Action potential-independent and nicotinic receptor-mediated concerted release of multiple quanta at hippocampal CA3-mossy fiber synapses

Nicotine Motivated Behavior in C. elegans

To maximize the advantages offered by Caenorhabditis elegans as a high-throughput (HTP) model for nicotine dependence studies, utilizing its well-defined neuroconnectome as a robust platform, and to unravel the genetic basis of nicotine-motivated behaviors, we established the nicotine conditioned cue preference (CCP) paradigm. Nicotine CCP enables the assessment of nicotine preference and seeking, revealing a parallel to fundamental aspects of nicotine-dependent behaviors observed in mammals. We demonstrated that nicotine-elicited cue preference in worms is mediated by nicotinic acetylcholine receptors and requires dopamine for CCP development. Subsequently, we pinpointed nAChR subunits associated with nicotine preference and validated human GWAS candidates linked to nicotine dependence involved in nAChRs. Functional validation involves assessing the loss-of-function strain of the CACNA2D3 ortholog and the knock-out (KO) strain of the CACNA2D2 ortholog, closely related to CACNA2D3 and sharing human smoking phenotypes. Our orthogonal approach substantiates the functional conservation of the α2δ subunit of the calcium channel in nicotine-motivated behavior. Nicotine CCP in C. elegans serves as a potent affirmation of the cross-species functional relevance of GWAS candidate genes involved in nicotine seeking associated with tobacco abuse, providing a streamlined yet comprehensive system for investigating intricate behavioral paradigms within a simplified and reliable framework.

Neuromodulatory Control of Early Visual Processing in Macaque

Visual processing is dynamically controlled by multiple neuromodulatory molecules that modify the responsiveness of neurons and the strength of the connections between them. In particular, modulatory control of processing in the lateral geniculate nucleus of the thalamus, V1, and V2 will alter the outcome of all subsequent processing of visual information, including the extent to and manner in which individual inputs contribute to perception and decision making and are stored in memory. This review addresses five small-molecule neuromodulators-acetylcholine, dopamine, serotonin, noradrenaline, and histamine-considering the structural basis for their action, and the effects of their release, in the early visual pathway of the macaque monkey. Traditionally, neuromodulators are studied in isolation and in discrete circuits; this review makes a case for considering the joint action of modulatory molecules and differences in modulatory effects across brain areas as a better means of understanding the diverse roles that these molecules serve.

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

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

Nicotine Increases Spontaneous Glutamate Release in the Rostromedial Tegmental Nucleus

The rostromedial tegmental nucleus (RMTg) is a bilateral structure localized in the brainstem and comprise of mainly GABAergic neurons. One of the main functions of the RMTg is to regulate the activity of dopamine neurons of the mesoaccumbens pathway. Therefore, the RMTg has been proposed as a modulator of the reward system and adaptive behaviors associated to reward learning. The RMTg receives an important glutamatergic input from the lateral habenula. Also, it receives cholinergic inputs from the laterodorsal and pedunculopontine tegmental nuclei. Previously, it was reported that nicotine increases glutamate release, evoked by electric stimulation, in the RMTg nucleus. However, the mechanisms by which nicotine induces this effect were not explored. In the present work, we performed electrophysiological experiments in brainstem slices to study the effect of nicotine on spontaneous excitatory postsynaptic currents recorded from immunocytochemically identified RMTg neurons. Also, we used calcium imaging techniques to explore the effects of nicotine on multiple RMTg neurons simultaneously. We found that nicotine promotes the persistent release of glutamate through the activation of α7 nicotinic acetylcholine receptors present on glutamatergic afferents and by a mechanism involving calcium release from intracellular stores. Through these mechanisms, nicotine increases the excitability and synchronizes the activity of RMTg neurons. Our results suggest that the RMTg nucleus mediates the noxious effects of the nicotine, and it could be a potential therapeutic target against tobacco addiction.

Ca2+ Dyshomeostasis Disrupts Neuronal and Synaptic Function in Alzheimer's Disease

Ca²⁺ homeostasis is essential for multiple neuronal functions and thus, Ca²⁺ dyshomeostasis can lead to widespread impairment of cellular and synaptic signaling, subsequently contributing to dementia and Alzheimer's disease (AD). While numerous studies implicate Ca²⁺ mishandling in AD, the cellular basis for loss of cognitive function remains under investigation. The process of synaptic degradation and degeneration in AD is slow, and constitutes a series of maladaptive processes each contributing to a further destabilization of the Ca²⁺ homeostatic machinery. Ca²⁺ homeostasis involves precise maintenance of cytosolic Ca²⁺ levels, despite extracellular influx via multiple synaptic Ca²⁺ channels, and intracellular release via organelles such as the endoplasmic reticulum (ER) via ryanodine receptor (RyRs) and IP₃R, lysosomes via transient receptor potential mucolipin channel (TRPML) and two pore channel (TPC), and mitochondria via the permeability transition pore (PTP). Furthermore, functioning of these organelles relies upon regulated inter-organelle Ca²⁺ handling, with aberrant signaling resulting in synaptic dysfunction, protein mishandling, oxidative stress and defective bioenergetics, among other consequences consistent with AD. With few effective treatments currently available to mitigate AD, the past few years have seen a significant increase in the study of synaptic and cellular mechanisms as drivers of AD, including Ca²⁺ dyshomeostasis. Here, we detail some key findings and discuss implications for future AD treatments.

Nicotine evoked efferent transmitter release onto immature cochlear inner hair cells

Olivocochlear neurons make temporary cholinergic synapses on inner hair cells of the rodent cochlea in the first 2 to 3 wk after birth. Repetitive stimulation of these efferent neurons causes facilitation of evoked release and increased spontaneous release that continues for seconds to minutes. Presynaptic nicotinic acetylcholine receptors (nAChRs) are known to modulate neurotransmitter release from brain neurons. The present study explores the hypothesis that presynaptic nAChRs help to increase spontaneous release from efferent terminals on cochlear hair cells. Direct application of nicotine (which does not activate the hair cells' α9α10-containing nAChRs) produces sustained efferent transmitter release, implicating presynaptic nAChRs in this response. The effect of nicotine was reduced by application of ryanodine that reduces release of calcium from intraterminal stores.NEW & NOTEWORTHY Sensory organs exhibit spontaneous activity before the onset of response to external stimuli. Such activity in the cochlea is subject to modulation by cholinergic efferent neurons that directly inhibit sensory hair cells (inner hair cells). Those efferent neurons are themselves subject to various modulatory mechanisms. One such mechanism is positive feedback by released acetylcholine onto presynaptic nicotinic acetylcholine receptors causing further release of acetylcholine.

Alpha7 nicotinic acetylcholine receptors and neural network synaptic transmission in human induced pluripotent stem cell-derived neurons

The α7 nicotinic acetylcholine receptor has been extensively researched as a target for treatment of cognitive impairment in Alzheimer's disease and schizophrenia. Investigation of the α7 receptor is commonly performed in animals but it is critical to increase the biologically relevance of the model systems to fully capture the physiological role of the α7 receptor in humans. For example most humans, in contrast to animals, express the hybrid gene CHRFAM7A, the product of which modulates α7 receptor activity. In the present study, we used human induced pluripotent stem cell (hiPSC) derived neurons to establish a humanized α7 model. We established a cryobank of neural stem cells (NSCs) that could reproducibly be matured into neurons expressing neuronal markers and CHRNA7 and CHRFAM7A. The neurons responded to NMDA, GABA, and acetylcholine and exhibited synchronized spontaneous calcium oscillations. Gene expression studies and application of a range of α7 positive allosteric modulators (PNU-120595, TQS, JNJ-39393406 and AF58801) together with the α7 agonist PNU-282987 during measurement of intracellular calcium levels demonstrated the presence of functional α7 receptors in matured hiPSC-derived neuronal cultures. Pharmacological α7 activation also resulted in intracellular signaling as measured by ERK 1/2 phosphorylation and c-Fos protein expression. Moreover, PNU-120596 increased the frequency of the spontaneous calcium oscillations demonstrating implication of α7 receptors in human synaptic networks activity. Overall, we show that hiPSC derived neurons are an advanced in vitro model for studying human α7 receptor pharmacology and the involvement of this receptor in cellular processes as intracellular signaling and synaptic transmission.

A dominant role for the beta 4 nicotinic receptor subunit in nicotinic modulation of glomerular microcircuits in the mouse olfactory bulb

Nicotinic acetylcholine receptors (nAChRs) regulate information transfer across the main olfactory bulb by instituting a high-pass intensity filter allowing for the filtering out of weak inputs. Excitation-driven inhibition of the glomerular microcircuit via GABA release from periglomerular cells appears to underlie this effect of nAChR activation. The multiplicity of nAChR subtypes and cellular locations raises questions about their respective roles in mediating their effects on the glomerular output. In this study, we address this issue by targeting heteromeric nAChRs using receptor knockouts (KOs) for the two dominant nAChR β-subunit genes known to be expressed in the central nervous system. KOs of the β₂-nAChR subunit did not affect nAChR currents from mitral cells (MCs) but attenuated those from the external tufted (ET) cells. In slices from these animals, activation of nAChRs still effectively inhibited excitatory postsynaptic currents (EPSCs) and firing on MCs evoked by the olfactory nerve (ON) stimulation, thereby indicating that the filter mechanism was intact. On the other hand, recordings from β₄-KOs showed that nAChR responses from MCs were abolished and those from ET cells were attenuated. Excitation-driven feedback was abolished as was the effect of nAChR activation on ON-evoked EPSCs. Experiments using calcium imaging showed that one possible consequence of the β₂-subunit activation might be to alter the time course of calcium transients in juxtaglomerular neurons suggesting a role for these receptors in calcium signaling. Our results indicate that nAChRs containing the β₄-subunit are critical in the filtering of odor inputs and play a determinant role in the cholinergic modulation of glomerular output. NEW & NOTEWORTHY In this study, using receptor gene knockouts we examine the relative contributions of heteromeric nAChR subtypes located on different cell types to this effect of receptor activation. Our results demonstrate that nAChRs containing the β₄-subunit activate MCs resulting in feedback inhibition from glomerular interneurons. This period of inhibition results in the selective filtering of weak odor inputs providing one mechanism by which nAChRs can enhance discrimination between two closely related odors.

Beyond the Channel: Metabotropic Signaling by Nicotinic Receptors

The α7 nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel (LGIC) that plays an important role in cellular calcium signaling and contributes to several neurological diseases. Agonist binding to the α7 nAChR induces fast channel activation followed by inactivation and prolonged desensitization while triggering long-lasting calcium signaling. These activities foster neurotransmitter release, synaptic plasticity, and somatodendritic regulation in the brain. We discuss here the ability of α7 nAChRs to operate in ionotropic (α7ⁱ) and metabotropic (α7^m) modes, leading to calcium-induced calcium release (CICR) and G protein-associated inositol trisphosphate (IP₃)-induced calcium release (IICR), respectively. Metabotropic activity extends the spatial and temporal aspects of calcium signaling by the α7 channel beyond its ionotropic limits, persisting into the desensitized state. Delineation of the ionotropic and metabotropic properties of the α7 nAChR will provide definitive indicators of moment-to-moment receptor functional status that will, in turn, spearhead new drug development.

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

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

Neuromodulation of the Feedforward Dentate Gyrus-CA3 Microcircuit

The feedforward dentate gyrus-CA3 microcircuit in the hippocampus is thought to activate ensembles of CA3 pyramidal cells and interneurons to encode and retrieve episodic memories. The creation of these CA3 ensembles depends on neuromodulatory input and synaptic plasticity within this microcircuit. Here we review the mechanisms by which the neuromodulators aceylcholine, noradrenaline, dopamine, and serotonin reconfigure this microcircuit and thereby infer the net effect of these modulators on the processes of episodic memory encoding and retrieval.

Modulatory compartments in cortex and local regulation of cholinergic tone

Neuromodulatory signaling is generally considered broad in its impact across cortex. However, variations in the characteristics of cortical circuits may introduce regionally-specific responses to diffuse modulatory signals. Features such as patterns of axonal innervation, tissue tortuosity and molecular diffusion, effectiveness of degradation pathways, subcellular receptor localization, and patterns of receptor expression can lead to local modification of modulatory inputs. We propose that modulatory compartments exist in cortex and can be defined by variation in structural features of local circuits. Further, we argue that these compartments are responsible for local regulation of neuromodulatory tone. For the cholinergic system, these modulatory compartments are regions of cortical tissue within which signaling conditions for acetylcholine are relatively uniform, but between which signaling can vary profoundly. In the visual system, evidence for the existence of compartments indicates that cholinergic modulation likely differs across the visual pathway. We argue that the existence of these compartments calls for thinking about cholinergic modulation in terms of finer-grained control of local cortical circuits than is implied by the traditional view of this system as a diffuse modulator. Further, an understanding of modulatory compartments provides an opportunity to better understand and perhaps correct signal modifications that lead to pathological states.

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

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

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

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

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Tonic ACh preferentially activates α7 nAChRs at glutamatergic nerve terminals.

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

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.

Cholinergic modulation of dopamine pathways through nicotinic acetylcholine receptors

Nicotine addiction is highly prevalent in current society and is often comorbid with other diseases. In the central nervous system, nicotine acts as an agonist for nicotinic acetylcholine receptors (nAChRs) and its effects depend on location and receptor composition. Although nicotinic receptors are found in most brain regions, many studies on addiction have focused on the mesolimbic system and its reported behavioral correlates such as reward processing and reinforcement learning. Profound modulatory cholinergic input from the pedunculopontine and laterodorsal tegmentum to dopaminergic midbrain nuclei as well as local cholinergic interneuron projections to dopamine neuron axons in the striatum may play a major role in the effects of nicotine. Moreover, an indirect mesocorticolimbic feedback loop involving the medial prefrontal cortex may be involved in behavioral characteristics of nicotine addiction. Therefore, this review will highlight current understanding of the effects of nicotine on the function of mesolimbic and mesocortical dopamine projections in the mesocorticolimbic circuit.

Signaling between periglomerular cells reveals a bimodal role for GABA in modulating glomerular microcircuitry in the olfactory bulb

In the mouse olfactory bulb glomerulus, the GABAergic periglomerular (PG) cells provide a major inhibitory drive within the microcircuit. Here we examine GABAergic synapses between these interneurons. At these synapses, GABA is depolarizing and exerts a bimodal control on excitability. In quiescent cells, activation of GABAA receptors can induce the cells to fire, thereby providing a means for amplification of GABA release in the glomerular microcircuit via GABA-induced GABA release. In contrast, GABA is inhibitory in neurons that are induced to fire tonically. PG-PG interactions are modulated by nicotinic acetylcholine receptors (nAChRs), and our data suggest that changes in intracellular calcium concentrations triggered by nAChR activation can be amplified by GABA release. Our results suggest that bidirectional control of inhibition in PG neurons can allow for modulatory inputs, like the cholinergic inputs from the basal forebrain, to determine threshold set points for filtering out weak olfactory inputs in the glomerular layer of the olfactory bulb via the activation of nAChRs.

Induction of long-term oscillations in the γ frequency band by nAChR activation in rat hippocampal CA3 area

The hippocampal neuronal network oscillation at γ frequency band (γ oscillation) is generated by the precise interaction between interneurons and principle cells. γ oscillation is associated with attention, learning and memory and is impaired in the diseased conditions such as Alzheimer's disease (AD) and schizophrenia. Nicotinic acetylcholine receptor (nAChR) plays an important role in the regulation of hippocampal neurotransmission and network activity. It is not known whether nicotine modulates plasticity of network activity at γ oscillations in the hippocampus. In this study we investigated the effects of nicotine on the long-term changes of KA-induced γ oscillations. We found that hippocampal γ oscillations can be enhanced by a low concentration of nicotine (1μM), such an enhancement lasts for hours after washing out of nicotine, suggesting a form of synaptic plasticity, named as long-term oscillation at γ frequency band (LTOγ). Nicotine-induced LTOγ was mimicked by the selective α4β2 but not by α7 nAChR agonist and was involved in N-methyl-d-aspartate (NMDA) receptor activation as well as depended on excitatory and inhibitory neurotransmission. Our results indicate that nAChR activation induced plasticity in γ oscillation, which may be beneficial for the improvement of cognitive deficiency in AD and schizophrenia.

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.

Acetylcholine Acts through Nicotinic Receptors to Enhance the Firing Rate of a Subset of Hypocretin Neurons in the Mouse Hypothalamus through Distinct Presynaptic and Postsynaptic Mechanisms

Neurons expressing the neuropeptide hypocretin regulate many behavioral functions, including sleep, motivation, and behaviors related to addiction. The ability of nicotine to stimulate nicotinic acetylcholine receptors (nAChRs) is essential for its addictive properties, but little is known about whether, and how, nicotine and the endogenous neurotransmitter acetylcholine affect hypocretin neurons.

Hypocretin/orexin neurons regulate many behavioral functions, including addiction. Nicotine acts through nicotinic acetylcholine receptors (nAChRs) to alter firing rate of neurons throughout the brain, leading to addiction-related behaviors. While nAChRs are expressed in the hypothalamus and cholinergic fibers project to this structure, it is unclear how acetylcholine modulates the activity of hypocretin neurons. In this study, we stimulated hypocretin neurons in mouse brain slices with ACh in the presence of atropine to dissect presynaptic and postsynaptic modulation of these neurons through nAChRs. Approximately one-third of tested hypocretin neurons responded to pressure application of ACh (1 mM) with an increase in firing frequency. Stimulation of postsynaptic nAChRs with ACh or nicotine resulted in a highly variable inward current in approximately one-third of hypocretin neurons. In contrast, ACh or nicotine (1 µM) reliably decreased the frequency of miniature EPSCs (mEPSCs). Antagonism of nAChRs with mecamylamine also suppressed mEPSC frequency, suggesting that an endogenous, tonic activation of presynaptic nAChRs might be required for maintaining functional mEPSC frequency. Antagonism of heteromeric (α4β2) or homomeric (α7) nAChRs alone suppressed mEPSCs to a lesser extent. Finally, blocking internal calcium release reduced the frequency of mEPSCs, occluding the suppressive effect of presynaptic ACh. Taken together, these data provide a mechanism by which phasic ACh release enhances the firing of a subset of hypocretin neurons through postsynaptic nAChRs, but disrupts tonic, presynaptic nAChR-mediated glutamatergic inputs to the overall population of hypocretin neurons, potentially enhancing the signal-to-noise ratio during the response of the nAChR-positive subset of neurons.

Nicotine increases GABAergic input on rat dorsal raphe serotonergic neurons through alpha7 nicotinic acetylcholine receptor

The dorsal raphe nucleus (DRN) contains large populations of serotonergic (5-HT) neurons. This nucleus receives GABAergic inhibitory afferents from many brain areas and from DRN interneurons. Both GABAergic and 5-HT DRN neurons express functional nicotinic acetylcholine receptors (nAChRs). Previous studies have demonstrated that nicotine increases 5-HT release and 5-HT DRN neuron discharge rate by stimulating postsynaptic nAChRs and by increasing glutamate and norepinephrine release inside DRN. However, the influence of nicotine on the GABAergic input to 5-HT DRN neurons was poorly investigated. Therefore, the aim of this work was to determine the effect of nicotine on GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) of 5-HT DRN neurons and the subtype of nAChR(s) involved in this response. Experiments were performed in coronal slices obtained from young Wistar rats. GABAergic sIPSCs were recorded from post hoc-identified 5-HT DRN neurons with the whole cell voltage patch-clamp technique. Administration of nicotine (1 μM) increased sIPSC frequency in 72% of identified 5-HT DRN neurons. This effect was not reproduced by the α4β2 nAChR agonist RJR-2403 and was not influenced by TTX (1 μM). It was mimicked by the selective agonist for α7 nAChR, PNU-282987, and exacerbated by the positive allosteric modulator of the same receptor, PNU-120596. The nicotine-induced increase in sIPSC frequency was independent on voltage-gated calcium channels and dependent on Ca2+-induced Ca2+ release (CICR). These results demonstrate that nicotine increases the GABAergic input to most 5-HT DRN neurons, by activating α7 nAChRs and producing CICR in DRN GABAergic terminals.

Exocytosis and synaptic vesicle function

Synaptic vesicles release their vesicular contents to the extracellular space by Ca(2+)-triggered exocytosis. The Ca(2+)-triggered exocytotic process is regulated by synaptotagmin (Syt), a vesicular Ca(2+)-binding C2 domain protein. Synaptotagmin 1 (Syt1), the most studied major isoform among 16 Syt isoforms, mediates Ca(2+)-triggered synaptic vesicle exocytosis by interacting with the target membranes and SNARE/complexin complex. In synapses of the central nervous system, synaptobrevin 2, a major vesicular SNARE protein, forms a ternary SNARE complex with the plasma membrane SNARE proteins, syntaxin 1 and SNAP25. The affinities of Ca(2+)-dependent interactions between Syt1 and its targets (i.e., SNARE complexes and membranes) are well correlated with the efficacies of the corresponding exocytotic processes. Therefore, different SNARE protein isoforms and membrane lipids, which interact with Syt1 with various affinities, are capable of regulating the efficacy of Syt1-mediated exocytosis. Otoferlin, another type of vesicular C2 domain protein that binds to the membrane in a Ca(2+)-dependent manner, is also involved in the Ca(2+)-triggered synaptic vesicle exocytosis in auditory hair cells. However, the functions of otoferlin in the exocytotic process are not well understood. In addition, at least five different types of synaptic vesicle proteins such as synaptic vesicle protein 2, cysteine string protein α, rab3, synapsin, and a group of proteins containing four transmembrane regions, which includes synaptophysin, synaptogyrin, and secretory carrier membrane protein, are involved in modulating the exocytotic process by regulating the formation and trafficking of synaptic vesicles.

Presynaptic α7 nicotinic acetylcholine receptors enhance hippocampal mossy fiber glutamatergic transmission via PKA activation

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 mechanism of nicotine's action on cognitive function remains elusive. We performed patch-clamp recordings from hippocampal CA3 pyramidal neurons to determine the effect of nicotine on mossy fiber glutamatergic synaptic transmission. We found that nicotine in combination with NS1738, an α7 nAChR-positive allosteric modulator, strongly potentiated the amplitude of evoked EPSCs (eEPSCs), and reduced the EPSC paired-pulse ratio. The action of nicotine and NS1738 was mimicked by PNU-282987 (an α7 nAChR agonist), and was absent in α7 nAChR knock-out mice. These data indicate that activation of α7 nAChRs was both necessary and sufficient to enhance the amplitude of eEPSCs. BAPTA applied postsynaptically failed to block the action of nicotine and NS1738, suggesting again a presynaptic action of the α7 nAChRs. We also observed α7 nAChR-mediated calcium rises at mossy fiber giant terminals, indicating the presence of functional α7 nAChRs at presynaptic terminals. Furthermore, the addition of PNU-282987 enhanced action potential-dependent calcium transient at these terminals. Last, the potentiating effect of PNU-282987 on eEPSCs was abolished by inhibition of protein kinase A (PKA). Our findings indicate that activation of α7 nAChRs at presynaptic sites, via a mechanism involving PKA, plays a critical role in enhancing synaptic efficiency of hippocampal mossy fiber transmission.

Nicotinic modulation of serotonergic activity in the dorsal raphe nucleus

Cholinergic signaling mediated by nicotinic receptors has been associated to a large number of physiological and behavioral processes such as learning, memory, attention, food-intake and mood disorders. Although it is well established that many nicotinic actions are mediated through an increase in serotonin (5-HT) release, the physiological mechanisms by which nicotine produces these effects are still unclear. The dorsal raphe nucleus (DRN) contains the major amount of 5-HT neurons projecting to different parts of the brain. DRN also contains nicotinic acetylcholine receptors (nAChRs) located at somatic and presynaptic elements. Nicotine produces both inhibitory and excitatory effects on different subpopulations of 5-HT DRN neurons. In this review, we describe the presynaptic and postsynaptic mechanisms by which nicotine increases the excitability of DRN neurons as well as the subtypes of nAChRs involved. We also describe the inhibitory effects of nicotine and the role of 5-HT1A receptors in this effect. These nicotinic actions modulate the activity of different neuronal subpopulations in the DRN, changing the 5-HT tone in the brain areas where these groups of neurons project. Some of the physiological implications of nicotine-induced 5-HT release are discussed.

Nicotine elicits prolonged calcium signaling along ventral hippocampal axons

Presynaptic nicotinic acetylcholine receptors (nAChRs) have long been implicated in the modulation of CNS circuits. We previously reported that brief exposure to low concentrations of nicotine induced sustained potentiation of glutamatergic transmission at ventral hippocampal (vHipp)-striatal synapses. Here, we exploited nAChR subtype-selective antagonists and agonists and α7*nAChR knockout mutant mice (α7-/-) to elucidate the signaling mechanisms underlying nAChR-mediated modulation of synaptic transmission. Using a combination of micro-slices culture from WT and α7-/-mice, calcium imaging, and immuno-histochemical techniques, we found that nicotine elicits localized and oscillatory increases in intracellular Ca(2+) along vHipp axons that persists for up to 30 minutes. The sustained phase of the nicotine-induced Ca(2+) response was blocked by α-BgTx but not by DHβE and was mimicked by α7*nAChR agonists but not by non-α7*nAChR agonists. In vHipp slices from α7-/- mice, nicotine elicited only transient increases of axonal Ca(2+) signals and did not activate CaMKII. The sustained phase of the nicotine-induced Ca(2+) response required localized activation of CaMKII, phospholipase C, and IP3 receptor mediated Ca(2+)-induced Ca(2+) release (CICR). In conclusion, activation of presynaptic nAChRs by nicotine elicits Ca(2+) influx into the presynaptic axons, the sustained phase of the nicotine-induced Ca(2+) response requires that axonal α7*nAChR activate a downstream signaling network in the vHipp axons.

Nicotinic receptors in neurodegeneration

Many studies have focused on expanding our knowledge of the structure and diversity of peripheral and central nicotinic receptors. Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop superfamily of pentameric ligand-gated ion channels, which include GABA (A and C), serotonin, and glycine receptors. Currently, 9 alpha (α2-α10) and 3 beta (β2-β4) subunits have been identified in the central nervous system (CNS), and these subunits assemble to form a variety of functional nAChRs. The pentameric combination of several alpha and beta subunits leads to a great number of nicotinic receptors that vary in their properties, including their sensitivity to nicotine, permeability to calcium and propensity to desensitize.

In the CNS, nAChRs play crucial roles in modulating presynaptic, postsynaptic, and extrasynaptic signaling, and have been found to be involved in a complex range of CNS disorders including Alzheimer’s disease (AD), Parkinson’s disease (PD), schizophrenia, Tourette´s syndrome, anxiety, depression and epilepsy. Therefore, there is growing interest in the development of drugs that modulate nAChR functions with optimal benefits and minimal adverse effects. The present review describes the main characteristics of nAChRs in the CNS and focuses on the various compounds that have been tested and are currently in phase I and phase II trials for the treatment of neurodegenerative diseases including PD, AD and age-associated memory and mild cognitive impairment.

Contribution of CA3 and CA1 pyramidal neurons to the tonic α7 nAChR-dependent glutamatergic input to CA1 pyramidal neurons

The Schaffer collaterals are among the major glutamatergic inputs to CA1 pyramidal neurons, the primary output of the hippocampus, which also receive sparse recurrent inputs from pyramidal neurons in the CA1 field. Although tonically active α7 nicotinic acetylcholine receptors (nAChRs) have been shown to sustain spontaneous glutamate transmission to CA1 pyramidal neurons in hippocampal slices under resting conditions, it remains to be determined whether these receptors are those expressed by CA3 or CA1 pyramidal neurons. This study was designed to test the hypothesis that the CA3 field of the hippocampus is a significant source of α7 nAChR-sustained glutamatergic transmission to CA1 pyramidal neurons. To this end, spontaneous excitatory postsynaptic currents (EPSCs) were recorded from CA1 and CA3 pyramidal neurons in intact rat hippocampal slices as well as from CA1 pyramidal neurons in CA3-ablated slices under various experimental conditions. Surgical removal of the CA3 region from the slices reduced by 20% the frequency of spontaneous EPSCs recorded from CA1 pyramidal neurons. This finding is in agreement with the concept that the CA3 field contributes significantly to the maintenance of spontaneous glutamatergic synaptic activity in CA1 pyramidal neurons. In addition, the α7 nAChR antagonist methyllycaconitine (MLA, 10nM) reduced the frequency of spontaneous EPSCs recorded from CA1 pyramidal neurons by 30% in intact slices and 12% in CA3-ablated slices. Taken together, these results demonstrate that tonically active α7 nAChRs in CA3 pyramidal neurons and/or in the Mossy fibers that innervate the CA3 pyramidal neurons do in fact contribute to the maintenance of glutamatergic synaptic activity in CA1 pyramidal neurons of hippocampal slices under resting conditions.

EPAC inhibition of SUR1 receptor increases glutamate release and seizure vulnerability

EPAC (Exchange Proteins Activated by cAMP) regulates glutamate transmitter release in the central neurons, but a role underlying this regulation has yet to be identified. Here we show that EPAC binds directly to the intracellular loop of an ATP-sensitive potassium (KATP) channel type-1 sulfonylurea receptor (SUR1) receptor consisting of amino acids 859-881 (SUR1(859-881)). Ablation of EPAC or expression of SUR1(859-881), which intercepts EPAC-SUR1 binding, increases the open probability of KATP channels consisting of the Kir6.1 subunit and SUR1. Opening of KATP channels inhibits glutamate release and reduces seizure vulnerability in adult mice. Therefore, EPAC interaction with SUR1 controls seizure susceptibility and possibly acts via regulation of glutamate release.

Nicotinic receptors modulate olfactory bulb external tufted cells via an excitation-dependent inhibitory mechanism

Olfactory bulb (OB) glomeruli, the initial sites of synaptic processing of odor information, exhibit high levels of nicotinic acetylcholine receptor (nAChR) expression and receive strong cholinergic input from the basal forebrain. The role of glomerular nAChRs in olfactory processing, however, remains to be elucidated. External tufted (ET) cells are a major source of excitation in the glomerulus and an important component of OB physiology. We have examined the role of nAChRs in modulating ET cell activity using whole-cell electrophysiology in mouse OB slices. We show here that the activation of glomerular nAChRs leads to direct ET cell excitation, as well as an increase in the frequency of spontaneous postsynaptic GABAergic currents. β2-containing nAChRs, likely the α4β2*-nAChR subtype (* represents the possible presence of other subunits), were significant contributors to these effects. The nAChR-mediated increase in spontaneous postsynaptic GABAergic current frequency on ET cells was, for the most part, dependent on glutamate receptor activation, thus implicating a role for excitation-dependent inhibition within the glomerulus. β2-containing nAChRs also regulate the frequency of miniature inhibitory postsynaptic currents on ET cells, implying nicotinic modulation of dendrodendritic signaling between ET and periglomerular cells. Our data also indicate that nAChR activation does not affect spontaneous or evoked transmission at the olfactory nerve-to-ET cell synapse. The results from this study suggest that ET cells, along with mitral cells, play an important role in the nicotinic modulation of glomerular inhibition.

Acetylcholinesterase inhibition reveals endogenous nicotinic modulation of glutamate inputs to CA1 stratum radiatum interneurons in hippocampal slices

The involvement of brain nicotinic acetylcholine receptors (nAChRs) in the neurotoxicological effects of soman, a potent acetylcholinesterase (AChE) inhibitor and a chemical warfare agent, is not clear. This is partly due to a poor understanding of the role of AChE in brain nAChR-mediated functions. To test the hypothesis that AChE inhibition builds sufficient acetylcholine (ACh) in the brain and facilitates nAChR-dependent glutamate transmission, we used whole-cell patch-clamp technique to record spontaneous glutamate excitatory postsynaptic currents (EPSCs) from CA1 stratum radiatum interneurons (SRI) in hippocampal slices. First, the frequency, amplitude and kinetics of EPSCs recorded from slices of control guinea pigs were compared to those recorded from slices of guinea pigs after a single injection of the irreversible AChE inhibitor soman (25.2μg/kg, s.c.). Second, EPSCs were recorded from rat hippocampal slices before and after their superfusion with the reversible AChE inhibitor donepezil (100nM). The frequency of EPSCs was significantly higher in slices taken from guinea pigs 24h but not 7 days after the soman injection than in slices from control animals. In 52% of the rat hippocampal slices tested, bath application of donepezil increased the frequency of EPSCs. Further, exposure to donepezil increased both burst-like and large-amplitude EPSCs, and increased the proportion of short (20-100ms) inter-event intervals. Donepezil's effects were suppressed significantly in presence of 10μM mecamylamine or 10nM methyllycaconitine. These results support the concept that AChE inhibition is able to recruit nAChR-dependent glutamate transmission in the hippocampus and such a mechanism can contribute to the acute neurotoxicological actions of soman.

Presynaptic α4β2 nicotinic acetylcholine receptors increase glutamate release and serotonin neuron excitability in the dorsal raphe nucleus

Several behavioral effects of nicotine are mediated by changes in serotonin (5-HT) release in brain areas that receive serotonergic afferents from the dorsal raphe nucleus (DRN). In vitro experiments have demonstrated that nicotine increases the firing activity in the majority of DRN 5-HT neurons and that DRN contains nicotinic acetylcholine receptors (nAChRs) located at both somata and presynaptic elements. One of the most common presynaptic effects of nicotine is to increase glutamate release. Although DRN receives profuse glutamatergic afferents, the effect of nicotine on glutamate release in the DRN has not been studied in detail. Using whole-cell recording techniques, we investigated the effects of nicotine on the glutamatergic input to 5-HT DRN neurons in rat midbrain slices. Low nicotine concentrations, in the presence of bicuculline and tetrodotoxin (TTX), increased the frequency but did not change the amplitude of glutamate-induced EPSCs, recorded from identified 5-HT neurons. Nicotine-induced increase of glutamatergic EPSC frequency persisted 10-20 min after drug withdrawal. This nicotinic effect was mimicked by exogenous administration of acetylcholine (ACh) or inhibition of ACh metabolism. In addition, the nicotine-induced increase in EPSC frequency was abolished by blockade of α4β2 nAChRs, voltage-gated calcium channels, or intracellular calcium signaling but not by α7 nAChR antagonists. These data suggest that both nicotine and endogenous ACh can increase glutamate release through activation of presynaptic α4β2 but not α7 nAChRs in the DRN. The effect involves long-term changes in synaptic function, and it is dependent on voltage-gated calcium channels and presynaptic calcium stores.

The dominant functional nicotinic receptor in progenitor cells in the rostral migratory stream is the α3β4 subtype

Addition of newly generated neurons into mature neural circuits in the adult CNS responds to changes in neurotransmitter levels and is tightly coupled to the activity of specific brain regions. This postnatal neurogenesis contributes to plasticity of the olfactory bulb and hippocampus and is thought to play a role in learning and memory, context and odor discrimination, as well as perceptual learning. While acetylcholine plays an important role in odor discrimination and perceptual learning, its role in adult neurogenesis in the olfactory bulb has not been elucidated. In this study, I have examined the functional expression of nAChRs in progenitor cells of the rostral migratory stream (RMS) in the adult olfactory bulb of mice. I show that most of these cells in the RMS exhibit large nAChR-mediated calcium transients upon application of acetylcholine (ACh). Unlike in the hippocampus, the predominant functional nAChRs on progenitor cells are of α3β4 subtype. Interestingly, functional receptor expression is lost once progenitor cells mature, and are incorporated into the granule cell layer. Instead, nAChRs are now expressed on some presynaptic terminals and modulate glutamate release onto granule cells. My results imply that ACh is a part of the permissive niche and likely plays a role in development of progenitor cells.

Genetic inhibition of CaMKII in dorsal striatal medium spiny neurons reduces functional excitatory synapses and enhances intrinsic excitability

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is abundant in striatal medium spiny neurons (MSNs). CaMKII is dynamically regulated by changes in dopamine signaling, as occurs in Parkinson's disease as well as addiction. Although CaMKII has been extensively studied in the hippocampus where it regulates excitatory synaptic transmission, relatively little is known about how it modulates neuronal function in the striatum. Therefore, we examined the impact of selectively overexpressing an EGFP-fused CaMKII inhibitory peptide (EAC3I) in striatal medium spiny neurons (MSNs) using a novel transgenic mouse model. EAC3I-expressing cells exhibited markedly decreased excitatory transmission, indicated by a decrease in the frequency of spontaneous excitatory postsynaptic currents (sEPSCs). This decrease was not accompanied by changes in the probability of release, levels of glutamate at the synapse, or changes in dendritic spine density. CaMKII regulation of the AMPA receptor subunit GluA1 is a major means by which the kinase regulates neuronal function in the hippocampus. We found that the decrease in striatal excitatory transmission seen in the EAC3I mice is mimicked by deletion of GluA1. Further, while CaMKII inhibition decreased excitatory transmission onto MSNs, it increased their intrinsic excitability. These data suggest that CaMKII plays a critical role in setting the excitability rheostat of striatal MSNs by coordinating excitatory synaptic drive and the resulting depolarization response.

Glutamatergic and GABAergic metabolism in mouse brain under chronic nicotine exposure: implications for addiction

Background and Purpose

The effects of nicotine on cerebral metabolism and its influence on smoking behavior is poorly understood. An understanding of the chronic effects of nicotine on excitatory and inhibitory metabolic demand, and corresponding neurotransmission may provide clues for designing strategies for the optimal smoking cessation intervention. The objective of the current study was to investigate neuronal and astroglial metabolism in mice exposed to nicotine (0.5 and 2.0 mg/kg, sc) three times in a day for 4 weeks.

Experimental Approach/Principal Findings

Metabolic measurements were carried out by co-infusing [U-¹³C₆]glucose and [2-¹³C]acetate, and monitoring ¹³C labeling of amino acids in brain tissue extract using ¹H-[¹³C] and ¹³C-[¹H]-NMR spectroscopy. Concentration of ¹³C-labeled glutamate-C4 was increased significantly from glucose and acetate with chronic nicotine treatment indicating an increase in glucose oxidation by glutamatergic neurons in all brain regions and glutamate-glutamine neurotransmitter cycle in cortical and subcortical regions. However, chronic nicotine treatment led to increased labeling of GABA-C2 from glucose only in the cortical region. Further, increased labeling of glutamine-C4 from [2-¹³C]acetate is suggestive of increased astroglial activity in subcortical and cerebellum regions of brain with chronic nicotine treatment.

Conclusions and Significance

Chronic nicotine exposure enhanced excitatory activity in the majority of brain regions while inhibitory and astroglial functions were enhanced only in selected brain regions.

M2 muscarinic acetylcholine receptors regulate long-term potentiation at hippocampal CA3 pyramidal cell synapses in an input-specific fashion

Muscarinic receptors have long been known as crucial players in hippocampus-dependent learning and memory, but our understanding of the cellular underpinnings and the receptor subtypes involved lags well behind. This holds in particular for the hippocampal CA3 region, where the mechanisms of synaptic plasticity depend on the type of afferent input. Williams and Johnston (Williams S, Johnston D. Science 242: 84-87, 1988; Williams S, Johnston D. J Neurophysiol 64: 1089-1097, 1990) demonstrated muscarinic depression of mossy fiber (MF) long-term potentiation (LTP) through a presynaptic site of action and Maeda et al. (Maeda T, Kaneko S, Satoh M. Brain Res 619: 324-330, 1993) proposed a bidirectional modulation of MF LTP by muscarinic receptor subtypes. Since then, this issue, as well as muscarinic regulation of plasticity at associational/commissural (A/C) fiber-CA3 synapses has remained largely neglected, not least because of the lack of highly selective ligands for the different muscarinic receptor subtypes. In the present study, we performed field potential and whole cell recordings from the hippocampal CA3 region of M(2) receptor knockout mice to determine the role of M(2) receptors in short-term and long-term plasticity at A/C and MF inputs to CA3 pyramidal cells. At the A/C synapse, M(2) receptors promoted short-term facilitation and LTP. Unexpectedly, M(2) receptors mediated the opposite effect on LTP at the MF synapse, which was significantly reduced, most likely involving a depressant effect of M(2) receptors on adenylyl cyclase activity in MF terminals. Our data demonstrate that cholinergic projections recruit M(2) receptors to redistribute the gain of LTP in CA3 pyramidal cells in an input-specific manner.

Vesicular zinc regulates the Ca2+ sensitivity of a subpopulation of presynaptic vesicles at hippocampal mossy fiber terminals

Synaptic vesicles segregate into functionally diverse subpopulations within presynaptic terminals, yet there is no information about how this may occur. Here we demonstrate that a distinct subgroup of vesicles within individual glutamatergic, mossy fiber terminals contain vesicular zinc that is critical for the rapid release of a subgroup of synaptic vesicles during increased activity in mice. In particular, vesicular zinc dictates the Ca(2+) sensitivity of release during high-frequency firing. Intense synaptic activity alters the subcellular distribution of zinc in presynaptic terminals and decreases the number of zinc-containing vesicles. Zinc staining also appears in endosomes, an observation that is consistent with the preferential replenishment of zinc-enriched vesicles by bulk endocytosis. We propose that functionally diverse vesicle pools with unique membrane protein composition support different modes of transmission and are generated via distinct recycling pathways.

An update on cholinergic regulation of cholecystokinin-expressing basket cells

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

Nerve terminal nicotinic acetylcholine receptors initiate quantal GABA release from perisomatic interneurons by activating axonal T-type (Cav3) Ca²⁺ channels and Ca²⁺ release from stores

Release of conventional neurotransmitters is mainly controlled by calcium (Ca²⁺) influx via high-voltage-activated (HVA), Ca(v)2, channels ("N-, P/Q-, or R-types") that are opened by action potentials. Regulation of transmission by subthreshold depolarizations does occur, but there is little evidence that low-voltage-activated, Ca(v)3 ("T-type"), channels take part. GABA release from cortical perisomatic-targeting interneurons affects numerous physiological processes, and yet its underlying control mechanisms are not fully understood. We investigated whether T-type Ca²⁺ channels are involved in regulating GABA transmission from these cells in rat hippocampal CA1 using a combination of whole-cell voltage-clamp, multiple-fluorescence confocal microscopy, dual-immunolabeling electron-microscopy, and optogenetic methods. We show that Ca(v)3.1, T-type Ca²⁺ channels can be activated by α3β4 nicotinic acetylcholine receptors (nAChRs) that are located on the synaptic regions of the GABAergic perisomatic-targeting interneuronal axons, including the parvalbumin-expressing cells. Asynchronous, quantal GABA release can be triggered by Ca²⁺ influx through presynaptic T-type Ca²⁺ channels, augmented by Ca²⁺ from internal stores, following focal microiontophoretic activation of the α3β4 nAChRs. The resulting GABA release can inhibit pyramidal cells. The T-type Ca²⁺ channel-dependent mechanism is not dependent on, or accompanied by, HVA channel Ca²⁺ influx, and is insensitive to agonists of cannabinoid, μ-opioid, or GABA(B) receptors. It may therefore operate in parallel with the normal HVA-dependent processes. The results reveal new aspects of the regulation of GABA transmission and contribute to a deeper understanding of ACh and nicotine actions in CNS.

Promoter methylation and tissue-specific transcription of the α7 nicotinic receptor gene, CHRNA7

The α7 nicotinic acetylcholine receptor is known to regulate a wide variety of developmental and secretory functions in neural and non-neural tissues. The mechanisms that regulate its transcription in these varied tissues are not well understood. Epigenetic processes may play a role in the tissue-specific regulation of mRNA expression from the α7 nicotinic receptor subunit gene, CHRNA7. Promoter methylation was correlated with CHRNA7 mRNA expression in various tissue types and the role of DNA methylation in regulating transcription from the gene was tested by using DNA methyltransferase (DNMT1) inhibitors and methyl donors. CHRNA7 mRNA expression was silenced in SH-EP1 cells and bisulfite sequencing PCR revealed the CHRNA7 proximal promoter was hypermethylated. The proximal promoter was hypomethylated in the cell lines HeLa, SH-SY5Y, and SK-N-BE which express varying levels of CHRNA7 mRNA. Expression of CHRNA7 mRNA was present in SH-EP1 cells after treatment with the methylation inhibitor, 5-aza-2-deoxycytidine (5-Aza-CdR), and increased in SH-EP1 and HeLa cells using another methylation inhibitor, zebularine (ZEB). Transcription from the CHRNA7 promoter in HeLa cells was increased when the methyl donor methionine (MET) was absent from the media. Using methylation-sensitive restriction enzyme analysis (MSRE), there was a strong inverse correlation between CHRNA7 mRNA levels and promoter DNA methylation across several human tissue types. The results support a role for DNA methylation of the proximal promoter in regulation of CHRNA7 transcription.

Nicotine: specific role in angiogenesis, proliferation and apoptosis

Nowadays, tobacco smoking is the cause of ~5-6 million deaths per year, counting 31% and 6% of all cancer deaths (affecting 18 different organs) in middle-aged men and women, respectively. Nicotine is the addictive component of tobacco acting on neuronal nicotinic receptors (nAChR). Functional nAChR, are also present on endothelial, haematological and epithelial cells. Although nicotine itself is regularly not referred to as a carcinogen, there is an ongoing debate whether nicotine functions as a 'tumour promoter'. Nicotine, with its specific binding to nAChR, deregulates essential biological processes like regulation of cell proliferation, apoptosis, migration, invasion, angiogenesis, inflammation and cell-mediated immunity in a wide variety of cells including foetal (regulation of development), embryonic and adult stem cells, adult tissues as well as cancer cells. Nicotine seems involved in fundamental aspects of the biology of malignant diseases, as well as of neurodegeneration. Investigating the biological effects of nicotine may provide new tools for therapeutic interventions and for the understanding of neurodegenerative diseases and tumour biology.

A transgenic mouse model reveals fast nicotinic transmission in hippocampal pyramidal neurons

The relative contribution to brain cholinergic signaling by synaptic- and diffusion-based mechanisms remains to be elucidated. In this study, we examined the prevalence of fast nicotinic signaling in the hippocampus. We describe a mouse model where cholinergic axons are labeled with the tauGFP fusion protein driven by the choline acetyltransferase promoter. The model provides for the visualization of individual cholinergic axons at greater resolution than other available models and techniques, even in thick, live, slices. Combining calcium imaging and electrophysiology, we demonstrate that local stimulation of visualized cholinergic fibers results in rapid excitatory postsynaptic currents mediated by the activation of α7-subunit-containing nicotinic acetylcholine receptors (α7-nAChRs) on CA3 pyramidal neurons. These responses were blocked by the α7-nAChR antagonist methyllycaconitine and potentiated by the receptor-specific allosteric modulator 1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxanol-3-yl)-urea (PNU-120596). Our results suggest, for the first time, that synaptic nAChRs can modulate pyramidal cell plasticity and development. Fast nicotinic transmission might play a greater role in cholinergic signaling than previously assumed. We provide a model for the examination of synaptic properties of basal forebrain cholinergic innervation in the brain.

Immediate and sustained improvements in working memory after selective stimulation of α7 nicotinic acetylcholine receptors

BACKGROUND

Nicotine improves cognition in humans and animal models of neuropsychiatric disorders. Here, we sought to establish whether selective stimulation of the neuronal nicotinic α7 receptor could improve spatial working memory in nonhuman primates.

METHODS

Beginning with an estimated dose range from rodent studies, the dose of the α7 agonist AZD0328 was titrated for a significant impact on working memory in rhesus macaques after acute administration. After training to stability on the spatial delayed response task, subjects were administered AZD0328 (1.6 ng/kg-.48 mg/kg; intramuscular) or vehicle 30 min before cognitive testing. AZD0328 (1 ng/kg-1.0 μg/kg; intramuscular) was then administered in a repeated, intermittent ascending dose regimen where each dose was given in two bouts for 4 days with a 1-week washout in between bouts, followed by 2-week washout.

RESULTS

Acute AZD0328 improved cognitive performance when the dose was titrated down to .0016 and .00048 mg/kg from a cognitively impairing dose of .48 mg/kg. In a subgroup, sustained enhancement of working memory was evident for 1 month or more after acute treatment. Immediate and sustained cognitive enhancement was also found during and after repeated administration of AZD0328 at .001 mg/kg.

CONCLUSIONS

These findings demonstrate that extremely low doses of a nicotinic α7 agonist can have profound acute and long-lasting beneficial consequences for cognition, dependent upon the integrity of dorsolateral prefrontal cortex. Thus, the α7 receptor might have a fundamental role in the neural circuitry of working memory and in the synaptic plasticity upon which it might depend.

Mossy fiber long-term potentiation deficits in BACE1 knock-outs can be rescued by activation of alpha7 nicotinic acetylcholine receptors

β-Site amyloid precursor protein-cleaving enzyme 1 (BACE1)-the neuronal β-secretase responsible for producing β-amyloid (Aβ) peptides-emerged as one of the key therapeutic targets of Alzheimer's disease (AD). Although complete ablation of the BACE1 gene prevents Aβ formation, we reported that BACE1 knock-out mice display severe presynaptic deficits at mossy fiber (MF)-to-CA3 synapses in the hippocampus, a major locus of BACE1 expression. We also found that the deficits are likely due to abnormal presynaptic Ca(2+) regulation. Cholinergic system has been implicated in AD, in some cases involving Ca(2+)-permeable α7-nicotinic acetylcholine receptors (nAChRs). Here we report that brief application of nicotine, via α7-nAChRs, can restore MF long-term potentiation in BACE1 knock-outs. Our data suggest that activating α7-nAChRs can recover the presynaptic deficits in BACE1 knock-outs.

Pregnenolone sulfate enhances spontaneous glutamate release by inducing presynaptic Ca2+-induced Ca2+ release

Pregnenolone sulfate (PS) acts as an excitatory neuromodulator and has a variety of neuropharmacological actions, such as memory enhancement and convulsant effects. In the present study, we investigated the effect of PS on glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs) in acutely isolated dentate gyrus (DG) hilar neurons by use of a conventional whole-cell patch-clamp technique. PS significantly increased sEPSC frequency in a concentration-dependent manner without affecting the current amplitude, suggesting that PS acts presynaptically to increase the probability of spontaneous glutamate release. However, known molecular targets of PS, such as α7 nicotinic ACh, NMDA, σ1 receptors and voltage-dependent Ca(2+) channels, were not responsible for the PS-induced increase in sEPSC frequency. In contrast, the PS-induced increase in sEPSC frequency was completely occluded in a Ca(2+)-free external solution, and was significantly reduced by either the depletion of presynaptic Ca(2+) stores or the blockade of ryanodine receptors, suggesting that PS elicits Ca(2+)-induced Ca(2+) release (CICR) within glutamatergic nerve terminals. In addition, the PS-induced increase in sEPSC frequency was completely occluded by transient receptor potential (TRP) channel blockers. These data suggest that PS increases spontaneous glutamate release onto acutely isolated hilar neurons via presynaptic CICR, which was triggered by the influx of Ca(2+) through presynaptic TRP channels. The PS-induced modulation of excitatory transmission onto hilar neurons could have a broad impact on the excitability of hilar neurons and affect the pathophysiological functions mediated by the hippocampus.

beta-Amyloid activates presynaptic alpha7 nicotinic acetylcholine receptors reconstituted into a model nerve cell system: involvement of lipid rafts

Beta amyloid (Abeta) plays a central role in the pathogenesis of Alzheimer's disease. Abeta is the major constituent of senile plaques, but there is a significant presence of Abeta in the brain in soluble forms. The results of functional studies indicate that soluble Abeta interacts with the alpha7 nicotinic acetylcholine receptor (nAChR) complex with apparent high affinity. However, conflicting data exist as to the nature of the Abeta-alpha7 nAChR interaction, and whether it is the result of specific binding. Moreover, both agonist-like and antagonist-like effects have been reported. In particular, agonist-like effects have been observed for presynaptic nAChRs. Here, we demonstrate Abeta(1-42)-evoked stimulatory changes in presynaptic Ca(2+) level via exogenous alpha7 nAChRs expressed in the axonal varicosities of differentiated hybrid neuroblastoma NG108-15 cells as a model, presynaptic system. The Abeta(1-42)-evoked responses were concentration-dependent and were sensitive to the highly selective alpha7 nAChR antagonist alpha-bungarotoxin. Voltage-gated Ca(2+) channels and internal Ca(2+) stores were both involved in Abeta(1-42)-evoked increases in presynaptic Ca(2+) following activation of alpha7 nAChRs. In addition, disruption of lipid rafts by cholesterol depletion led to substantially attenuated responses to Abeta(1-42), whereas responses to nicotine were largely intact. These results directly implicate the nicotinic receptor complex as a target for the agonist-like action of pico- to nanomolar concentrations of soluble Abeta(1-42) on the presynaptic nerve terminal, including the possible involvement of receptor-associated lipid rafts. This interaction probably plays an important neuromodulatory role in synaptic dynamics.

Presynaptic roles of intracellular Ca(2+) stores in signalling and exocytosis

The signalling roles of Ca(2+)(ic) (intracellular Ca(2+)) stores are well established in non-neuronal and neuronal cells. In neurons, although Ca(2+)(ic) stores have been assigned a pivotal role in postsynaptic responses to G(q)-coupled receptors, or secondarily to extracellular Ca(2+) influx, the functions of dynamic Ca(2+)(ic) stores in presynaptic terminals remain to be fully elucidated. In the present paper, we review some of the recent evidence supporting an involvement of Ca(2+)(ic) in presynaptic function, and discuss loci at which this source of Ca(2+) may impinge. Nerve terminal preparations provide good models for functionally examining putative Ca(2+)(ic) stores under physiological and pathophysiological stimulation paradigms, using Ca(2+)-dependent activation of resident protein kinases as sensors for fine changes in intracellular Ca(2+) levels. We conclude that intraterminal Ca(2+)(ic) stores may, directly or indirectly, enhance neurotransmitter release following nerve terminal depolarization and/or G-protein-coupled receptor activation. During conditions that prevail following neuronal ischaemia, increased glutamate release instigated by Ca(2+)(ic) store activation may thereby contribute to excitotoxicity and eventual synaptopathy.

Alexa Fluor 546-ArIB[V11L;V16A] is a potent ligand for selectively labeling alpha 7 nicotinic acetylcholine receptors

The alpha7* (*denotes the possible presence of additional subunits) nicotinic acetylcholine receptor (nAChR) subtype is widely expressed in the vertebrate nervous system and implicated in neuropsychiatric disorders that compromise thought and cognition. In this report, we demonstrate that the recently developed fluorescent ligand Cy3-ArIB[V11L;V16A] labels alpha7 nAChRs in cultured hippocampal neurons. However, photobleaching of this ligand during long image acquisition times prompted us to develop a new derivative. In photostability studies, this new ligand, Alexa Fluor 546-ArIB[V11L;V16A], was significantly more resistant to bleaching than the Cy3 derivative. The classic alpha7 ligand alpha-bungarotoxin binds to alpha1* and alpha9* nAChRs. In contrast, Alexa Fluor 546-ArIB[V11L;V16A] potently (IC(50) 1.8 nM) and selectively blocked alpha7 nAChRs but not alpha1* or alpha9* nAChRs expressed in Xenopus oocytes. Selectivity was further confirmed by competition binding studies of native nAChRs in rat brain membranes. The fluorescence properties of Alexa Fluor 546-ArIB[V11L;V16A] were assessed using human embryonic kidney-293 cells stably transfected with nAChRs; labeling was observed on cells expressing alpha7 but not cells expressing alpha3beta2, alpha3beta4, or alpha4beta2 nAChRs. Further imaging studies demonstrate that Alexa Fluor 546-ArIB[V11L;V16A] labels hippocampal neurons from wild-type mice but not from nAChR alpha7 subunit-null mice. Thus, Alexa Fluor 546-ArIB[V11L;V16A] represents a potent and selective ligand for imaging alpha7 nAChRs.

Ultra-fast versus sustained cholinergic transmission: a variety of different mechanisms

Although synaptic transmission was assumed to use the same mechanisms in the case of different synapses of the central and peripheral nervous system, recent research revealed a great variety of different processes. Time might be a crucial factor to be considered in this diversity. It is recalled that the speed of a chemical reaction is inversely related to affinity. "Time is gained at the expense of sensitivity" as noticed by Bernard Katz (1989). Therefore, synaptic transmission will occur at a high speed only if it is supported by low affinity reactions. In the present work, we compare two examples of ultra-rapid transmission (the Torpedo nerve electroplaque synapse and the rat hippocampus mossy fiber/CA3 synapses), with a cholinergic process operating with high affinity but at a low speed: the release of glutamate elicited by nicotine from mossy fibers of the rat hippocampus.

Activation of presynaptic alpha7 nicotinic receptors evokes an excitatory response in hippocampal CA3 neurones in anaesthetized rats: an in vivo iontophoretic study

BACKGROUND AND PURPOSE

alpha7 Nicotinic receptors have been suggested to play an important role in hippocampal learning and memory. However, the direct action of this receptor subtype on hippocampal pyramidal neurones in vivo has not yet been fully investigated. The availability of selective agonists for alpha7 receptors [AR-R17779 and (R)-(-)-5'-phenylspiro[1-azabicyclo[2.2.2] octane-3,2'-(3'H)furo[2,3-b]pyridine (PSAB-OFP)] has now allowed this role to be investigated.

EXPERIMENTAL APPROACH

Single-cell extracellular recordings were made from hippocampal CA3 pyramidal neurones in anaesthetized rats. The effects of nicotine, AR-R17779 and PSAB-OFP, applied either systemically or iontophoretically, were studied on the activity of these neurones.

KEY RESULTS

Intravenous injection of cumulative doses of nicotine and PSAB-OFP induced dose-related, significant increases in neuronal firing in the majority of neurones tested. This excitation could be inhibited by intravenous administration of methyllycaconitine (MLA), a selective alpha7 nicotinic receptor antagonist. Furthermore, iontophoretic application of nicotine, AR-R17779 and PSAB-OFP each evoked current-dependent excitation of most CA3 pyramidal neurones studied, and this excitation was antagonized by co-iontophoretic application of MLA. In addition, the excitation induced by iontophoretic application of nicotine, AR-R17779 or PSAB-OFP was also blocked by co-iontophoretic application of either 6,7-dinitroquinoxaline-2,3-dione (DNQX) or D(2)-2-amino-5-phosphonopentanoate (D-AP5), selective N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor antagonists respectively.

CONCLUSIONS AND IMPLICATIONS

CA3 pyramidal neurones are modulated by activation of presynaptic alpha7 nicotinic receptors, which, at least in part, enhances glutamate release onto post-synaptic (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid and NMDA receptors on these CA3 neurones. This mechanism probably contributes to the effects of nicotine on hippocampal learning and memory.