[Non-cholinergic projections from basal forebrain to medial limbic cortex of rat]

Laminar distribution and synaptic organization of non-cholinergic projections to the medial limbic cortex from the basal forebrain (BF) were studied in rats. To eliminate BF cholinergic neurons selectively, a immunotoxin, 192IgG-saporin, was injected into lateral ventricle, and 7 days later, to label the remaining non-cholinergic elements, Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected into BF. In rats injected with the toxin, PHA-L labeled fibers, presumably originating from the remaining non-cholinergic neurons, were distributed in layers V-VI of the medial limbic cortex. Ultrastructural observation showed that the PHA-L labeled fibers were classified into two categories. One class of the axons provided with numerous passant boutons, whereas the other carried fewer passant varicosities. Both classes of terminals were found to make symmetrical synaptic contacts predominantly with dendritic shafts, together with a large number of degenerating terminals making asymmetrical synapses with dendritic spines, presumably cholinergic in nature. These results suggest that non-cholinergic projections from the BF exert a modulatory effect directly on cortical neurons in the medial limbic cortex.