Fear conditioning and latent inhibition in mice lacking the high affinity subclass of nicotinic acetylcholine receptors in the brain

Fear Conditioning by Proxy: The Role of High Affinity Nicotinic Acetylcholine Receptors

Observational fear-learning studies in genetically modified animals enable the investigation of the mechanisms underlying the social transmission of fear-related information. Here, we used a three-day protocol to examine fear conditioning by proxy (FCbP) in wild-type mice (C57BL/6J) and mice lacking the β2-subunit of the nicotinic acetylcholine receptor (nAChR). Male animals of both genotypes were exposed to a previously fear-conditioned (FC) cage mate during the presentation of the conditioned stimulus (CS, tone). On the following day, observer (FCbP) mice were tested for fear reactions to the tone: none of the β2-KO mice froze to the stimulus, while 30% of the wild-type mice expressed significant freezing. An investigation of the possible factors that predicted the fear response revealed that only wild-type mice that exhibited enhanced and more flexible social interaction with the FC cage mate during tone presentations (Day 2) expressed fear toward the CS (Day-3). Our results indicate that (i) FCbP is possible in mice; (ii) the social transmission of fear depends on the interaction pattern between animals during the FCbP session and (iii) β2-KO mice display a more rigid interaction pattern compared to wild-type mice and are unable to acquire such information. These data suggest that β2-nAChRs influence observational fear learning indirectly through their effect on social behaviour.

Nicotinic acetylcholine receptors and learning and memory deficits in Neuroinflammatory diseases

Animal survival depends on cognitive abilities such as learning and memory to adapt to environmental changes. Memory functions require an enhanced activity and connectivity of a particular arrangement of engram neurons, supported by the concerted action of neurons, glia, and vascular cells. The deterioration of the cholinergic system is a common occurrence in neurological conditions exacerbated by aging such as traumatic brain injury (TBI), posttraumatic stress disorder (PTSD), Alzheimer's disease (AD), and Parkinson's disease (PD). Cotinine is a cholinergic modulator with neuroprotective, antidepressant, anti-inflammatory, antioxidant, and memory-enhancing effects. Current evidence suggests Cotinine's beneficial effects on cognition results from the positive modulation of the α7-nicotinic acetylcholine receptors (nAChRs) and the inhibition of the toll-like receptors (TLRs). The α7nAChR affects brain functions by modulating the function of neurons, glia, endothelial, immune, and dendritic cells and regulates inhibitory and excitatory neurotransmission throughout the GABA interneurons. In addition, Cotinine acting on the α7 nAChRs and TLR reduces neuroinflammation by inhibiting the release of pro-inflammatory cytokines by the immune cells. Also, α7nAChRs stimulate signaling pathways supporting structural, biochemical, electrochemical, and cellular changes in the Central nervous system during the cognitive processes, including Neurogenesis. Here, the mechanisms of memory formation as well as potential mechanisms of action of Cotinine on memory preservation in aging and neurological diseases are discussed.

Implications of Oligomeric Amyloid-Beta (oAβ42) Signaling through α7β2-Nicotinic Acetylcholine Receptors (nAChRs) on Basal Forebrain Cholinergic Neuronal Intrinsic Excitability and Cognitive Decline

Neuronal and network-level hyperexcitability is commonly associated with increased levels of amyloid-β (Aβ) and contribute to cognitive deficits associated with Alzheimer's disease (AD). However, the mechanistic complexity underlying the selective loss of basal forebrain cholinergic neurons (BFCNs), a well-recognized characteristic of AD, remains poorly understood. In this study, we tested the hypothesis that the oligomeric form of amyloid-β (oAβ₄₂), interacting with α7-containing nicotinic acetylcholine receptor (nAChR) subtypes, leads to subnucleus-specific alterations in BFCN excitability and impaired cognition. We used single-channel electrophysiology to show that oAβ₄₂ activates both homomeric α7- and heteromeric α7β2-nAChR subtypes while preferentially enhancing α7β2-nAChR open-dwell times. Organotypic slice cultures were prepared from male and female ChAT-EGFP mice, and current-clamp recordings obtained from BFCNs chronically exposed to pathophysiologically relevant level of oAβ₄₂ showed enhanced neuronal intrinsic excitability and action potential firing rates. These resulted from a reduction in action potential afterhyperpolarization and alterations in the maximal rates of voltage change during spike depolarization and repolarization. These effects were observed in BFCNs from the medial septum diagonal band and horizontal diagonal band, but not the nucleus basalis. Last, aged male and female APP/PS1 transgenic mice, genetically null for the β2 nAChR subunit gene, showed improved spatial reference memory compared with APP/PS1 aged-matched littermates. Combined, these data provide a molecular mechanism supporting a role for α7β2-nAChR in mediating the effects of oAβ₄₂ on excitability of specific populations of cholinergic neurons and provide a framework for understanding the role of α7β2-nAChR in oAβ₄₂-induced cognitive decline.

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.

A threshold model for opposing actions of acetylcholine on reward behavior: Molecular mechanisms and implications for treatment of substance abuse disorders

The cholinergic system plays important roles in both learning and addiction. Medications that modify cholinergic tone can have pronounced effects on behaviors reinforced by natural and drug reinforcers. Importantly, enhancing the action of acetylcholine (ACh) in the nucleus accumbens and ventral tegmental area (VTA) dopamine system can either augment or diminish these behaviors. A threshold model is presented that can explain these seemingly contradictory results. Relatively low levels of ACh rise above a lower threshold, facilitating behaviors supported by drugs or natural reinforcers. Further increases in cholinergic tone that rise above a second upper threshold oppose the same behaviors. Accordingly, cholinesterase inhibitors, or agonists for nicotinic or muscarinic receptors, each have the potential to produce biphasic effects on reward behaviors. Pretreatment with either nicotinic or muscarinic antagonists can block drug- or food- reinforced behavior by maintaining cholinergic tone below its lower threshold. Potential threshold mediators include desensitization of nicotinic receptors and biphasic effects of ACh on the firing of medium spiny neurons. Nicotinic receptors with high- and low- affinity appear to play greater roles in reward enhancement and inhibition, respectively. Cholinergic inhibition of natural and drug rewards may serve as mediators of previously described opponent processes. Future studies should evaluate cholinergic agents across a broader range of doses, and include a variety of reinforced behaviors.

Nicotinic receptors, memory, and hippocampus

Nicotinic acetylcholine receptors (nAChRs) modulate the neurobiological processes underlying hippocampal learning and memory. In addition, nicotine's ability to desensitize and upregulate certain nAChRs may alter hippocampus-dependent memory processes. Numerous studies have examined the effects of nicotine on hippocampus-dependent learning, as well as the roles of low- and high-affinity nAChRs in mediating nicotine's effects on hippocampus-dependent learning and memory. These studies suggested that while acute nicotine generally acts as a cognitive enhancer for hippocampus-dependent learning, withdrawal from chronic nicotine results in deficits in hippocampus-dependent memory. Furthermore, these studies demonstrated that low- and high-affinity nAChRs functionally differ in their involvement in nicotine's effects on hippocampus-dependent learning. In the present chapter, we reviewed studies using systemic or local injections of acute or chronic nicotine, nAChR subunit agonists or antagonists; genetically modified mice; and molecular biological techniques to characterize the effects of nicotine on hippocampus-dependent learning.

Sleep pattern and learning in knockdown mice with reduced cholinergic neurotransmission

Impaired cholinergic neurotransmission can affect memory formation and influence sleep-wake cycles (SWC). In the present study, we describe the SWC in mice with a deficient vesicular acetylcholine transporter (VAChT) system, previously characterized as presenting reduced acetylcholine release and cognitive and behavioral dysfunctions. Continuous, chronic ECoG and EMG recordings were used to evaluate the SWC pattern during light and dark phases in VAChT knockdown heterozygous (VAChT-KDHET, n=7) and wild-type (WT, n=7) mice. SWC were evaluated for sleep efficiency, total amount and mean duration of slow-wave, intermediate and paradoxical sleep, as well as the number of awakenings from sleep. After recording SWC, contextual fear-conditioning tests were used as an acetylcholine-dependent learning paradigm. The results showed that sleep efficiency in VAChT-KDHET animals was similar to that of WT mice, but that the SWC was more fragmented. Fragmentation was characterized by an increase in the number of awakenings, mainly during intermediate sleep. VAChT-KDHET animals performed poorly in the contextual fear-conditioning paradigm (mean freezing time: 34.4±3.1 and 44.5±3.3 s for WT and VAChT-KDHET animals, respectively), which was followed by a 45% reduction in the number of paradoxical sleep episodes after the training session. Taken together, the results show that reduced cholinergic transmission led to sleep fragmentation and learning impairment. We discuss the results on the basis of cholinergic plasticity and its relevance to sleep homeostasis. We suggest that VAChT-KDHET mice could be a useful model to test cholinergic drugs used to treat sleep dysfunction in neurodegenerative disorders.

Cellular, molecular, and genetic substrates underlying the impact of nicotine on learning

Addiction is a chronic disorder marked by long-lasting maladaptive changes in behavior and in reward system function. However, the factors that contribute to the behavioral and biological changes that occur with addiction are complex and go beyond reward. Addiction involves changes in cognitive control and the development of disruptive drug-stimuli associations that can drive behavior. A reason for the strong influence drugs of abuse can exert on cognition may be the striking overlap between the neurobiological substrates of addiction and of learning and memory, especially areas involved in declarative memory. Declarative memories are critically involved in the formation of autobiographical memories, and the ability of drugs of abuse to alter these memories could be particularly detrimental. A key structure in this memory system is the hippocampus, which is critically involved in binding multimodal stimuli together to form complex long-term memories. While all drugs of abuse can alter hippocampal function, this review focuses on nicotine. Addiction to tobacco products is insidious, with the majority of smokers wanting to quit; yet the majority of those that attempt to quit fail. Nicotine addiction is associated with the presence of drug-context and drug-cue associations that trigger drug seeking behavior and altered cognition during periods of abstinence, which contributes to relapse. This suggests that understanding the effects of nicotine on learning and memory will advance understanding and potentially facilitate treating nicotine addiction. The following sections examine: (1) how the effects of nicotine on hippocampus-dependent learning change as nicotine administration transitions from acute to chronic and then to withdrawal from chronic treatment and the potential impact of these changes on addiction, (2) how nicotine usurps the cellular mechanisms of synaptic plasticity, (3) the physiological changes in the hippocampus that may contribute to nicotine withdrawal deficits in learning, and (4) the role of genetics and developmental stage (i.e., adolescence) in these effects.

Examining the sex- and circadian dependency of a learning phenotype in mice with glycine transporter 1 deletion in two Pavlovian conditioning paradigms

Behavioural characterisation of transgenic mice has been instrumental in search of therapeutic targets for the modulation of cognitive function. However, little effort has been devoted to phenotypic characterisation across environmental conditions and genomic differences such as sex and strain, which is essential to translational research. The present study is an effort in this direction. It scrutinised the stability and robustness of the phenotype of enhanced Pavlovian conditioning reported in mice with forebrain neuronal deletion of glycine transporter 1 by evaluating the possible presence of sex and circadian dependency, and its consistency across aversive and appetitive conditioning paradigms. The Pavlovian phenotype was essentially unaffected by the time of testing between the two circadian phases, but it was modified by sex in both conditioning paradigms. We observed that the effect size of the phenotype was strongest in female mice tested during the dark phase in the aversive paradigm. Critically, the presence of the phenotype in female mutants was accompanied by an increase in resistance to extinction. Similarly, enhanced conditioned responding once again emerged solely in female mutants in the appetitive conditioning experiment, which was again associated with an increased resistance to extinction across days, but male mutants exhibited an opposite trend towards facilitation of extinction. The present study has thus added hitherto unknown qualifications and specifications of a previously reported memory enhancing phenotype in this mouse line by identifying the determinants of the magnitude and direction of the expressed phenotype. This in-depth comparative approach is of value to the interpretation of behavioural findings in general.

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.

Transient exposure of neonatal mice to neuregulin-1 results in hyperdopaminergic states in adulthood: implication in neurodevelopmental hypothesis for schizophrenia

Neuregulin-1 (NRG1) is implicated in the etiology or pathology of schizophrenia, although its biological roles in this illness are not fully understood. Human midbrain dopaminergic neurons highly express NRG1 receptors (ErbB4). To test its neuropathological role in the neurodevelopmental hypothesis of schizophrenia, we administered type-1 NRG1 protein to neonatal mice and evaluated the immediate and subsequent effects on dopaminergic neurons and their associated behaviors. Peripheral NRG1 administration activated midbrain ErbB4 and elevated the expression, phosphorylation and enzyme activity of tyrosine hydroxylase (TH), which ultimately increased dopamine levels. The hyperdopaminergic state was sustained in the medial prefrontal cortex after puberty. There were marked increases in dopaminergic terminals and TH levels. In agreement, higher amounts of dopamine were released from this brain region of NRG1-treated mice following high potassium stimulation. Furthermore, NRG1-treated mice exhibited behavioral impairments in prepulse inhibition, latent inhibition, social behaviors and hypersensitivity to methamphetamine. However, there were no gross abnormalities in brain structures or other phenotypic features of neurons and glial cells. Collectively, our findings provide novel insights into neurotrophic contribution of NRG1 to dopaminergic maldevelopment and schizophrenia pathogenesis.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Nicotine addiction and nicotinic receptors: lessons from genetically modified mice

The past decades have seen a revolution in our understanding of brain diseases and in particular of drug addiction. This has been largely due to the identification of neurotransmitter receptors and the development of animal models, which together have enabled the investigation of brain functions from the molecular to the cognitive level. Tobacco smoking, the principal - yet avoidable - cause of lung cancer is associated with nicotine addiction. Recent studies in mice involving deletion and replacement of nicotinic acetylcholine receptor subunits have begun to identify the molecular mechanisms underlying nicotine addiction and might offer new therapeutic strategies to treat this addiction.

Involvement of hippocampal jun-N terminal kinase pathway in the enhancement of learning and memory by nicotine

Despite intense scrutiny over the past 20 years, the reasons for the high addictive liability of nicotine and extreme rates of relapse in smokers have remained elusive. One factor that contributes to the development and maintenance of nicotine addiction is the ability of nicotine to produce long-lasting modifications of behavior, yet little is known about the mechanisms by which nicotine alters the underlying synaptic plasticity responsible for behavioral changes. This study is the first to explore how nicotine interacts with learning to alter gene transcription, which is a process necessary for long-term memory consolidation. Transcriptional upregulation of hippocampal jun-N terminal kinase 1 (JNK1) mRNA was found in mice that learned contextual fear conditioning (FC) in the presence of nicotine, whereas neither learning alone nor nicotine administration alone exerted an effect. Furthermore, the upregulation of JNK1 was absent in beta2 nicotinic receptor subunit knockout mice, which are mice that do not show enhanced learning by nicotine. Finally, hippocampal JNK activation was increased in mice that were administered nicotine before conditioning, and the inhibition of JNK during consolidation prevented the nicotine-induced enhancement of contextual FC. These data suggest that nicotine and learning interact to alter hippocampal JNK1 gene expression and related signaling processes, thus resulting in strengthened contextual memories.

Interleukin-1 receptor null mutant mice show decreased anxiety-like behavior and enhanced fear memory

IL-1beta is a proinflammatory cytokine that contributes to psychological stress responses and has been implicated in various psychiatric disorders most notably depression. Preclinical studies also demonstrate that IL-1beta modulates anxiety- and fear-related behaviors, although these findings are difficult to assess because IL-1beta infusions influence locomotor activity and nociception. Here we demonstrate that IL-1RI null mice exhibit a behavioral phenotype consistent with a decrease in anxiety-related behaviors. This includes significant effects in the elevated plus maze, light-dark, and novelty-induced hypophagia tests compared to wild-type mice, with no differences in locomotor activity. With regard to fear conditioning, IL-1RI null mice showed more freezing in auditory and contextual fear conditioning tests, and there was no effect on pain sensitivity. Taken together, the results indicate that the IL-1beta/IL-1RI signaling pathway induces anxiety-related behaviors and impairs fear memory.

Towards an animal model of an antipsychotic drug-resistant cognitive impairment in schizophrenia: scopolamine induces abnormally persistent latent inhibition, which can be reversed by cognitive enhancers but not by antipsychotic drugs

Schizophrenia symptoms segregate into positive, negative and cognitive, which exhibit differential sensitivity to drugs. Recent efforts to identify treatments targeting cognitive impairments in schizophrenia have directed attention to the cholinergic system for its well documented role in cognition. Relatedly, muscarinic antagonists (e.g. scopolamine) produce an 'antimuscarinic syndrome', characterized by psychosis and cognitive impairments. Latent inhibition (LI) is the poorer conditioning to a stimulus resulting from its non-reinforced pre-exposure. LI indexes the ability to ignore irrelevant stimuli and aberrations of this capacity produced by pro-psychotic agents (e.g. amphetamine, MK-801) are used extensively to model attentional impairments in schizophrenia. We recently showed that LI was disrupted by scopolamine at low doses, and this was reversed by typical and atypical antipsychotic drugs (APDs) and the acetylcholinesterase inhibitor physostigmine. Here, at a higher dose (1.5 mg/kg), scopolamine produced an opposite pole of attentional impairment, namely, attentional perseveration, whereby scopolamine-treated rats persisted in expressing LI under strong conditioning that prevented LI expression in controls. Scopolamine-induced persistent LI was reversed by cholinergic and glycinergic cognitive enhancers (physostigmine and glycine) but was resistant to both typical and atypical APDs (haloperidol and clozapine). The latter sets scopolamine-induced persistent LI apart from scopolamine- and amphetamine-induced disrupted LI, which are reversed by both typical and atypical APDs, as well as from other cases of abnormally persistent LI including MK-801-induced persistent LI, which is reversed by atypical APDs. Thus, scopolamine-induced persistent LI may provide a pharmacological LI model for screening cognitive enhancers that are efficient for the treatment of APD-resistant cognitive impairments in schizophrenia.

Sex differences in nicotine action

Accumulating evidence suggests that the antecedents, consequences, and mechanisms of drug abuse and dependence are not identical in males and females and that gender may be an important variable in treatment and prevention. Although there has been a decline in smoking prevalence in developed countries, females are less successful in quitting. Tobacco use is accepted to be a form of addiction, which manifests sex differences. There is also evidence for sex differences in the central effects of nicotine in laboratory animals. Although social factors impact smoking substantially in humans, findings from nonhuman subjects in controlled experiments provide support that sex differences in nicotine/tobacco addiction have a biological basis. Differences in the pharmacokinetic properties of nicotine or the effect of gonadal hormones may underlie some but not all sex differences observed. Laboratory-based information is very important in developing treatment strategies. Literature findings suggest that including sex as a factor in nicotine/tobacco-related studies will improve our success rates in individually tailored smoking cessation programs.

Nicotine self-medication of cognitive-attentional processing

This article selectively reviews research concerning nicotine's effects on cognition, including the neurobiological mechanism for these effects, task and experimental features that may be important for elucidating these effects, and why these effects may have amplified motivational significance among smokers with cognitive deficit. Nicotine has effects on various cognitive processes, though most studies in humans have focused on the amelioration of cognitive deficits experienced during drug withdrawal. The direct cognitive-enhancing effect of nicotine remains a controversial topic. The relationship between attentional and non-attentional cognitive effects of nicotine is discussed in the context of cognitive self-medication. Further research should include theory-driven examination of cognitive effects of nicotine, and develop targeted smoking cessation programs based on an improved understanding of the role of cognitive self-medication in high-risk individuals.

Fluctuation of latent inhibition along the estrous cycle in the rat: modeling the cyclicity of symptoms in schizophrenic women?

Latent inhibition (LI) is a cross-species selective attention phenomenon manifested as poorer conditioning of stimuli that had been experienced as irrelevant prior to conditioning. Disruption of LI by pro-psychotic agents such as amphetamine and its restoration by antipsychotic drugs (APDs) is a well-established model of psychotic symptoms of schizophrenia. There is evidence that in schizophrenic women symptom severity and treatment response fluctuate along the menstrual cycle. Here we tested whether hormonal fluctuation along the estrous cycle in female rats (as determined indirectly via the cellular composition of the vaginal smears) would modulate the expression of LI and its response to APDs. The results showed that LI was seen if rats were in estrus during pre-exposure stage and in metestrus during the conditioning stage of the LI procedure (estrus-metestrus) but not along the remaining sequential phases of the cycle (metestrus-diestrus, diestrus-proestrus and proestrus-estrus). Additionally, the efficacy of typical and atypical APDs, haloperidol and clozapine, respectively, in restoring LI depended on estrous condition. Only LI disruption in proestrus-estrus exhibited sensitivity to both APDs, whereas LI disruption in the other two phases was alleviated by clozapine but not haloperidol. Our results show for the first time that both the expression of LI and its sensitivity to APDs are modulated along the estrous cycle, consistent with fluctuations in psychotic symptoms and response to APDs seen along women's menstrual cycle. Importantly, the results indicate that although both low and high levels of hormones may give rise to psychotic-like behavior as manifested in LI loss, the pro-psychotic state associated with low hormonal level is more severe due to reduced sensitivity to typical APDs. The latter constellation may mimic states of increased vulnerability to psychosis coupled with reduced treatment response documented in schizophrenic women during periods associated with low levels of hormones.

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.

Modulation of hippocampus-dependent learning and synaptic plasticity by nicotine

A long-standing relationship between nicotinic acetylcholine receptors (nAChRs) and cognition exists. Drugs that act at nAChRs can have cognitive-enhancing effects and diseases that disrupt cognition such as Alzheimer's disease and schizophrenia are associated with altered nAChR function. Specifically, hippocampus-dependent learning is particularly sensitive to the effects of nicotine. However, the effects of nicotine on hippocampus-dependent learning vary not only with the doses of nicotine used and whether nicotine is administered acutely, chronically, or withdrawn after chronic nicotine treatment but also vary across different hippocampus-dependent tasks such as the Morris water maze, the radial arm maze, and contextual fear conditioning. In addition, nicotine has variable effects across different types of hippocampal long-term potentiation (LTP). Because different types of hippocampus-dependent learning and LTP involve different neural and molecular substrates, comparing the effects of nicotine across these paradigms can yield insights into the mechanisms that may underlie the effects of nicotine on learning and memory and aid in understanding the variable effects of nicotine on cognitive processes. This review compares and contrasts the effects of nicotine on hippocampus-dependent learning and LTP and briefly discusses how the effects of nicotine on learning could contribute to nicotine addiction.

The subtypes of nicotinic acetylcholine receptors on dopaminergic terminals of mouse striatum

This review summarizes studies that attempted to determine the subtypes of nicotinic acetylcholine receptors (nAChR) expressed in the dopaminergic nerve terminals in the mouse. A variety of experimental approaches has been necessary to reach current knowledge of these subtypes, including in situ hybridization, agonist and antagonist binding, function measured by neurotransmitter release from synaptosomal preparations, and immunoprecipitation by selective antibodies. Early developments that facilitated this effort include the radioactive labeling of selective binding agents, such as [(125)I]-alpha-bungarotoxin and [(3)H]-nicotine, advances in cloning the subunits, and expression and evaluation of function of combinations of subunits in Xenopus oocytes. The discovery of epibatidine and alpha-conotoxin MII (alpha-CtxMII), and the development of nAChR subunit null mutant mice have been invaluable in determining which nAChR subunits are important for expression and function in mice, as well as allowing validation of the specificity of subunit specific antibodies. These approaches have identified five nAChR subtypes of nAChR that are expressed on dopaminergic nerve terminals. Three of these contain the alpha6 subunit (alpha4alpha6beta2beta3, alpha6beta2beta3, alpha6beta2) and bind alpha-CtxMII with high affinity. One of these three subtypes (alpha4alpha6beta2beta3) also has the highest sensitivity to nicotine of any native nAChR that has been studied, to date. The two subtypes that do not have high affinity for alpha-CtxMII (alpha4beta2, alpha4alpha5beta2) are somewhat more numerous than the alpha6* subtypes, but do bind nicotine with high affinity. Given that our first studies detected readily measured differences in sensitivity to agonists and antagonists among these five nAChR subtypes, it seems likely that subtype selective compounds could be developed that would allow therapeutic manipulation of diverse nAChRs that have been implicated in a number of human conditions.

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.

Beta2 subunit containing acetylcholine receptors mediate nicotine withdrawal deficits in the acquisition of contextual fear conditioning

Acute nicotine enhances contextual fear conditioning, whereas withdrawal from chronic nicotine produces impairments. However, the nicotinic acetylcholine receptors (nAChR) that are involved in nicotine withdrawal deficits in contextual fear conditioning are unknown. The present study used genetic and pharmacological techniques to investigate the nAChR subtype(s) involved in the effects of nicotine withdrawal on contextual fear conditioning. beta2 or alpha 7 nAChR subunit knockout (KO) and corresponding wild-type (WT) mice were withdrawn from 12 days of chronic nicotine treatment (6.3mg/kg/day), and trained with 2 conditioned stimulus (CS; 85 dB white noise)--unconditioned stimulus (US; 0.57 mA footshock) pairings on day 13. On day 14, mice were tested for contextual and cued freezing. beta2 KO mice did not show nicotine withdrawal-related deficits in contextual fear conditioning, in contrast to WT mice and alpha 7 KO mice. A follow-up study investigated if nicotine withdrawal disrupts acquisition or recall of contextual fear conditioning. The high affinity nAChR antagonist dihydro-beta-erythroidine (DH beta E; 3mg/kg) was administered prior to training or testing to precipitate withdrawal in chronic nicotine-treated C57BL/6 mice. Deficits in contextual fear conditioning were observed in chronic nicotine-treated mice when DH beta E was administered prior to training, but not when administered at testing. These results indicate that beta2-containing nAChRs, such as the alpha 4 beta 2 receptor, mediate nicotine withdrawal deficits in contextual fear conditioning. In addition, nicotine withdrawal selectively affects acquisition but not recall or expression of the learned response.

Nicotine and hippocampus-dependent learning: implications for addiction

Addiction is a complex disorder because many factors contribute to the development and maintenance of addiction. One factor is learning. For example, drug-context associations that develop during drug use could facilitate drug craving upon re-exposure to contexts previously associated with drugs. Additionally, deficits in cognitive processes associated with withdrawal could precipitate relapse in attempts to ameliorate those deficits. Because addiction and learning involve common neural areas and cell signaling cascades, addiction-related changes in processes underlying plasticity may contribute to addiction. This article examines similarities between addiction and learning at the behavioral, neural, and cellular levels, with emphasis on the neural substrates underlying the effects of acute nicotine, chronic nicotine, and withdrawal from chronic nicotine on hippocampus-dependent contextual learning.

beta2 subunit-containing nicotinic receptors mediate the enhancing effect of nicotine on trace cued fear conditioning in C57BL/6 mice

RATIONALE

Previous research indicates that acute nicotine administration enhances the acquisition of contextual fear conditioning and trace cued fear conditioning. Pharmacological inhibition of alpha4beta2 nicotinic acetylcholine receptors (nAChRs), but not alpha7 nAChRs, blocked the enhancing effect of nicotine on contextual fear conditioning. Similarly, genetic deletion of the beta2 nAChR subunit but not the alpha7 nAChR subunit blocked the enhancing effect of nicotine on contextual fear conditioning.

OBJECTIVES

In the present study, nAChR subunit knockout mice were used to compare the involvement of beta2 subunit-containing nAChRs and alpha7 subunit-containing nAChRs in the effects of nicotine on hippocampus-dependent trace cued fear conditioning and contextual fear conditioning.

METHODS

beta2 nAChR subunit knockout mice, alpha7 nAChR subunit knockout mice, and their wild-type littermates received either nicotine or saline 5 minutes before training and testing. Mice were trained using five conditioned stimulus (CS; 30 s, 85 dB white noise)--trace (30 s)--unconditioned stimulus (US; 2 s footshock) pairings. Freezing to the context and freezing to the CS were assessed 24 h later.

RESULTS

Both contextual and trace cued fear conditioning were enhanced by nicotine administration in wild-type littermates and in alpha7 nAChR subunit knockout mice. In contrast, neither contextual fear conditioning nor trace cued fear conditioning was enhanced by nicotine administration in beta2 nAChR subunit knockout mice.

CONCLUSIONS

These results suggest that beta2 subunit-containing nAChRs but not alpha7 nAChR subunit-containing nAChRs are critically involved in the enhancing effect of nicotine on contextual and trace cued fear conditioning.

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.

Cholinergic modulation of Pavlovian fear conditioning in rats: differential effects of intrahippocampal infusion of mecamylamine and methyllycaconitine

The cholinergic system has consistently been implicated in Pavlovian fear conditioning. Considerable work has been done to localize specific nicotinic receptor subtypes in the hippocampus and determine their functional importance; however, the specific function of many of these subtypes has yet to be determined. An alpha7 nicotinic antagonist methyllycaconitine (MLA) (35 microg), and a broad spectrum non-alpha7 nicotinic antagonist mecamylamine (35 microg) was injected directly into the dorsal hippocampus or overlying cortex either 15 min pre-, 1 min post-, or 6h post-fear conditioning. One week after conditioning, retention of contextual and cue (tone) conditioning were assessed. A significant impairment in retention of contextual fear was observed when mecamylamine was injected 15 min pre- and 1 min post-conditioning. No significant impairment was observed when mecamylamine was injected 6h post-conditioning. Likewise, a significant impairment in retention of contextual fear was observed when MLA was injected 1 min post-conditioning; however, in contrast, MLA did not show any significant impairments when injected 15 min pre-conditioning, but did show a significant impairment when injected 6h post-conditioning. There were no significant impairments observed when either drug was injected into overlying cortex. No significant impairments were observed in cue conditioning for either drug. In general, specific temporal dynamics involved in nicotinic receptor function were found relative to time of receptor dysfunction. The results indicate that the greatest deficits in long-term retention (1 week) of contextual fear are produced by central infusion of MLA minutes to hours post-conditioning or mecamylamine within minutes of conditioning.

Schizophrenia-relevant behavioral testing in rodent models: a uniquely human disorder?

Animal models are extremely useful tools in defining pathogenesis and treatment of human disease. Creating adequate animal models of complex neuropsychiatric disorders such as schizophrenia represents a particularly difficult challenge. In the case of schizophrenia, little is certain regarding the etiology or pathophysiology of the human disease. In addition, many symptoms of the disorder are difficult to measure directly in rodents. These challenges have not daunted neuroscientists who are capitalizing on even subtle overlaps between this uniquely human disorder and rodent behavior. In this perspective, we detail the features of ideal animal models of schizophrenia, the potential utility of such models, and the rodent behaviors used to model certain aspects of schizophrenia. The development of such models will provide critical tools to understand the pathogenesis of schizophrenia and novel insights into therapeutic approaches to this complex disorder.

TNFalpha signaling in depression and anxiety: behavioral consequences of individual receptor targeting

BACKGROUND

Increased serum levels of TNFalpha and other pro-inflammatory cytokines have been found in patients with major depression and several other psychiatric conditions. In rodents, these cytokines produce symptoms commonly referred to as "sickness behavior." Some of these, including reduced feeding and decreased social and exploratory behavior, are reminiscent of those seen in depressed patients. Interpretation of these effects is complicated by the malaise caused by acute injections of pro-inflammatory cytokines, however. Thus, it is unclear whether cytokines are involved in the etiology of depressive symptoms.

METHODS

We used a panel of behavioral assays to assess TNFR1(-/-) and TNFR2(-/-) mice for anxiety and depression-like behaviors.

RESULTS

We show that deletion of either TNFR1 or TNFR2 leads to an antidepressant-like response in the forced swim test and that mice lacking TNFR2 demonstrate a hedonic response in a sucrose drinking test compared with wildtype littermates. In addition, deletion of TNFR1 leads to decreased fear conditioning. There were no differences in behavior in anxiety tests for either null mutant.

CONCLUSIONS

These results are consistent with the hypothesis that TNFalpha can induce depression-like symptoms even in the absence of malaise and demonstrate that both receptor subtypes can be involved in this response.

The effects of DHBE and MLA on nicotine-induced enhancement of contextual fear conditioning in C57BL/6 mice

RATIONALE

Previous research indicates that nicotine administration enhances hippocampus-dependent forms of learning, including contextual fear conditioning. This effect is blocked by mecamylamine, a noncompetitive, broad-spectrum nicotinic receptor antagonist.

OBJECTIVES

The present study extends previous research by further characterizing the nicotinic acetylcholinergic receptor (nAChR) subtypes through which nicotine acts to enhance contextual fear conditioning.

METHODS

C57BL/6J mice were trained with two conditioned stimulus (CS; 30 s, 85-dB white noise)-unconditioned stimulus (US; 2 s, 0.57-mA foot shock) pairings and tested 24 h later for contextual and cued fear conditioning. The effects of the alpha7 nAChR antagonist methyllycaconitine (MLA; 1.00, 10.00, and 20.00 mg/kg) and the effects of the alpha4beta2 nAChR antagonist dihydro-beta-erythroidine (DHBE; 1.00, 3.00, and 6.00 mg/kg) on cued and contextual fear conditioning and on the enhancement of contextual fear conditioning by nicotine (0.25 mg/kg) were examined.

RESULTS

We demonstrate that DHBE (all doses) administration attenuates the enhancing effect of nicotine on contextual fear conditioning, and MLA administration has no significant effect on the enhancement of contextual fear conditioning by nicotine.

CONCLUSIONS

The data suggest that non-alpha7 nAChRs (most likely alpha4beta2 nAChRs) underlie the enhancement of contextual fear conditioning by nicotine.

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.

Impaired synaptic plasticity and learning in mice lacking beta-adducin, an actin-regulating protein

The adducin family of proteins interacts with the actin cytoskeleton and the plasma membrane in a calcium- and cAMP-dependent manner. Thus, adducins may be involved in changes in cytoskeletal organization resulting from synaptic stimulation. beta-Adducin knock-out mice were examined in physiological and behavioral paradigms related to synaptic plasticity to elucidate the role the adducin family plays in processes underlying learning and memory. In situ hybridization for alpha- and beta-adducin demonstrates that these mRNAs are found throughout the brain, with high levels of expression in the hippocampus. Schaffer collateral-CA1 tetanic long-term potentiation decayed rapidly in acute hippocampal slices from beta-adducin knock-out mice, although baseline spine morphology and postsynaptic density were normal. Interestingly, the input-output relationship was significantly increased in hippocampal slices from beta-adducin knock-out mice. Furthermore, beta-adducin knock-out mice were impaired in performance of fear conditioning and the water maze paradigm. The current results indicate that beta-adducin may play an important role in the cellular mechanisms underlying activity-dependent synaptic plasticity associated with learning and memory.

Coantagonism of glutamate receptors and nicotinic acetylcholinergic receptors disrupts fear conditioning and latent inhibition of fear conditioning

The present study investigated the hypothesis that both nicotinic acetylcholinergic receptors (nAChRs) and glutamate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs) and N-methyl-d-aspartate glutamate receptors (NMDARs)) are involved in fear conditioning, and may modulate similar processes. The effects of the nAChR antagonist mecamylamine administered alone, the AMPAR antagonist NBQX administered alone, and the NMDAR antagonist MK-801 administered alone on cued fear conditioning, contextual fear conditioning, and latent inhibition of cued fear conditioning were examined. In addition, the effects of coadministration of either mecamylamine and NBQX or mecamylamine and MK-801 on these behaviors were examined. Consistent with previous studies, neither mecamylamine nor NBQX administered alone disrupted any of the tasks. However, coadministration of mecamylamine and NBQX disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. In addition, coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting either task disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. Coadministration of mecamylamine and NBQX, and coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting fear conditioning had little effect on cued fear conditioning. These results suggest that nAChRs and glutamate receptors may support similar processes mediating acquisition of contextual fear conditioning and latent inhibition of fear conditioning.

Performance of alpha7 nicotinic receptor null mutants is impaired in appetitive learning measured in a signaled nose poke task

Wild-type and mutant mice lacking expression of alpha5, alpha7, beta2, beta3, or beta4 neuronal nicotinic cholinergic receptors (nAChRs) were compared on a signaled nose poke task, a multi-phased task used to measure appetitive learning and impulsivity. In the early phases of training, mutants of all nicotinic lines did not differ compared to wild types in the days to reach criterion when mice were required to nose poke for a sucrose reward on FR1 or FR3 schedules, or in their ability to respond to an auditory clicker to receive a sucrose reward. However, mutants lacking alpha7 nAChRs, but not lines lacking other nAChRs, showed impairments when task difficulty was increased such that an auditory stimulus was presented on a variable schedule and mice were required to withhold their responses until the presentation of the auditory cue to obtain a reward. alpha7 mutants were impaired compared to wild types in appetitive learning as measured by the percentage of conditioned responses but overcame their deficits with extensive training for 10 days. However, when efficiency ratios were used to measure impulsivity, alpha7 mutants exhibited lower efficiency ratios even after 10 days of training. These results support a role of the alpha7 nicotinic receptor in mediating appetitive learning and suggest a potential role for the alpha7 nAChRs in the regulation of behavioral disinhibition.

The interactive effects of nicotinic and muscarinic cholinergic receptor inhibition on fear conditioning in young and aged C57BL/6 mice

Both normal aging and age-related disease, such as Alzheimer's disease, have diverse effects on forebrain-dependent cognitive tasks as well as the underlying neurobiological substrates. The purpose of the current study was to investigate if age-related alterations in the function of the cholinergic system are associated with memory impairments in auditory-cued and contextual fear conditioning. Young (2-3 months) and aged (19-20 months) C57BL/6 mice were administered scopolamine (0.1, 0.3, 0.5, or 1.0 mg/kg), a muscarinic cholinergic receptor antagonist, mecamylamine (1.0 and 2.0 mg/kg), a nicotinic cholinergic receptor antagonist, both scopolamine and mecamylamine (0.1 and 1.0 mg/kg, respectively), or saline prior to training. Training consisted of two white-noise CS (85 dB, 30 s)-footshock US (0.57 mA, 2 s) presentations. Testing occurred 48 h post-training. Scopolamine administration impaired contextual and cued fear conditioning in young and aged mice, although the aged mice were less sensitive to disruption by scopolamine. Mecamylamine did not disrupt conditioned fear in the young or aged mice. Scopolamine and mecamylamine co-administration, at doses sub-threshold for disrupting fear conditioning with separate administration, disrupted contextual and auditory-cued fear conditioning in the young mice, indicating that in the young mice the muscarinic and nicotinic cholinergic processes interact in the formation and maintenance of long-term memories for conditioned fear. Co-administration of both antagonists did not disrupt fear conditioning in the aged mice, indicating that age-related alterations in the cholinergic receptor subtypes may occur.

Decreased amphetamine-induced locomotion and improved latent inhibition in mice mutant for the M5 muscarinic receptor gene found in the human 15q schizophrenia region

M5 muscarinic receptors are coexpressed with D2 dopamine receptors in the ventral tegmentum and striatum, and are important for reward in rodents. Previously, we reported that disruption of the M5 receptor gene in mice reduced dopamine release in the nucleus accumbens. In this study, we established a polymerase chain reaction (PCR) genotyping method for M5 mutant mice, and, using RT-PCR, found that M5 mRNA expression was highest in the ventral tegmentum, striatum, and thalamus in wild-type mice. In the M5 mutant mice, D2 mRNA expression was increased in several brain structures, including the striatum. Genome mapping studies showed the M5 gene is localized to chromosome 2E4 in mice, and to 15q13 in humans in the region that has been linked to schizophrenia. Amphetamine-induced locomotion, but not baseline locomotion or motor functions, decreased in M5 mutant mice, consistent with lower accumbal dopamine release. Previous reports found latent inhibition improvement in rats following nucleus accumbens lesions, or blockade of dopamine D2 receptors with neuroleptic drugs. Here, latent inhibition was significantly increased in M5 mutant mice as compared with controls, consistent with reduced dopamine function in the nucleus accumbens. In summary, our results showed that M5 gene disruption in mice decreased amphetamine-induced locomotion and increased latent inhibition, suggesting that increased M5 mesolimbic function may be relevant to schizophrenia.

Alteration of hippocampal cell proliferation in mice lacking the ?2 subunit of the neuronal nicotinic acetylcholine receptor

Adult hippocampal neurogenesis declines with age in parallel with decreased performance on a variety of hippocampal-dependent tasks. We measured the rate of cellular proliferation in the hippocampus of mice lacking the beta 2-subunit of the nicotinic acetylcholine receptor (beta 2-/- mice) at three ages: young adult (3 months old), fully adult (7-10 months old), and aged (22-24 months old). Consistent with previous studies, we observed an age-related decline in hippocampal proliferation in both groups. However, in fully adult beta 2-/- mice a 43% reduction of granule cell proliferation was detected compared to age-matched controls. This was accompanied by a significant decrease in dentate gyrus area/section and the length of the granule cell layer in beta 2-/- mice. These alterations were not the result of a change in plasma corticosterone levels or expression of the neurotrophic factor BDNF in the dentate gyrus, two known regulators of hippocampal cell proliferation. Similarly, there was no increase in gliosis, abnormal myelination, or apoptotic cell death in the beta 2-/- animals, although there was a significant shift in the location of apoptotic cells in the dentate gyrus indicative of a change in neuronal survival. These results suggest that the beta 2-subunit containing nicotinic acetylcholine receptors play an important role in regulating cell proliferation in the hippocampus and that endogenous acetylcholine may act to oppose the negative effects of normal aging and stress on cellular proliferation.

Behavioural and cardiovascular responses during latent inhibition of conditioned fear: measurement by telemetry and conditioned freezing

This study assessed freezing behaviour and cardiovascular responses during the expression of latent inhibition of conditioned fear. Animals that were either repeatedly preexposed (PE) to a tone conditioned stimulus (CS) or naive to the tone (non-preexposed; NPE) subsequently experienced three presentations of the tone paired with footshock. Animals were tested 24 h later in the context of the footshock chamber, and on the following day, in the presence of the tone CS. Changes in heart rate and blood pressure were recorded by radio-telemetry. The PE rats spent more time freezing to the conditioned contextual cues and exhibited higher blood pressures during the last half of the context test session than did the NPE animals. During the tone test, the PE rats exhibited less conditioned freezing to the tone CS compared with the NPE animals, i.e. expression of the latent inhibition. This behavioural effect was associated with a significant increase in heart rate, but not blood pressure, in the PE but not the NPE animals. Our results suggest that the increased blood pressures of the PE rats during the context test directly reflect their greater fear of the conditioning context. In contrast, the increased heart rate response but decreased freezing shown by PE rats in response to the tone CS may be due to the fact that lower stress levels (e.g. PE condition) elicit sympathetically-mediated increases in heart rate, whereas higher stress levels (e.g. NPE condition) activate both sympathetic and parasympathetic systems, thus eliminating any CS-induced increase in heart rate in the NPE rats.

Knockout and knockin mice to investigate the role of nicotinic receptors in the central nervous system

The recent use of genetically engineered knockout (Ko) and knockin (Kin) animals for neurotransmitter receptor genes, in particular, nicotinic acetylcholine receptors (nAChRs) in the brain, has provided a powerful alternative to the classical pharmacological approach. These animal models are not only useful in order to reexamine and refine the results derived from pharmacological studies, but they do also provide a unique opportunity to determine the subunit composition of the nicotinic receptors which modulate various brain functions. Ultimately, this knowledge will be valuable in the process of designing new drugs that will mimic the effects of nicotine on several important pathologies or on smoking cessation therapies. In this review, we present recent data obtained from the studies of mutant animals that contributed to our understanding of the role and composition of nAChRs in the central nervous system (CNS). The advantages and pitfalls of Ko animal models will also be discussed.

Role of neuronal nicotinic receptors in the effects of nicotine and ethanol on contextual fear conditioning

Nicotine can enhance contextual learning while ethanol impairs some forms of learning. Nicotine can overcome some of the impairing effects of ethanol when the two drugs are co-administered. The specific brain nicotinic acetylcholine receptors (nAChRs) that mediate nicotine's effects on contextual learning and whether any of ethanol's actions are mediated by nAChRs are unknown. The potential roles of nAChRs in contextual and cued fear conditioning as well as the effects of nicotine, ethanol, or co-administration of nicotine and ethanol were examined in wild type and homozygous null mutant mice from alpha7, beta2, beta3, and beta4 mouse lines at 24 h after training. Nicotine was given prior to training and testing, whereas ethanol was given only before training. Nicotine enhanced contextual learning in both alpha7 wild types and mutants when mice were trained at 0.17 mA, but not 0.35 mA. Mutants lacking the alpha7 subunit were less sensitive to the memory impairing effects of ethanol trained at 0.35 mA. beta2 Null mutants receiving saline showed a small, but significant, impairment in contextual learning compared with wild type littermates when the shock stimulus was 0.35 mA. Beta2 Null mutant mice also did not respond to the cognitive enhancing effects of nicotine alone, or after ethanol administration. beta3 and beta4 null mutants did not differ from wild types either after saline or any of drug treatments. These results show that beta2-containing nAChRs, but not beta3- or beta4-containing receptors, mediate the enhancing effects of nicotine on contextual learning and confirm previous studies implicating beta2 in other forms of learning. A new role for alpha7 nAChRs in regulating sensitivity to the cognitive disrupting effects of ethanol is proposed.

Improvements in hippocampal-dependent learning and decremental attention in 5-HT(3) receptor overexpressing mice

The 5-HT3 receptor for serotonin is expressed within limbic structures and is known to modulate neurotransmitter release, suggesting that this receptor may influence learning and memory. Perturbations in serotonergic neurotransmission lead to changes in the ability to attend, learn, and remember. To examine the role of 5-HT3 receptors in learning, memory, and attention, 5-HT3 receptor overexpressing (5-HT3-OE) transgenic mice and their wild-type littermates (WT) were tested in Pavlovian contextual and cued fear conditioning, fear extinction, and latent inhibition (LI) paradigms. Prepulse inhibition (PPI) was assessed to reveal changes in sensorimotor gating. Additionally, anxious behaviors, shock sensitivity, and reactions to novel stimuli were evaluated. 5-HT3-OE mice displayed enhanced contextual conditioning, whereas cued conditioning remained the same as that of WT mice. 5-HT3-OE mice did not differ from WT in extinction rates to either the context or cue. LI was enhanced for 5-HT3-OE mice compared to WT. PPI remained unchanged. No differences in sensitivity to footshock or startle were found. However, 5-HT3-OE mice demonstrated heightened exploratory behavior in response to novel environmental stimuli and decreased anxiety as measured in the elevated plus-maze. Results indicate that overexpression of the 5-HT3 receptor in mouse forebrain results in enhanced hippocampal-dependent learning and attention. Enhanced inspective behavior in response to novelty may contribute to the observed improvements in learning, memory, and attention due to 5-HT3 receptor overexpression.

Identifying loci for behavioral traits using genome-tagged mice

Identification of behavioral loci through complex trait mapping remains a widely employed approach but suffers from poor gene localization and low replicability. Genome-tagged mice (GTMs) are overlapping sets of congenic strains spanning the whole genome and offer the possibilities of superior mapping power and reproducibility. In this study, three GTM strains each consisting of an average approximately 27 cM DBA/2J genomic intervals introgressed onto a C57BL/6J background were employed for localization of behavioral traits. These GTMs were chosen because the corresponding chromosomal regions had been previously identified as containing loci for learning and memory. Analysis of the GTMs allowed confirmation of the learning and memory loci, and one on chromosome 3 was in addition fine mapped to an 8.8-cM region of overlap between two of the GTMs. Moreover, loci for prepulse inhibition of the startle response, acoustic startle response, and spontaneous locomotor activity were also mapped. These results suggest that the GTMs should be a valuable resource for mapping and confirmation of loci contributing to complex behavioral traits in the mouse.

Conditional calcineurin knockout mice exhibit multiple abnormal behaviors related to schizophrenia

Calcineurin (CN), a calcium- and calmodulin-dependent protein phosphatase, plays a significant role in the central nervous system. Previously, we reported that forebrain-specific CN knockout mice (CN mutant mice) have impaired working memory. To further analyze the behavioral effects of CN deficiency, we subjected CN mutant mice to a comprehensive behavioral test battery. Mutant mice showed increased locomotor activity, decreased social interaction, and impairments in prepulse inhibition and latent inhibition. In addition, CN mutant mice displayed an increased response to the locomotor stimulating effects of MK-801. Collectively, the abnormalities of CN mutant mice are strikingly similar to those described for schizophrenia. We propose that alterations affecting CN signaling could comprise a contributing factor in schizophrenia pathogenesis.

Acetylcholine receptors and thresholds for convulsions from flurothyl and 1,2-dichlorohexafluorocyclobutane

There are acetylcholine receptors throughout the central nervous system, and they may mediate some forms and aspects of convulsive activity. Most high-affinity binding sites on nicotinic acetylcholine receptors for nicotine, cytisine, and epibatidine in the brain contain the beta2 subunit of the receptor. Transitional inhaled compounds (compounds less potent than predicted from their lipophilicity and the Meyer-Overton hypothesis) and nonimmobilizers (compounds that do not produce immobility despite a lipophilicity that suggests anesthetic qualities as predicted from the Meyer-Overton hypothesis) can produce convulsions. The nonimmobilizer flurothyl [di-(2,2,2,-trifluoroethyl)ether] blocks the action of gamma-aminobutyric acid on gamma-aminobutyric acid(A) receptors, whereas the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N, also called F6) does not. 2N can block the action of acetylcholine on nicotinic acetylcholine receptors. We examined the relative capacities of these compounds to cause convulsions in mice having and lacking the beta2 subunit of the acetylcholine receptor. The partial pressure causing convulsions in half the mice (the 50% effective concentration [EC(50)]) was the same as in control mice. For the knockout mice, the EC(50) for flurothyl was 0.00170 +/- 0.00030 atm (mean +/- SD), and for 2N, it was 0.0345 +/- 0.0041 atm. For the control mice, the respective values were 0.00172 +/- 0.00057 atm and 0.0341 +/- 0.0048 atm. The ratio of the 2N to flurothyl EC(50) values was 20.8 +/- 3.5 for the knockout mice and 21.7 +/- 7.0 for the control mice. These results do not support the notion that acetylcholine receptors are important mediators of the capacity of 2N or flurothyl to cause convulsions. However, we also found that both nonimmobilizers inhibit rat alpha4beta2 neuronal nicotinic acetylcholine receptors at EC(50) partial pressures (0.00091 atm and 0.062 atm for flurothyl and 2N, respectively) that approximate those that produce convulsions (0.0015 atm and 0.04 atm).

IMPLICATIONS

The results from the present study provide conflicting data concerning the notion that acetylcholine receptors mediate the capacity of nonimmobilizers to produce convulsions.

Nicotinic receptors in aging and dementia

Activation of neuronal nicotinic acetylcholine receptors (nAChRs) has been shown to maintain cognitive function following aging or the development of dementia. Nicotine and nicotinic agonists have been shown to improve cognitive function in aged or impaired subjects. Smoking has also been shown in some epidemiological studies to be protective against the development of neurodegenerative diseases. This is supported by animal studies that have shown nicotine to be neuroprotective both in vivo and in vitro. Treatment with nicotinic agonists may therefore be useful in both slowing the progression of neurodegenerative illnesses, and improving function in patients with the disease. While increased nicotinic function has been shown to be beneficial, loss of cholinergic markers is often seen in patients with dementia, suggesting that decreased cholinergic function could contribute to both the cognitive deficits, and perhaps the neuronal degeneration, associated with dementia. In this article we will review the literature on each of these areas. We will also present hypotheses that might address the mechanisms underlying the ability of nAChR function to protect against neurodegeneration or improve cognition, two potentially distinct actions of nicotine.

Decreased synaptic vesicle recycling efficiency and cognitive deficits in amphiphysin 1 knockout mice

The function of the clathrin coat in synaptic vesicle endocytosis is assisted by a variety of accessory factors, among which amphiphysin (amphiphysin 1 and 2) is one of the best characterized. A putative endocytic function of amphiphysin was supported by dominant-negative interference studies. We have now generated amphiphysin 1 knockout mice and found that lack of amphiphysin 1 causes a parallel dramatic reduction of amphiphysin 2 selectively in brain. Cell-free assembly of endocytic protein scaffolds is defective in mutant brain extracts. Knockout mice exhibit defects in synaptic vesicle recycling that are unmasked by stimulation and suggest impairments at multiple stages of the cycle. These defects correlate with increased mortality due to rare irreversible seizures and with major learning deficits, suggesting a critical role of amphiphysin for higher brain functions.

Nicotine and neurodegeneration in ageing

Impairment in cholinergic systems is a highly consistent finding in human dementia. Among cholinergic markers, marked decreases in nicotine binding have been most consistently observed in the telencephalic regions of demented patients and are thought to contribute to the cognitive deficits associated with ageing and age-related neurodegenerative diseases. New evidence that the cholinergic system has a specific pathogenic role in the neurodegenerative alterations of aged and, especially, demented patients is fast accumulating. Both in vivo and in culture, nicotine protects striatal, hippocampal and cortical neurons against the neurotoxicity induced by excitotoxic amino acids as well as the toxicity caused by beta-amyloid, the major component of senile plaques. Further support for the implication of nicotinic receptors in brain ageing is come from recent studies on transgenic animals lacking nicotinic receptor subtypes, which shed light on the mechanisms of nicotine neuroprotection and neurotoxicity.