Examination of GABAergic and dopaminergic compounds in the acquisition of nicotine-conditioned hyperactivity in rats

D2 receptor antagonism enhances cocaine-induced behavioral sensitization in female, but not male Japanese quail ( Coturnix japonica )

Sex differences in cocaine-induced behaviors are well established. In rodents, females show enhanced locomotion to cocaine over multiple trials compared with males, a behavioral response known as sensitization. Estradiol enhances cocaine-induced sensitization in female rats by agonizing dopaminergic activity within the brain. In female quail, cocaine does not increase locomotion regardless of increased estradiol. A higher D2:D1 dopamine receptor ratio in quail compared with rodents may explain this sex and species difference. The goal of the present work was to investigate the role of D2 receptors in cocaine-induced locomotion and sensitization in Japanese quail and to determine whether a greater D2 receptor availability contributed to the lack of cocaine-induced sensitization in female quail found in previous studies. Male and female quail were administered 0, 0.03, 0.05, or 0.07 mg/kg of eticlopride (Eti) followed by 10 mg/kg of cocaine or saline then immediately placed in open-field chambers. Distance traveled was recorded for 30 min daily for 7 days. In female quail, cocaine-induced sensitization was observed with 0.03 or 0.05 mg/kg Eti, but not in cocaine-only females. In male quail, cocaine-induced sensitization was observed similar to previous research. However, Eti did not enhance cocaine-induced locomotion or produce sensitization in male quail. The D2 receptor likely mediates cocaine's motor stimulating effects in quail. In females, this effect is more pronounced. Since high D2 availability is protective against stimulant abuse, Japanese quail may be a useful model for investigating the role of the D2 receptor in cocaine addiction, but further research is needed.

Extending the Role of Associative Learning Processes in Nicotine Addiction

Compulsive smoking is a worldwide public health problem. Although research has confirmed the importance of associative learning processes in nicotine addiction, therapies targeting nicotine-associated cues still have a high relapse rate. Most theories conceptualize nicotine as an ‘outcome’ that reinforces behaviors and/or changes the affective value of stimuli. Albeit important, this view does not capture the complexity of associative processes involved in nicotine addiction. For example, nicotine serves as a conditional stimulus acquiring new appetitive/affective properties when paired with a non-drug reward. Also, nicotine functions as an occasion setter that participates in higher-order associative processes that likely permit a more pervasive influence of conditioned cues that are resistant to typically cue-exposure therapy techniques. Finally, nicotine appears to amplify the salience of other stimuli that have some incentive value resulting in enhanced nicotine selfadministration and conditioned reinforcement processes. Future smoking intervention strategies should take into consideration these additional associative learning processes.

Nicotine-induced locomotor sensitization: pharmacological analyses with candidate smoking cessation aids

There are a number of approved therapeutics for the management of alcohol dependence, which might also convey the potential as smoking cessation aids. The present study investigated the effect of a few of these therapeutics and potential candidates (non-peptide vasopressin V1b antagonists) on the expression of nicotine-induced behavioral sensitization in Wistar rats. The following compounds were included in this evaluation: rimonabant, bupropion, topiramate, acamprosate, naltrexone, mecamylamine, nelivaptan (SSR-149415, V1b antagonist) and two novel V1b antagonists. Following the development of nicotine-induced locomotor sensitization and a withdrawal period, the expression of sensitization was assessed in the presence of one of the examined agents given 30 minutes prior to the nicotine challenge injection. Acamprosate, naltrexone, rimonabant, mecamylamine, nelivaptan and V1b antagonist 'compound 2' significantly antagonized the expression of nicotine-induced sensitization. Whereas topiramate showed a trend for effects, the V1b antagonist 'compound 1' did not show any significant effects. Bupropion failed to block sensitization but increased activity alone and was therefore tested in development and cross-sensitization studies. Taken together, these findings provide pre-clinical evidence that these molecules attenuated the expression of nicotine-induced sensitization and should be further investigated as putative treatments for nicotine addiction. Moreover, V1b antagonists should be further investigated as a potential novel smoking cessation aid.

The effect of varenicline on the development and expression of nicotine-induced behavioral sensitization and cross-sensitization in rats

The present study focused on the evaluation of behavioral sensitization and cross-sensitization induced by nicotine and varenicline in rats. Furthermore, it examined the influence of varenicline, a partial alpha4beta2 nicotinic receptor agonist, on nicotine-induced sensitization. To assess the development of behavioral sensitization, rats were chronically treated with vehicle, varenicline (0.03-3.0 mg/kg), nicotine (0.4 mg/kg) or combinations for 5 days and locomotor activity was measured. The expression of sensitization was assessed following a withdrawal period (17-26 days). The present results confirmed previous data showing the development and expression of nicotine-induced sensitization of locomotor activity in the rat. Varenicline did not induce sensitization on its own. When varenicline and nicotine were repeatedly administered sequentially, varenicline blocked the development and expression of nicotine-induced sensitization. Acute varenicline blocked the expression of nicotine-induced sensitization in a dose-dependent manner. Acute varenicline did not significantly increase locomotor activity, nor did it attenuate nicotine-induced sensitization. However, varenicline did cross-sensitize to the effects of nicotine, and vice versa. The present study showed that varenicline produced a dose-dependent bidirectional cross-sensitization with nicotine. Taken together, these findings provide pre-clinical evidence that varenicline is able to attenuate the effects of nicotine, yet simultaneously 'substitutes' for the effects of nicotine in the rat. Longitudinal studies would be needed to see if similar effects are seen in the clinical setting, and whether such effects contribute to the actions of varenicline as a smoking cessation aid.

The incentive amplifying effects of nicotine are reduced by selective and non-selective dopamine antagonists in rats

Nicotine is a psychomotor stimulant with 'reinforcement enhancing' effects--the actions of nicotine in the brain increase responding for non-nicotine rewards. We hypothesized that this latter effect of nicotine depends on increased incentive properties of anticipatory cues; consistent with this hypothesis, multiple laboratories have reported that nicotine increases sign tracking, i.e. approach to a conditioned stimulus (CS), in Pavlovian conditioned-approach tasks. Incentive motivation and sign tracking are mediated by mesolimbic dopamine (DA) transmission and nicotine facilitates mesolimbic DA release. Therefore, we hypothesized that the incentive-promoting effects of nicotine would be impaired by DA antagonists. To test this hypothesis, separate groups of rats were injected with nicotine (0.4mg/kg base) or saline prior to Pavlovian conditioning sessions in which a CS (30s illumination of a light or presentation of a lever) was immediately followed by a sweet reward delivered in an adjacent location. Both saline and nicotine pretreated rats exhibited similar levels of conditioned approach to the reward location (goal tracking), but nicotine pretreatment significantly increased approach to the CS (sign tracking), regardless of type (lever or light). The DAD1 antagonist SCH-23390 and the DAD2/3 antagonist eticlopride reduced conditioned approach in all rats, but specifically reduced goal tracking in the saline pretreated rats and sign tracking in the nicotine pretreated rats. The non-selective DA antagonist flupenthixol reduced sign-tracking in nicotine rats at all doses tested; however, only the highest dose of flupenthixol reduced goal tracking in both nicotine and saline groups. The reductions in conditioned approach behavior, especially those by SCH-23390, were dissociated from simple motor suppressant effects of the antagonists. These experiments are the first to investigate the effects of dopaminergic drugs on the facilitation of sign-tracking engendered by nicotine and they implicate dopaminergic systems both in conditioned approach as well as the incentive-promoting effects of nicotine.

Anxiety status affects nicotine- and baclofen-induced locomotor activity, anxiety, and single-trial conditioned place preference in male adolescent rats

Adolescents have an increased vulnerability to nicotine and anxiety may play a role in the development of nicotine abuse. One possible treatment for anxiety disorders and substance abuse is the GABAB agonist, baclofen. The aim of the present study was to determine the effect of anxiety-like behavior on single-trial nicotine conditioned place preference in adolescent rats, and to assess the action of baclofen. Baclofen was shown to have effects on locomotor and anxiety-like behavior in rats divided into high-anxiety and low-anxiety groups. Baclofen decreased locomotor behavior in high-anxiety rats. Baclofen alone failed to produce differences in anxiety-like behavior, but nicotine and baclofen + nicotine administration were anxiolytic. High- and low-anxiety groups also showed differences in single-trial nicotine-induced place preference. Only high-anxiety rats formed place preference to nicotine, while rats in the low-anxiety group formed no conditioned place preference. These results suggest that among adolescents, high-anxiety individuals are more likely to show preference for nicotine than low-anxiety individuals.

An investigation of bupropion substitution for the interoceptive stimulus effects of nicotine

Although the exact mechanism that makes bupropion hydrochloride (Zyban) effective as a smoking cessation aid has not been fully elucidated, studies have found that bupropion and nicotine share behavioural and neurophysiological properties suggesting that bupropion might serve as a substitute for nicotine. In fact, bupropion prompts nicotine-appropriate responding in operant and Pavlovian drug discrimination studies with rats. A majority of the literature examining this substitution pattern has been done with an operant paradigm. The present research extended this literature by further characterising the behavioural and neuropharmacological properties underlying the substitution for a nicotine conditioned stimulus (CS). Examination of the dose-effect function and temporal dynamics of this substitution pattern showed that bupropion (20 mg/kg) produced conditioned responding similar to nicotine (0.4 mg base/kg) (ED(50) = 9.9 mg/kg) at 15 and 30 min after injection and partially substituted 5 and 60 min post-injection. Bupropion produced a pattern of conditioned responding similar to nicotine during a 60-min extinction test. Additionally, it has been hypothesised that bupropion and nicotine have an overlapping dopaminergic mechanism. We tested the effects of bupropion pretreatment, the nicotine dose-effect function and the ability of dopamine antagonist to block the substitution of bupropion for nicotine. Pretreatment with doses of bupropion that did not substitute for the nicotine stimulus (5 and 10 mg/kg) did not affect nicotine-conditioned responding; pretreatment with 20 mg/kg attenuated nicotine-evoked responding. Pretreatment with the dopamine antagonists SCH-23390 and eticlopride blocked the substitution. Finally, S,S-hydroxybupropion, the major metabolite of bupropion in humans, did not substitute for the nicotine CS.

Baclofen prevents drug-induced reinstatement of extinguished nicotine-seeking behaviour and nicotine place preference in rodents

The gamma-aminobutyric acid(GABA)-B receptor agonist baclofen is known to reduce drug intake in both animals and humans and to prevent reinstatement of cocaine-, opioid-, and alcohol-seeking in rats after a period of extinction, but its effect on nicotine reinstatement is unknown. This study investigated the effect of baclofen on nicotine-seeking reinstatement both using the extinction/reinstatement model of nicotine self-administration and conditioned place preference (CPP). Results showed that in rats previously trained to intravenously self-administer nicotine (30 microg/kg/inf) under a FR-1 schedule of reinforcement, acute nicotine (0.15 mg/kg) priming effectively reinstates nicotine-seeking behaviour following extinction. At doses used in this study (up to 2.5 mg/kg) baclofen alone did not affect locomotor activity and did not reinstate responding. However, baclofen dose-dependently attenuated drug-induced reinstatement of nicotine-seeking in rats. Moreover, baclofen (1.25 mg/kg) completely blocked nicotine-induced reinstatement of extinguished nicotine (0.3 mg/kg) CPP in mice. Altogether, our results showed that baclofen is able to antagonise reinstatement of nicotine-seeking and CPP triggered by nicotine primings, suggesting its potential clinical utility as an anti-relapse agent.

Effects of mecamylamine on nicotine-induced conditioned hyperactivity and sensitization in differentially reared rats

Rats reared in an enriched condition (EC) display less sensitization to nicotine than rats reared in an impoverished condition (IC). However, it is unknown what effect differential rearing has on nicotine-induced conditioned hyperactivity. The present study determined whether differential rearing affects nicotine-induced conditioned hyperactivity. This study also examined the effects of mecamylamine on conditioned hyperactivity and sensitization. EC, IC, and social condition (SC) rats were reared from 21 to 51 days of age before receiving repeated nicotine injections (.4 mg/kg) prior to 1-h locomotor sessions. Following the conditioned-hyperactivity test, rats received additional training sessions followed by a drug-free rest period before the sensitization test. Mecamylamine (1.0 mg/kg) was administered prior to the conditioned-hyperactivity test and sensitization test. Nicotine treatment resulted in sensitization and conditioned hyperactivity in all differential rearing groups. EC rats displayed less locomotor activity in response to nicotine than both IC and SC rats. Pretreatment with mecamylamine blocked the expression of conditioned hyperactivity only in EC and SC rats and attenuated sensitization in all three rearing groups. These findings suggest that environmental enrichment may alter nicotinic acetylcholine receptors during development and may be a protective factor in the initiation and relapse of smoking behavior.

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.

Molecular mechanisms underlying the motivational effects of nicotine

In addition to the primary rewarding properties of nicotine and the alleviation of withdrawal symptoms, cues associated with smoking are critical contributors to maintenance of smoking behavior. Nicotine-paired cues are also critical for precipitating relapse after smoking cessation. An accumulation of evidence suggests that repeated exposure to tobacco, including the primary psychoactive ingredient, nicotine, changes brain neurochemistry in a way that promotes the control that cues associated with smoking or other rewards have over behavior. This chapter will consider the neurochemical mechanisms underlying these neuroadaptations. Targeting these molecular alterations may provide novel treatments for smoking cessation.

Translational research in medication development for nicotine dependence

A major obstacle to the development of medications for nicotine dependence is the lack of animal and human laboratory models with sufficient predictive clinical validity to support the translation of knowledge from laboratory studies to clinical research. This Review describes the animal and human laboratory paradigms commonly used to investigate the pathophysiology of nicotine dependence, and proposes how their predictive validity might be determined and improved, thereby enhancing the development of new medications.

Acute stress and nicotine cues interact to unveil locomotor arousal and activity-dependent gene expression in the prefrontal cortex

BACKGROUND

This study examines the interactive effects of acute stress and nicotine-associated contextual cues on locomotor activity and activity-dependent gene expression in subregions of the prefrontal cortex.

METHODS

Locomotor activity of rats was measured in a context associated with either low-dose nicotine or saline administration with or without 5 minutes of pre-exposure to ferrets, a nonphysical stressor. After 45 minutes in the test environment, plasma corticosterone levels and mRNA levels of the immediate-early genes Arc, NGFI-B, and c-Fos in prefrontal and primary motor cortical subregions were measured.

RESULTS

Stress alone increased plasma corticosterone and prefrontal cortex gene expression. Low-dose nicotine cues had no effect on corticosterone levels nor did they elicit conditioned motor activation, and they caused minor elevations in gene expression. Stress and low-dose nicotine cues, however, interacted to elicit conditioned motor activation and further increases in early response gene expression in prefrontal but not in the primary motor cortical subregions.

CONCLUSIONS

Stress interacts with nicotine-associated cues to uncover locomotor arousal, a state associated with prefrontal neuronal activation and immediate early gene expression. Thus, in nicotine-experienced individuals, stress may be an important determinant of subjective reactivity and prefrontal cortex activation that occurs in response to nicotine-associated cues.

The selective dopamine D3 receptor antagonist SB-277011A reduces nicotine-enhanced brain reward and nicotine-paired environmental cue functions

Increasing evidence suggests that enhanced dopamine (DA) neurotransmission in the nucleus accumbens (NAc) may play a role in mediating the reward and reinforcement produced by addictive drugs and in the attentional processing of drug-associated environmental cues. The meso-accumbens DA system is selectively enriched with DA D3 receptors, a DA receptor subtype increasingly implicated in reward-related brain and behavioural processes. From a variety of evidence, it has been suggested that selective DA D3 receptor antagonism may be a useful pharmacotherapeutic approach for treating addiction. The present experiments tested the efficacy of SB-277011A, a selective DA D3 receptor antagonist, in rat models of nicotine-enhanced electrical brain-stimulation reward (BSR), nicotine-induced conditioned locomotor activity (LMA), and nicotine-induced conditioned place preference (CPP). Nicotine was given subcutaneously within the dose range of 0.25-0.6 mg/kg (nicotine-free base). SB-277011A, given intraperitoneally within the dose range of 1-12 mg/kg, dose-dependently reduced nicotine-enhanced BSR, nicotine-induced conditioned LMA, and nicotine-induced CPP. The results suggest that selective D3 receptor antagonism constitutes a new and promising pharmacotherapeutic approach to the treatment of nicotine dependence.

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.

Timing of conditioned responding in a nicotine locomotor conditioning preparation: manipulations of the temporal arrangement between context cues and drug administration

Using a locomotor conditioning preparation, we examined whether manipulating time between exposure to distinct environmental cues and nicotine administration affected conditioned responding. Rats that received nicotine (0.42 mg/kg base) immediately before placement in an environment for 30 min on eight separate occasions displayed hyperactivity relative to controls in a subsequent injection/drug-free test. This conditioned hyperactivity was weaker if nicotine was administered 15 min before environment exposure. Conditioning was not evidenced when nicotine was administered 15 min after placement or upon removal from the environment. In a follow-up experiment, rats received 45 min in the environment; nicotine was administered 15 min after placement. This group showed conditioning that was localized to the last two-thirds of a 45 min test indicating that a 15 min delay did not prevent conditioning given 30 min of environment/nicotine overlap. This apparent timing of conditioned responding was not due to increasing environment exposure to 45 min. Further, a state-dependent environmental familiarization account of locomotor hyperactivity during testing was eliminated by the finding that rats displayed temporally specific increases in activity on the test day despite the fact that the context was previously experienced without drug for 15 min on eight consecutive days.

Nicotine-conditioned locomotor sensitization in rats: assessment of the US-preexposure effect

In rats, stimulus-nicotine associations can be altered by preexposure to the nicotine US (unconditioned stimulus). This alteration differs with the conditioning preparation. In a conditioned taste avoidance preparation, preexposure to nicotine weakens conditioning. In contrast, nicotine preexposure enhances acquisition of a nicotine-conditioned place preference. No one has examined the effects of US preexposure on nicotine locomotor conditioning. In three separate experiments, we assessed the effects of nicotine preexposure on the subsequent expression of conditioned hyperactivity produced by a nicotine US. We found evidence for nicotine-conditioned locomotor sensitization in non-preexposed rats that received repeated pairings of a distinct context with the psychomotor effects of a 0.42 mg/kg dose of nicotine (free base). Conditioning was not observed at lower nicotine doses (0.18 and 0.11 mg/kg) in non-preexposed rats. Preexposure to the 0.42 and 0.18 mg/kg doses of nicotine (3 or 9 days) attenuated acute locomotor suppression and enhanced the development of locomotor sensitization to that same dose. Despite similar qualitative shifts in the locomotor profile induced by preexposure to the nicotine US, conditioned hyperactivity was only altered after 3 or 9 days of preexposure at the 0.18 mg/kg dose. Thus, similar to place conditioning, nicotine preexposure can enhance the subsequent effectiveness of the nicotine US in a locomotor conditioning preparation.

Effects of chronic caffeine pre-exposure on conditioned and unconditioned psychomotor activity induced by nicotine and amphetamine in rats

Three experiments examined the effects of chronic pre-exposure to caffeine on the subsequent conditioned and unconditioned locomotor activating effects of nicotine or amphetamine in rats. Rats were given daily intraperitoneal injections of caffeine anhydrous (0, 10 or 30 mg/kg base) for 30 days. Conditioning (environment-drug pairings) began after the last day of caffeine pre-exposure. Pre-exposure to 30 mg/kg of caffeine enhanced the acute and chronic locomotor effects of amphetamine (0.5 mg/kg). A similar enhancement of activity was not seen with the high (0.421 mg/kg base) or low dose (0.175 mg/kg) of nicotine. In a drug-free test, the distinct environment paired with amphetamine and the high dose of nicotine evoked increases in activity relative to controls. Caffeine pre-exposure did not affect expression of this conditioned hyperactivity. These effects of caffeine pre-exposure on amphetamine-induced activity could not be attributed to non-specific effects of caffeine.