Topographic, non-collateralized basal forebrain projections to amygdala, hippocampus, and anterior cingulate cortex...

Projections of the basal forebrain magnocellular complex to the limbic telencephalon of the primate were studied by combining double-retrograde tracing with immunocytochemistry. Tracers were injected into anterior cingulate cortex and hippocampus or into hippocampus and amygdala. Retrogradely labeled populations of neurons were topographically arranged but intermingled peripherally. Double-labeled neurons, found only after amygdala-hippocampus injections, were very rare. Approximately 30% of hippocampopetal, 50-70% of amygdalopetal, and 50-90% of cingulopetal neurons were cholinergic; percentages varied among different regions of basal forebrain. These findings further support the concept of a system with a highly organized efferent circuitry.