Inhibition of monoamine oxidases desensitizes 5-HT1A autoreceptors and allows nicotine to induce a neurochemical and behavioral sensitization

Monoamine oxidase inhibition in cigarette smokers: From preclinical studies to tobacco product regulation

Monoamine oxidase (MAO) activity is reduced in cigarette smokers and this may promote the reinforcing actions of nicotine, thereby enhancing the addictive properties of cigarettes. At present, it is unclear how cigarette smoking leads to MAO inhibition, but preclinical studies in rodents show that MAO inhibition increases nicotine self-administration, especially at low doses of nicotine. This effect of MAO inhibition develops slowly, likely due to plasticity of brain monoamine systems; studies relying on acute MAO inhibition are unlikely to replicate what happens with smoking. Given that MAO inhibition may reduce the threshold level at which nicotine becomes reinforcing, it is important to consider this in the context of very low nicotine content (VLNC) cigarettes and potential tobacco product regulation. It is also important to consider how this interaction between MAO inhibition and the reinforcing actions of nicotine may be modified in populations that are particularly vulnerable to nicotine dependence. In the context of these issues, we show that the MAO-inhibiting action of cigarette smoke extract (CSE) is similar in VLNC cigarettes and cigarettes with a standard nicotine content. In addition, we present evidence that in a rodent model of schizophrenia the effect of MAO inhibition to enhance nicotine self-administration is absent, and speculate how this may relate to brain serotonin systems. These issues are relevant to the MAO-inhibiting effect of cigarette smoking and its implications to tobacco product regulation.

Cell-type- and region-specific modulation of cocaine seeking by micro-RNA-1 in striatal projection neurons

The persistent and experience-dependent nature of drug addiction may result in part from epigenetic alterations, including non-coding micro-RNAs (miRNAs), which are both critical for neuronal function and modulated by cocaine in the striatum. Two major striatal cell populations, the striato-nigral and striato-pallidal projection neurons, express, respectively, the D1 (D1-SPNs) and D2 (D2-SPNs) dopamine receptor, and display distinct but complementary functions in drug-evoked responses. However, a cell-type-specific role for miRNAs action has yet to be clarified. Here, we evaluated the expression of a subset of miRNAs proposed to modulate cocaine effects in the nucleus accumbens (NAc) and dorsal striatum (DS) upon sustained cocaine exposure in mice and showed that these selected miRNAs were preferentially upregulated in the NAc. We focused on miR-1 considering the important role of some of its predicted mRNA targets, Fosb and Npas4, in the effects of cocaine. We validated these targets in vitro and in vivo. We explored the potential of miR-1 to regulate cocaine-induced behavior by overexpressing it in specific striatal cell populations. In DS D1-SPNs miR-1 overexpression downregulated Fosb and Npas4 and reduced cocaine-induced CPP reinstatement, but increased cue-induced cocaine seeking. In DS D2-SPNs miR-1 overexpression reduced the motivation to self-administer cocaine. Our results indicate a role of miR1 and its target genes, Fosb and Npas4, in these behaviors and highlight a precise cell-type- and region-specific modulatory role of miR-1, illustrating the importance of cell-specific investigations.

Behavioral and Noradrenergic Sensitizations in Vulnerable Traumatized Rats Suggest Common Bases with Substance Use Disorders

The aim of the present study was to strengthen our hypothesis of a common physiological basis for post-traumatic stress disorder (PTSD) and substance use disorders. This paper investigates the possibility that rats exposed to a PTSD model exhibit noradrenergic and behavioral sensitization, as observed following repeated drugs of abuse injections. First, rats received a single prolonged stress (SPS), combining three consecutive stressors. They were then tested, 2 weeks after the trauma for PTSD-like symptoms to discriminate between vulnerable and resilient rats. When microdialysis was performed in the prelimbic cortex (Experiment 1), larger increases of noradrenaline (NA) release in response to amphetamine were observed in vulnerable rats when compared to control and resilient animals. Experiment 2 showed that trauma-vulnerable rats exhibited increases in locomotor activity relative to controls, in response to an exposure to trauma-associated cues. These data demonstrate that a single trauma exposure induces in vulnerable animals both, a noradrenergic sensitization evidenced within the prelimbic cortex and behavioral sensitization obtained after a physiologic activation of the noradrenergic system. However, Experiment 3 showed that when NA system was activated by amphetamine (1 mg/kg), a decrease in behavioral sensitization was obtained in vulnerable rats. We proposed that this decreased locomotor activity results from an additional stress-induced increased reactivity of mesocortical dopaminergic neurons, known to counteract the consequences of cortical noradrenergic release in rats. These results support our hypothesis that noradrenergic sensitization represents a common physiological basis, involved both in PTSD and drug addiction and suggest new common therapeutic approaches for these pathologies.

The Combination of Marketed Antagonists of α1b-Adrenergic and 5-HT2A Receptors Inhibits Behavioral Sensitization and Preference to Alcohol in Mice: A Promising Approach for the Treatment of Alcohol Dependence

Alcohol-dependence is a chronic disease with a dramatic and expensive social impact. Previous studies have indicated that the blockade of two monoaminergic receptors, α1b-adrenergic and 5-HT2A, could inhibit the development of behavioral sensitization to drugs of abuse, a hallmark of drug-seeking and drug-taking behaviors in rodents. Here, in order to develop a potential therapeutic treatment of alcohol dependence in humans, we have blocked these two monoaminergic receptors by a combination of antagonists already approved by Health Agencies. We show that the association of ifenprodil (1 mg/kg) and cyproheptadine (1 mg/kg) (α1-adrenergic and 5-HT2 receptor antagonists marketed as Vadilex ® and Periactine ® in France, respectively) blocks behavioral sensitization to amphetamine in C57Bl6 mice and to alcohol in DBA2 mice. Moreover, this combination of antagonists inhibits alcohol intake in mice habituated to alcohol (10% v/v) and reverses their alcohol preference. Finally, in order to verify that the effect of ifenprodil was not due to its anti-NMDA receptors property, we have shown that a combination of prazosin (0.5 mg/kg, an α1b-adrenergic antagonist, Mini-Press ® in France) and cyproheptadine (1 mg/kg) could also reverse alcohol preference. Altogether these findings strongly suggest that combined prazosin and cyproheptadine could be efficient as a therapy to treat alcoholism in humans. Finally, because α1b-adrenergic and 5-HT2A receptors blockade also inhibits behavioral sensitization to psychostimulants, opioids and tobacco, it cannot be excluded that this combination will exhibit some efficacy in the treatment of addiction to other abused drugs.

Inhibition of apomorphine-induced conditioned place preference in rats co-injected with buspirone: relationship with serotonin and dopamine in the striatum

Apomorphine is a non-narcotic derivative of morphine, which acts as a dopamine agonist to produce psychostimulant like effects. Currently, apomorphine is used in patients with advanced Parkinson׳s disease, for the treatment of persistent and disabling motor fluctuations, but a constellation of addictive syndromes such as excessive over use of medication, compulsive behaviors, and disturbances of impulse control are noticed in certain patients. Research on rodent models using conditioned place preference (CPP) paradigm also shows that the drug is rewarding. Previously we have shown that repeated administration of apomorphine produces behavioral sensitization which is prevented in rats co-injected with a low (1.0mg/kg) but not higher (2.0mg/kg) dose of buspirone. The present study shows that rewarding effects of apomorphine (1.0mg/kg) in a CPP paradigm are also blocked in rats co-injected with a low (1.0mg/kg) but not higher (2.0mg/kg) dose of buspirone. The levels of serotonin and its metabolite are decreased in the caudate as well as nucleus accumbens of rats exhibiting CPP and the decreases do not occur in animals co-injected with low or higher dose of buspirone. The levels of dopamine and its metabolites are not affected in animals exhibiting CPP; administration as well as co-administration of higher dose of buspirone decreased dopamine metabolism in the caudate as well as nucleus accumbens. The findings suggest a critical role of serotonin in the rewarding effects of apomorphine and imply that co-use of buspirone at low doses can help to control addictive syndromes in Parkinson׳s disease patients on apomorphine therapy.

Repeated exposure to MDMA triggers long-term plasticity of noradrenergic and serotonergic neurons

3,4-Methylenedioxymethamphetamine (MDMA or 'ecstasy') is a psychostimulant drug, widely used recreationally among young people in Europe and North America. Although its neurotoxicity has been extensively described, little is known about its ability to strengthen neural circuits when administered in a manner that reproduces human abuse (i.e. repeated exposure to a low dose). C57BL/6J mice were repeatedly injected with MDMA (10 mg kg(-1), intraperitoneally) and studied after a 4-day or a 1-month withdrawal. We show, using in vivo microdialysis and locomotor activity monitoring, that repeated injections of MDMA induce a long-term sensitization of noradrenergic and serotonergic neurons, which correlates with behavioral sensitization. The development of this phenomenon, which lasts for at least 1 month after withdrawal, requires repeated stimulation of α(1B)-adrenergic and 5-hydroxytryptamine (5-HT)(2A) receptors. Moreover, behavioral and neuroendocrine assays indicate that hyper-reactivity of noradrenergic and serotonergic networks is associated with a persistent desensitization of somatodendritic α(2A)-adrenergic and 5-HT1A autoreceptor function. Finally, molecular analysis including radiolabeling, western blot and quantitative reverse transcription-polymerase chain reaction reveals that mice repeatedly treated with MDMA exhibit normal α(2A)-adrenergic and 5-HT(1A) receptor binding, but a long-lasting downregulation of Gαi proteins expression in both locus coeruleus and dorsal raphe nucleus. Altogether, our results show that repeated MDMA exposure causes strong neural and behavioral adaptations and that inhibitory feedback mediated by α(2A)-adrenergic and 5-HT(1A) autoreceptors has an important role in the physiopathology of addictive behaviors.

Neurochemical dysfunction in treated and nontreated schizophrenia - a retrospective analysis of in vivo imaging studies

To evaluate the contribution of individual synaptic constituents, all available in vivo imaging studies on schizophrenic patients were subjected to a retrospective analysis. For the pool of drug-naïve, drug-free, and acutely medicated patients, major findings were increases in neostriatal dopamine (DA) synthesis and release and decreases in neostriatal DA transporters and D1 receptors, neostriatal, thalamic, frontal, and parietal D2 receptors, mesencephalic/pontine and temporal 5-HT1A receptors, frontal and temporal HT2A and μ-amino butyric acid (GABA)A receptors. Based on the findings on drug-naïve and drug-free patients, it may be hypothesized that schizophrenia initially is characterized by an impaired mechanism of D2 autoreceptor and heteroreceptor sensitization leading to sensitization instead of desensitization in response to increased levels of neostriatal DA. Neuroleptic medication blocks neostriatal D2 autoreceptor and heteroreceptors, reducing neostriatal DA and disinhibiting DA action mediated by D2 heteroreceptor binding sites. Ultimately, this may result in a restitution of GABA function, leading to a recovery of inhibitory input to the target regions of the descending corticothalamostriatal efferents. Furthermore, a blockade of inhibitory and excitatory neocortical 5-HT function may be inferred, which is likely to reduce (excitatory) DAergic input to the mesolimbic target regions of corticothalamostriatal projections.

Update on the pharmacology of selective inhibitors of MAO-A and MAO-B: focus on modulation of CNS monoamine neurotransmitter release

Inhibitors of monoamine oxidase (MAO) were initially used in medicine following the discovery of their antidepressant action. Subsequently their ability to potentiate the effects of an indirectly-acting sympathomimetic amine such as tyramine was discovered, leading to their limitation in clinical use, except for cases of treatment-resistant depression. More recently, the understanding that: a) potentiation of indirectly-acting sympathomimetic amines is caused by inhibitors of MAO-A but not by inhibitors of MAO-B, and b) that reversible inhibitors of MAO-A cause minimal tyramine potentiation, has led to their re-introduction to clinical use for treatment of depression (reversible MAO-A inhibitors and new dose form MAO-B inhibitor) and treatment of Parkinson's disease (MAO-B inhibitors). The profound neuroprotective properties of propargyl-based inhibitors of MAO-B in preclinical experiments have drawn attention to the possibility of employing these drugs for their neuroprotective effect in neurodegenerative diseases, and have raised the question of the involvement of the MAO-mediated reaction as a source of reactive free radicals. Despite the long-standing history of MAO inhibitors in medicine, the way in which they affect neuronal release of monoamine neurotransmitters is still poorly understood. In recent years, the detailed chemical structure of MAO-B and MAO-A has become available, providing new possibilities for synthesis of mechanism-based inhibitors. This review describes the latest advances in understanding the way in which MAO inhibitors affect the release of the monoamine neurotransmitters dopamine, noradrenaline and serotonin (5-HT) in the CNS, with an accent on the importance of these effects for the clinical actions of the drugs.

Tranylcypromine reduces herpes simplex virus 1 infection in mice

Herpes simplex virus 1 (HSV-1) infects the majority of the human population and establishes latency by maintaining viral genomes in neurons of sensory ganglia. Latent virus can undergo reactivation to cause recurrent infection. Both primary and recurrent infections can cause devastating diseases, including encephalitis and corneal blindness. Acyclovir is used to treat patients, but virus resistance to acyclovir is frequently reported. Recent in vitro findings reveal that pretreatment of cells with tranylcypromine (TCP), a drug widely used in the clinic to treat neurological disorders, restrains HSV-1 gene transcription by inhibiting the histone-modifying enzyme lysine-specific demethylase 1. The present study was designed to examine the anti-HSV-1 efficacy of TCP in vivo because of the paucity of reports on this issue. Using the murine model, we found that TCP decreased the severity of wild-type-virus-induced encephalitis and corneal blindness, infection with the acyclovir-resistant (thymidine kinase-negative) HSV-1 mutant, and tissue viral loads. Additionally, TCP blocked in vivo viral reactivation in trigeminal ganglia. These results support the therapeutic potential of TCP for controlling HSV-1 infection.

Extending therapeutic use of psychostimulants: focus on serotonin-1A receptor

INTRODUCTION

Despite a number of medicinally important pharmacological effects, the therapeutic use of psychostimulants is limited because of abuse potential and psychosis following long term use. Development of pharmacological agents for improving and extending therapeutic use of psychostimulants in narcolepsy, attention deficit hyperactivity disorder, Parkinson's disease, obesity and as cognitive enhancer is an important research imperative. In this regard, one potential target system is the 5-hydroxytryptamine (5-HT; serotonin) neurotransmitter system. The focus of the present article is to evaluate a potential role of 5-HT-1A receptor in the alleviation of abuse potential and psychosis-induced by prescription psychostimulants amphetamines and apomorphine.

METHOD

Synaptic contacts between dopamine systems and 5-HT-1A receptors are traced. Studies on serotonin-1A influences on the modulation of dopamine neurotransmission and psychostimulant-induced behavioral sensitization are accumulated.

RESULTS

Inhibition of amphetamine and apomorphine-induced behavioral sensitization by co administration of 5-HT-1A agonists cannot be explained in terms of direct activation of 5-HT-1A receptors, because activation of pre- as well as postsynaptic 5-HT-1A receptors tends to increase dopamine neurotransmission.

CONCLUSION

Long term use of amphetamine and apomorphine produces adaptive changes in 5-HT-1A receptor mediated functions, which are prevented by the co-use of 5-HT-1A agonists. In view of extending medicinal use of psychostimulants, it is important to evaluate the effects of co-use of 5-HT-1A agonists on potential therapeutic profile of amphetamine and apomorphine in preclinical research. It is also important to evaluate the functional significance of 5-HT-1A receptors on psychostimulant-induced behaviors in other addiction models such as drug self-administration and reinstatement of drug seeking behavior.

Inhibition of monoamine oxidase isoforms modulates nicotine withdrawal syndrome in the rat

AIMS

There have been many reports of monoamine oxidase (MAO) inhibition by non-nicotine ingredients in tobacco smoke, persisting for days after smoking cessation. This study determined the effect of inhibiting MAO and its isoforms on nicotine withdrawal syndrome.

MAIN METHODS

Rats were rendered nicotine-dependent by seven days of subcutaneous (s.c.) 9 mg/kg/day infusion of nicotine bitartrate. Twenty-two hours after termination of infusion, they were observed over 20 min for somatically expressed nicotine withdrawal signs. Three hours before observation, rats were injected intraperitoneally (i.p.) with 4 mg/kg each of the MAO A antagonist clorgyline and the MAO B antagonist deprenyl, or with saline alone. A similar experiment was performed with non-dependent, saline-infused rats. Another experiment compared nicotine-dependent rats that received injections of either saline or 4 mg/kg clorgyline alone. A further experiment compared rats receiving either saline or 4 mg/kg deprenyl alone.

KEY FINDINGS

Combined treatment with both MAO inhibitors markedly and significantly exacerbated somatically expressed nicotine withdrawal signs in nicotine infused rats, while having no significant effects in saline-infused rats. Rats injected s.c. with 4 mg/kg clorgyline alone had significantly more withdrawal signs than saline-injected rats, while deprenyl-injected rats had significantly fewer signs than saline controls. Assays confirmed that clorgyline thoroughly reduced MAO A enzymatic activity and deprenyl thoroughly reduced MAO B activity.

SIGNIFICANCE

The results suggest that inhibition of MAO A may contribute to the intensity of withdrawal syndrome in smoking cessation.

Nicotine-, tobacco particulate matter- and methamphetamine-produced locomotor sensitisation in rats

RATIONALE

Repeated nicotine exposure produces a weak and transient sensitised locomotor response in rats. Since tobacco smoke contains thousands of non-nicotine chemical constituents, these could alter the sensitised response.

OBJECTIVES

This study aims to compare the magnitude, persistence and spatial distribution of locomotor sensitisation produced by repeated doses of nicotine, aqueous tobacco particulate matter (TPM) and a positive methamphetamine control.

METHODS

Male Sprague-Dawley rats received five nicotine (0.0, 0.2 or 0.4 mg/kg), TPM (containing 0.2 or 0.4 mg/kg nicotine) or methamphetamine (0.5 mg/kg) injections every second day, followed by a 4-day withdrawal before the first challenge (Challenge 1, C1). The animals were re-challenged again at 15 days post C1 to test for the persistence of sensitisation (Challenge 2, C2).

RESULTS

There were no major differences in sensitisation profile between nicotine and TPM. At the lowest 0.2 mg/kg nicotine/TPM dose, however, small differences emerged on select test days.

CONCLUSIONS

The results indicated that the non-nicotinic agents in TPM did not greatly impact the nicotine-produced locomotor-sensitised response. These findings might suggest that the differential pharmacological properties of TPM do not have major clinical significance. Alternatively, the locomotor model might not expose effects of non-nicotinic constituents, and furthermore, might not closely relate to human tobacco dependence. Different reward-related behavioural models should also be utilised to assess potential effects of non-nicotinic constituents before a role in dependence is discounted.

Sustained impairment of α2A-adrenergic autoreceptor signaling mediates neurochemical and behavioral sensitization to amphetamine

BACKGROUND

In rodents, drugs of abuse induce locomotor hyperactivity, and repeating injections enhance this response. This effect, called behavioral sensitization, persists months after the last administration. It has been shown that behavioral sensitization to amphetamine develops parallel to an increased release of norepinephrine (NE) in the prefrontal cortex (PFC).

METHODS

Rats and mice were repeatedly treated with amphetamine (1 or 2 mg/kg intraperitoneally, respectively) to obtain sensitized animals. The NE release in the PFC was measured by microdialysis in freely moving mice (n = 55). Activity of locus coeruleus (LC) noradrenergic neurons was determined in anaesthetized rats (n = 15) by in vivo extracellular electrophysiology. The α2A-adrenergic autoreceptor (α2A-AR) expression was assessed by autoradiography on brain slices, and Gαi proteins expression was measured by western blot analysis of LC punches.

RESULTS

In sensitized rats LC neurons had a higher spontaneous firing rate, and clonidine-an α2A-adrenergic agonist-inhibited LC neuronal firing less efficiently than in control animals. Clonidine also induced lower levels of NE release in the PFC of sensitized mice. This desensitization was maintained by a lower density of Gαi1 and Gαi2 proteins in the LC of sensitized mice rather than weaker α2A-AR expression. Behavioral sensitization was facilitated by α2A-AR antagonist, efaroxan, during amphetamine injections and abolished by clonidine treatment.

CONCLUSIONS

Our data indicate that noradrenergic inhibitory feedback is impaired for at least 1 month in rats and mice repeatedly treated with amphetamine. This work highlights the key role of noradrenergic autoreceptor signaling in the persistent modifications induced by repeated amphetamine administration.

A selective reversible monoamine oxidase B inhibitor in smoking cessation: effects on its own and in association with transdermal nicotine patch

RATIONALE

Monoamine oxidase B (MAO-B) activity is reduced in smokers. A MAO-B inhibitor alone or co-administered with nicotine may mimic the effects of smoking and be a candidate drug for smoking cessation.

OBJECTIVE

This study aims to determine the efficacy and safety of EVT302, a selective reversible MAO-B inhibitor, alone and on top of nicotine patch (NP) in smoking cessation.

METHODS

This was a randomised, double blind, placebo-controlled phase II, multicentre trial. Smokers (≥10 cigarettes/day) received either EVT302 (N = 145) or placebo (N = 145), or EVT302 (N = 61) or placebo (N = 61) on top of open label NP 21 mg/day for 8 weeks. The main comparison was between EVT302 and placebo without NP. The primary outcome measure was end-of-treatment 4-week continuous abstinence rate (CAR).

SECONDARY OUTCOME MEASURES

point prevalence abstinence rate, saliva cotinine concentrations in the groups without NP, urge to smoke, nicotine withdrawal symptoms and assessment of subjective effects of cigarettes.

RESULTS

The 4-week CAR was 15.2 % in the placebo, 17.2 % in the EVT302, 26.8 % in the NP + placebo and 32.8 % in the NP + EVT302 groups, respectively. There was no difference between EVT302 and placebo either alone (adjusted OR: 1.45, 95 % CI: 0.65-3.26) or when co-administered with NP. No statistically significant difference occurred for the secondary outcome measures.

CONCLUSIONS

The selective, reversible MAO-B inhibitor EVT302 was not superior to placebo in helping smokers quit, in line with data with selegiline and confirms that MAO-B inhibitors are not effective in smoking cessation. Co-administration of NP does not provide a supplementary benefit.

Early exposure to ethanol differentially affects ethanol preference at adult age in two inbred mouse strains

Although the acute effects of ethanol exposure on brain development have been extensively studied, the long term consequences of juvenile ethanol intake on behavior at adult age, regarding especially ethanol consumption, are still poorly known. The aim of this study was to analyze the consequences of ethanol ingestion in juvenile C57BL/6J and DBA/2J mice on ethanol intake and neurobiological regulations at adulthood. Mice were given intragastric ethanol at 4 weeks of age under different protocols and their spontaneous ethanol consumption was assessed in a free choice paradigm at adulthood. Both serotonin 5-HT(1A) and cannabinoid CB1 receptors were investigated using [(35)S]GTP-γ-S binding assay for the juvenile ethanol regimens which modified adult ethanol consumption. In DBA/2J mice, juvenile ethanol ingestion dose-dependently promoted adult spontaneous ethanol consumption. This early ethanol exposure enhanced 5-HT(1A) autoreceptor-mediated [(35)S]GTP-γ-S binding in the dorsal raphe nucleus and reduced CB1 receptor-mediated G protein coupling in both the striatum and the globus pallidus at adult age. In contrast, early ethanol ingestion by C57BL/6J mice transiently lowered spontaneous ethanol consumption and increased G protein coupling of postsynaptic 5-HT(1A) receptors in the hippocampus but had no effect on CB1 receptors at adulthood. These results show that a brief and early exposure to ethanol can induce strain-dependent long-lasting changes in both behavior toward ethanol and key receptors of central 5-HT and CB systems in mice.

Markov model of smoking cessation

Survival functions from smoking cessation interventions are described by a three-state Markov model. On quitting, smokers transit through a state of withdrawal characterized by a high rate of relapse, and then into a more secure state of long-term abstinence. The Markov model embodies the dynamic nature of the cessation/relapse process; it permits stronger inference to long-term abstinence rates, provides measures of treatment efficacy, describes the outcomes of new quit attempts, and suggests mechanisms for the survival process.

Nicotinic excitation of serotonergic projections from dorsal raphe to the nucleus accumbens

Tobacco use is a major public health problem, and although many smokers report that they want to quit, only a small percentage succeed. Side effects associated with nicotine withdrawal, including depression, anxiety, and restlessness, certainly contribute to the low success rate. The dorsal raphe nucleus (DRN) is a serotonergic center with many functions, including control of mood and emotional state. We investigated the effect of nicotine on DRN neurons that project to the nucleus accumbens (NAc), an area involved in reward-related behaviors. Using a retrograde labeling method, we found that 75% of DRN-NAc projection neurons are serotonergic. In coronal slices that include the DRN, whole cell recordings were conducted on neurons identified by fluorescent backlabeling from NAc or randomly selected within the nucleus. Nicotine increased action potential firing rates in a subset of DRN neurons. Voltage-clamp recording revealed nicotinic acetylcholine receptor (nAChR)-mediated inward currents that contribute to the nicotine-induced excitation. Nicotinic receptors also indirectly affect excitability by modulating synaptic inputs to these neurons. Nicotine enhanced excitatory glutamatergic inputs to a subset of DRN-NAc projection neurons, while inhibitory γ-aminobutyric acid (GABA)ergic inputs were modulated either positively or negatively in a subset of these neurons. The net effect of nAChR activation is enhancement of serotonergic output from DRN to the NAc, which may contribute to the effects of nicotine on mood and affect.

Serotonergic mechanism underlying tranylcypromine enhancement of nicotine self-administration

Although nicotine is generally considered to be the main psychoactive component of tobacco, growing evidence highlights the importance of nonnicotine compounds in smoking reinforcement. Monoamine oxidase (MAO) inhibition is a major consequence of smoking and MAO inhibitors, such as tranylcypromine, increase nicotine reinforcement. Tranylcypromine has multiple pharmacological effects, increasing monoamine release for a few hours immediately after its administration and blocking MAO activity for several days. To assess the relative role of these two actions, adult male rats were tested in consecutive daily 3-h sessions for self-administration of nicotine (3 μg kg⁻¹) inj⁻¹, i.v.) either 20 or 1 h following administration of tranylcypromine (3 mg kg⁻¹). Both paradigms were shown to produce highly significant inhibition of MAO activity. However, whereas animals readily acquired self-administration when pretreated with tranylcypromine 1 h prior to testing, they did not with the longer pretreatment interval. Such animals did immediately acquire nicotine self-administration when the tranylcypromine pretreatment interval was switched to 1 h prior to testing on Day 4, indicating that an acute effect of the MAO inhibitor was responsible for enhanced nicotine reinforcement. Several lines of evidence implicate serotonin (5-HT) as the mediator of this enhancement: (1) Tranyclypromine-enhanced nicotine reinforcement was blocked by the 5-HT₂ receptor antagonists, ritanserin and ketanserin; (2) parachloroamphetamine (PCA), a 5-HT releaser, also enhanced nicotine self-administration in animals in which MAO activity was inhibited; (3) pretreatment with tranylcypromine increased PCA-induced 5-HT overflow in the nucleus accumbens. These findings suggest that MAO inhibition enhances serotonergic transmission, which serves a critical role in the reinforcing effects of nicotine.

Smoking, nicotine and neuropsychiatric disorders

Tobacco smoking is an extremely addictive and harmful form of nicotine (NIC) consumption, but unfortunately also the most prevalent. Although disproportionately high frequencies of smoking and its health consequences among psychiatric patients are widely known, the neurobiological background of this epidemiological association is still obscure. The diverse neuroactive effects of NIC and some other major tobacco smoke constituents in the central nervous system may underlie this association. This present paper summarizes the pharmacology of NIC and its receptors (nAChR) based on a systematic review of the literature. The role of the brain's reward system(s) in NIC addiction and the results of functional and structural neuroimaging studies on smoking-related states and behaviors (i.e. dependence, craving, withdrawal) are also discussed. In addition, the epidemiological, neurobiological, and genetic aspects of smoking in several specific neuropsychiatric disorders are reviewed and the clinical relevance of smoking in these disease states addressed.