TDK conference 2020

Areas of the cerebral cortex, such as the memory encoding hippocampus, are highly dependent on the control function of the excitatory and inhibitory pathways from the basal forebrain regions. The basal forebrain consists of the medial septum (MS) and the horizontal limbs of the diagonal band of Broca (HDB), among other areas. Their ascending fibers innervate cortical areas with different cognitive functions, and they are fundamental for the functioning of attention, motivation, and memory processes. Although it is widely accepted that cholinergic innervation of the hippocampus is provided exclusively by the MS, we discovered that it was also innervated by the hitherto unrecognized cholinergic and non-cholinergic cells of the HDB. We examined these cells by combining double retrograde tracing with immunohistochemical experiments as well. We found that HDB contains cholinergic and non-cholinergic cells that innervate the hippocampal sub-regions, where MS cells arborize only sparsely. In addition, HDB collaterals also innervate the ventral and dorsal hippocampus and the prefrontal cortical areas, which all have different functions. Our results may shed new light on the cholinergic regulation of hippocampal learning and memory processes and may provide new insights into neurodegenerative diseases, in which the cholinergic system is strongly involved, e.g. the Alzheimer’s disease.