Conditions have been optimized for an immunohistochemical procedure for the localization of nerve growth factor receptor-containing cells in the brain. Using this immunohistochemical procedure, the normal morphology and distribution of the nerve growth factor receptor-containing neurons of the adult rat forebrain have been studied, and the findings compared with observations on the choline acetyltransferase-positive neurons present either in the immediately-adjacent sections or in the medially-divided half of the same section. Unlike in the peripheral nervous system, only neurons showed immunoreactivity to the nerve growth factor receptor in the brain. Both the nerve growth factor receptor-and choline acetyltransferase-immunoreactive cells appear to form a continuous anteroposterior band, which includes the olfactory tubercle, the medial septal nucleus, the vertical and horizontal limbs of the diagonal band and the basal nucleus. In each subdivision of the basal forebrain, the topographic organization, the localization, the intensity of the immunoreaction and the total cell number of nerve growth factor receptor- and of choline acetyltransferase-immunoreactive neurons were strikingly similar, indicating that nearly all nerve growth factor receptor-containing cells were cholinergic neurons. However, in the striatum, only about half the number of the choline acetyltransferase-positive cells showed immunopositive reactions to the nerve growth factor receptor, and, also, in the nerve growth factor receptor-containing neurons the intensity of the reaction product was much less than the choline acetyltransferase immunoreactivity. In the neurons of the basal forebrain nuclei, the choline acetyl-transferase immunoreaction product was uniformly distributed on the cell bodies, while the nerve growth factor receptor immunoreaction product was present also as intensely stained granules on the cell somata and the dendrites. The mean diameter and the mean cross-sectional area of the nerve growth factor receptor-containing neurons were least in the medial septal nucleus and were greatest in the basal nucleus, and showed a gradation in cell size going from the medial septal nucleus through the nucleus of the diagonal band and extending more posteriorly to the basal nucleus.
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