CHOLINERGIC BASAL FOREBRAIN PROJECTIONS TO NITRIC-OXIDE SYNTHASE-CONTAINING NEURONS IN THE RAT CEREBRAL-CORTEX

Stimulation of basal forebrain neurons elicits regional cerebral blood flow increases which are reportedly mediated by acetylcholine and nit ric oxide. However, the modality of interaction between these two medi ators remains unclear. Particularly, little is known about the source, i.e. endothelial, glial and/or neuronal, of the potent gaseous vasodi lator nitric oxide. In the present study, we examined, by double immun ocytochemical labelling of nitric oxide synthase and choline acteyltra nsferase at the light and electron microscopic level, the existence of morphological relationships between cortical nitric oxide synthase-co ntaining neurons and cholinergic cells or nerve fibres. Using anterogr ade tract tracing and selective basal forebrain lesions, we further in vestigated the origin of the cholinergic input to cortical nitric oxid e synthase neurons. The results confirm that cortical nitric oxide syn thase-immunoreactive neurons are often associated with the local micro vascular bed, show that intracortical neurons immunostained for nitric oxide synthase and choline acetyltransferase belong to two distinct n euronal populations and, further, that a subset of nitric oxide syntha se-containing cell bodies and their proximal dendrites receive a choli nergic input which originates primarily from basalocortical projection s. Altogether, these findings suggest that cholinergic basal forebrain neurons could increase cortical blood flow partly via a local nitric oxide relay neuron whereby the freely diffusing gas would be the direc t smooth muscle vasodilator agent. It is concluded that this interacti on might contribute to the complex relationships between the basal for ebrain and the cortical microcirculation, interactions which result in fine regulation of cortical perfusion. (C) 1997 IBRO. Published by El sevier Science Ltd.