The nicotinergic receptor as a target for cognitive enhancement in schizophrenia: barking up the wrong tree?

The relationship between immune and cognitive dysfunction in mood and psychotic disorder: a systematic review and a meta-analysis

BACKGROUND

In psychotic and mood disorders, immune alterations are hypothesized to underlie cognitive symptoms, as they have been associated with elevated blood levels of inflammatory cytokines, kynurenine metabolites, and markers of microglial activation. The current meta-analysis synthesizes all available clinical evidence on the associations between immunomarkers (IMs) and cognition in these psychiatric illnesses.

METHODS

Pubmed, Web of Science, and Psycinfo were searched for peer-reviewed studies on schizophrenia spectrum disorder (SZ), bipolar disorder (BD), or major depressive disorder (MDD) including an association analysis between at least one baseline neuropsychological outcome measure (NP) and one IM (PROSPERO ID:CRD42021278371). Quality assessment was performed using BIOCROSS. Correlation meta-analyses, and random effect models, were conducted in Comprehensive Meta-Analysis version 3 investigating the association between eight cognitive domains and pro-inflammatory and anti-inflammatory indices (PII and AII) as well as individual IM.

RESULTS

Seventy-five studies (n = 29,104) revealed global cognitive performance (GCP) to be very weakly associated to PII (r = -0.076; p = 0.003; I² = 77.4) or AII (r = 0.067; p = 0.334; I² = 38.0) in the combined patient sample. Very weak associations between blood-based immune markers and global or domain-specific GCP were found, either combined or stratified by diagnostic subgroup (GCP x PII: SZ: r = -0.036, p = 0.370, I² = 70.4; BD: r = -0.095, p = 0.013, I² = 44.0; MDD: r = -0.133, p = 0.040, I² = 83.5). We found evidence of publication bias.

DISCUSSION

There is evidence of only a weak association between blood-based immune markers and cognition in mood and psychotic disorders. Significant publication and reporting biases were observed and most likely underlie the inflation of such associations in individual studies.

The effects of acute nicotine administration on cognitive and early sensory processes in schizophrenia: a systematic review

Nicotine use, which is mostly done through smoking tobacco, is among the most burdensome comorbidities of schizophrenia. However, the ways in which nicotine affects the cognitive and early sensory alterations found in this illness are still debated. After conducting a systematic literature search, 29 studies were selected. These studies involve individuals with schizophrenia who underwent cognitive and/or early sensory function assessments after acute nicotine administration and include 560 schizophrenia subjects and 346 non-schizophrenia controls. The findings highlight that a single dose of nicotine can improve a range of cognitive functions in schizophrenia subjects, such as attention, working memory, and executive functions, with attention being the most responsive domain. In addition, nicotine can modulate early detection of changes in the sensory environment at both the auditory and visual levels. Nevertheless, effects vary strongly depending on the type of neuropsychological assessment and nicotine intake conditions used in each study. The current findings suggest the need to consider a potential decrease of cognitive and early sensory performance when patients with schizophrenia quit smoking.

A review of the effects of nicotine on social functioning

Many smokers are aware that smoking is a dangerous health behavior and eventually try to quit smoking. Unfortunately, most quit attempts end in failure. Traditionally, the addictive nature of smoking has been attributed to the pharmacologic effects of nicotine. In an effort to offer a more comprehensive, biobehavioral analysis of smoking behavior and motivation, some researchers have begun to consider the role of social factors in smoking. In line with recent recommendations to integrate social and pharmacological analyses of smoking, we reviewed the experimental literature examining the effects of nicotine and nicotine withdrawal on social functioning. The review identified 13 studies that experimentally manipulated nicotine and assessed social functioning, 12 of which found support for nicotine's enhancement of social functioning. Although few experiments have investigated social functioning, they nevertheless offer compelling evidence that nicotine enhances social functioning in smokers and suggest that nicotine deprivation may hamper social functioning in those dependent on nicotine. Future directions for investigating social outcomes and context in those who use nicotine products are discussed with a focus on leveraging advances in social and developmental psychology, animal research, sociology, and neuroimaging to more comprehensively understand smoking behavior. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Designing selective modulators for the nicotinic receptor subtypes: challenges and opportunities

Nicotinic receptors are membrane proteins involved in several physiological processes. They are considered suitable drug targets for various CNS disorders or conditions, as shown by the large number of compounds which have entered clinical trials. In recent years, nonconventional agonists have been discovered: positive allosteric modulators, allosteric agonists, site-specific agonists and silent desensitizers are compounds able to modulate the receptor interacting at sites different from the orthodox one, or to desensitize the receptor without prior opening. While these new findings can further complicate the pharmacology of these proteins and the design and optimization of ligands, they undoubtedly offer new opportunities to find drugs for the many therapeutic indications involving nicotinic receptors.

Effects of smoking and smoking abstinence on spatial vision in chronic heavy smokers

Cigarette smoke is a complex chemical mixture, involving health-damaging components such as carbon monoxide, ammonia, pyridine, toluene and nicotine. While cognitive functions have been well documented in heavy smokers, spatial vision has been less characterized. In the article, we investigated smoking effects through contrast sensitivity function (CSF), a rigorous procedure that measures the spatial vision. Data were recorded from 48 participants, a group of non-smokers (n = 16), a group of chronic and heavy cigarette smokers (n = 16) and deprived smokers (n = 16); age range 20-45 years. Sinewave gratings with spatial frequencies ranging from 0.25 to 20 cycles per degree were used. All subjects were free from any neurological disorder, identifiable ocular disease and had normal acuity. No abnormalities were detected in the fundoscopic examination and in the optical coherence tomography exam. Contrary to expectations, performance on CSF differed between groups. Both smokers and deprived smokers presented a loss of contrast sensitivity compared to non-smokers. Post-hoc analyses suggest that deprived smokers were less sensitive at all spatial frequencies. These results suggest that not only chronic exposure to cigarette compounds but also withdrawal from nicotine affected spatial vision. This highlights the importance of understanding diffuse effects of smoking compounds on visual spatial processing.

Comparison of color discrimination in chronic heavy smokers and healthy subjects

Background: Cigarette smoke is probably the most significant source of exposure to toxic chemicals for humans, involving health-damaging components, such as nicotine, hydrogen cyanide and formaldehyde. The aim of the present study was to assess the influence of chronic heavy smoking on color discrimination (CD). Methods: All subjects were free of any neuropsychiatric disorder, identifiable ocular disease and had normal acuity. No abnormalities were detected in the fundoscopic examination and in the optical coherence tomography exam. We assessed color vision for healthy heavy smokers ( n = 15; age range, 20-45 years), deprived smokers ( n = 15, age range 20-45 years) and healthy non-smokers ( n = 15; age range, 20-45 years), using the psychophysical forced-choice method. All groups were matched for gender and education level. In this test, the volunteers had to choose the pseudoisochromatic stimulus containing a test frequency at four directions (e.g., up, down, right and left) in the subtest of Cambridge Colour Test (CCT): Trivector. Results: Performance on CCT differed between groups, and the observed pattern was that smokers had lower discrimination compared to non-smokers. In addition, deprived smokers presented lower discrimination to smokers and non-smokers. Contrary to expectation, the largest differences were observed for medium and long wavelengths. Conclusions: These results suggests that cigarette smoking, chronic exposure to its compounds, and withdrawal from nicotine affect color discrimination. This highlights the importance of understanding the diverse effects of nicotine on attentional bias.

The effects of cigarette smoking behavior and psychosis history on general and social cognition in bipolar disorder

OBJECTIVES

Several studies have documented the prevalence and effects of cigarette smoking on cognition in psychotic disorders; fewer have focused on bipolar disorder (BD). Cognitive and social dysfunction are common in BD, and the severity of these deficits may be related both to illness features (e.g., current symptoms, psychosis history) and health-related behaviors (e.g., smoking, alcohol use). The current study assessed the influence of cigarette smoking on general and social cognition in a BD cohort, accounting for illness features with a focus on psychosis history.

METHODS

We assessed smoking status in 105 euthymic patients with BD, who completed a comprehensive battery including social (facial affect recognition, emotional problem-solving, and theory of mind) and general (the MATRICS Consensus Cognitive Battery and executive functioning) cognitive measures. We compared smokers vs nonsmokers on cognitive performance and tested for the effects of psychosis history, premorbid intellectual functioning, substance use, and current affective symptoms.

RESULTS

Within the nonpsychotic subgroup with BD (n=45), smokers generally outperformed nonsmokers; by contrast, for subjects with BD with a history of psychosis (n=41), nonsmokers outperformed smokers. This pattern was noted more globally using a general composite cognitive score and on social/affective measures assessing patients' ability to identify emotions of facial stimuli and solve emotional problems.

CONCLUSIONS

Cigarette smoking differentially affects performance on both general and social cognition in patients with BD as a function of psychosis history. These results suggest that there may be at least partially divergent underlying neurobiological causes for cognitive dysfunction in patients with BD with and without psychosis.

Drugs, games, and devices for enhancing cognition: implications for work and society

As work environments change, the demands on working people change. Cognitive abilities in particular are becoming progressively more important for work performance and successful competition in a global environment. However, work-related stress, performance over long hours, lack of sleep, shift work, and jet lag affect cognitive functions. Therefore, an increasing number of healthy people are reported to use cognitive-enhancing drugs, as well as other interventions, such as noninvasive brain stimulation, to maintain or improve work performance. This review summarizes research on pharmacological and technical methods as well as cognitive training, including game apps for the brain, in healthy people. In neuropsychiatric disorders, impairments in cognitive functions can drastically reduce the chances of returning to work; therefore, this review also summarizes findings from pharmacological and cognitive-training studies in neuropsychiatric disorders.

The Role of Nicotine in Schizophrenia

Schizophrenia is associated with by severe disruptions in thought, cognition, emotion, and behavior. Patients show a marked increase in rates of smoking and nicotine dependence relative to nonaffected individuals, a finding commonly ascribed to the potential ameliorative effects of nicotine on symptom severity and cognitive impairment. Indeed, many studies have demonstrated improvement in patients following the administration of nicotine. Such findings have led to an increased emphasis on the development of therapeutic agents to target the nicotinic system as well as increasing the impetus to understand the genetic basis for nicotinic dysfunction in schizophrenia. The goal of this review article is to provide a critical summary of evidence for the role of the nicotinic system in schizophrenia. The first part will review the role of nicotine in normalization of primary dysfunctions and endophenotypical changes found in schizophrenia. The second part will provide a summary of genetic evidence linking polymorphisms in nicotinic receptor genes to smoking and schizophrenia. The third part will summarize attempts to treat schizophrenia using agents specifically targeting nicotinic and nicotinic receptor subtypes. Although currently available antipsychotic treatments are generally able to manage some aspects of schizophrenia (e.g., positive symptoms) they fail to address several other critically effected aspects of the disease. As such, the search for novel mechanisms to treat this disease is necessary.

Nicotine effects in adolescence and adulthood on cognition and α₄β₂-nicotinic receptors in the neonatal ventral hippocampal lesion rat model of schizophrenia

RATIONAL

Nicotine use in schizophrenia has traditionally been explained as "self-medication" of cognitive and/or nicotinic acetylcholinergic receptor (nAChR) abnormalities.

OBJECTIVES

We test this hypothesis in a neurodevelopmental rat model of schizophrenia that shows increased addiction behaviors including enhanced nicotine reinforcement and drug-seeking.

METHODS

Nicotine transdermal patch (5 mg/kg/day vs. placebo × 10 days in adolescence or adulthood) effects on subsequent radial-arm maze learning (15 sessions) and frontal-cortical-striatal nAChR densities (α4β2; [3H]-epibatidine binding) were examined in neonatal ventral hippocampal lesion (NVHL) and SHAM-operated rats.

RESULTS

NVHL cognitive deficits were not differentially affected by nicotine history compared to SHAMs. Nicotine history produced minimal cognitive effects while increasing food-reward consumption on the maze, compounding with NVHL-induced overconsumption. Acute nicotine (0.5 mg/kg) delivered before the final maze sessions produced modest improvements in maze performance in rats with nicotine patch histories only, but not differentially so in NVHLs. Consistent with in vivo neuroimaging of β2 nAChR binding in schizophrenia smokers vs. non-smokers and healthy controls, adult NVHLs showed 12% reductions in nAChR binding in MPFC (p < 0.05) but not ventral striatum (<5% changes, p > .40), whereas nicotine history elevated nAChRs across both regions (>30%, p < 0.001) without interacting with NVHLs. Adolescent vs. adult nicotine exposure did not alter nAChRs differentially.

CONCLUSIONS

Although replicating nicotine-induced upregulation of nAChRs in human smokers and demonstrating NVHL validity in terms of schizophrenia-associated nAChR density patterns, these findings do not support hypotheses explaining increased nicotine use in schizophrenia as reflecting illness-specific effects of nicotine to therapeutically alter cognition or nAChR densities.

Dementia praecox redux: a systematic review of the nicotinic receptor as a target for cognitive symptoms of schizophrenia

Most individuals with schizophrenia suffer some cognitive dysfunction: such deficits are predictive of longer-term functioning; and current dopamine-blocking antipsychotics have made little impact on this domain. There is a pressing need to develop novel pharmacological agents to tackle this insidious but most disabling of problems. The acetylcholinergic system is involved in cognitive and attentional processing, and its metabotropic and nicotinic receptors are widespread throughout the brain. Deficits in acetylcholinergic functioning occur in schizophrenia, and high rates of tobacco smoking have been posited to represent a form of self-medication. The nicotinic acetylcholine receptor (nAChR) has emerged as a putative target to improve cognitive deficits in schizophrenia, and this study systematically reviewed the emerging data. Nineteen studies were identified, covering three compound classes: agonists at the α7 and α 4β2 nAChRs, and positive allosteric modulators. Overall data are underwhelming: some studies showed significant improvements in cognition but as many studies had negative findings. It remains unclear if this represents drug limitations or nascent study methodology problems. The literature is particularly hindered by variability in inclusion of smokers, generally small sample sizes, and a lack of consensus on cognitive test batteries. Future work should evaluate longer-term outcomes, and, particularly, the effects of concomitant cognitive training.

CDP-choline: effects of the procholine supplement on sensory gating and executive function in healthy volunteers stratified for low, medium and high P50 suppression

Diminished auditory sensory gating and associated neurocognitive deficits in schizophrenia have been linked to altered expression and function of the alpha-7 nicotinic acetycholinergic receptor (α7 nAChR), the targeting of which may have treatment potential. Choline is a selective α7 nAChR agonist and the aim of this study was to determine whether cytidine 5'-diphosphocholine (CDP-choline), or citicoline, a dietary source of choline, increases sensory gating and cognition in healthy volunteers stratified for gating level. In a randomized, placebo-controlled, double-blind design involving acute administration of low, moderate doses (500 mg, 1000 mg) of CDP-choline, 24 healthy volunteers were assessed for auditory gating as indexed by suppression of the P50 event-related potential (ERP) in a paired-stimulus (S1, S2) paradigm, and for executive function as measured by the Groton Maze Learning Task (GMLT) of the CogState Schizophrenia Battery. CDP-choline improved gating (1000 mg) and suppression of the S2 P50 response (500 mg, 1000 mg), with the effects being selective for individuals with low gating (suppression) levels. Tentative support was also shown for increased GMLT performance (500 mg) in low suppressors. These preliminary findings with CDP-choline in a healthy, schizophrenia-like surrogate sample are consistent with a α7 nAChR mechanism and support further trials with choline as a pro-cognitive strategy.

Hyper-coupling between working memory task-evoked activations and amplitude of spontaneous fluctuations in first-episode schizophrenia

Working memory (WM) deficit is an important component of impaired cognition in schizophrenia. However, between-studies inconsistencies as to the specific functional substrate imply that inter-individual variability (IIV) in the WM performance is associated with IIV in brain activity in schizophrenia. To examine the neural substrate of this WM IIV, we studied whether the neural mechanisms that underlie individual differences in WM capacity are the same in schizophrenia patients and healthy people. We correlated the IIV of the task-evoked brain activity and task performance during an n-back WM task with the IIV of the moment-to-moment variability in intrinsic resting-state activity, as measured by the amplitude of low-frequency fluctuations (ALFFs) and further compared this relationship between 17 patients with first-episode schizophrenia (FES) and 18 healthy controls. Between-group comparisons of the correlation patterns indicated aberrant ALFF-WM activation correlations and ALFF-WM performance correlations in the FES patients, but no significant changes were detected in any single measurement of these three characteristics. Specifically, we found increased positive ALFF-WM activation correlations in the bilateral lateral prefrontal cortices, posterior parietal cortices and fusiform gyri in the FES patients. We also observed significant increases in positive ALFF-WM performance correlations in the bilateral ventromedial prefrontal cortices in the FES patients. This hyper-coupling between the ALFF and fMRI measures during a WM task may indicate that it was difficult for the patients to detach themselves from one state to transition to another and suggests that the inefficient cortical function in schizophrenia stems from the intrinsic functional architecture of the brain.

The effects of nicotine on cognition are dependent on baseline performance

Since cholinergic neurotransmission plays a major role in cognition, stimulation of the nicotinic acetylcholine receptor may be a target for cognitive enhancement. While nicotine improves performance on several cognitive domains, results of individual studies vary. A possible explanation for these findings is that the effect of nicotine administration may be dependent on baseline cognitive function, where subjects with a suboptimal cognitive performance may benefit from nicotine, while subjects who already perform optimally may show a decline in performance after nicotinic stimulation. We conducted a double-blind randomised placebo-controlled crossover trial, examining the effects of placebo, 1, and 2mg of nicotine on cognition in young (n=16, age 18-30 years) and healthy elderly (n=16, age 60-75 years) subjects. We hypothesised that the elderly would benefit more from nicotine compared to young subjects, as normal ageing is associated with decreases in cognitive function. Attention, working memory, visual memory, information-processing speed, psychomotor function, stereotypy, and emotion recognition were assessed. Compared to the young volunteers, the elderly performed significantly worse on psychomotor function and emotion recognition in the placebo condition. Nicotine had no effect in the young volunteers and decreased performance on working memory and visual memory in the elderly. Contrary to our hypothesis, the effect of nicotine was dependent on baseline performance in both the groups, with subjects with lower baseline performance benefiting from nicotine administration, while those with higher baseline performance performed worse after nicotine administration. This suggests that subjects with lower cognitive performance, irrespective of age, may benefit from nicotine.