Ultrastructural localization of N-acetylaspartylglutamate in synaptic vesicles of retinal neurons

Localization of N-acetylaspartylglutamate (NAAG) in a variety of central and peripheral neurons, as well as its receptor-mediated activation of membrane conductance have led to speculation that this peptide has a role in chemical neurotransmission. We previously identified NAAG in retinal neurons of several species, including the grass frog, and now have determined its ultrastructural distribution within the plexiform layers of this amphibian retina. NAAG immunoreactivity was localized within vesicles in synaptic endings of presumptive amacrine and bipolar neurons in the inner plexiform layer. Additionally, the peptide was present in vesicles within ribbon synapses in the outer plexiform layer, a result suggestive of release from photoreceptor cells. These data support the hypothesis that NAAG is secreted at points of synaptic contact between neurons, including retinal amacrine, bipolar and photoreceptor cells.