Stimulation of the muscarinic receptor M4 activates quiescent neural precursor cells and ameliorates medial septum cholinergic lesion-induced impairments in adult hippocampal neurogenesis

Cholinergic signaling plays a crucial role in the regulation of adult hippocampal neurogenesis and hippocampus-dependent cognitive and mood-related functions. However, the contribution of basal forebrain medial septum (MS) and diagonal band of Broca (DBB) cholinergic neurons that innervate the hippocampus and the identity of the cholinergic receptor(s) that regulate the production and maturation of new neurons are not completely understood. Using a targeted, selective ablation approach, we show that MS/DBB cholinergic neurons support both the survival and morphological maturation of adult-born neurons in the mouse hippocampus. We demonstrate that the muscarinic acetycholine receptor subtype M4 (M4 mAChR) is expressed on a population of quiescent neural precursor cells (NPCs) and that its pharmacological stimulation via intra-hippocampal or systemic administration of M4-selective modulators leads to their activation, thereby enhancing neurogenesis in vivo . Furthermore, we show that the activation of M4 mAChR-expressing quiescent NPCs ameliorates the MS/DBB cholinergic lesion-induced decrease in hippocampal neurogenesis. In contrast, the impairment in the morphological maturation of adult-born neurons due to MS/DBB cholinergic neuron loss is further exacerbated by the systemic administration of an M4-selective allosteric potentiator. These findings reveal novel and stage-specific roles of cholinergic signaling in regulating adult hippocampal neurogenesis. They also uncouple the positive role of selective M4 potentiators in enhancing the production of new neurons from the M4-induced inhibition of their morphological maturation, at least in the context of cholinergic dysfunction. Significance statement Cholinergic signaling plays an important role in the regulation of adult hippocampal neurogenesis and cognitive function, with impairments in these processes reported as early pathogenic events in age-related dementia. Here, we uncover the presence of cholinergic-responsive hippocampal precursor cells that are are directly activated by selective stimulation of the muscarinic receptor subtype M4. Furthermore, M4-mediated stimulation rescues the decrease in the level of hippocampal neurogenesis following the sepal-hippocampal cholinergic neuron loss. We also reveal that septal-hippocampal cholinergic dysfunction impairs the structural maturation of hippocampal adult-born neurons, an effect which is further exacerbated by M4 receptor modulators. These findings reveal stage-specific roles of cholinergic signaling in regulating functionally relevant adult hippocampal neurogenesis. ### Competing Interest Statement The authors have declared no competing interest.