N-acetyl-aspartyl-glutamate (NAAG) elicits rapid increase in intraneuronal Ca2+ in vitro

N-acetyl-aspartylglutamate (NAAG) is an acidic dipeptide found at high concentrations almost exclusively in the brain and spinal cord. There is evidence that NAAG is an excitatory neurotransmitter/neuromodulator. On the other hand, the identification and characterization of an enzyme in the nervous system which hydrolyzes NAAG to liberate glutamic acid (Glu) has led to an alternative hypothesis that the dipeptide might serve as a precursor of the excitatory amino acid. Using an interactive laser cytometer to quantitate changes in intraneuronal Ca2+ in individual neurons, we demonstrate that NAAG, at the concentrations at which it is found in the brain, promotes a rapid increase in intraneuronal Ca2+. NAAG-induced effects are completely dependent on the presence of Ca2+ in the bathing medium and are inhibited by NMDA receptor and channel antagonists. Several factors have led us to conclude that it is NAAG itself, and not NAAG-derived Glu, which is responsible for the observed effects in this system.