Consequences of low levels of nicotinic acetylcholine receptors in schizophrenia for drug development

Dose-dependent effects of Varenicline on tobacco craving and withdrawal in tobacco smokers with and without schizophrenia

BACKGROUND

People with schizophrenia (SCZ) have significantly higher tobacco smoking rates and lower quit rates than the general population. Varenicline, a partial agonist at α4β2 nicotinic acetylcholine receptors (nAChRs) is an effective smoking cessation pharmacotherapy, however, investigation into its effects in SCZ are less well-studied and mechanisms may differ from non-psychiatric controls due to dysregulation in nAChR neurotransmission associated with SCZ. Here, we investigate whether Varenicline attenuates acute abstinence-induced increases in craving and withdrawal in participants with and without SCZ.

METHODS

Following biochemically-verified overnight abstinence and subsequent smoking reinstatement, individuals with nicotine-dependence (n = 13 SCZ or schizoaffective; n = 12 controls) were assessed on the Minnesota Nicotine Withdrawal Scale (MNWS) and Tiffany Questionnaire for Smoking Urges (TQSU). Participants were pretreated in a double-blind, counterbalanced manner with Varenicline (0, 1 or 2 mg/day x 3 days) over three separate weeks. Data were analyzed using linear mixed-effects modelling and estimated marginal means.

RESULTS

Robust effects of smoking abstinence were observed on TQSU and MNWS scores in SCZ and control participants. Relative to 1 mg, 2 mg/day of Varenicline attenuated abstinence-induced increases in craving (TQSU Factor 1 d=-0.47, p = .006; TQSU Factor 2 d=-0.42, p = .008) and withdrawal (MNWS d=-0.35, p = .03) in both groups.

CONCLUSION

Our preliminary findings suggest that subacute Varenicline treatment reduces abstinence-induced craving and withdrawal in participants with and without SCZ. The efficacy of Varenicline on tobacco withdrawal and craving requires further study.

Analyzing the genes related to nicotine addiction or schizophrenia via a pathway and network based approach

The prevalence of tobacco use in people with schizophrenia is much higher than in general population, which indicates a close relationship between nicotine addiction and schizophrenia. However, the molecular mechanism underlying the high comorbidity of tobacco smoking and schizophrenia remains largely unclear. In this study, we conducted a pathway and network analysis on the genes potentially associated with nicotine addiction or schizophrenia to reveal the functional feature of these genes and their interactions. Of the 276 genes associated with nicotine addiction and 331 genes associated with schizophrenia, 52 genes were shared. From these genes, 12 significantly enriched pathways associated with both diseases were identified. These pathways included those related to synapse function and signaling transduction, and drug addiction. Further, we constructed a nicotine addiction-specific and schizophrenia-specific sub-network, identifying 11 novel candidate genes potentially associated with the two diseases. Finally, we built a schematic molecular network for nicotine addiction and schizophrenia based on the results of pathway and network analysis, providing a systematic view to understand the relationship between these two disorders. Our results illustrated that the biological processes underlying the comorbidity of nicotine addiction and schizophrenia was complex, and was likely induced by the dysfunction of multiple molecules and pathways.

Smoking in schizophrenia: cognitive impact of nicotine and relationship to smoking motivators

Smoking prevalence in schizophrenia is significantly elevated relative to other clinical and to non-clinical groups. The cognitive self-medication hypothesis attributes this to the beneficial effects of nicotine on illness-related cognitive deficits. Significant effects of nicotine have been observed on visual spatial working memory (VSWM), sustained attention (Continuous Performance Test - Identical Pairs; CPT-IP) and prepulse inhibition (PPI). It remains unclear whether these neurophysiological and neurocognitive effects of nicotine influence self-reported smoking motivation. To explore the relationship between nicotine effects on cognition and self-reported smoking motivation in schizophrenia and non-psychiatric control smokers, the impact of smoking abstinence and reinstatement was examined across three cognitive indices (VSWM, CPT-IP, PPI) and compared to self-reported smoking motivation (Modified Reasons for Smoking Scale revised to include 'cognitive motivators'). Cognitive function was assessed after 'typical' smoking and overnight abstinence. Schizophrenia smokers (but not controls) demonstrated significantly less error on the VSWM task in the smoking relative to abstinent condition. Control (but not schizophrenia) smokers, showed evidence of CPT-IP improvement in the smoking relative to abstinent condition. The overall profile of smoking motivation was comparable between groups. However, significant relationships between subjective and objective indices of smoking related cognitive change were observed for controls. Differential effects of nicotine on cognition have been hypothesised to influence the pattern and persistence of smoking in schizophrenia. These preliminary findings indicate that evidence for such effects is apparent even in small samples - particularly for VSWM. This is the first study to show that neurocognitive effects of smoking may influence self-reported smoking motivation.

Structure-activity studies of 7-heteroaryl-3-azabicyclo[3.3.1]non-6-enes: a novel class of highly potent nicotinic receptor ligands

The potential for nicotinic ligands with affinity for the α4β2 or α7 subtypes to treat such diverse diseases as nicotine addiction, neuropathic pain, and neurodegenerative and cognitive disorders has been exhibited clinically for several compounds while preclinical activity in relevant in vivo models has been demonstrated for many more. For several therapeutic programs, we sought nicotinic ligands with various combinations of affinity and function across both subtypes, with an emphasis on dual α4β2-α7 ligands, to explore the possibility of synergistic effects. We report here the structure-activity relationships (SAR) for a novel series of 7-heteroaryl-3-azabicyclo[3.3.1]non-6-enes and characterize many of the analogues for activity at multiple nicotinic subtypes.

The chimeric gene CHRFAM7A, a partial duplication of the CHRNA7 gene, is a dominant negative regulator of α7*nAChR function

The human α7 neuronal nicotinic acetylcholine receptor gene (CHRNA7) is a candidate gene for schizophrenia and an important drug target for cognitive deficits in the disorder. Activation of the α7*nAChR, results in opening of the channel and entry of mono- and divalent cations, including Ca(2+), that presynaptically participates to neurotransmitter release and postsynaptically to down-stream changes in gene expression. Schizophrenic patients have low levels of α7*nAChR, as measured by binding of the ligand [(125)I]-α-bungarotoxin (I-BTX). The structure of the gene, CHRNA7, is complex. During evolution, CHRNA7 was partially duplicated as a chimeric gene (CHRFAM7A), which is expressed in the human brain and elsewhere in the body. The association between a 2bp deletion in CHRFAM7A and schizophrenia suggested that this duplicate gene might contribute to cognitive impairment. To examine the putative contribution of CHRFAM7A on receptor function, co-expression of α7 and the duplicate genes was carried out in cell lines and Xenopus oocytes. Expression of the duplicate alone yielded protein expression but no functional receptor and co-expression with α7 caused a significant reduction of the amplitude of the ACh-evoked currents. Reduced current amplitude was not correlated with a reduction of I-BTX binding, suggesting the presence of non-functional (ACh-silent) receptors. This hypothesis is supported by a larger increase of the ACh-evoked current by the allosteric modulator 1-(5-chloro-2,4-dimethoxy-phenyl)-3-(5-methyl-isoxazol-3-yl)-urea (PNU-120596) in cells expressing the duplicate than in the control. These results suggest that CHRFAM7A acts as a dominant negative modulator of CHRNA7 function and is critical for receptor regulation in humans.

Differential regulation of alpha7 nicotinic receptor gene (CHRNA7) expression in schizophrenic smokers

The alpha7 neuronal nicotinic receptor gene (CHRNA7) has been implicated in the pathophysiology of schizophrenia by genetic and pharmacological studies. Expression of the alpha7* receptor, as measured by [(125)I]alpha-bungarotoxin autoradiography, is decreased in postmortem brain of schizophrenic subjects compared to non-mentally ill controls. Most schizophrenic patients are heavy smokers, with high levels of serum cotinine. Smoking changes the expression of multiple genes and differentially regulates gene expression in schizophrenic hippocampus. We examined the effects of smoking on CHRNA7 expression in the same tissue and find that smoking differentially regulates expression of both mRNA and protein for this gene. CHRNA7 mRNA and protein levels are significantly lower in schizophrenic nonsmokers compared to control nonsmokers and are brought to control levels in schizophrenic smokers. Sufficient protein but low surface expression of the alpha7* receptor, seen in the autoradiographic studies, suggests aberrant assembly or trafficking of the receptor.

Association of the 5'-upstream regulatory region of the alpha7 nicotinic acetylcholine receptor subunit gene (CHRNA7) with schizophrenia

BACKGROUND

The alpha7 neuronal nicotinic acetylcholine receptor subunit gene (CHRNA7) is localized in a chromosomal region (15q14) linked to schizophrenia in multiple independent studies. CHRNA7 was selected as the best candidate gene in the region for a well-documented endophenotype of schizophrenia, the P50 sensory processing deficit, by genetic linkage and biochemical studies.

METHODS

Subjects included Caucasian-Non Hispanic and African-American case-control subjects collected in Denver, and schizophrenic subjects from families in the NIMH Genetics Initiative on Schizophrenia. Thirty-five single nucleotide polymorphisms (SNPs) in the 5'-upstream regulatory region of CHRNA7 were genotyped for association with schizophrenia, and for smoking in schizophrenia.

RESULTS

The rs3087454 SNP, located at position -1831 bp in the upstream regulatory region of CHRNA7, was significantly associated with schizophrenia in the case-control samples after multiple-testing correction (P=0.0009, African American; P=0.013, Caucasian-Non Hispanic); the association was supported in family members. There was nominal association of this SNP with smoking in schizophrenia.

CONCLUSIONS

The data support association of regulatory region polymorphisms in the CHRNA7 gene with schizophrenia.

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.

Genetic interaction between alpha4 and beta2 subunits of high affinity nicotinic receptor: analysis in schizophrenia

Cholinergic dysfunction is one of the hypotheses for the cognitive deficits of schizophrenia. Neurocognitive deficits, which are well-described clinical features of schizophrenia, may be remediated by nicotine; therefore investigations of nicotinic receptor subtypes is of considerable clinical interest. We typed polymorphisms in CHRNA4 and CHRNB2 genes controlling the expression of neuronal high-affinity nicotinic receptors in 117 Canadian families having at least one schizophrenic patient. Using a family-based association strategy, we performed allele, haplotype and interaction analysis of these two loci. In the families tested, the two cholinergic genes interact to affect schizophrenia in combination (P=0.010), while neither was sufficient alone to confer susceptibility. Our present study provided the first line of direct evidence suggesting that the CHRNA4 gene combined with CHRNB2 receptor gene may be linked to schizophrenia.

Chronic nicotine interactions with clozapine and risperidone and attentional function in rats

Although antipsychotic drugs are therapeutically effective in attenuating the hallmark symptoms of schizophrenia, these improvements do not return most patients to normative standards of cognitive function. Thus, complementary drug treatment may be needed to treat the attentional deficits of schizophrenia as well as to counteract the potential attentional impairments caused by some antipsychotic drugs. Nicotine, a drug commonly self-administered by a great majority of individuals with schizophrenia, has been shown to significantly improve cognitive function in some studies. The current study was conducted to determine the interactive effects of the atypical antipsychotic drugs clozapine and risperidone with chronic nicotine administration on attentional performance. Adult female Sprague-Dawley rats (N=35) were trained to perform an attentional task using an operant visual signal detection task. After training, rats were infused with a dose of 5 mg/kg/day (s.c.) nicotine base (n=18) or saline (n=17) for 28 consecutive days via osmotic pump. In Exp. 1, while being administered chronic nicotine or saline, rats were given acute doses of clozapine (0, 0.625, 1.25 and 2.5 mg/kg, s.c.) and were tested for attentional function. In Exp. 2, while on chronic nicotine or saline, other rats were challenged with acute doses of risperidone (0, 0.025, 0.05 and 0.1 mg/kg, s.c.) and were tested for attentional function. Results showed that acute administration of clozapine caused a significant dose-dependent impairment in choice accuracy (percent hit) in animals treated with chronic saline. Chronic nicotine treatment itself lowered accuracy, but attenuated further declines with acute clozapine treatment. Acute administration of risperidone at high dose significantly reduced performance (percent correct rejection) in chronically saline-treated rats, but in a similar fashion as in Exp. 1, chronic nicotine lowered accuracy but attenuated further impairment with acute risperidone. In summary, atypical antipsychotic drugs clozapine and risperidone significantly impaired choice accuracy in the visual signal detection task. Clozapine was more detrimental than risperidone but the adverse effects of both clozapine and risperidone on attentional performance were masked in rats chronically treated with nicotine.

Differential modulation of gene expression in the NMDA postsynaptic density of schizophrenic and control smokers

Nicotine is known to induce the release of multiple neurotransmitters, including glutamate and dopamine, through activation of nicotinic receptors. Gene expression in the N-methyl-d-aspartate postsynaptic density (NMDA-PSD), as well as other functional groups, was compared in postmortem hippocampus of schizophrenic and nonmentally ill smokers and nonsmokers utilizing a microarray and quantitative RT-PCR approach. The expression of 277 genes was significantly changed between all smokers and nonsmokers. Specific gene groups, most notably genes expressed in the NMDA-PSD, were prevalent among these transcripts. Analysis of the interaction between smoking and schizophrenia identified several genes in the NMDA-PSD that were differentially affected by smoking in patients. The present findings suggest that smoking may differentially modulate glutamatergic function in schizophrenic patients and control subjects. The biological mechanisms underlying chronic tobacco use are likely to differ substantially between these two groups.

Tropisetron improves deficient inhibitory auditory processing in DBA/2 mice: role of alpha 7 nicotinic acetylcholine receptors

RATIONALE

Deficient inhibitory processing of the P50 auditory evoked potential is a pathophysiological feature of schizophrenia. Several lines of evidence suggest that alpha 7 nicotinic receptors play a critical role in this phenomenon. Similar to schizophrenic patients, DBA/2 mice spontaneously exhibit a deficit in inhibitory processing of the P20-N40 auditory evoked potential, which is thought to be a rodent analog of the human P50 auditory evoked potential.

OBJECTIVE

The present study was undertaken to examine whether tropisetron, a partial agonist at alpha 7 nicotinic receptors and an antagonist at 5-hydroxytryptamine-3 receptors, improves this deficit in DBA/2 mice.

RESULTS

Administration of tropisetron (1 mg/kg i.p.) significantly improved the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice. Coadministration of methyllycaconitine (MLA; 3 mg/kg i.p.), a partially selective antagonist at alpha 7 nicotinic receptors, significantly blocked the normalizing effect of tropisetron. Furthermore, MLA alone did not alter the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice.

CONCLUSIONS

The data suggest that tropisetron improves the deficient inhibitory processing of the P20-N40 auditory evoked potential in DBA/2 mice by effects on alpha 7 and perhaps alpha 4 beta 2 nicotinic receptors. Tropisetron may be useful for the treatment of deficient inhibitory processing in schizophrenia.