Cholinergic neurons in the basal forebrain of the cat have direct projections to the sensorimotor cortex

Abstract

Previous studies showed that projections from the cells in the substantia innominata to the sensorimotor cortex exist in the monkey and rat and that this group of cortical afferent fibers utilizes acetylcholine as a neurotransmitter. Because similar studies have not been made in the cat, the following study was undertaken to analyze the extent and distribution of this projection from the basal forebrain. Horseradish peroxidase (HRP) injections were made into the anterior and posterior sigmoid gyri 24 h before, and diisopropyl fluorophosphate was injected intramuscularly 3 h before the anesthetized cats were perfused. Brain sections from these cats were incubated in solutions to simultaneously detect retrogradely transported HRP and acetylcholinesterase (AChE) reaction product within the somata. The cell bodies that contained reaction product for both HRP and AChE were mostly large (25 to 30 μm) and multipolar. These double-labeled cells were located in (i) the nucleus of the diagonal band in rostral sections, (ii) the globus pallidus, entopeduncular nucleus, and substantia innominata at the level of the anterior commissure, and (iii) the lateral hypothalamus in the most caudal sections. Many of these sites corresponded to that for the basal nucleus of Meynert, an aggregation of large multipolar neurons scattered throughout the basal forebrain. Although the presence of AChE within a cell does not define a cholinergic neuron, recent studies indicated that its presence is a requirement for this neurotransmitter. These data together with biochemical and immunocytochemical data indicate that a cholinergic projection to the sensorimotor cortex of cats arises from the basal forebrain. This pathway may play a vital role in memory and cognition.