Neurocircuitry of the nicotinic cholinergic system

Cholesterol´s role in membrane organization and nicotinic acetylcholine receptor function: Implications for aging and Alzheimer's disease

Biological membranes are complex entities composed of various molecules exhibiting lateral and transbilayer lipid asymmetries, along with a selective spatial distribution of different membrane proteins. This dynamic orchestration is crucial for proper physiological functions, undergoes changes with aging, and is disturbed in several neurological disorders. In this review, we analyze the impact of disruption in this equilibrium on physiological aging and the onset of pathological conditions. Alzheimer´s disease (AD) is a multifactorial neurodegenerative disorder in the elderly, characterized by the increased presence of the Aβ peptide, which supports the amyloid hypothesis of the disease. However, AD also involves a progressive loss of cholinergic innervation, leading to the cholinergic hypothesis of the disease. Nicotinic acetylcholine receptors (nAChRs) are transmembrane proteins, and Aβ peptides, their oligomeric and fibrillar species, which increase in hydrophobicity as they develop, interact with membranes. Therefore, a membrane hypothesis of the disease emerges as a bridge between the other two. Here, we discuss the impact of the membrane environment, through direct or indirect mechanisms, on cholinergic signaling and Aβ formation and subsequent incorporation into the membrane, with a special focus on the crucial role of cholesterol in these processes.

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.

Brain nicotinic acetylcholine receptors are involved in stress-induced potentiation of nicotine reward in rats

The aim of the present study was to examine the possible role of nicotinic acetylcholine receptors of the dorsal hippocampus (CA1 regions), the medial prefrontal cortex or the basolateral amygdala in the effect of acute or sub-chronic stress on nicotine-induced conditioned place preference. Our results indicated that subcutaneous administration of nicotine (0.2 mg/kg) induced significant conditioned place preference. Exposure to acute or sub-chronic elevated platform stress potentiated the response of an ineffective dose of nicotine. Pre-conditioning intra-CA1 (0.5-4 µg/rat) or intra-medial prefrontal cortex (0.2-0.3 µg/rat) microinjection of mecamylamine (a non-selective nicotinic acetylcholine receptor antagonist) reversed acute stress-induced potentiation of nicotine reward as measured in the conditioned place preference paradigm. By contrast, pre-conditioning intra-basolateral amygdala microinjection of mecamylamine (4 µg/rat) potentiated the effects of acute stress on nicotine reward. Our findings also showed that intra-CA1 or intra-medial prefrontal cortex, but not intra-basolateral amygdala, microinjection of mecamylamine (4 µg/rat) prevented the effect of sub-chronic stress on nicotine reward. These findings suggest that exposure to elevated platform stress potentiates the rewarding effect of nicotine which may be associated with the involvement of nicotinic acetylcholine receptors. It seems that there is a different contribution of the basolateral amygdala, the medial prefrontal cortex or the CA1 nicotinic acetylcholine receptors in stress-induced potentiation of nicotine-induced conditioned place preference.

Resting-state EEG, impulsiveness, and personality in daily and nondaily smokers

OBJECTIVES

Resting EEG is sensitive to transient, acute effects of nicotine administration and abstinence, but the chronic effects of smoking on EEG are poorly characterized. This study measures the resting EEG profile of chronic smokers in a non-deprived, non-peak state to test whether differences in smoking behavior and personality traits affect pharmaco-EEG response.

METHODS

Resting EEG, impulsiveness, and personality measures were collected from daily smokers (n=22), nondaily smokers (n=31), and non-smokers (n=30).

RESULTS

Daily smokers had reduced resting delta and alpha EEG power and higher impulsiveness (Barratt Impulsiveness Scale) compared to nondaily smokers and non-smokers. Both daily and nondaily smokers discounted delayed rewards more steeply, reported lower conscientiousness (NEO-FFI), and reported greater disinhibition and experience seeking (Sensation Seeking Scale) than non-smokers. Nondaily smokers reported greater sensory hedonia than nonsmokers.

CONCLUSIONS

Altered resting EEG power in daily smokers demonstrates differences in neural signaling that correlated with greater smoking behavior and dependence. Although nondaily smokers share some characteristics with daily smokers that may predict smoking initiation and maintenance, they differ on measures of impulsiveness and resting EEG power.

SIGNIFICANCE

Resting EEG in non-deprived chronic smokers provides a standard for comparison to peak and trough nicotine states and may serve as a biomarker for nicotine dependence, relapse risk, and recovery.

Bacoside A: Role in Cigarette Smoking Induced Changes in Brain

Cigarette smoking (CS) is a major health hazard that exerts diverse physiologic and biochemical effects mediated by the components present and generated during smoking. Recent experimental studies have shown predisposition to several biological consequences from both active and passive cigarette smoke exposure. In particular, passive smoking is linked to a number of adverse health effects which are equally harmful as active smoking. A pragmatic approach should be considered for designing a pharmacological intervention to combat the adverse effects of passive smoking. This review describes the results from a controlled experimental condition, testing the effect of bacoside A (BA) on the causal role of passive/secondhand smoke exposure that caused pathological and neurological changes in rat brain. Chronic exposure to cigarette smoke induced significant changes in rat brain histologically and at the neurotransmitter level, lipid peroxidation states, mitochondrial functions, membrane alterations, and apoptotic damage in rat brain. Bacoside A is a neuroactive agent isolated from Bacopa monnieri. As a neuroactive agent, BA was effective in combating these changes. Future research should examine the effects of BA at molecular level and assess its functional effects on neurobiological and behavioral processes associated with passive smoke.

Partial blockade of nicotinic acetylcholine receptors improves the counterregulatory response to hypoglycemia in recurrently hypoglycemic rats

Recurrent exposure to hypoglycemia can impair the normal counterregulatory hormonal responses that guard against hypoglycemia, leading to hypoglycemia unawareness. This pathological condition known as hypoglycemia-associated autonomic failure (HAAF) is the main adverse consequence that prevents individuals with type 1 diabetes mellitus from attaining the long-term health benefits of tight glycemic control. The underlying molecular mechanisms responsible for the progressive loss of the epinephrine response to subsequent bouts of hypoglycemia, a hallmark sign of HAAF, are largely unknown. Normally, hypoglycemia triggers both the release and biosynthesis of epinephrine through activation of nicotinic acetylcholine receptors (nAChR) on the adrenal glands. We hypothesize that excessive cholinergic stimulation may contribute to impaired counterregulation. Here, we tested whether administration of the nAChR partial agonist cytisine to reduce postganglionic synaptic activity can preserve the counterregulatory hormone responses in an animal model of HAAF. Compared with nicotine, cytisine has limited efficacy to activate nAChRs and stimulate epinephrine release and synthesis. We evaluated adrenal catecholamine production and secretion in nondiabetic rats subjected to two daily episodes of hypoglycemia for 3 days, followed by a hyperinsulinemic hypoglycemic clamp on day 4. Recurrent hypoglycemia decreased epinephrine responses, and this was associated with suppressed TH mRNA induction (a measure of adrenal catecholamine synthetic capacity). Treatment with cytisine improved glucagon responses as well as epinephrine release and production in recurrently hypoglycemic animals. These data suggest that pharmacological manipulation of ganglionic nAChRs may be promising as a translational adjunctive therapy to avoid HAAF in type 1 diabetes mellitus.

Structural and molecular modeling features of P2X receptors

Currently, adenosine 5'-triphosphate (ATP) is recognized as the extracellular messenger that acts through P2 receptors. P2 receptors are divided into two subtypes: P2Y metabotropic receptors and P2X ionotropic receptors, both of which are found in virtually all mammalian cell types studied. Due to the difficulty in studying membrane protein structures by X-ray crystallography or NMR techniques, there is little information about these structures available in the literature. Two structures of the P2X4 receptor in truncated form have been solved by crystallography. Molecular modeling has proven to be an excellent tool for studying ionotropic receptors. Recently, modeling studies carried out on P2X receptors have advanced our knowledge of the P2X receptor structure-function relationships. This review presents a brief history of ion channel structural studies and shows how modeling approaches can be used to address relevant questions about P2X receptors.

How cigarette smoking may increase the risk of anxiety symptoms and anxiety disorders: a critical review of biological pathways

Multiple studies have demonstrated an association between cigarette smoking and increased anxiety symptoms or disorders, with early life exposures potentially predisposing to enhanced anxiety responses in later life. Explanatory models support a potential role for neurotransmitter systems, inflammation, oxidative and nitrosative stress, mitochondrial dysfunction, neurotrophins and neurogenesis, and epigenetic effects, in anxiety pathogenesis. All of these pathways are affected by exposure to cigarette smoke components, including nicotine and free radicals. This review critically examines and summarizes the literature exploring the role of these systems in increased anxiety and how exposure to cigarette smoke may contribute to this pathology at a biological level. Further, this review explores the effects of cigarette smoke on normal neurodevelopment and anxiety control, suggesting how exposure in early life (prenatal, infancy, and adolescence) may predispose to higher anxiety in later life. A large heterogenous literature was reviewed that detailed the association between cigarette smoking and anxiety symptoms and disorders with structural brain changes, inflammation, and cell-mediated immune markers, markers of oxidative and nitrosative stress, mitochondrial function, neurotransmitter systems, neurotrophins and neurogenesis. Some preliminary data were found for potential epigenetic effects. The literature provides some support for a potential interaction between cigarette smoking, anxiety symptoms and disorders, and the above pathways; however, limitations exist particularly in delineating causative effects. The literature also provides insight into potential effects of cigarette smoke, in particular nicotine, on neurodevelopment. The potential treatment implications of these findings are discussed in regards to future therapeutic targets for anxiety. The aforementioned pathways may help mediate increased anxiety seen in people who smoke. Further research into the specific actions of nicotine and other cigarette components on these pathways, and how these pathways interact, may provide insights that lead to new treatment for anxiety and a greater understanding of anxiety pathogenesis.

Reduced Chrna7 expression in mice is associated with decreases in hippocampal markers of inhibitory function: implications for neuropsychiatric diseases

The α7* nicotinic acetylcholine receptor encoded by CHRNA7 (human)/Chrna7 (mice) regulates the release of both the inhibitory neurotransmitter GABA and the excitatory neurotransmitter glutamate in the hippocampal formation. A heterozygous (Het) deletion at 15q13.3 containing CHRNA7 is associated with increased risk for schizophrenia, autism, and epilepsy. Each of these diseases are characterized by abnormalities in excitatory and inhibitory hippocampal circuit function. Reduced Chrna7 expression results in decreased hippocampal α7* receptor density, abnormal hippocampal auditory sensory processing, and increased hippocampal CA3 pyramidal neuron activity in C3H mice Het for a null mutation in Chrna7. These abnormalities demonstrate that decreased Chrna7 expression alters hippocampal inhibitory circuit function. The current study examined the specific impact of reduced Chrna7 expression on hippocampal inhibitory circuits by measuring the levels of GABA, GABA(A) receptors, the GABA synthetic enzyme l-glutamic acid decarboxylase-65 (GAD-65), and the vesicular GABA transporter 1 (GAT-1) in wild-type (Chrna7 +/+) and Het (Chrna7 +/-) C3H α7 mice of both genders. GAD-65 levels were significantly decreased in male and female Het C3H α7 mice, whereas GABA(A) receptors were significantly reduced only in male Het C3H α7 mice. No changes in GABA and GAT-1 levels were detected. These data suggest that reduced CHRNA7 expression may contribute to the abnormalities in hippocampal inhibitory circuits observed in schizophrenia, autism, and/or epilepsy.