time period but on an explicitly non-paired schedul e, the conditioned emotional response was seen to the tone, but not to the light, showing that no association had been formed between the tw o stimuli during stage 1. In dialysis experiments using the same proce dure, we measured a two-fold rise in dopamine in the nucleus accumbens during paired presentation of flashing light and tone, but not during non-paired presentation of the two stimuli. On subsequent test presen tation of the two stimuli, we saw increases in accumbal dopamine on pr esentation of the tone in both groups, reflecting the formation of an association with the footshock in both. However the Hashing light elic ited an increase in dopamine only in the group which had received pair ed presentation at stage 1. Thus accumbal dopamine release at test is correlated to the ability of the stimulus to evoke a conditioned respo nse measured behaviourally. Hypotheses of the behavioural function of the mesolimbic dopamine system centre on its role in mediating the eff ects of biological reinforcers, both rewarding and aversive, condition ed and unconditioned. The present results, showing increases in extrac ellular dopamine in the nucleus accumbens when an association is forme d between two stimuli of which neither is a biological reinforcer nor, prior to Formation of the association, affects dopamine levels, sugge st a role for accumbal dopamine in the modulation of associative learn ing in general, not only that involving reinforcement. (C) 1998 IBRO. Published by Elsevier Science Ltd.