Scopolamine prevents the development of sensitization to methamphetamine

Pharmacological Treatments for Methamphetamine Use Disorder: Current Status and Future Targets

The illicit use of the psychostimulant methamphetamine (METH) is a major concern, with overdose deaths increasing substantially since the mid-2010s. One challenge to treating METH use disorder (MUD), as with other psychostimulant use disorders, is that there are no available pharmacotherapies that can reduce cravings and help individuals achieve abstinence. The purpose of the current review is to discuss the molecular targets that have been tested in assays measuring the physiological, the cognitive, and the reinforcing effects of METH in both animals and humans. Several drugs show promise as potential pharmacotherapies for MUD when tested in animals, but fail to produce long-term changes in METH use in dependent individuals (eg, modafinil, antipsychotic medications, baclofen). However, these drugs, plus medications like atomoxetine and varenicline, may be better served as treatments to ameliorate the psychotomimetic effects of METH or to reverse METH-induced cognitive deficits. Preclinical studies show that vesicular monoamine transporter 2 inhibitors, metabotropic glutamate receptor ligands, and trace amine-associated receptor agonists are efficacious in attenuating the reinforcing effects of METH; however, clinical studies are needed to determine if these drugs effectively treat MUD. In addition to screening these compounds in individuals with MUD, potential future directions include increased emphasis on sex differences in preclinical studies and utilization of pharmacogenetic approaches to determine if genetic variances are predictive of treatment outcomes. These future directions can help lead to better interventions for treating MUD.

Safety and efficacy of varenicline to reduce positive subjective effects produced by methamphetamine in methamphetamine-dependent volunteers

Methamphetamine use is increasing in the US. Although there are no Food and Drug Administration (FDA)-approved medications for methamphetamine dependence, preclinical and clinical studies suggest that methamphetamine users may benefit from treatments that enhance cholinergic neurotransmission. Consequently, we determined the safety and the efficacy of varenicline treatment, a partial agonist at α4β2 and a full agonist at α7 nicotinic acetylcholine receptors, to reduce positive subjective effects produced by smoked methamphetamine. Additionally, the effects of treatment with varenicline on the cardiovascular and reinforcing effects of methamphetamine were determined. We conducted a double-blind, placebo-controlled, within-subjects trial of varenicline vs. placebo in methamphetamine-dependent volunteers who were not seeking treatment. Participants were randomly assigned to receive one dose of varenicline (0, 1, or 2 mg) po BID, titrated up to the target dose over days 1-7, during each of three separate inpatient phases. Safety measures included the frequency, duration, severity, and relatedness of adverse events reported. Positive subjective effects included 'Any drug effect', 'High', 'Good effects', 'Stimulated', and 'Drug liking', which were rated by participants before and for 1 h after smoking methamphetamine (0, 10, and 30 mg). There were no serious adverse events and no differences in adverse events reported during the three phases. Varenicline (2 mg) significantly reduced ratings of 'Any drug effect' and 'Stimulated', as well as attenuated ratings of 'High', 'Drug liking', and 'Good effects', produced by methamphetamine (30 mg). The ability of varenicline to attenuate the positive subjective effects of methamphetamine in the laboratory suggests that varenicline should continue to be explored as a treatment for methamphetamine dependence.

Positive or negative allosteric modulation of metabotropic glutamate receptor 5 (mGluR5) does not alter expression of behavioral sensitization to methamphetamine

We investigated the role of metabotropic glutamate receptor type 5 (mGluR5) in methamphetamine-induced behavioral sensitization. The mGluR5 positive allosteric modulator (3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl) benzamide (CDPPB) and negative allosteric modulator fenobam were tested in separate experiments. Sprague-Dawley rats were repeatedly injected with 1 mg/kg methamphetamine or saline, and then given a locomotor challenge test using a dose of 0.5 mg/kg methamphetamine. Prior to the challenge test session, rats were injected with CDPPB, fenobam, or a vehicle.  Doses from previous studies showed reduced drug-conditioned behavior; however in this study neither CDPPB nor fenobam pretreatment resulted in an altered expression of behavioral sensitization, indicating a lack of mGluR5 involvement in sensitized methamphetamine-induced locomotion. Additionally, the high dose (30 mg/kg) of fenobam resulted in decreased methamphetamine-induced locomotion in rats regardless of drug exposure history, which suggests evidence of nonspecific behavioral inhibition.

Anxiolytic effects of environmental enrichment attenuate sex-related anxiogenic effects of scopolamine in rats

In groups of four same-sexed animals, PVG/c hooded rats were housed for 4.5 months in standard or enriched cages containing several objects that could be explored and manipulated. On separate occasions, each rat then experienced two consecutive daily trials in an open field, a light-dark box or a Y maze with arm inserts that enabled an acquisition trial comprising one black and one white arm to be changed for a retention trial consisting of two black arms. Before their trials in the open field and light-dark box, and following each acquisition trial in the Y maze, the rats received an intraperitoneal injection of 2 mg/kg scopolamine or isotonic saline. In the open field, enrichment led to higher levels of ambulation, walking, rearing and occupancy of the center of the apparatus and shorter emergence latencies from the dark into the light compartment of the light-dark box accompanied by more entries of this compartment. Enrichment also increased entries of and time spent in the changed (or novel) Y-maze arm only for male rats treated with scopolamine. The drug decreased rearing and increased grooming in the open field as well as increasing emergence latencies and decreasing entries of and the time spent on the light compartment of the light-dark box. The main results were interpreted as enrichment having attenuated anxiogenic effects of the behavioral testing and the action of scopolamine for male (but not female) rats in their choices of the novel arm in the Y maze.

Does Scopolamine Block the Development of Ethanol-Induced Behavioral Sensitization?

BACKGROUND

The cholinergic system is important in learning processes and probably influences behavioral sensitization to drugs. This study examined the effects of scopolamine (scop), a muscarinic antagonist, on the behavioral sensitization to ethanol (EtOH) stimulant effect in mice.

METHODS

In experiments 1 and 2, male Swiss albino mice received saline or 1.0 mg/kg scop (s.c.) + saline or EtOH (i.p). (1.0 g/kg in experiment 1 and 2.2 g/kg in experiment 2), for 21 days. Locomotor activity (LA) was recorded once a week, being the treatment withdrawn for 7 days after the last test. On the 28th day (challenge 1), they were evaluated under saline or EtOH. In experiment 2, 3 days after the first challenge, they were tested in an open-field arena, under saline or 2.2 g/kg EtOH. Three days after this, mice were tested under saline or 1.0 mg/kg scop in the activity cages.

RESULTS

Acutely, EtOH and scop did not alter the LA. However, when both drugs were coadministered, a significant reduction was observed. During the treatment, tolerance to the depressor effect was developed and behavioral sensitization observed only in the saline + 2.2 EtOH group. In challenge 1, the groups scop + 1.0 EtOH, saline + 2.2 EtOH, and scop + 2.2 EtOH presented higher levels of LA than that of the control groups. However, in challenge 2, conducted in a different setting, no differences between groups were observed. In challenge 3, when the animals received scop, both groups pretreated with 2.2 g/kg EtOH (saline + 2.2 EtOH and scop + 2.2 EtOH groups) presented higher levels of activity suggesting an interaction between EtOH and scop.

CONCLUSIONS

Although the coadministration of scopolamine had impaired the observation of sensitization on the 21st day test, when the group scop + EtOH was challenged with scop + EtOH, it seems that the scop just masked the observation, but did not impair the development, of the EtOH-sensitization observed in the challenge with EtOH alone. The higher levels of LA in groups pretreated with EtOH only in the cages but not in the open-field arena confirm the importance of environmental factors, such as the context of drug administration and testing.

Methamphetamine-induced sensitization differentially alters pCREB and DeltaFosB throughout the limbic circuit of the mammalian brain

Enhancements in behavior that accompany repeated, intermittent administration of abused drugs (sensitization) endure long after drug administration has ceased. Such persistence reflects changes in intracellular signaling cascades and associated gene transcription factors in brain regions that are engaged by abused drugs. This process is not characterized for the most potent psychomotor stimulant, methamphetamine. Using motor behavior as an index of brain state in rats, we verified that five once-daily injections of 2.5 mg/kg methamphetamine induced behavioral sensitization that was demonstrated (expressed) 3 and 14 days later. Using immunoblot procedures, limbic brain regions implicated in behavioral sensitization were assayed for extracellular signal-regulated kinase and its phosphorylated form (pERK/ERK, a signal transduction kinase), cAMP response element binding protein and its phosphorylated form (pCREB/CREB, a constitutively expressed transcriptional regulator), and DeltaFosB (a long-lasting transcription factor). pERK, ERK, and CREB levels were not changed for any region assayed. In the ventral tegmental area, pCREB and DeltaFosB also were not changed. pCREB (activated CREB) was elevated in the frontal cortex at 3 days withdrawal, but not at 14 days. pCREB levels were decreased at 14 days withdrawal in the nucleus accumbens and ventral pallidum. Accumbal and pallidal levels of DeltaFosB were increased at 3 days withdrawal, and this increase persisted to 14 days in the pallidum. Thus, only the ventral pallidum showed changes in molecular processes that consistently correlated with motor sensitization, revealing that this region may be associated with this enduring behavioral phenotype initiated by methamphetamine. The present findings expand our understanding of the neuroanatomical and molecular substrates that may play a role in the persistence of druginduced sensitization.

Valproate blocks high-dose methamphetamine-induced behavioral cross-sensitization to locomotion-inducing effect of dizocilpine (MK-801), but not methamphetamine

RATIONALE

Our group has recently shown that methamphetamine (METH) (2.5 mg/kg) induced delayed increases in glutamate (Glu) levels in the rat nucleus accumbens (NAC), and that its repeated administration leads to behavioral cross-sensitization to a selective uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (MK-801).

OBJECTIVES

The present study aims to examine whether valproate (VPA) would inhibit the delayed increases in Glu levels and prevent METH (2.5 mg/kg)-induced behavioral cross-sensitization to MK-801 (0.2 mg/kg).

MATERIALS AND METHODS

We examined the effects of post-treated VPA (50 mg/kg) on METH (2.5 mg/kg)-induced delayed increases in Glu levels. We injected VPA (50 mg/kg) at 120 min after each METH (2.5 mg/kg, once every other day, total of five times) administration and measured locomotor activity induced by challenge with MK-801 (0.2 mg/kg) or METH (0.15 mg/kg) after sufficient withdrawal period. Finally, we measured locomotion induced by MK-801 (0.2 mg/kg) after pretreatment of a competitive NMDA receptor antagonist, CPP (30 mg/kg). Effects of VPA on extracellular Glu levels were examined by using in vivo microdialysis. Locomotor activity was measured by using an infrared sensor.

RESULTS

VPA administered 120 min after METH injection had no effect on METH-induced hyperlocomotion, and inhibited METH-induced delayed increases in Glu levels. Repeated VPA administration prevented METH-induced behavioral cross-sensitization to MK-801, but not sensitization to METH. MK-801-induced hyperlocomotion was enhanced when pretreated with the competitive NMDA receptor antagonist, CPP.

CONCLUSIONS

These results suggest that VPA inhibits high-dose METH-induced delayed increases in Glu levels to prevent development of behavioral cross-sensitization to an NMDA antagonist, but not sensitization to METH.

Brain vesicular acetylcholine transporter in human users of drugs of abuse

Limited animal data suggest that the dopaminergic neurotoxin methamphetamine is not toxic to brain (striatal) cholinergic neurons. However, we previously reported that activity of choline acetyltransferase (ChAT), the cholinergic marker synthetic enzyme, can be very low in brain of some human high-dose methamphetamine users. We measured, by quantitative immunoblotting, concentrations of a second cholinergic marker, the vesicular acetylcholine transporter (VAChT), considered to be a "stable" marker of cholinergic neurons, in autopsied brain (caudate, hippocampus) of chronic users of methamphetamine and, for comparison, in brain of users of cocaine, heroin, and matched controls. Western blot analyses showed normal levels of VAChT immunoreactivity in hippocampus of all drug user groups, whereas in the dopamine-rich caudate VAChT levels were selectively elevated (+48%) in the methamphetamine group, including the three high-dose methamphetamine users who had severely reduced ChAT activity. To the extent that cholinergic neuron integrity can be inferred from VAChT concentration, our data suggest that methamphetamine does not cause loss of striatal cholinergic neurons, but might damage/downregulate brain ChAT in some high-dose users. However, the finding of increased VAChT levels suggests that brain VAChT concentration might be subject to up- and downregulation as part of a compensatory process to maintain homeostasis of neuronal cholinergic activity. This possibility should be taken into account when utilizing VAChT as a neuroimaging outcome marker for cholinergic neuron number in human studies.

Alteration in behavioral sensitivity to amphetamine after treatment with oxotremorine

Our earlier experiment revealed that rats pretreated once with an anticholinesterase develop hyposensitivity to amphetamine (AMPH). One of the likely causes of this effect might be a transient hyperexcitation of the central muscarinic receptors. It has appeared, however, that rats pretreated with oxotremorine (OX), a muscarinic agonist, show an augmented behavioral response to AMPH weeks later. The present experiments were performed in order to obtain more information on the relationship between the OX-induced sensitization to AMPH and the OX dose and dosing regime (single or repeated), and to find out whether the environment associated with the acute effects of OX could affect the response to AMPH. In experiment 1, adult male rats were given a single i.p. injection of OX in home cages at a moderate (0.5 mg/kg) or high (1.0 mg/kg) dose. In experiment 2, the rats received eight 1.0 mg/kg doses of OX in the course of three days. After each injection, some animals returned to their home cages, and some were placed in the test cages for 30 min. In both experiments, the response to AMPH was assessed on day 21 after the treatment. The obtained results indicate that: (i) a single i.p. exposure to OX results in an increase of the rat's behavioral sensitivity to AMPH but the moderate dose is more effective in inducing this effect; (ii) repeated exposure to OX at high doses, in a regime enabling development of tolerance to the acute OX effects, does not alter the rat sensitivity to AMPH, and (iii) expression of the AMPH response is suppressed in environment which has been associated with acute effects of OX.

Suppressive effects of trihexyphenidyl on methamphetamine-induced dopamine release as measured by in vivo microdialysis

Abuse of methamphetamine (MAP) and cocaine causes severe medical and social problems throughout the world. Our previous study found that trihexyphenidyl (THP), a muscarinic cholinergic receptor antagonist, specifically suppressed the rewarding properties of MAP but not of cocaine, as measured by conditioned place preference in mice. The present study examined using in vivo microdialysis whether THP differentially affects the extracellular dopamine (DA) levels in the nucleus accumbens and striatum of mice injected with MAP and cocaine in comparison with another antimuscarinic agent, scopolamine (SCP). In addition, locomotor activity was simultaneously measured during microdialysis. In vivo microdialysis experiments revealed that during the initial hour after injection of MAP (1 mg/kg) DA levels increased up to 698% in the nucleus accumbens and 367% in the striatum as compared to the basal level. These increases were reduced to 293% in the nucleus accumbens and 207% in the striatum by treatment with 5 mg/kg THP. However, SCP (3 mg/kg) had no effect on the increases in extracellular DA levels in both regions after MAP injection. Cocaine (10 mg/kg) increased DA levels during the initial hour to 254% in the nucleus accumbens and 220% in the striatum as compared to the basal level. These increases were unaffected by treatment with either THP or SCP. On the contrary, both THP and SCP enhanced the locomotor-stimulant action of MAP and cocaine. These results, together with our previous finding, suggest that THP may specifically antagonize the rewarding properties of MAP through suppression of DA release in the mesolimbic area without retarding locomotor activity.

Ontogenesis of behavioral sensitization and conditioned place preference induced by psychostimulants in laboratory rodents

The present review deals mainly with the ontogenesis of two important phenomena involved in vulnerability to several neuropsychiatric disorders, namely with drug-induced sensitization (both contextual and non-contextual) and with conditioned place preference. The term 'infancy' covers the first three postnatal weeks during development in rats and mice. Conversely, the term 'adolescence' may cover the whole postnatal period ranging from weaning (PND 21) to adulthood (at least PND 60) or specifically the period around the onset of puberty (animals aged 33-44 days). Recent studies in rats demonstrated that the establishment of a context-dependent sensitization appears during the first (for repeated drug administration) or during the second (for a single drug administration) postnatal week. However, the memory of drug-context association is transient in developing pups (lasting one or two days following the drug pretreatment). The long-term retention of drug-context associations matures progressively, and is complete by the third week of postnatal life. Finally, those mechanisms responsible for an adult-like profile of context-independent pharmacological sensitization appear later during ontogenesis, being mature by the fourth week of postnatal life. Another set of experiments extended this ontogenetic characterization by comparing adolescent and adult mice. When compared to the latter, the former subjects exhibit a greater amphetamine-induced locomotor sensitization, almost no sensitization of aversive stereotyped behaviors, and a less marked place conditioning. The strength of the drug-induced place conditioning was also directly compared with the unconditioned novelty-seeking drive. In conclusion, neonatal rats are able to show a relatively short-lasting retention of sensitized drug effects (short-term sensitization), whereas the ability to exhibit relatively long-lasting sensitized effects matures progressively during infancy (long-term sensitization). On the other hand, adolescent mice show a reduced sensitization of drug-induced psychotic symptoms, together with a more marked sensitization of arousing and euphorigenic properties of the drug and a reduced incentive memory of its hedonic effects. These age-related changes do imply very different degrees of vulnerability to drug addiction and several other neuropsychiatric disorders.

Cholinergic modulation of basal and amphetamine-induced dopamine release in rat medial prefrontal cortex and nucleus accumbens

Behavioral evidence suggests that muscarinic/cholinergic inhibition of brain dopaminergic activity may be a useful principle for developing novel antipsychotic drugs (APDs). Thus, oxotremorine, a muscarinic agonist, attenuates amphetamine-induced locomotor activity in rodents, an effect also produced by a wide variety of proven APDs, whereas scopolamine, a muscarinic antagonist, has the opposite effect. Since atypical APDs such as clozapine, olanzapine, risperidone, ziprasidone and quetiapine, increase brain acetylcholine as well as dopamine (DA) release in a region-specific manner, their effects on cholinergic and dopaminergic neurotransmission may also contribute to various actions of these drugs. Oxotremorine (0.5-1.5 mg/kg) dose-dependently and preferentially increased DA release in rat medial prefrontal cortex (mPFC), compared to the nucleus accumbens (NAC). However, S-(-)-scopolamine (0.5-1.5 mg/kg) produced similar increases in DA release in the mPFC, but the effect was much less than that of oxotremorine. Whereas a dose of S-(-)-scopolamine of 0.5 mg/kg comparably increased DA release in the mPFC and NAC, 1.5 mg/kg had no effect on DA release in the NAC. Oxotremorine-M (0.5 mg/kg), a M(1/4)-preferring agonist, also increased DA release in the mPFC, but not the NAC, an effect completely abolished by telenzepine (3 mg/kg), a M(1/4)-preferring antagonist, which by itself had no effect on DA release in either region. Oxotremorine (0.5, but not 1.5, mg/kg) attenuated amphetamine (1 mg/kg)-induced DA release in the NAC, whereas S-(-)-scopolamine did not. Oxotremorine (1.5 mg/kg) and S-(-)-scopolamine (0.5 mg/kg) modestly but significantly potentiated amphetamine (1 mg/kg)-induced DA release in the mPFC. These results suggest that stimulation of muscarinic receptors, in particular M(1/4), as indicated by the effect of oxotremorine-M and telenzepine, may preferentially increase cortical DA release and inhibit amphetamine-induced DA release in the NAC.

The role of benzodiazepine receptors in the acquisition and expression of behavioral sensitization to methamphetamine

The GABA-benzodiazepine neurotransmission has been reported to be implicated in various forms of plasticity such as kindling and learning. In a previous study, we have shown that clonazepam (CZP), a GABA-benzodiazepine agonist, prevents the acquisition of behavioral sensitization to methamphetamine (MA). The present study was conducted to extend this finding by examining the effect of flumazenil (Flu), a GABA-benzodiazepine antagonist on the prevention by CZP. Rats (male Wistar-King rats) treated with MA (1 mg/kg, SC) for 10 days showed significantly enhanced motor activity compared to those treated with saline when tested with MA (1 mg/kg) after a 7-8-day withdrawal, indicating the acquisition of behavioral sensitization. Representing the previous finding, pretreatment with CZP (0.5 mg/kg) prior to MA administration prevented the acquisition of the phenomenon. Pretreatment with Flu (10 mg/kg) prior to MA administration has no influence on the acquisition of sensitization. However, pretreatment with Flu prior to CZP administration reversed the inhibitory effect of CZP. CZP showed no effect on the expression of sensitization in the sensitized rats when given prior to the MA readministration. These results strengthen the suggestion that stimulation of GABA-benzodiazepine receptors plays a role in the acquisition but not in the expression of behavioral sensitization to MA.

Role of the laterodorsal tegmental nucleus in scopolamine- and amphetamine-induced locomotion and stereotypy

Scopolamine (1.5 mg/kg; i.p.) or amphetamine (3 mg/kg; i.p.) increases locomotion and stereotyped behavior patterns in rats. Previous studies suggest that scopolamine acts via muscarinic receptors near the midbrain-pons border. In this study, unilateral microinjections in N-methyl-scopolamine (2.5-10 microg) into the laterodorsal tegmental nucleus (LDT) increased locomotion. Bilateral ibotenate lesions of the LDT attenuated scopolamine-induced locomotion by 68% 7 days postlesion, and by 35% 28 days postlesion. LDT lesions reduced scopolamine-induced stereotypy less than locomotion. The sensitization to amphetamine observed on repeated tests was attenuated by LDT lesions for stereotypy, but not for locomotion. These findings suggest that scopolamine induces locomotion largely, but not exclusively, by blocking muscarinic receptors in LDT.

Effects of repeated administration of methamphetamine on P3-like potentials in rats

Effects of repeated administration of methamphetamine (MAP) on a component of the cortical event-related potential (ERP), P3-like potential which corresponds to the human P3b, were examined in rats performing an active discrimination task. Rats were trained to press a bar within 1200 ms after cessation of a target tone (1000 Hz) lasting for 800 ms, and to withhold an overt response to the standard tone (2000 Hz). The rats were given intracranial electrical stimulation to the medial forebrain bundle as a reward, only when they correctly responded to the target tone. ERPs before drug administration were recorded after the correct response ratio exceeded 85%. Thereafter, a daily dose of 4 mg/kg of MAP, or the same volume of saline in another group, was administered intraperitoneally 15 times. ERPs were recorded again 7-10 days after the last injection. In the rats which received MAP the amplitude of the P3-like potential decreased with no change in its latency, while the response latency of bar-pressing and the correct response ratio were not altered significantly. These results suggest some changes in catecholaminergic transmission induced by repeated MAP-administration affect a P3 generation mechanism. MAP-treated rats may be useful as an animal model to investigate neural mechanisms of MAP-psychosis and schizophrenia.

Dopamine and serotonin imbalances in the left anterior cingulate and pyriform cortices following the repeated intermittent administration of cocaine

Studies on the neurobiology of cocaine abuse suggest that cocaine directly modifies the activity of dopamine neurons projecting from the dopamine-synthesizing cells of the ventral tegmental area to the nucleus accumbens. The repeated use of cocaine produces persistent adaptations within the mesocorticolimbic system and the resulting changes in monoamine neurotransmission may lead to behavioral sensitization. The present series of experiments sought to determine the effects of the repeated, intermittent challenge that took place two days after discontinuation of the pretreatment regimen; (ii) the ex vivo levels of biogenic monoamines, choline and acetylcholine in the nucleus accumbens, the dorsolateral caudate nucleus, as well as the anterior cingulate, frontal motor, frontal somatosensory and pyriform cortices; and (iii) the degree of neurochemical relationship between the left and right hemispheres. The repeated administration of cocaine produced sensitized behavioral responses to a subsequent challenge. Neurochemical correlates of repeated cocaine administration were observed at the cortical level and included a significant decrease in serotonin levels in the left anterior cingulate and pyriform cortices and an increase in dopamine metabolism in the left pyriform cortex. Furthermore, a shift in the interhemispheric coupling coefficient matrix for dopamine neurotransmission was observed in both the pyriform cortex and nucleus accumbens of cocaine-sensitized animals suggesting that, in these structures, the two hemispheres are operating independently. These results demonstrate that cocaine produces alterations in specific dopaminergic and serotonergic pathways that arise from the mesencephalon and project towards both the anterior cingulate and pyriform cortices.

Importance of initial environments in the development of ambulatory sensitization to methamphetamine and cocaine in mice

Repeated administration of CNS stimulants such as amphetamines and cocaine induces behavioural sensitization which can be influenced by the animal's environment. This study has evaluated the effect of restraint on the development and maintenance of ambulatory sensitization to methamphetamine and cocaine in mice. Subcutaneous administration of the CNS stimulants methamphetamine (2 mg kg(-1)) and cocaine (20 mg kg(-1)) seven times at three-day intervals resulted in ambulatory sensitization when the mice were placed in 20-cm diameter activity cages after each dose of the drug. However, if methamphetamine or cocaine was administered when the mice were in small jars (6-cm diameter) in which expression of ambulation, but not of circling and rearing, was completely restricted, the development of ambulatory sensitization was retarded or inhibited, with circling behaviour concurrently increased, when subsequent repeated doses of the drug were administered in the activity cage. Subsequent repeated treatment of ambulatory-sensitized mice with the drug or saline when the mice were in the jars did not change the levels of the ambulatory sensitization or the circling behaviour. These results suggest that the mice are sensitized to the behavioural effect of CNS stimulants which can be expressed in the environment in which the drug is administered. It is also considered that the established sensitization is strongly retained and is responsible for retardation or suppression of the development of sensitization to other behavioural stimulant effects.

Methamphetamine modulates ACh-evoked currents in Xenopus occytes expressing the rat alpha7 receptors

Although the cholinergic system is proposed to be involved in methamphetamine (MeAMPH)-induced abnormal behaviors, no direct evidence has been provided yet. The present study investigated the effects of MeAMPH on acetylcholine (ACh)-evoked currents in the neuronal nicotinic ACh receptors (alpha7) expressed in Xenopus oocytes. MeAMPH enhanced the currents in a time- and dose-dependent manner at concentrations ranged from 1 nM to 3 microM, reaching a maximum of 150% 30 min after treatment. Lesser potentiation was observed at higher concentrations (>3 microM) and instead, the currents were inhibited at more than 10 microM MeAMPH with a slow reverse after washing-out of the drug. The current potentiation or depression was caused via a pathway independent of G-protein, protein kinase C or cAMP-dependent protein kinase. The ACh dose-response curve was shifted to the left and to the right after treatment with 1 and 100 microM MeAMPH, respectively, suggesting that MeAMPH potentiated or inhibited ACh-evoked currents by a change in the affinity for ACh. The actions of MeAMPH on the neuronal nicotinic ACh receptors may represent a cellular mechanism for MeAMPH-induced abnormal behaviors and sensitization.

Clonazepam prevents the development of sensitization to methamphetamine

The GABA-benzodiazepine neurotransmission has been implicated in various forms of plasticity such as kindling and learning. The present study examined the effects of clonazepam (CZP), a GABA-benzodiazepine agonist, on the development of behavioral sensitization to methamphetamine (MA). Rats treated with MA (1 mg/kg, S.C.) for 10 days displayed significantly enhanced motor activity when tested with MA (1 mg/kg) after a 7-8-day withdrawal, indicating the development of behavioral sensitization. Pretreatment with CZP (0.5 and 2.0 mg/kg) prior to MA administration prevented the development of the phenomenon. Rats treated with CZP alone showed no difference in the motor activity compared to those treated with saline. These results suggest that stimulation of GABA-benzodiazepine receptors plays a role in the development of behavioral sensitization.

Evidence for an involvement of muscarinic cholinergic systems in the induction but not expression of behavioral sensitization to cocaine

The present study was designed to determine whether the muscarinic cholinergic antagonist scopolamine can prevent the expression and induction of sensitization to the locomotor-activating effects of cocaine. Rats received one daily injection of cocaine (20 mg/kg i.p.) for 5 days. Two days after withdrawal of pretreatment, rats were pretreated with scopolamine (3.0 mg/kg s.c) or its vehicle and challenged 15 min later with either saline or cocaine (20-30 mg/kg i.p.). In second set of experiments, scopolamine (3.0 mg/kg s.c) or its vehicle was given in combination with either saline or cocaine (20 mg/kg i.p.) for 5 days. Activity in response to saline and to cocaine (20 mg/kg i.p.) was assessed on day 7. The effects of acute administration of scopolamine (3.0 mg/kg s.c.) on cocaine-induced locomotor activity were also assessed. Acute administration of scopolamine increased both distance traveled and time spent in stereotypy. When scopolamine was administered 15 min prior to an acute injection of cocaine, a significant increase in the behavioral response to cocaine was seen. Daily injections of cocaine for 5 days produced sensitized behavioral responses to a subsequent cocaine challenge. Acute administration of scopolamine to animals preexposed and sensitized to cocaine did not disrupt the expression of sensitization to the locomotor and stereotypic effects of cocaine. In contrast, when scopolamine was given in combination with cocaine for 5 days, sensitization to the locomotor-activating effects of cocaine was prevented. These results suggest an important role of cholinergic muscarinic systems in mediating sensitization to the locomotor-activating effects of cocaine, which occurred after the repeated context-independent administration of this agent. In contrast, the enhanced stereotypic effects in response to the repeated administration of cocaine seem to be independent of alterations in muscarine cholinergic transmission.

The role of glutamate in behavioral and neurotoxic effects of methamphetamine

Studies on the mechanisms of behavioral and neurochemical effects of amphetamine or methamphetamine (MA) have focused on the dopaminergic system. However, recent reports suggest that the glutamatergic system may be involved in the MA effects. Our laboratory has been conducting a series of experiments to further examine the role of glutamate in both behavioral and neurotoxic effects of MA. These studies include (1) behavioral studies on the effect of N-methyl-D-aspartate (NMDA) antagonists on the development of MA-induced behavioral sensitization, (2) neurochemical studies on the effects of NMDA antagonists on MA-induced neurotoxicity, and (3) in vivo microdialysis studies on the effects of MA on glutamate release. In the present paper, the authors comment on an important role of glutamatergic systems in the behavioral and toxic effects of MA.

Scopolamine prevents augmentation of stereotypy induced by chronic methamphetamine treatment

Cholinergic neurotransmission has been implicated in various forms of neural plasticity such as kindling and learning. We have previously shown that blockade of muscarinic cholinergic receptors prevents the development of locomotor sensitization to methamphetamine. The present study was conducted to examine whether scopolamine, a muscarinic cholinergic antagonist, would also block augmentation of stereotypy induced by chronic methamphetamine (MA) treatment. Rats treated with MA (2.5 mg/kg, SC) for 10 days indicated significantly enhanced stereotyped behavior when tested with MA (2.5 mg/kg) after a 7- to 8- day withdrawal. Pretreatment with scopolamine (3 mg/kg) prior to MA administration prevented the augmentation of stereotypy. Rats treated with scopolamine alone showed no difference in MA-induced stereotypy compared to those treated with saline. Scopolamine methylbromide, a derivative of scopolamine that does not easily cross the blood-brain barrier, had no effect on the augmentation of stereotypy. These results suggest that stimulation of central muscarinic cholinergic receptors plays a role in the development of sensitization to the stereotypy stimulating effect of methamphetamine.