Involvement of glutamatergic and GABAergic systems in nicotine dependence: Implications for novel pharmacotherapies for smoking cessation

A pilot study of ketamine among individuals with tobacco use disorder: tolerability and initial impact on tobacco use outcomes

OBJECTIVES

There is increasing evidence of ketamine's therapeutic potential in reducing substance use in individuals with substance use disorders. However, its effects on tobacco use disorder are unknown. We investigated the effect of a subanesthetic dose of ketamine on tobacco use.

METHODS

This randomized, single-blind, placebo-controlled, pilot study administered intravenous ketamine to individuals with tobacco use disorder recruited from the local community. Participants were randomized to receive either ketamine (0.5 mg/kg) (n = 6) or saline placebo (n = 4) over 20 min. Primary outcomes included measures of drug safety and tolerability during and within an hour after the infusion. Secondary outcomes included measures of tobacco use, craving, and withdrawal before, and 24-hours after, the drug infusion study day. A follow-up visit occurred eight days after the infusion.

RESULTS

Intravenous ketamine was well tolerated with transient side effects. No significant effects were noted on cigarette smoking, craving, or withdrawal symptoms on the post-infusion visit following overnight abstinence or on the follow-up visit (p's > 0.05).

CONCLUSIONS

Although limited by the small sample size, this pilot study extends previous research on ketamine for substance use disorders. While ketamine was well tolerated in this sample, additional research testing different ketamine doses and administration routes is necessary to determine whether ketamine has therapeutic potential for tobacco use disorder.

Impaired spatial dynamic functional network connectivity and neurophysiological correlates in functional hemiparesis

The present study investigated spatial dynamic functional network connectivity (dFNC) in patients with functional hemiparesis (i.e., functional stroke mimics, FSM). The aim of this work was to assess static functional connectivity (large-scale) networks and dynamic brain states, which represent distinct dFNC patterns that reoccur in time and across subjects. Resting-state fMRI data were collected from 15 patients with FSM (mean age = 42.3 ± 9.4, female = 80 %) and 52 age-matched healthy controls (HC, mean age = 42.1 ± 8.6, female = 73 %). Each patient underwent a resting-state functional MRI scan for spatial dFNC evaluation and transcranial magnetic stimulation protocols for indirect assessment of GABAergic and glutamatergic transmission. We considered three dynamic brain networks, i.e., the somatomotor network (SMN), the default mode network (DMN) and the salience network (SN), each summarized into four distinct recurring spatial configurations. Compared to HC, patients with FSM showed significant decreased dwell time, e.g. the time each individual spends in each spatial state of each network, in state 2 of the SMN (HC vs. FSM, 13.5 ± 27.1 vs. 1.9 ± 4.1, p = 0.044). Conversely, as compared to HC, FSM spent more time in state 1 of the DMN (10.8 ± 14.9 vs. 27.3 ± 38.9, p = 0.037) and in state 3 of the SN (23.1 ± 23.0 vs. 38.8 ± 38.2, p = 0.002). We found a significant correlation between the dwell time of impaired functional state of the SMN and measures of GABAergic neurotransmission (r = 0.581, p = 0.037). Specifically, longer impaired dwell time was associated with greater GABAergic inhibition. These findings demonstrate that FSM present altered functional brain network dynamics, which correlate with measures of GABAergic neurotransmission. Both dFNC and GABAergic neurotransmission may serve as potential targets for future intervention strategies.

Fluorocitrate inhibition of astrocytes reduces nicotine self-administration and alters extracellular levels of glutamate and dopamine within the nucleus accumbens in male wistar rats

Emerging evidence suggests an important role of astrocytes in mediating behavioral and molecular effects of commonly misused drugs. Passive exposure to nicotine alters molecular, morphological, and functional properties of astrocytes. However, a potential involvement of astrocytes in nicotine reinforcement remains largely unexplored. The overall hypothesis tested in the current study is that astrocytes play a critical role in nicotine reinforcement. Protein levels of the astrocyte marker glial fibrillary acidic protein (GFAP) were examined in key mesocorticolimbic regions following chronic nicotine intravenous self-administration. Fluorocitrate, a metabolic inhibitor of astrocytes, was tested for its effects on behaviors related to nicotine reinforcement and relapse. Effects of fluorocitrate on extracellular neurotransmitter levels, including glutamate, GABA, and dopamine, were determined with microdialysis. Chronic nicotine intravenous self-administration increased GFAP expression in the nucleus accumbens core (NACcr), but not other key mesocorticolimbic regions, compared to saline intravenous self-administration. Both intra-ventricular and intra-NACcr microinjection of fluorocitrate decreased nicotine self-administration. Intra-NACcr fluorocitrate microinjection also inhibited cue-induced reinstatement of nicotine seeking. Local perfusion of fluorocitrate decreased extracellular glutamate levels, elevated extracellular dopamine levels, but did not alter extracellular GABA levels in the NACcr. Fluorocitrate did not alter basal locomotor activity. These results indicate that nicotine reinforcement upregulates the astrocyte marker GFAP expression in the NACcr, metabolic inhibition of astrocytes attenuates nicotine reinforcement and relapse, and metabolic inhibition of astrocytes disrupts extracellular dopamine and glutamate transmission. Overall, these findings suggest that astrocytes play an important role in nicotine reinforcement and relapse, potentially through regulation of extracellular glutamate and dopamine neurotransmission.

Nicotine inhalant via E-cigarette facilitates sensorimotor function recovery by upregulating neuronal BDNF-TrkB signalling in traumatic brain injury

BACKGROUND AND PURPOSE

Traumatic brain injury (TBI) causes lifelong physical and psychological dysfunction in affected individuals. The current study investigated the effects of chronic nicotine exposure via E-cigarettes (E-cig) (vaping) on TBI-associated behavioural and biochemical changes.

EXPERIMENTAL APPROACH

Adult C57/BL6J male mice were subjected to controlled cortical impact (CCI) followed by daily exposure to E-cig vapour for 6 weeks. Sensorimotor functions, locomotion, and sociability were subsequently evaluated by nesting, open field, and social approach tests, respectively. Immunoblots were conducted to examine the expression of mature brain-derived neurotrophic factor (mBDNF) and associated downstream proteins (p-Erk, p-Akt). Histological analyses were performed to evaluate neuronal survival and neuroinflammation.

KEY RESULTS

Post-injury chronic nicotine exposure significantly improved nesting performance in CCI mice. Histological analysis revealed increased survival of cortical neurons in the perilesion cortex with chronic nicotine exposure. Immunoblots revealed that chronic nicotine exposure significantly up-regulated mBDNF, p-Erk and p-Akt expression in the perilesion cortex of CCI mice. Immunofluorescence microscopy indicated that elevated mBDNF and p-Akt expression were mainly localized within cortical neurons. Immunolabelling of Iba1 demonstrated that chronic nicotine exposure attenuated microglia-mediated neuroinflammation.

CONCLUSIONS AND IMPLICATIONS

Post-injury chronic nicotine exposure via vaping facilitates recovery of sensorimotor function by upregulating neuroprotective mBDNF/TrkB/Akt/Erk signalling. These findings suggest potential neuroprotective properties of nicotine despite its highly addictive nature. Thus, understanding the multifaceted effects of chronic nicotine exposure on TBI-associated symptoms is crucial for paving the way for informed and properly managed therapeutic interventions.

Brain dysfunctions and neurotoxicity induced by psychostimulants in experimental models and humans: an overview of recent findings

Preclinical and clinical studies indicate that psychostimulants, in addition to having abuse potential, may elicit brain dysfunctions and/or neurotoxic effects. Central toxicity induced by psychostimulants may pose serious health risks since the recreational use of these substances is on the rise among young people and adults. The present review provides an overview of recent research, conducted between 2018 and 2023, focusing on brain dysfunctions and neurotoxic effects elicited in experimental models and humans by amphetamine, cocaine, methamphetamine, 3,4-methylenedioxymethamphetamine, methylphenidate, caffeine, and nicotine. Detailed elucidation of factors and mechanisms that underlie psychostimulant-induced brain dysfunction and neurotoxicity is crucial for understanding the acute and enduring noxious brain effects that may occur in individuals who use psychostimulants for recreational and/or therapeutic purposes.

Neurometabolic profile of the amygdala in smokers assessed with 1H-magnetic resonance spectroscopy

Tobacco smoking is one of the main causes of premature death worldwide and quitting success remains low, highlighting the need to understand the neurobiological mechanisms underlying relapse. Preclinical models have shown that the amygdala and glutamate play an important role in nicotine addiction. The aims of this study were to compare glutamate and other metabolites in the amygdala between smokers and controls, and between different smoking states. Furthermore, associations between amygdalar metabolite levels and smoking characteristics were explored. A novel non-water-suppressed proton magnetic resonance spectroscopy protocol was applied to quantify neurometabolites in 28 male smokers (≥15 cigarettes/day) and 21 non-smoking controls, matched in age, education, verbal IQ, and weekly alcohol consumption. Controls were measured once (baseline) and smokers were measured in a baseline state (1-3 h abstinence), during withdrawal (24 h abstinence) and in a satiation state (directly after smoking). Baseline spectroscopy data were compared between groups by independent t-tests or Mann-Whitney-U tests. Smoking state differences were investigated by repeated-measures analyses of variance (ANOVAs). Associations between spectroscopy data and smoking characteristics were explored using Spearman correlations. Good spectral quality, high anatomical specificity (98% mean gray matter) and reliable quantification of most metabolites of interest were achieved in the amygdala. Metabolite levels did not differ between groups, but smokers showed significantly higher glutamine levels at baseline than satiation. Glx levels were negatively associated with pack-years and smoking duration. In summary, this study provides first insights into the neurometabolic profile of the amygdala in smokers with high anatomical specificity. By applying proton magnetic resonance spectroscopy, neurometabolites in smokers during different smoking states and non-smoking controls were quantified reliably. A significant shift in glutamine levels between smoking states was detected, with lower concentrations in satiation than baseline. The negative association between Glx levels and smoking quantity and duration may imply altered glutamate homeostasis with more severe nicotine addiction.

Neurobiological associations between smoking and internalizing disorders

People with severe mental disorders have a higher mortality rate due to preventable conditions like cardiovascular diseases and respiratory diseases. Nicotine addiction is a preventable risk factor, with tobacco use being twice as high in people with mental disorders. An integrative model that divides mental disorders into externalising, internalising, and thought disorders could be useful for identifying common causalities and risk factors. This review aims to examine the interface between smoking and internalising disorders, specifically schizophrenia, depressive disorders, and anxiety disorders. The review finds that there is a clear association between smoking behaviour and these disorders. Schizophrenia is associated with polymorphisms that result in an imbalance between glutamate and GABA release and abnormalities of dopaminergic pathways. Nicotine improves dopaminergic signalling and balances glutamatergic and GABAergic pathways, improving symptoms and increasing the risk of nicotine dependence. In depressive disorders, smoking is associated with functional changes in brain regions affected by smoking and self-medication. In anxiety disorders, there is a bidirectional relationship with smoking, involving the amygdala and changes in dopaminergic pathways and cortisol production. Smoking poses a threat to people living with psychiatric disorders and calls for further research to assess the interactions between nicotine dependence and internalising and thought disorders.

Effect of Cigarette Smoke Exposure and Aspirin Treatment on Neurotransmitters’ Tissue Content in Rats’ Hippocampus and Amygdala

Cigarette smoke withdrawal can cause anxiety-like behavior and modulate neurotransmitter-related proteins in the brain. We examined the effects of cigarette smoke with and without aspirin treatment on the concentrations of neurotransmitters, including dopamine, serotonin, glutamate, glutamine, and GABA in the amygdala and hippocampus. Sprague-Dawley rats were randomly assigned to four different groups: (1) control group exposed only to standard room air, (2) cigarette smoke exposed group treated with saline vehicle, (3) cigarette smoke exposed group treated with aspirin (30 mg/kg), and (4) control group treated only with aspirin (30 mg/kg). Cigarette smoke exposure was performed for 2 h/day, 5 days/week, for 31 days. Behavioral testing was carried out weekly, 24 h after cigarette smoke exposure, during acute withdrawal. At the end of week 4, rats were given either distilled water (1 mL) or aspirin 45 min before cigarette exposure for 11 days. Dopamine, serotonin, glutamate, glutamine, and GABA were extracted from both the amygdala and hippocampus and were separated and quantified using a developed and validated HPLC-MS/MS method. Cigarette smoke withdrawal induced anxiety behaviors, and aspirin treatment reduced this effect. Cigarette smoke exposure increased tissue content of dopamine, serotonin, glutamate, glutamine, and GABA, and aspirin treatment reversed this effect. Cigarette smoke caused an increase in tissue content of several neurotransmitters as well as anxiety-like behavior, and these effects were normalized by aspirin treatment.

Nicotine Use and Metabotropic Glutamate Receptor 5 in Individuals With Major Depressive and Posttraumatic Stress Disorders

Metabotropic glutamate receptor 5 (mGluR5) dysregulation has been implicated in the pathophysiology of many psychiatric disorders, as well as nicotine use and dependence. We used positron emission tomography with [18F]FPEB to measure mGluR5 availability in vivo in 6 groups: (1) nicotine users (NUs) without other psychiatric comorbidities (n = 23); (2) comparison controls (CCs) without nicotine use or psychiatric comorbidities (n = 38); (3) major depressive disorder subjects with concurrent nicotine use (MDD-NU; n = 19); (4) MDD subjects without concurrent nicotine use (MDD-CC; n = 20); (5) posttraumatic stress disorder subjects with concurrent nicotine use (PTSD-NU; n = 17); and (6) PTSD subjects without concurrent nicotine use (PTSD-CC; n = 16). The goal of the study was to test the hypothesis that mGluR5 availability in key corticolimbic regions of interest (ROIs) is different in NU with versus without comorbid psychiatric disorders (ROI: dorsolateral prefrontal cortex [dlPFC], orbitofrontal cortex [OFC], ventromedial prefrontal cortex [vmPFC], anterior cingulate cortex [ACC], amygdala, hippocampus). We found that NU had 11%-13% lower mGluR5 availability in OFC, vmPFC, dlPFC, and ACC as compared with CC, while PTSD-NU had 9%-11% higher mGluR5 availability in OFC, dlPFC, and ACC compared with PTSD. Furthermore, relationships between mGluR5 availability and psychiatric symptoms varied as a function of psychiatric diagnosis among NUs. NU showed a negative correlation between mGluR5 and smoking cravings and urges (r's = -0.58 to -0.70, p's = 0.011 - 0.047), while PTSD-NU had the reverse relationship (r's = 0.60-0.71, p's = 0.013-0.035 in ACC, vmPFC, and dlPFC). These findings have substantial implications for our understanding of glutamate homeostasis in psychiatric subgroups and for identifying key neural phenotypes among NU. mGluR5 is a potential treatment target for precision medicine in individuals with nicotine use.

Overnight Abstinence Is Associated With Smaller Secondary Somatosensory Cortical Volumes and Higher Somatosensory-Motor Cortical Functional Connectivity in Cigarette Smokers

INTRODUCTION

Abstinence symptoms present challenges to successful cessation of cigarette smoking. Chronic exposure to nicotine and long-term nicotine abstinence are associated with alterations in cortical and subcortical gray matter volumes (GMVs).

AIMS AND METHODS

We aimed at examining changes in regional GMVs following overnight abstinence and how these regional functions relate to abstinence symptoms. Here, in a sample of 31 regular smokers scanned both in a satiety state and after overnight abstinence, we employed voxel-wise morphometry and resting-state functional connectivity (rsFC) to investigate these issues. We processed imaging data with published routines and evaluated the results with a corrected threshold.

RESULTS

Smokers showed smaller GMVs of the left ventral hippocampus and right secondary somatosensory cortex (SII) after overnight abstinence as compared to satiety. The GMV alterations in right SII were positively correlated with changes in withdrawal symptom severity between states. Furthermore, right SII rsFC with the precentral gyrus was stronger in abstinence as compared to satiety. The inter-regional rsFC was positively correlated with motor impulsivity and withdrawal symptom severity during abstinence and negatively with craving to smoke during satiety.

CONCLUSIONS

These findings highlight for the first time the effects of overnight abstinence on cerebral volumetrics and changes in functional connectivity of a higher-order sensory cortex. These changes may dispose smokers to impulsive behaviors and aggravate the urge to smoke at the earliest stage of withdrawal from nicotine.

IMPLICATIONS

Overnight abstinence leads to changes in gray matter volumes and functional connectivity of the second somatosensory cortex in cigarette smokers. Higher somatosensory and motor cortical connectivity in abstinence is significantly correlated with trait motor impulsivity and withdrawal symptom severity. The findings add to the literature of neural markers of nicotine addiction.

Amino acid systems in the interpeduncular nucleus are altered in a sex-dependent manner during nicotine withdrawal

Prior work in male rodents established that the medial habenula-interpeduncular nucleus (MHb-IPN) pathway modulates nicotine withdrawal. Specifically, withdrawal severity has been closely associated with inhibitory tone in the IPN via interneurons that release γ-aminobutyric acid (GABA). Inhibitory tone in the IPN is regulated by projections from the MHb that co-release glutamate and acetylcholine. Within the IPN, inhibitory tone is also regulated via corticotropin-releasing factor type 1 (CRF1) receptors that control GABA release from local interneurons. This study extends previous work by comparing sex differences in GABA, glutamate, as well serotonin levels in the IPN during precipitated nicotine withdrawal. Sex differences in withdrawal-induced neurochemical effects were also compared following systemic administration of a CRF1 receptor antagonist. The results revealed that there were no group differences in serotonin levels in the IPN. A major finding was that females displayed a larger withdrawal-induced increases in GABA levels in the IPN than males. Also, withdrawal increased IPN glutamate levels in a similar manner in females and males. Blockade of CRF1 receptors produced a larger suppression of the withdrawal-induced increases in GABA levels in the IPN of females versus males, an effect that was likely related to the robust increase in glutamate following administration of the CRF1 receptor antagonist in females. These data suggest that amino acid systems in the IPN modulate sex differences in the behavioral effects of nicotine withdrawal. Furthermore, our data imply that medications that target stress-induced activation of the IPN may reduce withdrawal severity, particularly in females.

Quantifying GABA in Addiction: A Review of Proton Magnetic Resonance Spectroscopy Studies

Gamma-aminobutyric acid (GABA) signaling plays a crucial role in drug reward and the development of addiction. Historically, GABA neurochemistry in humans has been difficult to study due to methodological limitations. In recent years, proton magnetic resonance spectroscopy (1H-MRS, MRS) has emerged as a non-invasive imaging technique that can detect and quantify human brain metabolites in vivo. Novel sequencing and spectral editing methods have since been developed to allow for quantification of GABA. This review outlines the clinical research utilization of 1H-MRS in understanding GABA neurochemistry in addiction and summarizes current literature that reports GABA measurements by MRS in addiction. Research on alcohol, nicotine, cocaine, and cannabis addiction all suggest medications that modulate GABA signaling may be effective in reducing withdrawal, craving, and other addictive behaviors. Thus, we discuss how improvements in current MRS techniques and design can optimize GABA quantification in future studies and explore how monitoring changes to brain GABA could help identify risk factors, improve treatment efficacy, further characterize the nature of addiction, and provide crucial insights for future pharmacological development.

Neurometabolic alterations in the nucleus accumbens of smokers assessed with 1 H magnetic resonance spectroscopy: The role of glutamate and neuroinflammation

Tobacco use is one of the leading causes of premature death and morbidity worldwide. For smokers trying to quit, relapse rates are high, even after prolonged periods of abstinence. Recent findings in animal models highlight the role of alterations in glutamatergic projections from the prefrontal cortex onto the nucleus accumbens (NAc) in relapse vulnerability. Moreover, inflammatory responses in the NAc have been reported during withdrawal. A novel proton magnetic resonance spectroscopy (¹ H-MRS) protocol was applied in humans to measure molar concentrations for glutamate, its sum with glutamine (Glx), and myoinositol plus glycine (mI + Gly) in the NAc (19 smokers, 20 matched controls). Smokers were measured at baseline and during withdrawal and satiation. No difference between groups or smoking states was found for glutamate or Glx, but, in smokers, stronger craving and more severe nicotine dependence were associated with lower baseline glutamate and Glx levels, respectively. Interestingly, mI + Gly concentrations were higher during withdrawal than baseline and correlated negatively with nicotine dependence severity and pack years of smoking. The lack of glutamatergic changes between groups and smoking states may imply that glutamate homeostasis is not significantly altered in smokers or that changes are too small for detection by ¹ H-MRS. Moreover, the observed increase in mI + Gly may imply that neuroinflammatory processes occur in the NAc during nicotine withdrawal. These findings shed light on neurobiological relapse mechanisms in smokers and may provide the opportunity to develop more effective treatment options targeting the glutamate and neuroinflammation system.

Linalool attenuates acquisition and reinstatement and accelerates the extinction of nicotine-induced conditioned place preference in male mice

Background: Nicotine is the addictive agent in tobacco products. The monoterpene linalool is the main ingredient in the essential oils of various aromatic plants. It has previously been demonstrated that linalool has beneficial effects on some mechanisms that are important in drug addiction.Objectives: The goal of the current study was to investigate the effect of linalool on nicotine-induced conditioned place preference (CPP) in male mice.Methods: CPP was induced by administering intraperitoneal (i.p.) injection of nicotine (0.5 mg/kg) during the conditioning phase. The effects of nicotinic acetylcholine receptor partial agonist varenicline and linalool on the rewarding characteristics of nicotine were tested in mice with administration of linalool (12.5, 25, and 50 mg/kg, i.p.), varenicline (2 mg/kg, i.p.) or saline 30 minutes before nicotine injection. CPP was extinguished by repeated testing, during which conditioned mice were administered varenicline and linalool every day. One day after the last extinction trial, mice that received linalool, varenicline or saline 30 minutes before a priming injection of nicotine (0.1 mg/kg, i.p.) were immediately tested for reinstatement of CPP.Results: Linalool attenuated nicotine acquisition (50 mg/kg, p < .01) and reinstatement (25 and 50 mg/kg, respectively p < .05, p < .01) and accelerated the extinction of nicotine-induced CPP (50 mg/kg, p < .05). Linalool exhibited similar effects on the reference drug varenicline in the CPP phases.Conclusion: These results suggest that linalool may be helpful as an adjuvant for the treatment of nicotine use disorder.

The effects of quercetin on nicotine-induced reward effects in mice

OBJECTIVES

Tobacco smoking remains the primary cause of preventable mortality and morbidity in the world. The complexity of the nicotine dependency process included the withdrawal effect that triggers recurrence being the main problem. Quercetin, known as an antioxidant, binds free radicals and modulates endogenous antioxidants through Nrf2 activations is expected as a potential agent to reduce the risk of nicotine dependence. This research aims to evaluate quercetin's effects on reducing the risk of nicotine addiction.

METHODS

Conditioned Place Preference (CPP) with a biased design was used to evaluate nicotine's reward effects in male Balb/C mice. Preconditioning test was performed on day 1; conditioning test was done twice daily on day 2-4 by administering quercetin (i.p.) 50 mg/kg along with nicotine (s.c.) 0.5 mg/kg or Cigarette Smoke Extract (CSE) (s.c.) contained nicotine 0.5 mg/kg; and postconditioning test was performed on day 5 continue with extinction test on day 6, 8, 10, 12, and reinstatement test on day 13. The duration spent in each compartment was recorded and analyzed.

RESULTS

Nicotine 0.5 mg/kg and CSE 0.5 mg/kg significantly induced reward effects (p<0.05). There was no decrease of reward effect during the extinction-reinstatement stage of the postconditioning phase (p>0.05), while quercetin 50 mg/kg both induced along with nicotine or CSE was able to inhibit the reward effect of nicotine (p>0.05).

CONCLUSIONS

Quercetin reduced the risk of nicotine dependence and has a potential effect to use as a therapy for nicotine dependence, especially as a preventive agent.

Repeated nicotine exposure increases the intracellular interaction between ERK-mGluR5 in the nucleus accumbens more in adult than adolescent rats

Intracellular interactions between protein kinases and metabotropic receptors in the striatum regulate behavioral changes in response to drug exposure. We investigated the difference in the degree of interaction between extracellular signal-regulated kinase (ERK) and metabotropic glutamate receptor subtype 5 (mGluR5) in the nucleus accumbens (NAc) after repeated exposure to nicotine in adult and adolescent rats. The results showed that repeated exposure to nicotine (0.5 mg/kg/day, s.c.) for seven consecutive days increased ERK phosphorylation more in adults than in adolescents. Furthermore, membrane expression of mGluR5 in gamma-aminobutyric acid (GABA) medium spiny neurons was higher in adults than adolescents as a result of repeated exposure to nicotine. Blockade of mGluR5 with MPEP (0.5 nmol/side) decreased the repeated nicotine-induced increase in ERK phosphorylation. Either blockade of mGluR5 or inhibition of ERK with SL327 (150 nmol/side) decreased the repeated nicotine-induced increase in the level of inositol-1,4,5-triphosphate (IP₃ ), a key transducer associated with mGluR5-coupled signaling cascades. Similarly, interference of binding between activated ERK and mGluR5 by the blocking peptide, Tat-mGluR5-i (2 nmol/side), decreased the repeated nicotine-induced increases in IP₃ and locomotor activity in adults. These findings suggest that the intracellular interaction between ERK and mGluR5 in the NAc is stronger in adult than in adolescent rats, which enhances the understanding of age-associated behavioral changes that occur after repeated exposure to nicotine.

Exposure to Commercial Cigarette Smoke Produces Psychomotor Sensitization Via Hyperstimulation of Glutamate Response in the Dorsal Striatum

Cigarette smoke is a highly complex mixture of nicotine and non-nicotine constituents. Exposure to cigarette smoke enhances tobacco dependence by potentiating glutamatergic neurotransmission via stimulation of nicotinic acetylcholine receptors (nAChRs). We investigated the effects of nicotine and non-nicotine alkaloids in the cigarette smoke condensates extracted from two commercial cigarette brands in South Korea (KCSC A and KCSC B) on psychomotor behaviors and glutamate levels in the dorsal striatum. Repeated and challenge administration of KCSCs (nicotine content: 0.4 mg/kg, subcutaneous) increased psychomotor behaviors (ambulatory, rearing, and rotational activities) and time spent in psychoactive behavioral states compared to exposure to nicotine (0.4 mg/kg) alone. The increase in psychomotor behaviors lasted longer when exposed to repeated and challenge administration of KCSCs compared to nicotine alone. In parallel with sustained increase in psychomotor behaviors, repeated administration of KCSCs also caused long-lasting glutamate release in the dorsal striatum compared to nicotine alone. KCSC-induced changes in psychomotor behaviors and glutamate levels in the dorsal striatum were found to be strongly correlated. These findings suggest that non-nicotine alkaloids in commercial cigarette smoke synergistically act with nicotine on nAChRs, thereby upregulating glutamatergic response in the dorsal striatum, which contributes to the hypersensitization of psychomotor behaviors.

Short-term nicotine deprivation alters dorsal anterior cingulate glutamate concentration and concomitant cingulate-cortical functional connectivity

Most cigarette smokers who wish to quit too often relapse within the first few days of abstinence, primarily due to the aversive aspects of the nicotine withdrawal syndrome (NWS), which remains poorly understood. Considerable research has suggested that the dorsal anterior cingulate cortex (dACC) plays a key role in nicotine dependence, with its functional connections between other brain regions altered as a function of trait addiction and state withdrawal. The flow of information between dACC and fronto-striatal regions is secured through different pathways, the vast majority of which are glutamatergic. As such, we investigated dACC activity using resting state functional connectivity (rsFC) with functional magnetic resonance imaging (fMRI) and glutamate (Glu) concentration with magnetic resonance spectroscopy (MRS). We also investigated the changes in adenosine levels in plasma during withdrawal as a surrogate for brain adenosine, which plays a role in fine-tuning synaptic glutamate transmission. Using a double-blind, placebo-controlled, randomized crossover design, nontreatment seeking smoking participants (N = 30) completed two imaging sessions, one while nicotine sated and another after 36 h nicotine abstinence. We observed reduced dACC Glu (P = 0.029) along with a significant reduction in plasma adenosine (P = 0.03) and adenosine monophosphate (AMP; P < 0.0001) concentrations during nicotine withdrawal in comparison with nicotine sated state. This withdrawal state manipulation also led to an increase in rsFC strength (P < 0.05) between dACC and several frontal cortical regions, including left superior frontal gyrus (LSFG), and right middle frontal gyrus (RMFG). Moreover, the state-trait changes in dACC Glu and rsFC strength between the dACC and both SFG and MFG were positively correlated (P = 0.012, and P = 0.007, respectively). Finally, the change in circuit strength between dACC and LSFG was negatively correlated with the change in withdrawal symptom manifestations as measured by the Wisconsin Smoking Withdrawal Scale (P = 0.04) and Tobacco Craving Questionnaire (P = 0.014). These multimodal imaging-behavioral findings reveal the complex cascade of changes induced by acute nicotine deprivation and call for further investigation into the potential utility of adenosine- and glutamate-signaling as novel therapeutic targets to treat the NWS.

The role of the metabotropic glutamate receptor 5 in nicotine addiction

This review summarizes the evidence for the potential involvement of metabotropic glutamate receptor 5 (mGluR5) in the development of nicotine addiction. Nicotine is consumed worldwide and is highly addictive. Previous research has extensively investigated the role of dopamine in association with reward learning and addiction, which has provided strong evidence for the involvement of dopaminergic neuronal circuitry in nicotine addiction. More recently, researchers focused on glutamatergic transmission after nicotine abuse, and its involvement in the reinforcing and rewarding effects of nicotine addiction. A number of robust preclinical and clinical studies have shown mGluR5 signaling as a facilitating mechanism of nicotine addiction and nicotine withdrawal. Specifically, clinical studies have illustrated lower cortical mGluR5 density in smokers compared to nonsmokers in the human brain. In addition, mGluR5 might selectively regulate craving and withdrawal. This suggests that mGluR5 could be a key receptor in the development of nicotine addiction and therefore clinical trials to examine the therapeutic potential of mGluR5 agents could help to contribute to reduce nicotine addiction in society.

Neuropeptide and cytokine regulation of pain in the context of substance use disorders

Substance use disorders (SUDs) are frequently accompanied by affective symptoms that promote negative reinforcement mechanisms contributing to SUD maintenance or progression. Despite their widespread use as analgesics, chronic or excessive exposure to alcohol, opioids, and nicotine produces heightened nociceptive sensitivity, termed hyperalgesia. This review focuses on the contributions of neuropeptide (CRF, melanocortin, opioid peptide) and cytokine (IL-1β, TNF-α, chemokine) systems in the development and maintenance of substance-induced hyperalgesia. Few effective therapies exist for either chronic pain or SUD, and the common interaction of these disease states likely complicates their effective treatment. Here we highlight promising new discoveries as well as identify gaps in research that could lead to more effective and even simultaneous treatment of SUDs and co-morbid hyperalgesia symptoms.

Regional GABA Concentrations Modulate Inter-network Resting-state Functional Connectivity

Coordinated activity within and differential activity between large-scale neuronal networks such as the default mode network (DMN) and the control network (CN) is a critical feature of brain organization. The CN usually exhibits activations in response to cognitive tasks while the DMN shows deactivations; in addition, activity between the two networks is anti-correlated at rest. To address this issue, we used functional MRI to measure whole-brain BOLD signal during resting-state and task-evoked conditions, and MR spectroscopy (MRS) to quantify GABA and glutamate concentrations, in nodes within the DMN and CN (MPFC and DLPFC, respectively) in 19 healthy individuals at 3 Tesla. We found that GABA concentrations in the MPFC were significantly associated with DMN deactivation during a working memory task and with anti-correlation between DMN and CN at rest and during task performance, while GABA concentrations in the DLPFC weakly modulated DMN-CN anti-correlation in the opposite direction. Highlighting specificity, glutamate played a less significant role related to brain activity. These findings indicate that GABA in the MPFC is potentially involved in orchestrating between-network brain activity at rest and during task performance.

Pathogenesis and pathophysiology of autosomal dominant sleep-related hypermotor epilepsy with S284L-mutant α4 subunit of nicotinic ACh receptor

BACKGROUND AND PURPOSE

The mechanisms causing spontaneous epileptic seizures, including carbamazepine-resistant/zonisamide -sensitive seizures and comorbidity in autosomal dominant sleep-related hypermotor epilepsy (ADSHE) are unclear. This study investigated functional abnormalities in thalamocortical transmission in transgenic rats bearing rat S286L-mutant Chrna4 (S286L-TG) of α4 subunit of the nicotinic ACh receptor (nAChR) that corresponds to the human S284L-mutant CHRNA4.

EXPERIMENTAL APPROACH

Effects of carbamazepine and zonisamide on epileptic discharges of S286L-TG rat were measured using telemetry electrocorticogram. Transmission abnormalities of L-glutamate and GABA in thalamocortical pathway of S286L-TG rats were investigated using multiprobe microdialysis and ultra-high-performance liquid-chromatography.

KEY RESULTS

Epileptic discharges in S286L-TG rats were reduced by zonisamide but not by carbamazepine, similar to that of S284L-ADSHE patients. Carbamazepine unaffected functional abnormality in transmission of S286L-TG rats. However, zonisamide was able to compensate for the attenuated S286L-mutant nAChR induced GABA release in frontal-cortex, without affecting attenuated thalamocortical glutamatergic transmission. Excitatory effects of S286L-mutant nAChR on thalamocortical transmission were attenuated compared with those of wild-type nAChR. Loss-of-function of S286L-nAChR enhanced transmission in thalamocortical motor pathway by predominantly attenuating GABAergic transmission. However, it attenuated transmission in thalamocortical cognitive pathway by reducing inhibitory GABAergic and excitatory glutamatergic transmission.

CONCLUSION AND IMPLICATIONS

Our results suggest that functional abnormalities of S286L-nAChR are associated with intra-frontal and thalamocortical transmission, possibly contributing to the pathogenesis of ADSHE-seizure and comorbidity of S284L-ADSHE. Selective compensation of impaired GABAergic transmission by zonisamide (but not by carbamazepine) in frontal cortex may be involved, at least partially, in carbamazepine-resistant ADSHE-seizure of S284L-ADSHE patients.

Acute Nicotine Exposure Alters Ventral Tegmental Area Inhibitory Transmission and Promotes Diazepam Consumption

Nicotine use increases the risk for subsequent abuse of other addictive drugs, but the biological basis underlying this risk remains largely unknown. Interactions between nicotine and other drugs of abuse may arise from nicotine-induced neural adaptations in the mesolimbic dopamine (DA) system, a common pathway for the reinforcing effects of many addictive substances. Previous work identified nicotine-induced neuroadaptations that alter inhibitory transmission in the ventral tegmental area (VTA). Here, we test whether nicotine-induced dysregulation of GABAergic signaling within the VTA increases the vulnerability for benzodiazepine abuse that has been reported in smokers. We demonstrate in rats that nicotine exposure dysregulates diazepam-induced inhibition of VTA GABA neurons and increases diazepam consumption. In VTA GABA neurons, nicotine impaired KCC2-mediated chloride extrusion, depolarized the GABAA reversal potential, and shifted the pharmacological effect of diazepam on GABA neurons from inhibition toward excitation. In parallel, nicotine-related alterations in GABA signaling observed ex vivo were associated with enhanced diazepam-induced inhibition of lateral VTA DA neurons in vivo Targeting KCC2 with the agonist CLP290 normalized diazepam-induced effects on VTA GABA transmission and reduced diazepam consumption following nicotine administration to the control level. Together, our results provide insights into midbrain circuit alterations resulting from nicotine exposure that contribute to the abuse of other drugs, such as benzodiazepines.

Pathomechanism of nocturnal paroxysmal dystonia in autosomal dominant sleep-related hypermotor epilepsy with S284L-mutant α4 subunit of nicotinic ACh receptor

To study the pathomechanism and pathophysiology of nocturnal paroxysmal dystonia of autosomal dominant sleep-related hypermotor epilepsy (ADSHE), this study determined functional abnormalities in thalamic hyperdirect pathway from reticular thalamic nucleus (RTN), motor thalamic nuclei (MoTN), subthalamic nucleus (STN) to substantia nigra pars reticulata (SNr) of transgenic rats (S286L-TG) bearing S286 L missense mutation of rat Chrna4 gene, which corresponds to the S284 L mutation in the human CHRNA4 gene. The activation of α4β2-nAChR in the RTN increased GABA release in MoTN resulting in reduced glutamatergic transmission in thalamic hyperdirect pathway of wild-type. Contrary to wild-type, activation of S286L-mutant α4β2-nAChR (loss-of-function) in the RTN relatively enhanced glutamatergic transmission in thalamic hyperdirect pathway of S286L-TG via impaired GABAergic inhibition in intra-thalamic (RTN-MoTN) pathway. These functional abnormalities in glutamatergic transmission in hyperdirect pathway contribute to the pathomechanism of electrophysiologically negative nocturnal paroxysmal dystonia of S286L-TG. Therapeutic-relevant concentration of zonisamide (ZNS) inhibited the glutamatergic transmission in the hyperdirect pathway via activation of group II metabotropic glutamate receptor (II-mGluR) in MoTN and STN. The present results suggest that S286L-mutant α4β2-nAChR induces GABAergic disinhibition in intra-thalamic (RTN-MoTN) pathway and hyperactivation of glutamatergic transmission in thalamic hyperdirect pathway (MoTN-STN-SNr), possibly contributing to the pathomechanism of nocturnal paroxysmal dystonia of ADSHE patients with S284L mutant CHRNA4. Inhibition of glutamatergic transmission in thalamic hyperdirect pathway induced by ZNS via activation of II-mGluR may be involved, at least partially, in ZNS-sensitive nocturnal paroxysmal dystonia of ADSHE patients with S284L mutation.

Demonstration of critical role of GRIN3A in nicotine dependence through both genetic association and molecular functional studies

Nicotine dependence (ND) is a chronic disease with catastrophic effects on individual and public health. The glutamate receptor subunit gene, ionotropic N-methyl-d-aspartate 3A (GRIN3A), encodes a crucial subunit of N-methyl-d-aspartate receptors (NMDARs), which play an essential role in synaptic plasticity in the brain. Although various variants of GRIN3A have been associated with ND in European-American and African-American samples, no study has been reported for the association between GRIN3A and ND in Chinese Han population. We performed an association study of 16 single nucleotide polymorphisms (SNPs) in GRIN3A with ND in 2616 Chinese individuals. SNP-based association analysis indicated that SNP rs1323423 was significantly associated with the Fagerström Test for Nicotine Dependence (FTND) score after correction for multiple testing (P = 0.0026). Haplotype-based association analysis revealed that Block 3, formed by rs1323423-rs10989591, was significantly associated with the FTND score after correction for multiple testing (global P = 0.0183). Furthermore, luciferase reporter assay demonstrated that the DNA region containing rs1323423 was an enhancer element, the activity of which was significantly impacted by rs1323423 genotype. Considering that rs1323423 is located in a potential enhancer region, we performed GRIN3A editing in HEK293T cells with CRISPR/Cas9 and found that the DNA region around rs1323423 has a regulatory function and the expression of GRIN3A affects the expression of other NMDA subunits. Moreover, we demonstrated that nicotine at a concentration of 100 μM decreased expression of GRIN3A in SH-SY5Y and HEK293T cells at the RNA and protein level, respectively. This study provides novel evidence for the involvement of GRIN3A in ND.

Prefrontal GABA and glutamate levels correlate with impulsivity and cognitive function of prescription opioid addicts: A 1 H-magnetic resonance spectroscopy study

AIM

Prescription opioids are psychoactive substances that can elicit many neuropsychological effects. There are no studies that directly demonstrate the effects of prescription opioid addiction (POA) on the human brain. This study aimed to quantify γ-aminobutyric acid (GABA) and glutamate (Glu) levels in the prefrontal cortex (PFC) of POA patients using proton magnetic resonance spectroscopy (¹ H-MRS), and to explore their association with impulsive behavior and cognitive impairment.

METHODS

Thirty-five patients with a definitive clinical diagnosis of codeine-containing cough syrup dependence and 35 matched healthy controls underwent neuropsychological assessments, namely the Barratt Impulsiveness Scale (BIS-11) and the Montreal Cognitive Assessment Scale (MoCA). Point-resolved spectroscopy was performed to detect GABA and glutamate within the medial PFC, and the corresponding levels were estimated using jMRUI and corrected for fraction of cerebrospinal fluid in the ¹ H-MRS voxel. The difference in metabolite levels between groups and the correlation between metabolite levels and psychometric scores in patients were analyzed statistically.

RESULTS

The peak level predominantly consisting of GABA with a relatively small influence of other chemicals (GABA+) was lower and that of glutamate was higher in the PFC of POA patients than in healthy controls. GABA+ levels correlated negatively with BIS-11 scores but correlated positively with MoCA scores. In contrast, glutamate levels showed a positive correlation with BIS-11 scores but no significant correlation with MoCA scores.

CONCLUSION

The quantitative in vivo measurement of GABA and glutamate levels in the PFC by ¹ H-MRS could be a reliable way to evaluate impulsivity and cognitive function of POA.

Current advances in research in treatment and recovery: Nicotine addiction

The health harms of combusted tobacco use are undeniable. With market and regulatory pressures to reduce the harms of nicotine delivery by combustion, the tobacco product landscape has diversified to include smokeless, heated, and electronic nicotine vaping products. Products of tobacco combustion are the main cause of smoking-induced disease, and nicotine addiction sustains tobacco use. An understanding of the biology and clinical features of nicotine addiction and the conditioning of behavior that occurs via stimuli paired with frequent nicotine dosing, as with a smoked cigarette, is important for informing pharmacologic and behavioral treatment targets. We review current advances in research on nicotine addiction treatment and recovery, with a focus on conventional combustible cigarette use. Our review covers evidence-based methods to treat smoking in adults and policy approaches to prevent nicotine product initiation in youth. In closing, we discuss emerging areas of evidence and consider new directions for advancing the field.

Neonatal Nicotine Exposure Primes Midbrain Neurons to a Dopaminergic Phenotype and Increases Adult Drug Consumption

BACKGROUND

Nicotine intake induces addiction through neuroplasticity of the reward circuitry, altering the activity of dopaminergic neurons of the ventral tegmental area. Prior work demonstrated that altered circuit activity can change neurotransmitter expression in the developing and adult brain. Here we investigated the effects of neonatal nicotine exposure on the dopaminergic system and nicotine consumption in adulthood.

METHODS

Male and female mice were used for two-bottle-choice test, progressive ratio breakpoint test, immunohistochemistry, RNAscope, quantitative polymerase chain reaction, calcium imaging, and DREADD (designer receptor exclusively activated by designer drugs)-mediated chemogenic activation/inhibition experiments.

RESULTS

Neonatal nicotine exposure potentiates drug preference in adult mice, induces alterations in calcium spike activity of midbrain neurons, and increases the number of dopamine-expressing neurons in the ventral tegmental area. Specifically, glutamatergic neurons are first primed to express transcription factor Nurr1, then acquire the dopaminergic phenotype following nicotine re-exposure in adulthood. Enhanced neuronal activity combined with Nurr1 expression is both necessary and sufficient for the nicotine-mediated neurotransmitter plasticity to occur.

CONCLUSIONS

Our findings illuminate a new mechanism of neuroplasticity by which early nicotine exposure primes the reward system to display increased susceptibility to drug consumption in adulthood.

Neurobiological and Neurophysiological Mechanisms Underlying Nicotine Seeking and Smoking Relapse

Tobacco-related morbidity and mortality continue to be a significant public health concern. Unfortunately, current FDA-approved smoking cessation pharmacotherapies have limited efficacy and are associated with high rates of relapse. Therefore, a better understanding of the neurobiological and neurophysiological mechanisms that promote smoking relapse is needed to develop novel smoking cessation medications. Here, we review preclinical studies focused on identifying the neurotransmitter and neuromodulator systems that mediate nicotine relapse, often modeled in laboratory animals using the reinstatement paradigm, as well as the plasticity-dependent neurophysiological mechanisms that facilitate nicotine reinstatement. Particular emphasis is placed on how these neuroadaptations relate to smoking relapse in humans. We also highlight a number of important gaps in our understanding of the neural mechanisms underlying nicotine reinstatement and critical future directions, which may lead toward the development of novel, target pharmacotherapies for smoking cessation.

Brain and Cognition for Addiction Medicine: From Prevention to Recovery Neural Substrates for Treatment of Psychostimulant-Induced Cognitive Deficits

Addiction to psychostimulants like cocaine, methamphetamine, and nicotine poses a continuing medical and social challenge both in the United States and all over the world. Despite a desire to quit drug use, return to drug use after a period of abstinence is a common problem among individuals dependent on psychostimulants. Recovery for psychostimulant drug-dependent individuals is particularly challenging because psychostimulant drugs induce significant changes in brain regions associated with cognitive functions leading to cognitive deficits. These cognitive deficits include impairments in learning/memory, poor decision making, and impaired control of behavioral output. Importantly, these drug-induced cognitive deficits often impact adherence to addiction treatment programs and predispose abstinent addicts to drug use relapse. Additionally, these cognitive deficits impact effective social and professional rehabilitation of abstinent addicts. The goal of this paper is to review neural substrates based on animal studies that could be pharmacologically targeted to reverse psychostimulant-induced cognitive deficits such as impulsivity and impairment in learning and memory. Further, the review will discuss neural substrates that could be used to facilitate extinction learning and thus reduce emotional and behavioral responses to drug-associated cues. Moreover, the review will discuss some non-pharmacological approaches that could be used either alone or in combination with pharmacological compounds to treat the above-mentioned cognitive deficits. Psychostimulant addiction treatment, which includes treatment for cognitive deficits, will help promote abstinence and allow for better rehabilitation and integration of abstinent individuals into society.

Effective Use of Animal Models for Therapeutic Development in Psychiatric and Substance Use Disorders

Athina Markou and others argue forcefully for the adoption of a "translational-back translational strategy" for central nervous system drug discovery involving novel application of drugs with established safety profiles in proof-of-principle studies in humans, which in turn encourage parallel studies using experimental animals to provide vital data on the neural systems and neuropharmacological mechanisms related to the actions of the candidate drugs. Encouraged by the increasing adoption of drug-development strategies involving reciprocal information exchange between preclinical animal studies and related clinical research programs, this review presents additional compelling examples related to the following: 1) the treatment of cognitive deficits that define attention-deficit/hyperactivity disorder; 2) the development of fast-acting antidepressants based on promising clinical effects with low doses of the anesthetic ketamine; and 3) new and effective medications for the treatment of substance misuse. In the context of addressing the unmet medical need for new and effective drugs for treatment of mental ill health, now may be the time to launch major new academic-industry consortia committed to open access of all preclinical and clinical data generated by this research.

Metabotropic Glutamate Receptors 2 and 3 as Targets for Treating Nicotine Addiction

Tobacco smoking, driven by the addictive properties of nicotine, continues to be a worldwide health problem. Based on the well-established role of glutamatergic neurotransmission in drug addiction, novel medication development strategies seek to halt nicotine consumption and prevent relapse to tobacco smoking by modulating glutamate transmission. The presynaptic inhibitory metabotropic glutamate receptors 2 and 3 (mGluR2/3) are key autoreceptors on glutamatergic terminals that maintain glutamate homeostasis. Accumulating evidence suggests the critical role of mGluR2/3 in different aspects of nicotine addiction, including acquisition and maintenance of nicotine taking, nicotine withdrawal, and persistent nicotine seeking even after prolonged abstinence. The involvement of mGluR2/3 in other neuropsychiatric conditions, such as anxiety, depression, schizophrenia, Alzheimer's disease, Parkinson's disease, and pain, provides convincing evidence suggesting that mGluR2/3 may provide an effective therapeutic approach for comorbidity of smoking and these conditions. This focused review article highlights that mGluR2/3 provide a promising target in the search for smoking cessation medication with novel mechanisms of actions that differ from those of currently U.S. Food and Drug Administration-approved pharmacotherapies.

The Gamma-Aminobutyric Acid B Receptor in Depression and Reward

The metabotropic gamma-aminobutyric acid B (GABA_B) receptor was the first described obligate G protein-coupled receptor heterodimer and continues to set the stage for discoveries in G protein-coupled receptor signaling complexity. In this review, dedicated to the life and work of Athina Markou, we explore the role of GABA_B receptors in depression, reward, and the convergence of these domains in anhedonia, a shared symptom of major depressive disorder and withdrawal from drugs of abuse. GABA_B receptor expression and function are enhanced by antidepressants and reduced in animal models of depression. Generally, GABA_B receptor antagonists are antidepressant-like and agonists are pro-depressive. Exceptions to this rule likely reflect the differential influence of GABA_B1 isoforms in depression-related behavior and neurobiology, including the anhedonic effects of social stress. A wealth of data implicate GABA_B receptors in the rewarding effects of drugs of abuse. We focus on nicotine as an example. GABA_B receptor activation attenuates, and deactivation enhances, nicotine reward and associated neurobiological changes. In nicotine withdrawal, however, GABA_B receptor agonists, antagonists, and positive allosteric modulators enhance anhedonia, perhaps owing to differential effects of GABA_B1 isoforms on the dopaminergic system. Nicotine cue-induced reinstatement is more reliably attenuated by GABA_B receptor activation. Separation of desirable and undesirable side effects of agonists is achievable with positive allosteric modulators, which are poised to enter clinical studies for drug abuse. GABA_B1 isoforms are key to understanding the neurobiology of anhedonia, whereas allosteric modulators may offer a mechanism for targeting specific brain regions and processes associated with reward and depression.

GABAergic Neurons as Putative Neurochemical Substrate Mediating Aversive Effects of Nicotine

Nicotine, the main addictive component of tobacco smoke, has both rewarding and aversive properties. Recent studies have suggested that GABAergic neurons, one of the main neurochemical components of the reward-addiction circuitry, may also play a role in the aversive responses to nicotine. In the present study of transgenic mice expressing Green Fluorescent Protein (GFP) in Glutamate Decarboxylase 67 (GAD67) neurons, we hypothesized that a subpopulation of GABAergic neurons in the Ventral Tegmental Area (VTA) are the targets of aversive doses of nicotine in the CNS. We tested this hypothesis using c-Fos immunohistochemical techniques to identify GAD67-GFP positive cells within the VTA, that are activated by a single intraperitoneal (i.p.) injection of a low (40 ug/kg) or a high (2 mg/kg) dose of nicotine. We also assessed the anatomical location of GAD67-GFP positive cells with respect to tyrosine hydroxylase (TH) Immunoreactive (IR) dopaminergic cells in VTA. Consistent with our previous studies low- and high-dose nicotine both induced c-Fos activation of various intensities at multiple sites in VTA. Double labeling of c-Fos activated cells with GAD67-GFP positive cells identified a subpopulation of GABAergic neurons in Substantia Nigra Compact part Medial tier (SNCM) that were activated by high- but not by low-dose nicotine. Of 217 GABAergic cells counted at this site, 48.9% exhibited nicotine induced c-fos immunoreactivity. GAD67-GFP positive cells in other regions of VTA were not activated by the nicotine doses tested. Double labeling of GAD67-GFP positive cells with TH IR cells showed that the GABAergic neurons that were activated by high-dose nicotine were located in close proximity to the dopaminergic neurons of substantia nigra compact part and VTA. Dose-dependent activation of GAD67-GFP positive neurons in SNCM, by a nicotine dose known to produce aversive responses, implies that GABAergic neurons at these sites may be an important component of the nicotine aversive circuitry.

Repeated Administration of Cigarette Smoke Condensate Increases Glutamate Levels and Behavioral Sensitization

Nicotine, a nicotinic acetylcholine receptor agonist, produces the reinforcing effects of tobacco dependence by potentiating dopaminergic and glutamatergic neurotransmission. Non-nicotine alkaloids in tobacco also contribute to dependence by activating the cholinergic system. However, glutamatergic neurotransmission in the dorsal striatum associated with behavioral changes in response to cigarette smoking has not been investigated. In this study, the authors investigated alterations in glutamate levels in the rat dorsal striatum related to behavioral alterations after repeated administration of cigarette smoke condensate (CSC) using the real-time glutamate biosensing and an open-field behavioral assessment. Repeated administration of CSC including 0.4 mg nicotine (1.0 mL/kg/day, subcutaneous) for 14 days significantly increased extracellular glutamate concentrations more than repeated nicotine administration. In parallel with the hyperactivation of glutamate levels, repeated administration of CSC-evoked prolonged hypersensitization of psychomotor activity, including locomotor and rearing activities. These findings suggest that the CSC-induced psychomotor activities are closely associated with the elevation of glutamate concentrations in the rat dorsal striatum.

Mutually augmenting interactions of dextromethorphan and sazetidine-A for reducing nicotine self-administration in rats

A variety of nicotinic drug treatments have been found to decrease nicotine self-administration. However, interactions of drugs affecting different nicotinic receptor subtypes have not been much investigated. This study investigated the interactions between dextromethorphan, which blocks nicotinic α3β2 receptors as well as a variety of other receptors with sazetidine-A which is a potent and selective α4β2 nicotinic receptor partial agonist with desensitizing properties. This interaction was compared with dextromethorphan combination treatment with mecamylamine, which is a nonspecific nicotinic channel blocker. Co-administration of dextromethorphan (either 0.5 or 5 mg/kg) and lower dose of sazetidine-A (0.3 mg/kg) caused a significant reduction in nicotine SA. With regard to food-motivated responding, 3 mg/kg of sazetidine-A given alone caused a significant decrease in food intake. However, the lower 0.3 mg/kg sazetidine-A dose did not significantly affect food-motivated responding even when given in combination with the higher 5 mg/kg dextromethorphan dose which itself caused a significant decrease in food motivated responding. Interestingly, this higher dextromethorphan dose significantly attenuated the decrease in food motivated responding caused by 3 mg/kg of sazetidine-A. Locomotor activity was increased by the lower 0.3 mg/kg sazetidine-A dose and decreased by the 5 mg/kg dextromethorphan dose. Mecamylamine at the doses (0.1 and 1 mg/kg) did not affect nicotine SA, but at 1 mg/kg significantly decreased food-motivated responding. None of the mecamylamine doses augmented the effect of dextromethorphan in reducing nicotine self-administration. These studies showed that the combination of dextromethorphan and sazetidine-A had mutually potentiating effects, which could provide a better efficacy for promoting smoking cessation, however the strength of the interactions was fairly modest.

Behavioral changes after nicotine challenge are associated with α7 nicotinic acetylcholine receptor-stimulated glutamate release in the rat dorsal striatum

Neurochemical alterations associated with behavioral responses induced by re-exposure to nicotine have not been sufficiently characterized in the dorsal striatum. Herein, we report on changes in glutamate concentrations in the rat dorsal striatum associated with behavioral alterations after nicotine challenge. Nicotine challenge (0.4 mg/kg/day, subcutaneous) significantly increased extracellular glutamate concentrations up to the level observed with repeated nicotine administration. This increase occurred in parallel with an increase in behavioral changes in locomotor and rearing activities. In contrast, acute nicotine administration and nicotine withdrawal on days 1 and 6 did not alter glutamate levels or behavioral changes. Blockade of α7 nicotinic acetylcholine receptors (nAChRs) significantly decreased the nicotine challenge-induced increases in extracellular glutamate concentrations and locomotor and rearing activities. These findings suggest that behavioral changes in locomotor and rearing activities after re-exposure to nicotine are closely associated with hyperactivation of the glutamate response by stimulating α7 nAChRs in the rat dorsal striatum.

Prefrontal Glx and GABA concentrations and impulsivity in cigarette smokers and smoking polysubstance users

Glutamate and GABA play an important role in substance dependence. However, it remains unclear whether this holds true for different substance use disorders and how this is related to risk-related traits such as impulsivity. We, therefore, compared Glx (as a proxy measure for glutamate) and GABA concentrations in the dorsal anterior cingulate cortex (dACC) of 48 male cigarette smokers, 61 male smoking polysubstance users, and 90 male healthy controls, and investigated the relationship with self-reported impulsivity and substance use. Glx and GABA concentrations were measured using proton Magnetic Resonance Spectroscopy. Impulsivity, smoking, alcohol and cocaine use severity and cannabis use were measured using self-report instruments. Results indicate a trend towards group differences in Glx. Post-hoc analyses showed a difference between smokers and healthy controls (p=0.04) and a trend towards higher concentrations in smoking polysubstance users and healthy controls (p=0.09), but no differences between smokers and smoking polysubstance users. dACC GABA concentrations were not significantly different between groups. Smoking polysubstance users were more impulsive than smokers, and both groups were more impulsive than controls. No significant associations were observed between dACC neurotransmitter concentrations and impulsivity and level and severity of smoking, alcohol or cocaine use or the presence of cannabis use. The results indicate that differences in dACC Glx are unrelated to type and level of substance use. No final conclusion can be drawn on the lack of GABA differences due to assessment difficulties. The relationship between dACC neurotransmitter concentrations and cognitive impairments other than self-reported impulsivity should be further investigated.

KK-92A, a novel GABAB receptor positive allosteric modulator, attenuates nicotine self-administration and cue-induced nicotine seeking in rats

RATIONALE

GABA_B receptors (GABA_BR) play a critical role in GABAergic neurotransmission in the brain and are thought to be one of the most promising targets for the treatment of drug addiction. GABA_BR positive allosteric modulators (PAMs) have shown promise as potential anti-addictive therapies, as they lack the sedative and muscle relaxant properties of full GABA_B receptor agonists such as baclofen.

OBJECTIVES

The present study was aimed at developing novel, selective, and potent GABA_BR PAMs with efficacy on abuse-related effects of nicotine.

RESULTS

We synthetized ~100 analogs of BHF177, a GABA_BR PAM that has been shown to inhibit nicotine taking and seeking, and tested their activity in multiple cell-based functional assays. Among these compounds, KK-92A displayed superior PAM properties at the GABA_BR. Interestingly, our results revealed the existence of pathway-selective differential modulation of GABA_BR signaling by the structurally related GABA_BR allosteric modulators BHF177 and KK-92A. In vivo, similarly to BHF177, KK-92A inhibited intravenous nicotine self-administration under both fixed- and progressive-ratio schedules of reinforcement in rats. In contrast to BHF177, KK-92A had no effect on food self-administration. Furthermore, KK-92A decreased cue-induced nicotine-seeking behavior without affecting food seeking.

CONCLUSIONS

These results indicate that KK-92A is a selective GABA_BR PAM with efficacy in inhibition of the primary reinforcing and incentive motivational effects of nicotine, and attenuation of nicotine seeking, further confirming that GABA_BR PAMs may be useful antismoking medications.

Metabotropic glutamate receptor 5 as a potential target for smoking cessation

RATIONALE

Most habitual smokers find it difficult to quit smoking because they are dependent upon the nicotine present in tobacco smoke. Tobacco dependence is commonly treated pharmacologically using nicotine replacement therapy or drugs, such as varenicline, that target the nicotinic receptor. Relapse rates, however, remain high, and there remains a need to develop novel non-nicotinic pharmacotherapies for the dependence that are more effective than existing treatments.

OBJECTIVE

The purpose of this paper is to review the evidence from preclinical and clinical studies that drugs that antagonise the metabotropic glutamate receptor 5 (mGluR5) in the brain are likely to be efficacious as treatments for tobacco dependence.

RESULTS

Imaging studies reveal that chronic exposure to tobacco smoke reduces the density of mGluR5s in human brain. Preclinical results demonstrate that negative allosteric modulators (NAMs) at mGluR5 attenuate both nicotine self-administration and the reinstatement of responding evoked by exposure to conditioned cues paired with nicotine delivery. They also attenuate the effects of nicotine on brain dopamine pathways implicated in addiction.

CONCLUSIONS

Although mGluR5 NAMs attenuate most of the key facets of nicotine dependence, they potentiate the symptoms of nicotine withdrawal. This may limit their value as smoking cessation aids. The NAMs that have been employed most widely in preclinical studies of nicotine dependence have too many "off-target" effects to be used clinically. However, newer mGluR5 NAMs have been developed for clinical use in other indications. Future studies will determine if these agents can also be used effectively and safely to treat tobacco dependence.

Converging findings from linkage and association analyses on susceptibility genes for smoking and other addictions

Experimental approaches to genetic studies of complex traits evolve with technological advances. How do discoveries using different approaches advance our knowledge of the genetic architecture underlying complex diseases/traits? Do most of the findings of newer techniques, such as genome-wide association study (GWAS), provide more information than older ones, for example, genome-wide linkage study? In this review, we address these issues by developing a nicotine dependence (ND) genetic susceptibility map based on the results obtained by the approaches commonly used in recent years, namely, genome-wide linkage, candidate gene association, GWAS and targeted sequencing. Converging and diverging results from these empirical approaches have elucidated a preliminary genetic architecture of this intractable psychiatric disorder and yielded new hypotheses on ND etiology. The insights we obtained by putting together results from diverse approaches can be applied to other complex diseases/traits. In sum, developing a genetic susceptibility map and keeping it updated are effective ways to keep track of what we know about a disease/trait and what the next steps may be with new approaches.

Attenuation of nicotine-taking and nicotine-seeking behavior by the mGlu2 receptor positive allosteric modulators AZD8418 and AZD8529 in rats

RATIONALE

Numerous medication development strategies seek to decrease nicotine consumption and prevent relapse to tobacco smoking by blocking glutamate transmission. Decreasing glutamate release by activating presynaptic inhibitory metabotropic glutamate (mGlu)2/3 receptors inhibits the reinforcing effects of nicotine and blocks cue-induced reinstatement of nicotine-seeking behavior in rats. However, the relative contribution of mGlu2 receptors in nicotine dependence is still unknown.

OBJECTIVES

The present study evaluated the role of mGlu2 receptors in nicotine-taking and nicotine-seeking behavior using the novel, relatively selective mGlu2 positive allosteric modulators (PAMs) AZD8418 and AZD8529.

RESULTS

Acute treatment with AZD8418 (0.37, 1.12, 3.73, 7.46, and 14.92 mg/kg) and AZD8529 (1.75, 5.83, 17.5, and 58.3 mg/kg) deceased nicotine self-administration and had no effect on food-maintained responding. Chronic treatment with AZD8418 attenuated nicotine self-administration, but tolerance to this effect developed quickly. The inhibition of nicotine self-administration by chronic AZD8529 administration persisted throughout the 14 days of treatment. Chronic treatment with either PAMs inhibited food self-administration. AZD8418 (acute) and AZD8529 (acute and subchronic) blocked cue-induced reinstatement of nicotine- and food-seeking behavior.

CONCLUSIONS

These findings indicate an important role for mGlu2 receptors in the reinforcing properties of self-administered nicotine and the motivational impact of cues that were previously associated with nicotine administration (i.e., cue-induced reinstatement of nicotine-seeking behavior). Thus, mGlu2 PAMs may be useful medications to assist people to quit tobacco smoking and prevent relapse.

Genome-wide time-to-event analysis on smoking progression stages in a family-based study

BACKGROUND

Various pivotal stages in smoking behavior can be identified, including initiation, conversion from experimenting to established use, development of tolerance, and cessation. Previous studies have shown high heritability for age of smoking initiation and cessation; however, time-to-event genome-wide association studies aiming to identify underpinning genes that accelerate or delay these transitions are missing to date.

METHODS

We investigated which single nucleotide polymorphisms (SNPs) across the whole genome contribute to the hazard ratio of transition between different stages of smoking behavior by performing time-to-event analyses within a large Finnish twin family cohort (N = 1962), and further conducted mediation analyses of plausible intermediate traits for significant SNPs.

RESULTS

Genome-wide significant signals were detected for three of the four transitions: (1) for smoking cessation on 10p14 (P = 4.47e-08 for rs72779075 flanked by RP11-575N15 and GATA3), (2) for tolerance on 11p13 (P = 1.29e-08 for rs11031684 in RP1-65P5.1), mediated by smoking quantity, and on 9q34.12 (P = 3.81e-08 for rs2304808 in FUBP3), independent of smoking quantity, and (3) for smoking initiation on 19q13.33 (P = 3.37e-08 for rs73050610 flanked by TRPM4 and SLC6A16) in analysis adjusted for first time sensations. Although our top SNPs did not replicate, another SNP in the TRPM4-SLC6A16 gene region showed statistically significant association after region-based multiple testing correction in an independent Australian twin family sample.

CONCLUSION

Our results suggest that the functional effect of the TRPM4-SLC6A16 gene region deserves further investigation, and that complex neurotransmitter networks including dopamine and glutamate may play a critical role in smoking initiation. Moreover, comparison of these results implies that genetic contributions to the complex smoking behavioral phenotypes vary among the transitions.

Metabotropic Glutamate Receptor 7 (mGluR7) as a Target for the Treatment of Psychostimulant Dependence

Although few medications have been approved by the U.S. Food and Drug Administration (FDA) to assist people to quit tobacco smoking, there are no FDA-approved medications to treat dependence on other psychostimulant drugs, such as cocaine. The motivation to maintain psychostimulant drug seeking and self-administration involves alterations in glutamatergic neurotransmission. Thus, medications that modulate glutamate transmission may be effective treatments for psychostimulant dependence. One presynaptic inhibitory glutamate receptor that critically regulates glutamate transmission is the metabotropic glutamate 7 receptor (mGluR7). This review summarizes nonhuman experimental animal data that indicate a critical role for mGluR7 in drug-taking and drug-seeking behaviors for the psychostimulants cocaine and nicotine. AMN082, the only commercially available allosteric receptor agonist, has been used to investigate the role of mGluR7 in psychostimulant dependence. Systemic administration or microinjection of AMN082 into brain sites within the mesocorticolimbic system decreased self-administration and reinstatement of both cocaine and nicotine seeking. In vivo microdialysis results indicated that a nucleus accumbens-ventral pallidum γ-aminobutyric acid-ergic mechanism may underlie AMN082-induced antagonism of the reinforcing effects of cocaine, whereas a glutamate mGlu2/3 receptor mechanism underlies the AMN082-induced blockade of cocaine seeking. These findings indicate an important role for mGluR7 in mesolimbic areas in modulating the reinforcing effects of psychostimulant drugs, such as nicotine and cocaine, and the conditioned behaviors associated with drugs of abuse. Thus, selective mGluR7 agonists or positive allosteric modulators may have the potential to treat psychostimulant dependence.

Modulation of social deficits and repetitive behaviors in a mouse model of autism: the role of the nicotinic cholinergic system

RATIONALE

Accumulating evidence implicates the nicotinic cholinergic system in autism spectrum disorder (ASD) pathobiology. Neuropathologic studies suggest that nicotinic acetylcholine (ACh) receptor (nAChR) subtypes are altered in brain of autistic individuals. In addition, strategies that increase ACh, the neurotransmitter for nicotinic and muscarinic receptors, appear to improve cognitive deficits in neuropsychiatric disorders and ASD.

OBJECTIVE

The aim of this study is to examine the role of the nicotinic cholinergic system on social and repetitive behavior abnormalities and exploratory physical activity in a well-studied model of autism, the BTBR T(+) Itpr3 (tf) /J (BTBR) mouse.

METHODS

Using a protocol known to up-regulate expression of brain nAChR subtypes, we measured behavior outcomes before and after BTBR and C57BL/6J (B6) mice were treated (4 weeks) with vehicle or nicotine (50, 100, 200, or 400 μg/ml).

RESULTS

Increasing nicotine doses were associated with decreases in water intake, increases in plasma cotinine levels, and at the higher dose (400 μg/ml) with weight loss in BTBR mice. At lower (50, 100 μg/ml) but not higher (200, 400 μg/ml) doses, nicotine increased social interactions in BTBR and B6 mice and at higher, but not lower doses, it decreased repetitive behavior in BTBR. In the open-field test, nicotine at 200 and 400 μg/ml, but not 100 μg/ml compared with vehicle, decreased overall physical activity in BTBR mice.

CONCLUSIONS

These findings support the hypotheses that the nicotinic cholinergic system modulates social and repetitive behaviors and may be a therapeutic target to treat behavior deficits in ASD. Further, the BTBR mouse may be valuable for investigations of the role of nAChRs in social deficits and repetitive behavior.

Neuroscience of nicotine for addiction medicine: novel targets for smoking cessation medications

Morbidity and mortality associated with tobacco smoking constitutes a significant burden on healthcare budgets all over the world. Therefore, promoting smoking cessation is an important goal of health professionals and policy makers throughout the world. Nicotine is a major psychoactive component in tobacco that is largely responsible for the widespread addiction to tobacco. A majority of the currently available FDA-approved smoking cessation medications act via neuronal nicotinic receptors. These medications are effective in approximately half of all the smokers, who want to quit and relapse among abstinent smokers continues to be high. In addition to relapse among abstinent smokers, unpleasant effects associated with nicotine withdrawal are a major motivational factor in continued tobacco smoking. Over the last two decades, animal studies have helped in identifying several neural substrates that are involved in nicotine-dependent behaviors including those associated with nicotine withdrawal and relapse to tobacco smoking. In this review, first the role of specific brain regions/circuits that are involved in nicotine dependence will be discussed. Next, the review will describe the role of specific nicotinic receptor subunits in nicotine dependence. Finally, the review will discuss the role of classical neurotransmitters (dopamine, serotonin, noradrenaline, glutamate, and γ-aminobutyric acid) as well as endogenous opioid and endocannabinoid signaling in nicotine dependence. The nicotinic and nonnicotinic neural substrates involved in nicotine-dependent behaviors can serve as possible targets for future smoking cessation medications.