Evidence for a complex influence of nicotinic acetylcholine receptors on hippocampal serotonin release

BNC210, a negative allosteric modulator of the alpha 7 nicotinic acetylcholine receptor, demonstrates anxiolytic- and antidepressant-like effects in rodents

This work describes the characterization of BNC210 (6-[(2,3-dihydro-1H-inden-2-yl)amino]-1-ethyl-3-(4-morpholinylcarbonyl)-1,8-naphthyridin-4(1H)-one), a selective, small molecule, negative allosteric modulator (NAM) of α7 nicotinic acetylcholine receptors (α7 nAChR). With the aim to discover a non-sedating, anxiolytic compound, BNC210 was identified during phenotypic screening of a focused medicinal chemistry library using the mouse Light Dark (LD) box to evaluate anxiolytic-like activity and the mouse Open Field (OF) (dark) test to detect sedative and/or motor effects. BNC210 exhibited anxiolytic-like activity with no measurable sedative or motor effects. Electrophysiology showed that BNC210 did not induce α7 nAChR currents by itself but inhibited EC80 agonist-evoked currents in recombinant GH4C1 cell lines stably expressing the rat or human α7 nAChR. BNC210 was not active when tested on cell lines expressing other members of the cys-loop ligand-gated ion channel family. Screening over 400 other targets did not reveal any activity for BNC210 confirming its selectivity for α7 nAChR. Oral administration of BNC210 to male mice and rats in several tests of behavior related to anxiety- and stress- related disorders, demonstrated significant reduction of these behaviors over a broad therapeutic range up to 500 times the minimum effective dose. Further testing for potential adverse effects in suitable rat and mouse tests showed that BNC210 did not produce sedation, memory and motor impairment or physical dependence, symptoms associated with current anxiolytic therapeutics. These data suggest that allosteric inhibition of α7 nAChR function may represent a differentiated approach to treating anxiety- and stress- related disorders with an improved safety profile compared to current treatments.

Central cholinergic transmission affects the compulsive-like behavior of mice in marble-burying test

The presence of the cholinergic system in the brain areas implicated in the precipitation of obsessive-compulsive behavior (OCB) has been reported but the exact role of the central cholinergic system therein is still unexplored. Therefore, the current study assessed the effect of cholinergic analogs on central administration on the marble-burying behavior (MBB) of mice, a behavior correlated with OCB. The result reveals that the enhancement of central cholinergic transmission in mice achieved by intracerebroventricular (i.c.v.) injection of acetylcholine (0.01 µg) (Subeffective: 0.1 and 0.5 µg), cholinesterase inhibitor, neostigmine (0.1, 0.3, 0.5 µg/mouse) and neuronal nicotinic acetylcholine receptor agonist, nicotine (0.1, 2 µg/mouse) significantly attenuated the number of marbles buried by mice in MBB test without affecting basal locomotor activity. Similarly, central injection of mAChR antagonist, atropine (0.1, 0.5, 5 µg/mouse), nAChR antagonist, mecamylamine (0.1, 0.5, 3 µg/mouse) per se also reduced the MBB in mice, indicative of anti-OCB like effect of all the tested cholinergic mAChR or nAChR agonist and antagonist. Surprisingly, i.c.v. injection of acetylcholine (0.01 µg), and neostigmine (0.1 µg) failed to elicit an anti-OCB-like effect in mice pre-treated (i.c.v.) with atropine (0.1 µg), or mecamylamine (0.1 µg). Thus, the findings of the present investigationdelineate the role of central cholinergic transmission in the compulsive-like behavior of mice probably via mAChR or nAChR stimulation.

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.

Efficacy of nicotine administration on obsessions and compulsions in OCD: a systematic review

BACKGROUND

Preliminary studies have tested nicotine as a novel treatment for OCD patients who respond partially/incompletely or not at all to first and second-line treatment strategies, with the former represented by SSRIs or clomipramine, and the latter by switching to another SSRI, or augmentation with atypical antipsychotics, and/or combination with/switching to cognitive-behavioural therapy. Some studies found nicotine-induced reduction of obsessive thoughts and/or compulsive behaviour in OCD patients. We aimed to evaluate the efficacy of nicotine administration in OCD patients.

METHODS

We searched the PubMed, ScienceDirect Scopus, CINHAL, Cochrane, PsycINFO/PsycARTICLES, and EMBASE databases from inception to the present for relevant papers. The 'Preferred Reporting Items for Systematic Review and Meta-Analyses' (PRISMA) standards were used. We included all studies focusing on the effects of nicotine administration on OCD patients' obsessions or compulsions. Studies could be open-label, cross-sectional, randomized controlled trials, case series or case reports.

RESULTS

A total of five studies could be included. Nicotine administration may ameliorate behavioural features and recurrent thoughts of severe, treatment-resistant OCD patients; however, in one study it was not associated with OC symptom improvement or cognitive enhancement across various executive function subdomains.

CONCLUSIONS

Although encouraging, the initial positive response from the use of nicotine in OCD needs testing in large controlled studies. This, however, raises ethical issues related to nicotine administration, due to its addiction potential, which were not addressed in the limited literature we examined. As an alternative, novel treatments with drugs able to mimic only the positive effects of nicotine could be implemented.

Additive interaction between scopolamine and nitric oxide agents on immobility in the forced swim test but not exploratory activity in the hole-board

RATIONALE

The muscarinic cholinergic antagonist scopolamine has received an attention due to its unique antidepressant effects. However, the considerable adverse effects on nervous system limit the use of scopolamine as a psychiatric drug.

OBJECTIVE

In order to overcome the limitations and increase the therapeutic effects of scopolamine, we decided to examine the effects of joint administration of sub-effective dose of scopolamine and the sub-effective dose of a nitric oxide (NO) precursor L-Arginine or a non-selective nitric oxide synthase (NOS) inhibitor L-NAME on depression- and anxiety-related behaviors in male NMRI mice.

METHODS

To this aim, animal behavior was assessed in the forced swim test (FST) and hole-board apparatus.

RESULTS

Scopolamine (0.05 mg/kg) significantly decreased immobility time in the FST, suggesting an antidepressant-like effect. Moreover, L-Arginine (50 mg/kg) produced an antidepressant-like response in the FST and decreased head-dip counts in the hole-board apparatus, indicating an anxiety-like effect. The same doses of scopolamine and L-Arginine decreased the locomotor activity in mice. Joint administration of sub-effective dose of scopolamine (0.01 mg/kg) with a low dose of L-Arginine (25 mg/kg) or L-NAME (1 mg/kg) induced a profound antidepressant-like effect in the FST. These drug combinations did not influence on anxiety-related behaviors. Meanwhile, L-NAME alone did not alter the performance of mice in the FST and hole-board. Isobolographic analysis revealed an additive effect for scopolamine and L-Arginine or L-NAME.

CONCLUSION

Data suggests that NO agents could positively impact the therapeutic profile of scopolamine, because they might be useful for inducing antidepressant-like effect associated to scopolamine.

Cholinergic regulation of mood: from basic and clinical studies to emerging therapeutics

Mood disorders are highly prevalent and are the leading cause of disability worldwide. The neurobiological mechanisms underlying depression remain poorly understood, although theories regarding dysfunction within various neurotransmitter systems have been postulated. Over 50 years ago, clinical studies suggested that increases in central acetylcholine could lead to depressed mood. Evidence has continued to accumulate suggesting that the cholinergic system has a important role in mood regulation. In particular, the finding that the antimuscarinic agent, scopolamine, exerts fast-onset and sustained antidepressant effects in depressed humans has led to a renewal of interest in the cholinergic system as an important player in the neurochemistry of major depression and bipolar disorder. Here, we synthesize current knowledge regarding the modulation of mood by the central cholinergic system, drawing upon studies from human postmortem brain, neuroimaging, and drug challenge investigations, as well as animal model studies. First, we describe an illustrative series of early discoveries which suggest a role for acetylcholine in the pathophysiology of mood disorders. Then, we discuss more recent studies conducted in humans and/or animals which have identified roles for both acetylcholinergic muscarinic and nicotinic receptors in different mood states, and as targets for novel therapies.

T-type calcium channel enhancer SAK3 promotes dopamine and serotonin releases in the hippocampus in naive and amyloid precursor protein knock-in mice

T-type calcium channels in the brain mediate the pathophysiology of epilepsy, pain, and sleep. Recently, we developed a novel therapeutic candidate, SAK3 (ethyl 8'-methyl-2',4-dioxo-2-(piperidin-1-yl)-2'H-spiro[cyclopentane-1,3'-imidazo[1,2-a] pyridine]-2-ene-3-carboxylate), for Alzheimer's disease (AD). The cognitive improvement by SAK3 is closely associated with enhanced acetylcholine (ACh) release in the hippocampus. Since monoamines such as dopamine (DA), noradrenaline (NA), and serotonin (5-HT) are also involved in hippocampus-dependent learning and psychomotor behaviors in mice, we investigated the effects of SAK3 on these monoamine releases in the mouse brain. Oral administration of SAK3 (0.5 mg/kg, p.o.) significantly promoted DA and 5-HT releases in the naive mouse hippocampal CA1 region but not in the medial prefrontal cortex (mPFC), while SAK3 did not affect NA release in either brain region. The T-type calcium channel-specific inhibitor, NNC 55-0396 (1 μM) significantly antagonized SAK3-enhanced DA and 5-HT releases in the hippocampus. Interestingly, the α7 nicotinic ACh receptor (nAChR) antagonist, methyllycaconitine (1 nM) significantly inhibited DA release, and the α4 nAChR antagonist, dihydro-β-erythroidine (100 μM) significantly blocked both DA and 5-HT releases following SAK3 (0.5 mg/kg, p.o.) administration in the hippocampus. SAK3 did not alter basal monoamine contents both in the mPFC and hippocampus. SAK3 (0.5 mg/kg, p.o.) administration also significantly elevated DA and 5-HT releases in the hippocampal CA1 region of amyloid-precursor protein (APP)NL-GF knock-in (KI) mice. Moreover, hippocampal DA and 5-HT contents were significantly decreased in APPNL-GF KI mice. Taken together, our data suggest that SAK3 promotes monoamine DA and 5-HT releases by enhancing the T-type calcium channel and nAChR in the mouse hippocampus.

Regulator of G protein signaling 2 differentially regulates nicotine-induced anxiolytic- and antidepressant-like effects in mice

This study assessed the role of regulator of G protein signaling 2 (RGS2) in nicotine-induced anxiolytic- and antidepressant-like effects using RGS2 wildtype (WT) and RGS2 knockout (KO) mice. RGS2 negatively regulates monoaminergic neurotransmission, which is implicated in the pathology of anxiety and depression. We hypothesized that deletion of RGS2 would enhance nicotine-induced anxiolytic- and antidepressant-like effects, which were assessed using the elevated plus maze and tail suspension tests, respectively. Anxiolytic-like effects were observed in both RGS2 WT and KO mice after administration of low dose of nicotine (0.05 mg/kg, base) compared to respective saline controls. Additionally, administration of nicotine (0.1 mg/kg, base) compared to saline resulted in anxiolytic-like effects in RGS2 KO mice, but not RGS2 WT mice, suggesting genetic deletion of RGS2 facilitated anxiolytic-like effects of nicotine. Administration of nicotine (0.5 and 1 mg/kg, base) compared to saline resulted in antidepressant-like effects in RGS2 WT mice. Antidepressant-like effects were observed in RGS2 KO mice only at the highest tested dose of nicotine (1 mg/kg, base) compared to saline controls, suggesting that genetic deletion of RGS2 decreased sensitivity to antidepressant-like effects of nicotine. Together, the data suggest that RGS2 differentially regulated nicotine-induced affective behavioral responses. These data suggest that individuals with RGS2 polymorphisms may experience differential affective responses to tobacco smoking, which may make them vulnerable to developing nicotine addiction.

Functional Connectivity of the Raphe Nuclei: Link to Tobacco Withdrawal in Smokers

BACKGROUND

Although nicotine alters serotonergic neurochemistry, clinical trials of serotonergic medications for smoking cessation have provided mixed results. Understanding the role of serotonergic dysfunction in tobacco use disorder may advance development of novel pharmacotherapies.

METHODS

Functional magnetic resonance imaging was used to measure resting-state functional connectivity of the raphe nuclei as an indicator of serotonergic function. Connectivity of the dorsal and median raphe nuclei was compared between 18 young smokers (briefly abstinent, ~40 minutes post-smoking) and 19 young nonsmokers (16-21 years old); connectivity was also examined in a separate sample of overnight-abstinent smokers (18-25 years old), before and after smoking the first cigarette of the day. Relationships between connectivity of the raphe nuclei with psychological withdrawal and craving were tested in smokers.

RESULTS

Connectivity of the median raphe nucleus with the right hippocampal complex was weaker in smokers than in nonsmokers and was negatively correlated with psychological withdrawal in smokers. In overnight-abstinent smokers, smoking increased connectivity of the median raphe nucleus with the right hippocampal complex, and the increase was positively correlated with the decrease in psychological withdrawal.

CONCLUSIONS

Relief of withdrawal due to smoking is potentially linked to the serotonergic pathway that includes the median raphe nucleus and hippocampal complex. These results suggest that serotonergic medications may be especially beneficial for smokers who endorse strong psychological withdrawal during abstinence from smoking.

Pleiotropy of genetic variants on obesity and smoking phenotypes: Results from the Oncoarray Project of The International Lung Cancer Consortium

Obesity and cigarette smoking are correlated through complex relationships. Common genetic causes may contribute to these correlations. In this study, we selected 241 loci potentially associated with body mass index (BMI) based on the Genetic Investigation of ANthropometric Traits (GIANT) consortium data and calculated a BMI genetic risk score (BMI-GRS) for 17,037 individuals of European descent from the Oncoarray Project of the International Lung Cancer Consortium (ILCCO). Smokers had a significantly higher BMI-GRS than never-smokers (p = 0.016 and 0.010 before and after adjustment for BMI, respectively). The BMI-GRS was also positively correlated with pack-years of smoking (p<0.001) in smokers. Based on causal network inference analyses, seven and five of 241 SNPs were classified to pleiotropic models for BMI/smoking status and BMI/pack-years, respectively. Among them, three and four SNPs associated with smoking status and pack-years (p<0.05), respectively, were followed up in the ever-smoking data of the Tobacco, Alcohol and Genetics (TAG) consortium. Among these seven candidate SNPs, one SNP (rs11030104, BDNF) achieved statistical significance after Bonferroni correction for multiple testing, and three suggestive SNPs (rs13021737, TMEM18; rs11583200, ELAVL4; and rs6990042, SGCZ) achieved a nominal statistical significance. Our results suggest that there is a common genetic component between BMI and smoking, and pleiotropy analysis can be useful to identify novel genetic loci of complex phenotypes.

Temporal sequencing of nicotine dependence and major depressive disorder: A U.S. national study

Major Depressive Disorder (MDD) and Nicotine dependence (ND) often co-occur. However, little attention has been given to the temporal order between the two disorders. We compared the sociodemographic and clinical characteristics of individuals whose onset of ND preceded (ND-prior) or followed the onset of MDD (MDD-prior). Binary logistic regression models were computed to compare ND-prior (n=546) and MDD-prior (n=801) individuals from the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC, n=43,093). We found that MDD-prior were more likely to have a history of suicide attempts and a family history of both depression and antisocial behavior, to have had psychiatric hospitalization, and to have an earlier age of onset of the first depressive episode; but a later age of onset for both daily smoking and ND. On average, MDD-prior individuals showed a significantly longer transition time from daily smoking to ND (15.6±0.6 vs. 6.9±0.4 years, P<0.001). In contrast, ND-prior subjects had a significantly greater proportion of withdrawal symptoms, and of lifetime alcohol use or alcohol use disorder. We conclude that the phenomenology and course of ND and MDD vary significantly, depending on which disorder had earlier onset.

Blockade of cholinergic transmission elicits somatic signs in nicotine-naïve adolescent rats

High doses of the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine can elicit somatic signs resembling those associated with nicotine withdrawal in nicotine-naïve adult rats. Understanding this phenomenon, and its possible modulation by acute nicotine and age, could inform the use of mecamylamine as both an experimental tool and potential pharmacotherapy for tobacco dependence and other disorders. This study evaluated the ability of high-dose mecamylamine to elicit somatic signs in adolescent rats, and the potential for acute nicotine pretreatment to potentiate this effect as previously reported in adults. Single or repeated injections of mecamylamine (1.5 or 3.0 mg/kg, s.c.) elicited somatic signs in nicotine-naïve adolescents, but this effect was not influenced by 2 h pretreatment with acute nicotine (0.5 mg/kg, s.c.). In an initial evaluation of the effects of age in this model, mecamylamine (2.25 mg/kg, s.c.) elicited somatic signs in nicotine-naïve adolescents and adults. This effect was modestly enhanced following acute nicotine injections in adults but not in adolescents, even when a higher nicotine dose (1.0 rather than 0.5 mg/kg, s.c.) was used in adolescents to account for age differences in nicotine pharmacokinetics. These studies are the first to show that mecamylamine elicits somatic signs in nicotine-naïve adolescent rats, an effect that should be considered when designing and interpreting studies examining effects of high doses of mecamylamine in adolescents. Our findings also provide preliminary evidence that these signs may be differentially modulated by acute nicotine pretreatment in adolescents versus adults.

Effects of lobeline and reboxetine, fluoxetine, or bupropion combination on depression-like behaviors in mice

Evidence suggests that lobeline, a nicotinic acetylcholine receptor ligand, has antidepressant-like properties in mice. The present study investigated the possible additive or synergistic effects of lobeline in combination with commonly used antidepressants, such as reboxetine, fluoxetine, or bupropion, using the tail suspension test (TST) and the forced swim test (FST) in C57BL/6J mice. Reboxetine (5 or 10 mg/kg, i.p.), fluoxetine (5 or 10 mg/kg, i.p.), or bupropion (2 or 4 mg/kg, i.p.) were administered 30 min before TST or FST. A fixed dose of lobeline (1 mg/kg, i.p.) was injected 15 min prior to tests. Co-administration of lobeline and reboxetine, fluoxetine, or bupropion significantly reduced immobility time in the TST and FST in comparison to the antidepressants used alone. The results suggest that lobeline enhanced the effects of reboxetine, fluoxetine, or bupropion in mice. Therefore, lobeline or similar nicotinic receptor ligand may have therapeutic potential as an adjunct for the treatment of major depression.

α7 nicotinic ACh receptor-deficient mice exhibit sustained attention impairments that are reversed by β2 nicotinic ACh receptor activation

BACKGROUND AND PURPOSE

Disruptions of executive function, including attentional deficits, are a hallmark of a number of diseases. ACh in the prefrontal cortex regulates attentive behaviour; however, the role of α7 nicotinic ACh receptor (α7nAChR) in attention is contentious.

EXPERIMENTAL APPROACH

In order to probe attention, we trained both wild-type and α7nAChR knockout mice on a touch screen-based five-choice serial reaction time task (5-CSRT). Following training procedures, we then tested sustained attention using a probe trial experiment. To further differentiate the role of specific nicotinic receptors in attention, we then tested the effects of both α7nAChR and β2nAChR agonists on the performance of both wild-type and knockout mice on the 5-CSRT task.

KEY RESULTS

At low doses, α7nAChR agonists improved attentional performance of wild-type mice, while high doses had deleterious effects on attention. α7nAChR knockout mice displayed deficits in sustained attention that were not ameliorated by α7nAChR agonists. However, these deficits were completely reversed by the administration of a β2nAChR agonist. Furthermore, administration of a β2nAChR agonist in α7nAChR knockout mice elicited similar biochemical response in the prefrontal cortex as the administration of α7nAChR agonists in wild-type mice.

CONCLUSIONS AND IMPLICATIONS

Our experiments reveal an intricate relationship between distinct nicotinic receptors to regulate attentional performance and provide the basis for targeting β2nAChRs pharmacologically to decrease attentional deficits due to a dysfunction in α7nAChRs.

Association between smoking, nicotine dependence, and BDNF Val66Met polymorphism with BDNF concentrations in serum

INTRODUCTION

Nicotine use is associated with the upregulation of brain-derived neurotrophic factor (BDNF) in serum. An association between smoking and the BDNF Val(66)Met polymorphism has also been found. The aim of this study is to examine the levels of serum BDNF in never-smokers, former smokers, and current smokers-with and without nicotine dependence-and to examine the interaction of the polymorphism and smoking status with serum BDNF.

METHODS

We used baseline serum and gene data of BDNF on 2,088 participants from the Netherlands Study of Depression and Anxiety (NESDA) to investigate smoking-BDNF association while controlling for potential confounding variables. Nicotine dependence was assessed with the Fagerstrom Test for Nicotine Dependence (FTND).

RESULTS

Smokers with and without nicotine dependence had higher levels of serum BDNF than former and never-smokers. Nicotine dependence and number of cigarettes smoked per day did not add to the prediction of serum BDNF; however, total number of smoking years was a significant predictor of serum BDNF. There was no association of BDNF Val(66)Met, nor an interaction of this polymorphism and smoking status, with serum BDNF.

CONCLUSIONS

Current smoking and higher number of smoking years are associated with higher levels of serum BDNF, and this is independent of the BDNF genotype. Nicotine dependence itself is not associated with a further increase or decrease of serum BDNF. Longitudinal investigations that address changes in serum BDNF in incident smokers and/or in quitters may be useful to understand the association of smoking with BDNF.

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.

Expression of the 5-HT1A serotonin receptor in the hippocampus is required for social stress resilience and the antidepressant-like effects induced by the nicotinic partial agonist cytisine

Nicotinic acetylcholine receptor (nAChR) blockers potentiate the effects of selective serotonin reuptake inhibitors (SSRIs) in some treatment-resistant patients; however, it is not known whether these effects are independent, or whether the two neurotransmitter systems act synergistically. We first determined that the SSRI fluoxetine and the nicotinic partial agonist cytisine have synergistic effects in a mouse model of antidepressant efficacy, whereas serotonin depletion blocked the effects of cytisine. Using a pharmacological approach, we found that the 5-HT1A agonist 8-OH-DPAT also potentiated the antidepressant-like effects of cytisine, suggesting that this subtype might mediate the interaction between the serotonergic and cholinergic systems. The 5-HT1A receptors are located both presynaptically and postsynaptically. We therefore knocked down 5-HT1A receptors in either the dorsal raphe (presynaptic autoreceptors) or the hippocampus (a brain area with high expression of 5-HT1A heteroreceptors sensitive to cholinergic effects on affective behaviors). Knockdown of 5-HT1A receptors in hippocampus, but not dorsal raphe, significantly decreased the antidepressant-like effect of cytisine. This study suggests that serotonin signaling through postsynaptic 5-HT1A receptors in the hippocampus is critical for the antidepressant-like effects of a cholinergic drug and begins to elucidate the molecular mechanisms underlying interactions between the serotonergic and cholinergic systems related to mood disorders.

Mood and anxiety regulation by nicotinic acetylcholine receptors: A potential pathway to modulate aggression and related behavioral states

The co-morbidity between smoking and mood disorders is striking. Preclinical and clinical studies of nicotinic effects on mood, anxiety, aggression, and related behaviors, such as irritability and agitation, suggest that smokers may use the nicotine in tobacco products as an attempt to self-medicate symptoms of affective disorders. The role of nicotinic acetylcholine receptors (nAChRs) in circuits regulating mood and anxiety is beginning to be elucidated in animal models, but the mechanisms underlying the effects of nicotine on aggression-related behavioral states (ARBS) are still not understood. Clinical trials of nicotine or nicotinic medications for neurological and psychiatric disorders have often found effects of nicotinic medications on ARBS, but few trials have studied these outcomes systematically. Similarly, the increase in ARBS resulting from smoking cessation can be resolved by nicotinic agents, but the effects of nicotinic medications on these types of mental states and behaviors in non-smokers are less well understood. Here we review the literature on the role of nAChRs in regulating mood and anxiety, and subsequently on the closely related construct of ARBS. We suggest avenues for future study to identify how nAChRs and nicotinic agents may play a role in these clinically important areas. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Role of the endogenous cannabinoid system in nicotine addiction: novel insights

Several lines of evidence have shown that the endogenous cannabinoids are implicated in several neuropsychiatric diseases. Notably, preclinical and human clinical studies have shown a pivotal role of the cannabinoid system in nicotine addiction. The CB1 receptor inverse agonist/antagonist rimonabant (also known as SR141716) was effective to decrease nicotine-taking and nicotine-seeking in rodents, as well as the elevation of dopamine induced by nicotine in brain reward area. Rimonabant has been shown to improve the ability of smokers to quit smoking in randomized clinical trials. However, rimonabant was removed from the market due to increased risk of psychiatric side-effects observed in humans. Recently, other components of the endogenous cannabinoid system have been explored. Here, we present the recent advances on the understanding of the role of the different components of the cannabinoid system on nicotine's effects. Those recent findings suggest possible alternative ways of modulating the cannabinoid system that could have implication for nicotine dependence treatment.

Cadmium increases the sensitivity of adolescent female mice to nicotine-related behavioral deficits

This study investigates spatial and nonspatial working memory, anxiety related behavior, and motor activities in cadmium and/or nicotine exposed female adolescent mice. P28 female adolescent mice (albino strain) were divided into four groups of five (n = 5) mice each. A set of mice (Nic) received subcutaneous nicotine (2.0 mg/kg) while a separate set (Cd) was treated with 2.0 mg/kg cadmium (subcutaneous). For the combined treatments of cadmium and nicotine, we administered 2.0 mg/kg Nicotine and 2.0 mg/kg of Cd. Subsequently, a separate group of animals (n = 5; control) received normal saline. The total duration of treatment for all groups was 28 days (P28–P56). At P56, the treatment was discontinued, after which the animals were examined in behavioural tests. Nicotine and cadmium increased the metabolism and food intake in the female adolescent mice. This also corresponded to an increase in weight when compared with the control. However, a combined nicotine-cadmium treatment induced a decline in weight of the animals versus the control. Also, nicotine administration increased the motor function, while cadmium and nicotine-cadmium treatment caused a decline in motor activity. Both nicotine and cadmium induced a reduction in memory index; however, nicotine-cadmium treatment induced the most significant decrease in nonspatial working memory.

Temporal sequencing of nicotine dependence and bipolar disorder in the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC)

Bipolar disorder (BD) and nicotine dependence (ND) often co-occur. However, the mechanisms underlying this association remain unclear. We aimed to examine, for the first time in a national and representative sample, the magnitude and direction of the temporal relationship between BD and ND; and to compare, among individuals with lifetime ND and BD, the sociodemographic and clinical characteristics of individuals whose onset of ND preceded the onset of BD (ND-prior) with those whose onset of ND followed the onset of BD (BD-prior). The sample included individuals with lifetime BD type I or ND (n = 7958) from the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC, n = 43093). Survival analyses and logistic regression models were computed to study the temporal association between ND and BD, and to compare ND-prior (n = 135) and BD-prior (n = 386) individuals. We found that ND predicted the onset of BD and BD also predicted the onset of ND. Furthermore, the risk of developing one disorder following the other one was greatest early in the course of illness. Most individuals with lifetime ND and BD were BD-prior (72.6%). BD-prior individuals had an earlier onset of BD and a higher number of manic episodes. By contrast, ND-prior individuals had an earlier onset of both daily smoking and ND, and an increased prevalence of alcohol use disorder. In conclusion, ND and BD predict the development of each other. The phenomenology and course of ND and BD varied significantly depending on which disorder had earlier onset.

Cholinergic connectivity: it's implications for psychiatric disorders

Acetylcholine has been implicated in both the pathophysiology and treatment of a number of psychiatric disorders, with most of the data related to its role and therapeutic potential focusing on schizophrenia. However, there is little thought given to the consequences of the documented changes in the cholinergic system and how they may affect the functioning of the brain. This review looks at the cholinergic system and its interactions with the intrinsic neurotransmitters glutamate and gamma-amino butyric acid as well as those with the projection neurotransmitters most implicated in the pathophysiologies of psychiatric disorders; dopamine and serotonin. In addition, with the recent focus on the role of factors normally associated with inflammation in the pathophysiologies of psychiatric disorders, links between the cholinergic system and these factors will also be examined. These interfaces are put into context, primarily for schizophrenia, by looking at the changes in each of these systems in the disorder and exploring, theoretically, whether the changes are interconnected with those seen in the cholinergic system. Thus, this review will provide a comprehensive overview of the connectivity between the cholinergic system and some of the major areas of research into the pathophysiologies of psychiatric disorders, resulting in a critical appraisal of the potential outcomes of a dysregulated central cholinergic system.

5-HT denervation of the adult rat prefrontal cortex induces changes in the expression of α4 and α7 nicotinic acetylcholine receptor subtypes

INTRODUCTION

Nicotinic acetylcholine receptors (nAChRs) are widely expressed throughout several brain regions. Formation of the α4β2 and α7 subtypes in particular is involved in the organisation of different types of memory. Furthermore, due to their location, these receptors can control the release of various types of neurotransmitters and contribute to synaptic plasticity.

METHODS

Rats were divided into three groups, an experimental group (E), a sham-operated group, (S) and an intact group (T). In group E, stereotactic guidance was used to induce a chemical lesion with 1 μ/μL of 5,7-dihydroxytryptamine (5,7-DHT) in the anteroventral part of the dorsal raphe nucleus (DRN). In the sham-operated group (S), animals underwent surgery including delivery of the same excipient solution to the same site. The intact group (T) received no treatment whatsoever. Twenty days after surgery, animals in all groups were euthanised by decapitation to evaluate the expression of α4 and α7 nAChRs by means of molecular biology techniques.

RESULTS

5-HT denervation of the rat PFC differentially modified the expression of α4 and α7 receptors: while α4 receptor expression increased, α7 expression decreased.

CONCLUSION

Expression differences observed between the two subtypes may be due to their separate locations. The α4 subtype is found in postsynaptic locations and may be related to adaptive changes in postsynaptic cells, while the location of α7 is presynaptic. This explains why the lesion and the elimination of 5-HT fibres in the CPF would cause a decrease in α7 expression.

Prefrontal neuromodulation by nicotinic receptors for cognitive processes

RATIONALE

The prefrontal cortex (PFC) mediates executive functions, a set of control processes that optimize performance on cognitive tasks. It enables appropriate decision-making and mediates adapted behaviors, all processes impaired in psychiatric or degenerative disorders. Key players of normal functioning of the PFC are neurotransmitter (NT) systems arising from subcortical nuclei and targeting PFC subareas and, also, neuronal nicotinic acetylcholine receptors (nAChRs). These ion channels, located on multiple cell compartments in all brain areas, mediate direct cholinergic transmission and modulate the release of NTs that cross onto PFC neurons or interneurons.

OBJECTIVE

We compiled current knowledge concerning the role of nAChRs in NT release, focusing on the PFC. We point out plausible mechanisms of interaction among PFC circuits implicated in executive functions and emphasized the role of β2-containing nAChRs, the high-affinity receptors for acetylcholine (ACh). These receptors are more directly implicated in behavioral flexibility either when located on PFC neurons or in the monoaminergic or cholinergic systems targeting the PFC.

RESULTS

We shed light on potentially crucial roles played by nAChRs in complex interactions between local and afferent NTs. We show how they could act on cognition via PFC networks.

CONCLUSIONS

nAChRs are crucial for decision-making, during integration of emotional and motivational features, both mediated by different NT pathways in the PFC. We review the knowledge recently gained on cognitive functions in mice and our current understanding of PFC NT modulation. The combination of these data is expected to provide new hypotheses concerning the role of AChRs in cognitive processes.

Combined α7 nicotinic acetylcholine receptor agonism and partial serotonin transporter inhibition produce antidepressant-like effects in the mouse forced swim and tail suspension tests: a comparison of SSR180711 and PNU-282987

Emerging evidence points to an involvement of nicotinic acetylcholine receptors (nAChRs) in major depression. Nicotine improves symptoms of depression in humans and shows antidepressant-like effects in rodents. Monoamine release is facilitated by nAChR stimulation, and nicotine-evoked serotonin (5-HT) release has been shown to depend on α7 nAChR activation. The α7 nAChR agonist PNU-282987 shows no antidepressant-like activity when tested alone in the mouse forced swim (mFST) or tail suspension tests (mTST). However, in combination with a sub-active dose of the selective 5-HT reuptake inhibitor citalopram, inducing ~50% 5-HT reuptake inhibition, PNU-282987 has shown marked antidepressant-like effects in the mFST. SSR180711 is a recently described α7 nAChR agonist that has shown antidepressant-like activity in the rat forced swim test. To address the possibility that 5-HT reuptake inhibition contributes to the antidepressant-like profile of SSR180711, we compared the behavioural and biochemical profiles of PNU-282987 and SSR180711. In the mFST and mTST, SSR180711 (3-30 mg/kg, s.c.) showed dose-dependent antidepressant-like activity, while PNU-282987 (3-30 mg/kg, s.c.) showed no significant effect. The ED(50) to displace [³H]α-bungarotoxin binding was 1.7 and 5.5 mg/kg for SSR180711 and PNU-282987, respectively, suggesting that both compounds produce near-maximal α7 nAChR occupancy at the highest dose. While PNU-282987 did not affect ex vivo [³H]5-HT uptake, SSR180711 inhibited [³H]5-HT uptake with an ED₅₀ of 30 mg/kg. This degree of inhibition is similar to that observed with a citalopram dose of ~2.4 mg/kg, a dose that is normally not active in the mFST or mTST. This suggests that the antidepressant-like activity of SSR180711 may involve partial 5-HT reuptake inhibition. SSR180711 therefore represents a compound displaying the synergistic effect of α7 nAChR agonism combined with partial 5-HT reuptake inhibition previously described. The addition of α7 nAChR agonism to classical monoamine-based mechanisms may represent a novel option for the improved treatment of major depression.

Subtype-selective nicotinic acetylcholine receptor agonists enhance the responsiveness to citalopram and reboxetine in the mouse forced swim test

Nicotine increases serotonergic and noradrenergic neuronal activity and facilitates serotonin and noradrenaline release. Accordingly, nicotine enhances antidepressant-like actions of reuptake inhibitors selective for serotonin or noradrenaline in the mouse forced swim test and the mouse tail suspension test. Both high-affinity α4β2 and low-affinity α7 nicotinic acetylcholine receptor subtypes are implicated in nicotine-mediated release of serotonin and noradrenaline. The present study therefore investigated whether selective agonism of α4β2 or α7 nicotinic acetylcholine receptors would affect the mouse forced swim test activity of two antidepressants with distinct mechanisms of action, namely the selective serotonin reuptake inhibitor citalopram and the noradrenaline reuptake inhibitor reboxetine. Subthreshold and threshold doses of citalopram (3 and 10 mg/kg) or reboxetine (10 and 20 mg/kg) were tested alone and in combination with the novel α4β2-selective partial nicotinic acetylcholine receptor agonist, NS3956 (0.3 and 1.0 mg/kg) or the α7-selective nicotinic acetylcholine receptor agonist, PNU-282987 (10 and 30 mg/kg). Alone, NS3956 and PNU-282987 were devoid of activity in the mouse forced swim test, but both 1.0 mg/kg NS3956 and 30 mg/kg PNU-282987 enhanced the effect of citalopram and also reboxetine. The data suggest that the activity of citalopram and reboxetine in the mouse forced swim test can be enhanced by agonists at either α4β2 or α7 nicotinic acetylcholine receptors, suggesting that both nicotinic acetylcholine receptor subtypes may be involved in the nicotine-enhanced action of antidepressants.

Dissociation between duration of action in the forced swim test in mice and nicotinic acetylcholine receptor occupancy with sazetidine, varenicline, and 5-I-A85380

RATIONALE

Nicotinic acetylcholine receptor (nAChR) agonists, partial agonists, and antagonists have antidepressant-like effects in rodents and reduce symptoms of depression in humans.

OBJECTIVES

The study determined whether the antidepressant-like effect of the nAChR β2* partial agonist sazetidine-A (sazetidine) in the forced swim test was due to activation or desensitization of β2* nAChRs. The study also determined if sazetidine's behavioral responses in the forced swim test corresponded to β2* nAChRs receptor occupancy and drug bioavailability.

RESULTS

Acute antidepressant-like effects in the forced swim test were seen with sazetidine and the full β2* agonist 5-I-A8350 (BALB/cJ mice) and the less selective β2* partial agonist varenicline in C57BL/6J but not BALB/cJ mice. The role of β2* nAChRs was confirmed by results showing: (1) reversal of sazetidine's antidepressant-like effects in the forced swim test by nAChR antagonists mecamylamine and dihydro-β-erythroidine; (2) absence of sazetidine's effect in mice lacking the β2 subunit of the nAChR; and (3) a high correspondence between behaviorally active doses of sazetidine and β2* receptor occupancy. β2* receptor occupancy following acute sazetidine, varenicline, and 5-I-A8350 lasted beyond the duration of action in the forced swim test. Sazetidine's long lasting receptor occupancy did not diminish behavioral efficacy in the forced swim test following repeated dosing.

CONCLUSIONS

Results demonstrate that activation of a small population of β2* nAChRs (10-40%) is sufficient to elicit sazetidine's antidepressant-like actions without producing tolerance and suggest that ligands that activate β2* nAChRs would be promising targets for the development of a new class of antidepressant.

Effects of varenicline on the reinforcing and discriminative stimulus effects of cocaine in rhesus monkeys

Varenicline is a low-efficacy, α4β2* subtype-selective nicotinic acetylcholine receptor (nAChR) agonist that has shown success in smoking cessation and promise in preclinical assessments relating to other drugs of abuse. The primary goal of the present study was to examine the effects of varenicline on cocaine self-administration and cocaine discrimination and compare these effects with those of the nAChR agonist nicotine and antagonist mecamylamine. One limitation of agonist treatments is the potential for abuse. Thus, a second goal was to examine the abuse potential of varenicline in rhesus monkeys. In the first experiment, rhesus monkeys (n = 3) were trained to self-administer cocaine (saline, 0.01-0.56 mg/kg) under a progressive-ratio schedule of reinforcement; monkeys also earned all of their food by responding on another lever under a fixed-ratio 50 schedule of reinforcement. Chronic administration of varenicline (0.01-0.56 mg/kg p.o., salt) potentiated the reinforcing effects of cocaine, whereas mecamylamine (0.3-1.7 mg/kg p.o, i.m., i.v., salt) had no significant effects on cocaine self-administration up to doses that disrupted food-maintained responding. Neither varenicline (0.01-0.17 mg/kg, salt) nor nicotine (0.01-0.1 mg/kg, base) functioned as reinforcers when substituted for cocaine. Finally, in monkeys trained to discriminate self-administered 0.3 mg/kg cocaine, varenicline (0.1-0.3 mg/kg i.v.) did not substitute for cocaine but, along with mecamylamine (0.3-1.7 mg/kg i.v.) and nicotine (0.03-0.1 mg/kg i.v.), potentiated the discriminative stimulus effects of cocaine. These results suggest that varenicline has low abuse liability in monkey models of cocaine abuse, but would not be an effective medication for cocaine addiction.

Age-dependent effects of initial exposure to nicotine on serotonin neurons

Adolescence is a critical vulnerable period during which exposure to nicotine greatly enhances the possibility to develop drug addiction. Growing evidence suggests that serotonergic (5-HT) neurotransmission may contribute to the initiation and maintenance of addictive behavior. As the dorsal raphe (DR) and median raphe (MnR) nuclei are the primary 5-HT source to the forebrain, the current study tested the hypothesis that there are age-dependent effects of acute nicotine administration on activation of 5-HT neurons within these regions. Both adolescent (Postnatal day 30) and adult (Postnatal day 70) male Sprague-Dawley rats received subcutaneous injection of either saline or nicotine (0.2, 0.4, or 0.8 mg/kg). Subsequently, the number of 5-HT cells that were double-labeled for Fos and tryptophan hydroxylase was counted in seven subregions within the DR and the entire MnR. The results show that acute nicotine injection induces Fos expression in 5-HT neurons in a region-specific manner. In addition, adolescents show broader regional activations at either a lower (0.2 mg/kg) and a higher (0.8 mg/kg) dose of nicotine, displaying a unique U-shape response curve across doses. In contrast, 5-HT cells with activated Fos expression were restricted to fewer regions in adults, and the patterns of expression were more consistent across doses. The results reveal dose-dependent effects of nicotine during adolescence with apparent sensitization at different ends of the dosage spectrum examined compared to adults. These data indicate that initial exposure to nicotine may have unique effects in adolescence on the ascending 5-HT system, with the potential for consequences on the affective-motivational qualities of the drug and the subsequent propensity for repeated use.

Nicotine receptors and depression: revisiting and revising the cholinergic hypothesis

There is a well-established connection between smoking and depression. Depressed individuals are over-represented among smokers, and ex-smokers often experience increased depressive symptoms immediately after stopping smoking. Nicotine in tobacco binds, activates and desensitizes nicotinic acetylcholine receptors (nAChRs), but it is not known whether activation or desensitization is more important for the effects of nicotine on depressive symptoms. Here we review, based on clinical and preclinical studies of nicotinic drugs, the hypothesis that blockade (rather than activation) of neuronal nAChRs might be important for the effects of nicotinic agents on depressive symptoms. The endogenous neurotransmitter for nAChRs is acetylcholine, and the effects of nicotine on depression-like behaviors support the idea that dysregulation of the cholinergic system might contribute to the etiology of major depressive disorder. Thus, pharmacological agents that limit acetylcholine signaling through neuronal nAChRs might be promising for the development of novel antidepressant medications.

Long-term effects of methamphetamine exposure on cognitive function and muscarinic acetylcholine receptor levels in mice

Exposure to methamphetamine during brain development impairs cognition in humans and rodents. In mice, these impairments are more severe in females than males. Genetic factors, such as apolipoprotein E genotype, may modulate the cognitive effects of methamphetamine. Methamphetamine-induced alterations in the brain acetylcholine system may contribute to the cognitive effects of methamphetamine and may also be modulated by apolipoprotein E isoform. We assessed the long-term effects of methamphetamine exposure during brain development on cognitive function and muscarinic acetylcholine receptors in mice, and whether apolipoprotein E isoform modulates these effects. Mice expressing human apolipoprotein E3 or E4 were exposed to methamphetamine (5 mg/kg) or saline once a day from postnatal days 11-20 and behaviorally tested in adulthood. Muscarinic acetylcholine receptor binding was measured in the hippocampus and cortex. Methamphetamine exposure impaired novel location recognition in female, but not male, mice. Methamphetamine-exposed male and female mice showed impaired novel object recognition and increased number of muscarinic acetylcholine receptors in the hippocampus. The cognitive and cholinergic effects of methamphetamine were similar in apolipoprotein E3 and E4 mice. Thus, the cholinergic system, but not apolipoprotein E isoform, might play an important role in the long-term methamphetamine-induced cognitive deficits in adulthood.

Beyond the cholinergic hypothesis: do current drugs work in Alzheimer's disease?

Alzheimer's disease (AD) is a neurodegenerative disease characterized by memory and cognitive loss, and represents the leading cause of dementia in elderly people. Besides the complex biochemical processes involved in the neuronal degeneration (formation of senile plaques containing Abeta peptides, and development of neurofibrillary tangles), other molecular and neurochemical alterations, like cholinergic deficit due to basal forebrain degeneration, also occur. Because acetylcholine has been demonstrated to be involved in cognitive processes, the idea to increase acetylcholine levels to restore cognitive deficits has gained interest (the so-called cholinergic hypothesis). This has led to the development of drugs able to prevent acetylcholine hydrolysis (acetylcholinesterase inhibitors). However, the analysis of clinical efficacy of these drugs in alleviating symptoms of dementia showed unsatisfactory results. Despite such critical opinions on the efficacy of these drugs, it should be said that acetylcholinesterase inhibitors, and for some aspects memantine also, improve memory and other cognitive functions throughout most of the duration of the disease. The pharmacological activity of these drugs suggests an effect beyond the mere increase of acetylcholine levels. These considerations are in agreement with the idea that cognitive decline is the result of a complex and not fully elucidated interplay among different neurotransmitters. The role of each of the neurotransmitters implicated has to be related to a cognitive process and as a consequence to its decline. The current review aims to highlight the positive role of cholinergic drugs in alleviating cognitive deficits during wake as well as sleep. Moreover, we suggest that future therapeutic approaches have to be developed to restore the complex interplay between acetylcholine and other neurotransmitters systems, such as dopamine, serotonin, noradrenaline, or glutamate, that are likely involved in the progressive deterioration of several cognitive functions such as attention, memory, and learning.

Smoking, parent smoking, depressed mood, and suicidal ideation in teens

OBJECTIVES

We address whether smoking is related to suicidal ideation in teens and whether there is evidence of a causal pathway.

METHODS

We use data from the 2002 National Annenberg Survey of Youth and employ multivariate logistic regression to model each teen's risk of suicidal ideation as a function of self-report of depressive symptoms, own smoking, parent smoking, and demographic and household income variables.

RESULTS

Individuals reporting depressive symptoms have an increased risk of suicidal ideation (odds ratio [OR] = 13.13; 95% CI = 5.98-28.81). Relative to teens who do not smoke and whose parents do not smoke, teens who smoke and do not have a parent who smokes have increased risk of suicidal ideation (OR = 8.10; 95% CI = 2.88-22.80), whereas those with a parent who smokes do not have a statistically significant increased risk of suicidal ideation regardless of teen smoking behavior.

CONCLUSION

Relative to teens who do not smoke and do not have parents who smoke, suicidal ideation risk is increased in teens who smoke only if they do not have a parent who smokes. We find evidence that the smoking and suicidal ideation of the teens is likely due to common psychosocial causes rather than a causal pathway from smoking to suicidal ideation.

Mecamylamine - a nicotinic acetylcholine receptor antagonist with potential for the treatment of neuropsychiatric disorders

Mecamylamine (Inversine), the first orally available antihypertensive agent launched in the 1950s, is rarely used today for hypertension because of its widespread ganglionic side effects at antihypertensive doses (25 - 90 mg/day). However, more recent clinical studies suggest that mecamylamine is effective at much lower doses for blocking the central and peripheral effects of nicotine. Pharmacologically, mecamylamine has been well characterized as a nonselective and noncompetitive antagonist of nicotinic acetylcholine receptors (nAChRs). Because mecamylamine easily crosses the blood - brain barrier at relatively low doses (2.5 - 10 mg), it has been used by several research groups over the past two decades investigating the role of central nAChRs in the etiology and treatment of various neuropsychiatric disorders, including addiction disorders, Tourette's syndrome, schizophrenia and various cognitive and mood disorders. Two independent Phase II clinical trials recently confirmed mecamylamine's hypothesized antidepressant activity and suggest that it may be effective as an augmentation pharmacotherapy for SSRI treatment resistant major depression. These areas of investigation for mecamylamine are reviewed and recommendations for future research directions are proposed.

Antidepressant-like effects of nicotinic acetylcholine receptor antagonists, but not agonists, in the mouse forced swim and mouse tail suspension tests

Current literature suggests involvement of nicotinic acetylcholine receptors (nAChRs) in major depression. However, it is controversial whether the antidepressant-like effect of nAChR modulation is induced by activation, desensitization or inhibition of central nAChRs. In addition, the specific nAChR subtype/s involved remains unknown. In this study, we systematically compared the effects of non-selective and selective nicotinic agonists and antagonists in two different tests for antidepressant effects in mice: the tail suspension test and the forced swim test. Compounds: nicotine, RJR-2403 (alpha4beta2-selective agonist), PNU-282987 (alpha7-selective agonist), mecamylamine (non-selective antagonist), dihydro-beta-erythroidine (DHbetaE; alpha4beta2-selective antagonist), methyllycaconitine (MLA; alpha7-selective antagonist) and hexamethonium (non-brain-penetrant non-selective antagonist). All compounds were tested in a locomotor activity paradigm to rule out non-specific stimulant effects. The data show that blockade of nAChRs with mecamylamine, or selective antagonism of alpha4beta2 or alpha7 nAChRs with DHbetaE or MLA, respectively, has antidepressant-like effects. These effects were not confounded by motor stimulation. Hexamethonium did not show antidepressant-like activity, supporting the involvement of central nAChRs. At the dose levels tested, none of the nAChR agonists displayed antidepressant-like profiles. In conclusion, antagonism of central alpha4beta2 and/or alpha7 nAChRs induced antidepressant-like effects in mice. A strategy involving antagonism of central nAChRs could potentially lead to the development of novel antidepressant therapeutics.

Chronic oral nicotine increases brain [3H]epibatidine binding and responsiveness to antidepressant drugs, but not nicotine, in the mouse forced swim test

INTRODUCTION

Smoking rates among depressed individuals is higher than among healthy subjects, and nicotine alleviates depressive symptoms. Nicotine increases serotonergic and noradrenergic neuronal activity and facilitates serotonin and noradrenaline release. In mice, acute nicotine administration enhances the activity of antidepressants in the mouse forced swim (mFST) and tail suspension tests. Here, we investigated if this action of nicotine is also reflected in a chronic treatment regimen.

MATERIALS AND METHODS

After chronic treatment with nicotine in the drinking water, mice were challenged with nicotine, duloxetine, citalopram, and reboxetine in the mFST. Additionally, 8-OH-DPAT- and clonidine-induced hypothermia was tested in vehicle- and nicotine-pretreated mice, as a measure of 5-HT(1A) and alpha(2)-adrenoceptor function, respectively. Finally, the effects on the brain expression levels of high- and low-affinity nicotinic acetylcholine receptors (nAChRs) and the transporters for serotonin (SERT) and noradrenaline (NET) were assessed using [(3)H]epibatidine, [(3)H]alpha-bungarotoxin, [(3)H]citalopram, and [(3)H]nisoxetine binding, respectively.

RESULTS

In the mFST, nicotine-pretreated mice did not show altered response to the nicotine challenge, but increased responses to all three antidepressants tested were observed when compared to mice that had been administered drinking water without nicotine. There was no change in hypothermic responses to 8-OH-DPAT or clonidine. [(3)H]epibatidine binding was significantly increased in all brain regions investigated; whereas, [(3)H]alpha-bungarotoxin, [(3)H]citalopram, and [(3)H]nisoxetine binding were not altered, indicating that chronic oral nicotine increases the expression and/or affinity of high-affinity nAChRs, but not low-affinity nAChRs, SERT, or NET.

DISCUSSION

It is suggested that the increased sensitivity to antidepressants after chronic nicotine exposure involves increased high-affinity nAChR-mediated neurotransmission.

Nicotinic receptors differentially modulate the induction and expression of behavioral sensitization to methylphenidate in rats

RATIONALE

Nicotinic acetylcholine receptors (nAChRs) regulate sensitization to stimulant drugs such as d-amphetamine and cocaine.

OBJECTIVES

The current study determined if nAChRs modulate the induction and/or expression of behavioral sensitization to high methylphenidate doses.

METHODS

In experiment 1, rats received saline or mecamylamine (3 mg/kg, sc), followed by saline or methylphenidate (5.6 or 10 mg/kg, sc) during 10 daily sessions; the effect of methylphenidate (1-17 mg/kg, sc) alone was determined 14 days later. In experiment 2, rats received saline or dihydro-beta-erythroidine (DHbetaE; 3 mg/kg, sc), followed by saline or 5.6 mg/kg of methylphenidate. In experiment 3, rats received saline or methylphenidate (5.6 or 10 mg/kg, sc) alone for 10 days; the effect of acute mecamylamine (3 mg/kg, sc) on the response to methylphenidate (1-17 mg/kg, sc) was determined 14 days later. Locomotor activity, sniffing, rearing, grooming, and stereotypy ratings were dependent measures.

RESULTS

Methylphenidate produced dose-dependent increases in locomotor activity, sniffing, and stereotypy on day 1 and these effects were enhanced on day 10, indicative of sensitization. Mecamylamine attenuated methylphenidate-induced stereotypy only on day 1, but reduced locomotor activity, sniffing, rearing, and stereotypy on day 10 and during the methylphenidate challenge phase; similar results were obtained with DHbetaE. However, acute mecamylamine did not alter the effects of the methylphenidate challenge following the induction of sensitization to methylphenidate alone.

CONCLUSIONS

Although nAChRs do not appear to regulate the expression of methylphenidate-induced behavioral sensitization, inhibition of high-affinity beta2 subunit nAChRs attenuates the induction of behavioral sensitization to high doses of methylphenidate.

Novel acetylcholine and carbamoylcholine analogues: development of a functionally selective alpha4beta2 nicotinic acetylcholine receptor agonist

A series of carbamoylcholine and acetylcholine analogues were synthesized and characterized pharmacologically at neuronal nicotinic acetylcholine receptors (nAChRs). Several of the compounds displayed low nanomolar binding affinities to the alpha4beta2 nAChR and pronounced selectivity for this subtype over alpha3beta4, alpha4beta4, and alpha7 nAChRs. The high nAChR activity of carbamoylcholine analogue 5d was found to reside in its R-enantiomer, a characteristic most likely true for all other compounds in the series. Interestingly, the pronounced alpha4beta2 selectivities exhibited by some of the compounds in the binding assays translated into functional selectivity. Compound 5a was a fairly potent partial alpha4beta2 nAChR agonist with negligible activities at the alpha3beta4 and alpha7 subtypes, thus being one of the few truly functionally selective alpha4beta2 nAChR agonists published to date. Ligand-protein docking experiments using homology models of the amino-terminal domains of alpha4beta2 and alpha3beta4 nAChRs identified residues Val111(beta2)/Ile113(beta4), Phe119(beta2)/Gln121(beta4), and Thr155(alpha4)/Ser150(alpha3) as possible key determinants of the alpha4beta2/alpha3beta4-selectivity displayed by the analogues.

Effects of a cannabinoid receptor (CB) 1 antagonist AM251 on behavioral sensitization to nicotine in a rat model of novelty-seeking behavior: correlation with hippocampal 5HT

RATIONALE

There are marked individual differences in the efficacy of mainstream nicotine cessation agents in preventing relapse. A rat model of novelty-seeking phenotype was reported to have predictive value for psychostimulant taking behavior where locomotor reactivity to novelty is used to rank high (HR, highest 1/3) versus low (LR, lowest 1/3) responsiveness to novelty in outbred rats. We tested the hypothesis that a cannabinoid receptor (CB) 1 antagonist that is in clinical trials for smoking cessation may reverse behaviorally sensitizing effects of nicotine in HRs and repeated nicotine-induced elevations in hippocampal 5HT.

MATERIALS AND METHODS

Adolescent LRHR rats underwent intermittent behavioral sensitization to nicotine regimen with or without a CB1 receptor antagonist AM251 or bupropion treatment following nicotine training during 1 week of nicotine-free period. Expression of behavioral sensitization to nicotine was assessed in response to a low-dose nicotine challenge. Using the same sensitization regimen and therapeutic treatments, hippocampal 5HT levels were measured via in vivo microdialysis in response to the nicotine challenge.

RESULTS

HR but not LR animals showed behavioral sensitization to a low-dose nicotine challenge following intermittent nicotine training and 1 week of injection-free period. AM251 (5 mg/kg, i.p.) but not bupropion administration during injection-free period successfully reversed locomotor sensitization to nicotine challenge in HRs. AM251 treatment also reversed nicotine-induced elevations in extracellular 5HT in the HR hippocampal hilus.

CONCLUSION

These data suggest that CB1 antagonists may prevent locomotor sensitization to nicotine and reverse nicotine-induced elevations in hippocampal 5HT in high novelty seekers.

Quantitative Structure Activity Relationship between Diazabicyclo[4.2.0]octanes Derivatives and Nicotinic Acetylcholine Receptor Agonists

Three dimensional quantitative structure activity relationship between diazabicyclo[4.2.0]octanes and nicotinic acetylcholine receptor (halpha4beta2 and halpha3beta4) agonists was studied using comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA). From 11 CoMFA and CoMSIA models, CoMSIA with steric and electrostatic fields gave the best predictive models (q(2)=0.926 and 0.945, r(2) (ncv)=0.983 and 0.988). This study can be used to develop potent halpha4beta2 receptor agonists with low activity on halpha3beta4 subtype.

Biological basis for the co-morbidity between smoking and mood disorders

Nicotine dependence is still the major preventable cause of death in the developed world, and has strong comorbity with mood disorders including major depression. Depressed patients are more likely to smoke cigarettes, and quitting can precipitate an episode of depression in some subjects. Interestingly, antidepressants, particularly the atypical antidepressant buproprion, are therapeutics that can help smokers quit. Despite these observations, the underlying biological factors of the relationship between smoking and depression remain unclear. Results from clinical and pre-clinical studies have seemed somewhat paradoxical because heightened cholinergic activity can induce depression while both nicotine and nicotinic antagonists can be antidepressant-like. These observations can be reconciled by considering that high affinity nicotinic receptors in the brain can be desensitized by chronic nicotine use, leading to blunted cholinergic activity. Based on this hypothesis, nicotinic antagonists have recently been tested as treatments for depression in human subjects, particularly as adjunct therapy along with classical antidepressants. These data suggest that the relationship between smoking and depression may be partially explained by the fact that depressed patients smoke in an effort to self-medicate depressive symptoms by desensitizing their nicotinic receptors. This possibility suggests new avenues for treatment of both nicotine dependence and depressive disorders.

Desensitization of nicotinic acetylcholine receptors as a strategy for drug development

The specific pharmacological response evoked by a nicotinic acetylcholine receptor (nAChR) agonist is governed by the anatomical distribution and expression of each receptor subtype and by the stoichiometry of subunits comprising each subtype. Contributing to this complexity is the ability of agonists that bind to the orthosteric site of the receptor to alter the affinity state of the receptor and induce desensitization and the observation that, at low doses, some nAChR antagonists evoke agonist-like nicotinic responses. Brain concentrations of nicotine rarely increase to the low-mid micromolar concentrations that have been reported to evoke direct agonist-like responses, such as calcium influx or neurotransmitter release. Low microgram per kilogram doses of nicotine administered to humans or to nonhuman primates to improve cognition and working memory probably result only in low nanomolar brain concentrations--more in line with the ability of nicotine to induce receptor desensitization. Here we review data illustrating that nicotine, its major metabolite cotinine, and two novel analogs of choline, JWB1-84-1 [2-(4-(pyridin-3-ylmethyl)piperazin-1-yl)ethanol] and JAY2-22-33, JWB1-84-1 [2-(methyl(pyridine-3-ylmethyl)amino)-ethanol], improve working memory in macaques. The effectiveness of these four compounds in the task was linearly related to their effectiveness in producing desensitization of the pressor response to ganglionic stimulation evoked by a nAChR agonist in rats. Only nicotine evoked an agonist-like action (increased resting blood pressure). Therefore, it is possible to develop new chemical entities that have the ability to desensitize nAChRs without an antecedent agonist action. Because these "silent desensitizers" are probably acting allosterically, an additional degree of subtype specificity could be attained.