The cholinergic nuclei of the basal forebrain of the rat: normal structure, development and experimentally induced degeneration.

The normal morphology and distribution of the cholinergic neurones of the basal forebrain of the rat have been studied qualitatively and quantitatively after staining immunohistochemically with a monoclonal antibody to choline acetyl transferase (ChAT). This was done in order to provide an adequate control for the changes found in these cells on both sides of the brain in the experimental investigation of the reaction of the cells to damage of their axons. The cholinergic cells form a more or less continuous anteroposterior band, but they can be subdivided into distinct nuclear groups on the basis of the size and form of the cell bodies and dendrites, their position and arrangement. these nuclei conform closely to previous descriptions of Nissl-stained material: the medial septal nucleus, the vertical and horizontal nuclei of the diagonal band and the basal nucleus. Quantitative measurements of the cross-sectional areas of the cells in the different nuclei confirmed the conclusions drawn from the qualitative examination. Measurements of the ChAT cells at different ages showed that in all nuclei they are significantly larger in size in infancy than in the adult, and they shrink to the mature size by 46 days. The cells in the various cholinergic nuclei show distinctly different reactions to damage of their terminal axonal fields. After removal of a large part of the neocortex by removal of the overlying pia-arachnoid mater the cells in the basal nucleus in the operated hemisphere underwent retrograde cellular degeneration, being swollen and paler-staining up to 14 days, and thereafter shrinking by 20-30% (as compared with those in the brains of age- and sex-matched littermate controls). The degree of shrinkage was appreciably greater when the animals were operated upon at the neonate stage. No cell loss was found, qualitatively or quantitatively, in the basal nucleus. After removal of the hippocampus there is marked loss of cholinergic neurones in the medial septal nucleus and in the vertical nucleus of the diagonal band, and with severe shrinkage of the remaining cells. Removal of the olfactory bulb results in only slight shrinkage of the cells, and no cell loss, in the horizontal nucleus of the diagonal band.(ABSTRACT TRUNCATED AT 400 WORDS)