Effects of ingestion of a carbohydrate-fat meal on the levels and synthesis of 5-hydroxyindoles in various regions of the rat central nervous system.

—The concentrations of tryptophan, serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in spinal cord and most brain regions increase 2 h after fasted rats begin to consume a carbohydrate-fat meal: indole levels rise in all portions of the brain studied, but the increase is not statistically significant in the hypothalamus and corpus striatum. The rate at which the brain synthesizes 5-hydroxy-indoles (as estimated in vivo by measuring 5-hydroxytryptophan accumulation following an injection of the decarboxylase inhibitor RO4-4602) is also accelerated in all of the regions in which the experimental diet elevates tryptophan, 5-HT and 5-HIAA levels. These observations indicate that the previously reported increase in brain 5-hydroxyindole levels following consumption of a protein-free meal reflects accelerated serotonin synthesis, and occurs within both the cell bodies and the terminals of serotonin-containing neurons. It is possible that diet-induced changes in neuronal serotonin levels influence the quantities of the neurotransmitter released into synapses, either spontaneously or in response to drugs.