Selective release of N-acetylaspartylglutamate from rat optic nerve terminals in vivo

Glutamate (Glu) and aspartate (Asp) are considered to be the neurotransmitters of the optic pathway in submammalian species, but their roles in mammals is uncertain. Recently, N-acetylaspartylglutamate (NAAG) has been proposed as a neurotransmitter in mammalian optic pathway; however, the release of endogenous NAAG on stimulation of the optic pathway has not been demonstrated. Using an in vivo microdialysis technique, we now report that electrical stimulation of rat optic nerve markedly increased the extracellular concentration of NAAG but not Glu/Asp in superficial superior colliculus where retinal afferents terminate, whereas non-specific stimulation of neurotransmitter release by high potassium or veratridine increased both extracellular Glu/Asp and NAAG concentration in the perfusate. The release of NAAG was dependent on Ca²⁺ and the presence of optic terminals. We conclude that NAAG is a better candidate as a neurotransmitter of rat optic nerve terminals than Glu/Asp.