Monoamines tissue content analysis reveals restricted and site-specific correlations in brain regions involved in cognition.

The dopamine (DA), noradrenalin (NA) and serotonin (5-HT) monoaminergic systems are deeply involved in cognitive processes via their influence on cortical and subcortical regions. The widespread distribution of these monoaminergic networks is one of the main difficulties in analyzing their functions and interactions. To address this complexity, we assessed whether inter-individual differences in monoamine tissue contents of various brain areas could provide information about their functional relationships. We used a sensitive biochemical approach to map endogenous monoamine tissue content in 20 rat brain areas involved in cognition, including 10 cortical areas and examined correlations within and between the monoaminergic systems. Whereas DA content and its respective metabolite largely varied across brain regions, the NA and 5-HT contents were relatively homogenous. As expected, the tissue content varied among individuals. Our analyses revealed a few specific relationships (10%) between the tissue content of each monoamine in paired brain regions and even between monoamines in paired brain regions. The tissue contents of NA, 5-HT and DA were inter-correlated with a high incidence when looking at a specific brain region. Most correlations found between cortical areas were positive while some cortico-subcortical relationships regarding the DA, NA and 5-HT tissue contents were negative, in particular for DA content. In conclusion, this work provides a useful database of the monoamine tissue content in numerous brain regions. It suggests that the regulation of these neuromodulatory systems is achieved mainly at the terminals, and that each of these systems contributes to the regulation of the other two.