The role of monoamine neurotransmitter systems in the nicotine discriminative stimulus

The co-use of nicotine and prescription psychostimulants: A review of their behavioral and neuropharmacological interactions

BACKGROUND

Nicotine is commonly co-used with other psychostimulants. These high co-use rates have prompted much research on interactions between nicotine and psychostimulant drugs. These studies range from examination of illicitly used psychostimulants such as cocaine and methamphetamine to prescription psychostimulants used to treat attention deficit hyperactivity disorder (ADHD) such as methylphenidate (Ritalin™) and d-amphetamine (active ingredient of Adderall™). However, previous reviews largely focus on nicotine interactions with illicitly used psychostimulants with sparse mention of prescription psychostimulants. The currently available epidemiological and laboratory research, however, suggests high co-use between nicotine and prescription psychostimulants, and that these drugs interact to modulate use liability of either drug. The present review synthesizes epidemiological and experimental human and pre-clinical research assessing the behavioral and neuropharmacological interactions between nicotine and prescription psychostimulants that may contribute to high nicotine-prescription psychostimulant co-use.

METHODS

We searched databases for literature investigating acute and chronic nicotine and prescription psychostimulant interactions. Inclusion criteria were that participants/subjects had to experience nicotine and a prescription psychostimulant compound at least once in the study, in addition to assessment of their interaction.

RESULTS AND CONCLUSIONS

Nicotine clearly interacts with d-amphetamine and methylphenidate in a variety of behavioral tasks and neurochemical assays assessing co-use liability across preclinical, clinical, and epidemiological research. The currently available research suggests research gaps examining these interactions in women/female rodents, in consideration of ADHD symptoms, and how prescription psychostimulant exposure influences later nicotine-related outcomes. Nicotine has been less widely studied with alternative ADHD pharmacotherapy bupropion, but we also discuss this research.

Effects of the nicotinic acetylcholine receptor antagonist mecamylamine on the discriminative stimulus effects of cocaine in male rhesus monkeys

Preclinical drug discrimination procedures have been useful in understanding the pharmacological mechanisms of the subjective-like effects of abused drugs. Converging lines of evidence from neurochemical and behavioral studies implicate a potential role of nicotinic acetylcholine (nACh) receptors in the abuse-related effects of cocaine. The aim of the present study was to determine the effects of the nACh receptor antagonist mecamylamine on the discriminative stimulus effects of cocaine in nonhuman primates. The effects of mecamylamine on the cocaine-like discriminative stimulus effects of nicotine were also examined. Male rhesus monkeys (n = 5) were trained to discriminate 0.32 mg/kg, IM cocaine from saline in a 2-key, food-reinforced discrimination procedure. Initially, potency and time course of cocaine-like discriminative stimulus effects were determined for nicotine and mecamylamine alone. Test sessions were then conducted examining the effects of mecamylamine on cocaine or the cocaine-like discriminative stimulus effects of nicotine. Curiously, mecamylamine produced partial cocaine-like discriminative stimulus effects. Mecamylamine did not significantly alter the discriminative stimulus effects of cocaine up to doses that significantly decreased rates of operant responding. Mecamylamine and nicotine combinations were not different than saline. These results confirm previous nonhuman primate studies of partial substitution with nicotine and extend these findings with mecamylamine. Furthermore, these results extend previous results in rats suggesting cocaine may have nACh receptor antagonist properties.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

Discriminative and reinforcing stimulus effects of nicotine, cocaine, and cocaine + nicotine combinations in rhesus monkeys

Concurrent cigarette smoking and cocaine use is well documented. However, the behavioral pharmacology of cocaine and nicotine combinations is poorly understood, and there is a need for animal models to examine this form of polydrug abuse. The purpose of this study was twofold: first to assess the effects of nicotine on the discriminative stimulus effects of cocaine, and second, to study self-administration of nicotine/cocaine combinations in a novel polydrug abuse model. In drug discrimination experiments, nicotine increased the discriminative stimulus effects of low cocaine doses in two of three monkeys, but nicotine did not substitute for cocaine in any monkey. Self-administration of cocaine and nicotine alone, and cocaine + nicotine combinations was studied under a second-order fixed ratio 2, variable ratio 16 (FR2[VR16:S]) schedule of reinforcement. Cocaine and nicotine alone were self-administered in a dose-dependent manner. The combination of marginally reinforcing doses of cocaine and nicotine increased drug self-administration behavior above levels observed with the same dose of either cocaine or nicotine alone. These findings indicate that nicotine may increase cocaine's discriminative stimulus and reinforcing effects in rhesus monkeys, and illustrate the feasibility of combining cocaine and nicotine in a preclinical model of polydrug abuse. Further studies of the behavioral effects of nicotine + cocaine combinations will contribute to our understanding the pharmacology of dual nicotine and cocaine dependence, and will be useful for evaluation of new treatment medications.

MD-354 selectively antagonizes the antinociceptive effects of (-)nicotine in the mouse tail-flick assay

RATIONALE

(-)Nicotine produces antinociceptive effects in rodents. meta-Chlorophenylguanidine (MD-354), an analgesia-enhancing agent, binds at 5-HT(3) and alpha(2)-adrenoceptors and potentiates the antinociceptive effects of an "inactive" dose of clonidine. The present study examined the actions of MD-354 on (-)nicotine-induced antinociception.

MATERIALS AND METHODS

Mouse tail-flick and other assays were employed.

RESULTS

In the tail-flick assay, (-)nicotine (ED(50) = 1.66 mg/kg) but not MD-354 produced dose-related antinociceptive effects. Administered in combination with (-)nicotine (2.5 mg/kg), MD-354 (AD(50) = 3.4 mg/kg) did not potentiate, but effectively antagonized the antinociceptive actions of (-)nicotine. In a mouse hot-plate assay, MD-354 failed to modify (-)nicotine responses. In combination with a locomotor activity-suppressing dose of (-)nicotine, MD-354 (up to 17 mg/kg) failed to antagonize (-)nicotine-induced hypolocomotion. In a rat drug discrimination paradigm using (-)nicotine as training drug, MD-354 produced saline-appropriate responding; in combination with the training dose of (-)nicotine, MD-354 failed to antagonize the nicotine cue.

CONCLUSIONS

MD-354 selectively antagonizes the antinociceptive actions of (-)nicotine in the tail-flick, but not in the hot-plate assay, or either the motor effects, or discriminative stimulus effects of (-)nicotine. The most parsimonious explanation is that MD-354 might act as a negative allosteric modulator of alpha 7 nACh receptors, and radioligand binding and functional data are provided to support this conclusion.

Drug discrimination and neurochemical studies in alpha7 null mutant mice: tests for the role of nicotinic alpha7 receptors in dopamine release

RATIONALE

The nicotine discriminative stimulus has been linked to beta2-containing (beta2*) nicotinic receptors, with little evidence of a role for alpha7 nicotinic receptors, because nicotine discrimination was very weak in beta2 null mutant mice but normal in alpha7 mutants.

OBJECTIVES

As both alpha7 and beta2* nicotinic receptors have been implicated in nicotine-stimulated dopamine overflow, this study focused on the dopamine-mediated element in the nicotine stimulus by examining cross-generalisation between amphetamine and nicotine.

MATERIALS AND METHODS

Male alpha7 nicotinic receptor null mutant mice and wild-type controls were bred in-house and trained to discriminate nicotine (0.8 mg/kg) or (+)-amphetamine (0.6 mg/kg) from saline in a two-lever procedure with a tandem VI-30 FR-10 schedule of food reinforcement. Dopamine release from striatal slices was determined in parallel experiments.

RESULTS

An alpha7 nicotinic receptor-mediated component of dopamine release was demonstrated in tissue from wild-type mice using choline as a selective agonist. This response was absent in tissue from null mutant animals. The mutation did not influence acquisition of drug discriminations but subtly affected the results of cross-generalisation tests. In mice trained to discriminate nicotine or amphetamine, there was partial cross-generalisation in wild-type mice and, at certain doses, these effects were attenuated in mutants. Further support for an alpha7 nicotinic receptor-mediated component was provided by the ability of the alpha7 nicotinic receptor antagonist methyllycaconitine to attenuate responses to nicotine and amphetamine in wild-type mice.

CONCLUSIONS

These findings support the concept of an alpha7 nicotinic receptor-mediated dopaminergic element in nicotine discrimination, warranting further tests with selective dopamine agonists.

Recognising nicotine: the neurobiological basis of nicotine discrimination

Drug discrimination methodology makes possible the objective, quantitative study of the perception of psychoactive drug effects in either human or animal subjects. Investigations of the nicotine discriminative stimulus complex have contributed to our present understanding of nicotine psychopharmacology by defining the origin of its effects at specific subtypes of nicotinic receptor and the role of diverse neurotransmitter systems as mediating and modulating mechanisms. The evidence strongly supports central sites as the origins of the nicotine stimulus, and these are likely to be located in the mesocorticolimbic dopaminergic neurons; the medial prefrontal cortex is primarily involved, with the Nucleus accumbens and ventral tegmental area of secondary importance, while another element of the complex stimulus may arise in the dorsal hippocampus. Additionally, it appears that interactions of nicotine with the dopamine, serotonin, cannabinoid and probably glutamate systems all contribute to the final perceived stimulus. The resemblance between the nicotine discriminative stimulus and those of the psychomotor stimulant drugs amphetamine and cocaine contributes to defining the nature of the addictive properties of nicotine. It is particularly interesting that acute and chronic exposure to caffeine produce quantitative and qualitative changes in the characteristics of the nicotine stimulus. Interactions of nicotine with caffeine and cannabinoids strengthen proposals that the use of one substance serves as a "gateway" in sequential shifts of the target substance for drug-seeking behaviour, with profound implications for the human use of the substances concerned. Drug discrimination is also an important standard technique used in assessments of the abuse liability of novel psychoactive compounds, with relevance to attempts to develop novel nicotinic agonists for use as cognitive enhancers.

Methylphenidate enhances the abuse-related behavioral effects of nicotine in rats: intravenous self-administration, drug discrimination, and locomotor cross-sensitization

Stimulant drugs, including D-amphetamine, cocaine, and methylphenidate, increase cigarette smoking in controlled human laboratory experiments. Although the mechanism(s) underlying this effect are unknown, it is possible that stimulants may enhance directly the abuse-related effects of nicotine. In the present study, we characterized the behavioral pharmacological interactions between methylphenidate and nicotine in the intravenous self-administration, drug discrimination, and locomotor cross-sensitization procedures. Adult male Sprague-Dawley rats were trained to respond for intravenous nicotine (0.01 or 0.03 mg/kg/infusion) or sucrose, and the acute effects of methylphenidate (1.25-10 mg/kg) were determined; in addition, separate groups of rats were treated with methylphenidate (2.5 mg/kg) or saline before 12 consecutive nicotine (0.03 mg/kg/infusion) self-administration sessions. Next, the discriminative stimulus effects of nicotine (0.03-0.3 mg/kg) and methylphenidate (1.25-10 mg/kg), alone and in combination with a low nicotine dose (0.056 mg/kg), were tested in nicotine-trained rats. Finally, the locomotor effect of repeated methylphenidate (2.5 mg/kg) was tested in rats previously treated with nicotine (0.2-0.8 mg/kg). Results indicated that acute methylphenidate increased the rate of nicotine self-administration at doses that reduced sucrose-maintained responding; furthermore, tolerance to this effect was not apparent following repeated methylphenidate. Methylphenidate, while not substituting for nicotine alone, dose-dependently enhanced the discriminative stimulus effect of a low nicotine dose. In addition, repeated nicotine exposure promoted the development of locomotor sensitization to methylphenidate. Taken together with recent clinical findings, these results suggest that methylphenidate may enhance the abuse-related behavioral effects of nicotine, perhaps increasing vulnerability to tobacco dependence.

The conditional stimulus effects of nicotine vary as a function of training dose

Although past research has shown that the interoceptive effects of nicotine serve as a conditional stimulus using sucrose as the unconditioned stimulus, very little is known about the importance of dose. Accordingly, rats were assigned to 0.1, 0.2, or 0.4 mg nicotine base/kg as the training dose. Sucrose (4-s access) was delivered 36 times on nicotine sessions; sucrose was withheld on intermixed saline sessions. The discrimination was acquired for all groups, as measured by more photobeam breaks in the dipper receptacle before the first sucrose delivery on nicotine sessions, compared with a similar interval on saline sessions. Thirty nicotine sessions without sucrose deliveries (extinction) decreased conditioned responding with the 0.4 mg/kg dose maintaining higher responding than the lower doses. After reestablishing discrimination performance, rats were tested with their training dose at various injection-to-placement intervals. Conditioned responding diminished with longer intervals; 0.4 mg/kg nicotine-evoked conditioned responding at longer intervals. Subsequent generalization testing with nicotine or saline at the 5-min training interval found that conditioned responding was evoked by lower test doses in the 0.1 mg/kg group than in the other groups. Combined, this research demonstrates that the nicotine conditional stimulus shows some variation with training dose.

Nicotine and methamphetamine share discriminative stimulus effects

BACKGROUND

Nicotine and methamphetamine are both abused in similar settings, sometimes together. Because there are known interactions between central nicotinic acetylcholine receptors and dopamine receptors, it is of interest to characterize the nature of the interaction of these two compounds in vivo.

METHODS

The purpose of this study was to characterize the extent to which these two compounds produce similar discriminative stimulus effects and to identify pharmacological mechanisms for their interaction. Male Sprague-Dawley rats were trained to discriminate methamphetamine or nicotine from saline. First, the ability of methamphetamine and nicotine to cross-substitute in rats trained to the other compound was tested. Subsequently, the ability of a dopamine antagonist (haloperidol) and a centrally acting nicotinic antagonist (mecamylamine) to block the discriminative stimulus effects of methamphetamine and nicotine were also tested.

RESULTS

Nicotine fully substituted in methamphetamine-trained rats, but methamphetamine only partially substituted in nicotine-trained rats. In nicotine-trained rats, mecamylamine fully antagonized the discriminative stimulus effects of nicotine, but haloperidol had no effect. The partial substitution of methamphetamine was partially attenuated by haloperidol, but not altered by mecamylamine. In methamphetamine-trained rats, mecamylamine failed to antagonize the discriminative stimulus effects of methamphetamine, but haloperidol fully blocked the methamphetamine cue. Mecamylamine blocked the ability of nicotine to substitute for methamphetamine, but haloperidol had no effect.

CONCLUSIONS

These results indicate that nicotine and methamphetamine share discriminative stimulus effects in some subjects and that the two compounds do not act at the same site, but produce their interaction indirectly. These findings suggest that these two compounds might be at least partially interchangeable in human users, and that there are potentially interesting pharmacological reasons for the commonly observed co-administration of nicotine and methamphetamine.

The serotonin 2C receptor agonist Ro-60-0175 attenuates effects of nicotine in the five-choice serial reaction time task and in drug discrimination

RATIONALE

There is evidence that serotonin(2C) (5-HT(2C)) receptors can modulate some behavioural effects of nicotine, but the generality of this action is not known.

OBJECTIVE

To analyse the influence of the 5-HT(2C) agonist Ro-60-0175 on responses to nicotine in the five-choice serial reaction time task (5-CSRTT) and on its discriminative stimulus effect; these procedures constitute models for attention-enhancing and subjective effects of nicotine, respectively.

MATERIALS AND METHODS

In the 5-CSRTT, rats were trained to obtain food reinforcers by detecting light stimuli and then challenged with Ro-60-0175 (0.3-0.8 mg/kg) and nicotine (0.2 mg/kg). For drug discrimination studies, rats were trained to discriminate nicotine (0.2 mg/kg) from saline in a two-lever procedure using a tandem schedule of food reinforcement.

RESULTS

In the 5-CSRTT, nicotine positively influenced most response indices, confirming previous results. Ro-60-0175 increased response latencies and omission errors and reduced anticipatory responding but had little effect on response accuracy; importantly, it counteracted the effects of nicotine on response speed and omission errors. Pentobarbitone (10-14 mg/kg) also slowed performance of the 5-CSRTT but did not weaken the nicotine-induced enhancement of performance. In the drug discrimination procedure, Ro-60-0175 was not generalised with nicotine but shifted the nicotine dose-response curve to the right in a dose-related manner.

CONCLUSIONS

The data suggest that selective occupancy of 5-HT(2C) receptors can attenuate some effects of nicotine in the 5-CSRTT and weaken the nicotine discriminative stimulus; these effects cannot be explained by a sedative action of Ro-60-0175.

Behavioral and neuropharmacological characterization of nicotine as a conditional stimulus

In rats, the pharmacological (interoceptive) effects of 0.4 mg/kg nicotine can serve as a conditional stimulus in a Pavlovian conditioning task. Nicotine administration is paired with intermittent access to a liquid sucrose unconditional stimulus; sucrose is withheld on saline sessions. An increase in sucrose receptacle entries (goal tracking) on nicotine sessions indicates conditioning. Rats were trained on a nicotine dose ((-)-1-Methyl-2-(3-pyridyl)pyrrolidine; 0.1, 0.2, or 0.4 mg base/kg, s.c.). Generalization was examined using 0.025, 0.05, 0.1, 0.2, and 0.4 mg/kg nicotine and saline. Some behavioral effects of nicotine have been attributed to dopamine and glutamate. Accordingly, potential blockade of the nicotine cue via the dopamine system was examined by administering (R)-(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390; 0.005, 0.01, and 0.03 mg/kg), 3-Chloro-5-ethyl-N-[[(2S)-1-ethyl-2-pyrrolidinyl)methyl]-6-hydroxy-2-methoxy-benzamide hydrochloride (eticlopride; 0.01, 0.03, 0.1, and 0.3 mg/kg), or N-[(1-Butyl-2-pyrrolidinyl)methyl]-4-cyano-1-methoxy-2-naphthalenecarboxamide (nafadotride; 0.03, 0.1, 0.3, 1, and 3 mg/kg) before nicotine. 2-Methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP; 0.3, 1, and 3 mg/kg) and (5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801; 0.01, 0.03, 0.1, and 0.2 mg/kg; dizocilpine) were used to examine possible glutamatergic components. Substitution tests were conducted with MPEP and nafadotride. Differential conditioned responding was acquired in the 3 groups. Conditioned responding generally decreased as the nicotine test dose moved away from the training dose; responding increased when 0.4 mg/kg trained rats were tested with 0.2 mg/kg. SCH-23390, eticlopride, nafadotride, and MPEP decreased conditioned responding on nicotine at doses that also decreased chamber activity. In contrast, MK-801 decreased goal tracking on nicotine without decreasing chamber activity, indicating a role for N-methyl-D-aspartate receptors in expression of nicotine-evoked conditioned responding.

Extended blockade of the discriminative stimulus effects of nicotine with low doses of ethanol

The aim of the present study was to further evaluate effects of ethanol on nicotine discrimination and to correlate these effects with blood ethanol levels. Rats were trained to discriminate 0.3 mg/kg nicotine from its vehicle in the standard two-lever operant procedure. In antagonism tests, small doses of ethanol (0.25-0.5 g/kg) were injected either 5 or 50 min before nicotine. Both doses of ethanol partially antagonized the nicotine cue regardless of the pre-treatment time. Ethanol attenuated also inhibitory effects of nicotine on the rate of responding. Suppression of the cueing effects of nicotine was noted even 60 min after the injection of 0.25 g/kg ethanol, i.e. at the time point when the blood ethanol level was close to zero. Ethanol-induced antagonism of the nicotine cue disappeared when longer time (110 min) was allowed to elapse between the ethanol (0.5 g/kg) and nicotine injection. Concluding, the present results may indicate that the effects of ethanol on nicotine discrimination are not primarily related to blood ethanol levels.

Desensitized nicotinic receptors in brain

Desensitization is an intriguing characteristic of ligand-gated channels, whereby a decrease or loss of biological response occurs following prolonged or repetitive stimulation. Nicotinic acetylcholine receptors (nAChRs), as a member of transmitter gated ion channels family, also can be desensitized by continuous or repeated exposure to agonist. Desensitization of nicotinic receptors can occur as a result of extended nicotine exposure during smoking or prolonged acetylcholine when treatment of Alzheimer's disease (AD) with cholinesterase inhibitors, or anticholinesterase agent poisoning. Studies from our lab have shown that nAChRs desensitization is not a nonfunctional state and we proposed that desensitized nAChRs could increase sensitivity of brain muscarinic receptor to its agonists. Here, we will review the regulation of nicotinic receptor desensitization and discuss the important biological function of desensitized nicotinic receptors in light of our previous studies. These studies provide the critical information for understanding the importance of nicotinic receptors desensitization in both normal physiological processing and in various disease states.

Attenuation of nicotine's discriminative stimulus effects in rats and its locomotor activity effects in mice by serotonergic 5-HT2A/2C receptor agonists

RATIONALE

Reports have indicated that administration of nicotine inhibits, while withdrawal of chronically administered nicotine augments effects of serotonergic 5HT2A/2C agonists.

OBJECTIVE

It was our objective to determine whether 5HT2A/2C agonists can modulate the discriminative stimulus effects of nicotine in rats or its locomotor activity effects in mice.

METHODS

Adult male Sprague-Dawley rats were trained to discriminate 0.3 mg/kg nicotine base from saline in a two-lever, fixed-ratio (FR10), food-reinforced, operant-conditioning task during daily (Monday-Friday) 15-min experimental sessions. After characterizing a dose-response curve for nicotine, we tested the ability of the 5HT(2A/2C) agonists (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCL (DOI; 0.18-1.0 mg/kg) and 1-(4-bromo-2, 5-dimethoxyphenyl)-2-aminopropane (DOB; 0.1-1.0 mg/kg), the 5HT2C agonist 6-chloro-2-(1-piperazinyl)pyrazine hydrochloride (MK 212; 0.1 mg/kg-1.0 mg/kg), and the 5HT1A agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT; 0.01 mg/kg-1.0 mg/kg) to modulate nicotine's discriminative stimulus effects. After finding that DOI was able to attenuate the percentage nicotine lever responding (%NLR), we tested for it to also reverse nicotine's effects on locomotor activity in mice.

RESULTS

The 5HT2A/2C agonists-in particular DOI-dose dependently attenuated %NLR. The effects of DOI were reversed by the 5HT2A/2C antagonist ketanserin. MK 212 and 8-OH-DPAT had irregular effects among rats and only reduced %NLR to below 50% levels at doses markedly suppressing responding. DOI also dose dependently blocked nicotine's acute rate-lowering locomotor activity effects.

CONCLUSIONS

These results indicate that activation of serotonin 5HT2A/2C receptors can blunt the discriminative stimulus and locomotor activity effects of nicotine and presents the possibility that activation of these receptors might also be able to attenuate other effects of nicotine.

Stimulus properties of nicotine, amphetamine, and chlordiazepoxide as positive features in a pavlovian appetitive discrimination task in rats

Recent experiments from our laboratory have demonstrated that drug states can signal when environmental cues will be followed by rewarding outcomes (ie Pavlovian conditioning). However, little is known about the generality of this approach and whether it can be used for studying the pharmacological properties of drug states. Accordingly, the present experiments tested the pharmacological specificity of nicotine (0.4 mg/kg), amphetamine (1 mg/kg), and chlordiazepoxide (CDP, 5 mg/kg) in this Pavlovian drug discrimination procedure. Following drug administration, presentation of a conditional stimulus (CS) was followed by brief access to sucrose. When saline was administered, the same CS was presented but sucrose was withheld. In substitution tests, rats in each condition received varying doses of all training drugs and caffeine. Anticipatory food seeking developed during the CS on drug sessions but not on saline sessions for all drug features (ie drug state-specific conditional response (CR)). In generalization tests, this CR decreased as a function of decreases in the training dose. Median effective doses (ED50s) were calculated for nicotine (0.054 mg/kg), amphetamine (0.26 mg/kg), and CDP (2.48 mg/kg). No compound tested substituted for the CDP training drug. Partial substitution was evident between nicotine and amphetamine; CDP did not substitute for either of these drug features. Caffeine fully substituted for nicotine (ED50 = 15.45 mg/kg) and amphetamine (ED50 = 3.70 mg/kg), but not for CDP. These results are consistent with the hypothesis that drug states can occasion appetitive Pavlovian CRs in a pharmacologically specific manner.

The role of nicotinic receptor α7 subunits in nicotine discrimination

The subtypes of nicotinic receptors at which the behavioural effects of nicotine originate are not fully understood. The experiments described here use mice lacking the alpha7 subunit of nicotinic receptors to investigate the role of alpha7-containing receptors in nicotine discrimination. Wild-type and alpha7-knockout mice were trained in a two-lever nicotine discrimination procedure using a tandem schedule of food reinforcement. Mutant mice exhibited baseline rates of lever-pressing as low as 52.2% of rates in wild-type controls (n=21-24). Mutant and wild-type mice acquired discrimination of nicotine (0.4 or 0.8 mg/kg) at a similar rate (n=10-12) and reached similar final levels of accuracy (71.9 +/- 4.4% and 90.8 +/- 3.1% after 60 training sessions for 0.4 and 0.8 mg/kg training doses, respectively, in mutant mice, as compared with 75.0 +/- 6.5% and 87.6 +/- 4.8% for wild types). The genotypes exhibited similar steep dose-response curves for nicotine discrimination. In both genotypes, dose-response curves for mice trained with 0.8 mg/kg of nicotine were displaced three- to four-fold to the right as compared with those for the mice trained with the smaller dose. The predominant effect of nicotine on the overall rate of responding was a reduction at the largest doses tested and there was no difference between the genotypes. The results suggest that nicotinic receptors containing the alpha7 subunit do not contribute to the discriminative stimulus or response-rate-depressant effects of nicotine, although they may regulate baseline rates of operant responding.

Dopaminergic and cholinergic involvement in the discriminative stimulus effects of nicotine and cocaine in rats

RATIONALE

Previous work has demonstrated asymmetrical cross-generalization between the discriminative stimulus effects of nicotine and cocaine: nicotine fully substitutes for cocaine, whereas cocaine only partially substitutes for nicotine. The factors responsible for the similarities and differences between the two drugs remain unclear.

OBJECTIVE

The study tested the involvement of dopaminergic and/or cholinergic mechanisms in the discriminative stimulus effects of nicotine and cocaine.

METHODS

One set of rats was trained to discriminate cocaine (8.9 mg/kg) from saline, and two other sets of rats were trained to discriminate nicotine (0.1 mg/kg) from saline.

RESULTS

In cocaine-trained rats, among the cholinergic agonists studied only nicotine (0.01-0.56 mg/kg) produced full, dose-related substitution; nornicotine (1-5.6 mg/kg) substituted only partially, and lobeline (2.71-15.34 mg/kg) and pilocarpine (0.26-2.55 mg/kg) failed to engender any cocaine-appropriate responding. The nicotinic antagonist mecamylamine (1-5.6 mg/kg) failed to block cocaine's discriminative stimulus effects. The dopamine antagonist cis-flupentixol (0.48 mg/kg) blocked the substitution of nicotine for cocaine. In nicotine-trained rats, the dopamine uptake blockers cocaine, bupropion and nomifensine (0.2-26.1 mg/kg) each substituted only partially for nicotine, and cis-flupentixol (0.48-0.86 mg/kg) antagonized the discriminative stimulus effects of nicotine.

CONCLUSIONS

Nicotine fully substitutes for cocaine because of its effects on dopamine transmission, and not because the discriminative stimulus effects of cocaine incorporate a cholinergic component. Substitution of nicotine for cocaine may depend more on nicotine-induced dopamine release than does the nicotine-trained discriminative stimulus; there may be differential dopaminergic involvement after acute and repeated treatment with nicotine or cocaine.

Potential serotonergic and noradrenergic involvement in the discriminative stimulus effects of the selective imidazoline I2-site ligand 2-BFI

The functional significance of imidazoline I2 binding sites is unknown but microdialysis studies have indicated that the administration of I2-site ligands leads to an increase in extracellular levels of monoamines. The specific I2-site ligand 2-(-2-benzofuranyl)-2-imidazoline (2-BFI) generates a cue in drug discrimination, thereby indicating functional consequences of I2-site ligand binding. In the present work, we explored the ability of selective noradrenergic and serotonergic ligands to substitute for 2-BFI. Hooded Lister rats were trained in two-lever operant chambers with condensed milk reward to distinguish 2-BFI (7 mg/kg) from saline vehicle, by pressing the correct lever to a predetermined success criterion. Training sessions were then interspersed with sessions in which animals were administered test substances and the proportion of lever presses on the 2-BFI-associated lever (substitution) recorded. Several agents exhibited significant partial substitution for 2-BFI: The monoamine-releasing agents D-amphetamine and fenfluramine dose-dependently substituted for 2-BFI, while norepinephrine (desipramine, reboxetine) and serotonin (clomipramine, citalopram) reuptake inhibitors substituted at one or more doses. Further investigation using specific receptor agonists and antagonists indicated a possible role for activation of alpha1-adrenoceptors but failed to support involvement of alpha2-adenoceptor, beta-adrenoceptor or 5-HT1A receptor activation. These results support the concept that the 2-BFI cue may contain both noradrenergic and serotonergic components.

Amphetamine enhances Ca2+ entry and catecholamine release via nicotinic receptor activation in bovine adrenal chromaffin cells

Amphetamine, a psychostimulant, has been shown to act as a channel blocker of muscle nicotinic receptors and to induce a Ca(2+)-dependent secretion from adrenal chromaffin cells. In this study, the relationship between amphetamine and nicotinic receptors was studied using bovine adrenal chromaffin cells as a model system. Our results show that D-amphetamine sulfate alone induced an increase in the cytosolic Ca(2+) concentration ([Ca(2+)](c)) and [3H]norepinephrine release in a dose-dependent and extracellular Ca(2+)-dependent manner. Two common nicotinic receptor antagonists, hexamethonium and mecamylamine, suppressed the D-amphetamine sulfate-induced [Ca(2+)](c) rise and [3H]norepinephrine release. In addition, D-amphetamine sulfate inhibited the 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP)-induced [Ca(2+)](c) rise and [3H]norepinephrine release, but not the high K(+)- or veratridine-induced [Ca(2+)](c) increase and [3H]norepinephrine release. Antagonists, including alpha-bungarotoxin and choline, that are more specific for alpha7 nicotinic receptors were capable of inhibiting the D-amphetamine sulfate-induced [Ca(2+)](c) rise, while D-amphetamine sulfate was found to be capable of inhibiting the [Ca(2+)](c) rise induced by the alpha7-nicotinic receptor agonists, epibatidine and choline. Moreover, D-amphetamine sulfate dose-dependently suppressed [3H]nicotine binding to chromaffin cells. We, therefore, conclude that D-amphetamine sulfate acts as a nicotinic receptor agonist to induce [Ca(2+)](c) increase and [3H]norepinephrine release in bovine adrenal chromaffin cells.

SR141716, a central cannabinoid (CB(1)) receptor antagonist, blocks the motivational and dopamine-releasing effects of nicotine in rats

The central CB(1) cannabinoid receptor has recently been implicated in brain reward function. In the present study we evaluated first the effects of the selective CB(1) receptor antagonist, SR141716, on the motivational effects of nicotine in the rat. Administration of SR141716 (0.3 and 1 mg/kg) decreased nicotine self-administration (0.03 mg/kg/injection). SR141716 (0.3-3 mg/kg) neither substituted for nicotine nor antagonized the nicotine cue in a nicotine discrimination procedure, but dose-dependently (0.01-1 mg/kg) antagonized the substitution of nicotine for D-amphetamine, in rats trained to discriminate D-amphetamine. Secondly, using brain microdialysis, SR141716 (1-3 mg/kg) blocked nicotine-induced dopamine release in the shell of the nucleus accumbens (NAc) and the bed nucleus of the stria terminalis. To investigate whether SR141716 would block the dopamine-releasing effects of another drug of abuse, we extended the neurochemical study to the effect of ethanol, consumption of which in rodents is reduced by SR141716. Dopamine release induced by ethanol in the NAc was also reduced by SR141716 (3 mg/kg). These results suggest that activation of the endogenous cannabinoid system may participate in the motivational and dopamine-releasing effects of nicotine and ethanol. Thus, SR141716 may be effective in reduction of alcohol consumption, as previously suggested, and as an aid for smoking cessation.

Role of dopamine in the behavioural actions of nicotine related to addiction

Experimental impairment of dopamine function by 6-hydroxydopamine lesions or by dopamine receptor antagonists shows that dopamine is involved in nicotine's discriminative stimulus properties, nicotine-induced facilitation of intracranial self-stimulation, intravenous nicotine self-administration, nicotine conditioned place-preference and nicotine-induced disruption of latent inhibition. Therefore, nicotine depends on dopamine for those behavioural effects that are most relevant for its reinforcing properties and are likely to be the basis of the abuse liability of tobacco smoke. On the other hand, in vivo monitoring studies show that nicotine stimulates dopamine transmission in specific brain areas and in particular, in the shell of the nucleus accumbens and in areas of the extended amygdala. These effects of nicotine resemble those of a reward like food except that nicotine-induced release of dopamine does not undergo single-trial, long-lasting habituation. It is speculated that repeated non-habituating stimulation of dopamine release by nicotine in the nucleus accumbens shell abnormally facilitates associative stimulus-reward learning. Acute effects of nicotine on dopamine transmission undergo acute and chronic tolerance; with repeated, discontinuous exposure, sensitization of nicotine-induced stimulation of dopamine release in the nucleus accumbens core takes place while the response in the shell is reduced. It is speculated that these adaptive changes are the substrate of a switch from abnormal incentive responding controlled by consequences (action-outcome responding) into abnormal habit responding, triggered by conditional stimuli and automatically driven by action schemata relatively independent from nicotine reward. These two modalities might coexist, being utilized alternatively in relation to the availability of tobacco. Unavailability of tobacco disrupts the automatic, implicit modality of abnormal habit responding switching responding into the explicit, conscious modality of incentive drug-seeking and craving.

A pharmacologic strategy for the treatment of nicotine addiction

Like many psychostimulant drugs, nicotine elevates extracellular and synaptic dopamine (DA) concentrations in the nucleus accumbens (NAc). This elevation has been linked to its reinforcing properties. Dopaminergic transmission within the NAc is modulated by gamma-aminobutyric acid (GABA). Therefore, we examined the utility of gamma vinyl-GABA (GVG, Vigabatrin) for inhibiting nicotine's biochemical effects on NAc DA as well as its effects on behaviors associated with these biochemical changes. Given 2.5 hours prior to nicotine, GVG (75 mg/kg) had no effect on nicotine-induced increases in extracellular NAc DA. However, at 90 mg/kg, GVG significantly inhibited nicotine-induced increases by approximately 50% while at 100 or 150 mg/kg, GVG completely abolished nicotine-induced increases in both naive and chronically nicotine-treated animals. When given 12 or 24 hours prior to nicotine administration at a dose of 100 mg/kg, GVG-induced inhibition was diminished or abolished, respectively. In addition, at a dose of 18.75 mg/kg GVG abolished the expression of nicotine-induced conditioned place preference (CPP) while a dose of 75 mg/kg abolished the acquisition phase of CPP. Finally, using positron emission tomography (PET) and 11C-raclopride in primates, GVG (100 mg/kg) abolished nicotine-induced increases in synaptic DA while having no effect on the rate of metabolism of the radiotracer or its regional distribution. Together, these data suggest that GVG may be useful for the treatment of nicotine addiction and further support the strategy of targeting the GABAergic system with a suicide inhibitor of GABA-transaminase for the treatment of drug addiction.