 This raises the possibility that this extracellular glutamate plays an "endocrine-like" role as part of a larger homeostatic system.Glutamate also serves as the precursor for the synthesis of the inhibitory gamma-aminobutyric acid (GABA) in GABA-ergic neurons.This reaction is catalyzed by glutamate decarboxylase (GAD), which is most abundant in the cerebellum and pancreas.