Deletion of p75NTR enhances the cholinergic innervation pattern of the visual cortex

The cholinergic system is involved in cortical plasticity, attention, and learning. Within the visual cortex the cholinergic system seems to play a role in visual perception. The cholinergic neurons which project into the visual cortex are located in the basal forebrain. It has been shown that mice deficient for the low-affinity neurotrophin receptor p75NTR display increased numbers of cholinergic neurons in the basal forebrain and a denser cholinergic innervation of the hippocampus. This prompted us to analyze whether the cholinergic system is altered in adult p75NTR deficient mice. By analyzing the densities of cholinergic fibers within layer IV as well as within layer V of the visual cortex, we found that adult p75NTR deficient mice display increased cholinergic fiber densities. However, this increase was not accompanied by an increase in the density of local cholinergic neurons within the visual cortex. This indicates that the enhanced cholinergic innervation of the visual cortex is due to alteration of the cholinergic neurons located in the basal forebrain, projecting to the visual cortex. The increased cholinergic innervation of the visual cortex makes the p75NTR deficient mice an attractive model to study the necessity of the cholinergic system for the visual cortex.