Cholinergic stimulation modulates the functional composition of CA3 cell types in the hippocampus.

The functional heterogeneity of hippocampal CA3 pyramidal neurons has emerged as a key aspect of circuit function. Here, we explored the effects of long-term cholinergic activity on the functional heterogeneity of CA3 pyramidal neurons in organotypic slices obtained from male rat brains. Application of agonists to either acetylcholine receptors (AChRs) generally, or muscarinic AChRs (mAChRs) specifically, induced robust increases in network activity in the low-gamma range. Prolonged AChR stimulation for 48 hrs uncovered a population of hyperadapting CA3 pyramidal neurons that typically fired a single, early action potential in response to current injection. Although these neurons were present in control networks, their proportions were dramatically increased following long-term cholinergic activity. Characterised by the presence of a strong M-current, the hyperadaptation phenotype was abolished by acute application of either M-channel antagonists or the re-application of AChR agonists. We conclude that long-term mAChR activation modulates the intrinsic excitability of a subset of CA3 pyramidal cells, uncovering a highly plastic cohort of neurons that are sensitive to chronic ACh modulation. Our findings provide evidence for the activity-dependent plasticity of functional heterogeneity in the hippocampus. Significance statement: The large heterogeneity of neuron types in the brain, each with its own specific functional properties, provide the rich cellular tapestry needed to account for the vast diversity of behaviours. By studying the functional properties of neurons in the hippocampus, a region of the brain involved in learning and memory, we find that exposure to the neuromodulator acetylcholine can alter the relative number of functionally-defined neuron types. Our findings suggest that the heterogeneity of neurons in the brain is not a static feature but can be modified by the ongoing activity of the circuits they belong to.