Interaction of nicotine with dopaminergic mechanisms assessed through drug discrimination and rotational behaviour in rats

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.

Preclinical and clinical research on the discriminative stimulus effects of nicotine

Across species, nicotine can produce robust discriminative stimulus (DS) effects, as with other drugs of abuse, a feature that has been harnessed to advance our understanding on the neuropharmacological mechanisms of nicotine's actions. With the crucial role played by nicotine in supporting tobacco dependence, nicotine DS effects have presented an ideal platform to develop novel generation of smoking cessation compounds. Findings from preclinical strands of research have invigorated the field of human discrimination research to objectively assess nicotine's interoceptive stimulus effects. As such, translation studies provide proof of concept for nicotine DS research as a method to assess the subjective effects of nicotine per se, separate from non-nicotine stimuli involved in smoking. Recent clinical studies with low doses have demonstrated that perceiving nicotine's DS effects is necessary, yet not sufficient, for that dose to be reinforcing. These measures have been instrumental in developing novel strategies with regards to establishing threshold doses of nicotine contained in tobacco products, to then determine subthreshold doses that cannot be discriminated and, therefore, fail to maintain reinforcement. Findings from preclinical and clinical nicotine DS research could substantially inform public health policies aimed at regulating nicotine content of consumer products so that they minimize risks of dependency. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Discriminative Stimulus Properties of S(-)-Nicotine: "A Drug for All Seasons"

S(-)-Nicotine is the major pharmacologically active substance in tobacco and can function as an effective discriminative stimulus in both experimental animals and humans. In this model, subjects must detect and communicate the nicotine drug state versus the non-drug state. This review describes the usefulness of the procedure to study nicotine, presents a general overview of the model, and provides some relevant methodological details for the establishment of this drug as a stimulus. Once established, the (-)-nicotine stimulus can be characterized for dose response and time course effects. Moreover, tests can be conducted to determine the similarity of effects produced by test drugs to those produced by the training dose of nicotine. Such tests have shown that the stimulus effects of nicotine are stereoselective [S(-)-nicotine >R(+)-nicotine] and that other "natural" tobacco alkaloids and (-)-nicotine metabolites can produce (-)-nicotine-like effects, but these drugs are much less potent than (-)-nicotine. Stimulus antagonism tests with mecamylamine and DHβE (dihydro-β-erythroidine) indicate that the (-)-nicotine stimulus is mediated via α4β2 nicotinic acetylcholine receptors (nAChRs) in brain; dopamine systems also are likely involved. Individuals who try to cease their use of nicotine-based products are often unsuccessful. Bupropion (Zyban®) and varenicline (Chantix®) may be somewhat effective as anti-smoking medications because they probably produce stimulus effects that serve as suitable substitutes for (-)-nicotine in the individual who is motivated to quit smoking. Finally, it is proposed that future drug discrimination studies should apply the model to the issue of maintenance of abstinence from (-)-nicotine-based products.

Nicotine-induced behavioral sensitization in an adult rat model of attention deficit/hyperactivity disorder (ADHD)

Attention deficit hyperactivity disorder (ADHD) is associated with increased risk of tobacco dependence. Nicotine, the main psychoactive component of tobacco, appears to be implicated in ADHD-related tobacco dependence. However, the behavioral responsiveness to nicotine of the prevalent animal model of ADHD, the spontaneously hypertensive rat (SHR), is currently underinvestigated. The present study examined the activational effects of acute and chronic nicotine on the behavior of adult male SHRs, relative to Wistar Kyoto (WKY) controls. Experiment 1 verified baseline strain differences in open-field locomotor activity. Experiment 2 tested for baseline strain differences in rotational behavior using a Rotorat apparatus. Adult SHR and WKY rats were then exposed to a 7-day regimen of 0.6mg/kg/d s.c. nicotine, or saline, prior to each assessment. A separate group of SHRs underwent similar training, but was pre-treated with mecamylamine, a cholinergic antagonist. Nicotine sensitization, context conditioning, and mecamylamine effects were then tested. Baseline strain differences were observed in open-field performance and in the number of full rotations in the Rotorat apparatus, but not in the number of 90° rotations or direction changes. In these latter measures, SHRs displayed weaker nicotine-induced rotational suppression than WKYs. Both strains expressed nicotine-induced sensitization of rotational activity, but evidence for strain differences in sensitization was ambiguous; context conditioning was not observed. Mecamylamine reversed the effects of nicotine on SHR performance. These findings are consistent with the hypothesis that a reduced aversion to nicotine (expressed in rats as robust locomotion) may facilitate smoking among adults with ADHD.

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.

The endocannabinoid system: a new molecular target for the treatment of tobacco addiction

Tobacco addiction is one of the leading preventable causes of mortality in the world and nicotine appears to be the main critical psychoactive component in establishing and maintaining tobacco dependence. Several lines of evidence suggest that the rewarding effects of nicotine, which underlie its abuse potential, can be modulated by manipulating the endocannabinoid system. For example, pharmacological blockade or genetic deletion of cannabinoid CB(1) receptors reduces or eliminates many behavioral and neurochemical effects of nicotine that are related to its addictive potential. This review will focus on the recently published literature about the role of the endocannabinoid system in nicotine addiction and on the endocannabinoid system as a novel molecular target for the discovery of medications for tobacco dependence.

The role of NMDA receptor antagonists in nicotine tolerance, sensitization, and physical dependence: a preclinical review

Nicotine, the primary psychoactive component of tobacco products, produces diverse neurophysiological, motivational, and behavioral effects through several brain regions and neurochemical pathways. Various neurotransmitter systems have been explored to understand the mechanisms behind nicotine tolerance, dependence, and withdrawal. Recent evidence suggests that glutamate neurotransmission has an important role in this phenomenon. The aim of the present review is to discuss preclinical findings concerning the role of N-methyl-D-aspartate (NMDA) receptor neurotransmission in mediating the behavioral effects of nicotine, tolerance, sensitization, dependence, and withdrawal. Based on preclinical findings, it is hypothesized that NMDA receptors mediate the common adaptive processes that are involved in the development, maintenance, and expression of nicotine addiction. Modulation of glutamatergic neurotransmission with NMDA receptor antagonists may prove to be useful in alleviating the symptoms of nicotine abstinence and facilitate tobacco-smoking cessation.

Behavioural pharmacology of nicotine: implications for multiple brain nicotinic receptors

Behavioural studies can contribute to the characterization of receptors for psychoactive drugs, and attempts have been made to link behavioural effects of nicotinic agonists with the high affinity binding site for [3H] nicotine. Cueing (discriminative stimulus) effects of drugs enable trained humans or animals to recognize when a specific drug is administered. There was a correlation between the potencies of some compounds in the binding procedure and their ability to produce the nicotine discriminative stimulus in rats, supporting the view that the high affinity binding site was a functional receptor. Nicotine also produced complex changes in locomotor activity of rats, characterized acutely by transient depression and chronically by persistent stimulation. The abilities of nicotinic compounds to produce these locomotor effects were not always consistent with the studies on binding and the nicotine discriminative stimulus. Some compounds were relatively more potent in producing locomotor depression or stimulation than the discriminative effect. Some compounds also failed to produce chronic locomotor activation at doses that produced discriminative and acute depressant effects. These findings may be interpreted as preliminary evidence that different behavioural effects of nicotine may be mediated through different mechanisms, possibly involving multiple subtypes of nicotinic receptors.

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.

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.

Effects of dopamine receptor antagonists on nicotine-induced attentional enhancement

An understanding of the neuropharmacological mechanisms mediating attentional enhancement by nicotine would indicate whether these effects could be dissociated pharmacologically from other behavioural effects of nicotine. The aim of the present study was to examine the involvement of dopamine neurotransmission in the effects of nicotine on different response indices of an attentional paradigm. The effects of the D2-type dopamine receptor antagonist raclopride (0.025-0.1 mg/kg) and the D1-type receptor antagonist SCH23390 (0.006-0.024 mg/kg) were tested, in both the presence and absence of nicotine (0.1 mg/kg), in rats trained in a modified version of the five-choice serial reaction time task (5-CSRTT). Nicotine robustly enhanced the accuracy of signal detection, reduced omission errors and shortened response latencies. Neither raclopride nor SCH23390 altered the effects of nicotine on accuracy and omissions, but raclopride augmented accuracy and SCH23390 increased omissions when given alone. By contrast, raclopride, but not SCH23390, reversed the nicotine-induced reductions in response latencies, at doses that had no effect on their own. In the presence of nicotine, both antagonists had rate-disruptive effects at the highest dose. Both antagonists also reduced responding in the intertrial interval, and this effect was additive to the nicotine-induced decrease in this measure. The data indicate that D2-type dopamine receptors may be involved in the effects of nicotine on response speed. Neither the D1- nor the D2-type dopamine receptor antagonist affected nicotine-induced improvements in signal detection, at doses that reversed dependence-related behavioural effects of nicotine in previous studies. Thus these effects may be pharmacologically dissociable.

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.

Influence of training paradigm on specificity of drug mixture discriminations

Generalization to different drugs and drug mixtures has been examined in rats trained to discriminate a mixture of amphetamine (0.4 mg/kg) plus pentobarbitone (10 mg/kg) from saline (AND discrimination, n = 8) or to discriminate the same mixture from its component drugs alone (AND-OR discrimination, n = 9). The studies used two-lever operant procedures with a tandem variable interval 1-min fixed-ratio 10 schedule of food reinforcement. There was partial generalization to nicotine and midazolam and no generalization to cocaine, caffeine, or ethanol under AND-discrimination conditions and no generalization to any of these drugs in the AND-OR discrimination. Nicotine or midazolam coadministered with the training doses of pentobarbitone and amphetamine, respectively, produced full generalization in the AND discrimination and partial generalization under AND-OR conditions. Cocaine coadministered with pentobarbitone generalized fully under both procedures, but at larger doses in the AND-OR than in the AND discrimination. Mixtures of either nicotine plus midazolam or caffeine plus ethanol produced very marked generalization under AND-discrimination conditions, but were without significant effect in the AND-OR procedure. The results consistently supported the hypothesis that the AND-OR discrimination procedure increases the specificity of discriminations based on drug mixtures.

The role of monoamine neurotransmitter systems in the nicotine discriminative stimulus

Nicotine serves as a reinforcer and induces a robust discriminative stimulus which is primarily mediated by neuronal nicotinic receptors. As a secondary effect of nicotinic stimulation, nicotine elicits an enhanced release of the biogenic amine neurotransmitters dopamine, norepinephrine and serotonin. In particular, compounds with dopaminergic activity have been reported to modify both the reinforcing and discriminative stimulus properties of nicotine. The present study examined a number of dopaminergic, noradrenergic and serotonergic compounds for their effectiveness in reproducing or modifying the stimulus properties of nicotine in rats. The non-selective dopamine agonists amphetamine, cocaine and apomorphine produced partial substitution for nicotine, while the selective D2/D3 agonists bromocriptine and 7-OH-DPAT and the dopamine autoreceptor antagonist (+)-AJ-76 had little effect. The substitution of amphetamine for nicotine was not blocked by haloperidol, suggesting a minimal role for D2 receptors in the nicotine-like discriminative effects of stimulants. The selective D1 agonist SKF 81,297 produced partial substitution for nicotine (45% maximum), but further experiments with the D1 antagonist SCH 23,390 and with rats trained in a three-way discrimination procedure failed to support a primary role for this receptor in the substitution of dopaminergic drugs for nicotine. Finally, tests of compounds with effects on noradrenergic or serotonergic neurotransmission did not yield strong evidence for the involvement of these systems. Taken together, these data support earlier suggestions that activation of dopamine receptor subtypes plays a role in the nicotine-like stimulus properties of abused stimulants, but do not clearly identify a single subtype that is uniquely responsible.

3-(5-Alkylamino-4-isoxazolyl)-1,2,5,6-tetrahydropyridines: a novel class of central nicotinic receptor ligands

A novel class of central nicotinic acetylcholine receptor ligands, 3-(5-alkylamino-4-isoxazolyl)-1,2,5,6-tetrahydropyridine 4a-f, was synthesized. Several of the compounds showed high affinity for central nicotinic receptors (4c: IC50 = 50 nM), with more than a 100-fold selectivity for nicotinic over muscarinic receptors. The compounds showed up to a 10-fold selectivity for the central nicotinic subtype combination alpha 4 beta 2 (4c: IC50 = 4.6 nM), as compared to the major ganglionic subtype composed of alpha 3 containing subunits (4c: IC50 = 48 nM). The compounds were further evaluated in a dopamine release assay in vitro, and in a drug discrimination assay in vivo. Compound 4a is an effective nicotinic agonist with a potency 50-100 times lower than nicotine. Extending the alkylamino chain beyond one, compound (4b-f), changed the pharmacological profile of the compounds in an antagonistic direction.

Is dopamine important in nicotine dependence?

Nicotine, like other drugs when abused, can produce a wide array of behaviours, some of which collectively propel 'drug-seeking behaviour'. This review focuses on three stimulus properties of nicotine and examines the role of dopamine in mediating each effect with respect to D1 and D2 receptor subtypes. Dopamine appears to be critical in mediating the reinforcing effects of nicotine, which is in line with other commonly abused psychomotor stimulants. However, evidence derived from studies with local microinjections of nicotine suggests that the origin of nicotine action to produce its other stimulus properties may be via multiple neuroanatomical substrates. The aversive stimulus effects are resistant to dopamine receptor antagonists. The discriminative stimulus effects of nicotine, despite showing some modification with dopaminergic compounds, appear not to be solely mediated via the mesolimbic dopamine system. Taken together, the neurobiology of nicotine dependence remains complex. Nonetheless, such dissociation between stimulus properties may permit the development of more effective therapies in combating tobacco dependence.

Brain sites mediating the discriminative stimulus effects of nicotine in rats

Pharmacological studies suggest that the discriminative stimulus (DS) produced by nicotine is mediated centrally. The aim of the present study was to identify neuroanatomical substrates that mediate the DS properties of nicotine. Specifically, the nucleus accumbens, a brain region known to mediate the DS effects of amphetamine and cocaine, was investigated using a two-lever operant drug discrimination paradigm. Male hooded rats were trained to discriminate nicotine (0.2 mg/kg s.c.) from saline with a tandem schedule of food reinforcement. Once stimulus control was attained, a randomised sequence of nicotine microinjections (2-8 micrograms) was tested for generalisation during brief extinction tests. It was confirmed that the stimulus produced by the systemic administration of nicotine generalized to nicotine administered bilaterally into the dorsal hippocampus, with significant decreases in overall response rates. Microinjections of nicotine (1-8 micrograms) into the nucleus accumbens failed to produce any dose-related increases in responding on the nicotine-appropriate lever although these microinjections also produced significant decreases in response rates. Smaller doses (1-4 micrograms) of nicotine administered into the fourth ventricle produced characteristic prostration responses but these microinjections failed to produce generalization in tests carried out 20 min later, when the disabling effects of prostration had dissipated. These results suggest that the DS effects of nicotine may be mediated, at least in part, through the dorsal hippocampus. Results from intra-accumbens and intraventricular injections suggest that these regions may not be important in mediating the DS effects of nicotine.

ABT-418: discriminative stimulus properties and effect on ventral tegmental cell activity

Previous studies have established that ABT-418 [(S)-3-methyl-5-(1 methyl-2-pyrrolidinyl)isoxazole hydrochloride] is a novel neuronal nicotinic acetylcholine receptor (nAChR) ligand with cognitive enhancing and anxiolytic-like activity 3- to 10-fold more potent than (-)-nicotine in rodents. A series of experiments was conducted to determine the discriminative stimulus properties of ABT-418 in comparison with (-)-nicotine, and to determine the relative potencies of these compounds on ventral tegmental area (VTA) neurons. While rats were able to discriminate (-)-nicotine 1.9 mumol/kg in 39 days, they were not able to discriminate 1.9 or 6.2 mumol/kg ABT-418 from a saline solution during 50 days of training. In rats trained to discriminate 1.9 mumol/kg (-)-nicotine, a reduced generalization was induced by ABT-418 at 1.9 and 6.2 mumol/kg, an effect completely blocked by the cholinergic channel blocker mecamylamine (15 mumol/kg, IP). However, in extensively trained rats, intraperitoneal or subcutaneous injections of ABT-418 induced 78-82% generalization at the 6.2 mumol/kg dose. The predominant metabolites of (-)-nicotine and ABT-418 (continine and A-87770, respectively) were devoid of any effect in nicotine-trained rats. The reduced potency of ABT-418 in nicotine-trained rats is consistent with the electrophysiological findings showing that ABT-418 is 3-fold less potent than (-)-nicotine in activating dopamine-containing neurons in the VTA area.

Clozapine attenuates the discriminative stimulus properties of (-)-nicotine

Rats were trained to discriminate 1.9 mumol/kg (-)-nicotine (0.3 mg/kg) from saline in a standard two-bar operant conditioning paradigm with food reinforcement. The effect of neuronal nicotinic acetylcholine receptor (nAChR) agonists and antagonists were verified, and the participation of dopaminergic receptors subtypes in the expression of the (-)-nicotine cue was investigated with cis-flupentixol (D1-D2 antagonist), haloperidol (D2 antagonist) and clozapine (D4 antagonist). The stereoselectivity of the behavioral response was indicated by the 10-fold less sensitivity to (+)-nicotine in (-)-nicotine-trained rats. (+/-)-Anabasine and (-)-cytisine exhibited partial agonist profiles at the 1.9 mumol/kg dose while (-)-lobeline was devoid of any effect in doses up to 19 mumol/kg. (-)-Lobeline did not show antagonist properties in this paradigm. The nicotinic channel blockers mecamylamine, chlorisondamine and hexamethonium were inactive on their own but mecamylamine and chlorisondamine were able to block the effect of (-)-nicotine. Clozapine attenuated the (-)-nicotine cue while cis-flupentixol and haloperidol were ineffective. Similar doses of cis-flupentixol significantly blocked the locomotor stimulant effect of (-)-nicotine in rats indicating that blockade of dopaminergic receptors was achieved at the doses used in the drug discrimination studies. These data suggest that the discriminative stimulus properties of (-)-nicotine are mediated through neuronal nAChRs and involves the activation of dopaminergic receptors of the D4 subtype.

Injections of 6-hydroxydopamine into the ventral tegmental area destroy mesolimbic dopamine neurons but spare the locomotor activating effects of nicotine in the rat

The locomotor response to nicotine was assessed four weeks following destruction of mesolimbic dopamine (DA) neurons in rats by infusion of 6-hydroxydopamine into the ventral tegmental area. Resulting depletions of nucleus accumbens (N.Acc.) DA of up to 100% of control concentrations did not block the acute locomotor response to nicotine (0.4 mg/kg, base, s.c.). Such depletions also did not prevent the progressive enhancement of nicotine's locomotor effects when injections were repeated daily for nine days. These results suggest that mesolimbic DA is not necessary for the elicitation of locomotor activation by nicotine.

D-amphetamine-like stimulus properties are produced by morphine injections into the ventral tegmental area but not into the nucleus accumbens

We investigated whether injections of morphine into the ventral tegmental area (VTA) or the nucleus accumbens (NAS) could produce amphetamine-like stimulus properties and locomotor stimulant effects. Rats were trained to discriminate 1.0 mg/kg D-amphetamine from saline using both VI-30 and FR-16 reinforcement schedules and they were then tested following bilateral injections of morphine sulfate (2.5, 5.0 and 10.0 micrograms/side) either into the VTA or the NAS. Intra-VTA injections of morphine produced significant increases in amphetamine-lever responding that were comparable to increases observed following intra-NAS D-amphetamine (2.5, 5.0 and 10.0 micrograms/side). Such increases were not observed, however, following intra-NAS injections of morphine. Tests for the effects of intracerebral morphine and D-amphetamine on locomotor activity produced a similar pattern of results. Locomotor activity was increased following intra-VTA morphine and intra-NAS D-amphetamine injections, but not after intra-NAS morphine injections. Together, these findings indicate that activation of opioid receptors within the VTA, but not the NAS, can produce a behavioral state which mimics to some degree the state produced by systemic and intra-NAS injections of D-amphetamine.

Dopaminergic mediation of the stimulant generalization of nicotine

1. Experiments were conducted to investigate if the psychostimulant cathinone, like d-amphetamine, would produce generalization of the discriminative stimulus effects of nicotine. 2. Rats were trained to discriminate either 0.8 mg/kg cathinone from its vehicle or 0.8 mg/kg nicotine from its vehicle and, subsequently, administered various doses of the other compound. 3. Results of Exp 1 indicate that animals trained to discriminate cathinone only partially generalize to the effects of 0.8-1.6 mg/kg nicotine. In contrast, animals trained to discriminate nicotine dose-responsively generalize to cathinone doses ranging from 0.1-1.2 mg/kg. 4. Exp 2 served to investigate the effects of the dopamine release inhibiting drug CGS 10746B upon the observed cathinone generalization in nicotine-trained rats. Pretreatment with this compound at doses of 20 and 30 mg/kg significantly attenuated cathinone generalization in these animals. 5. The results are discussed in light of the growing evidence that nicotinic receptors reside upon mesolimbic dopamine neurons and the possibility that the consequent increase in extracellular dopamine may produce the discriminative stimuli, as well as the reinforcing properties, of nicotine.

Nicotinic and muscarinic components of rat brain dopamine synthesis stimulation induced by physostigmine

The role of a putative cholinergic control of ascending midbrain dopamine neurons was studied with biochemical methods in the unanaesthetized male albino rat. Post-mortem catechols were measured with high performance liquid chromatography with electrochemical detection. The acetylcholinesterase inhibitor physostigmine (0.5 mg/kg s.c.) enhanced L-dihydroxyphenylalanine (DOPA) and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in both the corpus striatum and limbic areas (nucleus accumbens) after inhibition of aromatic amino acid decarboxylase with NSD-1015, indicating an enhanced synthesis of dopamine in these brain regions. The effect of physostigmine was blocked both in the corpus striatum and in limbic areas by the centrally penetrating muscarinic antagonist scopolamine (1.0 mg/kg s.c.). In contrast, the nicotinic antagonist mecamylamine (1.0 mg/kg s.c.) significantly reduced the stimulatory effect of physostigmine in limbic areas, but not in the corpus striatum. The present results suggest that ascending dopamine neurons are influenced by cholinergic synaptic transmission being mediated mainly by muscarinic receptors as regards the nigrostriatal system, and by both nicotinic and muscarinic receptors as regards the mesolimbic system. The nicotinic influence appears to primarily control phasic activity of the dopamine neurons.

Further evidence for the mechanisms that may mediate nicotine discrimination

Rats were trained to discriminate the interoceptive stimuli produced by subcutaneously administered 0.4 mg/kg nicotine in a two-lever, food-motivated, operant task. Once criterion performance was attained, dose-response experiments indicated an ED50 value of 0.1 mg/kg and subsequent time course experiments showed a maximal effect between 10 and 30 min postadministration with a return to saline-like responding at 2 h. Pretreatment with the presynaptic dopamine release inhibitors CGS 10746B (30 mg/kg), as well as with the dihydropyridine calcium blocker isradipine (15 mg/kg), each produced a significant blockade of nicotine discrimination. In contrast, the 5-hydroxytryptamine (5-HT) receptor 5-HT3 antagonist ICS-205930 did not produce any effect upon nicotine discrimination. Thus, drugs that interfere with calcium influx, viz., isradipine, or with dopamine release (CGS 10746B) also interfere with nicotine discrimination and these results suggest that calcium influx and dopamine release may be necessary conditions for nicotine discrimination.

The neurobiology of tobacco addiction

The key property that makes nicotine addictive is an ability to support the drug-seeking behaviour that has been demonstrated in self-administration and place preference experiments. This reinforcing effect is complex, possibly involving subjective states of euphoria, cognitive enhancements, changed adaptation to stress, and relief from the nicotine withdrawal syndrome. The neural mechanisms, described here by Ian Stolerman and Mohammed Shoaib, include a primary action on central nicotinic acetylcholine receptors, associated with selective activation of the mesolimbic dopamine system that also mediates other sources of reinforcement. Structures such as the mesopontine tegmentum may also contribute to the reinforcing effect, whereas hippocampal and striatal regions seem to mediate other behavioural changes.

Selective dopamine antagonists reduce nicotine self-administration

The effects of selective D1 (SCH23390) and D2 (spiperone) dopamine antagonists, as well as of haloperidol, were examined on nicotine self-administration, food-maintained responding, and locomotor activity in rats. Antagonists reduced both operant responding and locomotor activity. Response patterns indicated that motor impairment was not the cause of the decreases, since responding was attenuated only in the latter half of operant sessions. Locomotor activity scores were significantly reduced by SCH23390, but not by spiperone. The effects of dopamine antagonists on nicotine self-administration are different from the effects of these antagonists on cocaine self-administration. Results are discussed in terms of the role of dopamine in drug reinforcement versus its role in sensorimotor integration.

Locomotor activity in rats after administration of nicotinic agonists intracerebrally

1. Nicotine (0.13 and 0.4 mg kg-1, s.c.) increased the ambulatory component of locomotor activity in rats previously exposed to the drug. Nicotine did not increase repeated movements reliably. 2. An infusion of either nicotine (8 micrograms) or the potent nicotinic agonist cytisine (4 micrograms) into the ventral tegmental area of the forebrain increased ambulation but not repeated movements. 3. An infusion of nicotine or cytisine into the nucleus accumbens, striatum, dorsal hippocampal formation or motor thalamus did not increase ambulatory or repeated movements. 4. Mecamylamine (0.1-1.0 mg kg-1, s.c.) blocked increases in locomotor activity produced by an infusion of nicotine or cytisine into the ventral tegmental area. 5. The locomotor activity produced by systemically administered nicotine may be mediated, in part, through nicotinic receptors located in the ventral tegmental area of the mesolimbic dopamine system.

Temporal factors in drug discrimination: Experiments with nicotine

The role of the presession interval (PI) in drug discrimination research has been studied in rats trained to discriminate nicotine from saline in a two-bar operant conditioning procedure. Different groups of rats were trained at different Pls, varying between 5 and 35 min, and tests were then carried out for qualitative and quantitative differences between the cues. There was complete generalization from nicotine cues trained at one time to tests carried out at other times. The sensitivity of the cues at different Pls to the nicotinic antagonist mecamylamine was very similar. Generalization to amphetamine was nearly complete when the nicotine cue was established with PI of 20-35 min and only partial when the PI for the nicotine was 5 min. Thus, there was no clear evidence for any qualitative difference between nicotine cues established with different PIs. However, the PI influenced quantitative aspects of the nicotine cue in a marked and complex manner. Increasing the PI during training produced a two- to three-fold decrease in the ED(50), whereas increasing the PI during testing produced a two- to three-fold increase in the ED(50). This shows that the effects of changing the PIs during training and testing were similar in magnitude but opposite in direction. These changes in ED(50) values can be explained by pharmacokinetic considerations in conjunction with knowledge of the role of training dose in the discrimination of nicotine. The quantitative sensitivity of the drug discrimination procedure can be substantially influenced by the choice of temporal parameters used in training and testing.

Discriminative stimulus effects of nicotine in rats trained under different schedules of reinforcement

There have been few comparisons between different schedules of reinforcement for establishing drugs as discriminative stimuli. Fixed-ratio (FR) 10 and tandem variable-interval 1-min FR-10 schedules have been compared directly in a conventional, nicotine-saline discrimination paradigm with food reinforcement in rats. The discrimination was acquired rapidly under both schedules, with stimulus control by nicotine (0.1 mg/kg SC) being very slightly superior under the FR schedule. In 5-min extinction tests with nicotine, rats maintained under the FR schedule yielded a clear dose-response curve with a bar-selection (quantal) index; in these rats, discrimination of nicotine appeared generally poor, and dose-response curves were shallow, when the percentage of drug-appropriate responding (quantitative index) was calculated. In contrast, rats under the tandem schedule yielded clear dose-response data with both indices. In tests with (+)-amphetamine full generalization was obtained with both schedules, and with both quantitative and quantal indices. Tests of generalization to morphine were negative regardless of the training schedule or index employed. In rats under the FR-10 schedule, overall response rates declined both within and across extinction tests; the relatively high rates of responding maintained by the tandem schedule were more sensitive to the response rate-decreasing effects of morphine and amphetamine. The results confirm that orderly data may be obtained with either a FR or a tandem schedule provided that an appropriate index of discriminative response is employed. The results generally support the validity of current practices, and there will probably be no marked differences between conclusions depending on which schedule is used.