THE IMMUNOCYTOCHEMICAL LOCALIZATION OF N-ACETYLASPARTYL GLUTAMATE, ITS HYDROLYZING ENZYME NAALADASE, AND THE NMDAR-1 RECEPTOR AT A VERTEBRATE NEUROMUSCULAR-JUNCTION

Although glutamate is thought to be the neurotransmitter at the invertebrate neuromuscular junction,(3) acetylcholine is accepted as the primary neurotransmitter of the vertebrate motoneurons.(4) N-acetylaspartylglutamate, a dipeptide localized in putative glutamatergic neurons in brain, is also found in high concentrations (>mM) in mammalian motoneurons and the ventral roots of spinal cord.(8) N-acetylaspartylglutamate, which is released from neurons by depolarization in a Ca2+-dependent fashion,(13) is implicated in glutamatergic transmission in two ways: it is a partial agonist at NMDA receptors,(10) and it is cleaved to yield extracellular glutamate and N-acetylasparate by the specific peptidase N-acetylated alpha-linked acidic dipeptidase.(12) Given the localization of N-acetylaspartylglutamate in motor neuronal perikarya and axons, we wondered whether N-acetylaspartylglutamate or glutamate cleaved from N-acetylaspartylglutamate by N-acetylated alpha-linked acidic dipeptidase may also play a role in neuromuscular transmission, Here we describe the immunocytochemical detection at the rat neuromuscular junction of N-acetylaspartylglutamate in terminals of motoneurons, of N-acetylated alpha-linked acidic dipeptidase in perisynaptic Schwann cells, and of the NMDAR-1 glutamate receptor subunit on postsynaptic muscle membranes, These results point to a potential role for N-acetylaspartylglutamate at the rat neuromuscular junction. Further, this is the first demonstration of a glutamate receptor protein at vertebrate neuromuscular synapses, Together with other recent findings,(6,14,15) our results suggest that glutamate-like molecules are involved in neuromuscular transmission not only in invertebrates but also in veretebrates where they may modulate signaling by acetylcholine.