Characterization of Channelrhodopsin and Archaerhodopsin in Cholinergic Neurons of Cre-Lox Transgenic Mice

Light-Driven Sodium Pump as a Potential Tool for the Control of Seizures in Epilepsy

The marine flavobacterium Krokinobactereikastus light-driven sodium pump (KR2) generates an outward sodium ion current under 530 nm light stimulation, representing a promising optogenetic tool for seizure control. However, the specifics of KR2 application to suppress epileptic activity have not yet been addressed. In the present study, we investigated the possibility of KR2 photostimulation to suppress epileptiform activity in mouse brain slices using the 4-aminopyrindine (4-AP) model. We injected the adeno-associated viral vector (AAV-PHP.eB-hSyn-KR2-YFP) containing the KR2 sodium pump gene enhanced with appropriate trafficking tags. KR2 expression was observed in the lateral entorhinal cortex and CA1 hippocampus. Using whole-cell patch clamp in mouse brain slices, we show that KR2, when stimulated with LED light, induces a substantial hyperpolarization of entorhinal neurons. However, continuous photostimulation of KR2 does not interrupt ictal discharges in mouse entorhinal cortex slices induced by a solution containing 4-AP. KR2-induced hyperpolarization strongly activates neuronal HCN channels. Consequently, turning off photostimulation resulted in HCN channel-mediated rebound depolarization accompanied by a transient increase in spontaneous network activity. Using low-frequency pulsed photostimulation, we induced the generation of short HCN channel-mediated discharges that occurred in response to the light stimulus being turned off; these discharges reliably interrupt ictal activity. Thus, low-frequency pulsed photostimulation of KR2 can be considered as a potential tool for controlling epileptic seizures.

Optogenetic stimulation of vagal nerves for enhanced glucose-stimulated insulin secretion and β cell proliferation

The enhancement of insulin secretion and of the proliferation of pancreatic β cells are promising therapeutic options for diabetes. Signals from the vagal nerve regulate both processes, yet the effectiveness of stimulating the nerve is unclear, owing to a lack of techniques for doing it so selectively and prolongedly. Here we report two optogenetic methods for vagal-nerve stimulation that led to enhanced glucose-stimulated insulin secretion and to β cell proliferation in mice expressing choline acetyltransferase-channelrhodopsin 2. One method involves subdiaphragmatic implantation of an optical fibre for the photostimulation of cholinergic neurons expressing a blue-light-sensitive opsin. The other method, which suppressed streptozotocin-induced hyperglycaemia in the mice, involves the selective activation of vagal fibres by placing blue-light-emitting lanthanide microparticles in the pancreatic ducts of opsin-expressing mice, followed by near-infrared illumination. The two methods show that signals from the vagal nerve, especially from nerve fibres innervating the pancreas, are sufficient to regulate insulin secretion and β cell proliferation.

Reward contingency gates selective cholinergic suppression of amygdala neurons

Basal forebrain cholinergic neurons modulate how organisms process and respond to environmental stimuli through impacts on arousal, attention, and memory. It is unknown, however, whether basal forebrain cholinergic neurons are directly involved in conditioned behavior, independent of secondary roles in the processing of external stimuli. Using fluorescent imaging, we found that cholinergic neurons are active during behavioral responding for a reward - even prior to reward delivery and in the absence of discrete stimuli. Photostimulation of basal forebrain cholinergic neurons, or their terminals in the basolateral amygdala (BLA), selectively promoted conditioned responding (licking), but not unconditioned behavior nor innate motor outputs. In vivo electrophysiological recordings during cholinergic photostimulation revealed reward-contingency-dependent suppression of BLA neural activity, but not prefrontal cortex. Finally, ex vivo experiments demonstrated that photostimulation of cholinergic terminals suppressed BLA projection neuron activity via monosynaptic muscarinic receptor signaling, while also facilitating firing in BLA GABAergic interneurons. Taken together, we show that the neural and behavioral effects of basal forebrain cholinergic activation are modulated by reward contingency in a target-specific manner.

Differential Regulation of Prelimbic and Thalamic Transmission to the Basolateral Amygdala by Acetylcholine Receptors

The amygdalar anterior basolateral nucleus (BLa) plays a vital role in emotional behaviors. This region receives dense cholinergic projections from basal forebrain which are critical in regulating neuronal activity in BLa. Cholinergic signaling in BLa has also been shown to modulate afferent glutamatergic inputs to this region. However, these studies, which have used cholinergic agonists or prolonged optogenetic stimulation of cholinergic fibers, may not reflect the effect of physiological acetylcholine release in the BLa. To better understand these effects of acetylcholine, we have used electrophysiology and optogenetics in male and female mouse brain slices to examine cholinergic regulation of afferent BLa input from cortex and midline thalamic nuclei. Phasic ACh release evoked by single pulse stimulation of cholinergic terminals had a biphasic effect on transmission at cortical input, producing rapid nicotinic receptor-mediated facilitation followed by slower mAChR-mediated depression. In contrast, at this same input, sustained ACh elevation through application of the cholinesterase inhibitor physostigmine suppressed glutamatergic transmission through mAChRs only. This suppression was not observed at midline thalamic nuclei inputs to BLa. In agreement with this pathway specificity, the mAChR agonist, muscarine more potently suppressed transmission at inputs from prelimbic cortex than thalamus. Muscarinic inhibition at prelimbic cortex input required presynaptic M4 mAChRs, while at thalamic input it depended on M3 mAChR-mediated stimulation of retrograde endocannabinoid signaling. Muscarinic inhibition at both pathways was frequency-dependent, allowing only high-frequency activity to pass. These findings demonstrate complex cholinergic regulation of afferent input to BLa that is pathway-specific and frequency-dependent.SIGNIFICANCE STATEMENT Cholinergic modulation of the basolateral amygdala regulates formation of emotional memories, but the underlying mechanisms are not well understood. Here, we show, using mouse brain slices, that ACh differentially regulates afferent transmission to the BLa from cortex and midline thalamic nuclei. Fast, phasic ACh release from a single optical stimulation biphasically regulates glutamatergic transmission at cortical inputs through nicotinic and muscarinic receptors, suggesting that cholinergic neuromodulation can serve precise, computational roles in the BLa. In contrast, sustained ACh elevation regulates cortical input through muscarinic receptors only. This muscarinic regulation is pathway-specific with cortical input inhibited more strongly than midline thalamic nuclei input. Specific targeting of these cholinergic receptors may thus provide a therapeutic strategy to bias amygdalar processing and regulate emotional memory.

Modeling optical design parameters for fine stimulation in sciatic nerve of optogenetic mice

Optogenetics presents an alternative method for interfacing with the nervous system over the gold-standard of electrical stimulation. While electrical stimulation requires electrodes to be surgically embedded in tissue for in vivo studies, optical stimulation offers a less-invasive approach that may yield more specific, localized stimulation. The advent of optogenetic laboratory animals-whose motor neurons can be activated when illuminated with blue light-enables research into refining optical stimulation of the mammalian nervous system where subsets of nerve fibers within a nerve may be stimulated without embedding any device directly into the nerve itself. However, optical stimulation has a major drawback in that light is readily scattered and absorbed in tissue thereby limiting the depth with which a single emission source can penetrate. We hypothesize that the use of multiple, focused light emissions deployed around the circumference of a nerve can overcome these light-scattering limitations. To understand the physical parameters necessary to produce pinpointed light stimulation within a single nerve, we employed a simplified Monte Carlo simulation to estimate the size of nerves where this technique may be successful, as well as the necessary optical lens design for emitters to be used during future in vivo studies. By modeling multiple focused beams, we find that only fascicles within a nerve diameter less than 1 mm are fully accessible to focused optical stimulation; a minimum of 4 light sources is required to generate a photon intensity at a point in a nerve over the initial contact along its surface. To elicit the same effect in larger nerves, focusing lenses would require a numerical aperture [Formula: see text]. These simulations inform on the design of instrumentation capable of stimulating disparate motor neurons in mouse sciatic nerve to control hindlimb movement.

Cholinergic upregulation by optogenetic stimulation of nucleus basalis after photothrombotic stroke in forelimb somatosensory cortex improves endpoint and motor but not sensory control of skilled reaching in mice

A network of cholinergic neurons in the basal forebrain innerve the forebrain and are proposed to contribute to a variety of functions including cortical plasticity, attention, and sensorimotor behavior. This study examined the contribution of the nucleus basalis cholinergic projection to the sensorimotor cortex on recovery on a skilled reach-to-eat task following photothrombotic stroke in the forelimb region of the somatosensory cortex. Mice were trained to perform a single pellet skilled reaching task and their pre and poststroke performance, from Day 4 to Day 28 poststroke, was assessed frame-by-frame by video analysis with endpoint, movement and sensorimotor integration measures. Somatosensory forelimb lesions produced impairments in endpoint and movement component measures of reaching and increased the incidence of fictive eating, a sensory impairment in mistaking a missed reach for a successful reach. Upregulated acetylcholine (ACh) release, as measured by local field potential recording, elicited via optogenetic stimulation of the nucleus basalis improved recovery of reaching and improved movement scores but did not affect sensorimotor integration impairment poststroke. The results show that the mouse cortical forelimb somatosensory region contributes to forelimb motor behavior and suggest that ACh upregulation could serve as an adjunct to behavioral therapy for acute treatment of stroke.

Alteration of the cholinergic system and motor deficits in cholinergic neuron-specific Dyt1 knockout mice

Dystonia is a neurological movement disorder characterized by sustained or intermittent muscle contractions, repetitive movement, and sometimes abnormal postures. DYT1 dystonia is one of the most common genetic dystonias, and most patients carry heterozygous DYT1 ∆GAG mutations causing a loss of a glutamic acid of the protein torsinA. Patients can be treated with anticholinergics, such as trihexyphenidyl, suggesting an abnormal cholinergic state. Early work on the cell-autonomous effects of Dyt1 deletion with ChI-specific Dyt1 conditional knockout mice (Dyt1 Ch1KO) revealed abnormal electrophysiological responses of striatal ChIs to muscarine and quinpirole, motor deficits, and no changes in the number or size of the ChIs. However, the Chat-cre line that was used to derive Dyt1 Ch1KO mice contained a neomycin cassette and was reported to have ectopic cre-mediated recombination. In this study, we generated a Dyt1 Ch2KO mouse line by removing the neomycin cassette in Dyt1 Ch1KO mice. The Dyt1 Ch2KO mice showed abnormal paw clenching behavior, motor coordination and balance deficits, impaired motor learning, reduced striatal choline acetyltransferase protein level, and a reduced number of striatal ChIs. Furthermore, the mutant striatal ChIs had a normal muscarinic inhibitory function, impaired quinpirole-mediated inhibition, and altered current density. Our findings demonstrate a cell-autonomous effect of Dyt1 deletion on the striatal ChIs and a critical role for the striatal ChIs and corticostriatal pathway in the pathogenesis of DYT1 dystonia.

Optogenetic Control of Cardiac Autonomic Neurons in Transgenic Mice

Optogenetic technology has enabled unparalleled insights into cellular and organ physiology by providing exquisite temporal and spatial control of biological pathways. Here, an optogenetic approach is presented for selective activation of the intrinsic cardiac nervous system in excised perfused mouse hearts. The breeding of transgenic mice that have selective expression of channelrhodopsin in either catecholaminergic or cholinergic neurons is described. An approach for perfusing hearts excised from those animals, recording the ECG to measure heart rate changes, and an illumination technique using a custom micro-LED light source to activate channelrhodopsin is explained. We have used these methods in ongoing studies of the kinetics of autonomic control of cardiac electrophysiology and contractility, demonstrating the proven utility of optogenetic technology to enable unparalleled spatiotemporal anatomic-functional probing of the intrinsic cardiac nervous system.

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.

Implications of Oligomeric Amyloid-Beta (oAβ42) Signaling through α7β2-Nicotinic Acetylcholine Receptors (nAChRs) on Basal Forebrain Cholinergic Neuronal Intrinsic Excitability and Cognitive Decline

Neuronal and network-level hyperexcitability is commonly associated with increased levels of amyloid-β (Aβ) and contribute to cognitive deficits associated with Alzheimer's disease (AD). However, the mechanistic complexity underlying the selective loss of basal forebrain cholinergic neurons (BFCNs), a well-recognized characteristic of AD, remains poorly understood. In this study, we tested the hypothesis that the oligomeric form of amyloid-β (oAβ₄₂), interacting with α7-containing nicotinic acetylcholine receptor (nAChR) subtypes, leads to subnucleus-specific alterations in BFCN excitability and impaired cognition. We used single-channel electrophysiology to show that oAβ₄₂ activates both homomeric α7- and heteromeric α7β2-nAChR subtypes while preferentially enhancing α7β2-nAChR open-dwell times. Organotypic slice cultures were prepared from male and female ChAT-EGFP mice, and current-clamp recordings obtained from BFCNs chronically exposed to pathophysiologically relevant level of oAβ₄₂ showed enhanced neuronal intrinsic excitability and action potential firing rates. These resulted from a reduction in action potential afterhyperpolarization and alterations in the maximal rates of voltage change during spike depolarization and repolarization. These effects were observed in BFCNs from the medial septum diagonal band and horizontal diagonal band, but not the nucleus basalis. Last, aged male and female APP/PS1 transgenic mice, genetically null for the β2 nAChR subunit gene, showed improved spatial reference memory compared with APP/PS1 aged-matched littermates. Combined, these data provide a molecular mechanism supporting a role for α7β2-nAChR in mediating the effects of oAβ₄₂ on excitability of specific populations of cholinergic neurons and provide a framework for understanding the role of α7β2-nAChR in oAβ₄₂-induced cognitive decline.

Tracing goes viral: Viruses that introduce expression of fluorescent proteins in chemically-specific neurons

Over the last century, there has been great progress in understanding how the brain works. In particular, the last two decades have been crucial in gaining more awareness over the complex functioning of neurotransmitter systems. The use of viral vectors in neuroscience has been pivotal for such development. Exploiting the properties of viral particles, modifying them according to the research needs, and making them target chemically-specific neurons, techniques such as optogenetics and chemogenetics have been developed, which could lead to a giant step toward gene therapy for brain disorders. In this review, we aim to provide an overview of some of the most widely used viral techniques in neuroscience. We will discuss advantages and disadvantages of these methods. In particular, attention is dedicated to the pivotal role played by the introduction of adeno-associated virus and the retrograde tracer canine-associated-2 Cre virus in order to achieve optimal visualization, and interrogation, of chemically-specific neuronal populations and their projections.

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.

Chrna5 is Essential for a Rapid and Protected Response to Optogenetic Release of Endogenous Acetylcholine in Prefrontal Cortex

Optimal attention performance requires cholinergic modulation of corticothalamic neurons in the prefrontal cortex. These pyramidal cells express specialized nicotinic acetylcholine receptors containing the α5 subunit encoded by Chrna5 Disruption of this gene impairs attention, but the advantage α5 confers on endogenous cholinergic signaling is unknown. To ascertain this underlying mechanism, we used optogenetics to stimulate cholinergic afferents in prefrontal cortex brain slices from compound-transgenic wild-type and Chrna5 knock-out mice of both sexes. These electrophysiological experiments identify that Chrna5 is critical for the rapid onset of the postsynaptic cholinergic response. Loss of α5 slows cholinergic excitation and delays its peak, and these effects are observed in two different optogenetic mouse lines. Disruption of Chrna5 does not otherwise perturb the magnitude of the response, which remains strongly mediated by nicotinic receptors and tightly controlled by autoinhibition via muscarinic M2 receptors. However, when conditions are altered to promote sustained cholinergic receptor stimulation, it becomes evident that α5 also works to protect nicotinic responses against desensitization. Rescuing Chrna5 disruption thus presents the double challenge of improving the onset of nicotinic signaling without triggering desensitization. Here, we identify that an agonist for the unorthodox α-α nicotinic binding site can allosterically enhance the cholinergic pathway considered vital for attention. Treatment with NS9283 restores the rapid onset of the postsynaptic cholinergic response without triggering desensitization. Together, this work demonstrates the advantages of speed and resilience that Chrna5 confers on endogenous cholinergic signaling, defining a critical window of interest for cue detection and attentional processing.SIGNIFICANCE STATEMENT The α5 nicotinic receptor subunit (Chrna5) is important for attention, but its advantage in detecting endogenous cholinergic signals is unknown. Here, we show that α5 subunits permit rapid cholinergic responses in prefrontal cortex and protect these responses from desensitization. Our findings clarify why Chrna5 is required for optimal attentional performance under demanding conditions. To treat the deficit arising from Chrna5 disruption without triggering desensitization, we enhanced nicotinic receptor affinity using NS9283 stimulation at the unorthodox α-α nicotinic binding site. This approach successfully restored the rapid-onset kinetics of endogenous cholinergic neurotransmission. In summary, we reveal a previously unknown role of Chrna5 as well as an effective approach to compensate for genetic disruption and permit fast cholinergic excitation of prefrontal attention circuits.

Dual recombinase fate mapping reveals a transient cholinergic phenotype in multiple populations of developing glutamatergic neurons

Cholinergic transmission shapes the maturation of glutamatergic circuits, yet the developmental sources of acetylcholine have not been systematically explored. Here, we have used Cre-recombinase-mediated genetic labeling to identify and map both mature and developing CNS neurons that express choline acetyltransferase (ChAT). Correction of a significant problem with a widely used ChatCre transgenic line ensures that this map does not contain expression artifacts. ChatCre marks all known cholinergic systems in the adult brain, but also identifies several brain areas not usually regarded as cholinergic, including specific thalamic and hypothalamic neurons, the subiculum, the lateral parabrachial nucleus, the cuneate/gracilis nuclei, and the pontocerebellar system. This ChatCre fate map suggests transient developmental expression of a cholinergic phenotype in areas important for cognition, motor control, and respiration. We therefore examined expression of ChAT and the vesicular acetylcholine transporter in the embryonic and early postnatal brain to determine the developmental timing of this transient cholinergic phenotype, and found that it mirrored the establishment of relevant glutamatergic projection pathways. We then used an intersectional genetic strategy combining ChatCre with Vglut2Flp to show that these neurons adopt a glutamatergic fate in the adult brain. The transient cholinergic phenotype of these glutamatergic neurons suggests a homosynaptic source of acetylcholine for the maturation of developing glutamatergic synapses. These findings thus define critical windows during which specific glutamatergic circuits may be vulnerable to disruption by nicotine in utero, and suggest new mechanisms for pediatric disorders associated with maternal smoking, such as sudden infant death syndrome.

Selective attenuation of Ether-a-go-go related K+ currents by endogenous acetylcholine reduces spike-frequency adaptation and network correlation

Most neurons do not simply convert inputs into firing rates. Instead, moment-to-moment firing rates reflect interactions between synaptic inputs and intrinsic currents. Few studies investigated how intrinsic currents function together to modulate output discharges and which of the currents attenuated by synthetic cholinergic ligands are actually modulated by endogenous acetylcholine (ACh). In this study we optogenetically stimulated cholinergic fibers in rat neocortex and find that ACh enhances excitability by reducing Ether-à-go-go Related Gene (ERG) K+ current. We find ERG mediates the late phase of spike-frequency adaptation in pyramidal cells and is recruited later than both SK and M currents. Attenuation of ERG during coincident depolarization and ACh release leads to reduced late phase spike-frequency adaptation and persistent firing. In neuronal ensembles, attenuating ERG enhanced signal-to-noise ratios and reduced signal correlation, suggesting that these two hallmarks of cholinergic function in vivo may result from modulation of intrinsic properties.

Sudden Heart Rate Reduction Upon Optogenetic Release of Acetylcholine From Cardiac Parasympathetic Neurons in Perfused Hearts

The balance of sympathetic and parasympathetic tone provides exquisite control of heart rate and contractility and has also been shown to modulate coronary flow and inflammation. Understanding how autonomic balance is altered by cardiac disease is an active area of research, and developing new ways to control this balance provides insights into disease therapies. However, achieving acute neuron-specific stimulation of autonomic neurons can be difficult in experiments that measure the acute effects of nerve stimulation on the heart. Conventional electrical and pharmacological approaches can be spatially and temporally non-selective. Cell-specific expression of light-activated channels (channelrhodopsin, ChR2) is a powerful approach that enables control of the timing and distribution of cellular stimulation using light. We present such an optogenetic approach where parasympathetic cardiac neurons are selectively photoactivated at high temporal precision to initiate cholinergic-mediated slowing of heart rate. Mice were crossbred to express ChR2 in peripheral cholinergic neurons using Cre-Lox recombination driven by a choline acetyltransferase (ChAT) promoter. Hearts from adult mice were excised, perfused, and the epicardium was illuminated (peak 460–465 nm) to photoactivate ChR2. In one set of studies, hearts were illuminated using a large-field LED light source. In other studies, a micro LED was placed on the right atrium to selectively illuminate the junction of the superior vena cava (SVC) and right atrium. The ECG was acquired before, during, and after tissue illumination to measure changes in heart rate. Upon illumination, hearts exhibited sudden and dramatic reductions in heart rate with restoration of normal heart rate after cessation of illumination. Delays in atrioventricular conduction were also observed. Heart rate reductions at the highest irradiance levels were similar to heart rate reductions caused by application of bethanechol (10 μM) or acetylcholine (800 μM). Atropine (50 nM) completely blocked the effect of ChR2 photoactivation, confirming cholinergic mediation. Optogenetic activation of intrinsic parasympathetic neurons reduced heart rate in an immediate, dose-dependent fashion, resembling the slowing of sinus rate in response to acetylcholine. Our results demonstrate a new approach for controlling parasympathetic modulation of cardiac function by selectively activating the endogenous release of acetylcholine from intrinsic cardiac cholinergic neurons.

Key Message: Optogenetic photoactivation of intrinsic cardiac neurons provides immediate, tissue-specific stimulation with minimal cross-reactivity. Our results demonstrate that selective expression of channelrhodopsin within cardiac cholinergic neurons enables photoactivated release of acetylcholine, thereby instantaneously slowing sinus rate and altering atrioventricular conduction. This provides for in-depth examination of the endogenous interplay between cardiac autonomic neurons and the functional outcomes of downstream post-synaptic receptor activation.

Discharge and Role of Acetylcholine Pontomesencephalic Neurons in Cortical Activity and Sleep-Wake States Examined by Optogenetics and Juxtacellular Recording in Mice

Acetylcholine (ACh) neurons in the pontomesencephalic tegmentum (PMT) are thought to play an important role in promoting cortical activation with waking (W) and paradoxical sleep [PS; or rapid eye movement (REM)], but have yet to be proven to do so by selective stimulation and simultaneous recording of identified ACh neurons. Here, we employed optogenetics combined with juxtacellular recording and labeling of neurons in transgenic (TG) mice expressing ChR2 in choline acetyltransferase (ChAT)-synthesizing neurons. We established in vitro then in vivo in anesthetized (A) and unanesthetized (UA), head-fixed mice that photostimulation elicited a spike with short latency in neurons which could be identified by immunohistochemical staining as ACh neurons within the laterodorsal (LDT)/sublaterodorsal (SubLDT) and pedunculopontine tegmental (PPT) nuclei. Continuous light pulse stimulation during sleep evoked tonic spiking by ACh neurons that elicited a shift from irregular slow wave activity to rhythmic θ and enhanced γ activity on the cortex without behavioral arousal. With θ frequency rhythmic light pulse stimulation, ACh neurons discharged in bursts that occurred in synchrony with evoked cortical θ. During natural sleep-wake states, they were virtually silent during slow wave sleep (SWS), discharged in bursts during PS and discharged tonically during W. Yet, their bursting during PS was not rhythmic or synchronized with cortical θ but associated with phasic whisker movements. We conclude that ACh PMT neurons promote θ and γ cortical activity during W and PS by their tonic or phasic discharge through release of ACh onto local neurons within the PMT and/or more distant targets in the hypothalamus and thalamus.

Nicotine aversion is mediated by GABAergic interpeduncular nucleus inputs to laterodorsal tegmentum

Nicotine use can lead to dependence through complex processes that are regulated by both its rewarding and aversive effects. Recent studies show that aversive nicotine doses activate excitatory inputs to the interpeduncular nucleus (IPN) from the medial habenula (MHb), but the downstream targets of the IPN that mediate aversion are unknown. Here we show that IPN projections to the laterodorsal tegmentum (LDTg) are GABAergic using optogenetics in tissue slices from mouse brain. Selective stimulation of these IPN axon terminals in LDTg in vivo elicits avoidance behavior, suggesting that these projections contribute to aversion. Nicotine modulates these synapses in a concentration-dependent manner, with strong enhancement only seen at higher concentrations that elicit aversive responses in behavioral tests. Optogenetic inhibition of the IPN-LDTg connection blocks nicotine conditioned place aversion, suggesting that the IPN-LDTg connection is a critical part of the circuitry that mediates the aversive effects of nicotine.

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.

Preferential cholinergic excitation of corticopontine neurons

KEY POINTS

Phasic activation of M1 muscarinic receptors generates transient inhibition followed by longer lasting excitation in neocortical pyramidal neurons. Corticopontine neurons in the mouse prefrontal cortex exhibit weaker cholinergic inhibition, but more robust and longer lasting excitation, than neighbouring callosal projection neurons. Optogenetic release of endogenous ACh in response to single flashes of light (5 ms) preferentially enhances the excitability of corticopontine neurons for many tens of seconds. Cholinergic excitation of corticopontine neurons involves at least three ionic mechanisms: suppression of K_V 7 currents, activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Preferential cholinergic excitation of prefrontal corticopontine neurons may facilitate top-down attentional processes and behaviours.

ABSTRACT

Pyramidal neurons in layer 5 of the neocortex comprise two broad classes of projection neurons: corticofugal neurons, including corticopontine (CPn) neurons, and intratelencephalic neurons, including commissural/callosal (COM) neurons. These non-overlapping neuron subpopulations represent discrete cortical output channels contributing to perception, decision making and behaviour. CPn and COM neurons have distinct morphological and physiological characteristics, and divergent responses to modulatory transmitters such as serotonin and acetylcholine (ACh). To better understand how ACh regulates cortical output, in slices of mouse prefrontal cortex (PFC) we compared the responsivity of CPn and COM neurons to transient exposure to exogenous or endogenous ACh. In both neuron subtypes, exogenous ACh generated qualitatively similar biphasic responses in which brief hyperpolarization was followed by longer lasting enhancement of excitability. However, cholinergic inhibition was more pronounced in COM neurons, while excitatory responses were larger and longer lasting in CPn neurons. Similarly, optically triggered release of endogenous ACh from cholinergic terminals preferentially and persistently (for ∼40 s) enhanced the excitability of CPn neurons, but had little impact on COM neurons. Cholinergic excitation of CPn neurons involved at least three distinct ionic mechanisms: suppression of K_V 7 channels (the 'M-current'), activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Our findings demonstrate projection-specific selectivity in cholinergic signalling in the PFC, and suggest that transient release of ACh during behaviour will preferentially promote corticofugal output.

Altered Baseline and Nicotine-Mediated Behavioral and Cholinergic Profiles in ChAT-Cre Mouse Lines

The recent development of transgenic rodent lines expressing cre recombinase in a cell-specific manner, along with advances in engineered viral vectors, has permitted in-depth investigations into circuit function. However, emerging evidence has begun to suggest that genetic modifications may introduce unexpected caveats. In the current studies, we sought to extensively characterize male and female mice from both the ChAT_(BAC)-Cre mouse line, created with the bacterial artificial chromosome (BAC) method, and ChAT_(IRES)-Cre mouse line, generated with the internal ribosome entry site (IRES) method. ChAT_(BAC)-Cre transgenic and wild-type mice did not differ in general locomotor behavior, anxiety measures, drug-induced cataplexy, nicotine-mediated hypolocomotion, or operant food training. However, ChAT_(BAC)-Cre transgenic mice did exhibit significant deficits in intravenous nicotine self-administration, which paralleled an increase in vesicular acetylcholine transporter and choline acetyltransferase (ChAT) hippocampal expression. For the ChAT_(IRES)-Cre line, transgenic mice exhibited deficits in baseline locomotor, nicotine-mediated hypolocomotion, and operant food training compared with wild-type and hemizygous littermates. No differences among ChAT_(IRES)-Cre wild-type, hemizygous, and transgenic littermates were found in anxiety measures, drug-induced cataplexy, and nicotine self-administration. Given that increased cre expression was present in the ChAT_(IRES)-Cre transgenic mice, as well as a decrease in ChAT expression in the hippocampus, altered neuronal function may underlie behavioral phenotypes. In contrast, ChAT_(IRES)-Cre hemizygous mice were more similar to wild-type mice in both protein expression and the majority of behavioral assessments. As such, interpretation of data derived from ChAT-Cre rodents must consider potential limitations dependent on the line and/or genotype used in research investigations.SIGNIFICANCE STATEMENT Altered baseline and/or nicotine-mediated behavioral profiles were discovered in transgenic mice from the ChAT_(BAC)-Cre and ChAT_(IRES)-Cre lines. Given that these cre-expressing mice have become increasingly used by the scientific community, either independently with chemicogenetic and optogenetic viral vectors or crossed with other transgenic lines, the current studies highlight important considerations for the interpretation of data from previous and future experimental investigations. Moreover, the current findings detail the behavioral effects of either increased or decreased baseline cholinergic signaling mechanisms on locomotor, anxiety, learning/memory, and intravenous nicotine self-administration behaviors.

Electrophysiological properties of basal forebrain cholinergic neurons identified by genetic and optogenetic tagging

Recent developments in the generation of neuronal population-specific, genetically modified mouse lines have allowed precise identification and selective stimulation of cholinergic neurons in vivo. Although considerably less laborious than studies conducted with post hoc identification of cholinergic neurons by immunostaining, it is not known whether the genetically based labeling procedures that permit in vivo identification are electrophysiologically benign. In this study, we use mice carrying a bacterial artificial chromosome transgene that drives expression of a tau-green fluorescent fusion protein specifically in cholinergic neurons. This allowed us to visualize basal forebrain cholinergic neurons in acute slice preparations. Using whole cell, patch clamp electrophysiological recording in acute brain slices, here we present original data about the basic electrical properties of these genetically tagged cholinergic neurons including firing rate, resting membrane potential, rheobase, and various characteristics of their action potentials and after-hyperpolarization potentials. The basic electrical properties are compared (i) with non-cholinergic neurons in the same brain regions; (ii) in cholinergic neurons between immature animals and young adults; and (iii) with cholinergic neurons that are expressing light-sensitive channels. Our conclusions based on these data are (i) cholinergic neurons are less excitable then their non-cholinergic neighbors, (ii) the basic properties of cholinergic neurons do not significantly change between adolescence and young adulthood and (iii) these properties are not significantly affected by chronic expression of the excitatory opsin, oChIEF. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.