Regional Distribution of Extracellular 5‐Hydroxytryptamine and 5‐Hydroxyindoleacetic Acid in the Brain of Freely Moving Rats

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Abstract
The extracellular concentrations of 5‐hydroxytryptamine (5‐HT) and its metabolite 5‐hydroxyindoleacetic acid (5‐HIAA) have been determined in six brain areas of awake rats (frontal cortex, striaturn, hypothalamus, hippocampus, inferior colliculus, and raphe nuclei) using intracerebral microdialysis. The extracellular levels of 5‐HT showed no significant differences among the brain regions studied. The tissue levels of 5‐HT and 5‐HIAA as well as the extracellular concentration of 5‐HIAA were significantly higher in raphe nuclei. The regional distribution of tissue and extracellular 5‐HIAA were very similar, suggesting that extracellular 5‐HIAA depends mainly on the output from the intracellular compartment. On the other hand, extracellular 5‐HT and tissue 5‐HT showed a different distribution pattern. The tissue/extracellular ratio for 5‐HT ranged from 739 in frontal cortex to 2,882 in raphe, whereas it only amounted to 1.8–3.6 for 5‐HIAA. The relationship between the present results and the density of 5‐HT uptake sites in these areas is discussed.

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