Cell type-specific cholinergic innervation of hippocampal interneurons

Neural interactions involving the synaptic release of the neurotransmitter acetylcholine onto inhibitory interneurons are essential for regulating normal brain function but are implicated to be dysfunctional in many disease states, including Alzheimer’s disease (AD), Autism Spectrum Disorders (ASD), and epilepsy. Clarifying the relationship between cholinergic inputs and GABAergic inhibition functioning may be central to understanding the driving forces of neurological dysfunction. By elucidating cholinergic targeting of specific interneuron populations, we can gain insight into a variety of neurodegenerative disease states that impact connectivity between specific populations of cholinergic and GABAergic interneurons. In our study, transgenic mouse lines that express fluorescent protein labeling in interneuron subtypes were utilized, focusing on GABAergic cells expressing somatostatin(SOM) and parvalbumin (PV). Hippocampal brain slices were counterstained with the vesicular acetylcholine transporter (vAChT) antibody, which enables visualization of acetylcholine release sites. After immunostaining, slices were imaged on a confocal microscope. Image processing involved deconvolution, which corrected for light distortion and blurring, and quantitation of bouton numbers, bouton surface area, cell surface area, overall cholinergic bouton density, and cell layer. Consistent with targeting of cholinergic fibers to a subset of interneurons, our findings demonstrate that the level of cholinergic innervation on hippocampal GABAergic interneuron subtypes is heterogenous, depending on both cell type and cell location within the hippocampus. Because both SOM and PV hippocampal cell types are selectively reduced in human cases and mouse models of AD, we predict that GABAergic interneuron populations that are the most heavily innervated by cholinergic synapses will be selectively vulnerable in disease states