Stimuli of the Same Nature Are Codified by the Brain as Different Events

Proceedings of the 9th International Multidisciplinary Conference ''Stress and Behavior'', Saint-Petersburg, Russia, 16–19 May 2005.It is known that novel stimuli that are behaviorally relevant activate the mesencephalic dopaminergic system. We started from the perspective that novelty, which is defined as the discrepancy between occurred and expected, is not a unitary phenomenon. Indeed, different kinds of novelty exist depending on the cognitive (kinds of information processing) and behavioral demand requested to correct the mismatch. Because, novel stimuli can activate dopaminergic (DA) neurons in the mesencephalon and, accordingly, increase the extracellular amount of DA (ECDA) in the prefrontal cortex and the nucleus accumbens core and shell, we tested the reactivity of the dopaminergic system in response to kinds of novelty whom detection is dependent on different kinds of information processes. Using the in vivo microdialysis technique, we measured extracellular amounts of dopamine (ECDA) in different dopaminergic terminal regions during a social learning task requiring the detection of a mismatch between familiar and unfamiliar stimuli. In the first session (40min) the rats were exposed to two juveniles (j1 and j2) never encountered before (i.e. unconditioned novelty). Afterward, the animals were divided into 3 groups: Group I was not exposed to any other stimulus; Group II was exposed to one of the known juvenile (j1) and to a new juvenile (j3) (i.e. knowledge-based novelty); Group III was exposed again to the two known juveniles (j1 and j2) (i.e. non-novelty condition). In both the medial prefrontal cortex and the nucleus accumbens shell ECDA increased in response to the first presentation of the juveniles (unconditioned novelty). When the rats were exposed to the new juvenile together with the known one (knowledge-based novelty) we found a different response in the two regions: a further increase was observed in the nucleus accumbens shell while no change was found in the medial prefrontal cortex. Finally, in the last group the re-exposition to the two known juveniles (non-novelty condition) induced a long lasting decrease of ECDA in nucleus accumbens shell. None of the stimuli presented affected ECDA in the nucleus accumbens core. The dissociation in the response of the different DA terminal regions to the distinct kinds of novelty provides evidence for separate mechanisms underlying these processes. Further, it suggests that DA in the prefrontal cortex might be activated to deal with possible threatening consequences of unconditional novelty presentation, while DA in the shell of the nucleus accumbens might play a role in the ability to focus on newly offered information. Alltogether, these data demonstrate that the brain codifies the two kinds of novelty as two different events