Immunologically distinct isoforms of ecto-5 '-nucleotidase in nerve terminals of different areas of the rat hippocampus

Ecto-5'-nucleotidase is regarded as being the key enzyme in the formation of the neuromodulator adenosine from released ATP, However, the association of ecto-5'-nucleotidase with nerve terminals is not consensual. Only enzyme histochemical and biochemical studies, but not immunocytochemical studies, agree on a general synaptic location of the enzyme, To clarify this issue further we tested the effect of an antibody against ecto-5'-nucleolidase, previously used in immunocytochemical studies, on the activity of ecto-5'-nucleotidase in fractions of nerve terminals isolated from different areas of rat hippocampus. The specific activity of extracellular AMP catabolism was higher in synaptosomes from the CA3 area (0.81 +/- 0.06 nmol/min/mg of protein) than from synaptosomes from the CAI area or the dentate gyrus or from the whole hippocampus (0.49-0.68 nmol/min/mg of protein). The catabolism of AMP (10 mu M) was equally inhibited (85-92%) in synaptosomes from whole hippocampus, CA1, CA3, or denlate gyrus by alpha,beta-methylene-ADP (100 mu M) and equally unaffected by p-nitrophenyl phosphate (0.5 mM) or rabbit IgGs (100 mu g/ml). However, the antiserum against ecto-5'-nucleotidase (100 mu g/ml) inhibited extracellular AMP catabolism by 44% in CA3 synaptosomes but had little or no effect in synaptosomes from CAI, dentate gyrus, or whole hippocampus, A similar difference in the inhibitory potential of the antibody was observed between fractions of isolated 5'-nucleotidase binding to concanavalin A-Sepharose (70%) and fractions not retained by the lectin column (18%), Taken together, these results suggest that immunological isoforms of ecto-5'-nucleotidase exist in the rat hippocampal nerve terminals, with predominance in the CA3 area.