Multiple Motivational Forces Contribute to Nicotine Dependence

Nicotine but not saline self-administering or yoked control conditions produces sustained neuroadaptations in the accumbens shell

Introduction

Using yoked animals as the control when monitoring operant drug-self-administration is considered the golden standard. However, instrumental learning per se recruits several neurocircuits that may produce distinct or overlapping neuroadaptations with drugs of abuse. The aim of this project was to assess if contingent responding for nicotine or saline in the presence of a light stimulus as a conditioned reinforcer is associated with sustained neurophysiological adaptations in the nucleus accumbens shell (nAcS), a brain region repeatedly associated with reward related behaviors.

Methods

To this end, nicotine-or saline-administrating rats and yoked-saline stimulus-unpaired training conditions were assessed in operant boxes over four consecutive weeks. After four additional weeks of home cage forced abstinence and subsequent cue reinforced responding under extinction conditions, ex vivo electrophysiology was performed in the nAcS medium spiny neurons (MSNs).

Results

Whole cell recordings conducted in voltage and current-clamp mode showed that excitatory synapses in the nAcS were altered after prolonged forced abstinence from nicotine self-administration. We observed an increase in sEPSC amplitude in animals with a history of contingent nicotine SA potentially indicating higher excitability of accumbal MSNs, which was further supported by current clamp recordings. Interestingly no sustained neuroadaptations were elicited in saline exposed rats from nicotine associated visual cues compared to the yoked controls.

Conclusion

The data presented here indicate that nicotine self-administration produces sustained neuroadaptations in the nAcS while operant responding driven by nicotine visual stimuli has no long-term effects on MSNs in nAcS.

Exploration of potential role of Rho GTPase in nicotine dependence-induced withdrawal syndrome in mice

The RhoA gene showed an important genotypic association with nicotine dependence and smoking initiation. The current study aims to investigate the effect of the Rho GTPase inhibitor ML141 in the progression of nicotine dependence in a mice model of precipitated nicotine withdrawal syndrome by mecamylamine.The experimental procedure involved administration of 2.5 mg/kg nicotine dissolved in normal saline subcutaneously (s.c) four times a day consecutively for 7 days and last single dose in the morning on 8th day. ML-141 was dissolved in dimethyl sulfoxide (DMSO) and was administered daily with nicotine as corrective treatment at a dose of 1,5 and 10 mg/kg (p < 0.05). An injection of 3 mg/kg of mecamylamine intraperitoneal (ip) was given an hour later than the last nicotine dose on the day 8 to precipitate withdrawal of nicotine and withdrawal severity was assessed by measuring hyperalgesia, piloerection, jumping frequency, tremors, and withdrawal severity score (WSS). Various behavioural changes such as hyperalgesia, piloerection, jumping frequency, and tremors were monitored and WSS was calculated. ML-141 a selective Rho GTPase inhibitor was found to show dose-dependent effect on all these parameters. Inhibition of Rho GTPase was found to reduce the severity of withdrawal syndrome; therefore, it can be concluded that Rho GTPase would serve as a suitable biological target by regulating the reward system in brain and could be used as new target for drug discovery.

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.

The Importance of Conditioned Stimuli in Cigarette and E-Cigarette Craving Reduction by E-Cigarettes

This study examined the impact of four variables pertaining to the use of e-cigarettes (e-cigs) on cravings for tobacco cigarettes and for e-cigs after an overnight abstinence period. The four variables were the nicotine level, the sensorimotor component, the visual aspect, and the aroma of the e-cig. In an experimental study, 81 participants without prior vaping experience first got acquainted with using e-cigs in a one-week tryout period, after which they participated in a lab session assessing the effect of five minutes of vaping following an abstinence period of 12 h. A mixed-effects model clearly showed the importance of nicotine in craving reduction. However, also non-nicotine factors, in particular the sensorimotor component, were shown to contribute to craving reduction. Handling cues interacted with the level (presence/absence) of nicotine: it was only when the standard hand-to-mouth action cues were omitted that the craving reducing effects of nicotine were observed. Effects of aroma or visual cues were not observed, or weak and difficult to interpret, respectively.

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.

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.

Interoception and learning: import to understanding and treating diseases and psychopathologies

Chemotherapeutic agents nauseate cancer patients. Some individuals with schizophrenia hear voices. Chronic pain can be reduced by analgesics. Nausea, voices, and pain are examples of internal (interoceptive) stimuli closely linked with a disease and/or its treatment. There is evidence that the perception and, hence, role of these internal stimuli can be modified by one's learning history. There is also increased awareness by researchers and practitioners of the potential import of learning involving internal states to some diseases and psychopathologies. Unfortunately, the science, theory, and practice appear to be trailing behind awareness. In this mini-review, we describe two examples: smoking and panic disorder. While doing so, we discuss the need to develop translationally relevant animal models that will allow investigators to better understand the behavioral and neural mechanisms underlying interoception and learning.

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

Tobacco smoking continues to be a major global health hazard despite significant public awareness of its harmful consequences. Although several treatment options are currently available for smoking cessation, these medications are effective in only a small subset of smokers, and relapse rates continue to be high. Therefore, a better understanding of the neurobiological mechanisms that mediate tobacco dependence is essential for the development of effective smoking cessation medications. Nicotine is the primary psychoactive component of tobacco that drives the harmful tobacco smoking habit. Nicotine binds to nicotinic acetylcholine receptors (nAChRs) in the brain, resulting in the release of a wide range of neurotransmitters, including glutamate and γ-aminobutyric acid (GABA). This review article focuses on the role of the excitatory glutamate system and inhibitory GABA system in nicotine dependence. Accumulating evidence suggests that blockade of glutamatergic transmission or facilitation of GABAergic transmission attenuates the positive reinforcing and incentive motivational aspects of nicotine, inhibits the reward-enhancing and conditioned rewarding effects of nicotine, and blocks nicotine-seeking behavior. Chronic nicotine exposure produced long-term neuroadaptations that contribute to nicotine withdrawal, but the role of GABA and glutamate transmission in nicotine withdrawal is less understood. Overall, the findings presented in this review provide strong converging evidence for the potential effectiveness of glutamatergic and GABAergic medications in nicotine dependence. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Development of novel pharmacotherapeutics for tobacco dependence: progress and future directions

INTRODUCTION

The vast majority of tobacco smokers seeking to quit will relapse within the first month of abstinence. Currently available smoking cessation agents have limited utility in increasing rates of smoking cessation and in some cases there are notable safety concerns related to their use. Hence, there is a pressing need to develop safer and more efficacious smoking cessation medications.

METHODS

Here, we provide an overview of current efforts to develop new pharmacotherapeutic agents to facilitate smoking cessation, identified from ongoing clinical trials and published reports.

RESULTS

Nicotine is considered the major addictive agent in tobacco smoke, and the vast majority of currently available smoking cessation agents act by modulating nicotinic acetylcholine receptor (nAChR) signaling. Accordingly, there is much effort directed toward developing novel small molecule therapeutics and biological agents such as nicotine vaccines for smoking cessation that act by modulating nAChR activity. Our increasing knowledge of the neurobiology of nicotine addiction has revealed new targets for novel smoking cessation therapeutics. Indeed, we highlight many examples of novel small molecule drug development around non-nAChR targets. Finally, there is a growing appreciation that medications already approved for other disease indications could show promise as smoking cessation agents, and we consider examples of such repurposing efforts.

CONCLUSION

Ongoing clinical assessment of potential smoking cessation agents offers the promise of new effective medications. Nevertheless, much of our current knowledge of molecular mechanisms of nicotine addiction derived from preclinical studies has not yet been leveraged for medications development.

Precipitated withdrawal from nicotine reduces reinforcing effects of a visual stimulus for rats

INTRODUCTION

Research has identified at least two positive reinforcement-related effects of nicotine: (a) primary reinforcement and (b) enhancement of reinforcement from concurrently available stimuli. Prior examples of the reinforcement-enhancing effects with rats showed that repeated, intermittent nicotine exposure increased responding for non-nicotine reinforcers, and this effect remained robust over several weeks. However, the effects of continuous nicotine exposure on responding for a non-nicotine reinforcer are unknown, as are the effects of abruptly withdrawing continuous nicotine on behavior maintained by the same reinforcer.

METHODS

Lever pressing for a visual reinforcer under a fixed ratio schedule was assessed while rats were maintained on a chronic, continuous infusion of nicotine (3.16 mg/kg/day; osmotic minipump). The effects of precipitated withdrawal on responding, following 16 days of continuous nicotine exposure, were assessed by pre-session subcutaneous injections of mecamylamine (1.0 mg/kg).

RESULTS

Continuous nicotine initially increased active responding for the visual reinforcer; however, continued exposure resulted in an attenuation of this effect. Precipitated withdrawal from nicotine resulted in a significant decline in active responding.

CONCLUSIONS

The initial increase in responding for the visual reinforcer with chronic nicotine exposure is consistent with prior research showing that intermittent exposure to nicotine acts as a reinforcement enhancer. However, the attenuation of this enhancement following prolonged nicotine exposure is in contrast with the persistent effects previously reported. Finally, the decrease in visual reinforcers below control levels (nicotine-naive animals) following nicotine withdrawal highlights a potential for affective withdrawal, which may serve as a motive for continued nicotine use.

Mouse model predicts effects of smoking and varenicline on event-related potentials in humans

BACKGROUND

Nicotine alters auditory event-related potentials (ERPs) in rodents and humans and is an effective treatment for smoking cessation. Less is known about the effects of the partial nicotine agonist varenicline on ERPs.

METHODS

We measured the effects of varenicline and nicotine on the mouse P20 and varenicline and smoking on the human P50 in a paired-click task. Eighteen mice were tested following nicotine, varenicline, and their combination. One hundred and fourteen current smokers enrolled in a placebo-controlled within-subject crossover study to test the effects of varenicline during smoking and abstinence. Thirty-two subjects participated in the ERP study, with half receiving placebo first and half varenicline first (VP).

RESULTS

Nicotine and varenicline enhanced mouse P20 amplitude, while nicotine improved P20 habituation by selectively increasing the first-click response. Similar to mice, abstinence reduced P50 habituation relative to smoking by reducing the first-click response. There was no effect of varenicline on P50 amplitude during abstinence across subjects. However, there was a significant effect of medication order on P50 amplitude during abstinence. Subjects in the PV group displayed reduced P50 during abstinence, which was blocked by varenicline. However, subjects in the VP group did not display abstinence-induced P50 reduction.

CONCLUSIONS

Data suggest that smoking improves sensory processing. Varenicline mimics amplitude changes associated with nicotine and smoking but fails to alter habituation. The effect of medication order suggests a possible carryover effect from the previous arm. This study supports the predictive validity of ERPs in mice as a marker of drug effects in human studies.

Positive modulation of GABA(B) receptors decreased nicotine self-administration and counteracted nicotine-induced enhancement of brain reward function in rats

Acute administration of gamma-aminobutyric acid (GABA)-B receptor agonists decreases nicotine, cocaine, ethanol, and heroin self-administration and also decreases food-maintained responding and suppresses locomotor activity at high doses. GABA(B) receptor-positive modulators may represent potentially improved therapeutic compounds because of their fewer side effects than receptor agonists. The present study investigated the effects of administration of the GABA(B) receptor-positive modulators 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) and N-[(1R,2R,4S)-bicyclo[2.2.1]hept-2-yl]-2-methyl-5-[4-(trifluoromethyl)phenyl]-4-pyrimidinamine (BHF177) and coadministration of the GABA(B) receptor-positive modulator N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine (GS39783) with the GABA(B) receptor agonist (3-amino-2[S]-hydroxypropyl)-methylphosphinic acid (CGP44532) on nicotine- and food-maintained responding under fixed ratio (FR) 5 and progressive ratio schedules of reinforcement. Furthermore, the effects of BHF177 and CGP44532 on nicotine-induced enhancement of brain reward function were evaluated. The results indicated that administration of CGP7930 decreased nicotine self-administration under an FR5 schedule. Administration of either GS39783 or CGP44532 selectively decreased nicotine self-administration, whereas coadministration of these compounds had additive effects. BHF177 administration selectively decreased nicotine- but not food-maintained responding under FR5 and progressive ratio schedules. The nicotine-induced enhancement of brain reward function was blocked by BHF177 or CGP44532, although the highest doses of both compounds, particularly CGP44532, decreased brain reward function when administered alone, suggesting an additive, rather than interactive, effect. Overall, the present results indicate that GABA(B) receptor-positive modulators, similarly to GABA(B) receptor agonists, attenuated the reinforcing and reward-enhancing effects of nicotine, perhaps with higher selectivity than GABA(B) receptor agonists. Thus, GABA(B) receptor-positive modulators may be useful antismoking medications.