MICROANATOMICAL AND ELECTROPHYSIOLOGICAL CHANGES OF THE RAT DENTATE GYRUS CAUSED BY LESIONS OF THE NUCLEUS BASALIS MAGNOCELLULARIS

The effect of unilateral or bilateral lesions of the nucleus basalis m agnocellularis (NBM) on the dentate gyrus of the hippocampus were asse ssed using microanatomical and electrophysiological techniques. NBM is the main cholinergic basal forebrain nucleus that supplies the fronto -parietal cortex. Lesions were induced using the neurotoxin ibotenic a cid or a radio-frequency system and did not affect glutamic acid decar boxylase activity both in the frontal cortex and in the hippocampus. A t 4 weeks after lesioning, a loss of choline acetyltransferase (ChAT) activity and of ChAT-immunoreactive fibres was observed in the frontal cortex but not in the hippocampus and no changes in the density of gr anule neurons of the dentate gyrus or in the hippocampal long-term pot entiation (LTP) were noticeable. At 8 weeks after lesioning the loss o f both ChAT activity and of ChAT-immunoreactive fibres persisted in th e frontal cortex of NBM-lesioned rats. Moreover, at this time a signif icant decrease in the density of granule neurons in the dentate gyrus accompanied by a reduced probability of dentate LTP induction were obs erved in both ibotenic acid- and radio-frequency-lesioned rats. These findings have shown that although NBM does not send direct cholinergic projections to the hippocampus, lesions of this cholinergic nucleus a re accompanied by delayed neurodegenerative changes involving the dent ate gyrus. This suggests the occurrence of indirect connections betwee n NBM and the hippocampus, the functional relevance of which should be explored.