Cholinergic-Sensitive Theta Oscillations in Memory Encoding in Mice

Cholinergic regulation of hippocampal theta oscillations has long been proposed to be a potential mechanism underlying hippocampus-dependent memory encoding processes. However, cholinergic transmission has been traditionally associated with type II theta under urethane anesthesia. The mechanisms and behavioral significance of cholinergic regulation of type I theta in freely exploring animals is much less clear. In this study, we examined the potential behavioral significance of cholinergic regulation of theta oscillations in the object location task in male mice that involves training and testing trials and provides an ideal behavioral task to study the underlying memory encoding and retrieval processes, respectively. Cholinergic regulation of hippocampal theta oscillations and the behavioral outcomes was examined by either intrahippocampal infusion of cholinergic receptor antagonists or knocking out cholinergic receptors in excitatory neurons or interneurons. We found that both muscarinic acetylcholine receptors (mAChRs) and α7 nicotinic AChRs (α7 nAChRs) regulated memory encoding by engaging excitatory neurons and interneurons, respectively. There is a transient upregulated theta oscillation at the beginning of individual object exploration events that only occurred in the training trials, but not in the testing trials. This transient upregulated theta is also the only theta component that significantly differed between training and testing trials and was sensitive to mAChR and α7 nAChR antagonists. Thus, our study has revealed a transient cholinergic-sensitive theta component that is specifically associated with memory encoding, but not memory retrieval, in the object location task, providing direct experimental evidence supporting a role for cholinergic-regulated theta oscillations in hippocampus-dependent memory encoding processes.

Entorhinal cortical delta oscillations drive memory consolidation

Long-term memories are formed by creating stable memory representations via memory consolidation, which mainly occurs during sleep following the encoding of labile memories in the hippocampus during waking. The entorhinal cortex (EC) has intricate connections with the hippocampus, but its role in memory consolidation is largely unknown. Using cell-type- and input-specific in vivo neural activity recordings, here we show that the temporoammonic pathway neurons in the EC, which directly innervate the output area of the hippocampus, exhibit potent oscillatory activities during anesthesia and sleep. Using in vivo individual and populational neuronal activity recordings, we demonstrate that a subpopulation of the temporoammonic pathway neurons, which we termed sleep cells, generate delta oscillations via hyperpolarization-activated cyclic-nucleotide-gated channels during sleep. The blockade of these oscillations significantly impaired the consolidation of hippocampus-dependent memory. Together, our findings uncover a key driver of delta oscillations and memory consolidation that are found in the EC.

Genetic deletion of α7 nAChRs reduces hippocampal granule and pyramidal cell number in both sexes but impairs pattern separation in males only

Introduction

Neurogenesis within the dentate gyrus is thought to play an important role in cognitive processes such as reversal learning and pattern separation. The α7 nicotinic acetylcholine receptor (α7 nAChR) is expressed early in newly formed granule cells of the dentate gyrus, though its role in neurogenesis and related cognitive function is not fully understood.

Methods

To better characterize relevant function of α7 nAChRs, we performed unbiased stereology to quantify hippocampal granule cells, pyramidal cells, and total volume and used a touchscreen operant spatial discrimination/reversal task to test pattern separation in a global α7 nAChR knockout mouse line.

Results

The knockout resulted in an ≈22% reduction in granule cells and a ≈ 20% reduction in pyramidal cells in both sexes, with no change in total hippocampal volume. However, the knockout impaired performance in the touchscreen task for males only. The sex-dependent difference in behavioral, but not stereological, results suggest a divergence in the structure-function relationship in males versus females. Detailed analyses revealed males were more biased by the initial reversal contingency relative to females indicating a potential source of the sex-specific interaction with the loss of α7 nAChRs.

Discussion

These findings argue that the α7 nAChR plays a critical role in hippocampal development, not just granule cell neurogenesis, and plays a sex-dependent role in cognitive function.

Loss of GABA co-transmission from cholinergic neurons impairs behaviors related to hippocampal, striatal, and medial prefrontal cortex functions

Introduction: Altered signaling or function of acetylcholine (ACh) has been reported in various neurological diseases, including Alzheimer's disease, Tourette syndrome, epilepsy among others. Many neurons that release ACh also co-transmit the neurotransmitter gamma-aminobutyrate (GABA) at synapses in the hippocampus, striatum, substantia nigra, and medial prefrontal cortex (mPFC). Although ACh transmission is crucial for higher brain functions such as learning and memory, the role of co-transmitted GABA from ACh neurons in brain function remains unknown. Thus, the overarching goal of this study was to investigate how a systemic loss of GABA co-transmission from ACh neurons affected the behavioral performance of mice. Methods: To do this, we used a conditional knock-out mouse of the vesicular GABA transporter (vGAT) crossed with the ChAT-Cre driver line to selectively ablate GABA co-transmission at ACh synapses. In a comprehensive series of standardized behavioral assays, we compared Cre-negative control mice with Cre-positive vGAT knock-out mice of both sexes. Results: Loss of GABA co-transmission from ACh neurons did not disrupt the animal's sociability, motor skills or sensation. However, in the absence of GABA co-transmission, we found significant alterations in social, spatial and fear memory as well as a reduced reliance on striatum-dependent response strategies in a T-maze. In addition, male conditional knockout (CKO) mice showed increased locomotion. Discussion: Taken together, the loss of GABA co-transmission leads to deficits in higher brain functions and behaviors. Therefore, we propose that ACh/GABA co-transmission modulates neural circuitry involved in the affected behaviors.

Nicotine exposure decreases likelihood of SARS-CoV-2 RNA expression and neuropathology in the hACE2 mouse brain but not moribundity

Individuals infected by SARS-CoV-2 are at risk of developing neurological-related post-acute disorders. Disputed epidemiological data indicated nicotine may reduce the severity of infection. Here we find exposure to nicotine in drinking water does not alter the moribundity of hACE2 mice. However, pre-exposure to nicotine decreased the likelihood of SARS-CoV-2 RNA expression and pathology in the brain. These results suggest mechanisms involving targets of nicotine could be leveraged to prevent the neurovirulence of SARS-CoV-2.

Recurring Cholinergic Inputs Induce Local Hippocampal Plasticity through Feedforward Disinhibition

The CA1 pyramidal neurons are embedded in an intricate local circuitry that contains a variety of interneurons. The roles these interneurons play in the regulation of the excitatory synaptic plasticity remains largely understudied. Recent experiments showed that recurring cholinergic activation of α7 nACh receptors expressed in oriens-lacunosum-moleculare (OLMα2) interneurons can directly induce LTP in Schaffer collateral (SC)-CA1 synapses. Here, we pair in vitro studies with biophysically based modeling to uncover the underlying mechanisms. According to our model, α7 nAChR activation increases OLM GABAergic activity. This results in the inhibition of the fast-spiking interneurons that provide feedforward inhibition onto CA1 pyramidal neurons. This disinhibition, paired with tightly timed SC stimulation, can induce potentiation at the excitatory synapses of CA1 pyramidal neurons. Our work details the role of cholinergic modulation in disinhibition-induced hippocampal plasticity. It relates the timing of cholinergic pairing found experimentally in previous studies with the timing between disinhibition and hippocampal stimulation necessary to induce potentiation and suggests the dynamics of the involved interneurons play a crucial role in determining this timing.Significance StatementWe use a combination of experiments and mechanistic modeling to uncover the key role for cholinergic neuromodulation of feedforward disinhibitory circuits in regulating hippocampal plasticity. We found that cholinergic activation of α7 nAChR on α7 nACh receptors expressed in oriens-lacunosum-moleculare interneurons, when tightly paired with stimulation of the Schaffer collaterals, can cancel feedforward inhibition onto CA1 pyramidal cells, enabling the potentiation of the SC-CA1 synapse. Our work details how cholinergic action on GABAergic interneurons can tightly regulate the excitability and plasticity of the hippocampal network, unraveling the intricate interplay of the hierarchal inhibitory circuitry and cholinergic neuromodulation as a mechanism for hippocampal plasticity.

Cholinergic Regulation of Hippocampal Theta Rhythm

Cholinergic regulation of hippocampal theta rhythm has been proposed as one of the central mechanisms underlying hippocampal functions including spatial memory encoding. However, cholinergic transmission has been traditionally associated with atropine-sensitive type II hippocampal theta oscillations that occur during alert immobility or in urethane-anesthetized animals. The role of cholinergic regulation of type I theta oscillations in behaving animals is much less clear. Recent studies strongly suggest that both cholinergic muscarinic and nicotinic receptors do actively regulate type I hippocampal theta oscillations and thus provide the cholinergic mechanism for theta-associated hippocampal learning. Septal cholinergic activation can regulate hippocampal circuit and theta expression either through direct septohippocampal cholinergic projections, or through septal glutamatergic and GABAergic neurons, that can precisely entrain hippocampal theta rhythmicity.

α7 nicotinic acetylcholine receptors in the hippocampal circuit: taming complexity

Cholinergic innervation of the hippocampus uses the neurotransmitter acetylcholine (ACh) to coordinate neuronal circuit activity while simultaneously influencing the function of non-neuronal cell types. The α7 nicotinic ACh receptor (nAChR) subtype is highly expressed throughout the hippocampus, has the highest calcium permeability compared with other subtypes of nAChRs, and is of high therapeutic interest due to its association with a variety of neurological disorders and neurodegenerative diseases. In this review, we synthesize research describing α7 nAChR properties, function, and relationship to cognitive dysfunction within the hippocampal circuit and highlight approaches to help improve therapeutic development.

Differential signalling induced by α7 nicotinic acetylcholine receptors in hippocampal dentate gyrus in vitro and in vivo

The activation of α7 nicotinic acetylcholine receptors (nAChRs) has been shown to improve hippocampus-dependent learning and memory. α7 nAChRs are densely expressed among several different cell types in the hippocampus, with high Ca²⁺  permeability, although it is unclear if α7 nAChRs mobilize differential signalling mechanisms among distinct neuronal populations. To address this question, we compared α7 nAChR agonist-induced responses (i.e. calcium and cAMP changes) between granule cells and GABAergic neurons in the hippocampal dentate gyrus both in vitro and in vivo. In cultured organotypic hippocampal slices, we observed robust intracellular calcium and cAMP increases in dentate granule cells upon activation of α7 nAChRs. In contrast, GABAergic interneurons displayed little change in either calcium or cAMP concentration after α7 nAChR activation, even though they displayed much larger α7 nAChR current responses than those of dentate granule cells. We found that this was due to smaller α7 nAChR-induced Ca²⁺ rises in GABAergic interneurons. Thus, the regulation of the Ca²⁺ transients in different cell types resulted in differential subsequent intracellular signalling cascades and likely the ultimate outcome of α7 nAChR activation. Furthermore, we monitored neuronal activities of dentate granule cells and GABAergic interneurons in vivo via optic fibre photometry. We observed enhancement of neuronal activities after nicotine administration in dentate granule cells, but not in GABAergic neurons, which was absent in α7 nAChR-deficient granule cells. In summary, we reveal a mechanism for α7 nAChR-mediated increase of neuronal activity via cell type-specific intracellular signalling pathways. KEY POINTS: α7 nicotinic acetylcholine receptors (nAChRs) are widely distributed throughout the central nervous system and regulate a variety of brain functions including learning and memory. Understanding the cellular signalling mechanisms of their activations among different neuronal populations is important for delineating their actions in cognitive function, and developing effective treatment strategies for cognitive deficits. We report that α7 nAChR activation leads to Ca²⁺ and cAMP increases in granule cells (but not in GABAergic interneurons) in hippocampal dentate gyrus in vitro, a key region for pattern separation during learning. We also found that nicotine enhanced granule cell (but not in GABAergic interneurons) activity in an α7 nAChR-dependent manner via in vivo fibre photometry recording. Based on our findings, we propose that differential responses to α7 nAChR activation between granule cells and GABAergic interneurons is responsible for the increase of excitation by α7 nAChR agonists in hippocampal circuits synergistically.

Hippocampal Interneuronal α7 nAChRs Modulate Theta Oscillations in Freely Moving Mice

Muscarinic acetylcholine receptors (mAChRs) are critically involved in hippocampal theta generation, but much less is known about the role of nicotinic AChRs (nAChRs). Here we provide evidence that α7 nAChRs expressed on interneurons, particularly those in oriens lacunosum moleculare (OLM), also regulate hippocampal theta generation. Local hippocampal infusion of a selective α7 nAChR antagonist significantly reduces hippocampal theta power and impairs Y-maze spontaneous alternation performance in freely moving mice. By knocking out receptors in different neuronal subpopulations, we find that α7 nAChRs expressed in OLM interneurons regulate theta generation. Our in vitro slice studies indicate that α7 nAChR activation increases OLM neuron activity that, in turn, enhances pyramidal cell excitatory postsynaptic currents (EPSCs). Our study also suggests that mAChR activation promotes transient theta generation, while α7 nAChR activation facilitates future theta generation by similar stimulations, revealing a complex mechanism whereby cholinergic signaling modulates different aspects of hippocampal theta oscillations through different receptor subtypes.

Regulation of hippocamposeptal input within the medial septum/diagonal band of Broca

The medial septum/diagonal band of Broca (MS/DBB) receives direct GABAergic input from the hippocampus via hippocamposeptal (HS) projection neurons as part of a reciprocal loop that mediates cognition and is altered in Alzheimer's disease. Cholinergic and GABAergic interactions occur throughout the MS/DBB, but it is not known how HS GABA release is impacted by these circuits. Most HS neurons contain somatostatin (SST), so to evoke HS GABA release we expressed Cre-dependent mCherry/channelrhodopisin-2 (ChR2) in the hippocampi of SST-IRES-Cre mice and then used optogenetics to stimulate HS fibers while performing whole-cell patch clamp recordings from MS/DBB neurons in acute slices. We found that the acetylcholine receptor (AChR) agonist carbachol and the GABA_B receptor (GABA_BR) agonist baclofen significantly decreased HS GABA release in the MS/DBB. Carbachol's effects were blocked by eliminating local GABAergic activity or inhibiting GABA_BRs, indicating that it was indirectly decreasing HS GABA release by increasing GABAergic tone. There was no effect of acute exposure to amyloid-β on HS GABA release. Repetitive stimulation of HS fibers increased spontaneous GABA release in the MS/DBB, revealing that HS projections can modulate local GABAergic tone. These results show that HS GABA release has far-reaching impacts on overall levels of inhibition in the MS/DBB and is under regulatory control by cholinergic and GABAergic activity. This bidirectional modulation of GABA release from local and HS projections in the MS/DBB will likely have profound impact not only on activity within the MS/DBB, but also on output to the hippocampus and hippocampal-dependent learning and memory.

A novel role of NLRP3-generated IL-1β in the acute-chronic transition of peripheral lipopolysaccharide-elicited neuroinflammation: implications for sepsis-associated neurodegeneration

BACKGROUND

Sepsis-associated acute brain inflammation, if unresolved, may cause chronic neuroinflammation and resultant neurodegenerative diseases. However, little is known how the transition from acute to chronic neuroinflammation, which is critical for the following progressive neurodegeneration, occurs in sepsis. The goal of this study was to investigate potential immune factors regulating the transition process using a widely used endotoxemia LPS mouse model. This model shows distinct acute and chronic phases of neuroinflammation and recapitulates many cardinal features of Parkinson's disease, thus, providing a unique opportunity for studying phase transition of neuroinflammation.

METHODS

C57BL/6 J, NLRP3-/-, and IL-1R1-/- mice were employed. Mild and severe endotoxemia were produced by LPS ip injection at 1 or 5 mg/kg. Neuroinflammation in vitro and in vivo was assessed with proinflammatory cytokine expression by qPCR or ELISA and microglial activation by immunohistochemical analysis. Neurodegeneration was measured by manual and stereological counts of nigral dopaminergic neurons and immunohistochemical analysis of protein nitrosylation and α-synuclein phosphorylation.

RESULTS

LPS-elicited initial increases in mouse brain mRNA levels of TNFα, IL-6, IL-1β, and MCP-1, and nigral microglial activation were not dose-related. By contrast, the delayed increase in brain mature IL-1β levels was dependent on LPS doses and protracted nigral microglial activation was only observed in high dose of LPS-treated mice. LPS-elicited increase in brain mature IL-1β but not IL-1α level was NLRP3-dependent. After high dose LPS treatment, deficiency of NLRP3 or IL-1R1 did not prevent the initiation of acute neuroinflammation but abolished chronic neuroinflammation. Genetic or pharmacological inhibition of the NLRP3-IL-1β axis repressed LPS-stimulated upregulation of chronic neuroinflammatory mediators including MHC-II, NOX2, and Mac1, and protected dopaminergic neurons. Ten months after LPS-elicited severe endotoxemia, nigral persisted microglial activation, elevated nitrosylated proteins and phosphorylated α-synuclein, and significant neuronal degeneration developed in wild-type mice but not in NLRP3-/- or IL-1R1-/- mice.

CONCLUSIONS

This study uncovers a novel role of the NLRP3-IL-1β signaling pathway in gauging the severity of sepsis-associated inflammation and determining whether acute neuroinflammation will resolve or transition to low grade chronic neuroinflammation. These findings also provide novel targets for developing therapy for severe systemic infection-related neurodegeneration.

Genetic identification of a population of noradrenergic neurons implicated in attenuation of stress-related responses

Noradrenergic signaling plays a well-established role in promoting the stress response. Here we identify a subpopulation of noradrenergic neurons, defined by developmental expression of Hoxb1, that has a unique role in modulating stress-related behavior. Using an intersectional chemogenetic strategy, in combination with behavioral and physiological analyses, we show that activation of Hoxb1-noradrenergic (Hoxb1-NE) neurons decreases anxiety-like behavior and promotes an active coping strategy in response to acute stressors. In addition, we use cerebral blood volume-weighted functional magnetic resonance imaging to show that chemoactivation of Hoxb1-NE neurons results in reduced activity in stress-related brain regions, including the bed nucleus of the stria terminalis, amygdala, and locus coeruleus. Thus, the actions of Hoxb1-NE neurons are distinct from the well-documented functions of the locus coeruleus in promoting the stress response, demonstrating that the noradrenergic system contains multiple functionally distinct subpopulations.

The α7 nicotinic acetylcholine receptors regulate hippocampal adult-neurogenesis in a sexually dimorphic fashion

Disruption in cholinergic signaling has been linked to many environmental and/or pathological conditions known to modify adult neurogenesis. The α7 nAChRs are in the family of cys-loop receptor channels which have been shown to be neuroprotective in adult neurons and are thought to be critical for survival and integration of immature neurons. However, in developing neurons, poor calcium buffering may cause α7 nAChR activation to be neurotoxic. To investigate whether the α7 nAChR regulates neurogenesis in the hippocampus, we used a combination of mouse genetics and imaging to quantify neural stem cell (NSC) densities located in the dentate gyrus of adult mice. In addition, we considered whether the loss of α7 nAChRs had functional consequences on a spatial discrimination task that is thought to rely on pattern separation mechanisms. We found that the loss of α7 nAChRs resulted in increased neurogenesis in male mice only, while female mice showed increased cell divisions and intermediate progenitors but no change in neurogenesis. Knocking out the α7 nAChR from nestin⁺ NSCs and their progeny showed signaling in these cells contributes to regulating neurogenesis. In addition, male, but not female, mice lacking α7 nAChRs performed significantly worse in the spatial discrimination task. This task was sexually dimorphic in wild-type mice, but not in the absence of α7 nAChRs. We conclude that α7 nAChRs regulate adult neurogenesis and impact spatial discrimination function in male, but not female mice, via a mechanism involving nestin⁺ NSCs and their progeny.

Hippocampus and Entorhinal Cortex Recruit Cholinergic and NMDA Receptors Separately to Generate Hippocampal Theta Oscillations

Although much progress has been made in understanding type II theta rhythm generation under urethane anesthesia, less is known about the mechanisms underlying type I theta generation during active exploration. To better understand the contributions of cholinergic and NMDA receptor activation to type I theta generation, we recorded hippocampal theta oscillations from freely moving mice with local infusion of cholinergic or NMDA receptor antagonists to either the hippocampus or the entorhinal cortex (EC). We found that cholinergic receptors in the hippocampus, but not the EC, and NMDA receptors in the EC, but not the hippocampus, are critical for open-field theta generation and Y-maze performance. We further found that muscarinic M1 receptors located on pyramidal neurons, but not interneurons, are critical for cholinergic modulation of hippocampal synapses, theta generation, and Y-maze performance. These results suggest that hippocampus and EC neurons recruit cholinergic-dependent and NMDA-receptor-dependent mechanisms, respectively, to generate theta oscillations to support behavioral performance.

Absolute Configuration and Pharmacology of the Poison Frog Alkaloid Phantasmidine

Phantasmidine, a rigid congener of the well-known nicotinic acetylcholine receptor agonist epibatidine, is found in the same species of poison frog ( Epipedobates anthonyi). Natural phantasmidine was found to be a 4:1 scalemic mixture, enriched in the (2a R,4a S,9a S) enantiomer by chiral-phase LC-MS comparison to the synthetic enantiomers whose absolute configurations were previously established by Mosher's amide analysis. The major enantiomer has the opposite S configuration at the benzylic carbon to natural epibatidine, whose benzylic carbon is R. Pharmacological characterization of the synthetic racemate and separated enantiomers established that phantasmidine is ∼10-fold less potent than epibatidine, but ∼100-fold more potent than nicotine in most receptors tested. Unlike epibatidine, phantasmidine is sharply enantioselective in its activity and the major natural enantiomer whose benzylic carbon has the 4a S configuration is more active. The stereoselective pharmacology of phantasmidine is ascribed to its rigid and asymmetric shape as compared to the nearly symmetric conformations previously suggested for epibatidine enantiomers. While phantasmidine itself is too toxic for direct therapeutic use, we believe it is a useful platform for the development of potent and selective nicotinic agonists, which may have value as pharmacological tools.

Levamisole: A Positive Allosteric Modulator for the α3β4 Nicotinic Acetylcholine Receptors Prevents Weight Gain in the CD-1 Mice on a High Fat Diet

Neuronal nicotinic acetylcholine receptors (nAChRs) regulate the function of multiple neurotransmitter pathways throughout the central nervous system. This includes nAChRs found on the proopiomelanocortin neurons in the hypothalamus. Activation of these nAChRs by nicotine causes a decrease in the consumption of food in rodents. This study tested the effect of subtype selective allosteric modulators for nAChRs on the body weight of CD-1 mice. Levamisole, an allosteric modulator for the α3β4 subtype of nAChRs, prevented weight gain in mice that were fed a high fat diet. PNU-120596 and desformylflustrabromine were observed to be selective PAMs for the α7 and α4β2 nAChR, respectively. Both of these compounds failed to prevent weight gain in the CD-1 mice. These results suggest that the modulation of hypothalamic α3β4 nAChRs is an important factor in regulating food intake, and the PAMs for these receptors need further investigation as potential therapeutic agents for controlling weight gain.

Cholinergic modulation of the hippocampal region and memory function

Acetylcholine (ACh) plays an important role in memory function and has been implicated in aging-related dementia, in which the impairment of hippocampus-dependent learning strongly manifests. Cholinergic neurons densely innervate the hippocampus, mediating the formation of episodic as well as semantic memory. Here, we will review recent findings on acetylcholine's modulation of memory function, with a particular focus on hippocampus-dependent learning, and the circuits involved. In addition, we will discuss the complexity of ACh actions in memory function to better understand the physiological role of ACh in memory. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Inducing theta oscillations in the entorhinal hippocampal network in vitro

The hippocampal theta rhythm emerges as rhythmic and synchronized activities among the hippocampus and hippocampus-associated brain regions during active exploration, providing a potential means for inter-regional communication. However, after decades of research, the origins of the theta rhythm remain elusive, at least partly due to the difficulty in recording from all three essential regions for theta generation, namely the hippocampus itself, the septum, and the entorhinal cortex. For this reason, we established an in vitro theta model in a septo-entorhinal-hippocampal brain slice tri-culture system by pairing septal cholinergic inputs with hippocampal local activities. Our study shows that the local entorhinal cortical circuit may play an active and critical role in hippocampal theta rhythm generation. Our study also reveals a potential mechanism for theta rhythms to emerge as the functional results of dynamic interactions among the septum, hippocampus, and the entorhinal cortex, in the absence of clear pace makers.

Heteromeric α7β2 Nicotinic Acetylcholine Receptors in the Brain

The α7 nicotinic acetylcholine receptor (α7 nAChR) is highly expressed in the brain, where it maintains various neuronal functions including (but not limited to) learning and memory. In addition, the protein expression levels of α7 nAChRs are altered in various brain disorders. The classic rule governing α7 nAChR assembly in the mammalian brain was that it was assembled from five α7 subunits to form a homomeric receptor pentamer. However, emerging evidence demonstrates the presence of heteromeric α7 nAChRs in heterologously expressed systems and naturally in brain neurons, where α7 subunits are co-assembled with β2 subunits to form a novel type of α7β2 nAChR. Interestingly, the α7β2 nAChR exhibits distinctive function and pharmacology from traditional homomeric α7 nAChRs. We review recent advances in probing the distribution, function, pharmacology, pathophysiology, and stoichiometry of the heteromeric α7β2 nAChR, which have provided new insights into the understanding of a novel target of cholinergic signaling.

The effect of α7 nicotinic receptor activation on glutamatergic transmission in the hippocampus

Nicotinic acetylcholine receptors (nAChRs) are expressed widely in the CNS, and mediate both synaptic and perisynaptic activities of endogenous cholinergic inputs and pharmacological actions of exogenous compounds (e.g., nicotine and choline). Behavioral studies indicate that nicotine improves such cognitive functions as learning and memory, however the cellular mechanism of these actions remains elusive. With help from newly developed biosensors and optogenetic tools, recent studies provide new insights on signaling mechanisms involved in the activation of nAChRs. Here we will review α7 nAChR's action in the tri-synaptic pathway in the hippocampus. The effects of α7 nAChR activation via either exogenous compounds or endogenous cholinergic innervation are detailed for spontaneous and evoked glutamatergic synaptic transmission and synaptic plasticity, as well as the underlying signaling mechanisms. In summary, α7 nAChRs trigger intracellular calcium rise and calcium-dependent signaling pathways to enhance glutamate release and induce glutamatergic synaptic plasticity.

Activation of α7 nicotinic acetylcholine receptors increases intracellular cAMP levels via activation of AC1 in hippocampal neurons

The activation of α7 nAChRs has been shown to improve hippocampal-dependent learning and memory. However, the molecular mechanism of α7 nAChRs' action remains elusive. We previously reported that activation of α7 nAChRs induced a prolonged enhancement of glutamatergic synaptic transmission in a PKA-dependent manner. Here, we investigated any connection between the activation of the α7 nAChR and cAMP signaling in hippocampal neurons. To address this question, we employed a FRET-based biosensor to measure the intracellular cAMP levels directly via live cell imaging. We found that application of the α7 nAChR-selective agonist choline, in the presence of the α7 nAChR positive allosteric modulator PNU-120596, induced a significant change in emission ratio of F535/F470, which indicated an increase in intracellular cAMP levels. This choline-induced increase was abolished by the α7 nAChR antagonist MLA and the calcium chelator BAPTA, suggesting that the cAMP increase depends on the α7 nAChR activation and subsequent intracellular calcium rise. The selective AC1 inhibitor CB-6673567 and siRNA-mediated deletion of AC1 both blocked the choline-induced cAMP increase, suggesting that calcium-dependent AC1 is required for choline's action. Furthermore, α7 nAChR activation stimulated the phosphorylation of synapsin, which serves as a downstream effector to regulate neurotransmitter release. Our findings provide the first direct evidence to link activation of α7 nAChRs to a cAMP rise via AC1, which defines a new signaling pathway employed by α7 nAChRs. Our study sheds light into potential molecular mechanisms of the positive cognitive actions of α7 nAChR agonists and development of therapeutic treatments for cognitive impairments.

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.

Research tool: Validation of floxed α7 nicotinic acetylcholine receptor conditional knockout mice using in vitro and in vivo approaches

There is much interest in α7 nicotinic acetylcholine receptors (nAChRs) in CNS function since they are found throughout peripheral tissues as well as being highly expressed in brain regions implicated in attention, learning and memory. As such, the role of these receptors in many aspects of CNS function and disease is being actively investigated. To date, only one null mouse model (A7KO) is available which is non-conditional and constitutive. Since α7 nAChRs are present on neurons and glia (including astrocytes), as well as being developmentally regulated, there is an unmet need for the technical capability to control α7 nAChR gene expression. Therefore we have generated mice in which the fourth exon of the α7 nAChR gene (Chrna7) is flanked by loxP sites (B6-Chrna7(LBDEx4007Ehs)) which we refer to as floxed α7 nAChR conditional knockout or α7nAChR(flox). We validated the chosen approach by mating α7nAChR(flox) with mice expressing Cre recombinase driven by the glial acidic fibrillary protein (GFAP)-Cre promoter (GFAP-A7KO) to test whether α7nAChR(flox), GFAP-A7KO and appropriate littermate controls performed equally in our standard Rodent In Vivo Assessment Core battery to assess general health, locomotion, emotional and cognitive behaviours. Neither α7nAChR(flox) nor GFAP-A7KO exhibited significant differences from littermate controls in any of the baseline behavioural assessments we conducted, similar to the 'first generation' non-conditional A7KO mice. We also determined that α7 nAChR binding sites were absent on GFAP-positive astrocytes in hippocampal slices obtained from GFAP-A7KO offspring from α7nAChR(flox) and GFAP-Cre crosses. Finally, we validated that Cre recombinase (Cre)-mediated excision led to functional, cell- and tissue-specific loss of α7 nAChRs by demonstrating that choline-induced α7 nAChR currents were present in Cre-negative, but not synapsin promoter-driven Cre-positive, CA1 pyramidal neurons. Additionally, electrophysiological characterization of α7 nAChR-mediated current traces was similar in terms of amplitude and time constants of decay (during desensitization) for the α7nAChR(flox) and wild-type (WT) mice. Thus, we have in vivo and in vitro evidence that the Chrna7 exon 4 targeting strategy does not alter behavioural, cognitive, or electrophysiological properties compared to WT and that Cre-mediated excision is an effective approach to delete α7 nAChR expression in a cell-specific manner.

Nicotinic ACh receptors in the hippocampal circuit; functional expression and role in synaptic plasticity

Acetylcholine (ACh) can regulate neuronal excitability in the hippocampus, an important area in the brain for learning and memory, by acting on both nicotinic (nAChRs) and muscarinic ACh receptors. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca (MS-DBB), and we investigated how their activation regulated hippocampal synaptic plasticity. We found that activation of these endogenous cholinergic inputs can directly induce different forms of hippocampal synaptic plasticity with a timing precision in the millisecond range. Furthermore, we observed a prolonged enhancement of excitability both pre- and postsynaptically. Lastly we found that the presence of the α7 nAChR subtype to both pre- and postsynaptic sites appeared to be required to induce this plasticity. We propose that α7 nAChRs coordinate pre- and postsynaptic activities to induce glutamatergic synaptic plasticity, and thus provide a novel mechanism underlying physiological neuronal communication that could lead to timing-dependent synaptic plasticity in the hippocampus.

A mutation in the extracellular domain of the α7 nAChR reduces calcium permeability

The α7 neuronal nicotinic acetylcholine receptor (nAChR) displays the highest calcium permeability among the different subtypes of nAChRs expressed in the mammalian brain and can impact cellular events including neurotransmitter release, second messenger cascades, cell survival, and apoptosis. The selectivity for cations in nAChRs is thought to be achieved in part by anionic residues which are located on either side of the channel mouth and increase relative cationic concentration. Mutagenesis studies have improved our understanding of the role of the second transmembrane domain and the intracellular loop of the channel in ion selectivity. However, little is known about the influence that the extracellular domain (ECD) plays in ion permeation. In the α7 nAChR, it has been found that the ECD contains a ring of ten aspartates (two per subunit) that is believed to face the lumen of the pore and could attract cations for permeation. Using mutagenesis and a combination of electrophysiology and imaging techniques, we tested the possible involvement of these aspartate residues in the calcium permeability of the rat α7 nAChR. We found that one of these residues (the aspartate at position 44) appears to be essential since mutating it to alanine resulted in a decrease in amplitude for both whole cell and single-channel responses and in the complete disappearance of detectable calcium changes in most cells, which indicates that the ECD of the α7 nAChR plays a key role in calcium permeation.

Effects of neuronal nicotinic acetylcholine receptor allosteric modulators in animal behavior studies

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation-conducting transmembrane channels from the cys-loop receptor superfamily. The neuronal subtypes of these receptors (e.g. the α7 and α4β2 subtypes) are involved in neurobehavioral processes such as anxiety, the central processing of pain, food intake, nicotine seeking behavior, and a number of cognitive functions like learning and memory. Neuronal nAChR dysfunction is involved in the pathophysiology of many neurological disorders, and behavioral studies in animals are useful models to assess the effects of compounds that act on these receptors. Allosteric modulators are ligands that bind to the receptors at sites other than the orthosteric site where acetylcholine, the endogenous agonist for the nAChRs, binds. While conventional ligands for the neuronal nAChRs have been studied for their behavioral effects in animals, allosteric modulators for these receptors have only recently gained attention, and research on their behavioral effects is growing rapidly. Here we will discuss the behavioral effects of allosteric modulators of the neuronal nAChRs.

Activation of the α7 nicotinic ACh receptor induces anxiogenic effects in rats which is blocked by a 5-HT₁a receptor antagonist

The α7 nicotinic acetylcholine receptor (nAChR) is highly expressed in different regions of the brain and is associated with cognitive function as well as anxiety. Agonists and positive allosteric modulators (PAMs) of the α7 subtype of nAChRs have been shown to improve cognition. Previously nicotine, which activates both α7 and non-α7 subtypes of nAChRs, has been shown to have an anxiogenic effect in behavioral tests. In this study, we compared the effects of the α7-selective agonist (PNU-282987) and PAM (PNU-120596) in a variety of behavioral tests in Sprague Dawley rats to look at their effects on learning and memory as well as anxiety. We found that neither PNU-282987 nor PNU-120596 improved spatial-learning or episodic memory by themselves. However when cognitive impairment was induced in the rats with scopolamine (1 mg/kg), both PNU-120596 and PNU-282987 were able to reverse this memory impairment and restore it back to normal levels. While PNU-120596 reversed the scopolamine-induced cognitive impairment, it did not have any adverse effect on anxiety. PNU-282987 on the other hand displayed an increase in anxiety-like behavior at a higher dose (10 mg/kg) that was significantly reduced by the serotonin 5-HT₁a receptor antagonist WAY-100135. However the α7 receptor antagonist methyllycaconitine was unable to reverse these anxiety-like effects seen with PNU-282987. These results suggest that α7 nAChR PAMs are pharmacologically advantageous over agonists, and should be considered for further development as therapeutic drugs targeting the α7 receptors.

Functional α7 nicotinic ACh receptors on astrocytes in rat hippocampal CA1 slices

Although much is known about the functional expression of the neuronal nicotinic acetylcholine receptors (nAChRs) in various neuronal populations in the brain and elsewhere, much less is known about their expression and functional relevance in glial cells. The expression of functional nAChRs has been reported for cultured astrocytes; however, previous work has failed to detect nAChR-mediated responses in astrocytes in acute slices. In the current study, functional α7 nAChRs on astrocytes in the CA1 region of the rat hippocampus were studied in situ using whole-cell patch-clamp recording and two-photon calcium imaging techniques in acute slices. We found that astrocytes and the chondroitin sulfate proteoglycan NG2-expressing (i.e., NG2) cells did express functional α7 nAChRs. Although the amplitudes of the responses were small, they could be enhanced by the α7-selective positive allosteric modulator PNU-120596. Under these conditions, we found that in comparing the properties of these responses between astrocytes, NG2, and interneurons, there were differences in the kinetics and increases in intracellular calcium levels. This is the first demonstration of functional α7 nAChR-mediated current responses in astrocytes in acute hippocampal slices, data which may shed light on the role of α7 nAChRs in neuroprotection.

Nicotinic ACh receptors in the hippocampus: role in excitability and plasticity

INTRODUCTION

The nicotinic ACh receptors (nAChRs) are in the cys-loop family of ligand-gated ion channels. They are widely expressed throughout the brain, including in the hippocampus where they are thought to be involved in regulating excitability, plasticity, and cognitive function. In addition, dysfunction in hippocampal nAChRs has been linked to a variety of neurological disorders and diseases, including Alzheimer's disease, schizophrenia, and epilepsy. In order to understand how to treat nAChR-related disorders and diseases, it is critical to understand how these receptors participate in normal brain function; this entails not only understanding the biophysical properties of ion channel function and their pattern of expression but also how these receptors are regulating excitability and circuit behavior.

DISCUSSION

The primary cholinergic input to the hippocampus comes from the medial septum and diagonal band of Broca; however, the mechanistic details are unknown of how activation of cholinergic receptors, either through exogenous nAChR ligands or the activation of endogenous acetylcholine release, regulates hippocampal network activity. This entails direct study of the excitatory and inhibitory neuronal networks, as well as the role of nonneuronal cells, in regulating hippocampal function.

CONCLUSIONS

Here, I will review the latest work from my laboratory in which we have attempted to do just that, with the overall goal of learning more about the role of the hippocampal nAChR in synaptic plasticity.

Timing-dependent septal cholinergic induction of dynamic hippocampal synaptic plasticity

Cholinergic modulation of hippocampal synaptic plasticity has been studied extensively by applying receptor agonists or blockers; however, the effect of rapid physiological cholinergic stimuli on plasticity is largely unknown. Here, we report that septal cholinergic input, activated either by electrical stimulation or via an optogenetic approach, induced different types of hippocampal Schaffer collateral (SC) to CA1 synaptic plasticity, depending on the timing of cholinergic input relative to the SC input. When the cholinergic input was activated 100 or 10 ms prior to SC stimulation, it resulted in α7 nAChR-dependent long-term potentiation (LTP) or short-term depression, respectively. When the cholinergic stimulation was delayed until 10 ms after the SC stimulation, a muscarinic AChR-dependent LTP was induced. Moreover, these various forms of plasticity were disrupted by Aβ exposure. These results have revealed the remarkable temporal precision of cholinergic functions, providing a novel mechanism for information processing in cholinergic-dependent higher cognitive functions.

The α7 nicotinic acetylcholine receptor function in hippocampal neurons is regulated by the lipid composition of the plasma membrane

The α7 nicotinic acetylcholine receptors (nAChRs) play an important role in cellular events such as neurotransmitter release, second messenger cascades, cell survival and apoptosis. In addition, they are a therapeutic target for the treatment of neurological disorders such as Alzheimer's disease and schizophrenia, and drugs that potentiate α7 nAChRs through the regulation of desensitization are currently being developed. Recently, these channels were found to be localized into lipid rafts. Here we show that the disruption of lipid rafts in rat primary hippocampal neurons, through cholesterol-scavenging drugs (methyl-β-cyclodextrin) and the enzymatic breakdown of sphingomyelin (sphingomyelinase), results in significant changes in the desensitization kinetics of native and expressed α7 nAChRs. These effects can be prevented by cotreatment with cholesterol and sphingomyelin, and can be mimicked by treatment with cholesterol and sphingomyelin synthesis inhibitors (mevastatin and myriocin, respectively), suggesting that the effects on desensitization kinetics are indeed due to changes in the levels of cholesterol and sphingomyelin in the plasma membrane. These data provide new insights into themechanism of desensitization of α7 nAChRs by providing evidence that the lipid composition of the plasma membrane can modulate the activity of the α7 nAChRs.

A structural and mutagenic blueprint for molecular recognition of strychnine and d-tubocurarine by different cys-loop receptors

Cys-loop receptors (CLR) are pentameric ligand-gated ion channels that mediate fast excitatory or inhibitory transmission in the nervous system. Strychnine and d-tubocurarine (d-TC) are neurotoxins that have been highly instrumental in decades of research on glycine receptors (GlyR) and nicotinic acetylcholine receptors (nAChR), respectively. In this study we addressed the question how the molecular recognition of strychnine and d-TC occurs with high affinity and yet low specificity towards diverse CLR family members. X-ray crystal structures of the complexes with AChBP, a well-described structural homolog of the extracellular domain of the nAChRs, revealed that strychnine and d-TC adopt multiple occupancies and different ligand orientations, stabilizing the homopentameric protein in an asymmetric state. This introduces a new level of structural diversity in CLRs. Unlike protein and peptide neurotoxins, strychnine and d-TC form a limited number of contacts in the binding pocket of AChBP, offering an explanation for their low selectivity. Based on the ligand interactions observed in strychnine- and d-TC-AChBP complexes we performed alanine-scanning mutagenesis in the binding pocket of the human α1 GlyR and α7 nAChR and showed the functional relevance of these residues in conferring high potency of strychnine and d-TC, respectively. Our results demonstrate that a limited number of ligand interactions in the binding pocket together with an energetic stabilization of the extracellular domain are key to the poor selective recognition of strychnine and d-TC by CLRs as diverse as the GlyR, nAChR, and 5-HT₃R.

Ligand-gated ion channels play an important role in fast electrochemical signaling in the brain. Cys-loop receptors are a class of pentameric ligand-gated ion channels that are activated by specific neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), glycine (Gly), and γ-aminobutyric acid (GABA). Each type of cys-loop receptor contains an extracellular domain that specifically recognizes only one of these four neurotransmitters and opens an ion-conducting channel pore upon ligand binding. In this study, we investigated the poor specificity with which two potent neurotoxic inhibitors, namely strychnine and d-tubocurarine, are recognized by different cys-loop receptors. Using X-ray crystallography we solved 3-dimensional structures of strychnine or d-tubocurarine in complex with ACh binding protein (AChBP), a well-recognized structural homolog of the nicotinic ACh receptor. Based on ligand-receptor interactions observed in AChBP structures we designed mutant GlyR and α7 nAChR to identify hot spots in the binding pocket of these receptors that define potent inhibition by strychnine and d-tubocurarine, respectively. Combined, our results offer detailed understanding of the molecular recognition of antagonists that have high affinity but poor specificity for different cys-loop receptors.

Allosteric modulation of related ligand-gated ion channels synergistically induces long-term potentiation in the hippocampus and enhances cognition

α5 Subunit-containing GABA(A) receptors (GABA(A)Rs) and α7 neuronal nicotinic-acetylcholine receptors (nAChRs) are members of the Cys-loop family of ligand-gated ion channels (LGICs) that mediate cognitive and attentional processes in the hippocampus. α5 GABA(A)Rs alter network activity by tonic inhibition of CA1/CA3 pyramidal cells of the hippocampus. Postsynaptic α7 nAChRs in the hippocampus regulate inhibitory GABAergic interneuron activity required for synchronization of pyramidal neurons in the CA1, whereas presynaptic α7 nAChRs regulate glutamate release. Can simultaneous allosteric modulation of these LGICs produce synergistic effects on cognition? We show that combined transient application of two allosteric modulators that individually 1) inhibit α5 GABA(A)Rs and 2) enhance α7 nAChRs causes long-term potentiation (LTP) of mossy fiber stimulation-induced excitatory postsynaptic currents (EPSC) from CA1 pyramidal neurons of rat hippocampal slices. The LTP effect evoked by two compounds is replicated by 3-(2,5-difluorophenyl)-6-(N-ethylindol-5-yl)-1,2,4-triazolo[4,3-b]pyridazine (522-054), a compound we designed to simultaneously inhibit α5 GABA(A)Rs and enhance α7 nAChRs. Selective antagonists for either receptor block sustained EPSC potentiation produced by 522-054. In vivo, 522-054 enhances performance in the radial arm maze and facilitates attentional states in the five-choice serial reaction time trial with similar receptor antagonist sensitivity. These observations may translate into therapeutic utility of dual action compounds in diseases of hippocampal-based cognitive impairment.

Crystal structures of a cysteine-modified mutant in loop D of acetylcholine-binding protein

Covalent modification of α7 W55C nicotinic acetylcholine receptors (nAChR) with the cysteine-modifying reagent [2-(trimethylammonium)ethyl] methanethiosulfonate (MTSET(+)) produces receptors that are unresponsive to acetylcholine, whereas methyl methanethiolsulfonate (MMTS) produces enhanced acetylcholine-gated currents. Here, we investigate structural changes that underlie the opposite effects of MTSET(+) and MMTS using acetylcholine-binding protein (AChBP), a homolog of the extracellular domain of the nAChR. Crystal structures of Y53C AChBP show that MTSET(+)-modification stabilizes loop C in an extended conformation that resembles the antagonist-bound state, which parallels our observation that MTSET(+) produces unresponsive W55C nAChRs. The MMTS-modified mutant in complex with acetylcholine is characterized by a contracted C-loop, similar to other agonist-bound complexes. Surprisingly, we find two acetylcholine molecules bound in the ligand-binding site, which might explain the potentiating effect of MMTS modification in W55C nAChRs. Unexpectedly, we observed in the MMTS-Y53C structure that ten phosphate ions arranged in two rings at adjacent sites are bound in the vestibule of AChBP. We mutated homologous residues in the vestibule of α1 GlyR and observed a reduction in the single channel conductance, suggesting a role of this site in ion permeation. Taken together, our results demonstrate that targeted modification of a conserved aromatic residue in loop D is sufficient for a conformational switch of AChBP and that a defined region in the vestibule of the extracellular domain contributes to ion conduction in anion-selective Cys-loop receptors.

Rapid desensitization of the rat α7 nAChR is facilitated by the presence of a proline residue in the outer β-sheet

The rat α7 nicotinic acetylcholine receptor (nAChR) has a proline residue near the middle of the β9 strand. The replacement of this proline residue at position 180 (P180) by either threonine (α7-P180T) or serine (α7-P180S) slowed the onset of desensitization dramatically, with half-times of ~930 and 700 ms, respectively, compared to 90 ms for the wild-type receptor. To investigate the importance of the hydroxyl group on the position 180 side-chains, the mutant receptors α7-P180Y and α7-P180F were studied and showed half-times of desensitization of 650 and 160 ms, respectively. While a position 180 side-chain OH group may contribute to the slow desensitization rates, α7-P180S and α7-P180V resulted in receptors with similar desensitization rates, suggesting that increased backbone to backbone H bonding expected in the absence of proline at position 180 would likely exert a great effect on desensitization. Single channel recordings indicated that for the α7-P180T receptor there was a significantly reduced closed time without any change in single channel conductance (as compared to wild-type). Kinetic simulations indicated that all changes observed for the mutant channel behaviour were reproduced by decreasing the rate of desensitization, and increasing the microscopic affinity to resting receptors. Molecular dynamics (MD) simulations on a homology model were used to provide insight into likely H bond interactions within the outer β-sheet that occur when the P180 residue is mutated. All mutations analysed increased about twofold the predicted number of H bonds between the residue at position 180 and the backbone of the β10 strand. Moreover, the α7-P180T and α7-P180S mutations also formed some intrastrand H bonds along the β9 strand, although H bonding of the OH groups of the threonine or serine side-chains was predicted to be infrequent. Our results indicate that rapid desensitization of the wild-type rat α7 nAChR is facilitated by the presence of the proline residue within the β9 strand.

The frequency-dependence of the nicotine-induced inhibition of dopamine is controlled by the α7 nicotinic receptor

Voltammetric analyses show that low (100-500 nM) doses of nicotine regulate striatal dopamine by inhibiting release evoked by a single stimulation to a greater extent than release evoked by high frequency stimulations. This frequency-dependent inhibition is because of nicotine desensitizing heteromeric β2 subunit-containing nicotinic acetylcholine receptor (nAChR) subtypes. Surprisingly, a high dose of nicotine (2 μM; capable of interacting with additional nAChR subtypes) produced an inhibition of dopamine evoked by high frequency stimulation, an effect that was not seen with the low dose of nicotine or the β2 antagonist, dihydro-β-erythroidine hydrobromide. This inhibition was replicated by application of α7 nAChR antagonists methyllcaconitine citrate or α-bungarotoxin in conjunction with the low dose of nicotine or dihydro-β-erythroidine hydrobromide. Blocking α7 receptor function alone produced a modest increase in dopamine evoked by single pulse stimulation while not affecting dopamine evoked by high frequency stimulation. The antagonist results were mimicked using selective α7 agonists PHA 543613 and PNU 282987. The frequency dependence of the low dose nicotine inhibition therefore requires functional α7 nAChRs, and may arise from differing levels of endogenous acetylcholine evoked by the stimulation.

Identification and functional characterization of a novel acetylcholine-binding protein from the marine annelid Capitella teleta

We identified a homologue of the molluscan acetylcholine-binding protein (AChBP) in the marine polychaete Capitella teleta, from the annelid phylum. The amino acid sequence of C. teleta AChBP (ct-AChBP) is 21-30% identical with those of known molluscan AChBPs. Sequence alignments indicate that ct-AChBP has a shortened Cys loop compared to other Cys loop receptors, and a variation on a conserved Cys loop triad, which is associated with ligand binding in other AChBPs and nicotinic ACh receptor (nAChR) alpha subunits. Within the D loop of ct-AChBP, a conserved aromatic residue (Tyr or Trp) in nAChRs and molluscan AChBPs, which has been implicated directly in ligand binding, is substituted with an isoleucine. Mass spectrometry results indicate that Asn122 and Asn216 of ct-AChBP are glycosylated when expressed using HEK293 cells. Small-angle X-ray scattering data suggest that the overall shape of ct-AChBP in the apo or unliganded state is similar to that of homologues with known pentameric crystal structures. NMR experiments show that acetylcholine, nicotine, and alpha-bungarotoxin bind to ct-AChBP with high affinity, with K(D) values of 28.7 microM, 209 nM, and 110 nM, respectively. Choline bound with a lower affinity (K(D) = 163 microM). Our finding of a functional AChBP in a marine annelid demonstrates that AChBPs may exhibit variations in hallmark motifs such as ligand-binding residues and Cys loop length and shows conclusively that this neurotransmitter binding protein is not limited to the phylum Mollusca.

Gating of nicotinic ACh receptors: latest insights into ligand binding and function

Nicotinic acetylcholine receptors (nAChRs) are in the superfamily of cys-loop receptors, and are widely expressed in the nervous system where they participate in a variety of physiological functions, including regulating excitability and neurotransmitter release, as well as neuromuscular contraction. Members of the cys-loop family of receptors, which also includes the molluscan ACh-binding protein (AChBP), a soluble protein that is analogous to the extracellular ligand-binding domain of the cys-loop receptors, are pentameric assemblies of five subunits, with each subunit arranged around a central pore. The binding of ACh to the extracellular interface between two subunits induces channel opening. With the recent 4 A resolution of the Torpedo nAChR, and the crystal structure of the AChBP, much has been learned about the structure of the ligand-binding domain and the channel pore, as well as major structural rearrangements that may confer channel opening, including a major rearrangement of the C-loop within the ligand binding pocket, and perhaps other regions including the F-loop (the beta8-beta9 linker), the beta1-beta2 linker and the cys-loop. Here I will review the latest findings from my lab aimed at a further understanding of the function of the neuronal nAChR channels (and in particular the role of desensitization), and our search for novel AChBP species that may lead to a further understanding of the function of the cys-loop receptor family.

Nicotinic acetylcholine receptor-mediated calcium signaling in the nervous system

Based on the composition of the five subunits forming functional neuronal nicotinic acetylcholine receptors (nAChRs), they are grouped into either heteromeric (comprising both alpha and beta subunits) or homomeric (comprising only alpha subunits) receptors. The nAChRs are known to be differentially permeable to calcium ions, with the alpha7 nAChR subtype having one of the highest permeabilities to calcium. Calcium influx through nAChRs, particularly through the alpha-bungarotoxin-sensitive alpha7-containing nAChRs, is a very efficient way to raise cytoplasmic calcium levels. The activation of nAChRs can mediate three types of cytoplasmic calcium signals: (1) direct calcium influx through the nAChRs, (2) indirect calcium influx through voltage-dependent calcium channels (VDCCs) which are activated by the nAChR-mediated depolarization, and (3) calcium-induced calcium release (CICR) (triggered by the first two sources) from the endoplasmic reticulum (ER) through the ryanodine receptors and inositol (1,4,5)-triphosphate receptors (IP(3)Rs). Downstream signaling events mediated by nAChR-mediated calcium responses can be grouped into instantaneous effects (such as neurotransmitter release, which can occur in milliseconds after nAChR activation), short-term effects (such as the recovery of nAChR desensitization through cellular signaling cascades), and long-term effects (such as neuroprotection via gene expression). In addition, nAChR activity can be regulated by cytoplasmic calcium levels, suggesting a complex reciprocal relationship. Further advances in imaging techniques, animal models, and more potent and subtype-selective ligands for neuronal nAChRs would help in understanding the neuronal nAChR-mediated calcium signaling, and lead to the development of improved therapeutic treatments.

Hippocampal infusions of apolipoprotein E peptides induce long-lasting cognitive impairment

The inheritance of the varepsilon4 allele of apolipoprotein E (ApoE4) and cholinergic system dysfunction have long been associated with the pathology of Alzheimer's disease (AD). Recently, in vitro studies have established a direct link between ApoE and cholinergic function in that synthetic peptides containing segments of the ApoE protein (ApoE(133-149) and ApoE(141-148)) interact with alpha7 nicotinic acetylcholine receptors (nAChRs) in the hippocampus. This raises the possibility that ApoE peptides may contribute to cognitive impairment in AD in that the hippocampus plays a key role in cognitive functioning. To test this, we acutely infused ApoE peptides into the ventral hippocampus of female Sprague-Dawley rats and assessed the resultant effects on radial-arm maze choice accuracy over a period of weeks after the infusion. Local ventral hippocampal infusion of ApoE peptides caused significant cognitive impairment in radial-arm maze learning that persisted several weeks after the acute infusion. This persisting deficit may be an important model for understanding the relationship between ApoE protein-induced neurotoxicity and cognitive impairment as well as serve as a platform for the development of new therapies to avoid neurotoxicity and cognitive decline.

Inhibition of alpha 7-containing nicotinic ACh receptors by muscarinic M1 ACh receptors in rat hippocampal CA1 interneurones in slices

Cys-loop ligand-gated nicotinic ACh receptors (nAChRs) and G protein-coupled muscarinic ACh receptors (mAChRs) are expressed on rat hippocampal interneurones where they can regulate excitability, synaptic communication and cognitive function. Even though both nAChRs and mAChRs appear to co-localize to the same interneurones, it is not clear whether there is crosstalk between them. We utilized patch-clamp techniques to investigate this issue in rat hippocampal CA1 interneurones in slices under conditions where synaptic transmission was blocked. The alpha7 nAChR-mediated currents were activated by choline, and when the activation of this receptor was preceded by the activation of the M(1) mAChR subtype, the amplitude of alpha7 responses was significantly reduced in a rapidly reversible and voltage-independent manner, without any change in the kinetics of responses. This M(1) mAChR-mediated inhibition of alpha7 nAChRs was through a PLC-, calcium- and PKC-dependent signal transduction cascade. These data show that M(1) mAChRs and alpha7 nAChRs are functionally co-localized on individual rat hippocampal interneurones where the activation of these particular mAChRs inhibits alpha7 nAChR function. This information will help to understand how these cholinergic receptor systems might be regulating neuronal excitability in the hippocampus in a manner that has relevance for synaptic plasticity and cognition.

Inhibition of native and recombinant nicotinic acetylcholine receptors by the myristoylated alanine-rich C kinase substrate peptide

A variety of peptide ligands are known to inhibit the function of neuronal nicotinic acetylcholine receptors (nAChRs), including small toxins and brain-derived peptides such as beta-amyloid(1-42) and synthetic apolipoproteinE peptides. The myristoylated alanine-rich C kinase substrate (MARCKS) protein is a major substrate of protein kinase C and is highly expressed in the developing and adult brain. The ability of a 25-amino acid synthetic MARCKS peptide, derived from the effector domain (ED), to modulate nAChR activity was tested. To determine the effects of the MARCKS ED peptide on nAChR function, receptors were expressed in Xenopus laevis oocytes, and two-electrode voltage-clamp experiments were performed. The MARCKS ED peptide completely inhibited acetylcholine (ACh)-evoked responses from alpha7 nAChRs in a dose-dependent manner, yielding an IC(50) value of 16 nM. Inhibition of ACh-induced responses was both activity- and voltage-independent. The MARCKS ED peptide was unable to block alpha-bungarotoxin binding. A MARCKS ED peptide in which four serine residues were replaced with aspartate residues was unable to inhibit alpha7 nAChR-mediated currents. The MARCKS ED peptide inhibited ACh-induced alpha4beta2 and alpha2beta2 responses, although with decreased potency. The effects of the MARCKS ED peptide on native nAChRs were tested using acutely isolated rat hippocampal slices. In hippocampal interneurons, the MARCKS ED peptide was able to block native alpha7 nAChRs in a dose-dependent manner. The MARCKS ED peptide represents a novel antagonist of neuronal nAChRs that has considerable utility as a research tool.

Functional alpha7-containing nicotinic acetylcholine receptors localize to cell bodies and proximal dendrites in the rat substantia nigra pars reticulata

The substantia nigra pars reticulata (SNr) is the primary output nucleus for the basal ganglia (BG) in the rat. The SNr is reciprocally connected with the pedunculopontine tegmental nucleus (PPN) in the brainstem, which provides cholinergic innervation to most BG nuclei. The cholinergic input into the BG is considered to be important because PPN activity is altered in Parkinson's disease (PD), a neurological disorder involving the BG, and cholinergic pharmacotherapy is beneficial in alleviating some of its symptoms. In order to better understand the role of cholinergic input to the BG, we examined the effects of nicotinic acetylcholine receptor (nAChR) activation in the GABAergic neurons in slices through juvenile rat SNr. With the aide of subtype selective antagonists, we found that SNr neurons express the alpha7 subtype of nAChRs, the function of which we assessed using the whole cell patch-clamp recording technique. Besides alpha7 nAChRs, GABAergic SNr neurons also contained functional non-alpha7 nAChRs. Using local photolysis of caged carbachol, a broad-spectrum cholinergic agonist, we mapped alpha7 nAChR-mediated currents along the visible extent of filled SNr neurons and found that alpha7 nAChRs can be functionally detected as far as 60 microm away from the soma. Our data are paving the way to a better understanding of the physiological roles of nAChRs in the BG.

Aromatic residues at position 55 of rat alpha7 nicotinic acetylcholine receptors are critical for maintaining rapid desensitization

The rat alpha7 nicotinic acetylcholine receptor (nAChR) can undergo rapid onset of desensitization; however, the mechanisms of desensitization are largely unknown. The contribution of a tryptophan (W) residue at position 55 of the rat alpha7 nAChR subunit, which lies within the beta2 strand, was studied by mutating it to other hydrophobic and/or aromatic amino acids, followed by voltage-clamp experiments in Xenopus oocytes. When mutated to alanine, the alpha7-W55A nAChR desensitized more slowly, and recovered from desensitization more rapidly, than wildtype alpha7 nAChRs. The contribution of desensitization was validated by kinetic modelling. Mutating W55 to other aromatic residues (phenylalanine or tyrosine) had no significant effect on the kinetics of desensitization, whereas mutation to various hydrophobic residues (alanine, cysteine or valine) significantly decreased the rate of onset and increased the rate of recovery from desensitization. To gain insight into possible structural rearrangements during desensitization, we probed the accessibility of W55 by mutating W55 to cysteine (alpha7-W55C) and testing the ability of various sulfhydryl reagents to react with this cysteine. Several positively charged sulfhydryl reagents blocked ACh-induced responses for alpha7-W55C nAChRs, whereas a neutral sulfhydryl reagent potentiated responses; residue C55 was not accessible for modification in the desensitized state. These data suggest that W55 plays an important role in both the onset and recovery from desensitization in the rat alpha7 nAChR, and that aromatic residues at position 55 are critical for maintaining rapid desensitization. Furthermore, these data suggest that W55 may be a potential target for modulatory agents operating via hydrophobic interactions.

Gating of nicotinic ACh receptors; new insights into structural transitions triggered by agonist binding that induce channel opening

Nicotinic acetylcholine receptors (nAChRs) are in the superfamily of Cys-loop ligand-gated ion channels, and are pentameric assemblies of five subunits, with each subunit arranged around the central ion-conducting pore. The binding of ACh to the extracellular interface between two subunits induces channel opening. With the recent 4 A resolution of the Torpedo nAChR, and the crystal structure of the related molluscan ACh binding protein, much has been learned about the structure of the ligand binding domain and the channel pore, as well as major structural rearrangements that may confer channel opening. For example, the putative pathway coupling agonist binding to channel gating may include a major rearrangement of the C-loop within the ligand binding pocket, and the disruption of a salt bridge between an arginine residue at the end of the beta10 strand and a glutamate residue in the beta1-beta2 linker. Here we will review and discuss the latest structural findings aiming to further refine the transduction pathway linking binding to gating for the nAChR channels, and discuss similarities and differences among the different members of this Cys-loop superfamily of receptors.

Structural determinates for apolipoprotein E-derived peptide interaction with the alpha7 nicotinic acetylcholine receptor

Neuronal nicotinic acetylcholine receptor (nAChR) signaling has been implicated in a variety of normal central nervous system (CNS) functions as well as an array of neuropathologies. Previous studies have demonstrated both neurotoxic and neuroprotective actions of peptides derived from apolipoprotein E (apoE). It has been discovered that apoE-derived peptides inhibit native and recombinant alpha7-containing nAChRs, indicating a direct interaction between apoE peptides and nAChRs. To probe the structure/function interaction between alpha7 nAChRs and the apoE peptide apoE(141-148), experiments were conducted in Xenopus laevis oocytes expressing wild-type and mutated nAChRs. Mutation of Trp55 to alanine blocks apoE peptide-induced inhibition of acetylcholine (ACh)-mediated alpha7 nAChR responses. Additional mutations at Trp55 suggest that hydrophobic interactions between the receptor and apoE(141-148) are essential for inhibition of alpha7 nAChR function. A mutated apoE peptide also demonstrated decreased inhibition at alpha7-W55A nAChRs as well as activity-dependent inhibition of both wild-type alpha7 nAChRs and alpha7-W55A receptors. Finally, a three-dimensional model of the alpha7 nAChR was developed based on the recently refined Torpedo marmorata nACh receptor. A structural model is proposed for the binding of apoE(141-148) to the alpha7 nAChR where the peptide binds at the interface between two subunits, near the ACh binding site. Similar to the functional data, the computational docking suggests the importance of hydrophobic interactions between the alpha7 nAChR and the apoE peptide for inhibition of receptor function. The current data suggest a mode for apoE peptide binding that directly blocks alpha7 nAChR activity and consequently may disrupt nAChR signaling.

Dendritic Ca2+ signalling due to activation of alpha 7-containing nicotinic acetylcholine receptors in rat hippocampal neurons

Neuronal nicotinic acetylcholine receptors (nAChRs) are in the superfamily of Cys-loop ligand-gated ion channels, which are widely expressed in the brain. Among the many different subtypes of nAChRs known to be expressed in the rat brain, the alpha 7-containing nAChRs are considered to be the most permeable to Ca2+. Utilizing highly localized and rapid iontophoretic agonist delivery, combined with patch-clamp electrophysiology and fura-2 fluorescence imaging techniques, we examined the alpha 7 nAChR-mediated currents and [Ca2+]i transients in the dendrites of rat hippocampal CA1 interneurons in the slice. We found that in the dendrites, whereas the amplitudes of the current responses were smaller and the decay kinetics faster than the responses in the soma, the amplitudes of the [Ca2+]i signals were significantly larger. Cultured hippocampal neurons were studied since the dendritic field lies in the same focal plane, which allowed for a broader investigation of the spatiotemporal dynamics of [Ca2+]i signalling. In cultured neurons, the [Ca2+]i signals in the dendrites were similar to those in slices. Interestingly in cultures, even though the amplitude of the alpha 7 nAChR-mediated currents dramatically decreased with distance from the soma (from approximately 20-250 microm), the amplitude of the [Ca2+]i signals did not correlate with distance. This indicates that the relative efficacy of alpha 7 nAChR activation to increase [Ca2+]i levels in dendrites increased severalfold with distance from the soma. These results may have implications for the role that alpha 7 nAChRs have in regulating various signal transduction cascades, synaptic plasticity, and memory processes, via significant changes in [Ca(2+)]i levels.

Functional somato-dendritic alpha7-containing nicotinic acetylcholine receptors in the rat basolateral amygdala complex

Multiple subtypes of nicotinic acetylcholine receptors (nAChRs) are expressed in the CNS. The amygdala complex, the limbic structure important for emotional memory formation, receives cholinergic innervation from the basal forebrain. Although cholinergic drugs have been shown to regulate passive avoidance performance via the amygdala, the neuronal subtypes and circuits involved in this regulation are unknown. In the present study, whole-cell patch-clamp electrophysiological techniques were used to identify and characterize the presence of functional somato-dendritic nAChRs within the basolateral complex of the amygdala. Pressure-application of acetylcholine (ACh; 2 mm) evoked inward current responses in a subset of neurons from both the lateral (49%) and basolateral nuclei (72%). All responses displayed rapid activation kinetics, and were blocked by the alpha7-selective antagonist methyllycaconitine. In addition, the alpha7-selective agonist choline induced inward current responses that were similar to ACh-evoked responses. Spiking patterns were consistent with pyramidal class I neurons (the major neuronal type in the basolateral complex); however, there was no correlation between firing frequency and the response to ACh. The local photolysis of caged carbachol demonstrated that the functional expression of nAChRs is located both on the soma and dendrites. This is the first report demonstrating the presence of functional nAChR-mediated current responses from rat amygdala slices, where they may be playing a significant role in fear and aversively motivated memory.