Neuronal nicotinic receptors in synaptic functions in humans and rats: physiological and clinical relevance

Nicotine affects mitochondrial structure and function in human airway smooth muscle cells

Exposure to cigarette smoke and e-cigarettes, with nicotine as the active constituent, contributes to increased health risks associated with asthma. Nicotine exerts its functional activity via nicotinic acetylcholine receptors (nAChRs), and the alpha7 subtype (α7nAChR) has recently been shown to adversely affect airway dynamics. The mechanisms of α7nAChR action in airways, particularly in the context of airway smooth muscle (ASM), a key cell type in asthma, are still under investigation. Mitochondria have garnered increasing interest for their role in regulating airway tone and adaptations to cellular stress. Here mitochondrial dynamics such as fusion versus fission, and mitochondrial Ca2+ ([Ca2+]m), play an important role in mitochondrial homeostasis. There is currently no information on effects and mechanisms by which nicotine regulates mitochondrial structure and function in ASM in the context of asthma. We hypothesized that nicotine disrupts mitochondrial morphology, fission-fusion balance, and [Ca2+]m regulation, with altered mitochondrial respiration and bioenergetics in the context of asthmatic ASM. Using human ASM (hASM) cells from nonasthmatics, asthmatics, and smokers, we examined the effects of nicotine on mitochondrial dynamics and [Ca2+]m. Fluorescence [Ca2+]m imaging of hASM cells with rhod-2 showed robust responses to 10 μM nicotine, particularly in asthmatics and smokers. In both asthmatics and smokers, nicotine increased the expression of fission proteins while decreasing fusion proteins. Seahorse analysis showed blunted oxidative phosphorylation parameters in response to nicotine in these groups. α7nAChR siRNA blunted nicotine effects, rescuing [Ca2+]m, changes in mitochondrial structural proteins, and mitochondrial dysfunction. These data highlight mitochondria as a target of nicotine effects on ASM, where mitochondrial disruption and impaired buffering could permit downstream effects of nicotine in the context of asthma.NEW & NOTEWORTHY Asthma is a major healthcare burden, which is further exacerbated by smoking. Recognizing the smoking risk of asthma, understanding the effects of nicotine on asthmatic airways becomes critical. Surprisingly, the mechanisms of nicotine action, even in normal and especially asthmatic airways, are understudied. Accordingly, the goal of this research is to investigate how nicotine influences asthmatic airways in terms of mitochondrial structure and function, via the a7nAChR.

The role of astrocytic α7 nicotinic acetylcholine receptors in Alzheimer disease

The ongoing search for therapeutic interventions in Alzheimer disease (AD) has highlighted the complexity of this condition and the need for additional biomarkers, beyond amyloid-β (Aβ) and tau, to improve clinical assessment. Astrocytes are brain cells that control metabolic and redox homeostasis, among other functions, and are emerging as an important focus of AD research owing to their swift response to brain pathology in the initial stages of the disease. Reactive astrogliosis - the morphological, molecular and functional transformation of astrocytes during disease - has been implicated in AD progression, and the definition of new astrocytic biomarkers could help to deepen our understanding of reactive astrogliosis along the AD continuum. As we highlight in this Review, one promising biomarker candidate is the astrocytic α7 nicotinic acetylcholine receptor (α7nAChR), upregulation of which correlates with Aβ pathology in the brain of individuals with AD. We revisit the past two decades of research into astrocytic α7nAChRs to shed light on their roles in the context of AD pathology and biomarkers. We discuss the involvement of astrocytic α7nAChRs in the instigation and potentiation of early Aβ pathology and explore their potential as a target for future reactive astrocyte-based therapeutics and imaging biomarkers in AD.

Chronic oral nicotine administration and withdrawal regulate the expression of neuropeptide Y and its receptors in the mesocorticolimbic system

Neuropeptide Y (NPY) and its receptors are involved in the regulation of mood, stress, and anxiety. In parallel, NPY signaling may play a vital role in the negative affective state induced by drug withdrawal. This study examined the changes in the transcript levels of NPY, Y1, Y2, and Y5 receptors in the mesocorticolimbic system during chronic nicotine exposure and withdrawal. Rats were administered with nicotine (initial dose: 25 μg/ml, maintenance dose: 50 μg/ml, free base) in drinking water for 12 weeks. Control group received only tap water. In the final week of the study, some of the nicotine-treated animals continued to receive nicotine (0-W), whereas some were withdrawn for either 24 (24-W) or 48 (48-W) h. All animals were decapitated after the evaluation of somatic signs (frequency of gasps, eye blinks, ptosis, shakes, teeth chatter) and the duration of locomotor activity and immobility. mRNA levels of NPY, Y1, Y2, and Y5 receptors in the mesocorticolimbic system were measured by quantitative real-time PCR (qRT-PCR). Results showed that nicotine withdrawal increased overall somatic signs. Moreover, chronic nicotine treatment increased the duration of locomotor activity, whereas withdrawal increased the duration of immobility. qRT-PCR analysis revealed that chronic nicotine treatment increased NPY mRNA levels in the hippocampus. On the other hand, 24- and 48-h withdrawals increased NPY mRNA levels in the amygdala and medial prefrontal cortex (mPFC), Y1 and Y2 mRNA levels in the nucleus accumbens and mPFC, and Y5 mRNA levels in the mPFC. These findings suggest that nicotine withdrawal enhances NPY signaling in the mesocorticolimbic system, which could be an important mechanism involved in regulating the negative affective state triggered during nicotine withdrawal.

Nicotinic α7 acetylcholine receptor (α7nAChR) in human airway smooth muscle

Diseases such as asthma are exacerbated by inflammation, cigarette smoke and even nicotine delivery devices such as e-cigarettes. However, there is currently little information on how nicotine affects airways, particularly in humans, and changes in the context of inflammation or asthma. Here, a longstanding assumption is that airway smooth muscle (ASM) that is key to bronchoconstriction has muscarinic receptors while nicotinic receptors (nAChRs) are only on airway neurons. In this study, we tested the hypothesis that human ASM expresses α7nAChR and explored its profile in inflammation and asthma using ASM of non-asthmatics vs. mild-moderate asthmatics. mRNA and western analysis showed the α7 subunit is most expressed in ASM cells and further increased in asthmatics and smokers, or by exposure to nicotine, cigarette smoke or pro-inflammatory cytokines TNFα and IL-13. In these effects, signaling pathways relevant to asthma such as NFκB, AP-1 and CREB are involved. These novel data demonstrate the expression of α7nAChR in human ASM and suggest their potential role in asthma pathophysiology in the context of nicotine exposure.

Cellular, Synaptic and Network Effects of Acetylcholine in the Neocortex

The neocortex is densely innervated by basal forebrain (BF) cholinergic neurons. Long-range axons of cholinergic neurons regulate higher-order cognitive function and dysfunction in the neocortex by releasing acetylcholine (ACh). ACh release dynamically reconfigures neocortical microcircuitry through differential spatiotemporal actions on cell-types and their synaptic connections. At the cellular level, ACh release controls neuronal excitability and firing rate, by hyperpolarizing or depolarizing target neurons. At the synaptic level, ACh impacts transmission dynamics not only by altering the presynaptic probability of release, but also the magnitude of the postsynaptic response. Despite the crucial role of ACh release in physiology and pathophysiology, a comprehensive understanding of the way it regulates the activity of diverse neocortical cell-types and synaptic connections has remained elusive. This review aims to summarize the state-of-the-art anatomical and physiological data to develop a functional map of the cellular, synaptic and microcircuit effects of ACh in the neocortex of rodents and non-human primates, and to serve as a quantitative reference for those intending to build data-driven computational models on the role of ACh in governing brain states.

Two distinct profiles of fMRI and neurophysiological activity elicited by acetylcholine in visual cortex

fMRI changes are typically assumed to be due to changes in neural activity, although whether this remains valid under the influence of neuromodulators is relatively unknown. Here, we found evidence that intracortical acetylcholine elicits distinct profiles of fMRI and electrophysiological activity in visual cortex. Two patterns of cholinergic activity were observed, depending on the distance to the injection site, although neurovascular coupling was preserved. Our results illustrate the effects of neuromodulators on fMRI and electrophysiological responses and show that these depend on neuromodulator concentration and kinetics.

Cholinergic neuromodulation is involved in all aspects of sensory processing and is crucial for processes such as attention, learning and memory, etc. However, despite the known roles of acetylcholine (ACh), we still do not how to disentangle ACh contributions from sensory or task-evoked changes in functional magnetic resonance imaging (fMRI). Here, we investigated the effects of local injection of ACh on fMRI and neural signals in the primary visual cortex (V1) of anesthetized macaques by combining pharmaco-based MRI (phMRI) with electrophysiological recordings, using single electrodes and electrode arrays. We found that local injection of ACh elicited two distinct profiles of fMRI and neurophysiological activity, depending on the distance from the injector. Near the injection site, we observed an increase in the baseline blood oxygen-level-dependent (BOLD) and cerebral blood flow (CBF) responses, while their visual modulation decreased. In contrast, further from the injection site, we observed an increase in the visually induced BOLD and CBF modulation without changes in baseline. Neurophysiological recordings suggest that the spatial correspondence between fMRI responses and neural activity does not change in the gamma, high-gamma, and multiunit activity (MUA) bands. The results near the injection site suggest increased inhibitory drive and decreased metabolism, contrasting to the far region. These changes are thought to reflect the kinetics of ACh and its metabolism to choline.

Neuronal life after death: electrophysiologic recordings from neurons in adult human brain tissue obtained through surgical resection or postmortem

Recordings from fresh human brain slices derived from surgically resected brain tissue are being used to unravel mechanisms underlying human neurophysiology and for the evaluation of potential therapeutic targets and compounds. Data resulting from these studies provide unique insights into physiologic properties of human neuronal microcircuits. However, substantial limitations still remain with this approach. First, the tissue is always resected from patients, never from healthy controls. Second, the patient population undergoing brain surgery with tissue resection is limited to epilepsy and tumor patients - never from patients with other neurologic disorders. Third, the vast majority of tissue resected is limited largely to temporal cortex and hippocampus, occasionally amygdala. Therefore, the possibility to study brain tissue: (1) from healthy controls; (2) from patients with different neuropathologies; (3) from different brain areas; and (4) from a wide spectrum of ages only exists through autopsy-derived brain tissue. Here we describe methods and results from physiologic recordings of adult human neurons and microcircuits in both surgically derived brain tissue as well as in tissue derived from autopsies. We define postmortem time windows during which physiologic recordings could match data obtained from surgical tissue.

Nicotinic Receptor Abnormalities in the Cerebellar Cortex of Sudden Unexplained Fetal and Infant Death Victims-Possible Correlation With Maternal Smoking

Nicotinic acetylcholine receptors (nAChRs) are cationic channels of the neuronal cell membrane, differentially expressed in the central nervous system which, when activated by endogenous acetylcholine or exogenous nicotine, are able to enhance cholinergic transmission. The aim of this study was to investigate in human perinatal age the immunohistochemical expression of the α7-nAChR subtype, given its involvement in neuronal differentiation and its significant vulnerability to the toxic effects of nicotine. Thirty fetuses (with a gestational age between 25 and 40 weeks) and 35 infants (1-6 months old), suddenly died of known (controls) and unknown causes (unexplained deaths), with smoking and nonsmoking mothers, were included in this study. A negative or low immunoexpression of α7-nAChRs, indicative of their inactivation, was observed in the granular layers of the cerebellar cortex in 66% of the sudden unexplained perinatal deaths and 11% of the controls. A high correlation was also observed between these findings and maternal smoking. Apart from the well-known adverse effects of nicotine exposure during pregnancy, it may also cause significant alterations in cerebellar cholinergic transmission in areas of the brain involved in vital functions. These events may give us insights into the pathogenetic mechanisms leading to sudden unexplained fetal and infant death.

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

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

Cytidine 5'-diphosphocholine differentially affects hemostatic parameters in diverse conditions in rats: an investigation via thromboelastography

Cytidine 5'-diphosphocholine (CDP-choline) has several physiological and pharmacological effects on various bodily functions, including hemostasis. This study determined the impact of CDP-choline on hemostasis in a trauma-hemorrhage (T-H) model in rats or under in vitro conditions or after chronic treatment via thromboelastography. Trauma-hemorrhage resuscitation was induced, and either saline (1 mL/kg) or CDP-choline (50 mg/kg) was injected intravenously just prior to resuscitation in the T-H group and at the same time point in the sham-control group. The effects of CDP-choline on thromboelastogram parameters, coagulation markers, and platelet aggregation were investigated under in vitro conditions (1.5 mM, 30- or 3-min incubation in blood or plasma) and after chronic use (50 mg/kg, i.p., 10 days). Acute CDP-choline treatment was shown to decrease the initial and maximum clot formation time, accelerate clotting rapidity, reduce the lysis percentage, and increase the coagulation index in the T-H resuscitation group, whereas the same treatment in the sham-control rats did not alter any of the thromboelastogram parameters. However, the incubation of whole blood with CDP-choline prolonged the initial and maximum clot formation time, and CDP-choline treatment significantly decreased the slopes of the disaggregation and aggregation curves when platelets were stimulated with ADP and collagen, respectively. Interestingly, the chronic use of this drug did not influence any of these hemostatic parameters. These data implicate that acute but not chronic CDP-choline administration may differentially alter the hemostatic parameters under diverse conditions. The drug may produce a hypercoagulable state in activated situations but cause opposite effects under normal in vitro conditions.

Advances in the pharmacology of lGICs auxiliary subunits

Ligand-gated ion channels (LGICs) are cell surface integral proteins that mediate the fast neurotransmission in the nervous system. LGICs require auxiliary subunits for their trafficking, assembly and pharmacological modulation. Auxiliary subunits do not form functional homomeric receptors, but are reported to assemble with the principal subunits in order to modulate their pharmacological profiles. For example, nACh receptors are built at least by co-assemble of α and β subunits, and the neuronal auxiliary subunits β3 and α5 and muscle type β, δ, γ, and ϵ determine the agonist affinity of these receptors. Serotonergic 5-HT3B, 5-HT3C, 5-HT3D and 5-HT3E are reported to assemble with the 5-HT3A subunit to modulate its pharmacological profile. Functional studies evaluating the role of γ2 and δ auxiliary subunits of GABAA receptors have made important advances in the understanding of the action of benzodiazepines, ethanol and neurosteroids. Glycine receptors are composed principally by α1-3 subunits and the auxiliary subunit β determines their synaptic location and their pharmacological response to propofol and ethanol. NMDA receptors appear to be functional as heterotetrameric channels. So far, the existence of NMDA auxiliary subunits is controversial. On the other hand, Kainate receptors are modulated by NETO 1 and 2. AMPA receptors are modulated by TARPs, Shisa 9, CKAMP44, CNIH2-3 auxiliary proteins reported that controls their trafficking, conductance and gating of channels. P2X receptors are able to associate with auxiliary Pannexin-1 protein to modulate P2X7 receptors. Considering the pharmacological relevance of different LGICs auxiliary subunits in the present work we will highlight the therapeutic potential of these modulator proteins.

The influence of sensory afferent input on local motor cortical excitatory circuitry in humans

In human, sensorimotor integration can be investigated by combining sensory input and transcranial magnetic stimulation (TMS). Short latency afferent inhibition (SAI) refers to motor cortical inhibition 20-25 ms after median nerve stimulation. We investigated the interaction between SAI and short-interval intracortical facilitation (SICF), an excitatory motor cortical circuit. Seven experiments were performed. Contrary to expectations, SICF was facilitated in the presence of SAI (SICF(SAI)). This effect is specific to SICF since there was no effect at SICF trough 1 when SICF was absent. Furthermore, the facilitatory SICF(SAI) interaction increased with stronger SICF or SAI. SAI and SICF correlated between individuals, and this relationship was maintained when SICF was delivered in the presence of SAI, suggesting an intrinsic relationship between SAI and SICF in sensorimotor integration. The interaction was present at rest and during muscle contraction, had a broad degree of somatotopic influence and was present in different interneuronal SICF circuits induced by posterior-anterior and anterior-posterior current directions. Our results are compatible with the finding that projections from sensory to motor cortex terminate in both superficial layers where late indirect (I-) waves are thought to originate, as well as deeper layers with more direct effect on pyramidal output. This interaction is likely to be relevant to sensorimotor integration and motor control.

Neurocognitive effects of acute choline supplementation in low, medium and high performer healthy volunteers

Novel pharmacological treatments targeting alpha 7 nicotinic acetylcholine receptor (α7 nAChR) hypofunction in schizophrenia have shown mixed success in ameliorating cognitive impairments associated with this disorder. Choline, a selective agonist at α7 receptors is increased with oral administration of cytidine 5'-diphosphocholine (CDP-choline), the cognitive effects of which were assessed in healthy volunteers. Using the CogState test battery, behavioral performance in schizophrenia-relevant cognitive domains was assessed in 24 male participants following a single low (500mg) and moderate (1000mg) dose of CDP-choline. Relative to placebo, CDP-choline improved processing speed, working memory, verbal learning, verbal memory, and executive function in low baseline performers, while exerting no effects in medium baseline performers, and diminishing cognition in high baseline performers. Dose effects varied with cognitive domain but were evident with both the 500mg and 1000mg doses. These preliminary findings of cognitive enhancement in relatively impaired performers are consistent with the α7 receptor mechanism and support further trials with CDP-choline as a potential pro-cognitive strategy for cognitive impairment in schizophrenia.

Diverse strategies targeting α7 homomeric and α6β2* heteromeric nicotinic acetylcholine receptors for smoking cessation

Preclinical studies suggest that a diversity of nicotinic acetylcholine receptors (nAChRs) with different sensitivities to nicotine may contribute to tobacco addiction. Using rodent intravenous nicotine self-administration as a preclinical model with good predictive validity for therapeutic efficacy for tobacco cessation, investigators have identified heteromeric α6β2* and homomeric α7 nAChRs as promising novel therapeutic targets to promote smoking abstinence (*denotes possible assembly with other subunits). The data suggest that diverse strategies that target these subclasses of nAChRs, namely inhibition of α6β2* nAChRs and stimulation of α7 nAChRs, will support tobacco cessation. α6β2* nAChRs, members of the high-affinity family of β2* nAChRs, function similarly to α4β2* nAChRs, the primary target of the FDA-approved drug varenicline, but have a much more selective neuroanatomical pattern of expression in catecholaminergic nuclei. Although activation of β2* nAChRs facilitates nicotine self-administration, stimulation of α7 nAChRs appears to negatively modulate both nicotine reinforcement and β2* nAChR function in the mesolimbic dopamine system. Although challenges and caveats must be considered in the development of therapeutics that target these nAChR subpopulations, an accumulation of data suggests that α7 nAChR agonists, partial agonists, or positive allosteric modulators and α6β2* nAChR antagonists, partial agonists, or negative allosteric modulators may prove to be effective therapeutics for tobacco cessation.

Polyunsaturated fatty acids and their metabolites in the pathobiology of schizophrenia

Schizophrenia can be considered as a low-grade systemic inflammatory disease with its origins in the perinatal period. It is likely that genetic, environmental, and nutritional factors interact to induce excess production of pro-inflammatory cytokines that, in turn, damage fetal neurons leading to the adult onset of schizophrenia. Polyunsaturated fatty acids (PUFAs) and their metabolites such as lipoxins, resolvins, protectins, maresins and nitrolipids not only have potent neuroprotective action but also are capable of inhibiting the production of pro-inflammatory cytokines. Decreased formation of PUFAs as a result of low activity of Δ(6) and Δ(5) desaturases can result in an increase in the production of pro-inflammatory cytokines due to the absence of negative control exerted by PUFAs and their anti-inflammatory metabolites that, in turn, may predispose to neuronal damage and development of schizophrenia in adult life. Furthermore, PUFAs are essential for brain growth and development. If this proposal is correct, this implies that perinatal and adult supplementation of PUFAs not only prevents but also helps in the treatment of schizophrenia. Furthermore, synthetic analogs of lipoxins, resolvins, and protectins may be of significant benefit in schizophrenia.

Nicotine related brain activity: the influence of smoking history and blood nicotine levels, an exploratory study

OBJECTIVE

In this study, we sought to explore brain activity in nicotine-dependent men in response to acute intravenous nicotine using pharmacological magnetic resonance imaging (phMRI).

METHODS

phMRI was used to evaluate brain activity in response to 1.5 mg/70 kg intravenous nicotine or saline. The nicotine and saline were administered on different visits. The time courses of individual subjects' nicotine levels were used as regressors to assess neural activity relating to the infusions. The influence of smoking history and physiological measures on the response to nicotine were also investigated.

RESULTS

Greater lifetime exposure to cigarette smoking was significantly correlated with higher peak serum nicotine levels. PhMRI analysis of the differential response of nicotine compared to the saline condition showed distinctive activation patterns when analyzed with the (a) nicotine time course, (b) nicotine time course controlling for smoking history (pack years), and (c) pack years controlling for nicotine.

CONCLUSIONS

These results suggest that smoking exposure history influences serum nicotine levels and the brain's response to nicotine. Alterations in brain activity may be a result of vascular and neuro-adaptations involved in drug exposure and addiction.

The effects of dietary choline

Food intake can influence neuronal functions through different modulators expressed in the brain. The present review is a report through relevant experimental findings on the effects of choline, a nutritional component found in the diet, to identify a safe and effective dietary solution that can offer some protection against neurotoxicity and neurological disorders and that can be implemented in animals and humans in a very short period of time.

α7 nicotinic ACh receptors as a ligand-gated source of Ca(2+) ions: the search for a Ca(2+) optimum

The spatiotemporal distribution of cytosolic Ca(2+) ions is a key determinant of neuronal behavior and survival. Distinct sources of Ca(2+) ions including ligand- and voltage-gated Ca(2+) channels contribute to intracellular Ca(2+) homeostasis. Many normal physiological and therapeutic neuronal functions are Ca(2+)-dependent, however an excess of cytosolic Ca(2+) or a lack of the appropriate balance between Ca(2+) entry and clearance may destroy cellular integrity and cause cellular death. Therefore, the existence of optimal spatiotemporal patterns of cytosolic Ca(2+) elevations and thus, optimal activation of ligand- and voltage-gated Ca(2+) ion channels are postulated to benefit neuronal function and survival. Alpha7 nicotinic -acetylcholine receptors (nAChRs) are highly permeable to Ca(2+) ions and play an important role in modulation of neurotransmitter release, gene expression and neuroprotection in a variety of neuronal and non-neuronal cells. In this review, the focus is placed on α7 nAChR-mediated currents and Ca(2+) influx and how this source of Ca(2+) entry compares to NMDA receptors in supporting cytosolic Ca(2+) homeostasis, neuronal function and survival.

Galanthamine plus estradiol treatment enhances cognitive performance in aged ovariectomized rats

We hypothesize that beneficial effects of estradiol on cognitive performance diminish with age and time following menopause due to a progressive decline in basal forebrain cholinergic function. This study tested whether galanthamine, a cholinesterase inhibitor used to treat memory impairment associated with Alzheimer's disease, could enhance or restore estradiol effects on cognitive performance in aged rats that had been ovariectomized in middle-age. Rats were ovariectomized at 16-17 months of age. At 21-22 months of age rats began receiving daily injections of galanthamine (5mg/day) or vehicle. After one week, half of each group also received 17ß-estradiol administered subcutaneously. Rats were then trained on a delayed matching to position (DMP) T-maze task, followed by an operant stimulus discrimination/reversal learning task. Treatment with galanthamine+estradiol significantly enhanced the rate of DMP acquisition and improved short-term delay-dependent spatial memory performance. Treatment with galanthamine or estradiol alone was without significant effect. Effects were task-specific in that galanthamine+estradiol treatment did not significantly improve performance on the stimulus discrimination/reversal learning task. In fact, estradiol was associated with a significant increase in incorrect responses on this task after reversal of the stimulus contingency. In addition, treatments did not significantly affect hippocampal choline acetyltransferase activity or acetylcholine release. This may be an effect of age, or possibly is related to compensatory changes associated with long-term cholinesterase inhibitor treatment. The data suggest that treating with a cholinesterase inhibitor can enhance the effects of estradiol on acquisition of a DMP task by old rats following a long period of hormone deprivation. This could be of particular benefit to older women who have not used hormone therapy for many years and are beginning to show signs of mild cognitive impairment. Potential mechanisms for these effects are discussed.

Naturally-expressed nicotinic acetylcholine receptor subtypes

Nicotinic acetylcholine receptors (nAChRs) warrant attention, as they play many critical roles in brain and body function and have been implicated in a number of neurological and psychiatric disorders, including nicotine dependence. nAChRs are composed as diverse subtypes containing specific combinations of genetically-distinct subunits and that have different functional properties, distributions, and pharmacological profiles. There had been confidence that the rules that define ranges of assembly partners for specific subunits were well-established, especially for the more prominent nAChR subtypes. However, we review here some newer findings indicating that nAChRs having largely the same, major subunits exist as isoforms with unexpectedly different properties. Moreover, we also summarize our own studies indicating that novel nAChR subtypes exist and/or have distributions not heretofore described. Importantly, the nAChRs that exist as new isoforms or subtypes or have interesting distributions require alteration in thinking about their roles in health and disease.

Historical and current perspective on tobacco use and nicotine addiction

Although the addictive influence of tobacco was recognized very early, the modern concepts of nicotine addiction have relied on knowledge of cholinergic neurotransmission and nicotinic acetylcholine receptors (nAChRs). The discovery of the 'receptive substance' by Langley, that would turn out to be nAChRs, and 'Vagusstoff' (acetylcholine) by Loewi, coincided with an exciting time when the concept of chemical synaptic transmission was being formulated. More recently, the application of more powerful techniques and the study of animal models that replicate key features of nicotine dependence have led to important advancements in our understanding of molecular, cellular and systems mechanisms of nicotine addiction. In this review, we present a historical perspective and overview of the research that has led to our present understanding of nicotine addiction.

Presynaptic nicotinic α7 and non-α7 receptors stimulate endogenous GABA release from rat hippocampal synaptosomes through two mechanisms of action

Background

Although converging evidence has suggested that nicotinic acetylcholine receptors (nAChR) play a role in the modulation of GABA release in rat hippocampus, the specific involvement of different nAChR subtypes at presynaptic level is still a matter of debate. In the present work we investigated, using selective α7 and α4β2 nAChR agonists, the presence of different nAChR subtypes on hippocampal GABA nerve endings to assess to what extent and through which mechanisms they stimulate endogenous GABA release.

Methodology/Findings

All agonists elicited GABA overflow. Choline (Ch)-evoked GABA overflow was dependent to external Ca²⁺, but unaltered in the presence of Cd²⁺, tetrodotoxin (TTX), dihydro-β-erythroidine (DHβE) and 1-(4,4-Diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride SKF 89976A. The effect of Ch was blocked by methyllycaconitine (MLA), α-bungarotoxin (α-BTX), dantrolene, thapsigargin and xestospongin C, suggesting that GABA release might be triggered by Ca²⁺ entry into synaptosomes through the α7 nAChR channel with the involvement of calcium from intracellular stores. Additionally, 5-Iodo-A-85380 dihydrochloride (5IA85380) elicited GABA overflow, which was Ca²⁺ dependent, blocked by Cd²⁺, and significantly inhibited by TTX and DHβE, but unaffected by MLA, SKF 89976A, thapsigargin and xestospongin C and dantrolene. These findings confirm the involvement of α4β2 nAChR in 5IA85380-induced GABA release that seems to occur following membrane depolarization and opening calcium channels.

Conclusions/Significance

Rat hippocampal synaptosomes possess both α7 and α4β2 nAChR subtypes, which can modulate GABA release via two distinct mechanisms of action. The finding that GABA release evoked by the mixture of sub-maximal concentration of 5IA85380 plus sub-threshold concentrations of Ch was significantly larger than that elicited by the sum of the effects of the two agonists is compatible with the possibility that they coexist on the same nerve terminals. These findings would provide the basis for possible selective pharmacological strategies to treat neuronal disorders that involve the dysfunction of hippocampal cholinergic system.

In vitro and in vivo characterization of a novel negative allosteric modulator of neuronal nAChRs

In this study, we compared the in vitro and in vivo neuronal nicotinic acetylcholine receptor (nAChR) properties of 1,2,3,3a,4,8b-hexahydro-2-benzyl-6-N,N-dimethylamino-1-methylindeno[1,2,-b]pyrrole (HDMP, 4) to that of negative allosteric modulator (NAM), PCP. Patch-clamp experiments showed that HDMP exhibited an inhibitory functional activity at α7 nAChRs with an IC(50) of 0.07 μM, and was 357- and 414-fold less potent at α4β2 and α3β4 nAChRs, with IC(50)s of 25.1 and 29.0 μM, respectively. Control patch-clamp experiments showed that PCP inhibited α7, α4β2 and α3β4 nAChRs with IC(50)s of to 1.3, 29.0 and 6.4 μM, respectively. Further, HDMP did not exhibit any appreciable binding affinity to either α7 or α4β2 nAChRs, suggesting its action via a non-competitive mechanism at these neuronal nAChR subtypes. The in vivo study showed that HDMP was a potent antagonist of nicotine-induced analgesia in the tail-flick (AD(50)=0.008 mg/kg), but not in the hot-plate test. All together, our in vitro and in vivo data suggest that HDMP is a novel NAM of neuronal nAChRs with potent inhibitory activity at α7 nAChR subtype at concentrations ≤ 1μM that are not effective for α4β2 and α3β4 nAChRs.

Increased glutamate in the hippocampus after galantamine treatment for Alzheimer disease

Galantamine is a cholinesterase inhibitor and allosteric potentiating ligand modulating presynaptic nicotinic acetylcholine receptors that is used in the treatment of Alzheimer disease (AD). The purpose of this study was to determine if galantamine treatment would result in detectable hippocampal metabolite changes that correlated with changes in cognition, as measured by the Mini-Mental State Examination (MMSE) and the Alzheimer Disease Assessment Scale-cognitive subscale (ADAS-cog). Short echo-time proton magnetic resonance (MR) spectra were acquired from within the right hippocampus of ten patients using a 4 Tesla magnetic resonance imaging (MRI) scanner. Spectra were used to quantify absolute metabolite levels for N-acetylaspartate (NAA), glutamate (Glu), choline (Cho), creatine (Cr), and myo-inositol (mI). Patient scans and cognitive tests were performed before and 4 months after beginning galantamine treatment, which consisted of an 8 mg daily dose for the first month and a 16 mg daily dose for the remaining three months. The levels of Glu, Glu/Cr, and Glu/NAA increased after four months of treatment, while there were no changes in MMSE or ADAS-cog scores. Additionally, changes (Delta) in Glu over the four months (DeltaGlu) correlated with DeltaNAA, and Delta(Glu/Cr) correlated with DeltaMMSE scores. Increased Glu and the ratio of Glu to Cr measured by MR spectroscopy after galantamine treatment were associated with increased cognitive performance. The increase in Glu may be related to the action of galantamine as an allosteric potentiating ligand for presynaptic nicotinic acetylcholine receptors, which increases glutamatergic neurotransmission.

Sexually diergic, dose-dependent hypothalamic-pituitary-adrenal axis responses to nicotine in a dynamic in vitro perfusion system

INTRODUCTION

The hypothalamic-pituitary-adrenal cortical (HPA) axis modulates physiological responses to stress. We previously reported sexually diergic, dose-dependent HPA responses in vivo following nicotine administration: Male rats had greater arginine vasopressin (AVP) responses than females, and female rats had greater adrenocorticotropic hormone (ACTH) and corticosterone (CORT) responses than males. The goal of the present study was to further investigate sexually diergic, dose-dependent HPA responses following nicotine addition to an in vitro model of the HPA axis, so that hormone output could be determined at each level of the axis.

METHODS

Hypothalami, pituitaries, and adrenal glands were harvested from male and female rats. One-half hypothalamus, one-half pituitary, and one adrenal gland were placed individually into three jacketed tissue baths connected by tubing and perfused in series with physiological medium. Sampling ports between tissue baths were used to collect buffer before and after addition of various doses of nicotine, for measurement of AVP and corticotropin-releasing hormone (CRH) from the hypothalamus bath, ACTH from the pituitary bath, and CORT from the adrenal bath. Hormones were measured by highly specific immunoassays.

RESULTS

Stable temperatures, flow rates, pH, and hormone baselines were achieved in the in vitro system. Consistent with our in vivo and earlier in vitro studies, nicotine added to the hypothalamus tissue bath significantly increased HPA responses in a sex- and dose-dependent manner: Males had greater AVP responses than did females, and females had greater CRH responses than did males. Sexually diergic ACTH and CORT responses were less apparent and were higher in females.

DISCUSSION

Our in vitro system accurately models in vivo HPA responses to nicotine in both sexes and thus represents a reliable method for investigating the effects of nicotine on components of the HPA axis. These studies may be pertinent to understanding the biological differences to nicotine between men and women smokers.

Evaluation of Ca2+ permeability of nicotinic acetylcholine receptors in hypothalamic histaminergic neurons

Hypothalamic histaminergic tuberomammillary (TM) neurons express nicotinic acetylcholine receptors (nAChRs) with kinetic and pharmacological properties resembling those of highly Ca(2+) permeable alpha7 nAChRs. However, the Ca(2+) permeability of TM nAChR channels has not been determined. To directly evaluate the Ca(2+) permeability of TM nAChRs, patch-clamp recordings were conducted using non-cultured acutely dissociated TM neurons and external solutions containing low (2 mM) and high (20 mM) concentrations of Ca(2+). A shift in the reversal potentials was determined from the current-voltage relationships and the permeability ratio, P(Ca)/P(Na), was estimated within the Goldman-Hodgkin-Katz constant field approximation. TM nAChRs were found to be highly Ca(2+) permeable with the permeability ratio, P(Ca)/P(Na)(nAChR) being approximately 5.9 and the fractional Ca(2+) current, P(f)(nAChR) being approximately 10.1% at -60 mV. As a positive control for the applied methods and analysis, the permeability ratio, P(Ca)/P(Na)(NMDAR) being approximately 8.3 and the fractional Ca(2+) current, P(f)(NMDAR) being approximately 13.6% at -60 mV for NMDA receptors were determined using non-cultured acutely dissociated hippocampal pyramidal neurons and found similar to previously reported values. Therefore, these results demonstrate that native TM nAChRs are highly Ca(2+) permeable, but approximately 1.4 fold less permeable to Ca(2+) than native hippocampal pyramidal NMDA receptors.

Structural and functional neuroprotection in glaucoma: role of galantamine-mediated activation of muscarinic acetylcholine receptors

Glaucoma is the leading cause of irreversible blindness worldwide. Loss of vision due to glaucoma is caused by the selective death of retinal ganglion cells (RGCs). Treatments for glaucoma, limited to drugs or surgery to lower intraocular pressure (IOP), are insufficient. Therefore, a pressing medical need exists for more effective therapies to prevent vision loss in glaucoma patients. In this in vivo study, we demonstrate that systemic administration of galantamine, an acetylcholinesterase inhibitor, promotes protection of RGC soma and axons in a rat glaucoma model. Functional deficits caused by high IOP, assessed by recording visual evoked potentials from the superior colliculus, were improved by galantamine. These effects were not related to a reduction in IOP because galantamine did not change the pressure in glaucomatous eyes and it promoted neuronal survival after optic nerve axotomy, a pressure-independent model of RGC death. Importantly, we demonstrate that galantamine-induced ganglion cell survival occurred by activation of types M1 and M4 muscarinic acetylcholine receptors, while nicotinic receptors were not involved. These data provide the first evidence of the clinical potential of galantamine as neuroprotectant for glaucoma and other optic neuropathies, and identify muscarinic receptors as potential therapeutic targets for preventing vision loss in these blinding diseases.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

Genome-wide linkage analyses of quantitative and categorical autism subphenotypes

BACKGROUND

The search for susceptibility genes in autism and autism spectrum disorders (ASD) has been hindered by the possible small effects of individual genes and by genetic (locus) heterogeneity. To overcome these obstacles, one method is to use autism-related subphenotypes instead of the categorical diagnosis of autism since they may be more directly related to the underlying susceptibility loci. Another strategy is to analyze subsets of families that meet certain clinical criteria to reduce genetic heterogeneity.

METHODS

In this study, using 976 multiplex families from the Autism Genome Project consortium, we performed genome-wide linkage analyses on two quantitative subphenotypes, the total scores of the reciprocal social interaction domain and the restricted, repetitive, and stereotyped patterns of behavior domain from the Autism Diagnostic Interview-Revised. We also selected subsets of ASD families based on four binary subphenotypes, delayed onset of first words, delayed onset of first phrases, verbal status, and IQ > or = 70.

RESULTS

When the ASD families with IQ > or = 70 were used, a logarithm of odds (LOD) score of 4.01 was obtained on chromosome 15q13.3-q14, which was previously linked to schizophrenia. We also obtained a LOD score of 3.40 on chromosome 11p15.4-p15.3 using the ASD families with delayed onset of first phrases. No significant evidence for linkage was obtained for the two quantitative traits.

CONCLUSIONS

This study demonstrates that selection of informative subphenotypes to define a homogeneous set of ASD families could be very important in detecting the susceptibility loci in autism.

3'-Fluoro substitution in the pyridine ring of epibatidine improves selectivity and efficacy for alpha4beta2 versus alpha3beta4 nAChRs

The analog of epibatidine having a fluoro substituent at the 3' position of the pyridine ring has been recently developed and shown to possess binding affinity in the pM range to alpha4beta2 nAChRs and in the nM range to alpha7 nAChRs and to exhibit potent agonist activity in nicotine-induced analgesia tests. Here we used patch-clamp technique in a whole-cell configuration to compare functional activity of 3'-fluoroepibatidine to that of epibatidine by itself on recombinant alpha4beta2, alpha7 and alpha3beta4 neuronal nAChRs. The agonist effect of (+/-)-epibatidine was partial and yielded comparable EC50s of 0.012 microM (72% efficacy) and 0.027 microM (81% efficacy) at alpha4beta2 and alpha3beta4 nAChRs, respectively, but was full at alpha7 nAChRs with an EC50 of 4.8 muM. Testing of the analog at different concentrations revealed that it acts as a full agonist with an EC50 of 0.36 microM at alpha4beta2 nAChRs and induces partial agonist effect (66% efficacy) at alpha7 nAChRs with an EC50 of 9.8 microM and an IC50 corresponding to 225 microM. In contrast, the analog caused only 24% maximal activation at the range of concentrations from 0.1 to 100 microM and, in addition, induced an inhibition of alpha3beta4 nAChR function with an IC50 of 8.3 microM. Our functional data, which are in agreement with previous binding and behavioral findings, demonstrate that 3'-fluoro substitution in the pyridine ring of epibatidine results in an improved pharmacological profile as observed by an increased efficacy and selectivity for alpha4beta2 versus alpha3beta4 nAChRs.

Transdermal nicotine modulates strategy-based attentional semantic processing in non-smokers

Nicotine has been shown to improve various aspects of cognitive processing such as attention and memory, however, its effects on lexical-semantic processing are relatively uncharted. Recent investigations of mnemonic processing in minimally deprived smokers suggest that nicotine might selectively modulate processes concerned with associative memory. This study investigated the effects of nicotine on lexical-semantic processing in non-smokers using a strategy-based lexical-decision priming paradigm. Transdermal nicotine patches (7 mg/24 h) were administered within a double-blind, placebo-controlled, cross-over design. Participants were trained to expect target words to come from a specified semantic category based on the prime word, although in some instances trained expectations were not met. Participants were presented with the stimuli at either a short or long stimulus onset asynchrony (SOA) to target automatic and attentional processing, respectively (n=12 and 17 for the short and long SOAs, respectively). Nicotine was found to selectively affect priming condition reaction times at the long SOA, indicating a nicotinic modulation of attentional mechanisms. Specifically, facilitation effects were dominant under placebo compared to a dominance of inhibition effects under nicotine. These results suggest that nicotine supports inhibitory attentional mechanisms in cognitively demanding semantic processing paradigms.

Immunohistochemical increase in cyclooxygenase-2 without apoptosis in different brain areas of subchronic nicotine- and D-amphetamine-treated rats

Cyclooxygenase-2 (COX-2) upregulation has been related to both neurodegeneration and physiological processes. To clarify whether nicotine-induced upregulation of COX-2 occurs, and to analyse its significance, a comparative immunohistochemical and Western blot study was performed on the frontoparietal cortex, hippocampus and cerebellar cortex of rats treated (14 days) with nicotine, D(+)amphetamine (0.35 and 1.16 mg free base/kg/day, respectively), or both drugs simultaneously. None of these treatments promoted neuronal apoptosis. Lipid peroxidation increased in the hippocampus of the nicotine-treated rats and in all the brain regions examined in the D(+)amphetamine rats, but not in the double-treated animals. Both molecules increased the COX-2 content (as determined by the number of immunopositive neurons and the intensity of their immunodeposits) in an area-, layer- and neuron type-dependent manner, in all brain regions in which a large number of COX-2 immunopositive neurons were observed in controls (the somatosensory cortical areas, CA-1, CA-3, the gyrus dentatus, the ectorhinal/perirhinal areas, and the gyrus cingularis). No increase was seen in the motor cortical areas, while a reduction was recorded in the cerebellar cortex; these regions had only a few immunopositive neurons in controls. Western blot analysis revealed a 50-80% increase in COX-2 in the brain cortex and hippocampus of nicotine-treated rats, and similar increases (150-200%) in the cortex of the D(+)amphetamine- and nicotine + D(+)amphetamine-treated rats. Nicotine-induced upregulation of COX-2 seems to be related to neuronal plasticity rather than neurodegeneration. Nicotine agonists might be useful in the treatment of cognitive disorders.

Cognitive deficits in schizophrenia: focus on neuronal nicotinic acetylcholine receptors and smoking

Patients with schizophrenia present with deficits in specific areas of cognition. These are quantifiable by neuropsychological testing and can be clinically observable as negative signs. Concomitantly, they self-administer nicotine in the form of cigarette smoking. Nicotine dependence is more prevalent in this patient population when compared to other psychiatric conditions or to non-mentally ill people. The target for nicotine is the neuronal nicotinic acetylcholine receptor (nAChR). There is ample evidence that these receptors are involved in normal cognitive operations within the brain. This review describes neuronal nAChR structure and function, focusing on both cholinergic agonist-induced nAChR desensitization and nAChR up-regulation. The several mechanisms proposed for the nAChR up-regulation are examined in detail. Desensitization and up-regulation of nAChRs may be relevant to the physiopathology of schizophrenia. The participation of several subtypes of neuronal nAChRs in the cognitive processing of non-mentally ill persons and schizophrenic patients is reviewed. The role of smoking is then examined as a possible cognitive remediator in this psychiatric condition. Finally, pharmacological strategies focused on neuronal nAChRs are discussed as possible therapeutic avenues that may ameliorate the cognitive deficits of schizophrenia.

Multiple brain pathways and receptors underlying tobacco addiction

Over the last 20 years much progress has been made in understanding the pharmacologic basis of tobacco addiction. In particular, the role of nicotine in reinforcing smoking behavior has been studied from a variety of perspectives. This article discusses two important aspects of this topic: (1) brain pathways underlying tobacco addiction; and (2) the actions of nicotine at nicotinic cholinergic receptors. Recent evidence will be reviewed indicating that nicotine reinforces smoking behavior by acting on more than one subtype of nicotinic receptor. Similarly, the role of several brain pathways in tobacco addiction will be considered. Tobacco addiction may thus be seen as a complex neuropsychopharmacological disorder; further progress in smoking cessation treatment may require that we address the multiple molecular and brain components of this addiction.

Nicotinic acetylcholine receptors and nicotinic cholinergic mechanisms of the central nervous system

Subtypes of neuronal nicotinic acetylcholine receptors (nAChRs) are constructed from numerous subunit combinations that compose channel-receptor complexes with varied functional and pharmacological characteristics. Structural and functional diversity and the broad presynaptic, postsynaptic, and nonsynaptic locations of nAChRs underlie their mainly modulatory roles throughout the mammalian brain. Presynaptic and preterminal nicotinic receptors enhance neurotransmitter release, postsynaptic nAChRs contribute a small minority of fast excitatory transmission, and nonsynaptic nAChRs modulate many neurotransmitter systems by influencing neuronal excitability. Nicotinic receptors have roles in development and synaptic plasticity, and nicotinic mechanisms participate in learning, memory, and attention. Decline, disruption, or alterations of nicotinic cholinergic mechanisms contribute to dysfunctions such as epilepsy, schizophrenia, Parkinson's disease, autism, dementia with Lewy bodies, Alzheimer's disease, and addiction.

Schizophrenia and the alpha7 nicotinic acetylcholine receptor

In addition to the devastating symptoms of psychosis, many people with schizophrenia also suffer from cognitive impairment. These cognitive symptoms lead to marked dysfunction and can impact employability, treatment adherence, and social skills. Deficits in P50 auditory gating are associated with attentional impairment and may contribute to cognitive symptoms and perceptual disturbances. This nicotinic cholinergic-mediated inhibitory process represents a potential new target for therapeutic intervention in schizophrenia. This chapter will review evidence implicating the nicotinic cholinergic, and specifically, the alpha7 nicotinic receptor system in the pathology of schizophrenia. Impaired auditory sensory gating has been linked to the alpha7 nicotinic receptor gene on the chromosome 15q14 locus. A majority of persons with schizophrenia are heavy smokers. Although nicotine can acutely reverse diminished auditory sensory gating in people with schizophrenia, this effect is lost on a chronic basis due to receptor desensitization. The alpha7 nicotinic agonist 3-(2,4 dimethoxy)benzylidene-anabaseine (DMXBA) can also enhance auditory sensory gating in animal models. DMXBA is well tolerated in humans and a new study in persons with schizophrenia has found that DMXBA enhances both P50 auditory gating and cognition. alpha7 Nicotinic acetylcholine receptor agonists appear to be viable candidates for the treatment of cognitive disturbances in schizophrenia.

Functional Pharmacology in Human Brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

Airway-related vagal preganglionic neurons express multiple nicotinic acetylcholine receptor subunits

Nicotine acting centrally increases bronchomotor tone and airway secretion, suggesting that airway-related vagal preganglionic neurons (AVPNs) within the rostral nucleus ambiguus (rNA) express nicotinic acetylcholine receptors (nAChRs). In the present study, we examined the three main functionally characterized subtypes of nAChRs in the CNS, the alpha7 homomeric and alpha4beta2 heteromeric receptors. First, we characterized the expression of these subunits at the message (mRNA) and protein levels in brain tissues taken from the rNA region, the site where AVPNs are located. In addition, double labeling fluorescent immunohistochemistry and confocal laser microscopy were used to define the presence of alpha7, alpha4, and beta2 nAChRs on AVPNs that were retrogradely labeled with cholera toxin beta subunit (CTb), injected into the upper lung lobe (n=4) or extrathoracic trachea (n=4). Our results revealed expression of all three studied subunits at mRNA and protein levels within the rNA region. Furthermore, virtually all identified AVPNs innervating intrapulmonary airways express alpha7 and alpha4 nAChR subunits. Similarly, a majority of labeled AVPNs projecting to extrathoracic trachea contain alpha7 and beta2 subunits, but less than half of them show detectable alpha4 nAChR traits. These results suggest that AVPNs express three major nAChR subunits (alpha7, alpha4, and beta2) that could assemble into functional homologous or heterologous pentameric receptors, mediating fast and sustained nicotinic effects on cholinergic outflow to the airways.

2-Fluoro-3-(4-nitro-phenyl)deschloroepibatidine is a novel potent competitive antagonist of human neuronal alpha4beta2 nAChRs

A patch-clamp technique in a whole-cell configuration was used to examine the functional activity of recently developed 2-fluoro-3-(substituted phenyl)deschloroepibatidine analogs on two major subtypes of neuronal nicotinic acetylcholine receptors (nAChRs), alpha4beta2 and alpha3beta4, that predominate in the central and peripheral nervous systems, respectively. These epibatidine analogs have been shown previously to possess high binding affinity to alpha4beta2 but not to alpha7 nAChRs and to inhibit nicotine-induced analgesia in behavioral pain tests. The 2-fluoro-3-(4-nitro-phenyl)deschloroepibatidine (4-nitro-PFEB) exhibited the most pronounced antagonist activity among these analogs when tested electrophysiologically on alpha4beta2 nAChRs. It inhibited acetylcholine (ACh)-induced currents in a concentration-dependent manner with an IC(50) value of 0.1 microM and produced complete inhibition at approximately 1 microM concentration. 4-Nitro-PFEB at 0.1 microM concentration produced a 4-fold rightward shift in the ACh concentration-response curve without altering maximum ACh-induced response. This inhibitory effect of 4-nitro-PFEB was voltage- and use-independent and was partially reversible at its 1 microM concentration. The rise and decay kinetics of ACh-induced currents was not altered in the presence of 4-nitro-PFEB. In contrast to alpha4beta2 nAChRs, this compound did not affect alpha3beta4 nAChR-mediated currents at < or =1 microM (IC(50) approximately 63.9 microM). Overall, these functional data agree with previous binding and behavioral findings and suggest collectively that 4-nitro-PFEB is the most effective and selective antagonist of alpha4beta2 versus alpha3beta4 and alpha7 nAChRs among the tested analogs, acting on alpha4beta2 nAChR through a competitive mechanism with a potency 17-fold higher than that of dihydro-beta-erythroidine.

Nicotinic cholinergic intercellular communication: implications for the developing auditory system

In this paper, research on the temporal and spatial distribution of cholinergic-related molecules in the lower auditory brainstem, with an emphasis on nicotinic acetylcholine receptors (nAChRs), is reviewed. The possible functions of acetylcholine (ACh) in driving selective auditory neurons before the onset of hearing, inducing glutamate receptor gene expression, synaptogenesis, differentiation, and cell survival are discussed. Experiments conducted in other neuronal and non-neuronal systems are drawn on extensively to discuss putative functions of ACh and nAChRs. Data from other systems may provide insight into the functions of ACh and nAChRs in auditory processing. The mismatch of presynaptic and postsynaptic markers and novel endogenous agonists of nAChRs are discussed in the context of non-classical interneuronal communication. The molecular mechanism that may underlie the many functions of ACh and its agonists is the regulation of intracellular calcium through nAChRs. The possible reorganization that may take place in the auditory system by the exposure to nicotine during critical developmental periods is also briefly considered.

Endogenous acetylcholine enhances synchronized interneuron activity in rat neocortex

Application of 4-aminopyridine (4-AP) along with EAA) receptor antagonists produces gamma-aminobutyric acid (GABAA) receptor-dependent synchronized activity in interneurons. This results in waves of activity propagating through upper cortical layers. Because interneurons in the neocortex are excited by nicotinic acetylcholine receptor (nAChR) agonists, ACh may influence synchronization of these local neocortical interneuronal networks. To study this possibility, we have used voltage-sensitive dye imaging using the fluorescent dye RH 414 (30 microM) in rat neocortical slices. Recordings were obtained in the presence of 4-AP (100 microM) and the EAA receptor antagonists D-2-amino-5-phosphonvaleric acid (20 microM) and 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM). In response to intracortical stimulation, localized or propagated activity restricted to upper cortical layers was seen. Bath application of the ACh esterase inhibitor neostigmine (10 microM) and the nAChR agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP; 10 microM) increased the response amplitude, the extent of spread, and the duration of this activity. These changes were seen in 13 of 16 slices tested with neostigmine (10 microM) and 4 of 7 slices tested with DMPP (10 microM). Application of the muscarinic AChR antagonist atropine (1 microM) did not block the enhancement of activity by neostigmine (n = 7). Application of dihydro-beta-erythroidine (10 microM), known, at this concentration, to selectively antagonize alpha4beta2-like nAChRs, blocked the effect of neostigmine (n = 5). The selective alpha7-like nAChR antagonist methyllycaconitine (50 nM) was ineffective (n = 5). These results suggest that activation of alpha4beta2-like nAChRs by endogenously released ACh can enhance synchronized activity in local neocortical inhibitory networks.

Nicotinic-receptor potentiator drugs, huprine X and galantamine, increase ACh release by blocking AChE activity but not acting on nicotinic receptors

The main goal of the present study was to analyse the effects of (+/-)-huprine X ((+/-)-HX) and galantamine (GAL), with potentiating action on nicotinic receptors, and huperzine A (HPA), devoid of nicotinic activity, on [3H]-acetylcholine ([3H]-ACh) release in striatal slices of rat brain. All compounds are non-covalent and reversible inhibitors of AChE. Addition of (+/-)-HX (0.01 microM), GAL (10 microM) and HPA (0.1 microM) to the superfusion medium decreased the release of the ACh neurotransmitter to a similar extent: 36%, 30% and 34%, respectively (P<0.01). This effect was reverted in the presence of atropine (ATR; 0.1 microM), which blocks the pre-synaptic muscarinic M2 receptor. After that, a wide range of concentrations of drugs, concomitantly with ATR (0.1 microM), was studied in the presence of haloperidol (HAL; 0.01 microM), a dopamine D2 antagonist. In these conditions, a dose-dependent increase of [3H]-ACh release was observed in the presence of (+/-)-HX, GAL and HPA. To test the role of nicotinic receptors in the drugs' effects on [3H]-ACh release, mecamylamine (MEC) 100 microM was used to block such receptors. MEC alone significantly decreased neurotransmitter release by 18% (P<0.05), but no change was obtained in the presence of both ATR and MEC. Under these conditions, (+/-)-HX, GAL and HPA increased the release of [3H]-ACh by 37%, 25% and 38%, respectively (P<0.01). Taking into account all of these data, the present results suggest that the effects induced by (+/-)-HX and GAL nicotinic-receptor potentiators seem to be mainly due to their ability in inhibiting acetylcholinesterase activity, but not by interaction on the nicotinic receptors.

Pro-inflammatory cytokines modify neuronal nicotinic acetylcholine receptor assembly

We have examined the impact of the inflammatory cytokines interleukin-1 beta (IL-1beta) and tumor necrosis factor alpha (TNFalpha) on assembly of nAChRs from subunit mixtures of nAChRalpha4, beta2 and beta4 transiently transfected into 293 cells. In control transfections approximately 55% of alpha4 associated preferentially with beta4, but less than 15% complexed with beta2 and the remainder was associated with both beta subunits. These relative ratios were modified by pro-inflammatory cytokines. IL-1beta strongly enhanced alpha4/beta2 association and decreased alpha4/beta4, whereas TNFalpha promoted mixed alpha4/beta2/beta4 interactions. These results show that the emerging rules governing assembly of nAChRs are subject to modification by the pro-inflammatory cytokine environment.

Effects of galantamine on working memory and global functioning in patients with mild cognitive impairment: a double-blind placebo-controlled study

Mild cognitive impairment (MCI) causes memory impairment and executive function deficits in those with the condition. There is also some evidence that MCI patients are impaired in their daily functioning. Cholinesterase inhibitors have been widely used for patients with Alzheimer's disease (AD), with evidence of improving cognitive function. There is currently no established treatment for MCI, and cholinesterase inhibitors are beginning to be studied in these patients. Galantamine is a cholinesterase inhibitor that also has nicotinic receptor-modulating properties that has been successful in improving AD patients. This study examined the effects of galantamine in patients with MCI in areas of memory, executive functioning, and global functioning. There was a significant improvement in scores on the Functional Activities Questionnaire, which is a measure of global functioning. There were also improvements in the galantamine group on two of six measures in the Cambridge Automated Neuropsychiatric Test Assessment Battery and in immediate free recall on the California Verbal Learning Test.