Laminar distribution of nicotinic receptor subtypes in cortical regions in schizophrenia

Differentiating nicotine- versus schizophrenia-associated decreases of the alpha7 nicotinic acetylcholine receptor transcript, CHRFAM7A, in peripheral blood lymphocytes

Nicotine addiction is prevalent in individuals with schizophrenia. Nicotine activation of nicotinic receptors (nAChRs) is time- and dose-dependent, but gene expression analyses often rely on qualitative self- or family-reported measures of smoking. We sought lymphocyte surrogates for cerebral alpha7-nAChR activity and tested if receptor transcription correlated with concurrently measured serum biomarkers for smoking [cotinine, C-reactive protein (CRP)]. PCR surveys to detect lymphocytic alpha7-related isoforms identified CHRFAM7A as the only consistently amplifiable transcript. In 20 smoking-matched people (n = 10 schizophrenia, n = 10 controls), we found significantly lower CHRFAM7A in cotinine and self-reported smokers versus nonsmokers (p <or= 0.001-0.03) and an inverse correlation of cotinine with CHRFAM7A (p <or= 0.04) in regression models. CHRFAM7A was not associated with diagnosis or CRP in any bi- or multi-variate analysis. Smoking-related CRP elevations only occurred in cotinine-based comparisons (p <or= 0.03), and not when smoking was self-reported. Including biochemical indicators of serum nicotine can help differentiate smoking- versus disease-associated changes in nAChR expression.

[11C]CHIBA-1001 as a novel PET ligand for alpha7 nicotinic receptors in the brain: a PET study in conscious monkeys

BACKGROUND

The alpha7 nicotinic acetylcholine receptors (nAChRs) play an important role in the pathophysiology of neuropsychiatric diseases such as schizophrenia and Alzheimer's disease. However, there are currently no suitable positron emission tomography (PET) radioligands for imaging alpha7 nAChRs in the intact human brain. Here we report the novel PET radioligand [11C]CHIBA-1001 for in vivo imaging of alpha7 nAChRs in the non-human primate brain.

METHODOLOGY/PRINCIPAL FINDINGS

A receptor binding assay showed that CHIBA-1001 was a highly selective ligand at alpha7 nAChRs. Using conscious monkeys, we found that the distribution of radioactivity in the monkey brain after intravenous administration of [11C]CHIBA-1001 was consistent with the regional distribution of alpha7 nAChRs in the monkey brain. The distribution of radioactivity in the brain regions after intravenous administration of [11C]CHIBA-1001 was blocked by pretreatment with the selective alpha7 nAChR agonist SSR180711 (5.0 mg/kg). However, the distribution of [11C]CHIBA-1001 was not altered by pretreatment with the selective alpha4beta2 nAChR agonist A85380 (1.0 mg/kg). Interestingly, the binding of [11C]CHIBA-1001 in the frontal cortex of the monkey brain was significantly decreased by subchronic administration of the N-methyl-D-aspartate (NMDA) receptor antagonist phencyclidine (0.3 mg/kg, twice a day for 13 days); which is a non-human primate model of schizophrenia.

CONCLUSIONS/SIGNIFICANCE

The present findings suggest that [11C]CHIBA-1001 could be a novel useful PET ligand for in vivo study of the receptor occupancy and pathophysiology of alpha7 nAChRs in the intact brain of patients with neuropsychiatric diseases such as schizophrenia and Alzheimer's disease.

Developmental excitation of corticothalamic neurons by nicotinic acetylcholine receptors

In this study, we show robust nicotinic excitation of pyramidal neurons in layer VI of prefrontal cortex. This layer contains the corticothalamic neurons, which gate thalamic activity and play a critical role in attention. Our experiments tested nicotinic excitation across postnatal development, using whole-cell recordings in prefrontal brain slices from rats. These experiments showed that layer VI neurons have peak nicotinic currents during the first postnatal month, a time period of intensive cortical development in rodents. We demonstrate that these currents are mediated directly by postsynaptic nicotinic receptors and can be suppressed by a competitive antagonist of alpha(4)beta(2)* nicotinic receptors. To record from identified corticothalamic neurons, we performed stereotaxic surgery to label the neurons projecting to medial dorsal thalamus. As hypothesized, recordings from these retrogradely labeled neurons in layer VI showed prominent nicotinic currents. Finally, we examined the effects of the drug nicotine on layer VI neurons and probed for the potential involvement of the accessory subunit, alpha(5), in their receptors. A level of nicotine similar to that found in the blood of smokers elicits a stable inward current in layer VI neurons, yet this exposure desensitizes approximately 50% of the subsequent current elicited by acetylcholine. An allosteric modulator of alpha(4)beta(2)alpha(5) receptors resulted in a 2.5-fold potentiation of submaximal nicotinic currents. This result is consistent with the expression of the relatively rare alpha(5) nicotinic subunit in layer VI. In summary, we show that layer VI corticothalamic neurons can be strongly excited during development by an unusual subtype of nicotinic receptor.

Nicotine effect on prepulse inhibition and prepulse facilitation in schizophrenia patients

Acoustic prepulse inhibition (PPI) is considered an important biomarker in animal studies of psychosis and a number of psychiatric conditions. Nicotine has been shown to improve acoustic PPI in some animal strains and in humans. However, there is little data on effects of nicotine on acoustic PPI in schizophrenia patients using a double-blind, placebo-controlled study design. The primary aim of the current study was to test the effect of nicotine nasal spray on acoustic PPI in schizophrenia patients. The secondary aim was to test nicotine effect on prepulse facilitation (PPF). The study included 18 schizophrenia patient smokers and 12 healthy control smokers, tested in a double-blind, placebo-controlled, crossover, randomized design immediately after nicotine or saline placebo nasal sprays. PPI was tested using 120 ms prepulse-pulse interval. PPF was tested using 4500 ms prepulse-pulse interval. The results showed a significant main effect of drug on PPI in that nicotine improved PPI compared to placebo (p=0.008) with no drug by diagnosis interaction (p=0.90). Improvement in PPI in response to nicotine was significantly correlated with the baseline severity of clinical symptoms (r=0.59, p=0.02) in patients. There was no significant drug or drug by diagnosis interaction for the 4500 ms prepulse-pulse interval condition. However, nicotine improved inhibition in a subgroup of subjects exhibiting PPF (p=0.002). In conclusion, the findings confirmed that nicotine transiently improves acoustic PPI in schizophrenia patients. Additionally, schizophrenia patients with more clinical symptoms may have benefited more from nicotinic effect on PPI.

Discovery of N-[(3R,5R)-1-azabicyclo[3.2.1]oct-3-yl]furo[2,3-c]pyridine-5-carboxamide as an agonist of the alpha7 nicotinic acetylcholine receptor: in vitro and in vivo activity

A novel alpha7 nAChR agonist, N-[(3R,5R)-1-azabicyclo[3.2.1]oct-3-yl]furo[2,3-c]pyridine-5-carboxamide (3a, PHA-709829), has been identified for the potential treatment of cognitive deficits in schizophrenia. The compound shows potent and selective alpha7 in vitro activity, excellent brain penetration, good rat oral bioavailability and robust in vivo efficacy in a rat auditory sensory gating model.

Prenatal exposure to maternal cigarette smoking and the adolescent cerebral cortex

Smoking during pregnancy is associated with long-term consequences on offspring behavior. We measured thickness of the cerebral cortex using magnetic resonance images obtained in 155 adolescents exposed in utero to maternal smoking and compared them with 159 non-exposed subjects matched by maternal education. Orbitofrontal, middle frontal, and parahippocampal cortices were thinner in exposed, as compared with non-exposed, individuals; these differences were more pronounced in female adolescents. In exposed females, the thickness of the orbitofrontal cortex correlated negatively with a self-rated assessment of caring, one of the components of a model of positive youth development. These findings provide evidence of the long-term impact of prenatal environment on a neural substrate of cognition and social behavior.

Effects of CDP-choline and the combination of CDP-choline and galantamine differ in an animal model of schizophrenia: development of a selective alpha7 nicotinic acetylcholine receptor agonist strategy

The regionally selective reduction of expression of the alpha7 nicotinic acetylcholine receptor (alpha7 nAChR) in schizophrenia underlies impaired sensory inhibition, a possible endophenotype of the disorder. This ligand-gated ion channel receptor has been proposed as a pharmacotherapeutic target in schizophrenia. The current study examined the effect of CDP-choline alone and the combination of CDP-choline and galantamine, administered acutely and once-daily for five consecutive days, in an animal model of NMDA receptor hypofunction that is relevant to schizophrenia. The results support the allosteric modulatory influence of galantamine on CDP-choline; however, individual doses of CDP-choline and galantamine must be carefully titrated in order to achieve optimal levels of alpha7 nAChR "agonism" that may be necessary for the desired therapeutic effect.

Preclinical characterization of A-582941: a novel alpha7 neuronal nicotinic receptor agonist with broad spectrum cognition-enhancing properties

Among the diverse sets of nicotinic acetylcholine receptors (nAChRs), the alpha7 subtype is highly expressed in the hippocampus and cortex and is thought to play important roles in a variety of cognitive processes. In this review, we describe the properties of a novel biaryl diamine alpha7 nAChR agonist, A-582941. A-582941 was found to exhibit high-affinity binding and partial agonism at alpha7 nAChRs, with acceptable pharmacokinetic properties and excellent distribution to the central nervous system (CNS). In vitro and in vivo studies indicated that A-582941 activates signaling pathways known to be involved in cognitive function such as ERK1/2 and CREB phosphorylation. A-582941 enhanced cognitive performance in behavioral models that capture domains of working memory, short-term recognition memory, memory consolidation, and sensory gating deficit. A-582941 exhibited a benign secondary pharmacodynamic and tolerability profile as assessed in a battery of assays of cardiovascular, gastrointestinal, and CNS function. The studies summarized in this review collectively provide preclinical validation that alpha7 nAChR agonism offers a mechanism with potential to improve cognitive deficits associated with various neurodegenerative and psychiatric disorders.

Smoking, Genetics and Schizophrenia: Evidence for Self Medication

Schizophrenia is a common mental illness with a high prevalence of smoking. More than 80% of schizophrenics smoke compared to 25% of the general population. Both schizophrenia and tobacco use have strong genetic components, which may overlap. It has been suggested that smoking in schizophrenia may be a form of self-medication in an attempt to treat an underlying biological pathology. Smoking normalizes auditory evoked potential and eye tracking deficits in schizophrenia, as well as improving cognitive function. Nicotine acts through a family of nicotinic receptors with either high or low affinity for nicotine. The loci for several of these receptors have been genetically linked to both smoking and to schizophrenia. Smoking changes gene expression for more than 200 genes in human hippocampus, and differentially normalizes aberrant gene expression in schizophrenia. The α7* nicotinic receptor, linked to schizophrenia and smoking, has been implicated in sensory processing deficits and is important for cognition and protection from neurotoxicity. Nicotine, however, has multiple health risks and desensitizes the receptor. A Phase I trial of DMXB-A, an α7* agonist, shows improvement in both P50 gating and in cognition, suggesting that further development of nicotinic cholinergic drugs is a promising direction in schizophrenia research.

Alpha7 nicotinic acetylcholine receptor mRNA expression and binding in postmortem human brain are associated with genetic variation in neuregulin 1

Studies in cell culture and in animals suggest that neuregulin 1 (NRG1), a probable schizophrenia susceptibility gene, regulates the expression of the alpha7 nicotinic acetylcholine receptors (nAChRs). We hypothesized that schizophrenia-associated allelic variations within the NRG1 gene, via their effects on NRG1 isoform expression, would be associated with alterations in nAChR alpha7 receptor levels. We examined the effects of four disease-associated single-nucleotide polymorphisms (SNPs) in the 5' region of the NRG1 gene on nAChR alpha7 mRNA transcript expression in both the dorsolateral prefrontal cortex (DLPFC) and hippocampus of normal controls and patients with schizophrenia using quantitative real-time PCR. NRG1 risk alleles at SNPs SNP8NRG221132 and rs6994992 predicted significantly lower nAChR alpha7 mRNA expression in the DLPFC. Haplotypes containing the risk alleles at the above SNPs were also associated with lower expression of nAChR alpha7 in the DLPFC. The genotype effect for rs6994992 and the haplotype effect were more pronounced within the schizophrenic patient group. To determine whether receptor levels follow that of mRNA expression, we performed receptor binding and autoradiography using [(125)I] alpha-bungarotoxin in the DLPFC. Consistent with the mRNA findings, we found a decrease in binding in risk allele carriers of SNP8NRG221132 as compared with heterozygous individuals. Together, these results suggest that the molecular mechanism of the association between NRG1 risk alleles and schizophrenia may include down-regulation of nAChR alpha7 expression.

Lack of RIC-3 congruence with β2 subunit-containing nicotinic acetylcholine receptors in bipolar disorder

Nicotinic acetylcholine receptor (nAChR) dysfunction occurs in individuals with schizophrenia (SZ) and may also affect individuals with bipolar disorder (BP). The molecular mechanisms for these disease-associated cholinergic deficits are not known. In vitro, the protein RIC-3 (resistance to inhibitors of cholinesterase-3) aids the assembly and trafficking of alpha7-nAChRs but has unclear action on the biogenesis of alpha4/beta2-nAChRs. To evaluate RIC-3/nAChR dynamics in diseased and normal human brain tissue, we measured RIC-3, alpha7-, alpha4- and beta2-nAChRs transcript levels in postmortem prefrontal cortex of individuals with SZ (n=31), BP (n=28) and unaffected controls (NC, n=33). Of the 28 individuals with BP, 20 had a history of psychotic symptoms. We compared relative message abundances between diagnostic groups and tested correlations of RIC-3 with each nAChR message subtype. RIC-3 and alpha4 messages were significantly increased in BP compared with NC (RIC-3, P< or =0.002; alpha4, P< or =0.04). RIC-3 message was also upregulated in SZ (P< or =0.04). In BP with psychoses, RIC-3 and alpha4 levels were increased compared with BP without psychoses (both P< or =0.02) and compared with NC (RIC-3, P< or =0.0003; alpha4, P< or =0.004). In correlation regression analyses, RIC-3 expression was very highly correlated to alpha7, alpha4 and beta2 in NC (alpha7, P< or =2.5e-05; alpha4, P< or =2.5e-09; beta2, P< or =0.003) and in SZ (alpha7, P< or =1e-07; alpha4, P< or =7e-07; beta2, P< or =3e-09). RIC-3 also strongly correlated with alpha7 and alpha4 in BP (alpha7, P< or =0.003; alpha4, P< or =3.5e-07). RIC-3 was modestly correlated with beta2 in BP overall (P< or =0.04), but showed no significant correlation in BP with psychoses (P< or =0.31) compared with a significant correlation in BP without psychoses (P< or =0.007). In conclusion, coordinated RIC-3/alpha4 upregulation and discordant RIC-3/beta2 levels suggest that alpha4/beta2 nAChR deficits in BP may occur from dysregulated RIC-3 chaperoning of the beta2 nAChR subunit in a subset of patients affected by psychotic features.

Treating schizophrenia symptoms with an alpha7 nicotinic agonist, from mice to men

Current antipsychotic treatments fail to fully address the range of symptoms of schizophrenia, particularly with respect to social and occupational dysfunctions. Recent work has highlighted the role of nicotinie in both cognitive and attentional deficits as well as deficient processing of repetitive sensory information. The predilection for schizophrenia patients to be extremely heavy cigarette smokers may be related to their attempt to compensate for a reduction in hippocampal alpha7 nicotinic cholinergic receptors by delivering exogenous ligand to the remaining receptors. Studies in rodent models of both learning and memory deficits and deficits in sensory inhibition have confirmed a role for the alpha7 subtype of the nicotinic cholinergic receptor in these processes. Rodent studies also demonstrated the efficacy of a selective partial alpha7 nicotinic agonist, DMXBA, to improve these deficits. Subsequent human clinical trials demonstrated improved sensory inhibition in 12 schizophrenia patients and showed improvement in several subtests of the RBANS learning and memory assessment instrument. These data suggest that therapeutic agents selected for alpha7 nicotinic activity may have utility in treating certain symptoms of schizophrenia.

Nicotine receptor gene CHRNA4 modulates early event-related potentials in auditory and visual oddball target detection tasks

The present study seeks to identify effects of a common genetic polymorphism in the human nicotinic alpha4beta2 receptor on components of the cognitive event-related potentials in auditory and visual modalities. The same sense thymine-to-cytosine polymorphism (c.1629T-C; Ser543Ser) was shown to preferentially modulate early components in both modalities. Specifically, the auditory N1 component amplitude was higher for T allele homozygotes than for C allele carriers. The visual P1 component revealed the same pattern of significant polymorphic modulation, but the later N1 amplitude differences were only marginally significant. There was no reliable indication of interactions between genotype and task factors. Parallel modulation of early latency modality-specific event-related potential (ERP) components in vision and audition may indicate that the CHRNA4 polymorphism affects factors that are common to top-down modulation of sensory processing across modalities.

Alpha-7 nicotinic acetylcholine receptor agonists selectively activate limbic regions of the rat forebrain: An effect similar to antipsychotics

It is considered that activation of nicotinic alpha7 receptors (alpha7 nAChR) is useful for the treatment of cognitive disturbances in schizophrenia and Alzheimer's disease. Recently, selective alpha7 nAChR agonists have been discovered and are used to validate the alpha7 nAChR as a drug target for the treatment of cognitive disturbances in schizophrenia. One important feature shared by all known antipsychotics is their capacity to induce expression of the neuronal immediate-early gene c-fos in the limbic forebrain. Using two novel and selective alpha7 nAChR agonists, PNU-282987 and SSR180711, we investigated their ability to induce c-Fos expression in the limbic forebrain with particular emphasis on the same regions reported to be activated by antipsychotics. Both alpha7 nAChR agonists increased c-Fos dose-dependently in the prefrontal cortex and the shell of nucleus accumbens, while leaving the core of nucleus accumbens and the dorsolateral striatum unaffected. The accumbal and cortical effect of SSR180711 was blocked completely by pre-administration of the alpha7 nAChR antagonist methyllycaconitine. Also, SSR180711 displayed no c-Fos-inducing effect in alpha7 nAChR knock-out mice. In conclusion, these results show that selective pharmacologic stimulation of alpha7 nAChR function results in activation of forebrain regions similar to conventional antipsychotics.

Novel alpha7 nicotinic receptor isoforms and deficient cholinergic transcription in schizophrenia

Abnormal alpha7 nicotinic acetylcholine receptor activity contributes to sensory gating and cognitive deficits in schizophrenic individuals. Negligible differences in alpha7 mRNA levels between disease and control states have led to conclusions that cholinergic dysfunction in schizophrenia (SZ) must occur post-transcriptionally. Alternatively, we propose that the dysregulation of splice variants of the alpha7 receptor could account for cholinergic deficiencies observed in this disease. Here, we isolated multiple alpha7 splice variants including exon deletions and those associated with a novel 124-127 base insertion following exon 4. Transcripts containing this new exon originated from sense strand-oriented RNA (vs. antisense), and in silico translations produced putative subunits with unique amino termini. Quantitative real-time polymerase chain reaction analyses indicated that one novel isoform was significantly downregulated (P < or = 0.03) in post-mortem prefrontal cortex of individuals with SZ (n = 35) compared with controls (n = 34). Ten brain regions (cerebellum, thalamus, corpus callosum, caudate, putamen and five areas of the cortex) were further screened for alpha7 isoforms in three individuals of each group. Semiquantitative analyses showed that each alpha7 mRNA subtype was present in each brain region, but all were particularly deficient in the corpus callosum in schizophrenics vs. controls (P < or = 0.0002 to 0.05 for different isoforms). Our data demonstrate that alpha7 transcription is altered in several ways in SZ, suggesting that transcription-level mechanisms could account in part for the impaired cholinergic neurotransmission observed in this disease.

Metabolism and Disposition of a Selective α7 Nicotinic Acetylcholine Receptor Agonist in Humans

The metabolism and disposition of N-(3R)-1-azabicyclo[2.2.2]oct-3-ylfuro[2,3-c]pyridine-5-carboxamide (1), an alpha(7) nicotinic acetylcholinergic receptor agonist, were elucidated in humans (4 female, 4 male; all white) after an oral dose of [(3)H]1. Overall, 1 was well tolerated, with >94% of administered radioactivity excreted renally by 48 h postdose; lyophilization of all urine and plasma samples confirmed (3)H stability within [(3)H]1. Across genders, 1 underwent low-to-moderate oral clearance comprising both renal (67%) and metabolic (33%) components, with the biotransformation of 1 occurring predominantly via oxidation of its furanopyridine moiety to carboxylic acid 2, and minimally by modification of its quinuclidine nitrogen to N-oxide 4 or N-glucuronide M5. Experiments using human in vitro systems were undertaken to better understand the enzyme(s) involved in the phase 1 biotransformation pathways. The formation of 2 was found to be mediated by CYP2D6, a polymorphically expressed enzyme absent in 5 to 10% of white people, whereas the generation of 4 was catalyzed by CYP2D6, FAD-containing monooxygenase 1 (FMO1), and FMO3. It is of interest that, although no overall gender-related differences in excretory routes, mass recoveries, pharmacokinetics, or metabolite profiles of 1 were evident, the observation of one of eight subjects (13%) showing disparate (relative to all other volunteers) systemic exposures to 1, and urinary and plasma quantitative profiles nearly devoid of 2 with the highest levels of 1, seem consistent with both the identification of CYP2D6 as the only major recombinant cytochrome P450 transforming 1 to 2 and the demographics of white CYP2D6 poor metabolizers. Data also reported herein suggest that 4 is generated predominantly by renal FMO1 in humans.

Schizophrenia and the alpha7 nicotinic acetylcholine receptor

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

Neurophysiological endophenotypes of schizophrenia: the viability of selected candidate measures

In an effort to reveal susceptibility genes, schizophrenia research has turned to the endophenotype strategy. Endophenotypes are characteristics that reflect the actions of genes predisposing an individual to a disorder, even in the absence of diagnosable pathology. Individual endophenotypes are presumably determined by fewer genes than the more complex phenotype of schizophrenia and would, therefore, reduce the complexity of genetic analyses. Unfortunately, despite there being rational criteria to define a viable endophenotype, the term is sometimes applied indiscriminately to characteristics that are deviant in affected individuals. Schizophrenia patients exhibit deficits in several neurophysiological measures of information processing that have been proposed as candidate endophenotypes. Successful processing of sensory inputs requires the ability to inhibit intrinsic responses to redundant stimuli and, reciprocally, to facilitate responses to less frequent salient stimuli. There is evidence to suggest that both these processes are "impaired" in schizophrenia. Measures of inhibitory failure include prepulse inhibition of the startle reflex, P50 auditory evoked potential suppression, and antisaccade eye movements. Measures of impaired deviance detection include mismatch negativity and the P300 event-related potential. The purpose of this review is to systematically evaluate the endophenotype candidacy of these key neurophysiological abilities. For each candidate, we describe typical experimental procedures, the current understanding of the underlying neurobiology, the nature of the abnormality in schizophrenia, the reliability, stability and heritability of the measure, and any reported gene associations. We conclude with a discussion of the few studies thus far that have employed a multivariate approach with these candidates.

Impaired performance of alpha7 nicotinic receptor knockout mice in the five-choice serial reaction time task

RATIONALE

Nicotinic receptors have been implicated in attentional performance. Nicotine can improve attention in animals and humans, but knowledge about relevant receptor subtypes is very limited.

OBJECTIVES

The aim was to examine the role of alpha7 receptors in attentional performance of mice and in effects of nicotine.

MATERIALS AND METHODS

Mice with targeted deletion of the gene coding for the alpha7 subunit of nicotinic receptors and wild-type controls were trained on a five-choice serial reaction time task with food reinforcers presented under varying parametric conditions. Nicotine was administered in a range of doses (0.001-1.0 mg/kg sc), including those reported to enhance attentional performance.

RESULTS

Initially the alpha7(-/-) (knockout) mice responded less accurately and made more anticipatory responses. After task parameters were altered so that the time allowed for responding was reduced and anticipatory (impulsive) responses were punished by a time-out, the pattern of performance deficits changed; there were increased omission errors in alpha7(-/-) mice but normal levels of accuracy and anticipatory responding. Nicotine did not improve any measure of performance, either with the original training parameters or after retraining; the largest dose used (1.0 mg/kg) produced a general impairment of responding in alpha7(-/-) and wild-type mice.

CONCLUSIONS

alpha7 nicotinic receptor knockout mice are impaired in performance of the 5-CSRTT, suggesting a possible role for alpha7 receptors in attentional processing. However, identification of a protocol for assessing attention-enhancing effects of nicotine in mice may require further modifications of test procedures or the use of different strains of animal.

Genetics of chromosome 15q13-q14 in schizophrenia

Positive genetic linkage to the 15q13-q14 region has been found in 11 studies, and several association reports support this locus as a candidate region for schizophrenia. The locus is unusual in that it was first linked to an endophenotype found in schizophrenia, the P50 deficit, and subsequently to schizophrenia. There is also biological data showing that a candidate gene in the region, the alpha7 nicotinic receptor CHRNA7, plays a seminal role in the linked endophenotype, and is decreased in expression in the patient population. The 15q13-q14 region is complicated by a partial duplication of the CHRNA7 gene that includes exons 5-10 and considerable sequence downstream. Evidence from multiple studies supports a broad region of genetic linkage around the marker D15S1360.

Epibatidine analogues as selective ligands for the αxβ2-containing subtypes of nicotinic acetylcholine receptors

A series of epibatidine analogues was synthesized and characterized in vitro. These compounds are high affinity ligands for the nicotinic acetylcholine receptors (nAChR). They display binding selectivity for the alpha(x)beta2 subtypes of nAChRs over the alpha(x)beta4 subtypes, and especially for the alpha4beta2 and alpha2beta2 subtypes. Furthermore, most of these new nicotinic compounds display little, if any, agonist activities at alpha3beta4 nAChR. As a result they might become lead structures for the design and synthesis of highly selective ligands for nAChR subtypes containing the beta2 subunit.

A novel positive allosteric modulator of the alpha7 neuronal nicotinic acetylcholine receptor: in vitro and in vivo characterization

Several lines of evidence suggest a link between the alpha7 neuronal nicotinic acetylcholine receptor (nAChR) and brain disorders including schizophrenia, Alzheimer's disease, and traumatic brain injury. The present work describes a novel molecule, 1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl)-urea (PNU-120596), which acts as a powerful positive allosteric modulator of the alpha7 nAChR. Discovered in a high-throughput screen, PNU-120596 increased agonist-evoked calcium flux mediated by an engineered variant of the human alpha7 nAChR. Electrophysiology studies confirmed that PNU-120596 increased peak agonist-evoked currents mediated by wild-type receptors and also demonstrated a pronounced prolongation of the evoked response in the continued presence of agonist. In contrast, PNU-120596 produced no detectable change in currents mediated by alpha4beta2, alpha3beta4, and alpha9alpha10 nAChRs. PNU-120596 increased the channel mean open time of alpha7 nAChRs but had no effect on ion selectivity and relatively little, if any, effect on unitary conductance. When applied to acute hippocampal slices, PNU-120596 increased the frequency of ACh-evoked GABAergic postsynaptic currents measured in pyramidal neurons; this effect was suppressed by TTX, suggesting that PNU-120596 modulated the function of alpha7 nAChRs located on the somatodendritic membrane of hippocampal interneurons. Accordingly, PNU-120596 greatly enhanced the ACh-evoked inward currents in these interneurons. Systemic administration of PNU-120596 to rats improved the auditory gating deficit caused by amphetamine, a model proposed to reflect a circuit level disturbance associated with schizophrenia. Together, these results suggest that PNU-120596 represents a new class of molecule that enhances alpha7 nAChR function and thus has the potential to treat psychiatric and neurological disorders.

Schizophrenia genes, gene expression, and neuropathology: on the matter of their convergence

This review critically summarizes the neuropathology and genetics of schizophrenia, the relationship between them, and speculates on their functional convergence. The morphological correlates of schizophrenia are subtle, and range from a slight reduction in brain size to localized alterations in the morphology and molecular composition of specific neuronal, synaptic, and glial populations in the hippocampus, dorsolateral prefrontal cortex, and dorsal thalamus. These findings have fostered the view of schizophrenia as a disorder of connectivity and of the synapse. Although attractive, such concepts are vague, and differentiating primary events from epiphenomena has been difficult. A way forward is provided by the recent identification of several putative susceptibility genes (including neuregulin, dysbindin, COMT, DISC1, RGS4, GRM3, and G72). We discuss the evidence for these and other genes, along with what is known of their expression profiles and biological roles in brain and how these may be altered in schizophrenia. The evidence for several of the genes is now strong. However, for none, with the likely exception of COMT, has a causative allele or the mechanism by which it predisposes to schizophrenia been identified. Nevertheless, we speculate that the genes may all converge functionally upon schizophrenia risk via an influence upon synaptic plasticity and the development and stabilization of cortical microcircuitry. NMDA receptor-mediated glutamate transmission may be especially implicated, though there are also direct and indirect links to dopamine and GABA signalling. Hence, there is a correspondence between the putative roles of the genes at the molecular and synaptic levels and the existing understanding of the disorder at the neural systems level. Characterization of a core molecular pathway and a 'genetic cytoarchitecture' would be a profound advance in understanding schizophrenia, and may have equally significant therapeutic implications.

Anabasine, a selective nicotinic acetylcholine receptor agonist, antagonizes MK-801-elicited mouse popping behavior, an animal model of schizophrenia

The expression of the alpha7-nicotinic acetylcholine receptor is diminished in selected brain areas of patients with schizophrenia. This diminished expression may account for the pathophysiological deficits of sensory inhibition and smooth pursuit eye movement performance in these patients. Furthermore, the deficits in sensory inhibition and smooth pursuit eye movement performance in schizophrenia appear to be inherited in an autosomal dominant fashion; thus, the "alpha7-nicotinic acetylcholine receptor-deficiency" may be a necessary condition for expression of schizophrenia. This deficit has encouraged speculation about the possible therapeutic benefit of selective alpha7-nicotinic acetylcholine receptor agonist interventions in this disorder. In view of this, we sought to examine the effect of anabasine, a selective alpha7-nicotinic acetylcholine receptor agonist, on popping behavior in mice elicited by MK-801. MK-801, a high affinity analogue of phencyclidine (PCP), is a noncompetitive N-methyl-D-aspartic acid (NMDA) receptor antagonist that binds to the hydrophobic domain of this ligand-gated channel. PCP is known to precipitate a schizophreniform psychosis in susceptible individuals, causing productive (e.g. hallucinations) deficit (e.g. affective blunting, amotivation, and social withdrawal), cognitive and motor symptoms similar to those seen in naturally-occurring schizophrenia. Behaviors elicited by MK-801 in mice reflect a pharmacologically-induced state of NMDA receptor hypofunction (NRH), which has been proposed to exist in schizophrenia. Compounds that attenuate MK-801-elicited behaviors, which are identified in this animal model, may have the potential to treat schizophrenia, including deficit and cognitive symptoms. In the current study, anabasine attenuated MK-801-elicited popping at a dose that did not cause clonic seizures. The development of alpha7-nicotinic acetylcholine receptor agonist interventions for schizophrenia must consider their potential liability to elicit seizure activity.

Nicotine reduces A beta in the brain and cerebral vessels of APPsw mice

Ten days treatment with nicotine reduced insoluble amyloid A beta 1-40 and Alpha beta 1-42 peptides by 80% in the cortex of 9-month-old APPsw mice, which is more than that observed in 14.5-month-old mice following nicotine treatment for 5.5 months. A reduction in A beta associated with cerebral vessels was observed in addition to that deposited as parenchymal plaques after 5.5 months treatment. The diminution in A beta peptides observed was not accompanied by changes in brain alpha, beta or gamma secretase-like activities, NGF or BDNF protein expression measured in brain homogenates. A significant increase in sAPP was observed after nicotine treatment of SH-SY5Yneuroblastoma cells that could be blocked by the nicotinic antagonist mecamylamine. Attenuation of elevated [(125)I]-alpha bungarotoxin binding (alpha 7) in APPsw mice was observed after 5.5 months nicotine treatment. Both these observations suggest that the reduction in insoluble A beta by nicotine might be in part mediated via the alpha 7 nicotinic receptor. Further studies are required to identify potential mechanisms of the nicotine's amyloid-reducing effect.

Effects of nicotine on cognitive deficits in schizophrenia

Several lines of evidence suggest a pathophysiological role for nicotinic receptors in schizophrenia. Activation by nicotine alters physiological dysfunctions, such as eye movement and sensory gating abnormalities, but effects on neuropsychological performance are just beginning to be investigated. Nicotine-induced desensitization and the well-known tachyphylaxis of nicotinic receptors may confound such efforts. In all, 20 schizophrenics, 10 smokers, and 10 nonsmokers were assessed following the administration of nicotine gum and placebo gum. The Repeatable Battery for the Assessment of Neuropsychological Status was administered. Nicotine affected only the Attention Index; there were no effects on learning and memory, language, or visuospatial/constructional abilities. Attentional function was increased in nonsmokers, but decreased in nicotine-abstinent smokers after nicotine administration. The effects of nicotine in schizophrenia do not extend to all areas of cognition. Effects on attention may be severely limited by tachyphylaxis, such that decremented performance occurs in smokers, while modest effects may be achieved in nonsmokers.

Nicotinic receptors and schizophrenia

The incidence of smoking is very high in non-schizophrenic subjects presenting various psychiatric disorders (35 to 54%). However, the incidence of smoking is extremely high in schizophrenic patients: 80% to 90%, versus 25% to 30% of the general population. Various studies have demonstrated that the use of tobacco transiently restores the schizophrenic patient's cognitive and sensory deficits. Smoking cessation also appears to exacerbate the symptoms of the disease. Post-mortem binding studies have revealed a disturbance of nicotinic receptor expression, affecting the alpha(7) and alpha(4)beta(2) subunits, in various cerebral areas. Genetic linkage studies have also shown that the alpha(7) subunit is involved in schizophrenia. This review assesses the involvement of the nicotinic system in schizophrenia and suggests ways in which this system may participate in the pathophysiology of this disease.

Alpha-7 nicotinic receptor agonists: potential new candidates for the treatment of schizophrenia

RATIONALE AND OBJECTIVE

Auditory sensory gating, a biological measurement of the ability to suppress the evoked response to the second of two auditory stimuli, is diminished in people with schizophrenia. Deficits in sensory gating are associated with attentional impairment, and may contribute to cognitive symptoms and perceptual disturbances. This inhibitory process, which involves the alpha(7) nicotinic receptor mediated release of gamma-aminobutyric acid (GABA) by hippocampal interneurons, represents a potential new target for therapeutic intervention in schizophrenia.

METHOD

This paper will review several lines of evidence implicating the nicotinic-cholinergic, and specifically, the alpha(7) nicotinic receptor system in the pathology of schizophrenia and the evidence that alpha(7) nicotinic receptor agonists may ameliorate some of these deficits.

RESULTS

Impaired auditory sensory gating has been linked to the alpha(7) nicotinic receptor gene on the chromosome 15q14 locus. Single nucleotide polymorphisms of the promoter region of this gene are more frequent in people with schizophrenia. Although nicotine can acutely reverse diminished auditory sensory gating in people with schizophrenia, this effect is lost on a chronic basis due to receptor desensitization. Clozapine is able to reverse auditory sensory gating impairment, probably through an alpha(7) nicotinic receptor mechanism, in both humans and animal models with repeated dosing. The alpha(7) nicotinic agonist 3-2,4 dimethoxybenzylidene anabaseine (DMXBA) can also enhance auditory sensory gating in animal models. DMXBA is well tolerated in humans and improves several cognitive measures.

CONCLUSION

Alpha-7 nicotinic receptor agonists appear to be reasonable candidates for the treatment of cognitive and perceptual disturbances in schizophrenia.

Role of α7 Nicotinic Acetylcholine Receptors in the Pressor Response to Intracerebroventricular Injection of Choline: Blockade by Amyloid Peptide Aβ1-42

Systemic blood pressure and cardiac function have long been known to be under the control of central autonomic and hormonal pathways that, in part, use cholinergic neural systems. Recently choline, a precursor and product of acetylcholine metabolism, has been shown to serve as a selective endogenous agonist for the alpha7 subtype of the nicotinic acetylcholine receptor (alpha7nAChR). This receptor subtype mediates several responses to nicotine in animals, most notably, neuroprotection and enhanced cognition. The purpose of this study was to determine whether the cardiovascular changes induced by central injection of choline in rats also were mediated by alpha7nAChRs. Moreover, we sought to determine whether these cardiovascular changes to choline could be blocked by central pretreatment with amyloid beta peptide (1-42) (Abeta1-42), a neurotoxic component of cerebral amyloid that is known to bind with high affinity to alpha7nAChRs. Central, i.c.v. injection of choline (50, 100, or 150 microg) produced dose-dependent (10-15-min duration) pressor response of up to about 20 mm Hg. The most consistent change in heart rate included a brief increase (up to 40 beats/min) that lasted 2 to 3 min, followed by a prolonged decrease averaging 50 beats/min that lasted up to 30 min. Pretreatment (i.c.v.) with the selective alpha7nAChR antagonists alpha-bungarotoxin and methyllycaconitine significantly inhibited the pressor and heart rate responses to subsequent injection of choline. Pretreatment with the non-alpha7-preferring antagonist dihydro-beta-erythroidin was not effective. These findings suggested that the cardiovascular response to i.c.v. injection of choline was mediated at least in part through alpha7nAChRs. Pretreatment (30 min) with low doses (1-100 pmol) of amyloid peptide Abeta1-42 (but not with Abeta40-1) administered by the i.c.v. route significantly inhibited the choline-induced blood pressure increase as well as the choline-induced decrease in heart rate.

Dementia rating and nicotinic receptor expression in the prefrontal cortex in schizophrenia

BACKGROUND

The etiology of dementia that occurs in patients with schizophrenia is not well understood. Nicotinic acetylcholine receptors have been implicated in cognitive function, and deficits in these receptors have been reported in schizophrenia.

METHODS

The present study investigates possible associations of nicotinic receptor subunit expression in the dorsal lateral prefrontal cortex, an area known to be affected in schizophrenia, and dementia rating.

RESULTS

alpha7 immunoreactivity was reduced by 20% to 28% and [(3)H]epibatidine binding was increased twofold in groups of patients with schizophrenia compared to normal control subjects matched for age, postmortem delay, and low levels of brain nicotine and cotinine. In contrast, no significant differences in alpha4, alpha3, or beta2 immunoreactivity or alpha7 messenger RNA expression were observed in schizophrenia patients compared with control subject values. Clinical dementia ratings in patients with schizophrenia were correlated with neither [(3)H]epibatidine binding nor nicotinic receptor subunit expression.

CONCLUSIONS

These data indicate no relationship between the trend for reduced neocortical alpha7 subunit protein expression in schizophrenia and dementia. Further investigations are required to establish whether the reduction in alpha7 protein in the dorsal lateral prefrontal cortex is associated with clinical features other than dementia in schizophrenia.

Progressive Worsening of Adaptive Functions in Down Syndrome May Be Mediated By the Complexing of Soluble Aβ Peptides With the α7 Nicotinic Acetylcholine Receptor: Therapeutic Implications

In persons with Down syndrome, soluble Abeta peptides, which result from the processing of the amyloid precursor protein, appear in the brain decades before the extracellular deposition of neuritic plaques. These soluble amyloidogenic peptides accumulate intraneuronally and can be secreted extracellularly. Their appearance has been reported in the brains of fetuses with Down syndrome. The extra gene dosage effect associated with trisomy 21 results in abnormalities of the processing of amyloid precursor protein in persons with Down syndrome. Abeta peptides, especially Abeta1-42, have been shown to form tight complexes with the alpha7 nicotinic acetylcholine receptor, interfering with transduction of the acetylcholine signal by this nicotinic receptor subtype. Furthermore, the selective binding of Abeta peptides by this nicotinic acetylcholine receptor subtype is associated with cytotoxicity. The alpha7 nicotinic acetylcholine receptor has unique electrophysiologic properties and plays a prominent role in normal psychophysiologic processes (eg, sensory inhibition) and cognition. In persons with Down syndrome there is a decrease in the ability to perform instrumental activities of daily living that worsen with aging. The progressive worsening of adaptive functions and cognition in persons with Down syndrome may be, at least in part, mediated by interference with this receptor by soluble Abeta peptides. In view of this complex formed by soluble Abeta peptides and the alpha7 nicotinic acetylcholine receptor, cholinergic interventions that have been developed for Alzheimer disease, including selective nicotinic ones, should be explored in Down syndrome. Ideally, selective cholinergic interventions would slow the progression of the worsening of adaptive function and emergence of dementia in persons with Down syndrome.

The genetics of sensory gating deficits in schizophrenia

Sensory gating abnormalities are an early clinical symptom of schizophrenia, and are characterized by a decrease in the brain's normal ability to inhibit the response to unimportant stimuli. Patients appear hypervigilant and have difficulty focusing their attention. A neurobiologic mechanism involved in these difficulties is nicotinic cholinergic modulation of inhibitory neuronal activity in the hippocampus. One measure of sensory gating abnormalities, diminished inhibition of the P50 evoked response to repeated auditory stimuli, has been linked to the chromosome 15q14 locus of the alpha-7-nicotinic receptor gene. This site is one of several that have shown evidence for linkage to schizophrenia, as well as to bipolar disorder, across several studies. Polymorphisms in the core promoter of the gene are associated with schizophrenia and also with diminished inhibition of the P50 response. These genetic data may identify a new pathophysiologic target for drug discovery.

Nicotinic receptors and Alzheimer's disease

Nicotinic receptors (NRs) belong to the group of polymeric receptors of the cell membrane and are key elements of cholinergic transmission. Numerous subtypes of NRs exist with the alpha 4 beta 2 and alpha 7 types being encountered most frequently. Deficiencies in NRs seem to play a role in Alzheimer's disease, which is characterised by accumulation of senile plaques, mainly composed of beta-amyloid peptide (beta A). Although the aetiology of this disease is unknown, different pathogenesis hypotheses implicating alpha 7 NRs have been proposed, with the receptors exerting a direct or indirect action on the mechanism of beta A toxicity. Allosteric modulators of NRs, such as the cholinesterase inhibitor galantamine, that facilitate the action of acetylcholine on these receptors may provide therapeutic benefits in the areas of cognition, attention and antineurodegenerative activity.

Selective changes in the levels of nicotinic acetylcholine receptor protein and of corresponding mRNA species in the brains of patients with Parkinson's disease

Reductions in the number of neuronal nicotinic acetylcholine receptors (nAChRs) have been shown to occur in connection with Parkinson's disease (PD), but it is still unclear which subtype of this receptor is affected. In the present study we examined various nAChR subtypes employing ligand binding, as well as levels of subunit protein and mRNA in the brains of PD patients and age-matched controls. Binding of [3H]epibatidine and levels of alpha3 mRNA in the caudate nucleus and temporal cortex, but not in the hippocampus were significantly decreased in the PD brain. The level of the alpha3 protein subunit was significantly reduced in all these brain regions but there was no change in the level of alpha4. The level of the beta2 protein subunit in the temporal cortex and hippocampus and the beta2 mRNA in the temporal cortex was lowered. Both the levels of the alpha7 subunit protein and [125I]alpha-bungarotoxin binding were significantly increased in the temporal cortex of PD patients whereas the alpha7 mRNA level was unchanged. These findings reveal selective losses of the alpha3- and beta2-containing nAChRs and an increase in the alpha7 nAChRs that might be related to the pathogenesis of PD.