The arborization pattern and postsynaptic targets of the GABAergic component of the basal forebrain projection to neo‐ and mesocortical areas have been studied by the combination of anterograde tracing and pre‐ and postembedding immunocytochemistry. Phaseolus vulgaris leucoagglutinin (PHAL) was iontophoretically delivered into the region of the diagonal band of Broca, with some spread of the tracer into the substantia innominata and ventral pallidum. A large number of anterogradely labelled varicose fibres were visualized in the cingulate and retrosplenial cortices, and a relatively sparse innervation was observed in frontal and occipital cortical areas. Most of the labelled axons were studded with large en passant varicosities (Type 1), whereas the others (Type 2) had smaller boutons often of the drumstick type. Type 1 axons were distributed in all layers of the mesocortex with slightly lower frequency in layers 1 and 4. In the neocortex, layer 4, and to a smaller extent upper layer 5 and layer 6 contained the largest number of labelled fibres, whereas only a few fibres were seen in the supragranular layers. Characteristic type 2 axons were very sparse but could be found in all layers. Most if not all boutons of PHAL‐labelled type 1 axons were shown to be GABA‐immunoreactive by immunogold staining for GABA, Altogether 73 boutons were serially sectioned and found to make symmetrical synaptic contacts mostly with dendritic shafts (66, 90% of total targets), cell bodies (6, 8.2% of total), and with one spine. All postsynaptic cell bodies, and the majority of the dendritic shafts (44, 60.3% of total targets) were immunoreactive for GABA. Thus at least 68.5% of the total targets were GABA‐positive, but the majority of the dendrites not characterized immunocytochemically for technical reasons (15.1%) also showed the fine structural characteristics of nonpyramidal neurons. The target interneurons included some of the somatostatin‐ and calbindin‐containing subpopulations, and a small number of parvalbumin‐containing neurons, as shown by double immunostaining for PHAL and calcium‐binding proteins or neuropeptides. We suggest that the innervation of inhibitory interneurons having extensive local axon arborizations may be a mechanism by which basal forebrain neurons–most notably those containing GABA–have a powerful global effect on the majority of principal cells in the entire cortical mantle.
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