REGIONAL STUDY OF THE COLOCALIZATION OF NEURONAL NITRIC-OXIDE SYNTHASE WITH MUSCARINIC RECEPTORS IN THE RAT CEREBRAL-CORTEX

There is increasing evidence that nitric oxide is an important molecul ar messenger involved in a wide variety of biological processes includ ing the regulation of the cerebral circulation. For instance, it has b een implicated in the vascular response to nucleus basalis magnocellul aris stimulation, a structure which is widely recognized as the predom inant source of cholinergic fibres projecting to the neocortex. The pr esent investigation was carried out to determine if muscarinic recepto rs are present on cortical neurons expressing neuronal nitric oxide sy nthase (nitric oxide-producing enzyme). To this aim, double labelling of both neuronal nitric oxide synthase/vessels and neuronal nitric oxi de synthase/muscarinic receptors was performed on free-floating cryose ctions obtained from rat brain. The observations were made by confocal laser scanning microscopy. The double labelling of neuronal nitric ox ide synthase with the arterioles demonstrated the presence of nitroxid ergic fibres in the wall of intraparenchymal vessels. A rich network o f nitroxidergic fibres independent of the vessels was also seen in the parenchyma. Since the maximal surface of a square of tissue without a ny nitroxidergic fibres corresponded to 1400 +/- 105 mu m(2), the dist ance separating any cortical point from its closest neuronal nitric ox ide synthase-positive fibre was never higher than 25 mu m (half diagon al of square). According to models of the diffusional spread of nitric oxide, it is likely that nitric oxide can reach the whole cortical vo lume. Our results on the regional study of neuronal nitric oxide synth ase/muscarinic receptors showed a high density of neuronal nitric oxid e synthase-positive neurons principally in the frontal and perirhinal cortices and a low density in the occipital cortex. These data fit wel l with the known pattern of cortical projections from the nucleus basa lis magnocellularis as revealed by anterogradely transported markers. The double labelling showed that about 10% of neuronai nitric oxide sy nthase-positive neurons were co-localized with muscarinic receptors in the frontoparietal cortex. In agreement with previous papers, the vas cular innervation by nitroxidergic neuronal processes was often found to lie near the branching points of arterioles. Such localization allo ws neuronal nitric oxide synthase-positive neurons an extensive contro l of the vascular tree without requiring a large number of neuronal co mmands. Therefore, despite the low level of neuronal nitric oxide synt hase/muscarinic receptor co-localization, this neuronal subpopulation could represent a possible relay implicated in the vascular effects of the nucleus basalis magnocellularis.