Bioavailability of Amino Acids and Amino Acid Precursors for Neurotransmitter Action: The Role of Hormones

In the context of neurotransmitter action, amino acids serve as inhibitory (taurine and glycine) and excitatory neurotransmitters (glutamate and aspartate), precursors of neurotransmitters (tyrosine and tryptophan), and transducers of transmitter components (glycine and serine in transmethylation processes). These amino acids derive exogenously from nutrients, and endogenously from the intermediary metabolism, and from the degradation of protein stores. They circulate in blood in the free from and bound to blood proteins (tryptophan), and blood cells (taurine). Hormones influence the amino acid bioavailability for neural actions in that they regulate the amino acid release from muscle proteins, their metabolism, and their transport across the plasma membrane. The latter step comprises several loci of action, e. g., influx into cellular systems of the gastrointestinal tract, efflux from muscle and metabolizing organs, and transport across the blood-brain barrier.