Morphology of cortically projecting basal forebrain neurons in the rat as revealed by intracellular iontophoresis of horseradish peroxidase

The intracellular horseradish peroxidase technique was employed to study the morphology of basal forebrain neurons that were identified as cortically projecting by antidromic invasion from the cerebral cortex. Four neurons were examined in detail; they were located at different rostrocaudal levels within the basal forebrain. Their somata were large, 30-50 μm in longest dimension, and gave rise to three to eight primary dendrites, which ramified into third- to fifth-order dendrites. The longest observed dendrite in each neuron terminated at a distance of 600-900 μm from the soma. The sizes of soma and dendritic field of the two most rostrally located cells were smaller than those of the other two cells located more caudally. Dendritic spines were seen in all four cortically projecting basal forebrain neurons. Spines had shafts of variable lengths and usually had spherical or elongated heads. The density of spines varied among the four neurons; one neuron, a type II cortically projecting basal forebrain neuron as defined physiologically by Reiner et al.,³⁷ had a much greater number of dendritic spines than the other three neurons, which were type I neurons. No somatic spines were observed. Presumptive axons were identified in three of the four cortically projecting basal forebrain neurons. These axons originated from either the soma or a primary dendrite, and two of them gave off local collaterals, which displayed occasional bouton-like swellings. The above observations confirm and extend previous findings that cortically projecting neurons in the basal forebrain are large multipolar cells, and provide evidence to support the conclusion that these cells, although somewhat variable in size, generally have extensive dendrites which display frequent spines.