Increased neurodegeneration during ageing in mice lacking high-affinity nicotine receptors

Why do hair cells and spiral ganglion neurons in the cochlea die during aging?

Age-related decline of cochlear function is mainly due to the loss of hair cells and spiral ganglion neurons (SGNs). Recent findings clearly indicate that survival of these two cell types during aging depends on genetic and environmental interactions, and this relationship is seen at the systemic, tissue, cellular, and molecular levels. At cellular and molecular levels, age-related loss of hair cells and SGNs can occur independently, suggesting distinct mechanisms for the death of each during aging. This mechanistic independence is also observed in the loss of medial olivocochlear efferent innervation and outer hair cells during aging, pointing to a universal independent cellular mechanism for age-related neuronal death in the peripheral auditory system. While several molecular signaling pathways are implicated in the age-related loss of hair cells and SGNs, studies with the ability to locally modify gene expression in these cell types are needed to address whether these signaling pathways have direct effects on hair cells and SGNs during aging. Finally, the issue of whether age-related loss of these cells occurs via typical apoptotic pathways requires further examination. As new studies in the field of aging reshape the framework for exploring these underpinnings, understanding of the loss of hair cells and SGNs associated with age and the interventions that can treat and prevent these changes will result in dramatic benefits for an aging population.

Mouse models of genetic effects on cognition: relevance to schizophrenia

Cognitive dysfunction is a core feature of schizophrenia. Growing evidence indicates that a wide variety of genetic mutations and polymorphisms impact cognition and may thus be implicated in various aspects of this mental disorder. Despite differences between human and rodent brain structure and function, genetic mouse models have contributed critical information about brain mechanisms involved in cognitive processes. Here, we summarize discoveries of genetic modifications in mice that impact cognition. Based on functional hypotheses, gene modifications within five model systems are described: 1) dopamine (D1, D2, D3, D4, D5, DAT, COMT, MAO); 2) glutamate (GluR-A, NR1, NR2A, NR2B, GRM2, GRM3, GLAST); 3) GABA (α(5), γ(2), α(4), δGABA(A), GABA(B(1)), GAT1); 4) acetylcholine (nAChRβ2, α7, CHRM1); and 5) calcium (CaMKII-α, neurogranin, CaMKKβ, CaMKIV). We also consider other risk-associated genes for schizophrenia such as dysbindin (DTNBP1), neuregulin (NRG1), disrupted-in-schizophrenia1 (DISC1), reelin and proline dehydrogenase (PRODH). Because of the presumed importance of environmental factors, we further consider genetic modifications within the stress-sensitive systems of corticotropin-releasing factor (CRF), brain-derived neurotrophic factor (BDNF) and the endocannabinoid systems. We highlight the missing information and limitations of cognitive assays in genetically modified mice models relevant to schizophrenia pathology.

Cholinergic receptor subtypes and their role in cognition, emotion, and vigilance control: an overview of preclinical and clinical findings

RATIONALE

The cholinergic system has long been linked to cognitive processes. Two main classes of acetylcholine (ACh) receptors exist in the human brain, namely muscarinic and nicotinic receptors, of which several subtypes occur.

OBJECTIVES

This review seeks to provide an overview of previous findings on the influence of cholinergic receptor manipulations on cognition in animals and humans, with particular emphasis on the role of selected cholinergic receptor subtypes. Furthermore, the involvement of these receptor subtypes in the regulation of emotion and brain electrical activity as measured by electroencephalography (EEG) shall be addressed since these domains are considered to be important modulators of cognitive functioning.

RESULTS

In regard to cognition, the muscarinic receptor subtypes have been implicated mainly in memory functions, but have also been linked to attentional processes. The nicotinic α7 receptor subtype is involved in working memory, whereas the α4β2* subtype has been linked to tests of attention. Both muscarinic and nicotinic cholinergic mechanisms play a role in modulating brain electrical activity. Nicotinic receptors have been strongly associated with the modulation of depression and anxiety.

CONCLUSIONS

Cholinergic receptor manipulations have an effect on cognition, emotion, and brain electrical activity as measured by EEG. Changes in cognition can result from direct cholinergic receptor manipulation or from cholinergically induced changes in vigilance or affective state.

Mouse mutants for the nicotinic acetylcholine receptor ß2 subunit display changes in cell adhesion and neurodegeneration response genes

Mice lacking expression of the ß2 subunit of the neuronal nicotinic acetylcholine receptor (CHRNB2) display abnormal retinal waves and a dispersed projection of retinal ganglion cell (RGC) axons to their dorsal lateral geniculate nuclei (dLGNs). Transcriptomes of LGN tissue from two independently generated Chrnb2−/− mutants and from wildtype mice were obtained at postnatal day 4 (P4), during the normal period of segregation of eye-specific afferents to the LGN. Microarray analysis reveals reduced expression of genes located on the cell membrane or in extracellular space, and of genes active in cell adhesion and calcium signaling. In particular, mRNA for cadherin 1 (Cdh1), a known axon growth regulator, is reduced to nearly undetectable levels in the LGN of P4 mutant mice and Lypd2 mRNA is similarly suppressed. Similar analysis of retinal tissue shows increased expression of crumbs 1 (Crb1) and chemokine (C-C motif) ligand 21 (Ccl21) mRNAs in Chrnb2−/− mutant animals. Mutations in these genes are associated with retinal neuronal degeneration. The retinas of Chrnb2−/− mutants are normal in appearance, but the increased expression of these genes may also be involved in the abnormal projection patterns of RGC to the LGN. These data may provide the tools to distinguish the interplay between neural activity and molecular expression. Finally, comparison of the transcriptomes of the two different Chrnb2−/− mutant strains reveals the effects of genetic background upon gene expression.

The effects of aging vs. α7 nAChR subunit deficiency on the mouse brain transcriptome: aging beats the deficiency

Aging is accompanied by expression changes in multiple genes, and the brain is one of the tissues most vulnerable to aging. Since the α7 nicotinic acetylcholine receptor (nAChR) subunit has been associated with neurodevelopmental disorders and cognitive decline during aging, we hypothesized that its absence might affect gene expression profiles in aged brains. To study whether transcriptional changes occur due to aging, α7 deficiency, or both, we analyzed whole-brain transcriptomes of young (8 weeks) and aged (2 years) α7-deficient and wild-type control mice, using Mouse Genome 430 2.0 microarray. Highly significant expression changes were detected in 47 and 1,543 genes [after Bonferroni and false discovery rate (FDR) correction] in the brains of aged mice compared to young mice, regardless of their genotype. These included genes involved in immune system function and ribosome structure, as well as genes that were previously demonstrated as differentially expressed in aging human brains. Genotype-dependent changes were detected in only three genes, Chrna7 which encodes the α7 nAChR subunit, and two closely linked genes, likely due to a "mouse background effect." Expression changes dependent on age-genotype interaction were detected in 207 genes (with a low significance threshold). Age-dependent differential expression levels were approved in all nine genes that were chosen for validation by real-time RT-PCR. Our results suggest that the robust effect of aging on brain transcription clearly overcomes the almost negligible effect of α7 nAChR subunit deletion and that germ line deficiency of this subunit has a minor effect on brain expression profile in aged mice.

Old mice lacking high-affinity nicotine receptors resist acoustic trauma

There is presently no clearly effective preventative medication against noise-induced hearing loss (NIHL). However, negative feedback systems that presumably evolved to modulate the sensitivity of the organ of Corti may incidentally confer protection. One feedback system implicated in protection from NIHL involves synaptic connections between the lateral olivocochlear efferent terminals and the afferent fibers of spiral ganglion neurons (SGNs). These connections operate via high-affinity nicotinic acetylcholine receptors containing the β2 subunit. We unexpectedly observed protection from NIHL in 9-month old knockout mice lacking the β2 subunit (β2(-/-)); however, the same protection was not observed in 2-month old β2(-/-) mice. This enigmatic observation led to the discovery that protection from acoustic trauma in older β2(-/-) mice is mainly mediated by an age-related increase of corticosterone, not disruption of efferent cholinergic transmission. Significant protection of inner hair cells after acoustic trauma in β2(-/-) mice was linked to the activation of glucocorticoid signaling pathways. However, significant loss of SGNs was observed in animals with chronically high systemic levels of corticosterone. These results suggested a "double-edge sword" nature of glucocorticoid signaling in neuronal protection, and a need for caution regarding when to apply synthetic glucocorticoid drugs to treat neural injury such as accompanies acoustic trauma.

Ryanodine receptor-2 upregulation and nicotine-mediated plasticity

Nicotine, the major psychoactive component of cigarette smoke, modulates neuronal activity to produce Ca2+-dependent changes in gene transcription. However, the downstream targets that underlie the long-term effects of nicotine on neuronal function, and hence behaviour, remain to be elucidated. Here, we demonstrate that nicotine administration to mice upregulates levels of the type 2 ryanodine receptor (RyR2), a Ca2+-release channel present on the endoplasmic reticulum, in a number of brain areas associated with cognition and addiction, notably the cortex and ventral midbrain. Nicotine-mediated RyR2 upregulation was driven by CREB, and caused a long-lasting reinforcement of Ca2+ signalling via the process of Ca2+-induced Ca2+ release. RyR2 upregulation was itself required for long-term phosphorylation of CREB in a positive-feedback signalling loop. We further demonstrate that inhibition of RyR-activation in vivo abolishes sensitization to nicotine-induced habituated locomotion, a well-characterised model for onset of drug dependence. Our findings, therefore, indicate that gene-dependent reprogramming of Ca2+ signalling is involved in nicotine-induced behavioural changes.

II. Cognitive performance of middle-aged female rats is influenced by capacity to metabolize progesterone in the prefrontal cortex and hippocampus

Cognitive decline can occur with aging; however, some individuals experience less cognitive decline than do others. Secretion of ovarian hormones is reduced post-menopause and may contribute to cognitive function. The extent to which hormonal effects may be parsed out from other age-related factors to influence cognition is of interest. Middle-aged (12-month-old) female rats that were retired breeders were categorized as maintaining or declining reproductive function based upon their estrous cyclicity (regular 4-5 day cycles), fertility (> 60 % successful pregnancy), and fecundity (>10 pups/litter). Performance in object recognition, Y-maze, water maze, inhibitory avoidance, and contextual-cued fear conditioning was evaluated. Estradiol, progesterone (P(4)), dihydroprogesterone, and 5α-pregnan-3α-ol-20-one (3α,5α-THP) were assessed in medial prefrontal cortex (mPFC) and hippocampus; corticosterone was assessed in plasma. Rats maintaining reproductive function performed significantly better on the object recognition, Y-maze, water maze, inhibitory avoidance, and cued fear conditioning tasks than did rats with declining reproductive function. Steroid concentrations varied greatly within groups. Higher levels of P(4) in mPFC and hippocampus were associated with better Y-maze performance. In mPFC, higher levels of P(4) were associated with poorer inhibitory avoidance performance; greater levels of 3α,5α-THP were associated with better object memory. Neither estradiol nor corticosterone levels significantly contributed to cognitive performance. Thus, the capacity for cortico-limbic P(4) utilization may influence cognitive performance in aging.

Age-related loss of spiral ganglion neurons

Spiral ganglion neurons (SGNs) are the relay station for auditory information between hair cells and central nervous system. Age-related decline of auditory function due to SGN loss can not be ameliorated by hearing aids or cochlear implants. Recent findings clearly indicate that survival of SGNs during aging depends on genetic and environmental interactions, which can be demonstrated at the systemic, tissue, cellular, and molecular levels. At the systemic level, both insulin/insulin-like growth factor-1 and lipophilic/steroid hormone pathways influence SGN survival during aging. At the level of organ of the Corti, it is difficult to determine whether age-related SGN loss is primary or secondary degeneration. However, a late stage of SGN degeneration may be independent of age-related loss of hair cells. At the cellular and molecular level, several pathways, particularly free radical and calcium signaling pathways, can influence age-related SGN loss, and further studies should determine how these pathways contribute to SGN loss, such as whether they directly or indirectly act on SGNs. With the advancement of recent genetic and pharmacologic tools, we should not only understand how SGNs die during aging, but also find ways to delay this loss.

NMR structure of the transmembrane domain of the n-acetylcholine receptor beta2 subunit

Nicotinic acetylcholine receptors (nAChRs) are involved in fast synaptic transmission in the central and peripheral nervous system. Among the many different types of subunits in nAChRs, the beta2 subunit often combines with the alpha4 subunit to form alpha4beta2 pentameric channels, the most abundant subtype of nAChRs in the brain. Besides computational predictions, there is limited experimental data available on the structure of the beta2 subunit. Using high-resolution NMR spectroscopy, we solved the structure of the entire transmembrane domain (TM1234) of the beta2 subunit. We found that TM1234 formed a four-helix bundle in the absence of the extracellular and intracellular domains. The structure exhibited many similarities to those previously determined for the Torpedo nAChR and the bacterial ion channel GLIC. We also assessed the influence of the fourth transmembrane helix (TM4) on the rest of the domain. Although secondary structures and tertiary arrangements were similar, the addition of TM4 caused dramatic changes in TM3 dynamics and subtle changes in TM1 and TM2. Taken together, this study suggests that the structures of the transmembrane domains of these proteins are largely shaped by determinants inherent in their sequence, but their dynamics may be sensitive to modulation by tertiary and quaternary contacts.

Object memory in young and aged mice after sevoflurane anaesthesia

Learning and memory are cognitive functions commonly impaired after surgery, especially in elderly patients. Our aim was to evaluate the effect of sevoflurane anaesthesia on episodic-like memory in young and aged wild-type mice and mice with altered nicotinic cholinergic neurotransmission (beta2KO). Mice learned objects before randomization to control, anaesthesia or sham groups. Anaesthesia was maintained at 2.6% sevoflurane for 2 h, starting immediately after training. Object memory testing was performed after 24 h, when one familiar object was replaced by a nonfamiliar object. While nonanaesthetized mice showed memory retention of the familiar object, anaesthetized wild-type and beta2KO mice showed impaired memory. Sevoflurane anaesthesia thus causes memory impairment in mice regardless of beta2 receptor-mediated nicotinic cholinergic neurotransmission.

The role of glucocorticoids for spiral ganglion neuron survival

Glucocorticoids, which are steroidal stress hormones, have a broad array of biological functions. Synthetic glucocorticoids are frequently used therapeutically for many pathologic conditions, including diseases of the inner ear; however, their exact functions in the cochlea are not completely understood. Recent work has clearly demonstrated the presence of glucocorticoid signaling pathways in the cochlea and elucidated their protective roles against noise-induced hearing loss. Furthermore, indirect evidence suggests the involvement of glucocorticoids in age-related loss of spiral ganglion neurons and extensive studies in the central nervous system demonstrate profound effects of glucocorticoids on neuronal functions. With the advancement of recent pharmacologic and genetic tools, the role of these pathways in the survival of spiral ganglion neurons after noise exposure and during aging should be revealed.

Neuronal nicotinic receptors as novel targets for inflammation and neuroprotection: mechanistic considerations and clinical relevance

A number of studies have confirmed the potential for neuronal nicotinic acetylcholine receptor (NNR)-mediated neuroprotection and, more recently, its anti-inflammatory effects. The mechanistic overlap between these pathways and the ubiquitous effects observed following diverse insults suggest that NNRs modulate fundamental pathways involved in cell survival. These results have wide-reaching implications for the design of experimental therapeutics that regulate inflammatory and anti-apoptotic responses through NNRs and represent an initial step toward understanding the benefits of novel therapeutic strategies for the management of central nervous system disorders that target neuronal survival and associated inflammatory processes.

Sexual function in adult male rats after prenatal modulation of the cholinergic system

Prenatal administration of the n-cholinolytic ganglerone to pregnant female rats at different periods of gestation was found to lead to long-term changes in sexual behavior in pubescent offspring: there was a reduced dynamic of acquiring sexual experience and a very low level of sexual activity, with significant impairment to the motivational and ejaculatory components of sexual behavior. The number of males with reduced sexual activity in the experimental groups was significantly greater than that in control offspring. The results obtained here provide evidence that impairments of sexual function in adult offspring induced by prenatal administration of the n-cholinolytic ganglerone at 9-11 and 12-14 days of gestation and, to a lesser extent, the m-cholinolytic metamyzil at 9-11 days of gestation, were due to impairment to the central mechanisms regulating sexual function due to stable changes in neurotransmitter activity in the hippocampus and hypothalamus, along with a significant reduction in the blood testosterone level.

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.

Nicotinic acetylcholine receptor expression in the hippocampus of 27 mouse strains reveals novel inhibitory circuitry

Mouse strains are well-characterized to exhibit differences in their physiological and behavioral responses to nicotine. This report examines the expression of the high-affinity nicotine binding receptor subunit, neuronal nicotinic receptor subunit alpha 4 (nAChR alpha 4), in the dorsal hippocampus of 27 inbred mouse strains. Multiple differences among mouse strains in the cellular expression of nAChR alpha 4 between subregions of the hippocampal field are evident. Differences that we describe in the expression of nAChR alpha 4 suggest mouse strains of diverse genetic origin could exhibit significant variation in how this receptor contributes to modulating intrahippocampal circuitry. These findings define a genetic frame-work in which the strain-specific responses to nicotine include underlying contributions by the varied anatomical context in which nAChRs are expressed.

Thirty years of Alzheimer's disease genetics: the implications of systematic meta-analyses

The genetic underpinnings of Alzheimer's disease (AD) remain largely elusive despite early successes in identifying three genes that cause early-onset familial AD (those that encode amyloid precursor protein (APP) and the presenilins (PSEN1 and PSEN2)), and one genetic risk factor for late-onset AD (the gene that encodes apolipoprotein E (APOE)). A large number of studies that aimed to help uncover the remaining disease-related loci have been published in recent decades, collectively proposing or refuting the involvement of over 500 different gene candidates. Systematic meta-analyses of these studies currently highlight more than 20 loci that have modest but significant effects on AD risk. This Review discusses the putative pathogenetic roles and common biochemical pathways of some of the most genetically and biologically compelling of these potential AD risk factors.

Synapsin-I- and synapsin-II-null mice display an increased age-dependent cognitive impairment

Synapsin I (SynI) and synapsin II (SynII) are major synaptic vesicle (SV) proteins that function in the regulation of the availability of SVs for release in mature neurons. SynI and SynII show a high level of sequence similarity and share many functions in vivo, although distinct physiological roles for the two proteins have been proposed. Both SynI–/– and SynII–/– mice have a normal lifespan, but exhibit a decreased number of SVs and synaptic depression upon high-frequency stimulation. Because of the role of the synapsin proteins in synaptic organization and plasticity, we studied the long-lasting effects of synapsin deletion on the phenotype of SynI–/– and SynII–/– mice during aging. Both SynI–/– and SynII–/– mice displayed behavioural defects that emerged during aging and involved emotional memory in both mutants, and spatial memory in SynII–/– mice. These abnormalities, which were more pronounced in SynII–/– mice, were associated with neuronal loss and gliosis in the cerebral cortex and hippocampus. The data indicate that SynI and SynII have specific and non-redundant functions, and that synaptic dysfunctions associated with synapsin mutations negatively modulate cognitive performances and neuronal survival during senescence.

Subtypes of nicotinic acetylcholine receptors in nicotine reward, dependence, and withdrawal: evidence from genetically modified mice

Neuronal nicotinic acetylcholine receptors (nAChRs) can regulate the activity of many neurotransmitter pathways throughout the central nervous system and are considered to be important modulators of cognition and emotion. nAChRs are also the primary site of action in the brain for nicotine, the major addictive component of tobacco smoke. nAChRs consist of five membrane-spanning subunits (alpha and beta isoforms) that can associate in various combinations to form functional nAChR ion channels. Owing to a dearth of nAChR subtype-selective ligands, the precise subunit composition of the nAChRs that regulate the rewarding effects of nicotine and the development of nicotine dependence are unknown. The advent of mice with genetic nAChR subunit modifications, however, has provided a useful experimental approach to assess the contribution of individual subunits in vivo. Here, we review data generated from nAChR subunit knockout and genetically modified mice supporting a role for discrete nAChR subunits in nicotine reinforcement and dependence processes. Importantly, the rates of tobacco dependence are far higher in patients suffering from comorbid psychiatric illnesses compared with the general population, which may at least partly reflect disease-associated alterations in nAChR signaling. An understanding of the role of nAChRs in psychiatric disorders associated with high rates of tobacco addiction, therefore, may reveal novel insights into mechanisms of nicotine dependence. Thus, we also briefly review data generated from genetically modified mice to support a role for discrete nAChR subunits in anxiety disorders, depression, and schizophrenia.

Chronic nicotine exposure has dissociable behavioural effects on control and beta2-/- mice

Nicotine exerts beneficial effects on various neurological and psychiatric pathologies, yet its effects on cognitive performance remain unclear. Mice lacking the beta2 subunit of the nicotinic receptor (beta2-/-) show characteristic deficits in executive functions and are suggested as reliable animal models for some specific endophenotypes of human pathologies, notably ADHD. We use beta2-/- and their controls to investigate the consequences of chronic nicotine exposure on cognitive behaviour. We show that in control mice, this treatment elicits somewhat slight effects, particularly affecting nocturnal activity and self-grooming. By contrast, in beta2-/- mice, chronic nicotine treatment had restorative effects on exploratory behaviour in the open-field and affected rearing, but did not modify motor functions. We confirmed that beta2-/- mice exhibit impaired exploratory and social behaviour, and further demonstrated their nocturnal hyperactivity. These data support the proposal that beta2-/- mice represent a relevant model for cognitive disorders in humans and that nicotine administered chronically at low dose may relieve some of these.

Cognitive and functional status in the extreme longevity

Usually, the effects of cognitive decline are not noted before the age of 70 years, which involve the intellectual capacities, the attention, the processes of elaboration and the memory. The studies on the cognitive disturbances of the elderly are numerous, and document the progressive increase of cerebral deterioration with advancing age. However, only a few studies refer to the significance of the cognitive disturbances in the clinical conditions and autonomy of the long living subjects. For this reason, we studied the cerebral deterioration of an adequate number of centenarians in correlation with their clinical conditions and autonomy. Our centenarian sample derived from the Italian multi-center study on centenarians (IMUSCE), which was an epidemiological study which identified 1173 centenarians (202 males, 971 females) in the age range of 100-109 years. From this sample, we analyzed 346 subjects as far as the cognitive functions and the degree of autonomy by using the psychometric tests of the mini-mental state examination (MMSE) and the instrumental activities of daily living (IADL) for the functional evaluations. In addition, we evaluated the clinical conditions of the subjects dividing them in three groups: Group A (those in good clinical conditions), Group B (those in discrete clinical conditions), and Group C (those in deteriorated clinical conditions). These analyses revealed that 187 (54.1%) of the 346 examined centenarians have shown an MMSE score in the normal range (score ratio from 1.0 to 0.63). The cognitive disorders are present in the centenarians in a clearly higher frequency (13.1%), than found in the common elderly (5.1%). The severe cognitive disorders do not allow a total autonomy or even a slight dependency. Only six subjects (1.7%) of the total sample were totally independent. These subjects had no cognitive disorders, and were in good clinical conditions. The results show that having an MMSE score in the normal range, and being in good clinical conditions are necessary but not sufficient prerequisites for a total autonomy in the IADL scores.

Inflammation, genes and zinc in Alzheimer's disease

Alzheimer's disease (AD) is a heterogeneous and progressive neurodegenerative disease which in Western society mainly accounts for clinical dementia. AD has been linked to inflammation and metal biological pathway. Neuro-pathological hallmarks are senile plaques, resulting from the accumulation of several proteins and an inflammatory reaction around deposits of amyloid, a fibrillar protein, Abeta, product of cleavage of a much larger protein, the beta-amyloid precursor protein (APP) and neurofibrillary tangles. Amyloid deposition, due to the accumulation of Abeta peptide, is the main pathogenetic mechanism. Inflammation clearly occurs in pathologically vulnerable regions of AD and several inflammatory factors influencing AD development, i.e. environmental factors (pro-inflammatory phenotype) and/or genetic factors (pro-inflammatory genotype) have been described. At the biochemical level metals such as zinc are known to accelerate the aggregation of the amyloid peptide and play a role in the control of inflammatory responses. In particular, zinc availability may regulate mRNA cytokine expression, so influencing inflammatory network phenotypic expression.

Acetylcholinesterase inhibitors may improve myelin integrity

Recent clinical trials have revealed that cholinergic treatments are efficacious in a wide spectrum of neuropsychiatric disorders that span the entire human lifespan and include disorders without cholinergic deficits. Furthermore, some clinical and epidemiological data suggest that cholinergic treatments have disease modifying/preventive effects. It is proposed that these observations can be usefully understood in a myelin-centered model of the human brain. The model proposes that the human brain's extensive myelination is the central evolutionary change that defines our uniqueness as a species and our unique vulnerability to highly prevalent neuropsychiatric disorders. Within the framework of this model the clinical, biochemical, and epidemiologic data can be reinterpreted to suggest that nonsynaptic effects of cholinergic treatments on the process of myelination and myelin repair contributes to their mechanism of action and especially to their disease modifying/preventive effects. The ability to test the model in human populations with safe and noninvasive imaging technologies makes it possible to undertake novel clinical trial efforts directed at primary prevention of some of the most prevalent and devastating of human disorders.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Loss of high‐affinity nicotinic receptors increases the vulnerability to excitotoxic lesion and decreases the positive effects of an enriched environment

Pharmacological activation of nicotinic acetylcholine receptors (nAChRs) exerts neuroprotective effects in cultured neurons and the intact animal. Much less is known about a physiological protective role of nAChRs. To understand whether endogenous activation of beta2* nAChRs contributes to the maintenance of the functional and morphological integrity of neural tissue, adult beta2-/- mice were subjected to in vivo challenges that cause neurodegeneration and cognitive impairment (intrahippocampal injection of the excitotoxin quinolinic acid), or neuroprotection and cognitive potentiation (2-month exposure to an enriched environment). The excitotoxic insult caused an increased deficit in the Morris water maze learning curve and increased loss of hippocampal pyramidal cells in beta2-/- mice. Exposure to an enriched environment improved performance in contextual and cued fear conditioning and object recognition tests in beta2+/+, whereas the improvement was absent in beta2-/- mice. In addition, beta2+/+, but not beta2-/-, mice exposed to an enriched environment showed a significant hypertrophy of the CA1/3 regions. Thus, lack of beta2* nAChRs increased susceptibility to an excitotoxic insult and diminished the positive effects of an enriched environment. These results may be relevant to understanding the pathophysiological consequences of the marked decrease in nAChRs that occurs in neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.

Genetics of nicotinic acetylcholine receptors: Relevance to nicotine addiction

Human twin studies have suggested that there is a substantial genetic component underlying nicotine dependence, ongoing smoking and ability to quit. Similarly, animal studies have identified a number of genes and gene products that are critical for behaviors related to nicotine addiction. Classical genetic approaches, gene association studies and genetic engineering techniques have been used to identify the gene products involved in nicotine dependence. One class of genes involved in nicotine-related behavior is the family of nicotinic acetylcholine receptors (nAChRs). These receptors are the primary targets for nicotine in the brain. Genetic engineering studies in mice have identified a number of subunits that are critical for the ability of nicotine to activate the reward system in the brain, consisting of the dopaminergic cell bodies in the ventral tegmental area and their terminals in the nucleus accumbens and other portions of the mesolimbic system. In this review we will discuss the various lines of evidence suggesting that nAChRs may be involved in smoking behavior, and will review the human and animal studies that have been performed to date examining the genetic basis for nicotine dependence and smoking.

Effects of TC-1734 (AZD3480), a selective neuronal nicotinic receptor agonist, on cognitive performance and the EEG of young healthy male volunteers

OBJECTIVES

The aim of this study was to get insight into the central effects of TC-1734 (renamed AZD3480), a selective agonist at the neuronal nicotinic receptor of the alpha4beta2 subtype.

MATERIALS AND METHODS

Electroencephalography (EEG) techniques and computerized cognitive tests were performed in young, healthy male volunteers during two double-blind and placebo-controlled studies: a rising single dose crossover study (from 2 to 320 mg) and a rising repeated dose study with a parallel group design (50, 100, and 200 mg).

RESULTS

In contrast to acute administration, administration of AZD3480 over 10 days produced statistically significant enhancement of several cognitive measures (attention and episodic memory) compared to placebo. Regarding EEG data, AZD3480 showed acceleration of the alpha centroid and of the alpha peak in the single-dose study. This EEG profile of the acceleration type was confirmed in the repeated dose study on both day 1 and day 10, with the greatest effect observed with the highest dose. The EEG pattern shown for AZD3480 was consistent with that previously described with other drugs known to improve attention and vigilance (including nicotine). In addition, subjects dosed with AZD3480 showed a statistically significant increase in mismatch negativity (MMN) amplitude at 50 and 200 mg while reducing MMN latency (200 mg only), suggesting an improvement of pre-attentional mechanisms.

CONCLUSION

These early data in healthy subjects provide encouragement to consider development of AZD3480 as a novel agent for the treatment of cognitive decline in the elderly, including age-associated memory impairment and/or dementia of the Alzheimer's type.

Nicotine gates long-term potentiation in the hippocampal CA1 region via the activation of alpha2* nicotinic ACh receptors

Hippocampal CA1 pyramidal cells receive two major excitatory synaptic inputs via the Schaffer collateral (SC) and temporoammonic (TA) pathways. Nicotine promotes induction of long-term potentiation (LTP) in the SC path; however, it is not known whether the modulatory effect of nicotine on LTP induction is pathway-specific. Here we show that nicotine suppresses LTP induction in the TA path. Interestingly, these opposing effects of nicotine were absent or greatly reduced in alpha2 nicotinic acetylcholine receptor (nAChR)-knockout (KO) mice, suggesting that an alpha2-containing nAChR subtype mediates these effects. Optical imaging with a voltage-sensitive dye revealed significantly stronger membrane depolarization in the presence of nicotine in the SC path, facilitating spread of excitatory neural activity along both the somatodendritic and the CA1 proximodistal axes. These effects of nicotine were also absent in alpha2 nAChR-KO mice, suggesting that the enhanced optical signal is related to the nicotine-induced facilitation of LTP induction. In contrast, in the TA path nicotine terminated depolarization more quickly and increased the delayed hyperpolarization in the termination zone of the TA path input. These inhibitory effects of nicotine were absent in alpha2 nAChR-KO mice and, thus, most probably underlie the nicotine-induced suppression of LTP induction. Our results suggest that nicotine influences the local balance between excitation and inhibition, gates LTP, and directs information flow through the hippocampal circuits via the activation of alpha2* nAChRs. These effects of nicotine may represent the cellular basis of nicotine-mediated cognitive enhancement.

Chronic nicotine-induced switch in Src-family kinase signaling for long-term potentiation induction in hippocampal CA1 pyramidal cells

Here, we show that chronic nicotine exposure induces changes in Src signaling for the modulation of N-methyl-D-aspartate receptor (NMDAR) function and LTP induction in CA1 pyramidal cells. Activation of muscarinic receptors normally potentiates NMDAR responses in pyramidal cells via a Gq/protein kinase C (PKC)/proline-rich tyrosine kinase 2/Src signaling cascade. However, muscarinic, PKC and Src stimulation had no effect on NMDAR responses after chronic nicotine treatment. The lack of effect was apparently due to enhanced tyrosine phosphorylation, and therefore further stimulation of the signaling cascade caused no effect on NMDAR responses. Interestingly, another Src-family kinase potentiated NMDAR responses after, but not before, chronic nicotine treatment. In control pyramidal cells, Src inhibitor peptides prevented tetanus-induced long-term potentiation (LTP). Conversely, in chronic nicotine-exposed cells, the inhibitor was ineffective in blocking tetanus-induced LTP. Furthermore, in control pyramidal cells, applying exogenous Src and administration of an endogenous Src-family kinase activator increased alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR)-mediated responses. This increase was blocked by Src inhibitor peptides and occluded tetanus-induced LTP, as reported previously. In contrast, in chronic nicotine-treated pyramidal cells, applying exogenous Src had no effect on AMPAR-mediated responses and a tetanus-induced LTP. Interestingly, however, administration of an endogenous Src-family kinase activator enhanced AMPAR-mediated responses, which occluded tetanus-induced LTP. This enhancement was not prevented by co-application of Src inhibitor peptides. Thus, it appears that chronic nicotine exposure recruits another member of the Src-family for the regulation of NMDAR function and LTP induction. The nicotine-induced distinct signaling cascades may be involved in long-lasting memories of nicotine misuse.

Emerging concepts: novel integration of in vivo approaches to localize the function of nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are important targets of the neuromodulator acetylcholine (ACh) and the drug nicotine. The role of their different subunits has been analysed for a decade by the creation of knock-out (KO) mice using homologous recombination. This technique shows that a given subunit is necessary for a given function. However, for ubiquitously expressed genes, it cannot demonstrate the localization for a given subunit in which its expression is sufficient, especially for behavioural phenotypes. Sufficient in this context means that the brain region requiring the expression of the gene product has been localized. Novel strategies have therefore been developed to re-express, region specifically, nAChR subunits on a KO background using lentiviral vectors. Localized regeneration of fully functional high-affinity nAChRs in defined brain regions has proven that these receptors are sufficient to restore a variety of functions: nicotine-induced dopamine release, nicotine self-administration in mice, dopamine neuron firing patterns, and exploratory and locomotor behaviours in a sequential locomotor task testing executive function were thus defined as depending exclusively on the 'knock-back' of beta2*-nAChRs into the ventral tegmental area. These analyses highlight the important role of endogenous cholinergic regulation of a variety of functions. The novel integrated use of restricted re-expressed nAChR subunits with in vivo electrophysiology and automated quantitative behavioural analysis enables the further analysis of defined neuronal circuits in nicotine addiction and higher cognitive function.

Impaired attention is central to the cognitive deficits observed in alpha 7 deficient mice

alpha7-Nicotinic acetylcholine receptors (alpha7-nAChR) have been implicated in a range of cognitive deficits in schizophrenia. Therefore we examined alpha7-nAChR knockout (KO), heterozygote (HT) and wildtype (WT) littermate mice in the 5-CSR (a rodent model of sustained attention) and odour span (a novel mouse working memory paradigm) tasks, and related performance to nAChR density. Whilst there was no difference between groups in baseline 5-CSR task performance, alpha7-nAChR KO's exhibited significantly higher omission levels compared to WT mice on increasing the attentional load, with HT mice performing at an intermediate level. Furthermore, alpha7-nAChR KO mice were significantly impaired in the odour span task when compared to WT mice, in a pattern consistent with impaired attention. These behavioural deficits were associated with the loss of alpha7-nAChRs, as alpha4beta2-nAChR density was unaltered in these mice. Thus these studies intimate that the attentional impairment in alpha7-nAChR transgenic mice maybe core to other deficits in cognition.

Smoking and structural brain deficits: a volumetric MR investigation

Growing evidence from animal studies indicates brain-damaging properties of nicotine exposure. Investigations in humans found a wide range of functional cerebral effects of nicotine and cigarette smoking, but studies focusing on brain damage are sparse. In 22 smokers and 23 never-smokers possible differences of the cerebral structures were investigated using magnetic resonance imaging and voxel-based morphometry. Significantly smaller grey matter volume and lower grey matter density (P = 0.05, corrected) were observed in the frontal regions (anterior cingulate, prefrontal and orbitofrontal cortex), the occipital lobe and the temporal lobe including parahippocampal gyrus, in smokers than in never-smokers. Group differences of either grey matter volume or grey matter density were also found in the thalamus, cerebellum and substantia nigra, among other regions. Smokers did not show greater volumes than never-smokers in any cerebral region. Magnitude of lifetime exposure to tobacco smoke (pack-years) was inversely correlated with volume of frontal and temporal lobes and cerebellum (P = 0.001, uncorrected). The data indicate structural deficits of several cortical and subcortical regions in smokers relative to never-smokers. The topographic profile of the group differences show some similarities to brain networks known to mediate drug reinforcement, attention and working memory processing. The present findings may explain in part the frequently reported cognitive dysfunctions in chronic cigarette consumers.

Reversible inhibition of GABAA receptors by alpha7-containing nicotinic receptors on the vertebrate postsynaptic neurons

Nicotinic acetylcholine receptors (nAChRs) are expressed throughout the central nervous system and influence a variety of higher order functions including learning and memory. While the effects of presynaptic nAChRs on transmitter release have been well documented, little is known about possible postsynaptic actions. A major species of neuronal nAChRs contains the alpha7 gene product and has a high relative permeability to calcium. Both on rodent hippocampal interneurons and on chick ciliary ganglion neurons these alpha7-nAChRs are often closely juxtaposed to GABAA receptors. We show here that in both cases activation of alpha7-nAChRs on the postsynaptic neuron acutely down-regulates GABA-induced currents. Nicotine application to dissociated ciliary ganglion neurons diminished subsequent GABAA receptor responses to GABA. The effect was blocked by alpha7-nAChR antagonists, by chelation of intracellular Ca2+ with BAPTA, and by inhibition of both Ca2+-calmodulin-dependent protein kinase II and mitogen-activated protein kinase. A similar outcome was obtained in the hippocampus where electrical stimulation to activate cholinergic fibres reduced the amplitude of subsequent GABAA receptor-mediated inhibitory postsynaptic currents. The reduction showed the same calcium and kinase dependence seen in ciliary ganglion neurons and was absent in hippocampal slices from alpha7-nAChR knockout mice. Moreover, alpha7-nAChR blockade in hippocampal slices reduced rundown of GABAA receptor-mediated whole-cell responses, indicating ongoing endogenous modulation. The results demonstrate regulation of GABAA receptors by alpha7-nAChRs on the postsynaptic neuron and identify a new mechanism by which nicotinic cholinergic signalling influences nervous system function.

The Ferrier Lecture 1998. The molecular biology of consciousness investigated with genetically modified mice

The question is raised of the relevance of experimental work with the mouse and some of its genetically modified individuals in the study of consciousness. Even if this species does not go far beyond the level of 'minimal consciousness', it may be a useful animal model to examine the elementary building blocks of consciousness using the methods of molecular biology jointly with investigations at the physiological and behavioural levels. These building blocks which are anticipated to be universally shared by higher organisms (from birds to humans) may include: (i) the access to multiple states of vigilance, like wakefulness, sleep, general anaesthesia, etc.; (ii) the capacity for global integration of several sensory and cognitive functions, together with behavioural flexibility resulting in what is referred to as exploratory behaviour, and possibly a minimal form of intentionality. In addition, the contribution of defined neuronal nicotinic receptors species to some of these processes is demonstrated and the data discussed within the framework of recent neurocomputational models for access to consciousness.

Pharmacokinetics and safety profile of ispronicline (TC-1734), a new brain nicotinic receptor partial agonist, in young healthy male volunteers

Recent research suggests that drugs activating nicotine acetylcholine receptors might be promising therapy in cognitive decline seen in the elderly, including Alzheimer's disease. Ispronicline (TC-1734), a brain-selective alpha4beta2 nicotine acetylcholine receptor partial agonist, has shown memory-enhancing properties in rodents and a good tolerability profile. The safety and the full pharmacokinetic profile of TC-1734 and its N-desalkylated metabolite, TC-1784, were investigated in 2 phase I studies, and results are reported in this article. Study A used a double-blind, placebo-controlled, crossover design with a rising single-dose scheme (2-320 mg). Study B used a double-blind, placebo-controlled, parallel-group design with a rising multiple-dose scheme (doses: 50, 100, and 200 mg, once daily, x 10 days). Cmax of TC-1734 was reached around 1 to 2 hours postdose, and mean terminal half-life (t1/2) ranged from 3 to 5.3 hours (single doses) and from 2.7 to 8.8 hours (repeated doses). No accumulation of TC-1734 was observed after 10 days. Renal clearance appeared to be a minor method of elimination of TC-1734 and TC-1784. A high interindividual variability was noted for all parameters. Across the dose ranges explored, TC-1734 was safe and well tolerated. No changes of clinical significance were seen on laboratory and cardiovascular parameters. Adverse events were generally of mild to moderate intensity, with dizziness and headache being reported most frequently.

Neuroprotective effects of behavioural training and nicotine on age-related deficits in spatial learning

Studies in humans and animals show a clear decline in spatial memory with age and several approaches have been adopted to alleviate this impairment. The purpose of our review is to assess the studies that have suggested the possible neuroprotective actions of behavioural training and nicotine-applied both independently and in conjunction-on age-related deficits in spatial learning. Both spatial pretraining and nonspatial experiences influence an animal's performance in spatial tasks. In aged rats, the experience of training in the water maze task increases the number of newly generated neurons in the hippocampus. The neuroprotective effects of nicotine have been demonstrated in both in-vitro and in-vivo models, although the molecular mechanisms underlying these actions are not yet fully understood. It had been concluded in different studies that nicotine can improve, impair or have no effect on performance in the water maze. Neurobiological data also suggest an interaction between nicotine and prior experience in complex tasks, although few studies have raised the question of whether nicotine treatment and training in spatial tasks may contribute in an interactive manner to alleviate spatial cognition impairment associated with the ageing process. Different findings suggest that past experience could be a confounding variable in longitudinal studies that aim to evaluate the neuroprotective effects of nicotine on age-related deficits in spatial learning.

The Prototoxin lynx1 Acts on Nicotinic Acetylcholine Receptors to Balance Neuronal Activity and Survival In Vivo

Nicotinic acetylcholine receptors (nAChRs) affect a wide array of biological processes, including learning and memory, attention, and addiction. lynx1, the founding member of a family of mammalian prototoxins, modulates nAChR function in vitro by altering agonist sensitivity and desensitization kinetics. Here we demonstrate, through the generation of lynx1 null mutant mice, that lynx1 modulates nAChR signaling in vivo. Its loss decreases the EC(50) for nicotine by approximately 10-fold, decreases receptor desensitization, elevates intracellular calcium levels in response to nicotine, and enhances synaptic efficacy. lynx1 null mutant mice exhibit enhanced performance in specific tests of learning and memory. Consistent with reports that mutations resulting in hyperactivation of nAChRs can lead to neurodegeneration, aging lynx1 null mutant mice exhibit a vacuolating degeneration that is exacerbated by nicotine and ameliorated by null mutations in nAChRs. We conclude that lynx1 functions as an allosteric modulator of nAChR function in vivo, balancing neuronal activity and survival in the CNS.

Requirement of nicotinic acetylcholine receptor subunit beta2 in the maintenance of spiral ganglion neurons during aging

Age-related hearing loss (presbycusis) is a major health concern for the elderly. Loss of spiral ganglion neurons (SGNs), the primary sensory relay of the auditory system, is associated consistently with presbycusis. The causative molecular events responsible for age-related loss of SGNs are unknown. Recent reports directly link age-related neuronal loss in cerebral cortex with the loss of high-affinity nicotine acetylcholine receptors (nAChRs). In cochlea, cholinergic synapses are made by olivocochlear efferent fibers on the outer hair cells that express alpha9 nAChR subunits and on the peripheral projections of SGNs that express alpha2, alpha4-7, and beta2-3 nAChR subunits. A significantly decreased expression of the beta2 nAChR subunit in SGNs was found specifically in mice susceptible to presbycusis. Furthermore, mice lacking the beta2 nAChR subunit (beta2-/-), but not mice lacking the alpha5 nAChR subunit (alpha5-/-), have dramatic hearing loss and significant reduction in the number of SGNs. Our findings clearly established a requirement for beta2 nAChR subunit in the maintenance of SGNs during aging.

The beta2 but not alpha7 subunit of the nicotinic acetylcholine receptor is required for nicotine-conditioned place preference in mice

RATIONALE

Tobacco use is implicated in approximately 440,000 deaths per year, making it the leading cause of preventable death in the United States. Although it is generally recognized that tobacco use is correlated with a variety of health-related complications, many smokers are unsuccessful in their efforts to stop smoking using current cessation therapies.

OBJECTIVES

Given that nicotine is the addictive component of tobacco, successful smoking cessation therapies must address the various processes, including reward, which contribute to nicotine addiction. As such, determining the nicotinic receptor subtypes involved in nicotine reward is of utmost importance to understanding how nicotine addiction progresses.

METHODS

Conditioned place preference (CPP) in three-chamber conditioning boxes was performed. For antagonist studies, drug was given on all conditioning sessions 10 min before nicotine or saline injection and placement in the boxes.

RESULTS

We have demonstrated that a pretreatment with the alpha4beta2 subunit of the nicotinic acetylcholine receptor (nAChR) antagonist dihydro-beta-erythroidine (2.0 mg/kg, s.c.) blocked nicotine (0.5 mg/kg, s.c.) CPP in wild-type mice (C57BL/6 mice). In contrast, pretreatment with an antagonist of the alpha7 subunit of the nAChR, methyllycaconitine (MLA, 5.0 or 10.0 mg/kg, s.c.), had no effect on this behavior. Finally, we showed that mice lacking the beta2 subunit of the nAChR did not exhibit nicotine CPP while alpha7 knock-out mice did.

CONCLUSION

Taken together, these data suggest that the beta2 subunit of the nAChR is critically involved in nicotine reward as measured by CPP.

The role of nicotinic acetylcholine receptors in lymphocyte development

The sizes of lymphocyte populations in lymphoid organs of nicotinic acetylcholine receptor knockout and chimera (knockout/wild-type) mice were studied by flow cytometry. The absence of beta2 subunit decreased, while nicotine treatment increased B lymphocyte numbers in the bone marrow. In chimera mice, either beta2 or alpha7 subunits influenced lymphocyte populations in primary lymphoid organs, while in the spleen, only alpha7 receptors were critical. More annexin V-positive B cells were found in the bone marrow of knockout than wild-type animals. We conclude that nicotinic receptors are involved in regulating lymphocyte development and control the B lymphocyte survival.

Attenuation of Abeta deposition in the entorhinal cortex of normal elderly individuals associated with tobacco smoking

Investigating correlates of tobacco smoking provides the only currently available opportunity of examining effects of long-term exposure of nicotinic receptors on a specific nicotinic agonist in human. Alzheimer-type pathology (Abeta and abnormally phosphorylated tau assessed on the basis of AT8 immunoreactivity) together with vascular markers has been compared in age-matched groups of normal elderly smokers and non-smokers in the entorhinal cortex, an area of noted age-related pathology. The density of total Abeta and diffuse Abeta immunoreactivity, together with formic acid-extractable Abeta42 but not Abeta40, was reduced in smokers (n = 10-18) compared with non-smokers (n = 10-20) (P < 0.05). There was also a reduced percentage of cortical and leptomeningeal vessels with associated Abeta immunoreactivity in smokers (n = 13) compared with non-smokers (n = 14) (P < 0.005 and 0.05, respectively). There was a significant inverse correlation between formic acid-extractable Abeta42 and pack years (n = 34, r = -0.389, P = 0.025), with a similar trend for total Abeta immunoreactivity which did not reach statistical significance (n = 30, r = -0.323, P = 0.082). In contrast, there were no significant group differences for vascular markers (collagen IV, alpha-actin or glucose transporter 1), AT8 immunoreactivity or phosphate-buffered saline-soluble Abeta peptides, and no significant associations with gender for any of the measured parameters. These findings are consistent with previously reported reductions in histologically assessed amyloid plaques in aged human brain associated with tobacco use and dramatic lessening of Abeta deposits in APPsw mice after nicotine treatment. Development of nicotinic drugs to protect against beta-amyloidosis as one of the principal pathological hallmarks of brain ageing and Alzheimer's disease is indicated.

Nicotine reinforcement and cognition restored by targeted expression of nicotinic receptors

Worldwide, 100 million people are expected to die this century from the consequences of nicotine addiction, but nicotine is also known to enhance cognitive performance. Identifying the molecular mechanisms involved in nicotine reinforcement and cognition is a priority and requires the development of new in vivo experimental paradigms. The ventral tegmental area (VTA) of the midbrain is thought to mediate the reinforcement properties of many drugs of abuse. Here we specifically re-expressed the beta2-subunit of the nicotinic acetylcholine receptor (nAChR) by stereotaxically injecting a lentiviral vector into the VTA of mice carrying beta2-subunit deletions. We demonstrate the efficient re-expression of electrophysiologically responsive, ligand-binding nicotinic acetylcholine receptors in dopamine-containing neurons of the VTA, together with the recovery of nicotine-elicited dopamine release and nicotine self-administration. We also quantified exploratory behaviours of the mice, and showed that beta2-subunit re-expression restored slow exploratory behaviour (a measure of cognitive function) to wild-type levels, but did not affect fast navigation behaviour. We thus demonstrate the sufficient role of the VTA in both nicotine reinforcement and endogenous cholinergic regulation of cognitive functions.

Repeated nicotine exposure in rats: effects on memory function, cholinergic markers and nerve growth factor

A decrease in the number of nicotinic-acetylcholine receptors (nAChRs) in the brain is thought to contribute to the cognitive dysfunction associated with diseases as diverse as Alzheimer's disease and schizophrenia. Interestingly, nicotine and similar compounds have been shown to enhance memory function and increase the expression of nAChRs and therefore, could have a therapeutic role in the aforementioned diseases. Nicotine has also been shown to exert positive effects on certain neurotrophins such as nerve growth factor (NGF), and therefore could play a role beyond mere symptomatic therapy. However, to date, comprehensive studies of nicotine's effects on the expression of specific acetylcholine (ACh) receptor subtypes, key cholinergic proteins (that are regulated by NGF) such as choline acetyltransferase (ChAT) and the vesicular ACh transporter (VAChT) are lacking. Studies to further investigate the effects of nicotine on NGF especially its high- and low-affinity receptors are also needed. In the present study, male Wistar rats exposed a relatively low dosage of nicotine (0.35 mg/kg every 12 h) for 14 days demonstrated improved memory performance (assessed in two separate water maze testing methods) when compared with controls. Autoradiographic experiments indicated that nicotine increased [3H]-epibatidine, [125I]-alpha-bungarotoxin and [3H]-AFDX384, but not [3H]-pirenzepine binding sites in several learning- and memory-related brain areas. The expression of ChAT, VAChT, as well as tropomyosin-receptor kinase A (TrkA) NGF receptors and phospho-TrK receptors was increased by nicotine in the hippocampus. No changes were observed in the levels of the NGF peptide or low affinity p75 neurotrophin receptors (p75NTR), however. These results suggest that repeated exposure to nicotine results in positive effects on central cholinergic markers and memory function, which may be mediated via effects on high-affinity NGF receptors.

Neuronal nicotinic receptors: from structure to pathology

Neuronal nicotinic receptors (NAChRs) form a heterogeneous family of ion channels that are differently expressed in many regions of the central nervous system (CNS) and peripheral nervous system. These different receptor subtypes, which have characteristic pharmacological and biophysical properties, have a pentameric structure consisting of the homomeric or heteromeric combination of 12 different subunits (alpha2-alpha10, beta2-beta4). By responding to the endogenous neurotransmitter acetylcholine, NAChRs contribute to a wide range of brain activities and influence a number of physiological functions. Furthermore, it is becoming evident that the perturbation of cholinergic nicotinic neurotransmission can lead to various diseases involving nAChR dysfunction during development, adulthood and ageing. In recent years, it has been discovered that NAChRs are present in a number of non-neuronal cells where they play a significant functional role and are the pathogenetic targets in several diseases. NAChRs are also the target of natural ligands and toxins including nicotine (Nic), the most widespread drug of abuse. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining their regional and cellular localisation and the molecular basis of their functional diversity mainly in pharmacological and biochemical terms. The recent availability of mice with the genetic ablation of single or double nicotinic subunits or point mutations have shed light on the role of nAChRs in major physiological functions, and we will here discuss recent data relating to their behavioural phenotypes. Finally, the role of NAChRs in disease will be considered in some details.

Effects of smoking and smoking abstinence on cognition in adolescent tobacco smokers

BACKGROUND

In adult animals and humans, nicotine can produce short-term cognitive enhancement and, in some cases, neuroprotection. Recent work in animals, however, suggests that exposure to nicotine during adolescence might be neurotoxic. We tested for evidence of acute and chronic effects of tobacco smoking on cognition in adolescents who smoked tobacco daily and were compared with adolescent nonsmokers.

METHODS

Verbal working memory, verbal learning and memory, selective, divided, sustained attention, mood, symptoms of nicotine withdrawal, and tobacco craving were examined in 41 adolescent daily smokers and 32 nonsmokers who were similar in age, gender, and education. Analyses were controlled for general intelligence, reading achievement, parental educational attainment, baseline affective symptoms, and lifetime exposure to alcohol and cannabis.

RESULTS

In adolescent smokers, cessation of tobacco use increased tobacco craving, symptoms of nicotine withdrawal, and depressed mood. Adolescent smokers were found to have impairments in accuracy of working memory performance irrespective of recency of smoking. Performance decrements were more severe with earlier age of onset of smoking. Adolescent smokers experienced further disruption of working memory and verbal memory during smoking cessation. As a group, male smokers initiated smoking at an earlier age than female smokers and were significantly more impaired during tests of selective and divided attention than female smokers and nonsmokers.

CONCLUSIONS

Adolescent daily tobacco smokers experience acute impairments of verbal memory and working memory after smoking cessation, along with chronic decrements in cognitive performance that are consistent with preclinical evidence that neurotoxic effects of nicotine are more severe when exposure to nicotine occurs at earlier periods in development.