Topographic, non-collateralized basal forebrain projections to amygdala, hippocampus, and anterior cingulate cortex in the rhesus monkey

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.