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Abstract
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.