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′‐nucleotidase, previously used in immunocyto‐chemical 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 CA1 area or the dentate gyrus or from the whole hippocampus (0.49‐0.68 nmol/min/mg of protein). The catabolism of AMP (10 μ M ) was equally inhibited (85‐92%) in synaptosomes from whole hippocampus, CA1, CA3, or dentate gyrus by α,β‐methylene‐ADP (100 μ M ) and equally unaffected by p ‐nitrophenyl phosphate (0.5 m M ) or rabbit IgGs (100 μg/ml). However, the antiserum against ecto‐5′‐nucleotidase (100 μg/ml) inhibited extracellular AMP catabolism by 44% in CA3 synaptosomes but had little or no effect in synaptosomes from CA1, 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.