Nicotine-induced Fos expression in the nucleus accumbens and the medial prefrontal cortex of the rat: role of nicotinic and NMDA receptors in the ventral tegmental area

Immediate early gene expression reveals interactions between social and nicotine rewards on brain activity in adolescent male rats

Smoking initiation predominantly occurs during adolescence, often in the presence of peers. Therefore, understanding the neural mechanisms underlying the rewarding effects of nicotine and social stimuli is vital. Using the conditioned place preference (CPP) procedure, we measured immediate early gene (IEG) expression in animals following exposure either to a reward-conditioned environment or to the unconditioned stimuli (US). Adolescent, male rats were assigned to the following CPP US conditions: (1) Saline+Isolated, (2) Nicotine+Isolated, (3) Saline+Social, or (4) Nicotine+Social. For Experiment 1, brain tissue was collected 90min following the CPP expression test and processed for Fos immunohistochemistry. We found that rats conditioned with nicotine with or without a social partner exhibited CPP; however, we found no group differences in Fos expression in any brain region analyzed, with the exception of the nucleus accumbens core that exhibited a social-induced attenuation in Fos expression. For Experiment 2, brain tissue was collected 90min following US exposure during the last conditioning session. We found social reward-induced increases in IEG expression in striatal and amydalar subregions. In contrast, nicotine reduced IEG expression in prefrontal and striatal subregions. Reward interactions were also found in the dorsolateral striatum, basolateral amygdala, and ventral tegmental area where nicotine alone attenuated IEG expression and social reward reversed this effect. These results suggest that in general social rewards enhance, whereas nicotine attenuates, activation of mesocorticolimbic regions; however, the rewards given together interact to enhance activation in some regions. The findings contribute to knowledge of how a social environment influences nicotine effects.

Contribution of NMDA glutamate and nicotinic acetylcholine receptor mechanisms in the discrimination of ethanol-nicotine mixtures

Ethanol and nicotine are commonly coabused drugs, and the incidence of codependence is greater than would be expected on the basis of the summed probability of dependence on each drug alone. Previous findings from our laboratory and others suggest that interactive mechanisms at the level of discriminative stimulus (S(D)) effects may contribute to this coabuse phenomenon. Specifically, ethanol overshadows the nicotine S(D) whereas nicotine potentiates the stimulus salience of ethanol when the two drugs are conditioned as a drug mixture. The goal of the current study was to begin to delineate the pharmacological bases of these ethanol-nicotine interactions. Three groups of C57BL/6J mice were trained to discriminate 0.8 mg/kg nicotine + 0.5 g/kg ethanol (0.8 N + 0.5 E), 0.8 N + 1.0 E, or 0.8 N + 2.0 E. An NMDA receptor antagonist (MK-801) and three nACh receptor ligands were tested for their ability to generalize from or antagonize, respectively, the drug mixtures. MK-801 fully generalized from the 0.8 N + 1.0 E and 0.8 N + 2.0 E mixtures and partially generalized from 0.8 N + 0.5 E. In contrast, nACh receptor ligands had minimal influence in blocking the perception of 0.8 N + 1.0 E and 0.8 N + 2.0 E mixtures, and only mecamylamine partially blocked 0.8 N+0.5 E. Reduced and enhanced contributions of nACh and NMDA receptors, respectively, in the discrimination of ethanol-nicotine mixtures may contribute to the overshadowing and potentiation phenomena observed previously.

Effects of acute and chronic inhalation of paint thinner in mice: behavioral and immunohistochemical study

Abuse of volatile inhalants has become a worldwide issue mainly among adolescents of low income social class. Acute and chronic exposure to these substances results in serious neurological and behavioral impairments. Although real exposure consists largely of simultaneous inhalation of multiple solvents, the vast majority of basic research studies have evaluated the actions of a single volatile component leaving the behavioral and neuronal effects of chemical mixture not fully understood. In this study, we investigated the acute behavioral effects of 300, 450 and 600 ppm of paint thinner inhalation on anxiety, locomotor activity and spatial memory. Additionally, the cognitive impairments related to chronic exposure of the same concentrations of thinner for 45 days were assessed. To understand the neuronal correlates of acute exposure to thinner, we used c-Fos immunohistochemistry as an endogenous marker of neuronal activation following 600 ppm of thinner. The results reveal that (i) chronically thinner exposed mice showed cognitive deficits in Morris water maze and object recognition tasks; (ii) acute inhalation of thinner induces a wide range of behavioral changes. These changes include an anxiolytic effect toward the aversive environmental bright light and a dose dependent effect on explorative locomotion. The wide range of behavioral alterations induced by acute thinner inhalation is consistent with the widespread distribution of thinner-induced c-Fos expression in multiple brain structures.

Dopamine D₂ and acetylcholine α7 nicotinic receptors have subcellular distributions favoring mediation of convergent signaling in the mouse ventral tegmental area

Alpha7 nicotinic acetylcholine receptors (α7nAChRs) mediate nicotine-induced burst-firing of dopamine neurons in the ventral tegmental area (VTA), a limbic brain region critically involved in reward and in dopamine D2 receptor (D2R)-related cortical dysfunctions associated with psychosis. The known presence of α7nAChRs and Gi-coupled D2Rs in dopamine neurons of the VTA suggests that these receptors are targeted to at least some of the same neurons in this brain region. To test this hypothesis, we used electron microscopic immunolabeling of antisera against peptide sequences of α7nACh and D2 receptors in the mouse VTA. Dual D2R and α7nAChR labeling was seen in many of the same somata (co-localization over 97%) and dendrites (co-localization over 49%), where immunoreactivity for each of the receptors was localized to endomembranes as well as to non-synaptic or synaptic plasma membranes often near excitatory-type synapses. In comparison with somata and dendrites, many more small axons and axon terminals were separately labeled for each of the receptors. Thus, single-labeled axon terminals were predominant for both α7nAChR (57.9%) and D2R (89.0%). The majority of the immunolabeled axonal profiles contained D2R-immunoreactivity (81.6%) and formed either symmetric or asymmetric synapses consistent with involvement in the release of both inhibitory and excitatory transmitters. Of 160 D2R-labeled terminals, 81.2% were presynaptic to dendrites that expressed α7nAChR alone or together with the D2R. Numerous glial processes inclusive of those enveloping either excitatory- or inhibitory-type synapses also contained single labeling for D2R (n=152) and α7nAChR (n=561). These results suggest that classic antipsychotic drugs, all of which block the D2R, may facilitate α7nAChR-mediated burst-firing by elimination of D2R-dependent inhibition in neurons expressing both receptors as well as by indirect pre-synaptic and glial mechanisms.

The α4β2 nicotine acetylcholine receptor agonist ispronicline induces c-Fos expression in selective regions of the rat forebrain

The dominant nicotine acetylcholine receptor (nAChR) subtype in the brain is the pentameric receptor containing both α4 and β2 subunits (α4β2). Due to the lack of selective agonists it has not been ruled out what neuronal circuits that are stimulated after systemic administration with nicotine. We used the novel and selective α4β2 receptor agonist ispronicline (10 and 30 mg/kg s.c.) to localise the activated neurons in the rat forebrain using c-Fos-immunoreactivity as a marker of immediate neuronal activity. In the hypothalamic paraventricular nucleus, a large increase of c-Fos-positive cells was found only within its medial part. In addition, an increased number of c-Fos-immunoreactive cells were observed in the central nucleus of the amygdala, and the dorsolateral part of the bed nucleus of the stria terminalis. The restricted distribution of c-Fos to these areas, all of which are directly or indirectly involved in acute stress regulation after a single dose of ispronicline, supports earlier studies that the α4β2 receptors are strongly involved in nicotine-dependent activation of the hypothalamo-pituitary adrenocortical axis.

A double-blind, placebo-controlled trial of the NMDA glycine site antagonist, GW468816, for prevention of relapse to smoking in females

Relapse to smoking is common after initial abstinence with pharmacotherapy and behavioral support and represents a major clinical challenge. Although mechanisms underlying relapse to smoking have not been elucidated, preclinical studies suggest that glutamate receptors may be involved. We sought to test a selective antagonist of the glycine coagonist site on the glutamate N-methyl-D-aspartate receptor, GW468816, for prevention of relapse in recently abstinent smokers. To do so, we enrolled 264 healthy female smokers in an open 8-week smoking cessation intervention with behavioral therapy and a standard dose of transdermal nicotine replacement therapy with taper and additional gum or lozenge as needed for nicotine withdrawal symptoms. Ninety-eight participants achieved 7-day point prevalence abstinence and were randomized into a 5-week double-blind, placebo-controlled, relapse-prevention trial of GW468816 (200 mg/d) and then followed for 60 days after randomization. There was no effect of treatment on abstinence rates at the end of treatment (χ² [1, n = 96] = 0.168, P = 0.838), on the rates of relapse (χ² [1, n = 98] = 0.031, P = 1.000) or lapse (χ² [1, n = 62] = 0.802, P = 0.423), or on time to relapse (χ² [1, n = 98) = 0.001, P = 0.972). No significant relationships were detected between plasma GW468816 concentrations and abstinence, time to relapse, or self-reported craving. In conclusion, despite promising preclinical data that support the use of a selective NMDA glycine site antagonist for prevention of relapse to smoking, we observed no effect of GW468816 on relapse or lapse rates, time to relapse, or craving compared to placebo.

Distinct neural pathways mediate α7 nicotinic acetylcholine receptor-dependent activation of the forebrain

alpha(7) nicotinic acetylcholine receptor (nAChR) agonists are candidates for the treatment of cognitive deficits in schizophrenia. Selective alpha(7) nAChR agonists, such as SSR180711, activate neurons in the medial prefrontal cortex (mPFC) and nucleus accumbens shell (ACCshell) in rats, regions important for cognitive function. However, the neural substrates involved in these effects remain elusive. Here we identify cortically projecting cholinergic neurons in the horizontal limb of the diagonal band of Broca (HDB) in the basal forebrain (BF) as important targets for alpha(7) nAChR activation, as measured by c-Fos immunoreactivity, a marker of neuronal activation. Selective depletion of these cholinergic neurons abolishes the SSR180711-induced activation of the mPFC but not the ACCshell, demonstrating their critical importance for alpha(7) nAChR-dependent activation of the mPFC. Contrarily, selective depletion of dopaminergic neurons in the ventral tegmental area abolishes the SSR180711-induced activation of the ACCshell but not the mPFC or HDB. These results demonstrate 2 distinct neural pathways activated by SSR180711. The BF and mPFC are important for attentional function and may subserve the procognitive effects of alpha(7) nAChR agonists, whereas activation of the ACCshell is implicated in the beneficial effect of antipsychotics on the positive symptoms of schizophrenia.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Alpha-7 nicotinic acetylcholine receptor agonists selectively activate limbic regions of the rat forebrain: An effect similar to antipsychotics

It is considered that activation of nicotinic alpha7 receptors (alpha7 nAChR) is useful for the treatment of cognitive disturbances in schizophrenia and Alzheimer's disease. Recently, selective alpha7 nAChR agonists have been discovered and are used to validate the alpha7 nAChR as a drug target for the treatment of cognitive disturbances in schizophrenia. One important feature shared by all known antipsychotics is their capacity to induce expression of the neuronal immediate-early gene c-fos in the limbic forebrain. Using two novel and selective alpha7 nAChR agonists, PNU-282987 and SSR180711, we investigated their ability to induce c-Fos expression in the limbic forebrain with particular emphasis on the same regions reported to be activated by antipsychotics. Both alpha7 nAChR agonists increased c-Fos dose-dependently in the prefrontal cortex and the shell of nucleus accumbens, while leaving the core of nucleus accumbens and the dorsolateral striatum unaffected. The accumbal and cortical effect of SSR180711 was blocked completely by pre-administration of the alpha7 nAChR antagonist methyllycaconitine. Also, SSR180711 displayed no c-Fos-inducing effect in alpha7 nAChR knock-out mice. In conclusion, these results show that selective pharmacologic stimulation of alpha7 nAChR function results in activation of forebrain regions similar to conventional antipsychotics.

Region-specific effects of nicotine on brain activity: a pharmacological MRI study in the drug-naïve rat

We have applied pharmacological magnetic resonance imaging (phMRI) methods to map the functional response to nicotine in drug-naïve rats. Nicotine (0.35 mg/kg intravenous (i.v.)) increased relative cerebral blood volume (rCBV) in cortical (including medial prefrontal, cingulate orbitofrontal, insular) and subcortical (including amygdala and dorsomedial hippocampus) structures. The pharmacological specificity of the effect was demonstrated by acute pretreatment with the nicotinic acetylcholine receptor (nAChR) ion-channel-blocking agent mecamylamine, which suppressed the rCBV response to nicotine. Control experiments with norepinephrine, a potent non-brain-penetrant vasopressor, at a dose that mimics the cardiovascular response induced by nicotine were performed to assess the potential confounding effects of peripheral blood pressure changes induced by nicotine. In an attempt to highlight the relative contribution of different nAChR subtypes to the observed activation pattern of nicotine, we also investigated the central phMRI response to an acute challenge with (R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide) (cpdA, at 5, 10, 20, and 30 mg/kg i.v.) and 5-iodo-A-85380 (5IA, 5 mg/kg i.v.). CpdA is a selective agonist at homomeric alpha7 nAChRs, while 5IA features high in vivo affinity for the alpha4beta2* and other less-abundant beta2-containing nicotinic receptors. CpdA did not produce significant rCBV changes at any of the doses tested, whereas 5IA induced a pattern of activation very similar to that induced by nicotine. The lack of phMRI response to cpdA together with the high spatial overlap between the activation profile of nicotine and 5IA, suggest that the acute functional response to nicotine in drug-naïve rats is mediated by beta2-containing nAChR isoforms, presumably belonging to the alpha4beta2* subtype.

Reinforcing effects of nicotine microinjections into the ventral tegmental area of mice: Dependence on cholinergic nicotinic and dopaminergic D1 receptors

We used an intracranial self-administration (ICSA) procedure to assess the involvement of the ventral tegmental area (VTA) nicotinic receptors in the rewarding effects of nicotine. We then challenged intra-VTA nicotine self-administration via systemic or local injections of dopamine (DA)-D1 and nicotinic receptor antagonists. C57BL/6J mice were stereotaxically implanted unilaterally with a guide cannula above the VTA. After 1 week of recovery, mice were allowed to discriminate between two arms of a Y-maze over seven daily sessions, one arm being reinforced by intracranial nicotine microinjection. Mice exhibited nicotine self-administration at both doses tested, i.e. 10 ng (21.6 pmol) and 100 ng (216 pmol)/50-nl injection. In contrast, mice receiving a 216-pmol nicotine dose 0.8 mm above VTA performed at chance level. Once the ICSA response was acquired, systemic pretreatment with the DA-D1 receptor antagonist SCH 23390 (25 microg/kg i.p.) or co-infusion of the nAChR antagonist DHbetaE with nicotine disrupted ICSA. Replacement of SCH 23390 by vehicle, or withdrawal of DHbetaE from nicotine/DHbetaE mixed solutions led to recovery of intra-VTA nicotine self-administration. We conclude that nicotinic receptors in the VTA, presumably alpha4beta2 nAChRs are critically to mediate the rewarding effects of nicotine and that DA-D1 receptors are also directly implicated.

Activation of orexin neurons by acute nicotine

The hypothalamus is a prominent central site of action of nicotine but the phenotype of nicotine-sensitive neurons in this region has not been fully described. Hypothalamic orexin neurons are important regulators of state-dependent behavior, arousal and feeding. Here, we treated rats with acute nicotine and quantitated Fos expression as a marker of neuronal activation. Nicotine increased the percentage of orexin neurons expressing Fos without a significant effect on non-orexin neurons. This effect was attenuated by the nicotinic antagonists mecamylamine and dihydro-beta-erythroidine, implicating alpha4beta2-containing nicotinic receptors. The orexin system is likely to play an important role in the coordination of physiological and behavioral responses to acute nicotine treatment.

TC-2559 excites dopaminergic neurones in the ventral tegmental area by stimulating alpha4beta2-like nicotinic acetylcholine receptors in anaesthetised rats

1. The in vivo effects of a selective partial agonist for neuronal nicotinic acetylcholine receptor (nAChRs) alpha4beta2 subtype, TC-2559, characterised recently in in vitro preparations, have been profiled. The brain bioavailability of TC-2559 and its effects on the spontaneous firing and bursting properties of the dopaminergic (DAergic) neurones recorded extracellularly in the ventral tegmental area (VTA) were studied following systemic administration in anaesthetised rats. 2. Cumulative doses of TC-2559 (0.021-1.36 mg kg(-1), i.v.) increased both the firing and bursting activities of VTA DA neurones. The effect of bolus doses of TC-2559 of 0.66 or 1.32 mg kg(-1), i.v., was approximately equivalent to that of 0.0665 mg kg(-1), i.v. nicotine. 3. The excitation evoked by both nicotine and TC-2559 was fully reversed by DHbetaE (0.39-0.77 mg kg(-1), i.v.), an alpha4beta2-subtype-preferring nicotinic antagonist, and application of nicotine after DHbetaE failed to evoke any excitation. MLA (0.23 mg kg(-1), i.v.), an alpha7 selective antagonist, failed to alter TC-2559-evoked excitation and bursting activities, and a novel alpha7 agonist (PSAB-OFP; 0.23 mg kg(-1), i.v.) was also without effect. 4. The present results indicated that TC-2559 fully mimics nicotine by increasing both the excitability and bursting behaviour of VTA DA neurones, effects that are predominantly due to activation of alpha4beta2-like nAChRs. 5. TC-2559 has been demonstrated to be a useful in vivo pharmacological tool for studying the alpha4beta2 subtype of nicotinic receptor.

Serotonin depletion results in a decrease of the neuronal activation caused by rivastigmine in the rat hippocampus

Interactions between the serotonergic and cholinergic systems are known to occur and are believed to play a role in the mechanism underlying both major depression and Alzheimer's disease. On a molecular level, studies suggest that acetylcholine (ACh) increases serotonin (5-HT) release through nicotinic receptors located at nerve terminals. The aim of the present study was to determine in which areas and to what extent 5-HT mediates the neuronal response to ACh release. For this purpose, neuronal activity was measured in rats with rivastigmine-induced elevated ACh levels after a 95% 5-HT depletion obtained by dosing p-chlorophenylalanine followed by D,L-fenfluramine. Neuronal activation was quantified by stereological measurements of c-Fos immunoreactivity. The brain areas examined were medial prefrontal cortex, septum, dorsal hippocampus, and dorsal raphe nucleus. Rivastigmine significantly increased c-Fos immunoreactivity in medial prefrontal cortex and the hippocampus, but not in the septum and dorsal raphe nucleus. 5-HT depletion decreased ACh-induced c-Fos immunoreactivity in the dentate gyrus. By contrast, 5-HT depletion had no effect on the ACh-induced activity in the other brain areas examined. It is concluded that 5-HT mediates part of the ACh-induced hippocampal neuronal activation, possibly mediated via locally released 5-HT.

Immunohistochemical localization of toluene-induced c-Fos protein expression in the rat brain

Although toluene is a widely abused substance, the neuronal populations and pathways mediating its effects are not well understood. Using c-Fos protein as a marker for neuronal activation, the present study investigated the pattern of c-Fos induction at 1h after various doses (0, 300, 750, and 1000 mg/kg, i.p.) of toluene injection in adult male rats. Quantitative analysis of Fos-immunoreactive neurons indicated toluene dose-related induced c-Fos immunoreactivity in the majority of structures examined. The structures included several cortex subareas (primary motor cortex, secondary motor cortex, somatosensory cortex, frontal association cortex, cigulate cortex area 1, cigulate cortex area 2, prelimbic cortex, infralimbic cortex, retrosplenial agranular cortex, ventral orbital cortex, lateral orbital cortex, and piriform cortex), ventral tegmental area, nucleus accumbens shell, thalamic nuclei (mediodorsal, lateral posterior, and laterodorsal ventrolateral) and pontine nuclei. However, the substantia nigra, caudate putamen, nucleus accumbens core, subthalamic nucleus, hippocampus and cerebellum were almost unaffected. The data demonstrate that toluene dose-related induced a unique pattern of c-Fos immunoreactivity. The widespread distribution of toluene-induced c-Fos expression seen in this study can be linked to the profound alterations in physiological function and behavior produced by this solvent.

Rapid delivery of nicotine promotes behavioral sensitization and alters its neurobiological impact

BACKGROUND

Nicotine is highly addictive when it is inhaled from tobacco smoke, whereas nicotine replacement products, which usually deliver nicotine orally or transdermally, rarely lead to addiction. It has been proposed that this is due in part to differences in the rate of nicotine delivery to the brain under the two conditions. However, the mechanism by which rapid nicotine delivery facilitates the transition to addiction is not known. The ability of drugs to alter gene regulation and to produce sensitization has been implicated in addiction. We hypothesized, therefore, that varying the rate of nicotine administration may modulate its ability to elicit this form of plasticity.

METHODS

Animals were treated with repeated intravenous infusions of nicotine over 5, 25, or 100 sec, and their locomotor responses were monitored over treatment days.

RESULTS

We found that increasing the rate of intravenous nicotine infusion potentiated its ability to produce locomotor sensitization, and to induce c-fos and arc mRNA expression in mesocorticolimbic structures.

CONCLUSIONS

We suggest that rapid administration may increase vulnerability to addiction by altering the neurobiological impact of nicotine and promoting a form of neurobehavioral plasticity (i.e., sensitization) that can lead to pathological incentive motivation for drugs.

Chronic nicotine and smoke treatment modulate dopaminergic activities in ventral tegmental area and nucleus accumbens and the ?-aminobutyric acid type B receptor expression of the rat prefrontal cortex

Dopaminergic afferents from the mesencephalic areas, such as ventral tegmental area (VTA), synapse with the gamma-aminobutyric acid (GABA)-ergic interneurons in the prefrontal cortex (PFC). Pharmacological and electrophysiological data show that the reinforcement, the dependence-producing properties, as well as the psychopharmacologic effects of nicotine depend to a great extent on activation of nicotinic receptors within the mesolimbocortical dopaminergic projection. To explore further the relationship between the mesencephalic dopaminergic neurons and PFC GABAergic neurons, we investigated the effects of nicotine and passive exposure to cigarette smoke on the regulation of tyrosine hydroxylase (TH) in VTA and substantia nigra (SNC) and dopamine (DA) D1 receptor levels in nucleus accumbens (NAc) and caudate-putamen (CPu). Also, the simultaneous changes in GABAB receptors mRNAs in the PFC were studied. The results showed that chronic nicotine and smoking treatment differentially changed the levels of TH protein in VTA and SNC and DA D1 receptor levels in Nac and CPu. GABAB1 and GABAB2 receptor mRNA levels also showed different change patterns. Ten and thirty minutes of smoke exposure increased GABAB1 receptor mRNA to a greater extent than that of GABAB2, whereas GABAB2 was greatly enhanced after 1 hr of smoke exposure. The TH levels in VTA were closely related to DA D1 receptor levels in NAc and with GABAB receptor mRNA changes in PFC. These results suggest that the mesolimbic pathway and GABAB receptor mRNA in PFC are modulated by nicotine and cigarette smoke, implying an important role in nicotine's psychopharmacological effects.

Nicotine enhances responding with conditioned reinforcement

RATIONALE

The mesolimbic dopamine system has been implicated in the primary reinforcing properties of drugs of abuse as well as in enhanced responding with conditioned reinforcement produced by psychomotor stimulant drugs. Despite clinical observations that nicotine self-administration (i.e. smoking) depends strongly upon conditioned reinforcement (i.e. cues support smoking behavior), little is known about whether nicotine directly affects motivational processes.

OBJECTIVE

In these experiments, we investigated whether acute nicotine would influence responding with conditioned reinforcement and the degree to which pretreatment with the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine would modify any nicotine-induced behavioral effects.

METHODS

After subjects had been trained to associate an initially neutral stimulus with water reward, they received acute nicotine (43,25-350 micro g/kg SC; -5 min) or saline injections and were tested on the acquisition of a new response for conditioned reinforcement paradigm. In separate experiments, the effect of pretreatment with the nicotinic acetylcholine receptor antagonist mecamylamine (300 or 1000 micro g/kg SC; -20 min) alone, or in combination with nicotine (350 micro g/kg SC; -5 min), on conditioned reinforcement was also examined.

RESULTS

Acute nicotine injection produced a selective enhancement of responding with conditioned reinforcement (i.e. on the CR lever), without producing non-selective increases in overall responding. The effect of nicotine (350 micro g/kg SC; -5 min) was selectively blocked by mecamylamine (300 micro g/kg).

CONCLUSIONS

These findings demonstrate that acute exposure to nicotine augments the control over behavior by a conditioned reinforcer, suggesting that nicotine may enhance motivational processes.

Active immunization against nicotine prevents reinstatement of nicotine-seeking behavior in rats

BACKGROUND

The presently available pharmaceutical aids in smoking cessation possess a rather limited effectiveness. Therefore, we have synthesized a series of immunoconjugates that stimulate the induction of antibodies which may bind nicotine in the blood, thereby preventing it from passing the blood-brain barrier. Thus, the reinforcing action of nicotine in the brain, which is the driving force in tobacco smoking, should be abolished.

OBJECTIVE

The present study was undertaken to test this notion in a long-term relapse model in rats, measuring the reinstatement of nicotine-seeking behavior, following active immunization with IP18-KLH, one of our immunoconjugates.

METHODS

Male Wistar rats were immunized with a nicotine-KLH conjugate (nicotine immunogen) and Freund's adjuvant after having been trained to meet the criteria of stable nicotine self-administration on a fixed ratio (FR3) schedule. The rats were subsequently extinguished from nicotine self-administration behavior and finally, as extinction was completed, they were exposed to small, priming doses of nicotine, which previously have been shown to reinstate the nicotine-seeking behavior. The antibody titers were measured by ELISA.

RESULTS

It was found that rats with high titers (>1:10,000) of antibodies against nicotine, in contrast to those with low/no nicotine selective antibodies, do not reinstate nicotine self-administration behavior when they are exposed to nicotine.

CONCLUSIONS

Our findings indicate that active immunization against nicotine may effectively abolish the reinforcing action of nicotine in brain, an effect which is critical for relapse in nicotine dependence. These data suggest the potential utility of active immunization in smoking cessation programs.

Induction of Fos-immunostaining by nicotine and nicotinic receptor antagonists in rat brain

Using Fos protein immunohistochemistry, we have studied the effects of acute nicotine (0.5 mg/kg s.c.) and nicotinic acetylcholine receptor (nAChR) antagonists in eleven rat brain areas. Acute nicotine elevated Fos-like immunostaining (Fos IS) significantly in all studied areas except the medial prefrontal cortex. Nicotine increased the Fos IS in cortical, limbic and hypothalamic areas by 2-10-fold, and in the interpeduncular nucleus as well as in the visual areas the increases were 15-150-fold. When given alone, the nAChR antagonists mecamylamine (1.0 or 5.0 mg/kg i.p.) and dihydro-beta-erythroidine (DHE; 1.4 or 2.8 mg/kg i.p.) increased Fos IS in most brain areas maximally by 2-10-fold, but methyllycaconitine (MLA; 4.0 mg/kg i.p.) only in three areas and maximally by 4-fold. The efficacy of nAChR antagonists in blocking nicotine's effects on Fos IS varied noticeably with respect to region and antagonist, and the combined effect of nicotine+antagonist did not exceed that of either treatment alone. Mecamylamine and DHE significantly reduced nicotine-induced Fos IS in most of the studied areas, and MLA only in two areas. Thus, nAChRs seem to mediate the effects of nicotine on Fos IS, and the differences in the effects of the antagonists studied suggest that more than one subtype of nAChRs are involved. The present experiments also provide evidence that nAChR blockade itself may result in increased Fos protein expression in the brain. This could be due to blockade of presynaptic nAChRs modulating transmitter release or interruption of complex polysynaptic feedback pathways.

Tonic function of nicotinic receptors in stress-induced release of L-DOPA from the nucleus accumbens in freely moving rats

We investigated whether stress induces the release of L-3,4-dihydroxyphenylalanine (DOPA) and dopamine from the nucleus accumbens in conscious rats and characterized the stress-induced response. Electrical foot-shock stress induced both DOPA and dopamine release, measured by microdialysis, from the nucleus accumbens in freely moving rats. Pretreatment of rats with mecamylamine completely blocked stress-induced DOPA release, but only partially blocked dopamine release. Diazepam did not affect the foot-shock-induced release of DOPA, while the same dose of diazepam partially blocked the stress-induced release of dopamine. These findings suggest a tonic function of central nicotinic receptors in stress-induced DOPA release from the nucleus accumbens in conscious rats.

Putative role of presynaptic alpha7* nicotinic receptors in nicotine stimulated increases of extracellular levels of glutamate and aspartate in the ventral tegmental area

We have previously provided evidence that the stimulatory action of systemic nicotine on dopamine release in the rat nucleus accumbens is initiated in the ventral tegmental area (VTA), and that it appears to be mediated partly through an indirect, presynaptic mechanism. Thus, it was found that blockade of N-methyl-D-aspartate (NMDA) receptors in the VTA attenuates the enhancing effect of nicotine on extracellular levels of dopamine in the nucleus accumbens. Moreover, the nicotine-induced dopamine output in the nucleus accumbens was found to be blocked by pretreatment with methyllycaconitine (MLA) in the VTA, indicating a role for alpha7* nicotinic acetylcholine receptors (nAChRs) in this mechanism. Thus, nicotine may exert its effects in the VTA through stimulation of alpha7* nAChRs localized on excitatory amino acid (EAA)ergic afferents. To test this hypothesis, we here measured extracellular concentrations of glutamate and aspartate in the VTA in response to systemic nicotine, with or without concurrent infusion of MLA in the VTA, using microdialysis in anaesthetized rats. Since the medial prefrontal cortex is an important source of EAA input to the VTA, we also assessed the density of alpha-bungarotoxin binding sites in the VTA in rats lesioned bilaterally in the prefrontal cortex with ibotenic acid and in sham-lesioned rats by means of quantitative autoradiography. Nicotine (0.5 mg/kg, s.c.) significantly increased extracellular levels of both aspartate and glutamate in the VTA. MLA (0.3 mM) infused locally in the VTA prevented the nicotine-induced increase in glutamate and aspartate levels. Ibotenic acid lesions of the prefrontal cortex decreased the density of alpha-bungarotoxin binding sites in the VTA by about 30%. These data indicate that nicotine increases the extracellular levels of excitatory amino acids in the VTA through stimulation of nAChRs in the VTA and that part of the alpha7* nAChR population in the VTA is localized on neurons originating in the prefrontal cortex.

Role of alpha7 nicotinic receptors in nicotine dependence and implications for psychiatric illness

It has previously been shown that the reinforcing and dependence-producing properties of nicotine depend to a great extent on activation of nicotinic receptors within the ventral tegmental area (VTA), i.e. the site of origin of the mesolimbocortical dopaminergic projection. Based on the data reviewed in the present study, it is suggested that nicotine by stimulating presynaptic alpha7 nicotinic receptors within the VTA, that are probably localized on glutamatergic afferents from the medial prefrontal cortex, produces sequentially an increase in glutamate concentrations, stimulation of NMDA receptors found on dopamine (DA)-containing neurons in the VTA, enhanced firing activity of VTA-DA neurons, augmented DA release in the nerve terminal regions, and enhanced c-fos expression in the dopaminergic projection areas through activation of D1-DA receptors. In addition, it appears that alpha7 nicotinic receptors within the VTA are directly involved in nicotine-related reward and withdrawal responses. These data may be instrumental in understanding how nicotine interacts with the mesolimbocortical dopaminergic system, which is perhaps the most important component of the neural mechanisms underlying nicotine dependence. These results may also contribute to unraveling the cellular basis of nicotine's association with neuropsychiatric disorders, thereby offering the prospect of new therapeutic advances for their treatment.