5 '-nucleotidase activity of mossy fibers in the dentate gyrus of normal and epileptic rats

Sprouting of mossy fibers in the hippocampus of rats that underwent limbic epileptogenesis by amygdala kindling qr kainate injection was studied at th e light microscopic and ultrastructural levels by cytochemical demonstratio n of the enzyme 5'-nucleotidase. This adenosine-producing ectoenzyme has pr eviously been shown to characterize malleable terminals during brain develo pment and lesion-induced synaptogenesis, but to be otherwise associated wit h glial membranes. At the light microscopic level, kainate-treated but not control or kindled rats showed 5'-nucleotidase activity in the CA3 region a nd in the inner molecular layer of the dentate gyms. At the ultrastructural level, in control animals, the synapses of the molecular and granular laye rs were enzyme negative. Only some mossy fiber boutons of the dentate hilus exhibited 5'-nucleotidase activity. In epileptic rats, synaptic labeling w ithin the hilus appeared more intense. Moreover, 5'-nucleotidase-containing terminals within the inner molecular layer, presumably ectopic mossy fiber boutons, were found in both kindled and kainate-treated rats. It is concluded that, in both the normal and epileptic hippocampus, 5'-nucl eotidase is associated with axons capable of a plastic sprouting response. The synaptic enzyme may attenuate the glutamatergic transmission of mossy f ibers, in particular of the aberrant mossy fibers in epileptic rats, by pro ducing the inhibitory neuromodulator adenosine. Alternatively, 5'-nucleotid ase may influence synapse formation by its putative non-enzymatic, adhesive functions. (C) 1999 IBRO. Published by Elsevier Science Ltd.