Differential increase of extracellular dopamine and serotonin in the 'prefrontal cortex' and striatum of pigeons during working memory.

Monoamines, such as dopamine (DA) and serotonin (5-HT), play a central role in the modulation of cognitive processes at the forebrain level. Experimental and clinical studies based on dopaminergic pathology, depletion or medication indicate that DA, in particular, is involved in working memory (WM). However, it is unclear whether DA is indeed related to WM, whether its function is specific to the prefrontal cortex (PFC), and whether other modulators, such as 5-HT, might have similar functions. Therefore, the aims of this study were threefold. First, we analysed whether increased prefrontal DA release is related to WM in general or only to its short-term memory component. Second, we examined whether the DA release during cognitive tasks is specific to prefrontal areas or also occurs in the striatum. Third, we analysed whether prefrontal or striatal 5-HT release accompanies working and short-term memory. We approached these questions by using in vivo microdialysis to analyse the extracellular DA and 5-HT release in the pigeons' 'PFC' and striatum during matching-to-sample tasks with or without a delay. Here, we show that DA has no unitary function but is differentially released during working as well as short-term memory in the pigeons' 'prefrontal' cortex. Striatal DA shows an increased efflux only during WM that involves a delay component. WM is also accompanied by a 'prefrontal' but not a striatal release of 5-HT, whose efflux pattern is thus partly different to that of DA. Our findings thus show a triple dissociation between transmitters, structures and tasks within the avian 'prefronto'-striatal system.