CHOLINERGIC MODULATION OF CELLULAR RESONANCE IN NON-HUMAN PRIMATE HIPPOCAMPUS.

Acetylcholine modulates the network physiology of the hippocampus, a crucial brain structure that supports cognition and memory formation in mammals . In this and adjacent regions, synchronized neuronal activity within theta-band oscillations (4-10Hz) is correlated with attentive processing that leads to successful memory encoding . Acetylcholine facilitates the hippocampus entering a theta oscillatory regime and modulates the temporal organization of activity within theta oscillations . Unlike rodents that exhibit constant theta oscillations during movement and exploration, primates only manifest theta oscillations in transient bouts during periods of acute attention-despite conserved hippocampal anatomy . The phasic nature of primate theta oscillations and their susceptibility to muscarinic antagonists , suggest that acetylcholine afferents acutely modulate local circuitry, resulting in a temporary shift in hippocampal rhythmic dynamics. However, we lack a mechanistic understanding that links cellular physiology to emergent theta-rhythmic network dynamics. We explored the hypothesis that acetylcholine induces a distinct modulation of cellular properties to facilitate synchronization within the theta band in non-human primate neurons. Here we show that non-human primate neurons from the CA1 region of monkey hippocampus are not homogeneous in their voltage response to inputs of varying frequencies, a phenomenon known as cellular resonance . We classified these neurons as 'resonant' or 'non-resonant'. Under the influence of carbachol, these two classes of neurons become indistinguishable in their resonance, suggesting that acetylcholine transiently creates a homogeneous susceptibility to inputs within the theta range. This change is mediated by metabotropic acetylcholine receptors that enhance sag potentials, indicating that acetylcholine acts on principal neurons to modulate Hyperpolarization-activated Cyclic Nucleotide-gated channels. Our results reveal a mechanism through which acetylcholine can rapidly modulate intrinsic properties of primate hippocampal neurons to facilitate synchronization within theta-rhythmic circuits, providing insight into the unique features of primate hippocampal physiology.