It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, however, we have discovered that the production and release in brain neurons of the neurotransmitter serotonin (neurotransmitters are compounds that neurons use to transmit signals to other cells) depend directly on the food that the body processes. Metabolism is a balancing act involving two kinds of activities that go on at the same time.

Our first studies sought to determine whether the increase in serotonin observed in rats given a large 93 injection of the amino acid tryptophan might also occur after rats ate meals that changed tryptophan levels in the blood. We found that immediately after the rats began to eat, parallel elevations occurred in blood tryptophan, brain tryptophan, and brain serotonin levels. These findings suggested that the production and release of serotonin in brain neurons were normally coupled with blood-tryptophan increases. In later studies, we found that injecting insulin into a rat’s bloodstream also caused parallel elevations in blood and brain tryptophan levels and in serotonin levels. We then decided to see whether the secretion of the animal’s own insulin similarly affected serotonin production. We gave the rats a carbohydrate-containing meal that we knew would induce insulin secretion. As we had hypothesised, the blood tryptophan level and the concentrations of tryptophan serotonin in the brain increased after the meal.