Basal forebrain neurons: Local circuits and their relationship to cortical activity with emphasis on the cholinergic NPY interactions

The basal forebrain (BF) is a heterogeneous structure located in the ventral aspect of the cerebral hemispheres. It contains cholinergic as well as different types of non-cholinergic corticopetal neurons and interneurons, including GABAergic and peptidergic cells. The BF constitutes an extrathalamic route to the cortex, and its activity is associated with an increase in cortical release of the neurotransmitter acetylcholine, concomitant with low voltage fast EEG activity (LVFA). However, the specific role of the different BF cell types has largely remained unknown due to the lack of chemical identification of the recorded neurons. Here it is shown that the firing rate of immunocytochemically identified cholinergic and parvalbumin-containing neurons increases during cortical LVFA while in contrast, in putative GABAergic neuropeptide Y (NPY) containing neurons increased firing is accompanied by cortical slow waves. The results further indicate that BF neurons exhibit a distinct temporal relationship to different EEG patterns and that neurochemically different neuronal populations may have distinct morphological features. A more dynamic interplay within the BF as well as between BF and cortical circuitries than previously proposed is suggested.; Single, juxtacellularly labeled (biocytin filled) neurons in combination with immunocytochemistry and retrograde tracing as well as traditional doubleimmunolabeling material were used both at the light and electron microscopic level to study the neural circuitry within the BF. Cholinergic neurons were found to exhibit cortically projecting axons and extensive local axon collaterals terminating on non-cholinergic, (possible GABAergic) neurons. Elaborate axon arbors confined to the BF region also originated from NPY BF neurons, some of which were interneurons. Terminals from local NPY neurons were found to contact local BF cholinergic neurons. The results allow the proposition that putative interneurons together with local collaterals from projection neurons contribute to regional integrative processing in the BF that may participate in selective functions, such as attention and cortical plasticity.