Cholinergic modulation amplifies the intrinsic oscillatory properties of CA1 hippocampal cholecystokinin-positive interneurons

Non-technical summary In the mammalian hippocampus, the neurotransmitter acetylcholine (ACh) promotes learning and memory storage. During sensory processing and learning, large ACh-dependent electrical oscillatory events are observed, which involve the synchronization of both inhibitory and excitatory neural circuits. While the actions of ACh are known on excitatory hippocampal circuits, its actions on specific inhibitory circuits are poorly understood. We show that two types of cholecystokinin-positive local circuit inhibitory interneuron, the so-called 'basket cells' and 'Schaffer collateral-associated' cells, which innervate separately the cell body and dendritic regions of principal cells, are modulated similarly by cholinergic receptor activation. In both cell types activation of their muscarinic receptors triggers a general increase of excitability and intrinsic oscillatory activity, and a more efficient engagement to slow network oscillations. Knowledge of how cholinergic neuromodulation acts on neurochemically identical but morphologically distinct inhibitory interneurons will allow us to understand the role played by this important neuromodulator during hippocampal-dependent tasks in vivo.Cholecystokinin (CCK)-containing interneuron subpopulations provide GABAergic inhibition to multiple surface domains of CA1 hippocampal pyramidal cells. CCK-basket cells (BCs) control AP initiation through perisomatic inhibition, whereas dendritically projecting Schaffer collateral-associated (SCAs) cells probably shape dendritic excitability and synaptic integration. We have shown previously that muscarinic receptor (mAChR) activation regulates firing properties and excitability of CCK-BCs; however, little is known about mAChR modulation of the related but anatomically distinct CCK-SCA population. Here, using whole-cell recordings and single-cell RT-PCR, we show that muscarine elicited a biphasic hyperpolarizing-depolarizing voltage response in CCK-SCAs, which was mediated by opposing actions of M1 and M3 mAChRs, respectively. In addition, like CCK-BCs, CCK-SCAs exhibited an M3-mediated increase in AP firing frequency and an afterdepolarization mediated synergistically by both M1 and M3 mAChRs. Spontaneous M3-mediated membrane potential oscillations (1-2 Hz) were observed in both CCK-SCAs and CCK-BCs. Using sinusoidal current injection we examined how mAChR activation of CCK interneurons alters intrinsic oscillatory properties and AP frequency preference. In CCK-BCs and CCK-SCAs, APs occurred preferentially at delta/theta frequencies (1-7 Hz) under control conditions. mAChR activation extended the AP phase-locking bandwidth into the theta and beta frequency range for CCK-SCAs and CCK-BCs respectively. This was accompanied by changes in both AP phase and precision. In conclusion, anatomically distinct CCK+ cells show similar changes in excitability, firing pattern activity and oscillatory preferences during cholinergic modulation suggesting that CCK+ interneurons probably act cooperatively to collectively synchronize the hippocampal network along the somatodendritic axis of the CA1 pyramidal cells during muscarinic receptor activation.