Relationship between discriminative stimulus effects and plasma methamphetamine and amphetamine levels of intramuscular methamphetamine in male rhesus monkeys

Effects of methamphetamine on actigraphy-based sleep parameters in female rhesus monkeys: Orexin receptor mechanisms

BACKGROUND

The orexin system has been implicated as a mechanism underlying insomnia and methamphetamine-induced sleep disruptions, with a potential role for OX2 receptors in the sleep-modulating effects of orexin. The aim of the present study was to investigate the extent to which orexin receptors mediate the effects of acute methamphetamine administration on actigraphy-based sleep in female rhesus monkeys.

METHODS

Actigraphy-based sleep measures were obtained in female rhesus monkeys (n=5) under baseline and acute test conditions. First, morning (10h) i.m. injections of methamphetamine (0.03 - 0.56mg/kg) were administered to determine the effects of methamphetamine alone. Then, saline or methamphetamine (0.3mg/kg) were administered at 10h, and evening (17h30) oral treatments with vehicle, the non-selective orexin receptor antagonist suvorexant (1 - 10mg/kg, p.o.), or the OX2-selective orexin receptor antagonist MK-1064 (1 - 10mg/kg, p.o.) were given. The ability of suvorexant and MK-1064 (10mg/kg, p.o.) to improve actigraphy-based sleep was also assessed in a group of female monkeys quantitatively identified with "short-duration sleep" (n=4).

RESULTS

Methamphetamine dose-dependently disrupted actigraphy-based sleep parameters. Treatment with either suvorexant or MK-1064 dose-dependently improved actigraphy-based sleep in monkeys treated with methamphetamine. Additionally, both suvorexant and MK-1064 promoted actigraphy-based sleep in a group of monkeys with baseline short actigraphy-based sleep.

CONCLUSIONS

These findings suggest that orexin-mediated mechanisms play a role in the effects of methamphetamine on actigraphy-based sleep in female monkeys. Targeting the orexin system, in particular OX2 receptors, could be an effective option for treating sleep disruptions observed in individuals with methamphetamine use disorder.

Methamphetamine-Induced Sleep Impairments and Subsequent Slow-Wave and Rapid Eye Movement Sleep Rebound in Male Rhesus Monkeys

Use of amphetamine-type stimulants is associated with numerous adverse health outcomes, with disturbed sleep being one of the most prominent consequences of methamphetamine use. However, the extent to which methamphetamine alters sleep architecture, and whether methamphetamine-induced sleep impairment is associated with next-day sleep rebound effects, has received relatively little investigation. In the present study, we investigated the effects of acute morning methamphetamine administration on sleep parameters in adult male rhesus monkeys (N = 4) using a fully-implantable telemetry system. Monkeys were prepared with telemetry devices that continuously monitored electroencephalography (EEG), electromyography (EMG) and electrooculography (EOG) throughout the night. We investigated the effects of morning (10h00) administration of methamphetamine (0.01-0.3 mg/kg, i.m.) on sleep during the night of the injection. In addition, we investigated sleep during the subsequent night in order to assess the possible emergence of sleep rebound effects. Methamphetamine administration dose-dependently increased sleep latency and wake time after sleep onset (WASO). Methamphetamine also decreased total sleep time, which was reflected by a decrease in total time spent in N2, slow-wave (N3) and REM sleep stages, while increasing the percentage of total sleep time spent in sleep stage N1. Importantly, methamphetamine decreased time spent in N3 and REM sleep even at doses that did not significantly decrease total sleep time. Sleep rebound effects were observed on the second night after methamphetamine administration, with increased total sleep time reflected by a selective increase in time spent in sleep stages N3 and REM, as well as a decrease in REM sleep latency. Our findings show that methamphetamine administered 8 h prior to the inactive (dark) phase induces marked changes in sleep architecture in rhesus monkeys, even at doses that do not change sleep duration, and that sleep rebound effects are observed the following day for both N3 and REM sleep stages.

The dual orexin receptor antagonist almorexant blocks the sleep-disrupting and daytime stimulant effects of methamphetamine in rhesus monkeys

BACKGROUND

The present study investigated the effects of the dual orexin receptor antagonist (DORA) almorexant, a sleep-modulating drug, on the sleep-disrupting effects of methamphetamine in adult rhesus monkeys.

METHODS

Monkeys were fitted with primate collars to which actigraphy monitors were attached. To determine the effects of methamphetamine on daytime activity and sleep-like parameters, monkeys were given acute injections of vehicle or methamphetamine (0.03, 0.1 or 0.3 mg/kg, i.m.) in the morning (9:00 h) (n = 4 males). We then determined the ability of almorexant to alter the daytime and/or sleep-like effects of the largest (effective) dose of methamphetamine. Vehicle or almorexant (1, 3 or 10 mg/kg, i.m.) were administered in the evening (16:30 h, 1.5 h before "lights off") following morning (9:00 h) administration of methamphetamine (0.3 mg/kg, i.m.), or as a pretreatment (8:30 h) before methamphetamine injections (9:00 h) (n = 4 males). The ability of almorexant (10 mg/kg) to improve sleep-like behaviors also was assessed in a group of monkeys quantitatively identified with short-duration sleep (n = 2 males, 2 females).

RESULTS

Morning methamphetamine administration dose-dependently impaired sleep in rhesus monkeys (0.3 mg/kg significantly increased sleep latency and decreased sleep efficiency). Administration of almorexant, both as a pretreatment or as an evening treatment, improved methamphetamine-induced sleep impairment in a dose dependent manner. Morning pretreatment with almorexant also blocked the daytime stimulant effects of methamphetamine. Evening, but not morning, treatment with almorexant in a group of monkeys with baseline short-duration sleep improved sleep measures.

CONCLUSIONS

Our findings indicate that orexin receptor systems are involved in methamphetamine-induced hyperarousal and sleep disruption.

Long-term exposure to ephedrine leads to neurotoxicity and neurobehavioral disorders accompanied by up-regulation of CRF in prefrontal cortex and hippocampus in rhesus macaques

Drug addiction continues to threaten the health and welfare of people worldwide, and ephedrine abuse is a serious drug problem in many areas of the world. Ephedrine toxicity is thought to induce behavioral effects primarily through actions on the central nervous system. The corticotropin-releasing factor (CRF) system plays an important role in regulating behavioral effects induced by addictive drugs, but whether CRF is related to ephedrine toxicity remains unclear. This study seeks to examine whether there is a correlation between the CRF and chronic ephedrine neurotoxicity. To this end, we established a chronic ephedrine (0.4-1.6 mg/kg/d) exposure model in rhesus macaques, assessed its effects on body weight and behavior, examined neuronal changes in the prefrontal cortex and hippocampus, and measured the CRF expression in the prefrontal cortex and hippocampus. After 8-weeks of exposure to ephedrine, the toxic effects of ephedrine included significant weight loss and induction of behavioral changes in rhesus macaques. In particular, in the modeling group, the abnormal behavioral changes mainly manifested as irritability and behavioral sensitization. Meanwhile, the histological abnormalities included neuronal morphological changes, pyknosis and irregular shapes of neurons in the prefrontal cortex and hippocampus. In addition, the expression levels of CRF mRNA and protein were increased in the prefrontal cortex and hippocampus of ephedrine-treated animals. In summary, the finding of this study indicated that ephedrine neurotoxicity can cause neuronal damage in cerebral cortex, which in turn can result in certain neurobehavioral abnormalities, and that CRF expression in prefrontal cortex and hippocampus is elevated in response to ephedrine exposure. These observations suggested that long-term exposure to ephedrine might be causing neurotoxicity and leading to neurobehavioral disorders accompanied by up-regulation of CRF in prefrontal cortex and hippocampus.

Evaluation of the Reinforcing Strength of Phendimetrazine Using a Progressive-Ratio Schedule of Reinforcement in Rhesus Monkeys

Stimulant abuse is a persistent public health problem with no Food and Drug Administration-approved pharmacotherapy. Although monoamine-releasing drugs such as d-amphetamine can decrease cocaine self-administration in human and animal laboratory studies, their potential for abuse limits clinical utility. "Abuse-deterrent" formulations of monoamine releasers, such as prodrugs, hold greater clinical promise if their abuse potential is, as theorized, lower than that of cocaine. In these studies, we determined the reinforcing strength of phendimetrazine (PDM), a prodrug for the amphetamine-like monoamine releaser phenmetrazine; both drugs have been shown to decrease cocaine self-administration in laboratory animals. To date, no study has directly compared PDM (Schedule III) with cocaine (Schedule II) under progressive-ratio (PR) schedules of reinforcement, which are better suited than fixed-ratio schedules to directly compare reinforcing strength of drugs. Dose-response curves for cocaine (saline, 0.001-0.3 mg/kg per injection) and PDM (0.1-1.0 mg/kg per injection) were generated in six cocaine-experienced male rhesus monkeys during 4-hour sessions with a 20-minute limited hold (LH). Under these conditions, the maximum number of injections was not significantly different between cocaine and PDM. The reinforcing strength of doses situated on the peaks of the cocaine and PDM dose-effect curves were redetermined with a 60-minute LH. The mean number of injections increased for both drugs, but not for saline. Cocaine presentations resulted in significantly higher peak injections than PDM with a 60-minute LH, which is consistent with the lower scheduling of PDM. These results support PDM as Schedule III and highlight the importance of schedule parameters when comparing reinforcing strength of drugs using a PR schedule of reinforcement. SIGNIFICANCE STATEMENT: One strategy for reducing cocaine use is to identify a treatment that substitutes for cocaine but has lower abuse potential. In a rhesus monkey model of drug abuse, this study compared the reinforcing strength of cocaine and phendimetrazine, a drug that has been shown to decrease cocaine use in some studies.

Assessment of tolerance to the effects of methamphetamine on daytime and nighttime activity evaluated with actigraphy in rhesus monkeys

RATIONALE

Methamphetamine is one of the most largely consumed illicit drugs, and its use is associated with abuse liability and several adverse health effects, such as sleep impairment. Importantly, sleep quality can influence addiction treatment outcomes. Evidence suggests that tolerance can develop to the sleep-disrupting effects of stimulant drugs.

OBJECTIVE

The aim of the present study was to investigate the development of tolerance to the actigraphy-based sleep-disrupting and stimulant effects of methamphetamine self-administration in rhesus monkeys.

METHODS

Methamphetamine (0.03 mg/kg/inf, i.v.) self-administration was carried out following three different protocols: 14 consecutive days of self-administration, 5 days/week for 3 weeks, with a 2-day interval between 5-day blocks of self-administration, and 3 days/week for 3 weeks, with a 4-day interval between 3-day blocks of self-administration. Daytime activity and activity-based sleep measures were evaluated with Actiwatch monitors a week before (baseline parameters) and throughout each protocol.

RESULTS

Methamphetamine self-administration markedly disrupted sleep-like measures and increased daytime activity. Tolerance developed to those effects with repeated methamphetamine intake exceeding five consecutive days. Inclusion of washout periods (2 or 4 days) between blocks of methamphetamine self-administration attenuated the development of tolerance, with longer breaks from methamphetamine intake being more effective in maintaining the sleep-disrupting and stimulant effects of methamphetamine.

CONCLUSIONS

Tolerance can develop to the stimulant and sleep-disrupting effects of methamphetamine self-administration. Interruption of drug intake extends the effects of methamphetamine on sleep-like measures and daytime activity.

Role of d-amphetamine and d-methamphetamine as active metabolites of benzphetamine: Evidence from drug discrimination and pharmacokinetic studies in male rhesus monkeys

Benzphetamine is a Schedule III anorectic agent that is a prodrug for d-amphetamine and d-methamphetamine and may have utility as an "agonist" medication for cocaine use disorder treatment. This study evaluated the pharmacokinetic-pharmacodynamic profile of benzphetamine using a drug discrimination procedure in rhesus monkeys. The potency and time course of cocaine-like discriminative stimulus effects were compared for benzphetamine (10-18mg/kg, intramuscular (IM)) and d-amphetamine (0.032-0.32mg/kg, IM) in monkeys (n=3-4) trained to discriminate IM cocaine (0.32mg/kg) from saline in a two-key food-reinforced discrimination procedure. Parallel pharmacokinetic studies in the same monkeys determined plasma benzphetamine, d-methamphetamine and/or d-amphetamine levels for correlation with behavioral effects. d-Amphetamine produced dose-dependent, time-dependent, and full cocaine-like effects, i.e. ≥90% cocaine-appropriate responding, in all monkeys without altering response rates. The time course of d-amphetamine's cocaine-like discriminative stimulus effects correlated with plasma d-amphetamine levels. Benzphetamine was 180-fold less potent than d-amphetamine and produced full cocaine-like effects in only 2 of 4 monkeys while significantly decreasing response rates. Benzphetamine administration increased plasma d-methamphetamine (peak at 100min) and d-amphetamine (peak at 24h) levels, but the time course of behavioral effects did not correlate with increased levels of benzphetamine, d-methamphetamine or d-amphetamine. These results suggest that benzphetamine yields d-amphetamine and d-methamphetamine as active metabolites in rhesus monkeys, but generation of these metabolites is not sufficient to account for benzphetamine behavioral effects. The incomplete cocaine substitution profile and protracted d-amphetamine plasma levels suggest that benzphetamine may still warrant further evaluation as a candidate pharmacotherapy for cocaine use disorder treatment.

Cocaine-like discriminative stimulus effects of amphetamine, cathinone, methamphetamine, and their 3,4-methylenedioxy analogs in male rhesus monkeys

RATIONALE

Synthetic cathinones have emerged as the newest class of abused monoamine transporter substrates. Structurally, these compounds are all beta-ketone amphetamine (cathinone) analogs. Whether synthetic cathinone analogs produce differential behavioral effects from their amphetamine analog counterparts has not been systematically examined. Preclinical drug discrimination procedures have been useful for determining the structure activity relationships (SARs) of abused drugs; however, direct comparisons between amphetamine and cathinone analogs are lacking and, in particular, in non-human primate models.

OBJECTIVES

The study aim was to determine the potency and time course of (±)-amphetamine, (±)-cathinone, and (±)-methamphetamine and their 3,4-methylenedioxy analogs (±)-MDA, (±)-MDC, and (±)-MDMA, respectively, to produce cocaine-like discriminative stimulus effects. If cathinone analogs have similar behavioral pharmacological properties to their amphetamine counterparts, then we would predict similar potencies and efficacies to produce cocaine-like discriminative stimulus effects.

METHODS

Male rhesus monkeys (n = 4) were trained to discriminate intramuscular cocaine (0.32 mg/kg) from saline in a two-key food-reinforced discrimination procedure.

RESULTS

Racemic amphetamine, cathinone, and methamphetamine produced dose-dependent and full substitution, ≥90 % cocaine-appropriate responding, in all monkeys. Addition of 3,4-methylenedioxy moiety attenuated both the potency and efficacy of amphetamine (MDA), cathinone (MDC), and methamphetamine (MDMA) to produce full cocaine-like effects. Moreover, the cocaine-like effects of amphetamine and cathinone were attenuated to a greater extent than those of methamphetamine or previously published methcathinone (Smith et al. 2016).

CONCLUSION

The presence of an N-methyl group blunted both the potency and the efficacy shift of the 3,4-methylenedioxy addition for both amphetamine and cathinone analogs.

Methamphetamine-like discriminative stimulus effects of bupropion and its two hydroxy metabolites in male rhesus monkeys

The dopamine transporter (DAT) inhibitor and nicotinic acetylcholine (nACh) receptor antagonist bupropion is being investigated as a candidate 'agonist' medication for methamphetamine addiction. In addition to its complex pharmacology, bupropion also has two distinct pharmacologically active metabolites. However, the mechanism by which bupropion produces methamphetamine-like 'agonist' effects remains unknown. The aim of the present study was to determine the role of DAT inhibition, nACh receptor antagonism, and the hydroxybupropion metabolites in the methamphetamine-like discriminative stimulus effects of bupropion in rhesus monkeys. In addition, varenicline, a partial agonist at the nACh receptor, and risperidone, a dopamine antagonist, were tested as controls. Monkeys (n=4) were trained to discriminate 0.18 mg/kg intramuscular methamphetamine from saline in a two-key food-reinforced discrimination procedure. The potency and time course of methamphetamine-like discriminative stimulus effects were determined for all compounds. Bupropion, methylphenidate, and 2S,3S-hydroxybupropion produced full, at least 90%, methamphetamine-like effects. 2R,3R-Hydroxybupropion, mecamylamine, and nicotine also produced full methamphetamine-like effects, but drug potency was more variable between monkeys. Varenicline produced partial methamphetamine-like effects, whereas risperidone did not. Overall, these results suggest DAT inhibition as the major mechanism of the methamphetamine-like 'agonist' effects of bupropion, although nACh receptor antagonism appeared, at least partially, to contribute. Furthermore, the contribution of the 2S,3S-hydroxybupropion metabolite could not be completely ruled out.

Potential Molecular Mechanisms on the Role of the Sigma-1 Receptor in the Action of Cocaine and Methamphetamine

The sigma-1 receptor (Sig-1R) is an endoplasmic reticulum membrane protein that involves a wide range of physiological functions. The Sig-1R has been shown to bind psychostimulants including cocaine and methamphetamine (METH) and thus has been implicated in the actions of those psychostimulants. For example, it has been demonstrated that the Sig-1R antagonists mitigate certain behavioral and cellular effects of psychostimulants including hyperactivity and neurotoxicity. Thus, the Sig-1R has become a potential therapeutic target of medication development against drug abuse that differs from traditional monoamine-related strategies. In this review, we will focus on the molecular mechanisms of the Sig-1R and discuss in such a manner with a hope to further understand or unveil unexplored relations between the Sig-1R and the actions of cocaine and METH, particularly in the context of cellular biological relevance.