NAAG and NAA appear to be phylogenetically ancient molecules, being found in the nervous system of invertebrates,3,21 and teleost vertebrates.2 No data are available on elasmobranch vertebrates, but both compounds have been found in the brains of amphibians and reptiles. NAA and NAAG, in general, exhibit a concentration ratio in birds and mammals of approximately 10:1 respectively.2 This ratio is reversed in amphibians such as the frog, where the ratio is approximately 1:10. In teleosts, such as the goldfish, the ratio is 100:1, with only trace amounts of NAAG 2 and only scattered NAAG-IR neurons present in the brain (Figure 10B, D and F). In contrast, NAA-IR was observed in most neurons throughout the goldfish brain (Figure 10A, C, and E).
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