Diurnal rhythm in extracellular levels of 5-hydroxyindoleacetic acid in the medial prefrontal cortex of freely moving rats: an in vivo microdialysis study

The cerebral monoaminergic nervous system was monitored for its diurnal changes to find whether these can be used as neurochemical indices for emotional states. Freely moving rats that were not anesthetized were used: by employing an in vivo microdialysis technique, the levels of extracellular monoamine metabolites were measured. Under an alternating 12 h light and dark (LD) cycle and subsequent constant light (LL) condition, the extracellular 5-hydroxyindoleacetic acid (5-HIAA) level was significantly higher in the dark phase than in the light phase in the medial prefrontal cortex (mPFC). However, the 5-HIAA levels in the light and dark phases in the striatum or hippocampus did not significantly differ. The homovanillic acid (HVA) level was significantly higher in the dark phase than in the light phase in the mPFC and striatum. These findings indicate that the extracellular 5-HIAA level in the mPFC has site-specific circadian rhythmicity. Furthermore, considering the relationship among diurnal rhythm, emotional disturbance and the mPFC, the site-specific diurnal rhythmicity of 5-HIAA in the mPFC may be a useful index in neurochemical studies on emotional states, such as seen in affective disorders and psychiatric diseases.