Nicotinic receptors in aging and dementia

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

Proteomic Signature and mRNA Expression in Hippocampus of SAMP8 and SAMR1 Mice during Aging

Aging is a complex process often accompanied by cognitive decline that represents a risk factor for many neurodegenerative disorders including Alzheimer's and Parkinson's disease. The molecular mechanisms involved in age-related cognitive decline are not yet fully understood, although increased neuroinflammation is considered to play a significant role. In this study, we characterized a proteomic view of the hippocampus of the senescence-accelerated mouse prone-8 (SAMP8), a model of enhanced senescence, in comparison with the senescence-accelerated-resistant mouse (SAMR1), a model of normal aging. We additionally investigated inflammatory cytokines and cholinergic components gene expression during aging in the mouse brain tissues. Proteomic data defined the expression of key proteins involved in metabolic and cellular processes in neuronal and glial cells of the hippocampus. Gene Ontology revealed that most of the differentially expressed proteins are involved in the cytoskeleton and cell motility regulation. Molecular analysis results showed that both inflammatory cytokines and cholinergic components are differentially expressed during aging, with a downward trend of cholinergic receptors and esterase enzymes expression, in contrast to an upward trend of inflammatory cytokines in the hippocampus of SAMP8. Together, our results support the important role of the cholinergic and cytokine systems in the aging of the murine brain.

Dissociating the involvement of muscarinic and nicotinic cholinergic receptors in object memory destabilization and reconsolidation

The content of long-term memory is neither fixed nor permanent. Reminder cues can destabilize consolidated memories, rendering them amenable to change before being reconsolidated. However, not all memories destabilize following reactivation. Characteristics of a memory, such as its age or strength, impose boundaries on destabilization. Previously, we demonstrated that presentation of salient novel information at the time of reactivation can readily destabilize resistant object memories in rats and this form of novelty-induced destabilization is dependent upon acetylcholine (ACh) activity at muscarinic receptors (mAChRs). In the present study, we sought to determine if this same mechanism for initiating destabilization of resistant object memories is present in mice and further expand our understanding of the mechanisms through which ACh modulates object memory destabilization by investigating the role of nicotinic receptors (nAChRs). We provide evidence that in mice mAChRs are necessary for destabilizing object memories that are readily destabilized and those that are resistant to destabilization. Conversely, nAChRs were found to be necessary only when memories are readily destabilized. We then investigated the role of both receptors in the reconsolidation of destabilized object memory traces and determined that nAChRs, but not mAChRs, are necessary for object memory reconsolidation. Together, these results suggest that nAChRs may play a more selective role in the re-storage of object memories following destabilization and that ACh acts through mAChRs to act as an override signal to initiate destabilization of resistant object memories following reactivation with novelty. These findings expand our current understanding of the role of ACh in the dynamic storage of long-term memory.

Applying a Fast-Scan Cyclic Voltammetry to Explore Dopamine Dynamics in Animal Models of Neuropsychiatric Disorders

Progress in the development of technologies for the real-time monitoring of neurotransmitter dynamics has provided researchers with effective tools for the exploration of etiology and molecular mechanisms of neuropsychiatric disorders. One of these powerful tools is fast-scan cyclic voltammetry (FSCV), a technique which has progressively been used in animal models of diverse pathological conditions associated with alterations in dopamine transmission. Indeed, for several decades FSCV studies have provided substantial insights into our understanding of the role of abnormal dopaminergic transmission in pathogenetic mechanisms of drug and alcohol addiction, Parkinson’s disease, schizophrenia, etc. Here we review the applications of FSCV to research neuropsychiatric disorders with particular attention to recent technological advances.

Possible Engagement of Nicotinic Acetylcholine Receptors in Pathophysiology of Brain Ischemia-Induced Cognitive Impairment

Post-stroke disabilities like cognitive impairment impose are complex conditions with great economic burdens on health care systems. For these comorbidities, no effective therapies have been identified yet. Nicotinic acetylcholine receptors (nAChRs) are multifunctional receptors participating in various behavioral and neurobiological functions. During brain ischemia, the increased glutamate accumulation leads to neuronal excitotoxicity as well as mitochondrial dysfunction. These abnormalities then cause the increased levels of oxidants, which play key roles in neuronal death and apoptosis in the infarct zone. Additionally, recall of cytokines and inflammatory factors play a prominent role in the exacerbation of ischemic injury. As well, neurotrophic factors' insufficiency results in synaptic dysfunction and cognitive impairments in ischemic brain. Of note, nAChRs through various signaling pathways can participate in therapeutic approaches such as cholinergic system's stimulation, and reduction of excitotoxicity, inflammation, apoptosis, oxidative stress, mitochondrial dysfunction, and autophagy. Moreover, the possible roles of nAChRs in neurogenesis, synaptogenesis, and stimulation of neurotrophic factors expression have been reported previously. On the other hand, the majority of the above-mentioned mechanisms were found to be common in both brain ischemia pathogenesis and cognitive function tuning. Therefore, it seems that nAChRs might be known as key regulators in the control of ischemia pathology, and their modulation could be considered as a new avenue in the multi-target treatment of post-stroke cognitive impairment.

Effects of nicotinic acetylcholine receptor-activating alkaloids on anxiety-like behavior in zebrafish

Alkaloids are a structurally complex group of natural products that have a diverse range of biological activities and significant therapeutic applications. In this study, we examined the acute, anxiolytic-like effects of nicotinic acetylcholine receptor (nAChR)-activating alkaloids with reported neuropharmacological effects but whose effects on anxiety are less well understood. Because α4β2 nAChRs can regulate anxiety, we first demonstrated the functional activities of alkaloids on these receptors in vitro. Their effects on anxiety-like behavior in zebrafish were then examined using the zebrafish novel tank test (NTT). The NTT is a relatively high-throughput behavioral paradigm that takes advantage of the natural tendency of fish to dive down when stressed or anxious. We report for the first time that cotinine, anatabine, and methylanatabine may suppress this anxiety-driven zebrafish behavior after a single 20-min treatment. Effective concentrations of these alkaloids were well above the concentrations naturally found in plants and the concentrations needed to induce anxiolytic-like effect by nicotine. These alkaloids showed good receptor interactions at the α4β2 nAChR agonist site as demonstrated by in vitro binding and in silico docking model, although somewhat weaker than that for nicotine. Minimal or no significant effect of other compounds may have been due to low bioavailability of these compounds in the brain, which is supported by the in silico prediction of blood-brain barrier permeability. Taken together, our findings indicate that nicotine, although not risk-free, is the most potent anxiolytic-like alkaloid tested in this study, and other natural alkaloids may regulate anxiety as well.

Aging, hormones and receptors

Ageing is accompanied by deterioration in physical condition and a number of physiological processes and thus a higher risk of a range of diseases and disorders. In particular, we focused on the changes associated with aging, especially the role of small molecules, their role in physiological and pathophysiological processes and potential treatment options. Our previously published results and data from other authors lead to the conclusion that these unwanted changes are mainly linked to the hypothalamic-pituitary-adrenal axis can be slowed down, stopped, or in some cases even reversed by an appropriate treatment, but especially by a life-management adjustment.

Effects of nicotinic antagonists on working memory performance in young rhesus monkeys

Acetylcholine plays a pivotal neuromodulatory role in the brain, influencing neuronal activity and cognitive function. Nicotinic receptors, particularly α7 and α4β2 receptors, modulate firing of dorsolateral prefrontal (dlPFC) excitatory networks that underlie successful working memory function. Minimal work however has been done examining working memory following systemic blockade of nicotinic receptor systems in nonhuman primates, limiting the ability to explore interactions of other neuromodulatory influences with working memory impairment caused by nicotinic antagonism. In this study, we investigated working memory performance after administering three nicotinic antagonists, mecamylamine, methyllycaconitine, and dihydro-β-erythroidine, in rhesus macaques tested in a spatial delayed response task. Surprisingly, we found that no nicotinic antagonist significantly impaired delayed response performance compared to vehicle. In contrast, the muscarinic antagonist scopolamine reliably impaired delayed response performance in all monkeys tested. These findings suggest there are some limitations on using systemic nicotinic antagonists to probe the involvement of nicotinic receptors in aspects of dlPFC-dependent working memory function, necessitating alternative strategies to understand the role of this system in cognitive deficits seen in aging and neurodegenerative disease.

Subchronic effects of plant alkaloids on anxiety-like behavior in zebrafish

Zebrafish provide a valuable emerging complementary model for neurobehavioral research. They offer a powerful way to screen for the potential therapeutic effects of neuroactive drugs. A variety of behavioral tests for zebrafish have been developed and validated for assessing neurobehavioral function. The novel tank diving test is a straightforward, reproducible way of measuring anxiety-like behavior in zebrafish. When introduced into a novel tank, zebrafish normally dive to the bottom of the tank and then gradually explore the higher levels of the water column as time progresses. Buspirone is an effective anxiolytic drug in humans, which has been found, with acute administration, to reduce this anxiety-like response in zebrafish. The current study used the zebrafish model to evaluate the potential anxiolytic effects of alkaloids, commonly found in Solanaceae plants, with known neuropharmacology relevant to mood regulation. In line with previous findings, acute treatment with anxiolytic positive controls buspirone and the plant alkaloid nicotine reduced the anxiety-like diving response in the zebrafish novel tank diving test. Further, both buspirone and nicotine continued to produce anxiolytic-like effects in zebrafish after 5 days of exposure. In the same treatment paradigm, the effects of five other alkaloids-cotinine, anatabine, anabasine, harmane, and norharmane-were investigated. Cotinine, the major metabolite of nicotine, also caused anxiolytic-like effects, albeit at a dose higher than the effective dose of nicotine. Nicotine's anxiolytic-like effect was not shared by the other nicotinic alkaloids, anabasine and anatabine, or by the naturally present monoamine oxidase inhibitors harmane and norharmane. We conclude that nicotine uniquely induces anxiolytic-like effects after acute and subchronic treatment in zebrafish. The zebrafish model with the novel tank diving test could be a useful complement to rodent models for screening candidate compounds for anxiolytic effects in nonclinical studies.

The selective alpha7 nicotinic acetylcholine receptor agonist AR-R17779 does not affect ischemia-reperfusion brain injury in mice

Inflammation plays a central role in stroke-induced brain injury. The alpha7 nicotinic acetylcholine receptor (α7nAChR) can modulate immune responses in both the periphery and the brain. The aims of the present study were to investigate α7nAChR expression in different brain regions and evaluate the potential effect of the selective α7nAChR agonist AR-R17779 on ischemia-reperfusion brain injury in mice. Droplet digital PCR (ddPCR) was used to evaluate the absolute expression of the gene encoding α7nAChR (Chrna7) in hippocampus, striatum, thalamus and cortex in adult, naïve mice. Mice subjected to transient middle cerebral artery occlusion (tMCAO) or sham surgery were treated with α7nAChR agonist AR-R17779 (12 mg/kg) or saline once daily for 5 days. Infarct size and microglial activation 7 days after tMCAO were analyzed using immunohistochemistry. Chrna7 expression was found in all analyzed brain regions in naïve mice with the highest expression in cortex and hippocampus. At sacrifice, white blood cell count was significantly decreased in AR-R17779 treated mice compared with saline controls in the sham groups, although, no effect was seen in the tMCAO groups. Brain injury and microglial activation were evident 7 days after tMCAO. However, no difference was found between mice treated with saline or AR-R17779. In conclusion, α7nAChR expression varies in different brain regions and, despite a decrease in white blood cells in sham mice receiving AR-R17779, this compound does not affect stroke-induced brain injury.

Differential effects of alkaloids on memory in rodents

Nicotinic acetylcholine receptors (nAChRs) play a critical role in the neuropharmacology of learning and memory. As such, naturally occurring alkaloids that regulate nAChR activity have gained interest for understanding and potentially improving memory function. In this study, we tested the acute effects of three known nicotinic alkaloids, nicotine, cotinine, and anatabine, in suppressing scopolamine-induced memory deficit in rodents by using two classic memory paradigms, Y-maze and novel object recognition (NOR) in mice and rats, respectively. We found that all compounds were able to suppress scopolamine-induced spatial memory deficit in the Y-maze spontaneous alternation paradigm. However, only nicotine was able to suppress the short-term object memory deficit in NOR, despite the higher doses of cotinine and anatabine used to account for their potential differences in nAChR activity. These results indicate that cotinine and anatabine can uniquely regulate short-term spatial memory, while nicotine seems to have more robust and general role in memory regulation in rodents. Thus, nAChR-activating alkaloids may possess distinct procognitive properties in rodents, depending on the memory types examined.

Nicotine and Cognition in Cognitively Normal Older Adults

The cholinergic system has been shown to be the primary neurotransmitter system which is responsible for the cognitive symptoms associated with dementia; its role in healthy non-demented older adults remains a gap in the literature. Understanding the effects of age-related functional changes on the nicotinic system will address this knowledge gap. As the older adult population grows and hence the importance of understanding cognitive changes that impact functional abilities and everyday life. In this article we examine the benefits of using nicotine as a method for improving cognition in non-demented healthy older adults which may have the potential for slowing neurodegeneration in aging. Furthermore, we discuss how nicotine can play a crucial role in maintaining cognitive abilities throughout normal cognitive aging.

Central Activation of Alpha7 Nicotinic Signaling Attenuates LPS-Induced Neuroinflammation and Sickness Behavior in Adult but Not in Aged Animals

We previously reported that lipopolysaccharide (LPS) challenge caused microglial-mediated neuroinflammation and sickness behavior that was amplified in aged mice. As α7 nAChRs are implicated in the "Cholinergic anti-inflammatory pathway", we aimed to determine how α7 nAChR stimulation modulates microglial phenotype in an LPS-induced neuroinflammation model in adult and aged mice. For this, BALB/c mice were injected intraperitoneally with LPS (0.33 mg/kg) and treated with the α7 nAChR agonist PNU282987, using different administration protocols. LPS challenge reduced body weight and induced lethargy and social withdrawal in adult mice. Peripheral (intraperitoneal) co-administration of the α7 nAChR agonist PNU282987 with LPS, attenuated body weight loss and sickness behavior associated with LPS challenge in adult mice, and reduced microglial activation with suppression of IL-1β and TNFα mRNA levels. Furthermore, central (intracerebroventricular) administration of the α7 nAChR agonist, even 2 h after LPS injection, attenuated the decrease in social exploratory behavior and microglial activation induced by peripheral administration of LPS, although this recovery was not achieved if activation of α7 nAChRs was performed peripherally. Finally, we observed that the positive results of central activation of α7 nAChRs were lost in aged mice. In conclusion, we provide evidence that stimulation of α7 nAChR signaling reduces microglial activation in an in vivo LPS-based model, but this cholinergic-dependent regulation seems to be dysfunctional in microglia of aged mice.

Interactions between reproductive transitions during aging and addiction: promoting translational crosstalk between different fields of research

Discovery of neural mechanisms underlying neuropsychiatric disorders within the aging and addiction fields has been a main focus of the National Institutes of Health. However, there is a dearth of knowledge regarding the biological interactions of aging and addiction, which may have important influences on progression of disease and treatment outcomes in aging individuals with a history of chronic drug use. Thus, there is a large gap in these fields of research, which has slowed progress in understanding and treating substance use disorders (SUDs) as well as age-related diseases, specifically in women who experience precipitous reproductive cycle transitions during aging. The goal of this review is to highlight overlap of SUDs and age-related processes with a specific focus on menopause and smoking, and identify critical gaps. We have narrowed the focus of the review to smoking, as the majority of findings on hormonal and aging influences on drug use have come from this area of research. Further, we highlight female-specific issues such as transitional menopause and exogenous estrogen use. These issues may impact drug use cessation as well as outcomes with aging and age-related neurodegenerative diseases in women. We first review clinical studies for smoking, normal aging, and pathological aging, and discuss the few aging-related studies taking smoking history into account. Conversely, we highlight the dearth of clinical smoking studies taking age as a biological variable into account. Preclinical and clinical literature show that aging, age-related pathological brain disease, and addiction engage overlapping neural mechanisms. We hypothesize that these putative drivers interact in meaningful ways that may exacerbate disease and hinder successful treatment outcomes in such comorbid populations. We highlight areas where preclinical studies are needed to uncover neural mechanisms in aging and addiction processes. Collectively, this review highlights the need for crosstalk between different fields of research to address medical complexities of older adults, and specifically women, who smoke.

Multidimensional Intersection of Nicotine, Gene Expression, and Behavior

The cholinergic system plays a crucial role in nervous system function with important effects on developmental processes, cognition, attention, motivation, reward, learning, and memory. Nicotine, the reinforcing component of tobacco and e-cigarettes, directly acts on the cholinergic system by targeting nicotinic acetylcholine receptors (nAChRs) in the brain. Activation of nAChRs leads to a multitude of immediate and long-lasting effects in specific cellular populations, thereby affecting the addictive properties of the drug. In addition to the direct actions of nicotine in binding to and opening nAChRs, the subsequent activation of circuits and downstream signaling cascades leads to a wide range of changes in gene expression, which can subsequently alter further behavioral expression. In this review, we provide an overview of the actions of nicotine that lead to changes in gene expression and further highlight evidence supporting how these changes can often be bidirectional, thereby inducing subsequent changes in behaviors associated with further drug intake.

Cholinergic Modulation of Glial Function During Aging and Chronic Neuroinflammation

Aging is a complex biological process that increases the risk of age-related cognitive degenerative diseases such as dementia, including Alzheimer’s disease (AD), Lewy Body Dementia (LBD), and mild cognitive impairment (MCI). Even non-pathological aging of the brain can involve chronic oxidative and inflammatory stress, which disrupts the communication and balance between the brain and the immune system. There has been an increasingly strong connection found between chronic neuroinflammation and impaired memory, especially in AD. While microglia and astrocytes, the resident immune cells of the central nervous system (CNS), exerting beneficial effects during the acute inflammatory phase, during chronic neuroinflammation they can become more detrimental. Central cholinergic circuits are involved in maintaining normal cognitive function and regulating signaling within the entire cerebral cortex. While neuronal-glial cholinergic signaling is anti-inflammatory and anti-oxidative, central cholinergic neuronal degeneration is implicated in impaired learning, memory sleep regulation, and attention. Although there is evidence of cholinergic involvement in memory, fewer studies have linked the cholinergic anti-inflammatory and anti-oxidant pathways to memory processes during development, normal aging, and disease states. This review will summarize the current knowledge of cholinergic effects on microglia and astroglia, and their role in both anti-inflammatory and anti-oxidant mechanisms, concerning normal aging and chronic neuroinflammation. We provided details on how stimulation of α7 nicotinic acetylcholine (α7nACh) receptors can be neuroprotective by increasing amyloid-β phagocytosis, decreasing inflammation and reducing oxidative stress by promoting the nuclear factor erythroid 2-related factor 2 (Nrf2) pathways and decreasing the release of pro-inflammatory cytokines. There is also evidence for astroglial α7nACh receptor stimulation mediating anti-inflammatory and antioxidant effects by inhibiting the nuclear factor-κB (NF-κB) pathway and activating the Nrf2 pathway respectively. We conclude that targeting cholinergic glial interactions between neurons and glial cells via α7nACh receptors could regulate neuroinflammation and oxidative stress, relevant to the treatment of several neurodegenerative diseases.

Smokeless tobacco induces toxicity and apoptosis in neuronal cells: a mechanistic evaluation

Smokeless tobacco (SLT) or chewing tobacco has been a highly addictive practice in India across ages, posing major threat to the systemic health and possibly neurodegeneration. Earlier studies showed components of SLT could be harmful to neuronal health. However, mechanism of SLT in neurodegeneration remained unexplored. This study investigated the detrimental role of SLT on differentiated neuronal cell lines, PC12 and SH-SY5Y by using graded doses of water soluble lyophilised SLT. Reduced cell viability, compromised mitochondrial structure and functions were observed when neuronal cell lines were treated with SLT (6 mg/mL) for 24 h. There was reduction of oxidative phosphorylation and aerobic glycolysis as determined by diminution of ATP production (2.5X) and basal respiration (1.9X). Mitochondrial membrane potential was dropped by 3.5 times. Bid, a pro-apoptotic Bcl-2 family protein, has imperative role in regulating mitochondrial outer membrane permeabilization and subsequent cytochrome c release leading to apoptosis. This article for the first time indicated the involvement of Bid in SLT mediated neurotoxicity and possibly neurodegeneration. SLT treatment enhanced expression of cleaved-Bid in time dependent manner. The involvement of Bid was further confirmed by using Bid specific shRNA which reversed the effects of SLT and conferred significant protection from apoptosis up to 72 h. Thus, our results clearly indicated that SLT induced neuronal cell death occurred via production of ROS, alteration of mitochondrial morphology, membrane potential and oxidative phosphorylation, inactivation of survival pathway and activation of apoptotic markers mediated by Bid. Therefore, Bid could be a potential future therapeutic target for SLT induced neurodegeneration.

Nicotinic Receptor Subunit Distribution in Auditory Cortex: Impact of Aging on Receptor Number and Function

The presence of novel or degraded communication sounds likely results in activation of basal forebrain cholinergic neurons increasing release of ACh onto presynaptic and postsynaptic nAChRs in primary auditory cortex (A1). nAChR subtypes include high-affinity heteromeric nAChRs commonly composed of α4 and β2 subunits and low-affinity homomeric nAChRs composed of α7 subunits. In young male FBN rats, we detail the following: (1) the distribution/expression of nAChR subunit transcripts in excitatory (VGluT1) and inhibitory (VGAT) neurons across A1 layers; (2) heteromeric nAChR binding across A1 layers; and (3) nAChR excitability in A1 layer (L) 5 cells. In aged rats, we detailed the impact of aging on A1 nAChR subunit expression across layers, heteromeric nAChR receptor binding, and nAChR excitability of A1 L5 cells. A majority of A1 cells coexpressed transcripts for β2 and α4 with or without α7, while dispersed subpopulations expressed β2 and α7 or α7 alone. nAChR subunit transcripts were expressed in young excitatory and inhibitory neurons across L2-L6. Transcript abundance varied across layers, and was highest for β2 and α4. Significant age-related decreases in nAChR subunit transcript expression (message) and receptor binding (protein) were observed in L2-6, most pronounced in infragranular layers. In vitro patch-clamp recordings from L5B pyramidal output neurons showed age-related nAChR subunit-selective reductions in postsynaptic responses to ACh. Age-related losses of nAChR subunits likely impact ways in which A1 neurons respond to ACh release. While the elderly require additional resources to disambiguate degraded speech codes, resources mediated by nAChRs may be compromised with aging.SIGNIFICANCE STATEMENT When attention is required, cholinergic basal forebrain neurons may trigger increased release of ACh onto auditory neurons in primary auditory cortex (A1). Laminar and phenotypic differences in neuronal nAChR expression determine ways in which A1 neurons respond to release of ACh in challenging acoustic environments. This study detailed the distribution and expression of nAChR subunit transcript and protein across A1 layers in young and aged rats. Results showed a differential distribution of nAChR subunits across A1 layers. Age-related decreases in transcript/protein expression were reflected in age-related subunit specific functional loss of nAChR signaling to ACh application in A1 layer 5. Together, these findings could reflect the age-related decline in selective attention observed in the elderly.

Mechanisms of GABAergic and cholinergic neurotransmission in auditory thalamus: Impact of aging

Age-related hearing loss is a complex disorder affecting a majority of the elderly population. As people age, speech understanding becomes a challenge especially in complex acoustic settings and negatively impacts the ability to accurately analyze the auditory scene. This is in part due to an inability to focus auditory attention on a particular stimulus source while simultaneously filtering out other sound stimuli. The present review examines the impact of aging on two neurotransmitter systems involved in accurate temporal processing and auditory gating in auditory thalamus (medial geniculate body; MGB), a critical brain region involved in the coding and filtering of auditory information. The inhibitory neurotransmitter GABA and its synaptic receptors (GABA_ARs) are key to maintaining accurate temporal coding of complex sounds, such as speech, throughout the central auditory system. In the MGB, synaptic and extrasynaptic GABA_ARs mediate fast phasic and slow tonic inhibition respectively, which in turn regulate MGB neuron excitability, firing modes, and engage thalamocortical oscillations that shape coding and gating of acoustic content. Acoustic coding properties of MGB neurons are further modulated through activation of tegmental cholinergic afferents that project to MGB to potentially modulate attention and help to disambiguate difficult to understand or novel sounds. Acetylcholine is released onto MGB neurons and presynaptic terminals in MGB activating neuronal nicotinic and muscarinic acetylcholine receptors (nAChRs, mAChRs) at a subset of MGB afferents to optimize top-down and bottom-up information flow. Both GABAergic and cholinergic neurotransmission is significantly altered with aging and this review will detail how age-related changes in these circuits within the MGB may impact coding of acoustic stimuli.

The role of pituitary adenylyl cyclase activating polypeptide in affective signs of nicotine withdrawal

Recent evidence implicates endogenous pituitary adenylyl cyclase activating polypeptide (PACAP) in the aversive effect of nicotine. In the present study, we assessed if nicotine-induced conditioned place preference (CPP) or affective signs of nicotine withdrawal would be altered in the absence of PACAP and if there were any sex-related differences in these responses. Male and female mice lacking PACAP and their wild-type controls were tested for baseline place preference on day 1, received conditioning with saline or nicotine (1 mg/kg) on alternate days for 6 days and were then tested for CPP the next day. Mice were then exposed to four additional conditioning and were tested again for nicotine-induced CPP 24 hr later. Controls were conditioned with saline in both chambers and tested similarly. All mice were then, 96 hr later, challenged with mecamylamine (3 mg/kg), and tested for anxiety-like behaviors 30 min later. Mice were then, 2 hr later, forced to swim for 15 min and then tested for depression-like behaviors 24 hr later. Our results showed that male but not female mice lacking PACAP expressed a significant CPP that was comparable to their wild-type controls. In contrast, male but not female mice lacking PACAP exhibited reduced anxiety- and depression-like behaviors compared to their wild-type controls following the mecamylamine challenge. These results suggest that endogenous PACAP is involved in affective signs of nicotine withdrawal, but there is a sex-related difference in this response.

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

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

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Prefrontal α7 nAChRs are critical for encoding of associative recognition memory

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Prefrontal α4β2 nAChRs are required for retrieval of associative recognition memory

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

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Bidirectional plasticity underlies the role of nAChR in associative recognition

Short-term auricular electrical stimulation rapidly elevated cortical blood flow and promoted the expression of nicotinic acetylcholine receptor α4 in the 2 vessel occlusion rats model

Background

Vascular dementia is the second dementing illness after Alzheimer’s disease and caused by reduced blood flow to the brain, and affects cognitive abilities. Our previous study found that auricular electrical stimulation (ES) improved motor and learning impairment, and this phenomenon related with nicotinic acetylcholine receptor (nAChR) expressed cells. However, the underlying mechanism was not clear. In the present study, we investigated the effects of auricular ES on cortical blood flow (CBF) and acetylcholine (ACh) - nAChRs expressed cells.

Methods

Vascular dementia rat animal model was established by permanent occlusions of common carotid arteries with 6–0 nylon suture filament. At 21 day after surgery, motor impairment was confirmed by rotarod test. 15-Hz auricular ES were applied to the ears for 20 min and CBF was recorded at the mean time. The brains were immediately dissected for immunohistochemical stain and western blot analysis.

Results

Our results showed that 15-Hz auricular ES rapidly elevated CBF in the middle cerebral artery. The numbers of nAChR α4 immuno-positive cells and western blot levels were significally increased by 15-Hz auricular ES in the hippocampal CA2 output cortex. The numbers of choline acetyltransferase (ChAT) – a key enzyme for biosynthesis of ACh – immuno-positive cells and western blot levels had no significant differences.

Conclusions

The present data suggested that the 15-Hz auricular ES for 20 min rapidly elevated cortical blood flow, promoted the expression of nAChR α4, and would be beneficial for the treatment of Alzheimer type and vascular type dementia.

The role of endogenous pituitary adenylyl cyclase activating polypeptide (PACAP) in nicotine self-administration, reward and aversion

Pituitary adenylyl cyclase activating polypeptide (PACAP) and its receptors (PAC1, VPAC1, and VPAC2) are localized in brain regions implicated in stress response, reward seeking and aversive responses, raising the possibility that PACAP may be involved in motivational effects of nicotine. To test this hypothesis, we used two-bottle choice (TBC) and place conditioning paradigms and assessed if nicotine preference or conditioned place preference (CPP) or aversion (CPA) induced by nicotine would be altered in mice lacking PACAP compared to their wild-type controls. In the TBC paradigm, mice had access to two water bottles during the first week and then one of the water bottles was switched to nicotine solution (20, 40 and then 80 μg/mL). The volume of water and nicotine consumed was measured every day. In the place conditioning paradigm, mice were tested for baseline place preference on day 1, received conditioning with saline versus a low (0.25) or high (1 mg/kg) dose nicotine and, respectively, tested for CPP or CPA 24 h following the last conditioning. We discovered that mice lacking PACAP compared to their wild-type controls exhibited more preference for nicotine over water in the TBC paradigm, particularly at the two higher concentrations of nicotine. While the rewarding action of the low dose nicotine was not altered in mice lacking PACAP, the aversive effect of the high dose nicotine was blunted in these mice compared to their wild-type controls. The present results suggest that endogenous PACAP may play a functional role in nicotine preference and its aversive effect.

Aging Affects Nicotinic Acetylcholine Receptors in Brain

BACKGROUND

Aging is a common and inevitable stage in the life cycle of higher organisms. Different organs, including the central nervous system, are affected by aging in different ways. Many processes are involved in aging, and neurodegeneration is one of the aging processes in which the central nervous system is engaged. Brain degeneration during normal aging underlies cognitive disorders experienced by older people. Not all molecular mechanisms associated with age-related neurodegeneration are fully understood; however, there is a whole range of data on the participation of nicotinic acetylcholine receptors in the processes of aging and neurodegeneration. Two main subtypes of nicotinic acetylcholine receptor α7 and α4β2 present in the central nervous system are affected by these processes. The loss of these receptor subtypes during normal aging is one of the reasons for the cognitive impairments. The decrease in nicotinic acetylcholine receptors is also very important for the pathogenesis of age-related neurodegenerative diseases. Thus, the drugs enhancing receptor functions may be considered promising for the treatment of cognitive dysfunction in the aged people.

CONCLUSION

To achieve healthy longevity, the molecular processes that occur during aging should be established. In this regard, the participation and role of nicotinic acetylcholine receptors in the brain aging and degeneration are considered in this review.

Endogenous acetylcholine regulates neuronal and astrocytic vascular endothelial growth factor expression levels via different acetylcholine receptor mechanisms

Vascular endothelial growth factor (VEGF), a signaling molecule involved in angiogenesis, plays an important role in neuroprotection and neurogenesis. In the present study, we aimed to elucidate the mechanisms underlying endogenous acetylcholine (ACh)-induced VEGF expression in neurons and astrocytes, and identify the neuronal cells contributing to its expression in the medial septal area, a nuclear origin of cholinergic neurons mainly projecting to the hippocampus. The mRNA expression and secretion of VEGF were measured by RT-PCR and ELISA using mouse primary cultured cortical neurons and astrocytes. VEGF expression in the medial septal area was assessed by RT-PCR and immunostaining using mice treated with tacrine [9-amino-1,2,3,4-tetrahydro-acridine HCl (THA); 2.5 mg/kg, i.p.] once daily for 7 days. The THA treatment increased VEGF mRNA expression in neurons in a manner that was reversed by mecamylamine, a nicotinic ACh receptor (AChR) antagonist, whereas in mouse primary cultured astrocytes, carbachol, but not THA dose-dependently increased VEGF mRNA expression and secretion in a manner that was inhibited by scopolamine, a muscarinic AChR inhibitor. In in vivo studies, the administration of THA significantly increased the expression of VEGF in medial septal cholinergic neurons and the effects of THA were significantly blocked by mecamylamine. THA also significantly increased the expression levels of a phosphorylated form of VEGF receptor 2 (p-VEGFR2), an activated form of VEGFR2. The present results suggest that endogenous ACh plays an up-regulatory role for VEGF expression in neurons and astrocytes via different mechanisms. Moreover, endogenous ACh-induced increases in VEGF levels appear to activate VEGFR2 on medial septal cholinergic neurons via an autocrine mechanism.

Nicotine alleviates chronic stress-induced anxiety and depressive-like behavior and hippocampal neuropathology via regulating autophagy signaling

Recently, we reported that chronic nicotine significantly improved chronic stress-induced impairments of cognition and the hippocampal synaptic plasticity in mice, however, the underlying mechanism still needs to be explored. In the present study, 32 male C57BL/6 mice were divided into four groups: control (CON), stress (CUS), stress with chronic nicotine administration (CUS + Nic) and chronic nicotine administration (Nic). The anxiety-like behavior and neuropathological alteration of DG neurons were examined. Moreover, PC12 cells were examined with corticosterone in the presence or absence of nicotine. Both cell viability and apoptosis were determined. When treated simultaneously with an unpredictable chronic mild stress (CUS), nicotine (0.2 mg/kg/d) attenuated behavioral deficits and neuropathological alterations of DG neurons. Moreover, Western blotting showed that chronic nicotine also elevated the level of autophagy makers including Beclin-1 and LC3 II triggered by CUS. In addition, concomitant treatment with nicotine (10 μM) significantly attenuated the loss of PC12 cell viability (p < .01) and apoptosis compared to that of corticosterone treatment alone. Besides, chronic nicotine also enhanced the protein and RNA expression levels of autophagy makers triggered by corticosterone, such as Beclin-1, LC3 II and p62/SQSTM1. However, the above improvements were significantly blocked by autophagy inhibitor 3-MA. Importantly, the activation of the PI3K/Akt/mTOR signaling was carefully tested to illuminate the effects of chronic nicotine. Consequently, chronic nicotine played a role of neuroprotection in either CUS mice or corticosterone cells associating with the enhancement of the autophagy signaling, which was involved in activating the PI3K/Akt/mTOR signaling.

Differential Effects of Nicotine Exposure on the Hippocampus Across Lifespan

BACKGROUND

Nicotine exposure affects the hippocampus through activation of hippocampal nicotinic acetylcholine receptors (nAChRs), which are present throughout excitatory and inhibitory hippocampal circuitry. The role of cholinergic functioning in the hippocampus varies across developmental stages so that nicotine exposure differentially affects this region depending upon timing of exposure, producing developmentally distinct changes in structure, function, and behavior.

METHODS

We synthesize findings across literature in this area to comprehensively review current understanding of the unique effects of nicotine exposure on the hippocampus throughout the lifespan with a focus on hippocampal morphology, cholinergic functioning, and hippocampusdependent learning and memory.

CONCLUSIONS

Chronic and acute nicotine exposure differentially affect hippocampus structure, functioning, and related learning and memory in the perinatal period, adolescence, and aging. Age-related differences in sensitivity to nicotine exposure should be considered in the research of nicotine addiction and the development of nicotine addiction treatments.

Human brain imaging of nicotinic acetylcholine α4β2* receptors using [18 F]Nifene: Selectivity, functional activity, toxicity, aging effects, gender effects, and extrathalamic pathways

Nicotinic acetylcholinergic receptors (nAChR's) have been implicated in several brain disorders, including addiction, Parkinson's disease, Alzheimer's disease and schizophrenia. Here we report in vitro selectivity and functional properties, toxicity in rats, in vivo evaluation in humans, and comparison across species of [¹⁸ F]Nifene, a fast acting PET imaging agent for α4β2* nAChRs. Nifene had subnanomolar affinities for hα2β2 (0.34 nM), hα3β2 (0.80 nM) and hα4β2 (0.83 nM) nAChR but weaker (27-219 nM) for hβ4 nAChR subtypes and 169 nM for hα7 nAChR. In functional assays, Nifene (100 μM) exhibited 14% agonist and >50% antagonist characteristics. In 14-day acute toxicity in rats, the maximum tolerated dose (MTD) and the no observed adverse effect level (NOAEL) were estimated to exceed 40 μg/kg/day (278 μg/m² /day). In human PET studies, [¹⁸ F]Nifene (185 MBq; <0.10 μg) was well tolerated with no adverse effects. Distribution volume ratios (DVR) of [¹⁸ F]Nifene in white matter thalamic radiations were ∼1.6 (anterior) and ∼1.5 (superior longitudinal fasciculus). Habenula known to contain α3β2 nAChR exhibited low levels of [¹⁸ F]Nifene binding while the red nucleus with α2β2 nAChR had DVR ∼1.6-1.7. Females had higher [¹⁸ F]Nifene binding in all brain regions, with thalamus showing >15% than males. No significant aging effect was observed in [¹⁸ F]Nifene binding over 5 decades. In all species (mice, rats, monkeys, and humans) thalamus showed highest [¹⁸ F]Nifene binding with reference region ratios >2 compared to extrathalamic regions. Our findings suggest that [¹⁸ F]Nifene PET may be used to study α4β2* nAChRs in various CNS disorders and for translational research.

Nicotine and networks: Potential for enhancement of mood and cognition in late-life depression

Late-life depression is characterized by both lower mood and poor cognitive performance, symptoms that often do not fully respond to current antidepressant medications. Nicotinic acetylcholine receptor (nAChR) agonists such as nicotine may serve as a novel therapeutic approach for this population. Both preclinical and preliminary clinical studies suggest that nAChR agonists can improve depressive behavior in animal models and improve mood in depressed individuals. Substantial literature also supports that nAChR agonists benefit cognitive performance, particularly in older populations. These potential benefits may be mediated by the effects of nAChR stimulation on neural network function and connectivity. Functional neuroimaging studies detail effects of nAChR agonists on the default mode network, central-executive network, and salience network that may oppose or reverse network changes seen in depression. We propose that, given the existent literature and the clinical presentation of late-life depression, nicotine or other nAChR agonists may have unique therapeutic benefits in this population and that clinical trials examining nicotine effects on mood, cognition, and network dynamics in late-life depression are justified.

Mechanisms of Cognitive Aging in the HIV-Positive Adult

PURPOSE OF REVIEW

As of the year 2016, an estimated 50% of the United States' HIV-Positive population is aged 50 years or older. Due to a combination of increased rates of infection in older adults, and successful anti-retroviral (ART) regimens allowing HIV-positive adults to survive for decades with the disease, we are now faced with a steadily graying HIV-positive population, with only limited knowledge of how the cognitive and physiological effects of aging intersect with those of chronic HIV-infection.

RECENT FINDINGS

Age-related changes to mood, cognition, and neurological health may be experienced differently in those living with HIV, and research concerning quality of life, mental health, and cognitive aging needs to account for and explore these factors more carefully in the coming years.

SUMMARY

This review will explore the topic of cognitive aging with HIV: 1. Central nervous system (CNS) infection of HIV and how the virus affects brain integrity and function; 2. Cognitive and behavioral symptoms of HIV-Associated Neurocognitive Disorders (HAND); 3. Neurobiological theories of Cognitive Aging and how these processes may be exacerbated by HIV-infection; 4: Clinical implications and complications of aging with HIV and factors that may result in poorer cognitive outcomes.

Heme-Oxygenase I and PCG-1α Regulate Mitochondrial Biogenesis via Microglial Activation of Alpha7 Nicotinic Acetylcholine Receptors Using PNU282987

AIMS

A loss in brain acetylcholine and cholinergic markers, subchronic inflammation, and impaired mitochondrial function, which lead to low-energy production and high oxidative stress, are common pathological factors in several neurodegenerative diseases (NDDs). Glial cells are important for brain homeostasis, and microglia controls the central immune response, where α7 acetylcholine nicotinic receptors (nAChR) seem to play a pivotal role; however, little is known about the effects of this receptor in metabolism. Therefore, the aim of this study was to evaluate if glial mitochondrial energetics could be regulated through α7 nAChR.

RESULTS

Primary glial cultures treated with the α7 nicotinic agonist PNU282987 increased their mitochondrial mass and their mitochondrial oxygen consumption without increasing oxidative stress; these changes were abolished when nuclear erythroid 2-related factor 2 (Nrf2) was absent, heme oxygenase-1 (HO-1) was inhibited, or peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) was silenced. More specifically, microglia of animals treated intraperitoneally with the α7 nAChR agonist PNU282987 (10 mg/kg) showed a significant increase in mitochondrial mass. Interestingly, LysM^cre-Hmox1^Δ/Δ and PGC-1α^-/- animals showed lower microglial mitochondrial levels and treatment with PNU282987 did not produce effects on mitochondrial levels.

INNOVATION

Increases in microglial mitochondrial mass and metabolism can be achieved via α7 nAChR by a mechanism that implicates Nrf2, HO-1, and PGC-1α. This signaling pathway could open a new strategy for the treatment of NDDs, such as Alzheimer's, characterized by a reduction of cholinergic markers.

CONCLUSION

α7 nAChR signaling increases glial mitochondrial mass, both in vitro and in vivo, via HO-1 and PCG-1α. These effects could be of potential benefit in the context of NDDs. Antioxid. Redox Signal. 27, 93-105.

Dehydration and Cognition in Geriatrics: A Hydromolecular Hypothesis

Dehydration is one of the ten most frequent diagnoses responsible for the hospital admission of elderly in the United States. It is associated with increased mortality, morbidity and an estimated cost of 1.14 billion per year (Xiao et al., 2004; Schlanger et al., 2010; Pretorius et al., 2013; Frangeskou et al., 2015). Older individuals are predisposed to dehydration encephalopathy as a result of decreased total body water (TBW) and diminished sensation of thirst. We hypothesize that thirst blunting in older individuals is the result of a defective microRNA-6842-3p failing to silence the expression of the vesicular GABA transporters (VGAT) and alpha 7 cholinergic nicotinic receptors in the subfornical organ (SFO) of the hypothalamus. We hypothesize further that resultant dehydration facilitates protein misfolding and aggregation, predisposing to neurocognitive disorders. We completed a search of predicted microRNA targets, utilizing the public domain tool miRDB and found that microRNA-6842-3p modulates the SLC6A1 and CHRNA7 genes both of which were previously hypothesized to inhibit the thirst sensation by their action on SFO. The primary aim of this article is to answer two questions: Can prevention and correction of dehydration in elderly lower age-related cognitive deterioration? Can exosomal miR-6842 in the peripheral blood predict dehydration encephalopathy in elderly?

Natural Phytochemicals in the Treatment and Prevention of Dementia: An Overview

The word dementia describes a class of heterogeneous diseases which etiopathogenetic mechanisms are not well understood. There are different types of dementia, among which, Alzheimer's disease (AD), vascular dementia (VaD), dementia with Lewy bodies (DLB) and frontotemporal dementia (FTD) are the more common. Currently approved pharmacological treatments for most forms of dementia seem to act only on symptoms without having profound disease-modifying effects. Thus, alternative strategies capable of preventing the progressive loss of specific neuronal populations are urgently required. In particular, the attention of researchers has been focused on phytochemical compounds that have shown antioxidative, anti-amyloidogenic, anti-inflammatory and anti-apoptotic properties and that could represent important resources in the discovery of drug candidates against dementia. In this review, we summarize the neuroprotective effects of the main phytochemicals belonging to the polyphenol, isothiocyanate, alkaloid and cannabinoid families in the prevention and treatment of the most common kinds of dementia. We believe that natural phytochemicals may represent a promising sources of alternative medicine, at least in association with therapies approved to date for dementia.

Tobacco nitrosamines as culprits in disease: mechanisms reviewed

The link between tobacco abuse and cancer is well-established. However, emerging data indicate that toxins in tobacco smoke cause cellular injury due to enhanced toxic/metabolic effects of metabolites, disruption of intracellular signaling mechanisms, and formation of DNA, protein, and lipid adducts that impair function and promote oxidative stress and inflammation. These effects of smoking, which are largely non-carcinogenic, can be produced by tobacco-specific nitrosamines and their metabolites. These factors could account for the increased rates of neurodegeneration and insulin resistance diseases among smokers. Herein, we review nicotine and tobacco-specific nitrosamine metabolism, mechanisms of adduct formation, DNA damage, mutagenesis, and potential mechanisms of disease.

Factors influencing discrepancies in self-reported memory and performance on memory recall in the Canadian Community Health Survey-Healthy Aging, 2008-09

OBJECTIVE

the objectives of this study were: (i) to estimate the rate of discrepancy between participant single-item self-reports of good memory and poor performance on a list-learning task and (ii) to identify the factors including age, gender and health status that influence these discrepant classifications.

STUDY DESIGN AND SETTINGS

in total, 14,172 individuals, aged 45-85, were selected from the 2008-09 Canadian Community Health Survey on Healthy Aging. We examined the individual characteristics of participants with and without discrepancies between memory self-reports and performance with a generalised linear model, adjusting for potential covariates.

RESULTS

the mean age of respondents was 62.9 years with 56.7% being female, 53.8% having post-secondary graduation and 83% being born in Canada. Higher discrepant classification rates we observed for younger people (6.77 versus 3.65 for lowest and highest group), female (5.90 versus 3.68) and with higher education (6.17 versus 3.52). Discrepant classification rates adjusted with all covariates were higher for those without chronic diseases (5.37 [95% Confidence Interval (CI): 4.16, 6.90] versus 4.05 95% CI: 3.38, 4.86; P = 0.0127), those who did not drink alcohol (5.87 95% CI: 4.69, 7.32 versus 3.70 95% CI: 3.00, 4.55; P < 0.0001), lonely participants (5.45 95% CI: 4.20, 7.04 versus 3.99 95% CI: 3.36, 4.77; P = 0.0081) and bilingual participants (5.67 95% CI: 4.18, 7.64 versus 3.83 95% CI: 3.27, 4.50; P = 0.0102).

CONCLUSION

the findings of this study suggest that the self-reported memory and memory performance differ in a substantial proportion of the population. Therefore, relying on a self-reported memory status may not accurately capture those experiencing memory difficulties.

Electric stimulation of the ears ameliorated learning and memory impairment in rats with cerebral ischemia-reperfusion injury

Ear acupuncture enhances the secretion of acetylcholine, which has anti-inflammatory effects. Here we want to investigate the effect of electric stimulation (ES) of the ears on learning and memory impairment in rats with cerebral ischemia-reperfusion injury. At 24 h after reperfusion, 2-Hz ES was applied to the ears for 20 min/day (10 min for each ear) for 7 days continuously. The step-through time of the passive avoidance test was greater in the ES group than in the control group (300.0 ± 0.0 s vs 45.0 ± 26.7 s, p < 0.05). Our results showed that neither neurological deficit score nor motor functions were improved after 2-Hz ES (4.0 ± 0 vs 4.5 ± 0.8, p > 0.05). The numbers of nicotinic acetylcholine receptor α4 positively stained cells in the CA2 and dentate gyrus of the hippocampus were 19.0 ± 11.5 and 269.2 ± 79.3, respectively, in the ES group, which were greater than those in the control group (7.0 ± 5.9 and 165.5 ± 30.8, respectively) (both p < 0.05). These results suggested that 2-Hz ES of the ears ameliorated learning and memory impairment in rats with ischemia-reperfusion injury. ES of the ears has neuroprotective effects, which are related to acetylcholine release.

High-Affinity Nicotinic Receptors Modulate Spontaneous Cortical Up States In Vitro

Nicotinic acetylcholine receptors (nAChRs) play an important role in the modulation of many cognitive functions but their role in integrated network activity remains unclear. This is at least partly because of the complexity of the cholinergic circuitry and the difficulty in comparing results from in vivo studies obtained under diverse experimental conditions and types of anesthetics. Hence the role of nAChRs in the synchronization of cortical activity during slow-wave sleep is still controversial, with some studies showing they are involved in ACh-dependent EEG desynchronization, and others suggesting that this effect is mediated exclusively by muscarinic receptors. Here we use an in vitro model of endogenous network activity, in the form of recurring self-maintained depolarized states (Up states), which allows us to examine the role of high-affinity nAChRs on network dynamics in a simpler form of the cortical microcircuit. We find that mice lacking nAChRs containing the β2-subunit (β2-nAChRs) have longer and more frequent Up states, and that this difference is eliminated when β2-nAChRs in wild-type mice are blocked. We further show that endogenously released ACh can modulate Up/Down states through the activation of both β2- and α7-containing nAChRs, but through distinct mechanisms: α7-nAChRs affect only the termination of spontaneous Up states, while β2-nAChRs also regulate their generation. Finally we provide evidence that the effects of β2-subunit-containing, but not α7-subunit-containing nAChRs, are mediated through GABAB receptors. To our knowledge this is the first study documenting direct nicotinic modulation of Up/Down state activity.

SIGNIFICANCE STATEMENT

Through our experiments we were able to uncover a clear and previously disputed effect of nicotinic signaling in synchronized activity of neuronal networks of the cortex. We show that both high-affinity receptors (containing the β2-subunit, β2-nAChRs) and low-affinity receptors (containing the α7-subunit, α7-nAChRs) can regulate cortical network function exhibited in the form of Up/Down states. We further show that the effects of β2-nAChRs, but not α7-nAChRs, are mediated through the activation of GABAB receptors. These results suggest a possible synthesis of seemingly contradictory results in the literature and could be valuable for informing computational models of cortical function and for guiding the search for therapeutic interventions.

Nicotinic receptors as targets for therapeutic discovery

Nicotinic acetylcholine receptors (nAChRs) represent a class of therapeutic targets with the potential to impact numerous diseases and disorders where significant unmet medical needs remain. The latter include cognitive and neurodegenerative diseases; psychotic disorders, such as schizophrenia; acute nociceptive, neuropathic and inflammatory pain; affective disorders, such as depression and inflammation, where nAChR subtypes modulate key cellular pathways involved in anti-inflammatory processes as well as cell survival. Our increased understanding of the heterogeneity of nAChR targets is defining the relationship of biologic effects to specific receptor subtypes, which in turn, will allow further refinement of desired therapeutic activities. Both preclinical and clinical evidence support the notion that novel compounds targeting specific nAChR subtypes will offer increased potency and efficacy, longer lasting effects, fewer side effects and a more rapid onset of action and less dependence, compared with existing therapies. Clinical proof-of-concept is rapidly emerging and will solidify the position of this new therapeutic approach.

Nicotine recruits glutamate receptors to postsynaptic sites

Cholinergic neurons project throughout the nervous system and activate nicotinic receptors to modulate synaptic function in ways that shape higher order brain function. The acute effects of nicotinic signaling on long-term synaptic plasticity have been well-characterized. Less well understood is how chronic exposure to low levels of nicotine, such as those encountered by habitual smokers, can alter neural connections to promote addiction and other lasting behavioral effects. We show here that chronic exposure of hippocampal neurons in culture to low levels of nicotine recruits AMPA and NMDA receptors to the cell surface and sequesters them at postsynaptic sites. The receptors include GluA2-containing AMPA receptors, which are responsible for most of the excitatory postsynaptic current mediated by AMPA receptors on the neurons, and include NMDA receptors containing GluN1 and GluN2B subunits. Moreover, we find that the nicotine treatment also increases expression of the presynaptic component synapsin 1 and arranges it in puncta juxtaposed to the additional AMPA and NMDA receptor puncta, suggestive of increases in synaptic contacts. Consistent with increased synaptic input, we find that the nicotine treatment leads to an increase in the excitatory postsynaptic currents mediated by AMPA and NMDA receptors. Further, the increases skew the ratio of excitatory-to-inhibitory input that the cell receives, and this holds both for pyramidal neurons and inhibitory neurons in the hippocampal CA1 region. The GluN2B-containing NMDA receptor redistribution at synapses is associated with a significant increase in GluN2B phosphorylation at Tyr1472, a site known to prevent GluN2B endocytosis. These results suggest that chronic exposure to low levels of nicotine not only alters functional connections but also is likely to change excitability levels across networks. Further, it may increase the propensity for synaptic plasticity, given the increase in synaptic NMDA receptors.

The HIV-1 transgenic rat model of neuroHIV

Despite the ability of current combination anti-retroviral therapy (cART) to limit the progression of HIV-1 to AIDS, HIV-positive individuals continue to experience neuroHIV in the form of HIV-associated neurological disorders (HAND), which can range from subtle to substantial neurocognitive impairment. NeuroHIV may also influence substance use, abuse, and dependence in HIV-positive individuals. Because of the nature of the virus, variables such as mental health co-morbidities make it difficult to study the interaction between HIV and substance abuse in human populations. Several rodent models have been developed in an attempt to study the transmission and pathogenesis of the HIV-1 virus. The HIV-1 transgenic (HIV-1Tg) rat is a reliable model of neuroHIV because it mimics the condition of HIV-infected patients on cART. Research using this model supports the hypothesis that the presence of HIV-1 viral proteins in the central nervous system increases the sensitivity and susceptibility of HIV-positive individuals to substance abuse.

Endogenous ACh suppresses LTD induction and nicotine relieves the suppression via different nicotinic ACh receptor subtypes in the mouse hippocampus

AIMS

Studying the normal role of nicotinic cholinergic systems in hippocampal synaptic plasticity is critical for understanding how cholinergic loss in Alzheimer's disease (AD) and tobacco use affect cognitive function. However, it is largely unknown how nicotinic cholinergic systems regulate the induction of long-term depression (LTD).

MAIN METHODS

Extracellular field potential recordings were performed in hippocampal slices prepared from wild-type, α2, α7, and β2 knockout (KO) mice. Effects of nicotine and nicotinic antagonists on LTD induction in wild-type, α2, α7, and β2 KO mice were compared.

KEY FINDINGS

Activation of α7 nicotinic acetylcholine receptors (nAChRs) occurs during LTD-inducing stimulation to suppress LTD induction at CA3-CA1 synapses. Nicotine relieves this suppression, causing larger LTD. This nicotine effect was mediated by the activation of non-α7 nAChR subtypes, which were not activated by ACh released during LTD-inducing stimulation, and requires the presence of endogenous ACh-induced α7 nAChR activation. Furthermore, the effect of nicotine was prevented in the presence of mecamylamine, but not dihydro-β-erythroidine, and was still observed in both α2 KO and β2 KO mice.

SIGNIFICANCE

This is the first report to evaluate the involvement of different nAChR subtypes in LTD induction. Findings indicate the involvement of unique non-α7 nAChR subtypes, which have not been considered in the nicotinic modulation of hippocampal long-term potentiation, in the control of LTD induction. The implication of our results is that the loss of cholinergic projections to the hippocampus, which reduces ACh release as seen in AD patients, and nicotine from tobacco smoking can differentially affect LTD induction.

Picomolar amyloid-β peptides enhance spontaneous astrocyte calcium transients

Amyloid-β (Aβ) peptides are constitutively produced in the brain throughout life via mechanisms that can be regulated by synaptic activity. Although Aβ has been extensively studied as the pathological plaque-forming protein species in Alzheimer's disease (AD), little is known about the normal physiological function(s) and signaling pathway(s). We previously discovered that physiologically-relevant, low picomolar amounts of Aβ can enhance synaptic plasticity and hippocampal-dependent cognition in mice. In this study, we demonstrated that astrocytes are cellular candidates for participating in this type of Aβ signaling. Using calcium imaging of primary astrocyte cultures, we observed that picomolar amounts of Aβ peptides can enhance spontaneous intracellular calcium transient signaling. After application of 200 pM Aβ42 peptides, the frequency and amplitude averages of spontaneous cytosolic calcium transients were significantly increased. These effects were dependent on α7 nicotinic acetylcholine receptors (α7-nAChRs), as the enhancement effects were blocked by a pharmacological α7-nAChR inhibitor and in astrocytes from an α7 deficient mouse strain. We additionally examined evoked intercellular calcium wave signaling but did not detect significant picomolar Aβ-induced alterations in propagation parameters. Overall, these results indicate that at a physiologically-relevant low picomolar concentration, Aβ peptides can enhance spontaneous astrocyte calcium transient signaling via α7-nAChRs. Since astrocyte-mediated gliotransmission has been previously found to have neuromodulatory roles, Aβ peptides may have a normal physiological function in regulating neuron-glia signaling. Dysfunction of this signaling process may underlie glia-based aspects of AD pathogenesis.

Premature Aging Phenotype in Mice Lacking High-Affinity Nicotinic Receptors: Region-Specific Changes in Layer V Pyramidal Cell Morphology

The mechanisms by which aging leads to alterations in brain structure and cognitive deficits are unclear. Α deficient cholinergic system has been implicated as one of the main factors that could confer a heightened vulnerability to the aging process, and mice lacking high-affinity nicotinic receptors (β2(-/-)) have been proposed as an animal model of accelerated cognitive aging. To date, however, age-related changes in neuronal microanatomy have not been studied in these mice. In the present study, we examine the neuronal structure of yellow fluorescent protein (YFP(+)) layer V neurons in 2 cytoarchitectonically distinct cortical regions in wild-type (WT) and β2(-/-) animals. We find that (1) substantial morphological differences exist between YFP(+) cells of the anterior cingulate cortex (ACC) and primary visual cortex (V1), in both genotypes; (2) in WT animals, ACC cells are more susceptible to aging compared with cells in V1; and (3) β2 deletion is associated with a regionally and temporally specific increase in vulnerability to aging. ACC cells exhibit a prematurely aged phenotype already at 4-6 months, whereas V1 cells are spared in adulthood but strongly affected in old animals. Collectively, our data reveal region-specific synergistic effects of aging and genotype and suggest distinct vulnerabilities in V1 and ACC neurons.

NeuroHIV and use of addictive substances

In the past three decades, substance abuse has been identified as a key comorbidity of human immunodeficiency virus-1 (HIV-1) infection. Many studies have found that the use and abuse of addictive substances hastens the progression of HIV-1 infection and HIV-associated neurocognitive disorders. Advances in highly active antiretroviral therapy (HAART) in the mid-1990s have been successful in limiting the HIV-1 viral load and maintaining a relatively healthy immune response, allowing the life expectancy of patients infected with HIV to approach that of the general population. However, even with HAART, HIV-1 viral proteins are still expressed and eradication of the virus, particularly in the brain, the key reservoir organ, does not occur. In the post-HAART era, the clinical challenge in the treatment of HIV infection is inflammation of the central nervous system (CNS) and its subsequent neurological disorders. To date, various explicit and implicit connections have been identified between the neuronal circuitry involved in immune responses and brain regions affected by and implicated in substance abuse. This chapter discusses past and current medical uses of prototypical substances of abuse, including morphine, alcohol, cocaine, methamphetamine, marijuana, and nicotine, and the evidence that systemic infections, particularly HIV-1 infection, cause neurological dysfunction as a result of inflammation in the CNS, which can increase the risk of substance abuse.