Reduction in methamphetamine induced sensitization and reinstatement after combined pergolide plus ondansetron treatment during withdrawal

Angiotensin II involvement in the development and persistence of amphetamine-induced sensitization: Striatal dopamine reuptake implications

Amphetamine (AMPH) exposure induces behavioural and neurochemical sensitization observed in rodents as hyperlocomotion and increased dopamine release in response to a subsequent dose. Brain Angiotensin II modulates dopaminergic neurotransmission through its AT1 receptors (AT1-R), positively regulating striatal dopamine synthesis and release. This work aims to evaluate the AT1-R role in the development and maintenance of AMPH-induced sensitization. Also, the AT1-R involvement in striatal dopamine reuptake was analysed. The sensitization protocol consisted of daily AMPH administration for 5 days and tested 21 days after withdrawal. An AT1-R antagonist, candesartan, was administered before or after AMPH exposure to evaluate the participation of AT1-R in the development and maintenance of sensitization, respectively. Sensitization was evaluated by locomotor activity and c-Fos immunostaining. Changes in dopamine reuptake kinetics were evaluated 1 day after AT1-R blockade withdrawal treatment, with or without the addition of AMPH in vitro. The social interaction test was performed as another behavioural output. Repeated AMPH exposure induced behavioural and neurochemical sensitization, which was prevented and reversed by candesartan. The AT1-R blockade increased the dopamine reuptake kinetics. Neither the AMPH administration nor the AT1-R blockade altered the performance of social interaction. Our results highlight the AT1-R's crucial role in AMPH sensitization. The enhancement of dopamine reuptake kinetics induced by the AT1-R blockade might attenuate the neuroadaptive changes that lead to AMPH sensitization and its self-perpetuation. Therefore, AT1-R is a prominent candidate as a target for pharmacological treatment of pathologies related to dopamine imbalance, including drug addiction and schizophrenia.

Intracerebroventricular microinjection of the 5-HT1F receptor agonist LY 344864 inhibits methamphetamine conditioned place preference reinstatement in rats

Relapse following a prolonged period of drug cessation is a key barrier in the treatment of methamphetamine (METH) addiction, for which pharmacological treatment exhibits little efficacy. Previous studies have suggested that this process involves alterations in levels of serotonin (5-HT) in the brain. Although the 5-HT1F receptor has been implicated in the reward pathway, its physiological functions remain unknown. In the present study, we examined the effect of the 5-HT1F agonist LY 344864 on the reinstatement of METH-seeking behavior in rats using a conditioned place preference (CPP) paradigm. The CPP paradigm was first used to determine the effective doses of LY and METH. Four groups were then conditioned with METH (5 mg/kg; i.p.), while the sham group received saline. METH-induced CPP was subsequently extinguished. On the 13th day of extinction, the rats received either METH (0, 1, or 2.5 mg/kg; i.p.) plus vehicle or priming METH plus LY (2 μg/5 μL; i.c.v.). On reinstatement day, preference scores were calculated as the difference in time spent in the drug-paired and vehicle-paired compartments. Rats conditioned with the lowest effective dose of METH (5 mg/kg) exhibited significant differences in pre- and post-testing preference scores. Preference scores were significantly higher in the saline + METH group than in the control group. Furthermore, preference scores were significantly higher in rats that had received priming METH treatment, and pre-treatment with LY significantly attenuated the reinstatement of METH-seeking behavior. These findings suggest that future studies should evaluate the therapeutic potential of 5-HT1F agonists for preventing relapse in individuals with METH addiction.

Abstinence to chronic methamphetamine switches connectivity between striatal, hippocampal and sensorimotor regions and increases cerebral blood volume response

Methamphetamine (meth), and other psychostimulants such as cocaine, present a persistent problem for society with chronic users being highly prone to relapse. We show, in a chronic methamphetamine administration model, that discontinuation of drug for more than a week produces much larger changes in overall meth-induced brain connectivity and cerebral blood volume (CBV) response than changes that occur immediately following meth administration. Areas showing the largest changes were hippocampal, limbic striatum and sensorimotor cortical regions as well as brain stem areas including the pedunculopontine tegmentum (PPTg) and pontine nuclei - regions known to be important in mediating reinstatement of drug-taking after abstinence. These changes occur concomitantly with behavioral sensitization and appear to be mediated through increases in dopamine D1 and D3 and decreases in D2 receptor protein and mRNA expression. We further identify a novel region of dorsal caudate/putamen, with a low density of calbindin neurons, that has an opposite hemodynamic response to meth than the rest of the caudate/putamen and accumbens and shows very strong correlation with dorsal CA1 and CA3 hippocampus. This correlation switches following meth abstinence from CA1/CA3 to strong connections with ventral hippocampus (ventral subiculum) and nucleus accumbens. These data provide novel evidence for temporal alterations in brain connectivity where chronic meth can subvert hippocampal - striatal interactions from cognitive control regions to regions that mediate drug reinstatement. Our results also demonstrate that the signs and magnitudes of the induced CBV changes following challenge with meth or a D3-preferring agonist are a complementary read out of the relative changes that occur in D1, D2 and D3 receptors using protein or mRNA levels.

Identification of Treatment Targets in a Genetic Mouse Model of Voluntary Methamphetamine Drinking

Methamphetamine has powerful stimulant and euphoric effects that are experienced as rewarding and encourage use. Methamphetamine addiction is associated with debilitating illnesses, destroyed relationships, child neglect, violence, and crime; but after many years of research, broadly effective medications have not been identified. Individual differences that may impact not only risk for developing a methamphetamine use disorder but also affect treatment response have not been fully considered. Human studies have identified candidate genes that may be relevant, but lack of control over drug history, the common use or coabuse of multiple addictive drugs, and restrictions on the types of data that can be collected in humans are barriers to progress. To overcome some of these issues, a genetic animal model comprised of lines of mice selectively bred for high and low voluntary methamphetamine intake was developed to identify risk and protective alleles for methamphetamine consumption, and identify therapeutic targets. The mu opioid receptor gene was supported as a target for genes within a top-ranked transcription factor network associated with level of methamphetamine intake. In addition, mice that consume high levels of methamphetamine were found to possess a nonfunctional form of the trace amine-associated receptor 1 (TAAR1). The Taar1 gene is within a mouse chromosome 10 quantitative trait locus for methamphetamine consumption, and TAAR1 function determines sensitivity to aversive effects of methamphetamine that may curb intake. The genes, gene interaction partners, and protein products identified in this genetic mouse model represent treatment target candidates for methamphetamine addiction.

Molecular changes in the medial prefrontal cortex and nucleus accumbens are associated with blocking the behavioral sensitization to cocaine

Previous studies have demonstrated that cocaine-induced behavioral sensitization is associated with persistent functional and structural alterations in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc); however, the molecular mechanisms underlying these changes have not been elucidated. In this study, the behavioral sensitization to cocaine was established in Sprague Dawley rats and was measured by locomotion and behavioral rating. The brain tissue homogenization was used for measuring the level of brain-derived neurotrophic factor (BDNF), the expression and activity of integrin-linked kinase (ILK), level of protein kinase B (Akt) phosphorylation at serine 473 and threonine 308, and the expression of p75(NTR), TrkA, and TrkB protein. The Results showed that cocaine sensitization was associated with increased BDNF, ILK activity, phospho-Akt Ser(473), p75(NTR), and TrkB protein levels in the mPFC and NAc core. The combination of pergolide and ondansetron normalized not only behavioral sensitization, but also the increases in these molecular markers. Dual immunofluoresence staining showed that ILK expression is co-distributed with p75(NTR) and TrkA expression in both the mPFC and NAc core. Results suggested that the BDNF-TrkA/p75(NTR)-ILK-Akt signaling pathway may be active in cocaine sensitization and associated neural plasticity in the mPFC and NAc core.

The role of serotonin in drug use and addiction

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

Mirtazapine, and mirtazapine-like compounds as possible pharmacotherapy for substance abuse disorders: evidence from the bench and the bedside

Understanding substance use disorders (SUDs) and the problems associated with abstinence has grown in recent years. Nonetheless, highly efficacious treatment targeting relapse prevention has remained elusive, and there remains no FDA-approved pharmacotherapy for psychostimulant dependence. Preclinical and clinical investigations assessing the utility of classical antidepressants, which block monoamine reuptake, show mixed and often contradictory results. Mirtazapine (Remeron®) is a unique FDA-approved antidepressant, with negligible affinity for reuptake proteins, indirectly augments monoamine transmission presumably through antagonist activity at multiple receptors including the norepinephrine (NE)(α2), and serotonin (5-HT)(2A/C) receptors. Historically, mirtazapine was also considered to be a 5-HT(2C) antagonist, but recent evidence indicates that mirtazapine is an inverse agonist at this receptor subtype. Suggesting a promising role for mixed-action serotonergic drugs for addiction pharmacotherapy, mirtazapine attenuates psychostimulant-induced behaviors in several rodent models of substance abuse, and antagonizes methamphetamine-induced biochemical and electrophysiological alterations in rats. Preclinical findings are confirmed through published case studies documenting successful outcomes with mirtazapine therapy across a number of SUDs. To date, a large scale clinical trial assessing the utility of mirtazapine in substance abuse pharmacotherapy has yet to be conducted. However, as reviewed here, accumulating preclinical and clinical evidence argues that mirtazapine, or compounds that emulate aspects of its pharmacological profile, may prove useful in helping treat addictions.

Pharmacologically-mediated reactivation and reconsolidation blockade of the psychostimulant-abuse circuit: a novel treatment strategy

Psychostimulant abuse continues to present legal, socioeconomic and medical challenges as a primary psychiatric disorder, and represents a significant comorbid factor in major psychiatric and medical illnesses. To date, monotherapeutic drug treatments have not proven effective in promoting long-term abstinence in psychostimulant abusers. In contrast to clinical trials utilizing monotherapies, combinations of dopamine (DA) agonists and selective 5-HT(3), 5HT(2A/2C), or NK(1) antagonists have shown robust efficacy in reversing behavioral and neurobiological alterations in animal models of psychostimulant abuse. One important temporal requirement for these treatments is that the 5-HT or NK(1) receptor antagonist be given at a critical time window after DA agonist administration. This requirement may reflect a necessary dosing regimen towards normalizing underlying dysfunctional neural circuits and "addiction memory" states. Indeed, chronic psychostimulant abuse can be conceptualized as a consolidated form of dysfunctional memory maintained by repeated drug- or cue-induced reactivation of neural circuit and subsequent reconsolidation. According to this concept, the DA agonist given first may reactivate this memory circuit, thereby rendering it transiently labile. The subsequent antagonist is hypothesized to disrupt reconsolidation necessary for restabilization, thus leading progressively to a therapeutically-mediated abolishment of dysfunctional synaptic plasticity. We propose that long-term abstinence in psychostimulant abusers may be achieved not only by targeting putative mechanistic pathways, but also by optimizing drug treatment regimens designed to disrupt the neural processes underlying the addicted state.

Ceftriaxone upregulates the glutamate transporter in medial prefrontal cortex and blocks reinstatement of methamphetamine seeking in a condition place preference paradigm

Glutamate signaling plays an essential role in drug-seeking behavior. Using reinstatement of conditioned place preference (CPP), we determined whether ceftriaxone, a β-lactam antibiotic known to increase the expression and activity of the glutamate transporter (EAAT₂) on glial cells, blocks methamphetamine-triggered reinstatement of CPP. Rats acquired methamphetamine CPP following 7 consecutive days of conditioning, during which each animal received pairings of alternating morning methamphetamine (2.5 mg/kg, IP) and afternoon saline (IP). Animals showing CPP were successfully extinguished with repeated twice daily saline administration over a 7-day period. Ceftriaxone (200 mg/kg, IP) was administered (vs. saline) once a day for 7 days during the extinction period. Upon successful extinction, animals received a single dose of methamphetamine (2.5 mg/kg, IP) for reinstatement and were tested for CPP one day later. Using real time PCR, EAAT₂ mRNA levels in the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) were quantified in response to ceftriaxone. Ceftriaxone blocked methamphetamine-triggered reinstatement of CPP and significantly increased EAAT₂ mRNA levels in the mPFC, with a trend towards significance in the NAc. In conclusion, Ceftriaxone modulated the expression of the glutamate transporter in a critical region of the cortico-striatal addiction circuitry and attenuated drug-seeking behavior in rats. Further research is needed to test the efficacy of compounds targeting the EAAT₂ in human methamphetamine-dependent users.

Evaluation of anticonvulsant and nootropic effect of ondansetron in mice

The role of serotonin receptors have been implicated in various types of experimentally induced seizures. Ondansetron is a highly selective 5-hydroxytryptamine 3 (5-HT3) receptor antagonist used as antiemetic agent for chemotherapy-, and radiotherapy-induced nausea and vomiting. The present study was carried out to examine the effect of ondansetron on electroshock, pentylenetetrazole (PTZ)-induced seizures and cognitive functions in mice. Ondansetron was administered intraperitoneally (i.p.) at doses of 0.5, 1.0 and 2.0 mg/kg (single dose) to observe its effect on the increasing current electroshock seizure (ICES) test and PTZ-induced seizure test. In addition, a chronic study (21 days) was also performed to assess the effects of ondansetron on electroshock-induced convulsions and cognitive functions. The effect on cognition was assessed by elevated plus maze and passive avoidance paradigms. Phenytoin (25 mg/kg, i.p.) was used as a standard anticonvulsant drug and piracetam (200 mg/kg) was administered as a standard nootropic drug. The results were compared with an acute study, wherein it was found that the administration of ondansetron (1.0 and 2.0 mg/kg) significantly raised the seizure-threshold current as compared to control group in the ICES test. Similar results were observed after chronic administration of ondansetron. In PTZ test, ondansetron in all the three tested doses failed to show protective effect against PTZ-induced seizure test. Administration of ondansetron for 21 days significantly decreased the transfer latency (TL) and prolonged the step-down latency (SDL). The results of present study suggest the anticonvulsant and memory-enhancing effect of ondansetron in mice.

Reversal of long-term methamphetamine sensitization by combination of pergolide with ondansetron or ketanserin, but not mirtazapine

Psychostimulant abuse represents a psychiatric disorder and societal concern that has been largely unamenable to therapeutic interventions. We have previously demonstrated that the 5-HT₃ antagonist ondansetron or non-selective 5-HT(₂A/₂C) antagonist ketanserin administered 3.5 h following daily pergolide, a non-selective DA agonist, reverses previously established cocaine sensitization. The present study was conducted to evaluate whether the same treatments or delayed pairing of pergolide with the antidepressant mirtazapine can also reverse consolidated methamphetamine (METH) behavioral sensitization. Sprague-Dawley rats received METH infusion via osmotic minipumps (25 mg/kg/day, s.c.) for 7 days, with accompanying daily injections of escalating METH doses (0-6 mg/kg, s.c.). This regimen takes into account the faster elimination of METH in rats, and is designed to replicate plasma METH concentrations with superimposed peak drug levels as observed during METH binging episodes in humans. Following a 7-day METH withdrawal, ondansetron (0.2 mg/kg, s.c.), ketanserin (1.0 mg/kg, s.c.), or mirtazapine (10mg/kg, i.p.) was administered 3.5 h after pergolide injections (0.1 mg/kg, s.c., qd) for 7 days. Behavioral sensitization as a model of METH abuse was assessed 14 days after the combination treatment cessation (i.e., day 28 of METH withdrawal) through an acute challenge with METH (0.5 mg/kg, i.p.). Pergolide combined with ondansetron or ketanserin reversed METH behavioral sensitization, but pergolide-mirtazapine combination was ineffective. The role of reactivation of addiction "circuit" by a non-selective DA agonist, and subsequent reconsolidation blockade through 5-HT₃ or 5-HT₂ antagonism in reversal of METH sensitization and treatment of METH addiction is discussed.

Treatment of the psychostimulant-sensitized animal model of schizophrenia

Behavioral sensitization to psychostimulants in rodents is associated with the alteration of dopaminergic neurotransmission, and has been proposed as a useful model of schizophrenia due to its progressively intensifying, easily relapsing, and long-lasting features. Pharmacological treatments that reverse the established sensitization may have potential therapeutic values for schizophrenia. The present aim is to review pharmacological treatments that induce the reversal of established sensitization to psychostimulants. In addition, we discuss possible mechanisms for the reversal of sensitization. Reversal of sensitization is induced by chronic dopamine D1 receptor agonism, D2 or D1/D2 receptor agonism combined with mild N-methyl-D-aspartate (NMDA) receptor antagonism or serotonin (5-HT(2A) or 5-HT(3) ) receptor antagonism, 5-HT(1A) receptor agonism, and 5-HT(2A) or 5-HT(3) receptor antagonism. Chronic treatments with these drugs likely adjust altered dopaminergic neurotransmission in sensitized animals. Especially, chronic dopamine D1 receptor agonism, which may adjust mesolimbic hyperdopaminergic and mesocortical hypodopaminergic functions in sensitized animals, is an attractive therapeutic approach for schizophrenia.

The glial cell modulator and phosphodiesterase inhibitor, AV411 (ibudilast), attenuates prime- and stress-induced methamphetamine relapse

Stress and renewed contact with drug (a "slip") have been linked to persisting relapse of methamphetamine abuse. Human brain microglial activation has been linked with methamphetamine abuse, and inhibitors of glial cell activation, certain phosphodiesterase (PDE) inhibitors, and glial cell derived neurotrophic factor (GDNF) have been reported to modulate drug abuse effects. Our objective was to determine whether the glial cell attenuator, 3-isobutyryl-2-isopropylpyrazolo-[1,5-a]pyridine (AV411, ibudilast), a non-selective PDE inhibitor and promoter of GDNF, could reduce stress- and methamphetamine prime-induced reinstatement of methamphetamine-seeking behavior. Male Long-Evans hooded rats were trained to lever press reinforced with 0.1 mg/kg i.v. methamphetamine infusion according to fixed-ratio 1 (FR1) reinforcement schedules during daily, 2-hour experimental sessions. After performance had stabilized, lever pressing was extinguished for 12 consecutive sessions and doses of 0 (vehicle), 2.5 and 7.5 mg/kg AV411 were then administered intraperitoneally b.i.d. on the last 2 days of extinction and then once on the testday to separate groups of 12 rats. During testing, the rats were given 15 min of intermittent footshock or a 1 mg/kg i.p. methamphetamine prime followed by a 2-hour reinstatement test session. AV411 significantly reduced response levels of footshock-induced (2.5 and 7.5 mg/kg) and prime-induced (7.5 mg/kg) reinstatement of extinguished methamphetamine-maintained responding. AV411 has properties consistent with the ability to attenuate relapse precipitated by stress and methamphetamine "slips" during abstinence. These results thus reinforce interest in atypical neurobiological mechanisms which could be exploited for developing novel medications for treating drug abuse disorders.

Systemic treatment with protein synthesis inhibitors attenuates the expression of cocaine memory

It has been proposed that a memory trace enters a labile phase each time it is retrieved. A reactivated memory relies on de novo protein synthesis to be faithfully reconsolidated and restored. Thus, in theory, a long-lasting and pathological memory associated with drug use may be disrupted by inhibiting its reconsolidation through use of protein synthesis inhibitors administered immediately following the memory retrieval. However, effective and efficient strategies to reactivate drug memory remained elusive. This study was undertaken to examine the effects of systemic cycloheximide and anisomycin treatment on the reconsolidation and maintenance of a reactivated cocaine-conditioned place preference (CPP) in mice using several strategies designed to reactivate the previously acquired memory. We found that anisomycin (50 mg/kg/injection) and cycloheximide (15 mg/kg/injection) administered immediately after the reactivation of cocaine-CPP ameliorated subsequent expression and maintenance of this memory. Likewise, when anisomycin and cycloheximide were administered immediately after additional cocaine and saline conditioning trials, the reactivated memory engendered by those extra training trials was also diminished. However, a similar anisomycin dosing regimen failed to affect subsequent expression of cocaine-CPP when additional cocaine conditioning trial was used in the absence of additional saline trial. Finally, cocaine and saline administration to mice in their home cages with or without anisomycin treatment had no effect on later cocaine-CPP expression. Taken together, these findings suggest that systemic treatment with protein synthesis inhibitors immediately after the reactivation of cocaine-CPP effectively diminished the reconsolidation and maintenance of such a cocaine memory. More importantly, reactivation of cocaine-CPP could be achieved by presentation of cocaine-conditioned cues as well as by administering additional cocaine and saline conditioning trials in a balanced fashion.

Anti-relapse medications: preclinical models for drug addiction treatment

Addiction is a chronic relapsing brain disease and treatment of relapse to drug-seeking is considered the most challenging part of treating addictive disorders. Relapse can be modeled in laboratory animals using reinstatement paradigms, whereby behavioral responding for a drug is extinguished and then reinstated by different trigger factors, such as environmental cues or stress. In this review, we first describe currently used animal models of relapse, different relapse triggering factors, and the validity of this model to assess relapse in humans. We further summarize the growing body of pharmacological interventions that have shown some promise in treating relapse to psychostimulant addiction. Moreover, we present an overview on the drugs tested in cocaine or methamphetamine addicts and examine the overlap of existing preclinical and clinical data. Finally, based on recent advances in our understanding of the neurobiology of relapse and published preclinical data, we highlight the most promising areas for future anti-relapse medication development.

Oxytocin decreases methamphetamine self-administration, methamphetamine hyperactivity, and relapse to methamphetamine-seeking behaviour in rats

There is emerging evidence that the neuropeptide oxytocin may be utilised as a treatment for various psychopathologies, including drug addictions. Here we used an animal model to assess whether oxytocin might be effective in the treatment of methamphetamine addiction. Sprague-Dawley rats were trained to lever press to intravenously self-administer methamphetamine under a progressive ratio schedule of reinforcement. Once responding had stabilised, one group of rats received escalating doses of oxytocin (0.001, 0.01, 0.1, 0.3, 1 mg/kg) administered intraperitoneally (IP) prior to daily self-administration tests, while other rats received vehicle. After these tests, lever-pressing was extinguished and the ability of methamphetamine primes (IP, 1 mg/kg) to reinstate responding was studied with and without co-administration of oxytocin (IP, 0.3 and 1 mg/kg). Results showed that oxytocin dose-dependently reduced responding for intravenous methamphetamine with an almost complete absence of responding at the highest oxytocin dose (1 mg/kg). Hyperactivity during methamphetamine self-administration was also dose-dependently reduced by oxytocin. Oxytocin (1 but not 0.3 mg/kg) also reduced the ability of methamphetamine to reinstate methamphetamine-seeking behaviour. In separate tests, oxytocin (IP, 0.3 and 1 mg/kg) robustly decreased the hyperactivity and rearing induced by methamphetamine challenge (IP, 1 mg/kg), producing activity levels similar to control animals. This study suggests that oxytocin may have a powerful inhibitory effect on the motivation to consume methamphetamine and on hyperactivity associated with acute methamphetamine intoxication. These results point to the potential utility of human trials of oxytocin as a therapeutic treatment for methamphetamine addiction.

Forced abstinence model of relapse to study pharmacological treatments of substance use disorder

Understanding and preventing relapse to drug use is one of the most difficult challenges faced by clinicians and practitioners in the struggle to help people remain abstinent. In this paper, we review basic preclinical research on forced abstinence periods that identify the neural substrates involved and neural adaptations that occur after a drug-free period. Our attention focuses on forced abstinence after self-administration because of its promise for translational research in the development of candidate medications to reduce relapse. This model requires subjects (often rats) to initially acquire drug self-administration. However, rather than extinguishing behavior with daily drug-free sessions as in the reinstatement model of drug seeking, subjects are removed from the self-administration situation and do not receive any exposure to the drug. Notably, the integrity of the drug-taking behavior and the drug-associated cues in the drug-taking environment are preserved because they are not experienced in the absence of the drug. Research shows time dependent increases in drug-seeking following forced abstinence periods. More so, neural substrates and adaptations within the mesocorticolimbic system and the nigrostriatal system have been identified that contribute to increased drug seeking following abstinence. From a translational perspective, behavioral and pharmacological treatment of substance use disorder often starts during this initial abstinence period (either forced or voluntary). The forced abstinence model simulates some of the features of this treatment situation and thus allows for the study of potential treatments that alter relapse of drug-seeking behaviors along with the accompanying neurobiological changes.

The 5-HT3 receptor antagonist, ondansetron, blocks the development and expression of ethanol-induced locomotor sensitization in mice

Manipulation of the serotonergic system has been shown to alter ethanol sensitization. Ondansetron is a 5-HT3 receptor antagonist, reported to attenuate cocaine and methamphetamine-induced behavioral sensitization, but no reports are available on its role in ethanol-induced behavioral sensitization. Therefore, an attempt has been made to assess this issue by using an earlier used animal model of ethanol-induced locomotor sensitization. Results indicated that ondansetron (0.25-1.0 mg/kg, subcutaneously) given before the challenge dose of ethanol (2.4 g/kg, intraperitoneally) injection, significantly and dose dependently attenuated the expression of sensitization. In addition, ondansetron (1.0 mg/kg, subcutaneously) given before ethanol injection on days 1, 4, 7, and 10 significantly blocked the development (days 1, 4, 7, and 10), and expression (day 15) of sensitization to the locomotor stimulant effect of ethanol injection. Ondansetron had no effect per se on locomotor activity and did not affect blood ethanol levels. Therefore, the results raise the possibility that ondansetron blocked the development and expression of ethanol-induced locomotor sensitization by acting on 5-HT3 receptors.

Racemic gamma vinyl-GABA (R,S-GVG) blocks methamphetamine-triggered reinstatement of conditioned place preference

Preventing relapse poses a significant challenge to the successful management of methamphetamine (METH) dependence. Although no effective medication currently exists for its treatment, racemic gamma vinyl-GABA (R,S-GVG, vigabatrin) shows enormous potential as it blocks both the neurochemical and behavioral effects of a variety of drugs, including METH, heroin, morphine, ethanol, nicotine, and cocaine. Using the reinstatement of a conditioned place preference (CPP) as an animal model of relapse, the present study specifically investigated the ability of an acute dose of R,S-GVG to block METH-triggered reinstatement of a METH-induced CPP. Animals acquired a METH CPP following a 20-day-period of conditioning, in which they received 10 pairings of alternating METH and saline injections. During conditioning, rats were assigned to one of four METH dosage groups: 1.0, 2.5, 5.0, or 10.0 mg/kg (i.p., n = 8/group). Animals in all dosage groups demonstrated a robust and consistent CPP. This CPP was subsequently extinguished in each dosage group with repeated saline administration. Upon extinction, all groups reinstated following an acute METH challenge. On the following day, an acute dose of R,S-GVG (300 mg/kg, i.p.) was administered 2.5 h prior to an identical METH challenge. R,S-GVG blocked METH-triggered reinstatement in all four groups. Given that drug re-exposure may potentiate relapse to drug-seeking behavior, the ability of R,S-GVG to block METH-triggered reinstatement offers further support for its use in the successful management of METH dependence.

The role of 5-HT3 receptors in drug abuse and as a target for pharmacotherapy

Alcohol and drug abuse continue to be a major public health problem in the United States and other industrialized nations. Extensive preclinical research indicates the mesolimbic dopamine (DA) pathway and associated regions mediate the rewarding and reinforcing effects of drugs of abuse and natural rewards, such as food and sex. The serotonergic (5-HT) system, in concert with others neurotransmitter systems, plays a key role in modulating neuronal systems within the mesolimbic pathway. A substantial portion of this modulation is mediated by activity at the 5-HT3 receptor. The 5-HT3 receptor is unique among the 5-HT receptors in that it directly gates an ion channel inducing rapid depolarization that, in turn, causes the release of neurotransmitters and/or peptides. Preclinical findings indicate that antagonism of the 5-HT3 receptor in the ventral tegmental area, nucleus accumbens or amygdala reduces alcohol self-administration and/or alcohol-associated effects. Less is known about the effects of 5-HT3 receptor activity on the self-administration of other drugs of abuse or their associated effects. Clinical findings parallel the preclinical findings such that antagonism of the 5-HT3 receptor reduces alcohol consumption and some of its subjective effects. This review provides an overview of the structure, function, and pharmacology of 5-HT3 receptors, the role of these receptors in regulating DA neurotransmission in mesolimbic brain areas, and discusses data from animal and human studies implicating 5-HT3 receptors as targets for the development of new pharmacological agents to treat addictions.

Neurochemical consequences of dysphoric state during amphetamine withdrawal in animal models: a review

Chronic abuse of amphetamines, such as d-amphetamine (AMPH) and d-methamphetamine, results in psychological dependence, a condition in which the drug produces a feeling of satisfaction and a drive that requires periodic or continuous administration of the drug to produce overwhelming pleasure or to avoid discomfort such as dysphoria. The dysphoric state of AMPH withdrawal has been recognized as depressive syndromes, such as anhedonia, depression, anxiety, and social inhibition, in early drug abstinence. Medication for treatment of the dysphoric state is important for AMPH abusers to avoid impulsive self-injurious behavior or acts that are committed with unconscious or uncontrolled suicidal ideation. However, successful treatments for AMPH withdrawal remain elusive, since the exact molecular basis of the expression of dysphoria has not been fully elucidated. This review focuses on the molecular aspects of AMPH withdrawal as indexed by neurochemical parameters under a variety of injection regimens (for example, levels of brain monoamines and their metabolites, and gamma-aminobutyric acid, expression of genes and proteins involved in neuronal activity, and monoamine metabolism and availability) in rodent models which exhibit significant phenotypic features relevant to the syndromes of AMPH withdrawal in humans.