Hippocampal localization of 5'-nucleotidase as revealed by immunocytochemistry

The distribution of binding sites for an antibody against ecto-5'-nucleotidase was investigated in the mouse hippocampus by light microscopical immunocytochemistry. The antibody selectively labels a band corresponding to the innervation area of mossy fibre terminals within area CA3. Area CA1 as well as the dendate gyrus are negative. In area CA3 only the proximal but not the distal parts of the apical dendrites of pyramidal cells are labelled. Labelling is in the form of large dots around dendrites of pyramidal cells suggesting that mossy fibre terminals are immunopositive. In contrast, an antibody against the ubiquitous synaptic vesicle protein SV2 labels the large mossy fibre terminals as well as fine and punctate structures in the dendritic and somatic regions throughout the hippocampus. Labelled astrocytes can be found in the entire hippocampus and are frequent in the stratum radiatum and stratum oriens of the CA1 region. Immunopositive astrocytic processes can be found in association with capillary walls. Our results suggest that ecto-5'-nucleotidase may play a crucial role in the hydrolysis of AMP to adenosine at the mossy fibre synapses. Thus, at these synapses, 5'-nucleotidases could function both in completing the extracellular hydrolysis of synaptically released ATP as well as in the extracellular formation of adenosine.