Cholinergic neuromodulation of hippocampal circuitry promotes network oscillations and facilitates learning and memory through cellular actions on both excitatory and inhibitory circuits. Despite widespread recognition that neurochemical content discriminates between functionally distinct interneuron populations, there has been no systematic examination of whether neurochemically distinct interneuron classes undergo differential cholinergic neuromodulation in the hippocampus. Using GFP transgenic mice that enable the visualization of perisomatically targeting parvalbumin-positive (PVϩ) or cholecystokinin-positive (CCKϩ) basket cells (BCs), we tested the hypothesis that neurochemically distinct interneuron populations are differentially engaged by muscarinic acetylcholine receptor (mAChR) activation. Cholinergic fiber activation revealed that CCK BCs were more sensitive to synaptic release of ACh than PV BCs. In response to depolarizing current steps, mAChR activation of PV BCs and CCK BCs also elicited distinct cholinergic response profiles, differing in mAChR-induced changes in action potential (AP) waveform, firing frequency, and intrinsic excitability. In contrast to PV BCs, CCK BCs exhibited a mAChR-induced afterdepolarization (mADP) that was frequency and activity-dependent. Pharmacological, molecular, and loss-of-function data converged on the presence of M3 mAChRs in distinguishing CCK BCs from PV BCs. Firing frequency of CCK BCs was controlled through M3 mAChRs but PV BC excitability was altered solely through M1 mAChRs. Finally, upon mAChR activation, glutamatergic transmission enhanced cellular excitability preferentially in CCK BCs but not in PV BCs. Our findings demonstrate that cell type-specific cholinergic specializations are present on neurochemically distinct interneuron subtypes in the hippocampus, revealing an organizing principle that cholinergic neuromodulation depends critically on neurochemical identity.

SUMMARY Because molecular mechanisms underlying refractory focal epilepsy are poorly defined, we performed transcriptome analysis on human epileptogenic tissue. Compared with controls, expression of Circadian Locomotor Output Cycles Kaput (CLOCK) is decreased in epileptogenic tissue. To define the function of CLOCK, we generated and tested the Emx-Cre; Clockflox/flox and PV-Cre; Clockflox/flox mouse lines with targeted deletions of the Clock gene in excitatory and parvalbumin (PV)-expressing inhibitory neurons, respectively. The Emx-Cre; Clockflox/flox mouse line alone has decreased seizure thresholds, but no laminar or dendritic defects in the cortex. However, excitatory neurons from the Emx-Cre; Clockflox/flox mouse have spontaneous epileptiform discharges. Both neurons from Emx-Cre; Clockflox/flox mouse and human epileptogenic tissue exhibit decreased spontaneous inhibitory post-synaptic currents. Finally, video-EEG of Emx-Cre; Clockflox/flox mice reveals epileptiform discharges during sleep and also seizures arising from sleep. Altogether, these data show that disruption of CLOCK alters cortical circuits and may lead to generation of focal epilepsy.

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.Abstract 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 sync...

Many neurological disorders, including neurodevelopmental disorders, report hypersynchrony of neuronal networks. These alterations in neuronal synchronization suggest a link to the function of inhibitory interneurons. In Fragile X Syndrome (FXS), it has been reported that altered synchronization may underlie hyperexcitability, cognitive dysfunction and provide a link to the increased incidence of epileptic seizures. Therefore, understanding the roles of inhibitory interneurons and how they control neuronal networks is of great importance in studying neurodevelopmental disorders such as FXS. Here, we present a review of how interneuron populations and inhibition are important contributors to the loss of excitatory/inhibitory balance seen in hypersynchronous and hyperexcitable networks from neurodevelopmental disorders, and specifically in FXS.

Successful completion of sensory decision-making requires focusing on relevant stimuli, adequate signal/noise ratio for stimulus discrimination, and stimulus valence evaluation. Different brain regions are postulated to play a role in these computations; however, evidence suggests that sensory and decision-making circuits are required to interact through a common neuronal pathway to elicit a context-adequate behavioral response. Recently, the basal forebrain (BF) region has emerged as a good candidate, since its heterogeneous projecting neurons innervate most of the cortical mantle and sensory processing circuits modulating different aspects of the sensory decisionmaking process. Moreover, evidence indicates that the BF plays an important role in attention and in fast modulation of neuronal activity that enhance visual and olfactory sensory perception. Here, we study in awake mice the involvement of BF in initiation and completion of trials in a reward-driven olfactory detection task. Using tetrode recordings, we find that BF neurons (including cholinergics) are recruited during sensory discrimination, reward, and interestingly slightly before trial initiation in successful discrimination trials. The precue neuronal activity was correlated with animal performance, indicating that this circuit could play an important role in adaptive context-dependent behavioral responses.

In this study, we analyzed the impact that spontaneous seizures might have on the plasma membrane expression, composition and function of GABAA receptors (GABAARs). For this, tissue of chronically epileptic rats was collected within 3 hours of seizure occurrence (≤3 hours group) or at least 24 hours after seizure occurrence (≥24 hours group). A retrospective analysis of seizure frequency revealed that selecting animals on the bases of seizure proximity also grouped animals in terms of overall seizure burden with a higher seizure burden observed in the ≤3 hours group. A biochemical analysis showed that although animals with more frequent/recent seizures (≤3 hours group) had similar levels of GABAAR at the plasma membrane they showed deficits in inhibitory neurotransmission. In contrast, tissue obtained from animals experiencing infrequent seizures (≥24 hours group) had increased plasma membrane levels of GABAAR and showed no deficit in inhibitory function. Together, our findings offer an initial insight into the molecular changes that might help to explain how alterations in GABAAR function can be associated with differential seizure burden. Our findings also suggest that increased plasma membrane levels of GABAAR might act as a compensatory mechanism to more effectively maintain inhibitory function, repress hyperexcitability and reduce seizure burden. This study is an initial step towards a fuller characterization of the molecular events that trigger alterations in GABAergic neurotransmission during chronic epilepsy.

Fragile X Syndrome (FXS) is a neurodevelopmental disorder characterized by intellectual disability, autism spectrum disorders (ASDs), and anxiety disorders. The disruption in the function of the FMR1 gene results in a range of alterations in cellular and synaptic function. Previous studies have identified dynamic alterations in inhibitory neurotransmission in early postnatal development in the amygdala of the mouse model of FXS. However, little is known about how these changes alter microcircuit development and plasticity in the lateral amygdala (LA). Using whole-cell patch clamp electrophysiology, we demonstrate that principal neurons (PNs) in the LA exhibit hyperexcitability with a concomitant increase in the synaptic strength of excitatory synapses in the BLA. Further, reduced feed-forward inhibition appears to enhance synaptic plasticity in the FXS amygdala. These results demonstrate that plasticity is enhanced in the amygdala of the juvenile Fmr1 KO mouse and that E/I imbalance may underpin anxiety disorders commonly seen in FXS and ASDs.

Information processing and transfer within cortical circuits requires precise spatiotemporal coordination of excitatory principal cell activity by a relatively small population of inhibitory interneurons that exhibit remarkable anatomical, molecular and electrophysiological diversity. One subtype of interneuron, the cholecystokinin-expressing basket cell (CCKBC), is particularly well suited to integrate and impart emotional features of an animal's physiological state to principal cell entrainment through the inhibitory network as CCKBCs are highly susceptible to neuromodulation by local and subcortically generated signals commonly associated with 'mood' such as cannabinoids, serotonin and acetylcholine. Here we briefly review recent studies that have elucidated the cellular mechanisms underlying cholinergic regulation of CCKBCs.