The efferent projections of neurons in the white matter of different cortical areas of the adult rat.

Injection of Fast Blue into different cortical areas (frontal, parietal, anterior and posterior cingulate cortex) revealed that neurons in the white matter (interstitial neurons) give rise to association fibers which project mostly to the gray matter of the overlying cytoarchitectonic area, but which may extend also over different cytoarchitectonic areas. The rostrocaudal extent of the projecting axons was up to 1 mm in the frontal and parietal cortex, and up to 3.5 mm in the cingulate cortex. Concurrent processing for dihydronicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry showed that 70% of cortically projecting interstitial neurons were NADPH-d-positive. An analysis of neuronal morphology suggests that the FasT-Blue-labeled, NADPH-d-negative neurons may represent displaced pyramidal neurons of layer VIb; the Fast-Blue-labeled and NADPH-d-positive neurons have bipolar or multipolar dendritic trees, constituting a population of nonpyramidal interstitial neurons that project into the cortical gray matter.