Modafinil reinstates a cocaine conditioned place preference following extinction in rats

Psychostimulant Use Disorder, an Unmet Therapeutic Goal: Can Modafinil Narrow the Gap?

The number of individuals affected by psychostimulant use disorder (PSUD) has increased rapidly over the last few decades resulting in economic, emotional, and physical burdens on our society. Further compounding this issue is the current lack of clinically approved medications to treat this disorder. The dopamine transporter (DAT) is a common target of psychostimulant actions related to their use and dependence, and the recent availability of atypical DAT inhibitors as a potential therapeutic option has garnered popularity in this research field. Modafinil (MOD), which is approved for clinical use for the treatment of narcolepsy and sleep disorders, blocks DAT just like commonly abused psychostimulants. However, preclinical and clinical studies have shown that it lacks the addictive properties (in both behavioral and neurochemical studies) associated with other abused DAT inhibitors. Clinical availability of MOD has facilitated its off-label use for several psychiatric disorders related to alteration of brain dopamine (DA) systems, including PSUD. In this review, we highlight clinical and preclinical research on MOD and its R-enantiomer, R-MOD, as potential medications for PSUD. Given the complexity of PSUD, we have also reported the effects of MOD on psychostimulant-induced appearance of several symptoms that could intensify the severity of the disease (i.e., sleep disorders and impairment of cognitive functions), besides the potential therapeutic effects of MOD on PSUD.

Modafinil potentiates cocaine self-administration by a dopamine-independent mechanism: possible involvement of gap junctions

Modafinil and methylphenidate are medications that inhibit the neuronal reuptake of dopamine, a mechanism shared with cocaine. Their use as "smart drugs" by healthy subjects poses health concerns and requires investigation. We show that methylphenidate, but not modafinil, maintained intravenous self-administration in Sprague-Dawley rats similar to cocaine. Both modafinil and methylphenidate pretreatments potentiated cocaine self-administration. Cocaine, at self-administered doses, stimulated mesolimbic dopamine levels. This effect was potentiated by methylphenidate, but not by modafinil pretreatments, indicating dopamine-dependent actions for methylphenidate, but not modafinil. Modafinil is known to facilitate electrotonic neuronal coupling by actions on gap junctions. Carbenoxolone, a gap junction inhibitor, antagonized modafinil, but not methylphenidate potentiation of cocaine self-administration. Our results indicate that modafinil shares mechanisms with cocaine and methylphenidate but has a unique pharmacological profile that includes facilitation of electrotonic coupling and lower abuse liability, which may be exploited in future therapeutic drug design for cocaine use disorder.

Role of venlafaxine in relapse to methamphetamine seeking. Potential treatment option for drug dependence

OBJECTIVES

To investigate the effects of venlafaxine (VEN) on the relapse of methamphetamine (METH)-induced conditioned place preference (CPP) in rats.

METHODS

This study was conducted at the Department of Pharmacology and Toxicology, and the Addiction and Neuroscience Research Unit, College of Pharmacy, Taif University, Taif, Kingdom of Saudi Arabia between May  2017 and October 2018. A total of 32 male Wistar rats were used in this study. Rats were divided to receive either METH or saline during the conditioning phase. Following the acquisition of METH-induced CPP, the animals were randomly divided to receive saline or VEN for 21 days instead of METH during the extinction training. The reinstatement was initiated by re-exposure to a single dose of METH to test any anti-reinstatement effects of VEN. Results: Two-way repeated measures Analysis of Variance (ANOVA) (time × chamber) was used to analyze time spent in conditioning chambers, while distance traveled was analyzed using one-way ANOVA. Saline or VEN, when administered alone, did not affect CPP or locomotor activity results. A priming intraperitoneal injection of METH reinstated CPP in the animals treated with saline during the extinction phase. Interestingly, VEN treatment blocked METH-induced CPP.

CONCLUSION

Venlafaxine effects on the reinstatement of METH-induced CPP are not likely due to nonspecific effects on locomotor activity. This beneficial effect of VEN on relapse to METH-induced CPP could be due to its antidepressant effects. Venlafaxine can thus be a potential therapeutic option in the treatment of relapse to METH-seeking behaviors.

Participation of Dopamine D1 and D2 Receptors in the Rapid-Onset Behavioral Sensitization to Modafinil

Studies on the abuse potential of modafinil, a psychostimulant-like drug used to treat narcolepsy, are still controversial. While some studies claim no potential for abuse, increasing evidence suggests that modafinil induces abuse-related effects, including rapid-onset behavioral sensitization (i.e., a type of sensitization that develops within hours from the drug priming administration). The rapid-onset sensitization paradigm is a valuable tool to study the neuroplastic changes that occur quickly after drug administration, and shares neuroadaptations with drug abuse in humans. However, the mechanisms involved in the rapid-onset behavioral sensitization induced by modafinil are uncertain. Our aim was to investigate the possible involvement of dopamine D1 and D2 receptors on acute modafinil-induced hyperlocomotion and on the induction and expression of rapid-onset behavioral sensitization induced by modafinil in male Swiss mice. Treatment with the D1 receptor antagonist SCH 23390 or the D2 receptor antagonist sulpiride attenuated the acute modafinil-induced hyperlocomotion in a dose-dependent manner. Pretreatment with either antagonist before the priming injection of modafinil prevented the development of sensitization in response to a modafinil challenge 4 h later. However, only SCH 23390 decreased the expression of modafinil-induced rapid-onset behavioral sensitization. Taken together, the present findings provide evidence of the participation of D1 and D2 receptors on the development of rapid-onset behavioral sensitization to modafinil, and point to a prominent role of D1 receptors on the expression of this phenomenon.

Modafinil Induces Rapid-Onset Behavioral Sensitization and Cross-Sensitization with Cocaine in Mice: Implications for the Addictive Potential of Modafinil

There is substantial controversy about the addictive potential of modafinil, a wake-promoting drug used to treat narcolepsy, proposed as pharmacotherapy for cocaine abuse, and used indiscriminately by healthy individuals due to its positive effects on arousal and cognition. The rapid-onset type of behavioral sensitization (i.e., a type of sensitization that develops within a few hours from the drug priming administration) has been emerged as a valuable tool to study binge-like patterns of drug abuse and the neuroplastic changes that occur quickly after drug administration that ultimately lead to drug abuse. Our aim was to investigate the possible development of rapid-onset behavioral sensitization to modafinil and bidirectional rapid-onset cross-sensitization with cocaine in male Swiss mice. A priming injection of a high dose of modafinil (64 mg/kg) induced rapid-onset behavioral sensitization to challenge injections of modafinil at the doses of 16, 32, and 64 mg/kg, administered 4 h later. Furthermore, rapid-onset cross-sensitization was developed between modafinil and cocaine (64 mg/kg modafinil and 20 mg/kg cocaine), in a bidirectional way. These results were not due to residual levels of modafinil as the behavioral effects of the priming injection of modafinil were no longer present and modafinil plasma concentration was reduced at 4 h post-administration. Taken together, the present findings provide preclinical evidence that modafinil can be reinforcing per se and can enhance the reinforcing effects of stimulants like cocaine within hours after administration.

Sex differences in dopamine binding and modafinil conditioned place preference in mice

BACKGROUND

Studies in humans and rodents have demonstrated under certain conditions some reinforcing properties of modafinil, a drug being examined clinically for its potential to treat psychostimulant abuse. However, the majority of rodent studies examining the abuse potential of modafinil have used high doses that may not be clinically relevant. In fact, recent work has indicated that doses similar to those administered to humans are not reinforcing in mice.

METHODS

The current study examined sex differences in the ability of low-dose modafinil (0.75mg/kg, IP) to induce a conditioned place preference in mice, and assessed sex-dependent alterations in dopamine D1, D2 and DAT binding sites in reward-related regions in naïve and modafinil-treated mice.

RESULTS

Low-dose modafinil failed to induce a conditioned place preference in male mice, while female mice demonstrated a significant modafinil place preference. Several dopamine binding differences were also detected in naïve and modafinil-treated mice, including sex differences in D1 and D2 availability in reward-related regions, and are discussed in relation to sex-dependent differences in the reinforcing effects of modafinil and psychostimulants in general.

CONCLUSIONS

These findings implicate sex differences in the reinforcing properties of modafinil in mice, and indicate that clinical evaluation of the sex dependence of the reinforcing properties of modafinil in humans is warranted.

Preclinical evaluation of the abuse potential of Pitolisant, a histamine H₃ receptor inverse agonist/antagonist compared with Modafinil

BACKGROUND AND PURPOSE

Pitolisant, a histamine H₃ receptor inverse agonist/antagonist is currently under Phase III clinical trials for treatment of excessive daytime sleepiness namely in narcoleptic patients. Its drug abuse potential was investigated using in vivo models in rodents and monkeys and compared with those of Modafinil, a psychostimulant currently used in the same indications.

EXPERIMENTAL APPROACH

Effects of Pitolisant on dopamine release in the nucleus accumbens, on spontaneous and cocaine-induced locomotion, locomotor sensitization were monitored. It was also tested in three standard drug abuse tests i.e. conditioned place preference in rats, self-administration in monkeys and cocaine discrimination in mice as well as in a physical dependence model.

KEY RESULTS

Pitolisant did not elicit any significant changes in dopaminergic indices in rat nucleus accumbens whereas Modafinil increased dopamine release. In rodents, Pitolisant was without any effect on locomotion and reduced the cocaine-induced hyperlocomotion. In addition, no locomotor sensitization and no conditioned hyperlocomotion were evidenced with this compound in rats whereas significant effects were elicited by Modafinil. Finally, Pitolisant was devoid of any significant effects in the three standard drug abuse tests (including self-administration in monkeys) and in the physical dependence model.

CONCLUSIONS AND IMPLICATIONS

No potential drug abuse liability for Pitolisant was evidenced in various in vivo rodent and primate models, whereas the same does not seem so clear in the case of Modafinil.

Modafinil attenuates reinstatement of cocaine seeking: role for cystine-glutamate exchange and metabotropic glutamate receptors

Modafinil may be useful for treating stimulant abuse, but the mechanisms by which it acts to do so are unknown. Indeed, a primary effect of modafinil is to inhibit dopamine transport, which typically promotes rather than inhibits motivated behavior. Therefore, we examined the role of nucleus accumbens extracellular glutamate and the group II metabotropic glutamate receptor (mGluR2/3) in modafinil effects. One group of rats was trained to self-administer cocaine for 10 days and extinguished, then given priming injections of cocaine to elicit reinstatement. Modafinil (300 mg/kg, intraperitoneal) inhibited reinstated cocaine seeking (but did not alter extinction responding by itself), and this effect was prevented by pre-treatment with bilateral microinjections of the mGluR2/3 antagonist LY-341495 (LY) into nucleus accumbens core. No reversal of modafinil effects was seen after unilateral accumbens core LY, or bilateral LY in the rostral pole of accumbens. Next, we sought to explore effects of modafinil on extracellular glutamate levels in accumbens after chronic cocaine. Separate rats were administered non-contingent cocaine, and after 3 weeks of withdrawal underwent accumbens microdialysis. Modafinil increased extracellular accumbens glutamate in chronic cocaine, but not chronic saline-pre-treated animals. This increase was prevented by reverse dialysis of cystine-glutamate exchange or voltage-dependent calcium channel antagonists. Voltage-dependent sodium channel blockade partly attenuated the increase in glutamate, but mGluR1 blockade did not. We conclude that modafinil increases extracellular glutamate in nucleus accumbens from glial and neuronal sources in cocaine-exposed rats, which may be important for its mGluR2/3-mediated antirelapse properties.

Rhodiola rosea Impairs Acquisition and Expression of Conditioned Place Preference Induced by Cocaine

A novel approach to the treatment of adverse effects of drugs of abuse is one which makes use of natural products. The present study investigated the effect of Rhodiola rosea L. hydroalcoholic extract (RHO) on cocaine-induced hyperactivity and conditioned place preference (CPP) in mice. In a first experiment, mice received RHO (15, 20 or 25 mg/kg, IG), cocaine (25 mg/kg, i.p.) (COC), or a combination of both drugs (COC + RHO15, COC + RHO20, and COC + RHO25), and their locomotor activity was evaluated. In a second experiment, the effects of RHO on the acquisition, expression, and reinstatement of cocaine CPP (induced by drug priming or social defeat stress) were evaluated. RHO alone did not increase activity but potentiated the hyperactivity induced by cocaine. Rhodiola did not induce motivational effects by itself but attenuated the acquisition and expression of cocaine-induced CPP. Moreover, it was found that RHO did not block reinstatement. The results indicate that RHO is effective in reducing the rewarding properties of cocaine but is ineffective in preventing priming or stress-induced cocaine reinstatement. In light of these findings, the benefits of Rhodiola rosea L. as a treatment of cocaine addiction would seem to be limited.

Combined effects of modafinil and d-amphetamine in male Sprague-Dawley rats trained to discriminate d-amphetamine

Modafinil is a novel wake-promoting drug with FDA approval for the treatment of sleep-related disorders that has recently been investigated as a potential agonist replacement therapy for psychostimulant dependence. Previous research in animals and humans indicates modafinil has a lower abuse liability than traditional psychostimulants, although few studies have carefully assessed modafinil's stimulus properties in combination with other psychostimulants. The current study trained male Sprague-Dawley rats to discriminate subcutaneous injections of 0.3 mg/kg (n=8) or 1.0 mg/kg d-amphetamine (n=8) from saline under an FR 20 schedule of food reinforcement and substitution tests were administered with d-amphetamine (0.03-1.0 mg/kg, s.c.), modafinil (32-256 mg/kg, i.g.), and a low modafinil dose (32 mg/kg, i.g.) in combination with d-amphetamine (0.03-1.0 mg/kg, s.c.) to determine if these drugs have additive effects. The selective D2 dopamine agonist, PNU-91356A, was also tested as a positive control and ethanol and morphine were tested as negative controls. Results indicate that modafinil produced dose-dependent and statistically significant d-amphetamine-lever responding in both groups and nearly complete substitution in animals trained to discriminate 1.0 mg/kg d-amphetamine. Modafinil pretreatment slightly increased the discrimination of low d-amphetamine doses in animals trained to discriminate 0.3 mg/kg d-amphetamine. These results support previous findings that modafinil and d-amphetamine may have additive effects. In consideration of recent interests in modafinil as an agonist treatment for psychostimulant dependence, additional preclinical investigations utilizing other methodologies to examine modafinil in combination with other stimulants, such as behavioral sensitization paradigms or drug self-administration, may be of interest.

A preclinical evaluation of the discriminative and reinforcing properties of lisdexamfetamine in comparison to D-amfetamine, methylphenidate and modafinil

Lisdexamfetamine dimesylate, which consists of L-lysine covalently bound to D-amfetamine, is the first prodrug for treating ADHD. Its metabolic conversion to yield D-amfetamine by rate-limited, enzymatic hydrolysis is unusual because it is performed by peptidases associated with red blood cells. Other stimulants shown to be effective in managing ADHD include D-amfetamine, methylphenidate and modafinil. All have the potential for misuse or recreational abuse. The discriminative and reinforcing effects of these compounds were determined in rats using a 2-choice, D-amfetamine (0.5 mg/kg, i.p.)-cued drug-discrimination test, and by substitution for intravenous cocaine in self-administration. Lisdexamfetamine (0.5-1.5 mg/kg [D-amfetamine base], p.o.) generalised to saline when tested 15 min post-dosing, but dose-dependently generalised to d-amfetamine at 60 min. At 120 min, its D-amfetamine-like effects were substantially diminished. At 15 min, methylphenidate (3.0-10 mg/kg, p.o.) and D-amfetamine (0.1-1.5 mg/kg, p.o.) dose-dependently generalised to the intraperitoneal D-amfetamine cue. Switching to the intraperitoneal route reduced the interval required for lisdexamfetamine to be recognised as D-amfetamine-like, but did not alter its potency. Switching to intraperitoneal injection increased the potency of methylphenidate and D-amfetamine by 3.4× and 2.2×, respectively. Modafinil (50-200 mg/kg, i.p.) generalised partially, but not fully, to d-amfetamine. Methylphenidate (0.1, 0.3, 1.0 mg/kg/injection, i.v.) maintained robust self-administration at the 2 highest doses. Neither lisdexamfetamine (0.05, 0.15 or 0.5 mg/kg/injection [D-amfetamine base], i.v.) nor modafinil (0.166, 0.498 or 1.66 mg/kg/injection, i.v.) served as reinforcers. The results reveal important differences between the profiles of these stimulants. Lisdexamfetamine did not serve as a positive reinforcer in cocaine-trained rats, and although it generalised fully to D-amfetamine, its discriminative effects were markedly influenced by its unusual pharmacokinetics.

Pharmacotherapy of amphetamine-type stimulant dependence: an update

ISSUES

Methamphetamine- or amphetamine-type stimulants are the second most frequently used illicit drug worldwide, second only to cannabis. Behavioural treatments are efficacious, but their impact is limited underscoring the need for other treatment options, notably, pharmacotherapy.

APPROACH

A review of randomised controlled trials of pharmacotherapies for methamphetamine- or amphetamine-type stimulants was performed using PubMed and Google Scholar databases. Evidence for efficacy of medications is reported.

KEY FINDINGS

Clinical trials have yielded no broadly effective pharmacotherapy. Promising signals have been observed for methylphenidate, naltrexone, bupropion and mirtazapine in subgroups of patients in reducing stimulant use (e.g. patients with less severe dependence at baseline and men who have sex with men), though none has produced an unambiguous, replicable signal of efficacy.

IMPLICATIONS

Problems in Phase II trials, including high dropout rates, missing data and a lack of agreement on outcomes, complicate efforts to find a broadly effective pharmacotherapy for amphetamine-type stimulant disorders. Efforts to address these problems include calls for better validation of pharmacological target exposure, receptor binding and functional modulation. As well, there is a need for agreement in using findings from preclinical and early phases of the medication development process for selecting better pharmacotherapy candidates.

CONCLUSION

After over 20 years of efforts worldwide to develop a broadly effective medication for dependence on methamphetamine- or amphetamine-type stimulants, no candidate has emerged. This highlights the need for new compounds, consistent and stringent research methods, better integration between preclinical and clinical stages of medication development, and improved collaboration between government, industry and researchers.

R-modafinil (armodafinil): a unique dopamine uptake inhibitor and potential medication for psychostimulant abuse

BACKGROUND

(±)-Modafinil has piqued interest as a treatment for attention-deficit/hyperactivity disorder and stimulant dependence. The R-enantiomer of modafinil might have unique pharmacological properties that should be further investigated.

METHODS

(±)-Modafinil and its R-(-)- and S-(+)-enantiomers were synthesized and tested for inhibition of [(3)H] dopamine (DA) uptake and [(3)H]WIN 35428 binding in human dopamine transporter (DAT) wild-type and mutants with altered conformational equilibria. Data were compared with cocaine and the atypical DA uptake inhibitor, JHW 007. R- and S-modafinil were also evaluated in microdialysis studies in the mouse nucleus accumbens shell and in a cocaine discrimination procedure.

RESULTS

(±)-, R-, and S-modafinil bind to the DAT and inhibit DA uptake less potently than cocaine, with R-modafinil having approximately threefold higher affinity than its S-enantiomer. Molecular docking studies revealed subtle differences in binding modes for the enantiomers. R-modafinil was significantly less potent in the DAT Y156F mutant compared with wild-type DAT, whereas S-modafinil was affected less. Studies with the Y335A DAT mutant showed that the R- and S-enantiomers tolerated the inward-facing conformation better than cocaine, which was further supported by [2-(trimethylammonium)ethyl]-methanethiosulfonate reactivity on the DAT E2C I159C. Microdialysis studies demonstrated that both R- and S-modafinil produced increases in extracellular DA concentrations in the nucleus accumbens shell less efficaciously than cocaine and with a longer duration of action. Both enantiomers fully substituted in mice trained to discriminate cocaine from saline.

CONCLUSIONS

R-modafinil displays an in vitro profile different from cocaine. Future trials with R-modafinil as a substitute therapy with the potential benefit of cognitive enhancement for psychostimulant addiction are warranted.

Reinstatement of methamphetamine conditioned place preference in nicotine-sensitized rats

The current experiments examined the effects of repeated nicotine prior to acquisition, extinction, and reinstatement of methamphetamine-induced conditioned place preference (CPP). Methamphetamine-induced (METH; 0.25, 0.5, or 1 mg/kg, s.c.) CPP was established using separate groups of adult male Sprague-Dawley rats with an unbiased conditioning procedure. Following extinction of METH CPP, drug-primed reinstatement (0, 0.25, 0.5 or 1 mg/kg, s.c.) of METH CPP was assessed in order to determine whether METH-induced reinstatement depends on the METH dose used to induce CPP. In a second experiment, separate groups of rats received nicotine (NIC; 0 or 0.2 mg/kg, s.c.) for 7 days prior to undergoing METH (0 or 0.5 mg/kg, s.c.) conditioning, extinction, and drug-primed reinstatement. Results indicate that METH-primed reinstatement varied as a function of dose such that priming with the conditioning dose did not reinstate CPP, but reinstatement was observed following priming doses of METH that were either lower or higher than the conditioning dose. Prior NIC exposure had no effect on METH CPP, extinction, or reinstatement. Interestingly, at a METH dose (0.5 mg/kg) that did not induce reinstatement alone, acute NIC (0.2 mg/kg) in combination with METH induced reinstatement, suggesting that NIC produced a leftward shift in the dose-response effect of METH to reinstate CPP. These studies indicate that prior NIC exposure may not be necessary for enhancement of the rewarding effects of METH, in contrast to previous self-administration reports.

Interactions between modafinil and cocaine during the induction of conditioned place preference and locomotor sensitization in mice: implications for addiction

Modafinil is a wake-promoting drug effective at enhancing alertness and attention with a variety of approved and off-label applications. The mechanism of modafinil is not well understood but initial studies indicated a limited abuse potential. A number of recent publications, however, have shown that modafinil can be rewarding under certain conditions. The present study assessed the reinforcing properties of modafinil using conditioned place preference and locomotor sensitization in mice. Experiment 1 examined a high dose of modafinil (75 mg/kg) as well as its interactions with cocaine (15 mg/kg). Cocaine alone and modafinil co-administered with cocaine induced sensitization of locomotor activity; modafinil alone showed little or no locomotor sensitization. Animals given modafinil alone, cocaine alone, and modafinil plus cocaine exhibited a strong and roughly equivalent place preference. When tested for sensitization using a low challenge dose of modafinil, cross-sensitization was observed in all cocaine-pretreated mice. Experiment 2 examined a low dose of modafinil that is similar to the dose administered to humans and has been shown to produce cognitive enhancements in mice. Low dose modafinil (0.75 mg/kg) did not produce conditioned place preference or locomotor sensitization. Together, these results suggest that modafinil has the potential to produce reward, particularly in cocaine addicts, and should be used with caution. However, the typical low dose administered likely moderates these effects and may account for lack of addiction seen in humans.

Chronic modafinil effects on drug-seeking following methamphetamine self-administration in rats

Acute administration of the cognitive enhancing drug, modafinil (Provigil®), reduces methamphetamine (Meth) seeking following withdrawal from daily self-administration. However, the more clinically relevant effects of modafinil on Meth-seeking after chronic treatment have not been explored. Here, we determined the impact of modafinil on Meth-seeking after chronic daily treatment during extinction or abstinence following Meth self-administration. Rats self-administered intravenous Meth during daily 2-h sessions for 14 d, followed by extinction sessions or abstinence. During this period, rats received daily injections of vehicle, 30, or 100 mg/kg modafinil and were then tested for Meth-seeking via cue, Meth-primed, and context-induced reinstatement at early and late withdrawal time-points. We found that chronic modafinil attenuated relapse to a Meth-paired context, decreased conditioned cue-induced and Meth-primed reinstatement, and resulted in enduring reductions in Meth-seeking even after discontinuation of treatment. Additionally, we determined that only a very high dose of modafinil (300 mg/kg) during maintenance of self-administration had an impact on Meth intake. These results validate and extend clinical and preclinical findings that modafinil may be a viable treatment option for Meth addiction.

Individual differences are critical in determining modafinil-induced behavioral sensitization and cross-sensitization with methamphetamine in mice

Modafinil is a non-amphetaminic psychostimulant used therapeutically for sleep and psychiatric disorders. However, some studies indicate that modafinil can have addictive properties. The present study examined whether modafinil can produce behavioral sensitization in mice, an experience and drug-dependent behavioral adaptation, and if individual differences play a role in this process. We further tested context-related factors and cross-sensitization between modafinil and methamphetamine. Important individual differences in the behavioral sensitization of Swiss Albino mice were observed after repeated administration of 50 mg/kg modafinil (Experiment 1), or 1 mg/kg methamphetamine (Experiment 2). Only mice classified as sensitized subgroup developed clear behavioral sensitization to the drugs. After a withdrawal period, mice received challenges of modafinil (Experiment 1), or methamphetamine (Experiment 2) and locomotor activity was evaluated in the activity cages (previous context) and in the open field arena (new context) in order to evaluate the context dependency of behavioral sensitization. The expression of sensitization to modafinil, but not to methamphetamine, was affected by contextual testing conditions, since modafinil-sensitized mice only expressed sensitization in the activity cage, but not in the open field. Subsequently, locomotor cross-sensitization between methamphetamine and modafinil was assessed by challenging modafinil-pretreated mice with 1mg/kg methamphetamine (Experiment 1), and methamphetamine-pretreated mice with 50mg/kg modafinil (Experiment 2). We observed a symmetrical cross-sensitization between the drugs only in those mice that were classified as sensitized subgroup. Our findings indicate that repeated exposure to modafinil induces behavioral sensitization only in some animals by similar neurobiological, but not contextual, mechanisms to those of methamphetamine.

Pharmacotherapeutics directed at deficiencies associated with cocaine dependence: focus on dopamine, norepinephrine and glutamate

Much effort has been devoted to research focused on pharmacotherapies for cocaine dependence yet there are no FDA-approved medications for this brain disease. Preclinical models have been essential to defining the central and peripheral effects produced by cocaine. Recent evidence suggests that cocaine exerts its reinforcing effects by acting on multiple neurotransmitter systems within mesocorticolimibic circuitry. Imaging studies in cocaine-dependent individuals have identified deficiencies in dopaminergic signaling primarily localized to corticolimbic areas. In addition to dysregulated striatal dopamine, norepinephrine and glutamate are also altered in cocaine dependence. In this review, we present these brain abnormalities as therapeutic targets for the treatment of cocaine dependence. We then survey promising medications that exert their therapeutic effects by presumably ameliorating these brain deficiencies. Correcting neurochemical deficits in cocaine-dependent individuals improves memory and impulse control, and reduces drug craving that may decrease cocaine use. We hypothesize that using medications aimed at reversing known neurochemical imbalances is likely to be more productive than current approaches. This view is also consistent with treatment paradigms used in neuropsychiatry and general medicine.

Addictive potential of modafinil and cross-sensitization with cocaine: a pre-clinical study

Repeated or even a single exposure to drugs of abuse can lead to persistent locomotor sensitization, which is the result of an abundance of neuroplastic changes occurring within the circuitry involved in motivational behavior and is thought to play a key role in certain aspects of drug addiction. There is substantial controversy about the addictive potential of modafinil, a wake-promoting drug used to treat narcolepsy that is increasingly being used as a cognitive enhancer and has been proposed as a pharmacotherapy for cocaine dependence. Male mice were used to investigate the ability of modafinil to induce locomotor sensitization after repeated or single administration in mice. Bidirectional cross-sensitization with cocaine and modafinil-induced conditioned place preference were also evaluated. Both repeated and single exposure to moderate and high doses of modafinil produced a pronounced locomotor sensitization that cross-sensitized in a bidirectional way with cocaine. Remarkably, when cocaine and modafinil were repeatedly administered sequentially, their behavioral sensitization was additive. Supporting these behavioral sensitization data, modafinil produced a pronounced conditioned place preference in the mouse. Taken together, the present findings provide pre-clinical evidence for the addictive potential of modafinil. Our data also strongly suggest that similar neural substrates are involved in the psychomotor/rewarding effects of modafinil and cocaine.

Modafinil-induced conditioned place preference via dopaminergic system in mice

Modafinil, a psychostimulant, is used in the treatment of narcolepsy, shift work sleep disorder, and excessive daytime sleepiness associated with obstructive sleep apnea. Preclinical and clinical studies suggest that modafinil may have reinforcing effects. However, a possible rewarding property of modafinil has not been fully investigated. In this study, we assessed the potential rewarding property of modafinil using the conditioned place preference (CPP) paradigm in mice. Using radiolabeled ligands, we observed changes in dopamine, glutamate, and GABA receptor binding in the brains of mice after treatment with modafinil. Modafinil produced significant CPP in mice at an intraperitoneal (i.p.) dose of 125 mg kg⁻¹ and prevented normal body weight gain of mice in a dose-dependent manner. A significant reduction in normal body weight gain was observed when mice were administrated 125 mg kg⁻¹ modafinil. In addition, there were widespread changes in receptor binding in the brains of modafinil-treated mice; Dopamine D₁ binding was increased in the caudate putamen, the accumbens, and the substantia nigra, while dopamine D₂ binding was decreased in the caudate putamen and the accumbens. Dopamine transporter (DAT) binding was increased in the prefrontal cortex, the caudate putamen, and the nucleus accumbens. No changes were observed in NMDA and GABA(A) receptor binding. These data indicate that modafinil had a significant rewarding property and could be abused as a recreational drug. Dopamine systems may play a key role in the rewarding property of modafinil.

Modafinil blocks reinstatement of extinguished opiate-seeking in rats: mediation by a glutamate mechanism

Opiate addiction is characterized by high rates of relapse even after long periods of abstinence, requiring new relapse-prevention treatments that do not have abuse potential. Recently, clinical studies suggested that the wake-promoting drug modafinil might decrease relapse in cocaine addicts. In addition, group II metabotropic glutamate receptors (mGlu2/3R) have been suggested as a new therapeutic target for drug addiction. Here, we investigated the ability of modafinil to prevent the acute morphine to promote reinstatement of extinguished preference for morphine, and the involvement of mGlu2/3Rs in this effect. Conditioned place preference (CPP) for morphine was induced in Sprague-Dawley rats, followed by extinction training. Preference for the morphine-paired side was reinstated following extinction by a morphine-priming injection. The results of our study showed that modafinil (300 mg/kg, i.p., but not 100 mg/kg) 30 min before the morphine-priming injection blocked reinstatement of extinguished CPP. The anti-reinstatement effect of modafinil was completely prevented by pretreatment with the selective mGlu2/3 antagonist LY341495. Additional experiments indicated that modafinil alone did not produce a preference, and that modafinil did not alter the expression of morphine CPP or the cueing properties of morphine either 1 or 14 days after morphine CPP conditioning. These data reveal a novel mechanism for modafinil actions, a role for mGlu2/3 receptors in reinstatement of opiate-seeking, and a new therapeutic option to treat relapse in opiate addiction.

Modafinil effects on reinstatement of methamphetamine seeking in a rat model of relapse

RATIONALE

Modafinil (Provigil) is a wake-promoting drug characterized by cognitive enhancing abilities. Recent clinical data have supported the use of modafinil for treatment of chronic psychostimulant addiction and relapse prevention.

MATERIALS AND METHODS

We used an intravenous methamphetamine (meth) self-administration procedure to assess the dose-dependent effects of modafinil on reinstatement following abstinence and after extinction on conditioned-cue and meth-primed reinstatement of meth seeking.

RESULTS

Modafinil attenuated active lever responding in multiple reinstatement conditions-context-induced, conditioned cue, and meth prime. The most pronounced and consistent effect was on meth-primed reinstatement, and modafinil did not reinstate meth seeking when tested alone.

DISCUSSION

These findings support clinical findings in humans that modafinil may be an effective therapeutic agent for the prevention of relapse in abstinent meth users.

Dopamine transporter-related effects of modafinil in rhesus monkeys

RATIONALE

Modafinil is currently used as a treatment for daytime sleepiness.

OBJECTIVES

The objectives of this study were to explore the dopamine transporter (DAT)-related effects of modafinil on behavior and in vivo neurochemistry in rhesus monkeys (Macaca mulatta).

METHODS

The effects of modafinil (3.0-10 mg/kg, i.v.) were evaluated on locomotor activity, reinstatement of cocaine-maintained behavior, extracellular dopamine levels in the caudate nucleus, and DAT occupancy in the dorsal striatum. Eight subjects were fitted with a collar-mounted activity monitor to evaluate sleep-activity cycles, with 4 days of baseline recording preceding an injection of saline or modafinil (3.0-10 mg/kg). The effects of modafinil (3.0-10 mg/kg) and cocaine (0.3 mg/kg) on reinstatement of behavior that was previously maintained under a second-order schedule of i.v. cocaine delivery were tested in a separate group of subjects (n = 6). Finally, the effects of modafinil (3.0-10 mg/kg) on extracellular dopamine levels and DAT occupancy in vivo were characterized using microdialysis and positron emission tomography, respectively, in a within-subjects design (n = 4).

RESULTS

Modafinil significantly increased nighttime locomotor activity and reinstated cocaine-maintained behavior but did not affect daytime locomotor activity. Modafinil significantly increased striatal extracellular dopamine levels at a dose that resulted in DAT occupancy of 64.4% (putamen) and 60.2% (caudate).

CONCLUSION

The behavioral and in vivo dopaminergic effects of modafinil are consistent with the profile of a low potency DAT inhibitor and may indicate potential for abuse at high doses.