Projections from basal forebrain to prefrontal cortex comprise cholinergic, GABAergic and glutamatergic inputs to pyramidal cells or interneurons

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Abstract
The present study was undertaken to characterize the pre‐ and postsynaptic constituents of the basal forebrain (BF) projection to the prefrontal cortex in the rat, and determine whether it includes glutamatergic in addition to established γ‐aminobutyric acid (GABA)ergic and cholinergic elements. BF fibres were labelled by anterograde transport using biotin dextran amine (BDA) and dual‐stained for the vesicular transporter proteins (VTPs) for glutamate (VGluT), GABA (VGAT) or acetylcholine (VAChT). Viewed by fluorescence microscopy and estimated by stereology, proportions of BDA‐labelled varicosities were found to be stained for VGluT2 (and not VGluT1 or 3), VGAT or VAChT (representing, respectively, ∼15%, ∼52% and ∼19% within the infralimbic cortex). Each type was present in all, though commonly most densely in deep, cortical layers. Material was triple‐stained for postsynaptic proteins to examine whether BDA+VTP+ varicosities might form excitatory or inhibitory synapses, respectively, labelled by postsynaptic density‐95 kDA (PSD‐95) or gephyrin (Geph). Viewed by confocal microscopy, a majority of BDA+/VGluT2+ varicosities were found to be apposed to PSD‐95+ elements, and a majority of BDA+/VGAT+ varicosities to be apposed to Geph+ elements. Other series were triple‐stained for cell marker proteins to assess whether the varicosities contacted interneurons or pyramidal cells. Viewed by confocal microscopy, BDA‐labelled VGluT2+, VGAT+ and VAChT+ BF terminals were all found in contact with calbindin+ interneurons, whereas VGAT+ BF terminals were also seen in contact with parvalbumin+ interneurons and non‐phosphorylated neurofilament+ pyramidal cells. Through distinct glutamatergic, GABAergic and cholinergic projections, the BF can thus influence cortical activity in a diverse manner.

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