The nicotinic acetylcholine receptor as a target for antidepressant drug development

Cytisine: State of the art in pharmacological activities and pharmacokinetics

Cytisine is a naturally occurring bioactive compound, an alkaloid mainly isolated from legume plants. In recent years, various biological activities of cytisine have been explored, showing certain effects in smoking cessation, reducing drinking behavior, anti-tumor, cardiovascular protection, blood sugar regulation, neuroprotection, osteoporosis prevention and treatment, etc. At the same time, cytisine has the advantages of high efficiency, safety, and low cost, has broad development prospects, and is a drug of great application value. However, a summary of cytisine's biological activities is currently lacking. Therefore, this paper summarizes the pharmacological action, mechanism, and pharmacokinetics of cytisine by referring to numerous databases, and analyzes the new and core targets of cytisine with the help of computer simulation technology, to provide reference for doctors.

Nicotinic acetylcholine receptors in neurological and psychiatric diseases

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that are widely distributed both pre- and post-synaptically in the mammalian brain. By modulating cation flux across cell membranes, neuronal nAChRs regulate neuronal excitability and the release of a variety of neurotransmitters to influence multiple physiologic and behavioral processes including synaptic plasticity, motor function, attention, learning and memory. Abnormalities of neuronal nAChRs have been implicated in the pathophysiology of neurologic disorders including Alzheimer's disease, Parkinson's disease, epilepsy, and Tourette´s syndrome, as well as psychiatric disorders including schizophrenia, depression, and anxiety. The potential role of nAChRs in a particular illness may be indicated by alterations in the expression of nAChRs in relevant brain regions, genetic variability in the genes encoding for nAChR subunit proteins, and/or clinical or preclinical observations where specific ligands showed a therapeutic effect. Over the past 25 years, extensive preclinical and some early clinical evidence suggested that ligands at nAChRs might have therapeutic potential for neurologic and psychiatric disorders. However, to date the only approved indications for nAChR ligands are smoking cessation and the treatment of dry eye disease. It has been argued that progress in nAChR drug discovery has been limited by translational gaps between the preclinical models and the human disease as well as unresolved questions regarding the pharmacological goal (i.e., agonism, antagonism or receptor desensitization) depending on the disease.

Vagally Mediated Heart Rate Variability Is Associated With Executive Function Changes in Patients With Treatment-Resistant Depression Following Magnetic Seizure Therapy

OBJECTIVES

Magnetic seizure therapy (MST) is a novel investigational brain stimulation modality for patients with treatment-resistant depression (TRD). MST is a potential alternative seizure-based treatment to electroconvulsive therapy (ECT), given that it may offer equivalent antidepressant efficacy, yet with a relative sparing of cognitive functioning. Heart rate variability (HRV) is a marker of central autonomic functioning. We aimed to explore the relationships among baseline HRV, age, clinical outcome, and executive function following MST, in patients with TRD.

MATERIALS AND METHODS

Eighty-eight TRD patients (55 females; 18-70 years) were enrolled and 48 patients completed a course of MST in an open-label study. Patients received MST treatments two to three times per week, using one of three stimulation frequencies (ie, 100 Hz, 50 Hz, or 25 Hz) at 100% stimulator output. Root mean square of the successive R-R differences (RMSSD), an index of HRV, was computed from a baseline electrocardiogram (ECG) recording. Clinical symptoms were assessed using the Hamilton Depression Rating Scale (HAM-D24) and the Quick Inventory of Depressive Symptomatology (QIDS16). Executive function was assessed using the Trail Making Test and the Mazes Test from the MATRICS battery.

RESULTS

Baseline RMSSD was correlated with baseline HAM-D24 (r = -0.340, p = 0.001) and baseline Mazes Test (r = 0.417, p = 0.0007) but not with baseline Trail Making Test. Furthermore, baseline RMSSD was not correlated with changes on the HAM-D24, QIDS16, or total scores on the Trail Making Test. However, there was a significant correlation between baseline RMSSD and improvement on the Mazes Test following MST (r = 0.502, p = 0.0004).

CONCLUSIONS

Since this is an open-label trial, the influence of the placebo effect cannot be excluded. However, our results suggest that baseline RMSSD may be a state-biomarker of depression and executive function impairment. Additionally, while baseline vagally mediated resting cardiac activity did not predict the outcome of depression, it may mediate executive function improvements following MST.

Structure of Gas Phase Monohydrated Nicotine: Implications for Nicotine's Native Structure in the Acetylcholine Binding Protein

We report a joint experimental-theoretical study of the never reported before structure and infrared spectra of gas phase monohydrated nicotine (NIC) and nornicotine (NOR) and use them to assign their protonation sites. NIC's biological activity is strongly affected by its protonation site, namely, the pyrrolidine (Pyrro-NICH⁺, anticipated active form) and pyridine (Pyri-NICH⁺) forms; however, these have yet to be directly experimentally determined in either the nicotinic acetylcholine receptor (nAChR, no water present) or the acetylcholine-binding protein (AChBP, a single water molecule is present) but can only be inferred to be Pyrro-NICH⁺ from the intermolecular distance to the neighboring residues (i.e., tryptophan). Our temperature-controlled double ion trap infrared spectroscopic experiments assisted by the collisional stripping method and high-level theoretical calculations yield the protonation ratio of Pyri:Pyrro = 8:2 at 240 K for the gas phase NICH⁺···(H₂O) complex, which resembles the molecular cluster present in the AChBP. Therefore, a single water molecule in the gas phase enhances this ratio in NICH⁺ relative to the 3:2 for the nonhydrated gas phase NICH⁺ in a trend that contrasts with the almost exclusive presence of Pyrro-NICH⁺ in aqueous solution. In contrast, the Pyri-NORH⁺ protomer is exclusively observed, a fact that may correlate with its weaker biological activity.

Ropanicant (SUVN-911), an α4β2 nicotinic acetylcholine receptor antagonist intended for the treatment of depressive disorders: pharmacological, behavioral, and neurochemical characterization

RATIONALE

Ropanicant (SUVN-911) (3-(6-Chloropyridine-3-yloxymethyl)-2-azabicyclo (3.1.0) hexane hydrochloride) is a novel α4β2 nicotinic acetylcholine receptor (nAChR) antagonist being developed for the treatment of depressive disorders.

OBJECTIVES

Pharmacological and neurochemical characterization of Ropanicant to support a potential molecule for the treatment of depressive disorders.

METHODS

Ropanicant was assessed for antidepressant-like activity using the rat forced swimming test (FST) and differential reinforcement of low rate -72 s (DRL-72 s). Alleviation of anhedonia was assessed in chronic mild stress model using sucrose preference test. To understand the mechanism of action, serotonin levels, ionized calcium-binding adaptor molecule 1 (Iba1), and brain-derived neurotrophic factor (BDNF) were determined. The onset of antidepressant-like activity was determined using the reduction in submissive behavior assay. The effects on cognition and sexual functions were assessed using the object recognition task and sexual dysfunction assay respectively. Interaction of Ropanicant, TC-5214, and methyllycaconitine (MLA) with citalopram was investigated individually in mice FST.

RESULTS

Ropanicant exhibited antidepressant like properties in the FST and DRL-72 s. A significant reduction in anhedonia was observed in the sucrose preference test. Oral administration of Ropanicant produced a significant increase in serotonin and BDNF levels, with a reduction in the Iba1 activity. The onset of antidepressant like effect with Ropanicant was within a week of treatment, and was devoid of cognitive dulling and sexual dysfunction. While Ropanicant potentiated the effect of citalopram in FST, such an effect was not observed with MLA or TC-5214.

CONCLUSIONS

Preclinical studies with Ropanicant support the likelihood of its therapeutic utility in the treatment of depressive disorders.

Cytisine and cytisine derivatives. More than smoking cessation aids

Cytisine, a natural bioactive compound that is mainly isolated from plants of the Leguminosae family (especially the seeds of Laburnum anagyroides), has been marketed in central and eastern Europe as an aid in the clinical management of smoking cessation for more than 50 years. Its main targets are neuronal nicotinic acetylcholine receptors (nAChRs), and pre-clinical studies have shown that its interactions with various nAChR subtypes located in different areas of the central and peripheral nervous systems are neuroprotective, have a wide range of biological effects on nicotine and alcohol addiction, regulate mood, food intake and motor activity, and influence the autonomic and cardiovascular systems. Its relatively rigid conformation makes it an attractive template for research of new derivatives. Recent studies of structurally modified cytisine have led to the development of new compounds and for some of them the biological activities are mediated by still unidentified targets other than nAChRs, whose mechanisms of action are still being investigated. The aim of this review is to describe and discuss: 1) the most recent pre-clinical results obtained with cytisine in the fields of neurological and non-neurological diseases; 2) the effects and possible mechanisms of action of the most recent cytisine derivatives; and 3) the main areas warranting further research.

Randomized Clinical Trials Investigating Innovative Interventions for Smoking Cessation in the Last Decade

Every year, billions of dollars are spent treating smoking and related conditions, yet smoking-related morbidity and mortality continue to rise. There are currently only three FDA-approved medications for smoking cessation: nicotine replacement therapy, bupropion, and varenicline. Although these medications increase abstinence rates, most individuals relapse following treatment. This chapter reviews clinical trials published within the past 10 years investigating novel smoking cessation pharmacotherapies. Among these pharmacotherapies, some showed promising results, such as cytisine and endocannabinoid modulators, whereas others failed to produce significant effects. More research is needed to develop drugs that produce higher rates of long-term abstinence and to determine which subgroups of patients benefit from a given treatment.

Quantitative separation of the depressive phase of bipolar disorder and major depressive disorder using electrovestibulography

Objectives: No electrophysiological, neuroimaging or genetic markers have been established that strongly relate to the diagnostic separation of bipolar disorder (BD) and major depressive disorder (MDD). This paper's objective is to describe the potential of features, extracted from the recording of electrical activity from the outer ear canal, in a process called electrovestibulography (EVestG), for identifying depressed and partly remitted/remitted MDD and BD patients from each other.Methods: From EVestG data four sensory vestibulo-acoustic features were extracted from both background (no movement) and using a single supine-vertical translation stimulus to distinguish 27 controls, 39 MDD and 43 BD patients.Results: Using leave-one-out-cross-validation, unbiased parametric and non-parametric classification routines resulted in 78-83% (2-3 features), 80-81% (1-2 features) and 66-68% (3 features) accuracies for separation of MDD from BD, controls from depressed (BD & MDD) and the 3-way separation of BD from MDD from control groups, respectively. The main limitations of this study were the inability to fully disentangle the impact of prescribed medication from the responses and also the limited sample size.Conclusions: EVestG features can reliably identify depressed and partly remitted/remitted MDD and BD patients from each other.

Translating GWAS findings into therapies for depression and anxiety disorders: gene-set analyses reveal enrichment of psychiatric drug classes and implications for drug repositioning

BACKGROUND

Depression and anxiety disorders (AD) are the first and sixth leading causes of disability worldwide. Despite their high prevalence and significant disability resulted, there are limited advances in new drug development. Recently, genome-wide association studies (GWAS) have greatly advanced our understanding of the genetic basis underlying psychiatric disorders.

METHODS

Here we employed gene-set analyses of GWAS summary statistics for drug repositioning. We explored five related GWAS datasets, including two on major depressive disorder (MDD2018 and MDD-CONVERGE, with the latter focusing on severe melancholic depression), one on AD, and two on depressive symptoms and neuroticism in the population. We extracted gene-sets associated with each drug from DSigDB and examined their association with each GWAS phenotype. We also performed repositioning analyses on meta-analyzed GWAS data, integrating evidence from all related phenotypes.

RESULTS

Importantly, we showed that the repositioning hits are generally enriched for known psychiatric medications or those considered in clinical trials. Enrichment was seen for antidepressants and anxiolytics but also for antipsychotics. We also revealed new candidates or drug classes for repositioning, some of which were supported by experimental or clinical studies. For example, the top repositioning hit using meta-analyzed p values was fendiline, which was shown to produce antidepressant-like effects in mouse models by inhibition of acid sphingomyelinase.

CONCLUSION

Taken together, our findings suggest that human genomic data such as GWAS are useful in guiding drug discoveries for depression and AD.

Selecting Potential Neuronal Drug Leads from Conotoxins of Various Venomous Marine Cone Snails in Bali, Indonesia

Many conotoxins, natural peptides of marine cone snails, have been identified to target neurons. Here, we provide data on pharmacological families of the conotoxins of 11 species of cone snails collected in Bali. The identified definitive pharmacological families possibly targeting neuronal tissues were α (alpha), ι (iota), κ (kappa), and ρ (rho). These classes shall target nicotinic acetylcholine receptors, voltage-gated Na channels, voltage-gated K channels, and α1-adrenoceptors, respectively. The VI/VII-O3 conotoxins might be prospected as an inhibitor of N-methyl-d-aspartate. Con-ikot-ikot could be applied as an α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor blocker medicine. The definitive pharmacology classes of conotoxins as well as those yet to be elucidated need to be further established and verified.

Bipolar disorder in the balance

Bipolar disorder (BD) is a severe mood disorder that lacks established electrophysiological, neuroimaging or biological markers to assist with both diagnosis and monitoring disease severity. This study's aim is to describe the potential of new neurophysiological features assistive in BD diagnosis and severity measurement utilizing the recording of electrical activity from the outer ear canal called Electrovestibulography (EVestG). From EVestG data sensory vestibulo-acoustic features were extracted from a single supine-vertical translation stimulus to distinguish 50 depressed and partly remitted/remitted bipolar disorder patients [18 symptomatic (BD-S, MADRS > 19), 32 reduced symptomatic (BD-R, MADRS ≤ 19)] and 31 age and gender matched healthy individuals (controls). Six features were extracted from the measured firing pattern interval histogram and the extracted shape of the average field potential response. Five of the six features had low but significant correlations (p < 0.05) with the MADRS assessment. Using leave-one-out-cross-validation, unbiased parametric and non-parametric classification routines resulted in 75-79%, 84-86%, 76-85% and 79-82% accuracy for separation of control from BD, BD-S and BD-R as well as BD-S from BD-R groups, respectively. The main limitation of this study was the inability to fully disentangle the impact of prescribed medication from the responses recorded. A mix of stationary and movement evoked EVestG features produced good discrimination between control and BD patients whether BD-S or BD-R. Moreover, BD-S and BD-R appear to have measurably different pathophysiological manifestations. The firing pattern features used were dissimilar to those observed in a prior major depressive disorder study.

SLPI Inhibits ATP-Mediated Maturation of IL-1β in Human Monocytic Leukocytes: A Novel Function of an Old Player

Interleukin-1β (IL-1β) is a potent, pro-inflammatory cytokine of the innate immune system that plays an essential role in host defense against infection. However, elevated circulating levels of IL-1β can cause life-threatening systemic inflammation. Hence, mechanisms controlling IL-1β maturation and release are of outstanding clinical interest. Secretory leukocyte protease inhibitor (SLPI), in addition to its well-described anti-protease function, controls the expression of several pro-inflammatory cytokines on the transcriptional level. In the present study, we tested the potential involvement of SLPI in the control of ATP-induced, inflammasome-dependent IL-1β maturation and release. We demonstrated that SLPI dose-dependently inhibits the ATP-mediated inflammasome activation and IL-1β release in human monocytic cells, without affecting the induction of pro-IL-1β mRNA by LPS. In contrast, the ATP-independent IL-1β release induced by the pore forming bacterial toxin nigericin is not impaired, and SLPI does not directly modulate the ion channel function of the human P2X₇ receptor heterologously expressed in Xenopus laevis oocytes. In human monocytic U937 cells, however, SLPI efficiently inhibits ATP-induced ion-currents. Using specific inhibitors and siRNA, we demonstrate that SLPI activates the calcium-independent phospholipase A2β (iPLA2β) and leads to the release of a low molecular mass factor that mediates the inhibition of IL-1β release. Signaling involves nicotinic acetylcholine receptor subunits α7, α9, α10, and Src kinase activation and results in an inhibition of ATP-induced caspase-1 activation. In conclusion, we propose a novel anti-inflammatory mechanism induced by SLPI, which inhibits the ATP-dependent maturation and secretion of IL-1β. This novel signaling pathway might lead to development of therapies that are urgently needed for the prevention and treatment of systemic inflammation.

CHRNA7 copy number gains are enriched in adolescents with major depressive and anxiety disorders

OBJECTIVE

Neuronal nicotinic acetylcholine receptors (nAChRs), specifically the α7 nAChR encoded by the gene CHRNA7, have been implicated in behavior regulation in animal models. In humans, copy number variants (CNVs) of CHRNA7 are found in a range of neuropsychiatric disorders, including mood and anxiety disorders. Here, we aimed to determine the prevalence of CHRNA7 CNVs among adolescents and young adults with major depressive disorder (MDD) and anxiety disorders.

METHODS

Twelve to 21 year-old participants with MDD and/or anxiety disorders (34% males, mean ± std age: 18.9 ± 1.8 years) were assessed for CHRNA7 copy number state using droplet digital PCR (ddPCR) and genomic quantitative PCR (qPCR). Demographic, anthropometric, and clinical data, including the Beck Anxiety Index (BAI), Beck Depression Inventory (BDI), and the Inventory of Depressive Symptoms (IDS) were collected and compared across individuals with and without a CHRNA7 CNV.

RESULTS

Of 205 individuals, five (2.4%) were found to carry a CHRNA7 gain, significantly higher than the general population. No CHRNA7 deletions were identified. Clinically, the individuals carrying CHRNA7 duplications did not differ significantly from copy neutral individuals with MDD and/or anxiety disorders.

CONCLUSIONS

CHRNA7 gains are relatively prevalent among young individuals with MDD and anxiety disorders (odds ratio = 4.032) without apparent distinguishing clinical features. Future studies should examine the therapeutic potential of α7 nAChR targeting drugs to ameliorate depressive and anxiety disorders.

Alpha-1 Antitrypsin Inhibits ATP-Mediated Release of Interleukin-1β via CD36 and Nicotinic Acetylcholine Receptors

While interleukin (IL)-1β is a potent pro-inflammatory cytokine involved in host defense, high levels can cause life-threatening sterile inflammation including systemic inflammatory response syndrome. Hence, the control of IL-1β secretion is of outstanding biomedical importance. In response to a first inflammatory stimulus such as lipopolysaccharide, pro-IL-1β is synthesized as a cytoplasmic inactive pro-form. Extracellular ATP originating from injured cells is a prototypical second signal for inflammasome-dependent maturation and release of IL-1β. The human anti-protease alpha-1 antitrypsin (AAT) and IL-1β regulate each other via mechanisms that are only partially understood. Here, we demonstrate that physiological concentrations of AAT efficiently inhibit ATP-induced release of IL-1β from primary human blood mononuclear cells, monocytic U937 cells, and rat lung tissue, whereas ATP-independent IL-1β release is not impaired. Both, native and oxidized AAT are active, suggesting that the inhibition of IL-1β release is independent of the anti-elastase activity of AAT. Signaling of AAT in monocytic cells involves the lipid scavenger receptor CD36, calcium-independent phospholipase A2β, and the release of a small soluble mediator. This mediator leads to the activation of nicotinic acetylcholine receptors, which efficiently inhibit ATP-induced P2X7 receptor activation and inflammasome assembly. We suggest that AAT controls ATP-induced IL-1β release from human mononuclear blood cells by a novel triple-membrane-passing signaling pathway. This pathway may have clinical implications for the prevention of sterile pulmonary and systemic inflammation.

Mecamylamine treatment for alcohol dependence: a randomized controlled trial

BACKGROUND AND AIMS

The nicotinic acetylcholine receptor antagonist, mecamylamine, is a potential novel pharmacotherapy for alcohol use disorder. The aims were to compare alcohol consumption between mecamylamine and placebo and test if smoking status modified treatment effects.

DESIGN

Out-patient, randomized, double-blind clinical trial for 12 weeks of treatment with mecamylamine (10 mg) (n = 65) versus placebo (n = 63).

SETTING

Connecticut, USA.

PARTICIPANTS

Individuals had current alcohol dependence (n = 128), had an average age of 48.5 [standard deviation (SD) = 9.4], 110 (85.9%) were men, and included 74 smokers (57.8%) and 54 non-smokers (42.2%). Participants were randomized to mecamylamine 10 mg per day or placebo. All subjects also received medical management therapy administered by trained research personnel.

MEASUREMENTS

Primary outcome was percentage of heavy drinking days during the last month of treatment; other outcomes included drinking days, drinks per drinking days, alcohol craving, smoking, symptoms of nicotine withdrawal and side effects.

FINDINGS

There were no significant differences in the percentage of heavy drinking days at 3 months between the mecamylamine (mean = 18.4, SD = 29.0) and placebo treatment groups (mean = 20.4, SD = 29.2) [F1, 100  = 1.3, P = 0.25; effect size d = 0.07; mean difference = 2.06, 95% confidence interval (CI) = -8.96 to 13.08]. There were no significant differences in percentage of drinking days or in drinks per drinking day at month 3 between the mecamylamine and placebo groups; there were no significant interactions.

CONCLUSIONS

Mecamylamine 10 mg per day did not reduce alcohol consumption significantly in treatment-seeking smokers and non-smokers with alcohol use disorder.

Electrophysiology-Based Assays to Detect Subtype-Selective Modulation of Human Nicotinic Acetylcholine Receptors

The Family Smoking Prevention and Tobacco Control Act of 2009 (Public Law 111-31) gave the US Food and Drug Administration (FDA) the responsibility for regulating tobacco products. Nicotine is the primary addictive component of tobacco and its effects can be modulated by additional ingredients in manufactured products. Nicotine acts by mimicking the neurotransmitter acetylcholine on neuronal nicotinic acetylcholine receptors (nAChRs), which function as ion channels in cholinergic modulation of neurotransmission. Subtypes within the family of neuronal nAChRs are defined by their α- and β-subunit composition. The subtype-selective profiles of tobacco constituents are largely unknown, but could be essential for understanding the physiological effects of tobacco products. In this report, we report the development and validation of electrophysiology-based high-throughput screens (e-HTS)for human nicotinic subtypes, α3β4, α3β4α5, α4β2, and α7 stably expressed in Chinese Hamster Ovary cells. Assessment of agonist sensitivity and acute desensitization gave results comparable to those obtained by conventional manual patch clamp electrophysiology assays. The potency of reference antagonists for inhibition of the receptor channels and selectivity of positive allosteric modulators also were very similar between e-HTS and conventional manual patch voltage clamp data. Further validation was obtained in pilot screening of a library of FDA-approved drugs that identified α7 subtype-selective positive allosteric modulation by novel compounds. These assays provide new tools for profiling of nicotinic receptor selectivity.

Canonical and Novel Non-Canonical Cholinergic Agonists Inhibit ATP-Induced Release of Monocytic Interleukin-1β via Different Combinations of Nicotinic Acetylcholine Receptor Subunits α7, α9 and α10

Recently, we discovered a cholinergic mechanism that inhibits the adenosine triphosphate (ATP)-dependent release of interleukin-1β (IL-1β) by human monocytes via nicotinic acetylcholine receptors (nAChRs) composed of α7, α9 and/or α10 subunits. Furthermore, we identified phosphocholine (PC) and dipalmitoylphosphatidylcholine (DPPC) as novel nicotinic agonists that elicit metabotropic activity at monocytic nAChR. Interestingly, PC does not provoke ion channel responses at conventional nAChRs composed of subunits α9 and α10. The purpose of this study is to determine the composition of nAChRs necessary for nicotinic signaling in monocytic cells and to test the hypothesis that common metabolites of phosphatidylcholines, lysophosphatidylcholine (LPC) and glycerophosphocholine (G-PC), function as nAChR agonists. In peripheral blood mononuclear cells from nAChR gene-deficient mice, we demonstrated that inhibition of ATP-dependent release of IL-1β by acetylcholine (ACh), nicotine and PC depends on subunits α7, α9 and α10. Using a panel of nAChR antagonists and siRNA technology, we confirmed the involvement of these subunits in the control of IL-1β release in the human monocytic cell line U937. Furthermore, we showed that LPC (C16:0) and G-PC efficiently inhibit ATP-dependent release of IL-1β. Of note, the inhibitory effects mediated by LPC and G-PC depend on nAChR subunits α9 and α10, but only to a small degree on α7. In Xenopuslaevis oocytes heterologously expressing different combinations of human α7, α9 or α10 subunits, ACh induced canonical ion channel activity, whereas LPC, G-PC and PC did not. In conclusion, we demonstrate that canonical nicotinic agonists and PC elicit metabotropic nAChR activity in monocytes via interaction of nAChR subunits α7, α9 and α10. For the metabotropic signaling of LPC and G-PC, nAChR subunits α9 and α10 are needed, whereas α7 is virtually dispensable. Furthermore, molecules bearing a PC group in general seem to regulate immune functions without perturbing canonical ion channel functions of nAChR.

PET imaging of α7 nicotinic acetylcholine receptors: a comparative study of [18F]ASEM and [18F]DBT-10 in nonhuman primates, and further evaluation of [18F]ASEM in humans

PURPOSE

The α₇ nicotinic acetylcholine receptor (nAChR) is implicated in many neuropsychiatric disorders, making it an important target for positron emission tomography (PET) imaging. The first aim of this work was to compare two α₇ nAChRs PET radioligands, [¹⁸F]ASEM (3-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-6-([¹⁸F]fluorodibenzo[b,d]thiophene 5,5-dioxide) and [¹⁸F]DBT-10 (7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-([¹⁸F]fluorodibenzo[b,d]thiophene 5,5-dioxide), in nonhuman primates. The second aim was to assess further the quantification and test-retest variability of [¹⁸F]ASEM in humans.

METHODS

PET scans with high specific activity [¹⁸F]ASEM or [¹⁸F]DBT-10 were acquired in three rhesus monkeys (one male, two female), and the kinetic properties of these radiotracers were compared. Additional [¹⁸F]ASEM PET scans with blocking doses of nicotine, varenicline, and cold ASEM were acquired separately in two animals. Next, six human subjects (five male, one female) were imaged with [¹⁸F]ASEM PET for 180 min, and arterial sampling was used to measure the parent input function. Different modeling approaches were compared to identify the optimal analysis method and scan duration for quantification of [¹⁸F]ASEM distribution volume (V _T). In addition, retest scans were acquired in four subjects (three male, one female), and the test-retest variability of V _T was assessed.

RESULTS

In the rhesus monkey brain [¹⁸F]ASEM and [¹⁸F]DBT-10 exhibited highly similar kinetic profiles. Dose-dependent blockade of [¹⁸F]ASEM binding was observed, while administration of either nicotine or varenicline did not change [¹⁸F]ASEM V _T. [¹⁸F]ASEM was selected for further validation because it has been used in humans. Accurate quantification of [¹⁸F]ASEM V _T in humans was achieved using multilinear analysis with at least 90 min of data acquisition, resulting in V _T values ranging from 19.6 ± 2.5 mL/cm³ in cerebellum to 25.9 ± 2.9 mL/cm³ in thalamus. Test-retest variability of V _T was 11.7 ± 9.8%.

CONCLUSIONS

These results confirm [¹⁸F]ASEM as a suitable radiotracer for the imaging and quantification of α₇ nAChRs in humans.

Major depression and electrovestibulography

OBJECTIVES

No electrophysiological neuroimaging or genetic markers have been established that strongly relate to a diagnosis of major depression or its severity. The objective of this paper is to describe the preliminary evaluation of a potential new biomarker for depression utilizing the recording of electrical activity from the outer ear canal referred to as electrovestibulography (EVestG).

METHODS

Sensory oto-acoustic features were extracted from EVestG data to compare 31 healthy age- and gender-matched individuals as controls to 43 major depressive disorder (MDD) subjects (22 symptomatic (MDD-S), 21 reduced symptomatic (MDD-R)). The stimulus was a single supine-vertical translation. The six features examined were based on the measured firing pattern interval histogram and the shape of the average field potential response.

RESULTS

An unbiased classification accuracy of 85, 87 and 77% was achieved for separating Control from MDD-S, Control from MDD, and MDD-S from MDD-R groups respectively. Features used showed low but significant correlations (P < 0.05) with MADRS and CORE assessments.

CONCLUSIONS

The results support the use of separate features for measuring MDD symptomatology versus diagnosing MDD, representing plausible different mechanisms of brain function in MDD-S and MDD-R. The first evidence of the successful application of sensory oto-acoustic features toward diagnosing and measuring the symptomatology of MDD is presented.

Cytisine, a Partial Agonist of α4β2 Nicotinic Acetylcholine Receptors, Reduced Unpredictable Chronic Mild Stress-Induced Depression-Like Behaviors

Cytisine (CYT), a partial agonist of α4β2-nicotinic receptors, has been used for antidepressant efficacy in several tests. Nicotinic receptors have been shown to be closely associated with depression. However, little is known about the effects of CYT on the depression. In the present study, a mouse model of depression, the unpredictable chronic mild stress (UCMS), was used to evaluate the activities of CYT. UCMS caused significant depression-like behaviors, as shown by the decrease of total distances in open field test, and the prolonged duration of immobility in tail suspension test and forced swimming test. Treatment with CYT for two weeks notably relieved the depression-like behaviors in the UCMS mice. Next, proteins related to depressive disorder in the brain region of hippocampus and amygdala were analyzed to elucidate the underlying mechanisms of CYT. CYT significantly reversed the decreases of 5-HT1A, BDNF, and mTOR levels in the hippocampus and amygdala. These results imply that CYT may act as a potential anti-depressant in the animals under chronic stress.

PET imaging evaluation of [(18)F]DBT-10, a novel radioligand specific to α7 nicotinic acetylcholine receptors, in nonhuman primates

PURPOSE

Positron emission tomography (PET) radioligands specific to α7 nicotinic acetylcholine receptors (nAChRs) afford in vivo imaging of this receptor for neuropathologies such as Alzheimer's disease, schizophrenia, and substance abuse. This work aims to characterize the kinetic properties of an α7-nAChR-specific radioligand, 7-(1,4-diazabicyclo[3.2.2]nonan-4-yl)-2-[(18)F]-fluorodibenzo[b,d]thiophene 5,5-dioxide ([(18)F]DBT-10), in nonhuman primates.

METHODS

[(18)F]DBT-10 was produced via nucleophilic substitution of the nitro-precursor. Four Macaca mulatta subjects were imaged with [(18)F]DBT-10 PET, with measurement of [(18)F]DBT-10 parent concentrations and metabolism in arterial plasma. Baseline PET scans were acquired for all subjects. Following one scan, ex vivo analysis of brain tissue was performed to inspect for radiolabeled metabolites in brain. Three blocking scans with 0.69 and 1.24 mg/kg of the α7-nAChR-specific ligand ASEM were also acquired to assess dose-dependent blockade of [(18)F]DBT-10 binding. Kinetic analysis of PET data was performed using the metabolite-corrected input function to calculate the parent fraction corrected total distribution volume (V T/f P).

RESULTS

[(18)F]DBT-10 was produced within 90 min at high specific activities of 428 ± 436 GBq/μmol at end of synthesis. Metabolism of [(18)F]DBT-10 varied across subjects, stabilizing by 120 min post-injection at parent fractions of 15-55%. Uptake of [(18)F]DBT-10 in brain occurred rapidly, reaching peak standardized uptake values (SUVs) of 2.9-3.7 within 30 min. The plasma-free fraction was 18.8 ± 3.4%. No evidence for radiolabeled [(18)F]DBT-10 metabolites was found in ex vivo brain tissue samples. Kinetic analysis of PET data was best described by the two-tissue compartment model. Estimated V T/f P values were 193-376 ml/cm(3) across regions, with regional rank order of thalamus > frontal cortex > striatum > hippocampus > occipital cortex > cerebellum > pons. Dose-dependent blockade of [(18)F]DBT-10 binding by structural analog ASEM was observed throughout the brain, and occupancy plots yielded a V ND/f P estimate of 20 ± 16 ml/cm(3).

CONCLUSION

These results demonstrate suitable kinetic properties of [(18)F]DBT-10 for in vivo quantification of α7-nAChR binding in nonhuman primates.

Behavioral effects of nicotinic antagonist mecamylamine in a rat model of depression: prefrontal cortex level of BDNF protein and monoaminergic neurotransmitters

Several studies have pointed to the nicotinic acetylcholine receptor (nAChR) antagonists, such as mecamylamine (MEC), as a potential therapeutic target for the treatment of depression. The present study evaluated the behavioral and neurochemical effects of chronic administration of MEC (1, 2, and 4 mg/kg/day, intraperitoneally (i.p.)) in Wistar rats exposed to chronic restraint stress (CRS, 4 h × 6 W). MEC prevented CRS-induced depressive-like behavior via increasing sucrose preference, body weight, and forced swim test (FST) struggling and swimming while reducing immobility in FST and hypothalamic-pituitary-adrenal (HPA) axis hyperactivity (adrenal gland weight and serum corticosterone). At the same time, MEC amended CRS-induced anxiety as indicated by decreasing central zone duration in open field test and increasing active interaction duration. Additionally, MEC modulated the prefrontal cortex (PFC) level of brain-derived neurotrophic factor (BDNF), 5-hydroxy tryptamine (5-HT), and norepinephrine (NE). In conclusion, the present data suggest that MEC possesses antidepressant and anxiolytic-like activities in rats exposed to CRS. These behavioral effects may be in part mediated by reducing HPA axis hyperactivity and increasing PFC level of BDNF and monoamines. Accordingly, these findings further support the hypothesis that nAChRs blockade might afford a novel promising strategy for pharmacotherapy of depression.

The diminished pipeline for medications to treat mental health and substance use disorders

OBJECTIVE

Psychotropic drug development is perceived to be lagging behind other pharmaceutical development, even though there is a need for more effective psychotropic medications. This study examined the state of the current psychotropic drug pipeline and potential barriers to psychotropic drug development.

METHODS

The authors scanned the recent academic and "grey" literature to evaluate psychotropic drug development and to identify experts in the fields of psychiatry and substance use disorder treatment and psychotropic drug development. On the basis of that preliminary research, the authors interviewed six experts and analyzed drugs being studied for treatment of major psychiatric disorders in phase III clinical trials.

RESULTS

Interviews and review of clinical trials of drugs in phase III of development confirmed that the psychotropic pipeline is slim and that a majority of the drugs in phase III trials are not very innovative. Among the barriers to development are incentives that encourage firms to focus on incremental innovation rather than take risks on radically new approaches. Other barriers include human brain complexity, failure of animal trials to translate well to human trials, and a drug approval threshold that is perceived as so high that it discourages development.

CONCLUSIONS

Drivers of innovation in psychotropic drug development largely parallel those for other drugs, yet crucial distinctions have led to slowing psychotropic development after a period of innovation and growth. Various factors have acted to dry up the pipeline for psychotropic drugs, with expert opinion suggesting that in the near term, this trend is likely to continue.

Faster, better, stronger: towards new antidepressant therapeutic strategies

Major depression is a highly prevalent disorder and is predicted to be the second leading cause of disease burden by 2020. Although many antidepressant drugs are currently available, they are far from optimal. Approximately 50% of patients do not respond to initial first line antidepressant treatment, while approximately one third fail to achieve remission following several pharmacological interventions. Furthermore, several weeks or months of treatment are often required before clinical improvement, if any, is reported. Moreover, most of the commonly used antidepressants have been primarily designed to increase synaptic availability of serotonin and/or noradrenaline and although they are of therapeutic benefit to many patients, it is clear that other therapeutic targets are required if we are going to improve the response and remission rates. It is clear that more effective, rapid-acting antidepressants with novel mechanisms of action are required. The purpose of this review is to outline the current strategies that are being taken in both preclinical and clinical settings for identifying superior antidepressant drugs. The realisation that ketamine has rapid antidepressant-like effects in treatment resistant patients has reenergised the field. Further, developing an understanding of the mechanisms underlying the rapid antidepressant effects in treatment-resistant patients by drugs such as ketamine may uncover novel therapeutic targets that can be exploited to meet the Olympian challenge of developing faster, better and stronger antidepressant drugs.

Synthesis and Pharmacological Evaluation of DHβE Analogues as Neuronal Nicotinic Acetylcholine Receptor Antagonists

Dihydro-β-erythroidine (DHβE) is a member of the Erythrina family of alkaloids and a potent competitive antagonist of the α4β2-subtype of the nicotinic acetylcholine receptors (nAChRs). Guided by an X-ray structure of DHβE in complex with an ACh binding protein, we detail the design, synthesis, and pharmacological characterization of a series of DHβE analogues in which two of the four rings in the natural product has been excluded. We found that the direct analogue of DHβE maintains affinity for the α4β2-subtype, but further modifications of the simplified analogues were detrimental to their activities on the nAChRs.

Recent developments in novel antidepressants targeting α4β2-nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) have been investigated for developing drugs that can potentially treat various central nervous system disorders. Considerable evidence supports the hypothesis that modulation of the cholinergic system through activation and/or desensitization/inactivation of nAChR holds promise for the development of new antidepressants. The introductory portion of this Miniperspective discusses the basic pharmacology that underpins the involvement of α4β2-nAChRs in depression, along with the structural features that are essential to ligand recognition by the α4β2-nAChRs. The remainder of this Miniperspective analyzes reported nicotinic ligands in terms of drug design considerations and their potency and selectivity, with a particular focus on compounds exhibiting antidepressant-like effects in preclinical or clinical studies. This Miniperspective aims to provide an in-depth analysis of the potential for using nicotinic ligands in the treatment of depression, which may hold some promise in addressing an unmet clinical need by providing relief from depressive symptoms in refractory patients.

Changes in the cholinergic system between bipolar depression and euthymia as measured with [123I]5IA single photon emission computed tomography

BACKGROUND

The cholinergic system is substantially altered in individuals with major depression and is partially restored when depression remits. We quantified the availability of β2-subunit-containing nicotinic acetylcholine receptors (β2*-nAChR) in subjects with bipolar disorder.

METHODS

Twenty-five subjects with bipolar disorder (15 depressed, 10 euthymic) and 25 sex- and age-matched control subjects had a [(123)I]5IA-85380 single photon emission computed tomography scan to quantify β2*-nAChR VT/fP (total volume of distribution, corrected for individual differences in metabolism and protein binding of the radiotracer). Average VT/fP was compared between groups and correlated with clinical characteristics. Postmortem analysis of β2*-nAChRs was conducted using equilibrium binding with [(125)I]5IA in subjects with bipolar disorder and matched control subjects.

RESULTS

We showed significantly lower β2*-nAChR availability (20%-38%) in subjects with bipolar depression compared with euthymic and control subjects across all brain regions assessed (frontal, parietal, temporal, and anterior cingulate cortex, hippocampus, amygdala, thalamus, striatum). The postmortem binding study in which endogenous acetylcholine was washed out did not show a statistically significant difference in β2*-nAChR number in temporal cortex of the bipolar depressed and control groups (15% difference; p = .2).

CONCLUSIONS

We show that the alteration in the cholinergic system observed during a depressive episode appears to resolve during euthymia. We suggest that lower VT/fP observed in vivo may be due to a combination of higher endogenous acetylcholine levels during depression, which could compete with radiotracer binding to the receptor in vivo, and lower receptor number in bipolar depression. Identification of differences in cholinergic signaling in subjects with bipolar depression may improve our understanding of its etiology and reveal new treatment targets.

Nicotinic modulation of intrinsic brain networks in schizophrenia

The nicotinic receptor is a promising drug target currently being investigated for the treatment of cognitive symptoms in schizophrenia. A key step in this process is the development of noninvasive functional neuroimaging biomarkers that can be used to determine if nicotinic agents are eliciting their targeted biological effect, ideally through modulation of a fundamental aspect of neuronal function. To that end, neuroimaging researchers are beginning to understand how nicotinic modulation affects "intrinsic" brain networks to elicit potentially therapeutic effects. An intrinsic network is a functionally and (often) structurally connected network of brain areas whose activity reflects a fundamental neurobiological organizational principle of the brain. This review summarizes findings of the effects of nicotinic drugs on three topics related to intrinsic brain network activity: (1) the default mode network, a group of brain areas for which activity is maximal at rest and reduced during cognitive tasks, (2) the salience network, which integrates incoming sensory data with prior internal representations to guide future actions and change predictive values, and (3) multi-scale complex network dynamics, which describe these brain's ability to efficiency integrate information while preserving local functional specialization. These early findings can be used to inform future neuroimaging studies that examine the network effects of nicotinic agents.

PPARα regulates cholinergic-driven activity of midbrain dopamine neurons via a novel mechanism involving α7 nicotinic acetylcholine receptors

Ventral tegmental area dopamine neurons control reward-driven learning, and their dysregulation can lead to psychiatric disorders. Tonic and phasic activity of these dopaminergic neurons depends on cholinergic tone and activation of nicotinic acetylcholine receptors (nAChRs), particularly those containing the β2 subunit (β2*-nAChRs). Nuclear peroxisome proliferator-activated receptors type-α (PPARα) tonically regulate β2*-nAChRs and thereby control dopamine neuron firing activity. However, it is unknown how and when PPARα endogenous ligands are synthesized by dopamine cells. Using ex vivo and in vivo electrophysiological techniques combined with biochemical and behavioral analysis, we show that activation of α7-nAChRs increases in the rat VTA both the tyrosine phosphorylation of the β2 subunit of nAChRs and the levels of two PPARα endogenous ligands in a Ca(2+)-dependent manner. Accordingly, in vivo production of endogenous PPARα ligands, triggered by α7-nAChR activation, blocks in rats nicotine-induced increased firing activity of dopamine neurons and displays antidepressant-like properties. These data demonstrate that endogenous PPARα ligands are effectors of α7-nAChRs and that their neuromodulatory properties depend on phosphorylation of β2*-nAChRs on VTA dopamine cells. This reveals an autoinhibitory mechanism aimed at reducing dopamine cell overexcitation engaged during hypercholinergic drive. Our results unveil important physiological functions of nAChR/PPARα signaling in dopamine neurons and how behavioral output can change after modifications of this signaling pathway. Overall, the present study suggests PPARα as new therapeutic targets for disorders associated with unbalanced dopamine-acetylcholine systems.

Neuroinflammation, neurodegeneration, and depression

Neurodegeneration and depression are two common co-morbid conditions, particularly within the aging population. Research has linked neuroinflammation as a major contributing factor to both of these diseases. The key to neuroinflammation effects on neurodegeneration and depression appears to lie within the dysregulation of the control and release of pro- and anti-inflammatory cytokines. This can come from an internal or external insult to the system, or from changes in the individual due to aging that culminate in immune dysregulation. The need to reduce neuroinflammation has led to extensive research into neuroprotectants. We discuss the efficacy found with nicotine, alcohol, resveratrol, curcumin, and ketamine. Our main focus will be on what research tells us about the connections between neuroinflammation, neurodegeneration, and depression, and the hope that neuroprotectants research gives people suffering from neurodegeneration and depression stemming from neuroinflammation. We will conclude by making suggestions for future research in this area.

The role of estrogen receptor β and nicotinic cholinergic receptors in postpartum depression

Postpartum depression (PPD) is a devastating disease occurring in approximately 20% of women. Women who suffer from PPD appear to be more sensitive to postpartum hormonal changes than women who do not experience this form of depression. Furthermore, women who quit smoking prior to or during pregnancy, and who develop PPD, are at an increased risk of smoking relapse. Unfortunately, the mechanistic relationship between the pathophysiology of PPD and smoking relapse is unknown. Here we review the roles of both estrogen receptor beta (ERβ) and cholinergic nicotinic receptors (nAChRs) in the pathogenesis of depression and propose a mechanistic rationale to explain the high rate of smoking relapse exhibited by women who develop PPD.