Mechanisms generating dual-component nicotinic EPSCs in cortical interneurons

Focal acetylcholinergic modulation of the human midcingulo-insular network during attention: Meta-analytic neuroimaging and behavioral evidence

The basal forebrain cholinergic neurons provide acetylcholine to the cortex via large projections. Recent molecular imaging work in humans indicates that the cortical cholinergic innervation is not uniformly distributed, but rather may disproportionately innervate cortical areas relevant to supervisory attention. In this study, we therefore reexamined the spatial relationship between acetylcholinergic modulation and attention in the human cortex using meta-analytic strategies targeting both pharmacological and non-pharmacological neuroimaging studies. We found that pharmaco-modulation of acetylcholine evoked both increased activity in the anterior cingulate and decreased activity in the opercular and insular cortex. In large independent meta-analyses of non-pharmacological neuroimaging research, we demonstrate that during attentional engagement these cortical areas exhibit (1) task-related co-activation with the basal forebrain, (2) task-related co-activation with one another, and (3) spatial overlap with dense cholinergic innervations originating from the basal forebrain, as estimated by multimodal positron emission tomography and magnetic resonance imaging. Finally, we provide meta-analytic evidence that pharmaco-modulation of acetylcholine also induces a speeding of responses to targets with no apparent tradeoff in accuracy. In sum, we demonstrate in humans that acetylcholinergic modulation of midcingulo-insular hubs of the ventral attention/salience network via basal forebrain afferents may coordinate selection of task relevant information, thereby facilitating cognition and behavior.

Synaptic-like axo-axonal transmission from striatal cholinergic interneurons onto dopaminergic fibers

Transmission from striatal cholinergic interneurons (CINs) controls dopamine release through nicotinic acetylcholine receptors (nAChRs) on dopaminergic axons. Anatomical studies suggest that cholinergic terminals signal predominantly through non-synaptic volume transmission. However, the influence of cholinergic transmission on electrical signaling in axons remains unclear. We examined axo-axonal transmission from CINs onto dopaminergic axons using perforated-patch recordings, which revealed rapid spontaneous EPSPs with properties characteristic of fast synapses. Pharmacology showed that axonal EPSPs (axEPSPs) were mediated primarily by high-affinity α6-containing receptors. Remarkably, axEPSPs triggered spontaneous action potentials, suggesting that these axons perform integration to convert synaptic input into spiking, a function associated with somatodendritic compartments. We investigated the cross-species validity of cholinergic axo-axonal transmission by recording dopaminergic axons in macaque putamen and found similar axEPSPs. Thus, we reveal that synaptic-like neurotransmission underlies cholinergic signaling onto dopaminergic axons, supporting the idea that striatal dopamine release can occur independently of somatic firing to provide distinct signaling.

Cell-Type Specific Neuromodulation of Excitatory and Inhibitory Neurons via Muscarinic Acetylcholine Receptors in Layer 4 of Rat Barrel Cortex

The neuromodulator acetylcholine (ACh) plays an important role in arousal, attention, vigilance, learning and memory. ACh is released during different behavioural states and affects the brain microcircuit by regulating neuronal and synaptic properties. Here, we investigated how a low concentration of ACh (30 μM) affects the intrinsic properties of electrophysiologically and morphologically identified excitatory and inhibitory neurons in layer 4 (L4) of rat barrel cortex. ACh altered the membrane potential of L4 neurons in a heterogeneous manner. Nearly all L4 regular spiking (RS) excitatory neurons responded to bath-application of ACh with a M4 muscarinic ACh receptor-mediated hyperpolarisation. In contrast, in the majority of L4 fast spiking (FS) and non-fast spiking (nFS) interneurons 30 μM ACh induced a depolarisation while the remainder showed a hyperpolarisation or no response. The ACh-induced depolarisation of L4 FS interneurons was much weaker than that in L4 nFS interneurons. There was no clear difference in the response to ACh for three morphological subtypes of L4 FS interneurons. However, in four morpho-electrophysiological subtypes of L4 nFS interneurons, VIP+-like interneurons showed the strongest ACh-induced depolarisation; occasionally, even action potential firing was elicited. The ACh-induced depolarisation in L4 FS interneurons was exclusively mediated by M1 muscarinic ACh receptors; in L4 nFS interneurons it was mainly mediated by M1 and/or M3/5 muscarinic ACh receptors. In a subset of L4 nFS interneurons, a co-operative activation of muscarinic and nicotinic ACh receptors was also observed. The present study demonstrates that low-concentrations of ACh affect different L4 neuron types in a cell-type specific way. These effects result from a specific expression of different muscarinic and/or nicotinic ACh receptors on the somatodendritic compartments of L4 neurons. This suggests that even at low concentrations ACh may tune the excitability of L4 excitatory and inhibitory neurons and their synaptic microcircuits differentially depending on the behavioural state during which ACh is released.

α3β4 ∗ Nicotinic Acetylcholine Receptors Strongly Modulate the Excitability of VIP Neurons in the Mouse Inferior Colliculus

The inferior colliculus (IC), the midbrain hub of the central auditory system, receives extensive cholinergic input from the pontomesencephalic tegmentum. Activation of nicotinic acetylcholine receptors (nAChRs) in the IC can alter acoustic processing and enhance auditory task performance. However, how nAChRs affect the excitability of specific classes of IC neurons remains unknown. Recently, we identified vasoactive intestinal peptide (VIP) neurons as a distinct class of glutamatergic principal neurons in the IC. Here, in experiments using male and female mice, we show that cholinergic terminals are routinely located adjacent to the somas and dendrites of VIP neurons. Using whole-cell electrophysiology in brain slices, we found that acetylcholine drives surprisingly strong and long-lasting excitation and inward currents in VIP neurons. This excitation was unaffected by the muscarinic receptor antagonist atropine. Application of nAChR antagonists revealed that acetylcholine excites VIP neurons mainly via activation of α₃β₄^∗ nAChRs, a nAChR subtype that is rare in the brain. Furthermore, we show that acetylcholine excites VIP neurons directly and does not require intermediate activation of presynaptic inputs that might express nAChRs. Lastly, we found that low frequency trains of acetylcholine puffs elicited temporal summation in VIP neurons, suggesting that in vivo-like patterns of cholinergic input can reshape activity for prolonged periods. These results reveal the first cellular mechanisms of nAChR regulation in the IC, identify a functional role for α₃β₄^∗ nAChRs in the auditory system, and suggest that cholinergic input can potently influence auditory processing by increasing excitability in VIP neurons and their postsynaptic targets.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Distinct proportions of cholinergic neurons in the rat prepositus hypoglossi nucleus according to their cerebellar projection targets

Cerebellar functions are modulated by cholinergic inputs, the density of which varies among cerebellar regions. Although the prepositus hypoglossi nucleus (PHN), a brainstem structure involved in controlling gaze holding, is known as one of the major sources of these cholinergic inputs, the proportions of cholinergic neurons in PHN projections to distinct cerebellar regions have not been quantitatively analyzed. In this study, we identified PHN neurons projecting to the cerebellum by applying retrograde labeling with dextran-conjugated Alexa 488 in choline acetyltransferase (ChAT)-tdTomato transgenic rats and compared the proportion of cholinergic PHN neurons in the PHN projections to four different regions of the cerebellum, namely the flocculus (FL), the uvula and nodulus (UN), lobules III-V in the vermis (VM), and the hemispheric paramedian lobule and crus 2 (PC). In the PHN, the percentage of cholinergic PHN neurons was lower in the projection to the FL than in the projection to the UN, VM or PC. Preposito-cerebellar neurons, except for preposito-FL neurons, included different proportions of cholinergic neurons at different rostrocaudal positions in the PHN. These results suggest that cholinergic PHN neurons project to not only the vestibulocerebellum but also the anterior vermis and posterior hemisphere and that the proportion of cholinergic neurons among projection neurons from the PHN differs depending on cerebellar target areas and the rostro-caudal regions of the PHN. This study provides insights regarding the involvement of cerebellar cholinergic networks in gaze holding.

Rapid multi-directed cholinergic transmission in the central nervous system

In many parts of the central nervous system, including the retina, it is unclear whether cholinergic transmission is mediated by rapid, point-to-point synaptic mechanisms, or slower, broad-scale 'non-synaptic' mechanisms. Here, we characterized the ultrastructural features of cholinergic connections between direction-selective starburst amacrine cells and downstream ganglion cells in an existing serial electron microscopy data set, as well as their functional properties using electrophysiology and two-photon acetylcholine (ACh) imaging. Correlative results demonstrate that a 'tripartite' structure facilitates a 'multi-directed' form of transmission, in which ACh released from a single vesicle rapidly (~1 ms) co-activates receptors expressed in multiple neurons located within ~1 µm of the release site. Cholinergic signals are direction-selective at a local, but not global scale, and facilitate the transfer of information from starburst to ganglion cell dendrites. These results suggest a distinct operational framework for cholinergic signaling that bears the hallmarks of synaptic and non-synaptic forms of transmission.

Cell-type-specific nicotinic input disinhibits mouse barrel cortex during active sensing

Fast synaptic transmission relies upon the activation of ionotropic receptors by neurotransmitter release to evoke postsynaptic potentials. Glutamate and GABA play dominant roles in driving highly dynamic activity in synaptically connected neuronal circuits, but ionotropic receptors for other neurotransmitters are also expressed in the neocortex, including nicotinic receptors, which are non-selective cation channels gated by acetylcholine. To study the function of non-glutamatergic excitation in neocortex, we used two-photon microscopy to target whole-cell membrane potential recordings to different types of genetically defined neurons in layer 2/3 of primary somatosensory barrel cortex in awake head-restrained mice combined with pharmacological and optogenetic manipulations. Here, we report a prominent nicotinic input, which selectively depolarizes a subtype of GABAergic neuron expressing vasoactive intestinal peptide leading to disinhibition during active sensorimotor processing. Nicotinic disinhibition of somatosensory cortex during active sensing might contribute importantly to integration of top-down and motor-related signals necessary for tactile perception and learning.

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Acetylcholine is released in the mouse barrel cortex during active whisker sensing

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Acetylcholine depolarizes inhibitory cells expressing vasoactive intestinal peptide

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Excitation of vasoactive intestinal peptide-expressing neurons causes disinhibition

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Cholinergic-driven disinhibition could gate sensorimotor integration and plasticity

Nicotinic receptor activation induces NMDA receptor independent long-term potentiation of glutamatergic signalling in hippocampal oriens interneurons

KEY POINTS

Long-term potentiation of glutamatergic transmission to hippocampal interneurons in stratum oriens does not require NMDA receptors and the induction mechanisms are incompletely understood. Extracellular stimulation, conventionally used to monitor synaptic strength and induce long-term potentiation (LTP), does not exclusively recruit glutamatergic axons. We used optogenetic stimulation of either glutamatergic or cholinergic afferents to probe the relative roles of different signalling mechanisms in LTP induction. Selective stimulation of cholinergic axons was sufficient to induce LTP, which was prevented by chelating postsynaptic Ca2+ or blocking nicotinic receptors. The present study adds nicotinic receptors to the list of sources of Ca2+ that induce NMDA receptor independent LTP in hippocampal oriens interneurons.

ABSTRACT

Many interneurons located in stratum oriens of the rodent hippocampus exhibit a form of long-term potentiation (LTP) of glutamatergic transmission that does not depend on NMDA receptors for its induction but, instead, requires Ca2+ -permeable AMPA receptors and group I metabotropic glutamate receptors. A role for cholinergic signalling has also been reported. However, electrical stimulation of presynaptic axons, conventionally used to evoke synaptic responses, does not allow the relative roles of glutamatergic and cholinergic synapses in the induction of LTP to be distinguished. Here, we show that repetitive optogenetic stimulation confined to cholinergic axons is sufficient to trigger a lasting potentiation of glutamatergic signalling. This phenomenon shows partial occlusion with LTP induced by electrical stimulation, and is sensitive to postsynaptic Ca2+ chelation and blockers of nicotinic receptors. ACh release from cholinergic axons is thus sufficient to trigger heterosynaptic potentiation of glutamatergic signalling to oriens interneurons in the hippocampus.

Endogenous Acetylcholine and Its Modulation of Cortical Microcircuits to Enhance Cognition

Acetylcholine regulates the cerebral cortex to sharpen sensory perception and enhance attentional focus. The cellular and circuit mechanisms of this cholinergic modulation are under active investigation in sensory and prefrontal cortex, but the universality of these mechanisms across the cerebral cortex is not clear. Anatomical maps suggest that the sensory and prefrontal cortices receive distinct cholinergic projections and have subtle differences in the expression of cholinergic receptors and the metabolic enzyme acetylcholinesterase. First, we briefly review this anatomical literature and the recent progress in the field. Next, we discuss in detail the electrophysiological effects of cholinergic receptor subtypes and the cell and circuit consequences of their stimulation by endogenous acetylcholine as established by recent optogenetic work. Finally, we explore the behavioral ramifications of in vivo manipulations of endogenous acetylcholine. We find broader similarities than we expected between the cholinergic regulation of sensory and prefrontal cortex, but there are some differences and some gaps in knowledge. In visual, auditory, and somatosensory cortex, the cell and circuit mechanisms of cholinergic sharpening of sensory perception have been probed in vivo with calcium imaging and optogenetic experiments to simultaneously test mechanism and measure the consequences of manipulation. By contrast, ascertaining the links between attentional performance and cholinergic modulation of specific prefrontal microcircuits is more complicated due to the nature of the required tasks. However, ex vivo optogenetic manipulations point to differences in the cholinergic modulation of sensory and prefrontal cortex. Understanding how and where acetylcholine acts within the cerebral cortex to shape cognition is essential to pinpoint novel treatment targets for the perceptual and attention deficits found in multiple psychiatric and neurological disorders.

Cortical ChAT+ neurons co-transmit acetylcholine and GABA in a target- and brain-region-specific manner

The mouse cerebral cortex contains neurons that express choline acetyltransferase (ChAT) and are a potential local source of acetylcholine. However, the neurotransmitters released by cortical ChAT⁺ neurons and their synaptic connectivity are unknown. We show that the nearly all cortical ChAT⁺ neurons in mice are specialized VIP⁺ interneurons that release GABA strongly onto other inhibitory interneurons and acetylcholine sparsely onto layer 1 interneurons and other VIP⁺/ChAT⁺ interneurons. This differential transmission of ACh and GABA based on the postsynaptic target neuron is reflected in VIP⁺/ChAT⁺ interneuron pre-synaptic terminals, as quantitative molecular analysis shows that only a subset of these are specialized to release acetylcholine. In addition, we identify a separate, sparse population of non-VIP ChAT⁺ neurons in the medial prefrontal cortex with a distinct developmental origin that robustly release acetylcholine in layer 1. These results demonstrate both cortex-region heterogeneity in cortical ChAT⁺ interneurons and target-specific co-release of acetylcholine and GABA.

Cholinergic and Adenosinergic Modulation of Synaptic Release

In this review we will discuss the effect of two neuromodulatory transmitters, acetylcholine (ACh) and adenosine, on the synaptic release probability and short-term synaptic plasticity. ACh and adenosine differ fundamentally in the way they are released into the extracellular space. ACh is released mostly from synaptic terminals and axonal bouton of cholinergic neurons in the basal forebrain (BF). Its mode of action on synaptic release probability is complex because it activate both ligand-gated ion channels, so-called nicotinic ACh receptors and G-protein coupled muscarinic ACh receptors. In contrast, adenosine is released from both neurons and glia via nucleoside transporters or diffusion over the cell membrane in a non-vesicular, non-synaptic fashion; its receptors are exclusively G-protein coupled receptors. We show that ACh and adenosine effects are highly specific for an identified synaptic connection and depend mostly on the presynaptic but also on the postsynaptic receptor type and discuss the functional implications of these differences.

Prefrontal cortical ChAT-VIP interneurons provide local excitation by cholinergic synaptic transmission and control attention

Neocortical choline acetyltransferase (ChAT)-expressing interneurons are a subclass of vasoactive intestinal peptide (ChAT-VIP) neurons of which circuit and behavioural function are unknown. Here, we show that ChAT-VIP neurons directly excite neighbouring neurons in several layers through fast synaptic transmission of acetylcholine (ACh) in rodent medial prefrontal cortex (mPFC). Both interneurons in layers (L)1-3 as well as pyramidal neurons in L2/3 and L6 receive direct inputs from ChAT-VIP neurons mediated by fast cholinergic transmission. A fraction (10-20%) of postsynaptic neurons that received cholinergic input from ChAT-VIP interneurons also received GABAergic input from these neurons. In contrast to regular VIP interneurons, ChAT-VIP neurons did not disinhibit pyramidal neurons. Finally, we show that activity of these neurons is relevant for behaviour and they control attention behaviour distinctly from basal forebrain ACh inputs. Thus, ChAT-VIP neurons are a local source of cortical ACh that directly excite neurons throughout cortical layers and contribute to attention.

Nicotinic Acetylcholine Receptors Control Encoding and Retrieval of Associative Recognition Memory through Plasticity in the Medial Prefrontal Cortex

Nicotinic acetylcholine receptors (nAChRs) expressed in the medial prefrontal cortex have critical roles in cognitive function. However, whether nAChRs are required for associative recognition memory and the mechanisms by which nAChRs may contribute to mnemonic processing are not known. We demonstrate that nAChRs in the prefrontal cortex exhibit subtype-specific roles in associative memory encoding and retrieval. We present evidence that these separate roles of nAChRs may rely on bidirectional modulation of plasticity at synaptic inputs to the prefrontal cortex that are essential for associative recognition memory.

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Prefrontal α7 nAChRs are critical for encoding of associative recognition memory

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Prefrontal α4β2 nAChRs are required for retrieval of associative recognition memory

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α7 and α4β2 nAChRs gate bidirectional plasticity at hippocampal-prefrontal synapses

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Bidirectional plasticity underlies the role of nAChR in associative recognition

Rapid Neuromodulation of Layer 1 Interneurons in Human Neocortex

Inhibitory interneurons govern virtually all computations in neocortical circuits and are in turn controlled by neuromodulation. While a detailed understanding of the distinct marker expression, physiology, and neuromodulator responses of different interneuron types exists for rodents and recent studies have highlighted the role of specific interneurons in converting rapid neuromodulatory signals into altered sensory processing during locomotion, attention, and associative learning, it remains little understood whether similar mechanisms exist in human neocortex. Here, we use whole-cell recordings combined with agonist application, transgenic mouse lines, in situ hybridization, and unbiased clustering to directly determine these features in human layer 1 interneurons (L1-INs). Our results indicate pronounced nicotinic recruitment of all L1-INs, whereas only a small subset co-expresses the ionotropic HTR3 receptor. In addition to human specializations, we observe two comparable physiologically and genetically distinct L1-IN types in both species, together indicating conserved rapid neuromodulation of human neocortical circuits through layer 1.

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Layer 1 interneurons in human and mouse neocortex respond strongly to acetylcholine

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These rapid responses are mediated by α7 and β2-containing nicotinic receptors

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Human layer 1 comprises neurogliaform cells expressing the conserved marker Ndnf

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Apart from conserved features, human L1 interneurons show a number of specializations

Muscarinic and Nicotinic Modulation of Neocortical Layer 6A Synaptic Microcircuits Is Cooperative and Cell-Specific

Acetylcholine (ACh) is known to regulate cortical activity during different behavioral states, for example, wakefulness and attention. Here we show a differential expression of muscarinic ACh receptors (mAChRs) and nicotinic ACh receptors (nAChRs) in different layer 6A (L6A) pyramidal cell (PC) types of somatosensory cortex. At low concentrations, ACh induced a persistent hyperpolarization in corticocortical (CC) but a depolarization in corticothalamic (CT) L6A PCs via M ₄ and M₁ mAChRs, respectively. At ~ 1 mM, ACh depolarized exclusively CT PCs via α₄β₂ subunit-containing nAChRs without affecting CC PCs. Miniature EPSC frequency in CC PCs was decreased by ACh but increased in CT PCs. In synaptic connections with a presynaptic CC PC, glutamate release was suppressed via M₄ mAChR activation but enhanced by nAChRs via α₄β₂ nAChRs when the presynaptic neuron was a CT PC. Thus, in L6A, the interaction of mAChRs and nAChRs results in an altered excitability and synaptic release, effectively strengthening CT output while weakening CC synaptic signaling.

Bioluminescence calcium imaging of network dynamics and their cholinergic modulation in slices of cerebral cortex from male rats

The activity of neuronal ensembles was monitored in neocortical slices from male rats using wide-field bioluminescence imaging of a calcium sensor formed with the fusion of green fluorescent protein and aequorin (GA) and expressed through viral transfer. GA expression was restricted to pyramidal neurons and did not conspicuously alter neuronal morphology or neocortical cytoarchitecture. Removal of extracellular magnesium or addition of GABA receptor antagonists triggered epileptiform flashes of variable amplitude and spatial extent, indicating that the excitatory and inhibitory networks were functionally preserved in GA-expressing slices. We found that agonists of muscarinic acetylcholine receptors largely increased the peak bioluminescence response to local electrical stimulation in layer I or white matter, and gave rise to a slowly decaying response persisting for tens of seconds. The peak increase involved layers II/III and V and did not result in marked alteration of response spatial properties. The persistent response involved essentially layer V and followed the time course of the muscarinic afterdischarge depolarizing plateau in layer V pyramidal cells. This plateau potential triggered spike firing in layer V, but not layer II/III pyramidal cells, and was accompanied by recurrent synaptic excitation in layer V. Our results indicate that wide-field imaging of GA bioluminescence is well suited to monitor local and global network activity patterns, involving different mechanisms of intracellular calcium increase, and occurring on various timescales.

A Dendritic Substrate for the Cholinergic Control of Neocortical Output Neurons

The ascending cholinergic system dynamically regulates sensory perception and cognitive function, but it remains unclear how this modulation is executed in neocortical circuits. Here, we demonstrate that the cholinergic system controls the integrative operations of neocortical principal neurons by modulating dendritic excitability. Direct dendritic recordings revealed that the optogenetic-evoked release of acetylcholine (ACh) transformed the pattern of dendritic integration in layer 5B pyramidal neurons, leading to the generation of dendritic plateau potentials which powerfully drove repetitive action potential output. In contrast, the synaptic release of ACh did not positively modulate axo-somatic excitability. Mechanistically, the transformation of dendritic integration was mediated by the muscarinic ACh receptor-dependent enhancement of dendritic R-type calcium channel activity, a compartment-dependent modulation which decisively controlled the associative computations executed by layer 5B pyramidal neurons. Our findings therefore reveal a biophysical mechanism by which the cholinergic system controls dendritic computations causally linked to perceptual detection.

Lateral inhibition by Martinotti interneurons is facilitated by cholinergic inputs in human and mouse neocortex

A variety of inhibitory pathways encompassing different interneuron types shape activity of neocortical pyramidal neurons. While basket cells (BCs) mediate fast lateral inhibition between pyramidal neurons, Somatostatin-positive Martinotti cells (MCs) mediate a delayed form of lateral inhibition. Neocortical circuits are under control of acetylcholine, which is crucial for cortical function and cognition. Acetylcholine modulates MC firing, however, precisely how cholinergic inputs affect cortical lateral inhibition is not known. Here, we find that cholinergic inputs selectively augment and speed up lateral inhibition between pyramidal neurons mediated by MCs, but not by BCs. Optogenetically activated cholinergic inputs depolarize MCs through activation of ß2 subunit-containing nicotinic AChRs, not muscarinic AChRs, without affecting glutamatergic inputs to MCs. We find that these mechanisms are conserved in human neocortex. Cholinergic inputs thus enable cortical pyramidal neurons to recruit more MCs, and can thereby dynamically highlight specific circuit motifs, favoring MC-mediated pathways over BC-mediated pathways.

Parvalbumin and somatostatin expressing interneurons mediate lateral inhibition between cortical neurons. Here the authors report the mechanisms by which acetylcholine from the basal forebrain selectively augments lateral inhibition via Martinotti cells and show that this is conserved in humans.

Learning-Related Plasticity in Dendrite-Targeting Layer 1 Interneurons

A wealth of data has elucidated the mechanisms by which sensory inputs are encoded in the neocortex, but how these processes are regulated by the behavioral relevance of sensory information is less understood. Here, we focus on neocortical layer 1 (L1), a key location for processing of such top-down information. Using Neuron-Derived Neurotrophic Factor (NDNF) as a selective marker of L1 interneurons (INs) and in vivo 2-photon calcium imaging, electrophysiology, viral tracing, optogenetics, and associative memory, we find that L1 NDNF-INs mediate a prolonged form of inhibition in distal pyramidal neuron dendrites that correlates with the strength of the memory trace. Conversely, inhibition from Martinotti cells remains unchanged after conditioning but in turn tightly controls sensory responses in NDNF-INs. These results define a genetically addressable form of dendritic inhibition that is highly experience dependent and indicate that in addition to disinhibition, salient stimuli are encoded at elevated levels of distal dendritic inhibition.

Video Abstract

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NDNF is a selective marker for neocortical layer 1 interneurons

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NDNF interneurons mediate prolonged inhibition of distal pyramidal neuron dendrites

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Inhibition from Martinotti cells tightly controls NDNF interneuron responses

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Dendritic inhibition by NDNF interneurons is highly experience dependent

Co-transmission of acetylcholine and GABA regulates hippocampal states

The basal forebrain cholinergic system is widely assumed to control cortical functions via non-synaptic transmission of a single neurotransmitter. Yet, we find that mouse hippocampal cholinergic terminals invariably establish GABAergic synapses, and their cholinergic vesicles dock at those synapses only. We demonstrate that these synapses do not co-release but co-transmit GABA and acetylcholine via different vesicles, whose release is triggered by distinct calcium channels. This co-transmission evokes composite postsynaptic potentials, which are mutually cross-regulated by presynaptic autoreceptors. Although postsynaptic cholinergic receptor distribution cannot be investigated, their response latencies suggest a focal, intra- and/or peri-synaptic localisation, while GABA_A receptors are detected intra-synaptically. The GABAergic component alone effectively suppresses hippocampal sharp wave-ripples and epileptiform activity. Therefore, the differentially regulated GABAergic and cholinergic co-transmission suggests a hitherto unrecognised level of control over cortical states. This novel model of hippocampal cholinergic neurotransmission may lead to alternative pharmacotherapies after cholinergic deinnervation seen in neurodegenerative disorders.

Synaptic Release of Acetylcholine Rapidly Suppresses Cortical Activity by Recruiting Muscarinic Receptors in Layer 4

Cholinergic afferents from the basal forebrain (BF) can influence cortical activity on rapid time scales, enabling sensory information processing and exploratory behavior. However, our understanding of how synaptically released acetylcholine (ACh) influences cellular targets in distinct cortical layers remains incomplete. Previous studies have shown that rapid changes in cortical dynamics induced by phasic BF activity can be mediated by the activation of nicotinic ACh receptors (nAChRs) expressed in distinct types of GABAergic interneurons. In contrast, muscarinic ACh receptors (mAChRs) are assumed to be involved in slower and more diffuse ACh signaling following sustained increases in afferent activity. Here, we examined the mechanisms underlying fast cholinergic control of cortical circuit dynamics by pairing optical stimulation of cholinergic afferents with evoked activity in somatosensory cortical slices of mice of either sex. ACh release evoked by single stimuli led to a rapid and persistent suppression of cortical activity, mediated by mAChRs expressed in layer 4 and to a lesser extent, by nAChRs in layers 1-3. In agreement, we found that cholinergic inputs to layer 4 evoked short-latency and long-lasting mAChR-dependent inhibition of the large majority of excitatory neurons, whereas inputs to layers 1-3 primarily evoked nAChR-dependent excitation of different classes of interneurons. Our results indicate that the rapid cholinergic control of cortical network dynamics is mediated by both nAChRs and mAChRs-dependent mechanisms, which are expressed in distinct cortical layers and cell types.SIGNIFICANCE STATEMENT Acetylcholine (ACh) release from basal forebrain (BF) afferents to cortex influences a variety of cognitive functions including attention, sensory processing, and learning. Cholinergic control occurs on the time scale of seconds and is mediated by BF neurons that generate action potentials at low rates, indicating that ACh acts as a point-to-point neurotransmitter. Our findings highlight that even brief activation of cholinergic afferents can recruit both nicotinic and muscarinic ACh receptors expressed in several cell types, leading to modulation of cortical activity on distinct time scales. Furthermore, they indicate that the initial stages of cortical sensory processing are under direct cholinergic control.

Stoichiometry of the Heteromeric Nicotinic Receptors of the Renshaw Cell

Neuronal nicotinic acetylcholine receptors (nAChRs) are pentamers built from a variety of subunits. Some are homomeric assemblies of α subunits, others heteromeric assemblies of α and β subunits which can adopt two stoichiometries (2α:3β or 3α:2β). There is evidence for the presence of heteromeric nAChRs with the two stoichiometries in the CNS, but it has not yet been possible to identify them at a given synapse. The 2α:3β receptors are highly sensitive to agonists, whereas the 3α:2β stoichiometric variants, initially described as low sensitivity receptors, are indeed activated by low and high concentrations of ACh. We have taken advantage of the discovery that two compounds (NS9283 and Zn) potentiate selectively the 3α:2β nAChRs to establish (in mice of either sex) the presence of these variants at the motoneuron-Renshaw cell (MN-RC) synapse. NS9283 prolonged the decay of the two-component EPSC mediated by heteromeric nAChRs. NS9283 and Zn also prolonged spontaneous EPSCs involving heteromeric nAChRs, and one could rule out prolongations resulting from AChE inhibition by NS9283. These results establish the presence of 3α:2β nAChRs at the MN-RC synapse. At the functional level, we had previously explained the duality of the EPSC by assuming that high ACh concentrations in the synaptic cleft account for the fast component and that spillover of ACh accounts for the slow component. The dual ACh sensitivity of 3α:2β nAChRs now allows to attribute to these receptors both components of the EPSC.SIGNIFICANCE STATEMENT Heteromeric nicotinic receptors assemble α and β subunits in pentameric structures, which can adopt two stoichiometries: 3α:2β or 2α:3β. Both stoichiometric variants are present in the CNS, but they have never been located and characterized functionally at the level of an identified synapse. Our data indicate that 3α:2β receptors are present at the spinal cord synapses between motoneurons and Renshaw cells, where their dual mode of activation (by high concentrations of ACh for synaptic receptors, by low concentrations of ACh for extrasynaptic receptors) likely accounts for the biphasic character of the synaptic current. More generally, 3α:2β nicotinic receptors appear unique by their capacity to operate both in the cleft of classical synapses and at extrasynaptic locations.

Normalization and the Cholinergic Microcircuit: A Unified Basis for Attention

Attention alters three key properties of population neural activity - firing rate, rate variability, and shared variability between neurons. All three properties are well explained by a single canonical computation - normalization - that acts across hierarchically integrated brain systems. Combining data from rodents and nonhuman primates, we argue that cortical cholinergic modulation originating from the basal forebrain closely mimics the effects of directed attention on these three properties of population neural activity. Cholinergic modulation of the cortical microcircuit underlying normalization may represent a key biological basis for the rapid and flexible changes in population neuronal coding that are required by directed attention.

Inhibitory circuit gating of auditory critical-period plasticity

Cortical sensory maps are remodeled during early life to adapt to the surrounding environment. Both sensory and contextual signals are important for induction of this plasticity, but how these signals converge to sculpt developing thalamocortical circuits remains largely unknown. Here we show that layer 1 (L1) of primary auditory cortex (A1) is a key hub where neuromodulatory and topographically organized thalamic inputs meet to tune the cortical layers below. Inhibitory interneurons in L1 send narrowly descending projections to differentially modulate thalamic drive to pyramidal and parvalbumin-expressing (PV) cells in L4, creating brief windows of intracolumnar activation. Silencing of L1 (but not VIP-expressing) cells abolishes map plasticity during the tonotopic critical period. Developmental transitions in nicotinic acetylcholine receptor (nAChR) sensitivity in these cells caused by Lynx1 protein can be overridden to extend critical-period closure. Notably, thalamocortical maps in L1 are themselves stable, and serve as a scaffold for cortical plasticity throughout life.

Layer- and Cell Type-Specific Modulation of Excitatory Neuronal Activity in the Neocortex

From an anatomical point of view the neocortex is subdivided into up to six layers depending on the cortical area. This subdivision has been described already by Meynert and Brodmann in the late 19/early 20. century and is mainly based on cytoarchitectonic features such as the size and location of the pyramidal cell bodies. Hence, cortical lamination is originally an anatomical concept based on the distribution of excitatory neuron. However, it has become apparent in recent years that apart from the layer-specific differences in morphological features, many functional properties of neurons are also dependent on cortical layer or cell type. Such functional differences include changes in neuronal excitability and synaptic activity by neuromodulatory transmitters. Many of these neuromodulators are released from axonal afferents from subcortical brain regions while others are released intrinsically. In this review we aim to describe layer- and cell-type specific differences in the effects of neuromodulator receptors in excitatory neurons in layers 2–6 of different cortical areas. We will focus on the neuromodulator systems using adenosine, acetylcholine, dopamine, and orexin/hypocretin as examples because these neuromodulator systems show important differences in receptor type and distribution, mode of release and functional mechanisms and effects. We try to summarize how layer- and cell type-specific neuromodulation may affect synaptic signaling in cortical microcircuits.

Cholinergic Modulation of Cortical Microcircuits Is Layer-Specific: Evidence from Rodent, Monkey and Human Brain

Acetylcholine (ACh) signaling shapes neuronal circuit development and underlies specific aspects of cognitive functions and behaviors, including attention, learning, memory and motivation. During behavior, activation of muscarinic and nicotinic acetylcholine receptors (mAChRs and nAChRs) by ACh alters the activation state of neurons, and neuronal circuits most likely process information differently with elevated levels of ACh. In several brain regions, ACh has been shown to alter synaptic strength as well. By changing the rules for synaptic plasticity, ACh can have prolonged effects on and rearrange connectivity between neurons that outlasts its presence. From recent discoveries in the mouse, rat, monkey and human brain, a picture emerges in which the basal forebrain (BF) cholinergic system targets the neocortex with much more spatial and temporal detail than previously considered. Fast cholinergic synapses acting on a millisecond time scale are abundant in the mammalian cerebral cortex, and provide BF cholinergic neurons with the possibility to rapidly alter information flow in cortical microcircuits. Finally, recent studies have outlined novel mechanisms of how cholinergic projections from the BF affect synaptic strength in several brain areas of the rodent brain, with behavioral consequences. This review highlights these exciting developments and discusses how these findings translate to human brain circuitries.

Molecular function of α7 nicotinic receptors as drug targets

Nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels involved in many physiological and pathological processes. In vertebrates, there are seventeen different nAChR subunits that combine to yield a variety of receptors with different pharmacology, function, and localization. The homomeric α7 receptor is one of the most abundant nAChRs in the nervous system and it is also present in non-neuronal cells. It plays important roles in cognition, memory, pain, neuroprotection, and inflammation. Its diverse physiological actions and associated disorders have made of α7 an attractive novel target for drug modulation. Potentiation of the α7 receptor has emerged as a novel therapeutic strategy for several neurological diseases, such as Alzheimer's and Parkinson's diseases, and inflammatory disorders. In contrast, increased α7 activity has been associated with cancer cell proliferation. The presence of different drug target sites offers a great potential for α7 modulation in different pathological contexts. In particular, compounds that target allosteric sites offer significant advantages over orthosteric agonists due to higher selectivity and a broader spectrum of degrees and mechanisms of modulation. Heterologous expression of α7, together with chaperone proteins, combined with patch clamp recordings have provided important advances in our knowledge of the molecular basis of α7 responses and their potential modulation for pathological processes. This review gives a synthetic view of α7 and its molecular function, focusing on how its unique activation and desensitization features can be modified by pharmacological agents. This fundamental information offers insights into therapeutic strategies.

Ultrafast and Slow Cholinergic Transmission. Different Involvement of Acetylcholinesterase Molecular Forms

Acetylcholine (ACh), an ubiquitous mediator substance broadly expressed in nature, acts as neurotransmitter in cholinergic synapses, generating specific communications with different time-courses. (1) Ultrafast transmission. Vertebrate neuromuscular junctions (NMJs) and nerve-electroplaque junctions (NEJs) are the fastest cholinergic synapses; able to transmit brief impulses (1-4 ms) at high frequencies. The collagen-tailed A12 acetylcholinesterase is concentrated in the synaptic cleft of NMJs and NEJs, were it curtails the postsynaptic response by ultrafast ACh hydrolysis. Here, additional processes contribute to make transmission so rapid. (2) Rapid transmission. At peripheral and central cholinergic neuro-neuronal synapses, transmission involves an initial, relatively rapid (10-50 ms) nicotinic response, followed by various muscarinic or nicotinic effects. Acetylcholinesterase (AChE) being not concentrated within these synapses, it does not curtail the initial rapid response. In contrast, the late responses are controlled by a globular form of AChE (mainly G4-AChE), which is membrane-bound and/or secreted. (3) SlowAChsignalling. In non-neuronal systems, in muscarinic domains, and in most regions of the central nervous system (CNS), many ACh-releasing structures (cells, axon terminals, varicosities, boutons) do not form true synaptic contacts, most muscarinic and also part of nicotinic receptors are extra-synaptic, often situated relatively far from ACh releasing spots. A12-AChE being virtually absent in CNS, G4-AChE is the most abundant form, whose function appears to modulate the "volume" transmission, keeping ACh concentration within limits in time and space.

Differential Control of Dopaminergic Excitability and Locomotion by Cholinergic Inputs in Mouse Substantia Nigra

Understanding how dopaminergic (DA) neurons of the substantia nigra pars compacta (SNc) govern movements requires a detailed knowledge of how different neurotransmitter systems modulate DA neuronal excitability. We report a heterogeneity of electrophysiological properties between medial and lateral SNc neurons modulated by cholinergic neurotransmission. Lateral DA neurons received mainly excitatory (nicotinic or glutamatergic) mediated cholinergic neurotransmission. Medial DA neurons received predominantly GABAergic currents mediated by presynaptic nicotinic receptors or biphasic GABAergic and nicotinic neurotransmission conveyed by GABA and ACh corelease, which inhibited DA neurons. To examine whether cholinergic signaling in the SNc controls mouse behavior, we used optogenetics in awake behaving mice and found that activation of cholinergic terminals in the medial SNc decreased locomotion, whereas activation in the lateral SNc increased locomotion. Our findings provide novel insights on how cholinergic inputs to subregions of the SNc regulate the excitability of DA neurons differentially, resulting in different patterns of motor behavior.

Segregation of glutamatergic and cholinergic transmission at the mixed motoneuron Renshaw cell synapse

In neonatal mice motoneurons excite Renshaw cells by releasing both acetylcholine (ACh) and glutamate. These two neurotransmitters activate two types of nicotinic receptors (nAChRs) (the homomeric α₇ receptors and the heteromeric α*ß* receptors) as well as the two types of glutamate receptors (GluRs) (AMPARs and NMDARs). Using paired recordings, we confirm that a single motoneuron can release both transmitters on a single post-synaptic Renshaw cell. We then show that co-transmission is preserved in adult animals. Kinetic analysis of miniature EPSCs revealed quantal release of mixed events associating AMPARs and NMDARs, as well as α₇ and α*ß* nAChRs, but no evidence was found for mEPSCs associating nAChRs with GluRs. Bayesian Quantal Analysis (BQA) of evoked EPSCs showed that the number of functional contacts on a single Renshaw cell is more than halved when the nicotinic receptors are blocked, confirming that the two neurotransmitters systems are segregated. Our observations can be explained if ACh and glutamate are released from common vesicles onto spatially segregated post-synaptic receptors clusters, but a pre-synaptic segregation of cholinergic and glutamatergic release sites is also possible.

Dendro-dendritic cholinergic excitation controls dendritic spike initiation in retinal ganglion cells

The retina processes visual images to compute features such as the direction of image motion. Starburst amacrine cells (SACs), axonless feed-forward interneurons, are essential components of the retinal direction-selective circuitry. Recent work has highlighted that SAC-mediated dendro-dendritic inhibition controls the action potential output of direction-selective ganglion cells (DSGCs) by vetoing dendritic spike initiation. However, SACs co-release GABA and the excitatory neurotransmitter acetylcholine at dendritic sites. Here we use direct dendritic recordings to show that preferred direction light stimuli evoke SAC-mediated acetylcholine release, which powerfully controls the stimulus sensitivity, receptive field size and action potential output of ON-DSGCs by acting as an excitatory drive for the initiation of dendritic spikes. Consistent with this, paired recordings reveal that the activation of single ON-SACs drove dendritic spike generation, because of predominate cholinergic excitation received on the preferred side of ON-DSGCs. Thus, dendro-dendritic release of neurotransmitters from SACs bi-directionally gate dendritic spike initiation to control the directionally selective action potential output of retinal ganglion cells.

Neural computations performed by the retinal microcircuit have been extensively studied. Here the authors report using dendritic recordings that the direction selective responses of retinal ganglion cells are controlled by dendro-dendritic cholinergic excitation from starburst amacrine cells.

Layer-specific cholinergic control of human and mouse cortical synaptic plasticity

Individual cortical layers have distinct roles in information processing. All layers receive cholinergic inputs from the basal forebrain (BF), which is crucial for cognition. Acetylcholinergic receptors are differentially distributed across cortical layers, and recent evidence suggests that different populations of BF cholinergic neurons may target specific prefrontal cortical (PFC) layers, raising the question of whether cholinergic control of the PFC is layer dependent. Here we address this issue and reveal dendritic mechanisms by which endogenous cholinergic modulation of synaptic plasticity is opposite in superficial and deep layers of both mouse and human neocortex. Our results show that in different cortical layers, spike timing-dependent plasticity is oppositely regulated by the activation of nicotinic acetylcholine receptors (nAChRs) either located on dendrites of principal neurons or on GABAergic interneurons. Thus, layer-specific nAChR expression allows functional layer-specific control of cortical processing and plasticity by the BF cholinergic system, which is evolutionarily conserved from mice to humans.

Ethanol-Induced Motor Impairment Mediated by Inhibition of α7 Nicotinic Receptors

Nicotine and ethanol (EtOH) are among the most widely co-abused substances, and nicotinic acetylcholine receptors (nAChRs) contribute to the behavioral effects of both drugs. Along with their role in addiction, nAChRs also contribute to motor control circuitry. The α7 nAChR subtype is highly expressed in the laterodorsal tegmental nucleus (LDTg), a brainstem cholinergic center that contributes to motor performance through its projections to thalamic motor relay centers, including the mediodorsal thalamus. We demonstrate that EtOH concentrations just above the legal limits for intoxication in humans can inhibit α7 nAChRs in LDTg neurons from rats. This EtOH-induced inhibition is mediated by a decrease in cAMP/PKA signaling. The α7 nAChR-positive allosteric modulator PNU120596 [N-(5-chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)-urea], which interferes with receptor desensitization, completely eliminated EtOH modulation of these receptors. These data suggest that EtOH inhibits α7 responses through a PKA-dependent enhancement of receptor desensitization. EtOH also inhibited the effects of nicotine at presynaptic α7 nAChRs on glutamate terminals in the mediodorsal thalamus. In vivo administration of PNU120596 either into the cerebral ventricles or directly into the mediodorsal thalamus attenuated EtOH-induced motor impairment. Thus, α7 nAChRs are likely important mediators of the motor impairing effects of moderate EtOH consumption.

SIGNIFICANCE STATEMENT

The motor-impairing effects of ethanol contribute to intoxication-related injury and death. Here we explore the cellular and neural circuit mechanisms underlying ethanol-induced motor impairment. Physiologically relevant concentrations of ethanol inhibit activity of a nicotinic receptor subtype that is expressed in brain areas associated with motor control. That receptor inhibition is mediated by decreased receptor phosphorylation, suggesting an indirect modulation of cell signaling pathways to achieve the physiological effects.

Nicotinic acetylcholine receptor-mediated responses in medial vestibular and prepositus hypoglossi nuclei neurons showing distinct neurotransmitter phenotypes

Cholinergic transmission in both the medial vestibular nucleus (MVN) and prepositus hypoglossi nucleus (PHN) plays an important role in horizontal eye movements. We previously demonstrated that the current responses mediated via nicotinic acetylcholine receptors (nAChRs) were larger than those mediated via muscarinic acetylcholine receptors (mAChRs) in cholinergic MVN and PHN neurons that project to the cerebellum. In this study, to clarify the predominant nAChR responses and the expression patterns of nAChRs in MVN and PHN neurons that exhibit distinct neurotransmitter phenotypes, we identified cholinergic, inhibitory, and glutamatergic neurons using specific transgenic rats and investigated current responses to the application of acetylcholine (ACh) using whole cell recordings in brain stem slices. ACh application induced larger nAChR-mediated currents than mAChR-mediated currents in every neuronal phenotype. In the presence of an mAChR antagonist, we found three types of nAChR-mediated currents that exhibited different rise and decay times and designated these as fast (F)-, slow (S)-, and fast and slow (FS)-type currents. F-type currents were the predominant response in inhibitory MVN neurons, whereas S-type currents were observed in the majority of glutamatergic MVN and PHN neurons. No dominant response type was observed in cholinergic neurons. Pharmacological analyses revealed that the F-, S-, and FS-type currents were mainly mediated by α7, non-α7, and both α7 and non-α7 nAChRs, respectively. These findings suggest that cholinergic responses in the major neuronal populations of the MVN and PHN are predominantly mediated by nAChRs and that the expression of α7 and non-α7 nAChRs differ among the neuronal phenotypes.

Integration of inhibitory and excitatory effects of α7 nicotinic acetylcholine receptor activation in the prelimbic cortex regulates network activity and plasticity

Cognitive and attentional processes governed by the prefrontal cortex (PFC) are influenced by cholinergic innervation. Here we have explored the role of α7 nicotinic acetylcholine receptors (nAChRs) as mediators of cholinergic signalling in the dorsomedial (prelimbic) PFC, using mouse brain slice electrophysiology. Activation of α7 nAChRs located on glutamatergic terminals and cell soma of GABAergic interneurons increased excitation and inhibition, respectively, in layer V of the prelimbic cortex. These actions were distinguished by their differential dependence on local acetylcholine (ACh): potentiation of endogenous cholinergic signalling with the positive allosteric modulator, PNU-120596, enhanced spontaneous excitatory events, an effect that was further increased by inhibition of acetylcholinesterase. In contrast, α7 nicotinic modulation of inhibitory signalling required addition of exogenous agonist (PNU-282987) as well as PNU-120596, and was unaffected by acetylcholinesterase inhibition. Thus α7 nAChRs can bi-directionally regulate network activity in the prelimbic cortex, depending on the magnitude and localisation of cholinergic signalling. This bidirectional influence is manifest in dual effects of α7 nAChRs on theta-burst-induced long-term potentiation (LTP) in layer V of the prelimbic cortex. Antagonism of α7 nAChRs significantly decreased LTP implicating a contribution from endogenous ACh, consistent with the ability of local ACh to enhance glutamatergic signalling. Exogenous agonist plus potentiator also decreased LTP, indicative of the influence of this drug combination on inhibitory signalling. Thus α7 nAChRs make a complex contribution to network activity and synaptic plasticity in the prelimbic cortex.

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α7 nAChRs exist at glutamatergic nerve terminals in the prelimbic cortex.

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α7 nAChRs exist at GABAergic cell bodies in the prelimbic cortex.

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Tonic ACh preferentially activates α7 nAChRs at glutamatergic nerve terminals.

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α7 nAChRs exert bidirectional control of LTP in the prelimbic cortex.

Optogenetic Dissection of the Basal Forebrain Neuromodulatory Control of Cortical Activation, Plasticity, and Cognition

The basal forebrain (BF) houses major ascending projections to the entire neocortex that have long been implicated in arousal, learning, and attention. The disruption of the BF has been linked with major neurological disorders, such as coma and Alzheimer's disease, as well as in normal cognitive aging. Although it is best known for its cholinergic neurons, the BF is in fact an anatomically and neurochemically complex structure. Recent studies using transgenic mouse lines to target specific BF cell types have led to a renaissance in the study of the BF and are beginning to yield new insights about cell-type-specific circuit mechanisms during behavior. These approaches enable us to determine the behavioral conditions under which cholinergic and noncholinergic BF neurons are activated and how they control cortical processing to influence behavior. Here we discuss recent advances that have expanded our knowledge about this poorly understood brain region and laid the foundation for future cell-type-specific manipulations to modulate arousal, attention, and cortical plasticity in neurological disorders.

SIGNIFICANCE STATEMENT

Although the basal forebrain is best known for, and often equated with, acetylcholine-containing neurons that provide most of the cholinergic innervation of the neocortex, it is in fact an anatomically and neurochemically complex structure. Recent studies using transgenic mouse lines to target specific cell types in the basal forebrain have led to a renaissance in this field and are beginning to dissect circuit mechanisms in the basal forebrain during behavior. This review discusses recent advances in the roles of basal forebrain cholinergic and noncholinergic neurons in cognition via their dynamic modulation of cortical activity.

Cotransmission of acetylcholine and GABA

Neurons that produce acetylcholine (ACh) are positioned to broadly influence the brain, with axonal arborizations that extend throughout the cerebral cortex, striatum, and hippocampus. While the action of these neurons has typically been attributed entirely to ACh, neurons often release more than one primary neurotransmitter. Here, we review evidence for the cotransmission of the inhibitory neurotransmitter GABA from cholinergic neurons throughout the mammalian central nervous system. Functional cotransmission of ACh and GABA has been reported in the retina and cortex, and anatomical studies suggest that GABA cotransmission is a common feature of nearly all forebrain ACh-producing neurons. Further experiments are necessary to confirm the extent of GABA cotransmission from cholinergic neurons, and the contribution of GABA needs to be considered when studying the functional impact of activity in ACh-producing neurons. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

Nicotinic Transmission onto Layer 6 Cortical Neurons Relies on Synaptic Activation of Non-α7 Receptors

Nicotinic excitation in neocortex is mediated by low-affinity α7 receptors and by high-affinity α4β2 receptors. There is evidence that α7 receptors are synaptic, but it is unclear whether high-affinity receptors are activated by volume transmission or synaptic transmission. To address this issue, we characterized responses of excitatory layer 6 (L6) neurons to optogenetic release of acetylcholine (ACh) in cortical slices. L6 responses consisted in a slowly decaying α4β2 current and were devoid of α7 component. Evidence that these responses were mediated by synapses was 4-fold. 1) Channelrhodopsin-positive cholinergic varicosities made close appositions onto responsive neurons. 2) Inhibition of ACh degradation failed to alter onset kinetics and amplitude of currents. 3) Quasi-saturation of α4β2 receptors occurred upon ACh release. 4) Response kinetics were unchanged in low release probability conditions. Train stimulations increased amplitude and decay time of responses and these effects appeared to involve recruitment of extrasynaptic receptors. Finally, we found that the α5 subunit, known to be associated with α4β2 in L6, regulates short-term plasticity at L6 synapses. Our results are consistent with previous anatomical observations of widespread cholinergic synapses and suggest that a significant proportion of these small synapses operate via high-affinity nicotinic receptors.

Impact of basal forebrain cholinergic inputs on basolateral amygdala neurons

In addition to innervating the cerebral cortex, basal forebrain cholinergic (BFc) neurons send a dense projection to the basolateral nucleus of the amygdala (BLA). In this study, we investigated the effect of near physiological acetylcholine release on BLA neurons using optogenetic tools and in vitro patch-clamp recordings. Adult transgenic mice expressing cre-recombinase under the choline acetyltransferase promoter were used to selectively transduce BFc neurons with channelrhodopsin-2 and a reporter through the injection of an adeno-associated virus. Light-induced stimulation of BFc axons produced different effects depending on the BLA cell type. In late-firing interneurons, BFc inputs elicited fast nicotinic EPSPs. In contrast, no response could be detected in fast-spiking interneurons. In principal BLA neurons, two different effects were elicited depending on their activity level. When principal BLA neurons were quiescent or made to fire at low rates by depolarizing current injection, light-induced activation of BFc axons elicited muscarinic IPSPs. In contrast, with stronger depolarizing currents, eliciting firing above ∼ 6-8 Hz, these muscarinic IPSPs lost their efficacy because stimulation of BFc inputs prolonged current-evoked afterdepolarizations. All the effects observed in principal neurons were dependent on muscarinic receptors type 1, engaging different intracellular mechanisms in a state-dependent manner. Overall, our results suggest that acetylcholine enhances the signal-to-noise ratio in principal BLA neurons. Moreover, the cholinergic engagement of afterdepolarizations may contribute to the formation of stimulus associations during fear-conditioning tasks where the timing of conditioned and unconditioned stimuli is not optimal for the induction of synaptic plasticity.

Cholinergic control of gamma power in the midbrain spatial attention network

The modulation of gamma power (25-90 Hz) is associated with attention and has been observed across species and brain areas. However, mechanisms that control these modulations are poorly understood. The midbrain spatial attention network in birds generates high-amplitude gamma oscillations in the local field potential that are thought to represent the highest priority location for attention. Here we explore, in midbrain slices from chickens, mechanisms that regulate the power of these oscillations, using high-resolution techniques including intracellular recordings from neurons targeted by calcium imaging. The results identify a specific subtype of neuron, expressing non-α7 nicotinic acetylcholine receptors, that directly drives inhibition in the gamma-generating circuit and switches the network into a primed state capable of producing high-amplitude oscillations. The special properties of this mechanism enable rapid, persistent changes in gamma power. The brain may employ this mechanism wherever rapid modulations of gamma power are critical to information processing.

Corelease of acetylcholine and GABA from cholinergic forebrain neurons

Neurotransmitter corelease is emerging as a common theme of central neuromodulatory systems. Though corelease of glutamate or GABA with acetylcholine has been reported within the cholinergic system, the full extent is unknown. To explore synaptic signaling of cholinergic forebrain neurons, we activated choline acetyltransferase expressing neurons using channelrhodopsin while recording post-synaptic currents (PSCs) in layer 1 interneurons. Surprisingly, we observed PSCs mediated by GABA_A receptors in addition to nicotinic acetylcholine receptors. Based on PSC latency and pharmacological sensitivity, our results suggest monosynaptic release of both GABA and ACh. Anatomical analysis showed that forebrain cholinergic neurons express the GABA synthetic enzyme Gad2 and the vesicular GABA transporter (Slc32a1). We confirmed the direct release of GABA by knocking out Slc32a1 from cholinergic neurons. Our results identify GABA as an overlooked fast neurotransmitter utilized throughout the forebrain cholinergic system. GABA/ACh corelease may have major implications for modulation of cortical function by cholinergic neurons.

DOI:http://dx.doi.org/10.7554/eLife.06412.001

Neurons communicate with one another at junctions called synapses. When an electrical signal arrives at the presynaptic cell, it triggers the release of molecules called neurotransmitters into the synapse. These molecules then bind to receptor proteins on the postsynaptic cell, starting a chain of events that leads to the regeneration of the electrical signal in the second cell.

Broadly speaking, neurotransmitters are either excitatory, which means that they increase the electrical activity of the postsynaptic neurons, or they are inhibitory, meaning that they reduce postsynaptic activity. Initially, it was thought that neurons release only one type of neurotransmitter, but it is now known that this is not always the case. Many neurons within the spinal cord, for example, release two different inhibitory neurotransmitters, GABA and glycine, while some neurons in the midbrain release GABA and an excitatory neurotransmitter called glutamate.

Saunders, Granger, and Sabatini now provide the first direct evidence that cholinergic neurons in different regions of the forebrain also release two neurotransmitters. Collectively known as the ‘forebrain cholinergic system’, these cells are best known for producing the excitatory transmitter acetylcholine. However, Saunders et al. now show that this system also produces an enzyme that manufactures GABA, as well as a protein that pumps GABA into structures called vesicles, which are then released into the synapse.

Although this is not concrete evidence for the release of GABA, Saunders et al. also show—with a technique called optogenetics, which involves the use of light to control neuronal activity—that some of the neurons in this system can trigger inhibitory responses in postsynaptic cells. Moreover, these responses can be blocked using drugs that occupy GABA receptors, or by using genetic techniques to delete the GABA-pumping protein from cholinergic neurons.

Taken together, the results of these experiments strongly suggest that the cholinergic neurons throughout the forebrain—unlike, for example, the cholinergic neurons in the midbrain, the region of the brain that controls movement—possess the molecular machinery needed to produce and release GABA, in addition to acetylcholine. Given that the cholinergic system has a key role in cognition and is particularly susceptible to degeneration in Alzheimer's disease, the ability of these neurons to release GABA release could have widespread implications for the study and understanding of brain function.

DOI:http://dx.doi.org/10.7554/eLife.06412.002

Nicotinic ACh receptors as therapeutic targets in CNS disorders

The neurotransmitter acetylcholine (ACh) can regulate neuronal excitability by acting on the cys-loop cation-conducting ligand-gated nicotinic ACh receptor (nAChR) channels. These receptors are widely distributed throughout the central nervous system (CNS), being expressed on neurons and non-neuronal cells, where they participate in a variety of physiological responses such as anxiety, the central processing of pain, food intake, nicotine seeking behavior, and cognitive functions. In the mammalian brain, nine different subunits have been found thus far, which assemble into pentameric complexes with much subunit diversity; however, the α7 and α4β2 subtypes predominate in the CNS. Neuronal nAChR dysfunction is involved in the pathophysiology of many neurological disorders. Here we will briefly discuss the functional makeup and expression of the nAChRs in mammalian brain, and their role as targets in neurodegenerative diseases (in particular Alzheimer's disease, AD), neurodevelopmental disorders (in particular autism and schizophrenia), and neuropathic pain.

Acetylcholine excites neocortical pyramidal neurons via nicotinic receptors

The neuromodulator acetylcholine (ACh) shapes neocortical function during sensory perception, motor control, arousal, attention, learning, and memory. Here we investigate the mechanisms by which ACh affects neocortical pyramidal neurons in adult mice. Stimulation of cholinergic axons activated muscarinic and nicotinic ACh receptors on pyramidal neurons in all cortical layers and in multiple cortical areas. Nicotinic receptor activation evoked short-latency, depolarizing postsynaptic potentials (PSPs) in many pyramidal neurons. Nicotinic receptor-mediated PSPs promoted spiking of pyramidal neurons. The duration of the increase in spiking was membrane potential dependent, with nicotinic receptor activation triggering persistent spiking lasting many seconds in neurons close to threshold. Persistent spiking was blocked by intracellular BAPTA, indicating that nicotinic ACh receptor activation evoked persistent spiking via a long-lasting calcium-activated depolarizing current. We compared nicotinic PSPs in primary motor cortex (M1), prefrontal cortex (PFC), and visual cortex. The laminar pattern of nicotinic excitation was not uniform but was broadly similar across areas, with stronger modulation in deep than superficial layers. Superimposed on this broad pattern were local differences, with nicotinic PSPs being particularly large and common in layer 5 of M1 but not layer 5 of PFC or primary visual cortex (V1). Hence, in addition to modulating the excitability of pyramidal neurons in all layers via muscarinic receptors, synaptically released ACh preferentially increases the activity of deep-layer neocortical pyramidal neurons via nicotinic receptors, thereby adding laminar selectivity to the widespread enhancement of excitability mediated by muscarinic ACh receptors.

Illuminating the role of cholinergic signaling in circuits of attention and emotionally salient behaviors

Acetylcholine (ACh) signaling underlies specific aspects of cognitive functions and behaviors, including attention, learning, memory and motivation. Alterations in ACh signaling are involved in the pathophysiology of multiple neuropsychiatric disorders. In the central nervous system, ACh transmission is mainly guaranteed by dense innervation of select cortical and subcortical regions from disperse groups of cholinergic neurons within the basal forebrain (BF; e.g., diagonal band, medial septal, nucleus basalis) and the pontine-mesencephalic nuclei, respectively. Despite the fundamental role of cholinergic signaling in the CNS and the long standing knowledge of the organization of cholinergic circuitry, remarkably little is known about precisely how ACh release modulates cortical and subcortical neural activity and the behaviors these circuits subserve. Growing interest in cholinergic signaling in the CNS focuses on the mechanism(s) of action by which endogenously released ACh regulates cognitive functions, acting as a neuromodulator and/or as a direct transmitter via nicotinic and muscarinic receptors. The development of optogenetic techniques has provided a valuable toolbox with which we can address these questions, as it allows the selective manipulation of the excitability of cholinergic inputs to the diverse array of cholinergic target fields within cortical and subcortical domains. Here, we review recent papers that use the light-sensitive opsins in the cholinergic system to elucidate the role of ACh in circuits related to attention and emotionally salient behaviors. In particular, we highlight recent optogenetic studies which have tried to disentangle the precise role of ACh in the modulation of cortical-, hippocampal- and striatal-dependent functions.

Acetylcholine release and inhibitory interneuron activity in hippocampal CA1

Acetylcholine release in the central nervous system (CNS) has an important role in attention, recall, and memory formation. One region influenced by acetylcholine is the hippocampus, which receives inputs from the medial septum and diagonal band of Broca complex (MS/DBB). Release of acetylcholine from the MS/DBB can directly affect several elements of the hippocampus including glutamatergic and GABAergic neurons, presynaptic terminals, postsynaptic receptors, and astrocytes. A significant portion of acetylcholine's effect likely results from the modulation of GABAergic inhibitory interneurons, which have crucial roles in controlling excitatory inputs, synaptic integration, rhythmic coordination of principal neurons, and outputs in the hippocampus. Acetylcholine affects interneuron function in large part by altering their membrane potential via muscarinic and nicotinic receptor activation. This minireview describes recent data from mouse hippocampus that investigated changes in CA1 interneuron membrane potentials following acetylcholine release. The interneuron subtypes affected, the receptor subtypes activated, and the potential outcome on hippocampal CA1 network function is discussed.

Orexin-dependent activation of layer VIb enhances cortical network activity and integration of non-specific thalamocortical inputs

Neocortical layer VI is critically involved in thalamocortical activity changes during the sleep/wake cycle. It receives dense projections from thalamic nuclei sensitive to the wake-promoting neuropeptides orexins, and its deepest part, layer VIb, is the only cortical lamina reactive to orexins. This convergence of wake-promoting inputs prompted us to investigate how layer VIb can modulate cortical arousal, using patch-clamp recordings and optogenetics in rat brain slices. We found that the majority of layer VIb neurons were excited by nicotinic agonists and orexin through the activation of nicotinic receptors containing α4-α5-β2 subunits and OX2 receptor, respectively. Specific effects of orexin on layer VIb neurons were potentiated by low nicotine concentrations and we used this paradigm to explore their intracortical projections. Co-application of nicotine and orexin increased the frequency of excitatory post-synaptic currents in the ipsilateral cortex, with maximal effect in infragranular layers and minimal effect in layer IV, as well as in the contralateral cortex. The ability of layer VIb to relay thalamocortical inputs was tested using photostimulation of channelrhodopsin-expressing fibers from the orexin-sensitive rhomboid nucleus in the parietal cortex. Photostimulation induced robust excitatory currents in layer VIa neurons that were not pre-synaptically modulated by orexin, but exhibited a delayed, orexin-dependent, component. Activation of layer VIb by orexin enhanced the reliability and spike-timing precision of layer VIa responses to rhomboid inputs. These results indicate that layer VIb acts as an orexin-gated excitatory feedforward loop that potentiates thalamocortical arousal.