Ac-Asp-Glu (N-Acetylaspartylglutamate, NAAG) is an endogeneous peptide with high affinity for a brain Glutamate receptor. Also Ac-Asp-Glu is the most abundant peptide neurotransmitter in the mammalian central nervous system. One role for Ac-Asp-Glu following synaptic release is the activation of metabotropic autoreceptors that inhibit subsequent transmitter release. A second role is the production of extracellular glutamate following Ac-Asp-Glu hydrolysis. High concentrations of Ac-Asp-Glu selectively activate NMDA receptors on mouse spinal cord neurons in cell culture.