Repeated Methamphetamine Injections Research Articles

Striatal miR-183-5p inhibits methamphetamine-induced locomotion by regulating glucocorticoid receptor signaling.

MicroRNA (miRNA)-mediated striatal gene regulation may play an important role in methamphetamine (METH) addiction. This study aimed to identify changes in novel miRNAs and their target genes during METH self-administration and investigate their roles in METH-induced locomotion. RNA sequencing analysis revealed that mir-183-5p was upregulated in the striatum of METH self-administered rats, and target gene prediction revealed that the glucocorticoid receptor (GR) gene, Nr3c1, was a potential target gene for mir-183-5p. We confirmed that single and repeated METH administrations increased METH-induced locomotion and plasma corticosterone levels in rats. Additionally, increased miR-185-5p expression and decreased GR gene expression were observed only in the repeated-METH-injection group but not in the single-injection group. We then investigated the effects of miR-183-5p on METH-induced locomotion using a miR-183-5p mimic and inhibitor. Injection of a mir-183-5p mimic in the striatum of rats attenuated METH-induced locomotion, whereas injection of a miR-183-5p inhibitor enhanced the locomotor activity in METH-administered rats. Furthermore, the miR-183-5p mimic reduced the phosphorylation of tyrosine hydroxylase (TH) whereas the inhibitor increased it. Taken together, these results indicate that repeated METH injections increase striatal miR-183-5p expression and regulate METH-induced locomotion by regulating GR expression in rats, thereby suggesting a potential role of miR-183-5p as a novel regulator of METH-induced locomotion.

Behavioural and biochemical responses to methamphetamine are differentially regulated by mGlu2 and mGlu3 metabotropic glutamate receptors in male mice

Group II metabotropic glutamate receptors (mGlu2 and mGlu3 receptors) shape mechanisms of methamphetamine addiction, but the individual role played by the two subtypes is unclear. We measured methamphetamine-induced conditioned place preference (CPP) and motor responses to single or repeated injections of methamphetamine in wild-type, mGlu2−/−, and mGlu3−/− mice. Only mGlu3−/− mice showed methamphetamine preference in the CPP test. Motor response to the first methamphetamine injection was dramatically reduced in mGlu2−/− mice, unless these mice were treated with the mGlu5 receptor antagonist, MTEP. In contrast, methamphetamine-induced sensitization was increased in mGlu3−/− mice compared to wild-type mice. Only mGlu3−/− mice sensitized to methamphetamine showed increases in phospho-ERK1/2 levels in the nucleus accumbens (NAc) and free radical formation in the NAc and medial prefrontal cortex. These changes were not detected in mGlu2−/− mice.We also measured a series of biochemical parameters related to the mechanism of action of methamphetamine in naïve mice to disclose the nature of the differential behavioural responses of the three genotypes. We found a reduced expression and activity of dopamine transporter (DAT) and vesicular monoamine transporter-2 in the NAc and striatum of mGlu2−/− and mGlu3−/− mice, whereas expression of the DAT adaptor, syntaxin 1A, was selectively increased in the striatum of mGlu3−/− mice. Methamphetamine-stimulated dopamine release in striatal slices was largely reduced in mGlu2−/−, but not in mGlu3−/−, mice. These findings suggest that drugs that selectively enhance mGlu3 receptor activity or negatively modulate mGlu2 receptors might be beneficial in the treatment of methamphetamine addiction and associated brain damage.

HIV-1 TAT protein enhances sensitization to methamphetamine by affecting dopaminergic function

Methamphetamine abuse is common among humans with immunodeficiency virus (HIV). The HIV-1 regulatory protein TAT induces dysfunction of mesolimbic dopaminergic systems which may result in impaired reward processes and contribute to methamphetamine abuse. These studies investigated the impact of TAT expression on methamphetamine-induced locomotor sensitization, underlying changes in dopamine function and adenosine receptors in mesolimbic brain areas and neuroinflammation (microgliosis). Transgenic mice with doxycycline-induced TAT protein expression in the brain were tested for locomotor activity in response to repeated methamphetamine injections and methamphetamine challenge after a 7-day abstinence period. Dopamine function in the nucleus accumbens (Acb) was determined using high performance liquid chromatography. Expression of dopamine and/or adenosine A receptors (ADORA) in the Acb and caudate putamen (CPu) was assessed using RT-PCR and immunohistochemistry analyses. Microarrays with pathway analyses assessed dopamine and adenosine signaling in the CPu. Activity-dependent neurotransmitter switching of a reserve pool of non-dopaminergic neurons to a dopaminergic phenotype in the ventral tegmental area (VTA) was determined by immunohistochemistry and quantified with stereology. TAT expression enhanced methamphetamine-induced sensitization. TAT expression alone decreased striatal dopamine (D1, D2, D4, D5) and ADORA1A receptor expression, while increasing ADORA2A receptors expression. Moreover, TAT expression combined with methamphetamine exposure was associated with increased adenosine A receptors (ADORA1A) expression and increased recruitment of dopamine neurons in the VTA. TAT expression and methamphetamine exposure induced microglia activation with the largest effect after combined exposure. Our findings suggest that dopamine-adenosine receptor interactions and reserve pool neuronal recruitment may represent potential targets to develop new treatments for methamphetamine abuse in individuals with HIV.

Netrin-1 receptor-deficient mice show age-specific impairment in drug-induced locomotor hyperactivity but still self-administer methamphetamine

The mesocorticolimbic dopamine system undergoes significant reorganization of neuronal connectivity and functional refinement during adolescence. Deleted in colorectal cancer (DCC), a receptor for the guidance cue netrin-1, is involved in this reorganization. Previous studies have shown that adult mice with a heterozygous (het) loss-of-function mutation in DCC exhibit impairments in sensitization and conditioned place preference (CPP) to psychostimulants. However, the commonly abused psychostimulant methamphetamine (METH) has not been assessed, and the role of DCC in drug self-administration remains to be established. Using dcc het mice and wildtype (WT) littermates, we extended previous findings on dcc haplodeficiency by examining self-administration of METH in adult mice, including cue-induced drug seeking following abstinence. We also examined hyperactivity, sensitization, and CPP to a METH-paired context in adult and adolescent mice. While adult dcc het mice expressed largely similar METH self-administration and cue-induced drug seeking as WT littermates, they failed to modulate responding according to dose of METH. Compared to WT, both adult and adolescent dcc het mice expressed impaired locomotor hyperactivity to acute METH but nevertheless showed comparable behavioral sensitization. Conditioned hyperactivity increased with age in WT but not in dcc het mice. Impaired METH-induced hyperactivity and dose-related responding in adult dcc het mice suggest that reduced DCC alters METH-related behaviors. Adolescence is identified as a vulnerable period during which impairment in hyperactivity due to reduced DCC can be overcome with repeated METH injections. Nevertheless, DCC appears to have a somewhat limited role in METH-consumption and seeking following abstinence.

Epigenetics of Methamphetamine-Induced Changes in Glutamate Function

Addiction to methamphetamine (METH) is a relapsing neuropsychiatric disorder that is secondary, in part, to functional changes in limbic and striatal brain regions (reviewed in Krasnova and Cadet (2009)). Stimulant-induced plastic changes within the striatum are dependent on a series of events that include modifications in the number and subtypes of glutamate receptors (Wolf and Ferrario, 2010). Elucidating the basic mechanisms that maintain METH addiction is important because such an understanding will probably lead to the development of efficacious treatments. The accumulated evidence supports the notion that illicit drugs exert substantial transcriptional and epigenetic changes in the brain (Robison and Nestler, 2011). Gene transcription is regulated by complex epigenetic changes that include post-translational histone modifications and DNA methylation (Mehler, 2008). There is evidence that epigenetic phenomena are intimately involved in the development and the clinical course of complex neuropsychiatric diseases including addiction (Robison and Nestler, 2011). Therefore, we thought it likely that METH might engender transcriptional and epigenetic alterations that are unique to this clinically devastating drug. As a first step toward clarifying the effects of METH on glutamatergic function, we treated rats with an escalating METH dose paradigm that started at METH (0.5 mg/kg twice/day) and ended with METH (3 mg/kg four times per day) over a period of 2 weeks (McCoy et al., 2011). We found that chronic METH caused significant decreases in mRNA and protein levels of both GluA1 and GluA2 AMPA receptor subunits. We also found that METH caused significant decreases in acetylation of histone H4 at lysine 5 (H4K5), lysine 12 (H4K12), and lysine 16 (H4K16). Using chromatin immunoprecipitation-PCR assay, we found that repeated METH injections produced decreased binding of acetylated H4K5, H4K12, and H4K16 on GluA1 and GluA2 DNA sequences. In addition, chronic METH administration enhanced the recruitment of corepressor of RE1 silencing transcription (CoREST) factor onto GluA1 and GluA2 DNA sequences. METH also caused CoREST co-immunoprecipitation with histone deacetylase 2 (HDAC2) and sirtuin 2 (SIRT2). Moreover, METH increased enrichment of methyl CpG binding protein 2 (MeCP2) on the promoters of both GluA1 and GluA2, with co-immunoprecipitation studies revealing METH-induced MeCP2 interactions with HDAC2. Finally, we demonstrated that the FDA-approved HDAC inhibitor, valproic acid, prevented METH-induced downregulation of GluA1 and GluA2 mRNA levels. In summary, the present study provides direct evidence for epigenetic regulation of chronic transcriptional effects of METH in the dorsal striatum. Figure 1 provides a scheme that describes a potential role of CoREST, HDAC2, MeCP2, and SIRT2 in the mediation of METH-induced downregulation of GluA1 and GluA2 mRNA levels. Specifically, CoREST might recruit SIRT2 onto the chromatin, with resulting H4K16ac hypoacetylation and decreased H4K16ac binding onto GluA1 and GluA2 DNA sequences. In addition, a METH-induced MeCP2-CoREST-HDAC2 complex might preferentially be involved in the hypoacetylation of histone H4 at lysines 5 and 12. The present observations add to the accumulating evidence that psychostimulants can cause substantial transcriptional and epigenetic changes in the brain. These results also suggest that therapeutic approaches that involved the use of epigenetic agents might be important areas for future investigations. Figure 1 Models illustrating METH-induced epigenetic modifications in the dorsal striatum. Under normal conditions, a balance exists between histone acetyltransferases (HATs) and histone deacetylases (HDACs) that regulate the histone acetylation/deacetylation ...

Amphetamine and methamphetamine reduce striatal dopamine transporter function without concurrent dopamine transporter relocalization

Amphetamine (AMPH) and methamphetamine (METH) alter dopamine transporter (DAT) function. In vitro heterologous cell line and synaptosome studies demonstrate AMPH-induced DAT internalization, implicating relocalization in reduced DAT uptake following drug exposure. However, few studies have evaluated DAT localization following in vivo AMPH/METH administration. To determine DAT subcellular localization following drug administration, a centrifugation technique was developed to isolate striatal synaptosomal membrane and vesicle fractions. DAT was distributed between the synaptosomal membrane (60%) and endosomal vesicles (40%), and in vitro application of the protein kinase C activator phorbol 12-myristate 13-acetate to striatal synaptosomes caused DAT internalization into the vesicle fractions. In contrast, neither single nor repeated in vivo AMPH and/or METH administrations altered DAT localization 5, 15, 30, or 60 min post-treatment, despite reduced DAT uptake. Importantly, repeated METH injections uniformly decreased total DAT immunoreactivity within all fractions 7 days post-treatment. These findings suggest that factors other than internalization can contribute to the observed acute and persistent DAT dysfunction and dopaminergic deficits following in vivo AMPH or METH administration.

Protective Effect of Sauchinone on Methamphetamine-Induced Neurotoxicity in Mice

Sauchinone is a lignan isolated from Saururus chinensis known to suppress nitric oxide (NO) activity. Previous studies demonstrate that NO plays a key role in methamphetamine-induced neurotoxicity. Thus, we hypothesized that sauchinone could have a suppressive effect on the neurotoxicity induced by methamphetamine. Repeated injections of methamphetamine cause degeneration of dopaminergic nerve terminals, whereas sauchinone treatment significantly prevented this degeneration. Sauchinone treatment also inhibited the methamphetamine-induced activation of glia cells and the production of NO via a blockade of inducible NO synthase protein expression. Our results suggest that sauchinone can prevent methamphetamine-induced neurotoxicity through the suppression of NO production.

Repeated administration of methamphetamine blocked cholecystokinin-octapeptide injection-induced c-fos mRNA expression without change in capsaicin-induced junD mRNA expression in rat cerebellum

In the cerebellum, there are numerous cholecystokinin (CCK-8)-containing fibers. Since systemic CCK-8 injection-induced anxiety (psychological stress) activates the locus coeruleus cells that send mossy fiber inputs to the cerebellum, we examined whether systemic CCK-8 injections activate the rat and mouse cerebellum. First, injections of CCK-8 were found to induce c-fos mRNA expression in a vague patchy pattern that is different from single methamphetamine-induced Zebrin band-like c-fos mRNA expression, suggesting that the CCK-8 activating mossy fibers induce gene expression differently from the dopamine-containing mossy fibers in the ventral tegmental area. Second, since CCK-8 facilitates neural activity of dopamine in the midbrain, we examined whether repeated methamphetamine administration that induced behavioral sensitization had similar effects on the cerebellar CCK system. Repeated administration of methamphetamine suppressed the CCK-8-induced c-fos mRNA expression in the rat cerebellum. Third, capsaicin injections (physical stress) into a hind limb of the rat increased junD mRNA expression with no effect on c-fos mRNA expression, and repeated methamphetamine injections had no effect on the capsaicin-induced expression of junD mRNA. Fourth, either single injection of methamphetamine or CCK-8 to mice increased c-fos mRNA expression in the locus coeruleus, and so noradrenalin, but not dopamine, might interact with CCK-8-activating system. However, we considered the possibility unlikely. Thus, we conclude that repeated methamphetamine administration though dopamine selectively inhibits the c-fos mRNA expression after CCK-8 injection in the cerebellum.

Differential effects of methamphetamine and SCH23390 on the expression of members of IEG families of transcription factors in the rat striatum

Methamphetamine (METH) is a psychostimulant that can cause long-lasting neurodegenerative effects in humans and animals. These toxic effects appear to occur, in part, via activation of dopamine (DA) D1 receptors. This paper assessed the possibility that the DA D1 receptor antagonist, SCH23390, might inhibit METH-induced changes in the expression of several members of immediate early genes (IEGs) which are known to control more delayed expression of other genes. We found that injections of METH (4 × 10 mg/kg, given at 2 h intervals) caused significant increases in c-fos and fra-2 expression which lasted from 30 min to 4 h. Pre-treatment with SCH23390, given 30 min before each METH injection, completely blocked METH-induced expression of c-fos, but only partially inhibited fra-2 mRNA expression. These results were confirmed by Western blot analysis which showed METH-induced changes in c-Fos protein expression that were blocked by pretreatment with SCH23390. There were also delayed METH-induced DA D1 receptor-dependent effects on fosB mRNA expression. Even though fra-1 expression was not affected by pretreatment with METH alone, the repeated injections of SCH23390 caused substantial decreases in fra-1 mRNA expression in both the presence and absence of METH. The repeated injections of METH caused no changes in the mRNAs for c-jun, junB or junD. However, there were significant increases in the phosphorylation of c-Jun protein (ser63). Phosphorylation of c-Jun occurred in a delayed fashion (16 and 24 h after the last METH injections) and was attenuated by SCH23390 pretreatment. Interestingly, SCH23390 given alone caused significant decreases in phospho-c-Jun at all time-points. The METH injections also caused delayed induction in the expression of members of the Egr family of transcription factors in a DA D1 receptor-dependent fashion. Repeated injections of SCH23390 caused substantial suppression of basal striatal egr-1 and egr-2 mRNA expression but not of that of egr-3. Both crem and arc mRNA levels were induced by METH in a SCH23390-sensitive fashion. Moreover, multiple injections of SCH23390 given alone caused marked inhibition of basal arc expression. These results show that multiple injections of METH can differentially affect the expression of several IEGs, some of which occurred in a DA D1 receptor dependent fashion. The SCH23390-mediated suppression of basal fra-1, egr-1, and egr-2 mRNA levels suggests that their basal expression in the striatum might be dependent on tonic stimulation of the DA D1 receptor.

Protective Effects of Isoliquiritigenin Against Methamphetamine-Induced Neurotoxicity in Mice

Isoliquiritigenin (ISL) suppresses cocaine-induced extracellular dopamine levels and has a neuroprotective effect in cocaine-treated rat brain. Here, we examine the effect of ISL on methamphetamine-induced striatal neurotoxicity. Repeated injections of methamphetamine cause the loss of striatal dopamine transporter (DAT) and tyrosine hydroxylase (TH). Intraperitoneal injection of ISL prior to methamphetamine injection significantly prevented methamphetamine-induced reduction of DAT and TH. ISL also suppressed methamphetamine-induced activation of glial cells. Moreover, ISL impeded the expression of nitric oxide synthase and the activation of NF-κB through blockage of its phosphorylation. Our results suggest that ISL protects against methamphetamine-induced neurotoxicity by inhibition of NF-κB activation.

Neurotoxic doses of methamphetamine cause neurocognitive abnormalities in mice

Methamphetamine (METH) is a neurotoxic psychostimulant that causes damage to striatal dopamine (DA) terminals and to non-monoaminergic cells in the striatum (STR) and cortex of rodents. The aim of the present study was to investigate short- and long-term consequences of neurotoxic METH doses on mice behaviors. Male BALB/c mice, 12–14 weeks old, were injected with dl-METH (i.p., 7.5 mg/kg x 4 times, every 2 hours) or saline. Behaviors were accessed at 10 days after drug treatment. METH administration caused significant decreases in responding to object displacement using an Open Field Object Recognition test. In contrast, METH caused increased response to object novelty. There were no drug-induced effects on locomotor activity. Moreover, repeated METH injections resulted in significant decreases in dopamine (DA) levels in the STR, frontal cortex (FC) and olfactory bulb (OB) in mice sacrificed 10 days after drug treatment. Furthermore, as previously reported for the cortex and striatum, METH administration caused marked increases in the number of cells that were positive for TUNEL staining in the OB at 3 days, observations that are indicative of drug-induced enhancement of cell death in that structure. When taken together, these results suggest that METH caused impairments in spatial learning and/or memory as a consequence of its neurodegenerative effects. This research was supported by the Intramural Research Program of the NIH, NIDA.

Social Isolation Stress Significantly Enhanced the Disruption of Prepulse Inhibition in Mice Repeatedly Treated with Methamphetamine

Repeated administration of methamphetamine (METH) causes reverse tolerance or behavioral sensitization in mice. However, the effects of social isolation stress on the METH-caused reverse tolerance have not been studied until now. The aim of this study was to investigate the effects of social isolation stress on METH-caused reverse tolerance by examining the prepulse inhibition of startle response (PPI). PPI was tested in socially isolated and grouped mice after repeated METH injections. Locomotor activity and PPI were also examined just after a four-week isolation rearing period as a control experiment. After completing behavioral experiments, the mice were sacrificed, and the contents of monoamines, including histamine in the brain, were measured. Social isolation stress significantly lowered the locomotion and disrupted PPI. Repeated injections of METH enhanced the effects of social isolation on PPI. The content of dopamine and histamine significantly increased in the cortex, and the turnover rate of dopamine decreased significantly. These findings demonstrate that social isolation stress significantly enhances METH-induced behavioral sensitization and that the altered histaminergic neuron system might play an important role in METH-induced behavioral sensitization in addition to dopaminergic and serotoninergic neurotransmission. Our data suggest that social isolation is involved in the development of METH-induced psychosis, schizophrenia, and other related psychiatric disorders.

Chronic methamphetamine increases fighting in mice

A propensity for violent behaviors to develop in chronic methamphetamine (METH) abusers has been noted. The idea that increased aggressiveness might result from chronic METH administration was tested in mice after chronic (long-term intermittent, 8 weeks) or single exposures to the drug. A single injection of METH (6 mg/kg) did not augment fighting. In contrast, chronic METH administration significantly increased the number of animals that initiated bite attacks. This regimen also shortened the latency before the first attack. Latency before the first attack was shorter at 20 h after the METH injection than at 15 min after injection. Locomotor activity was not different at 20 h after METH injection, indicating that increased fighting was not secondary to METH-induced hyperactivity. METH-induced increases in fighting were not related to the duration of persistent sniffing after the initial encounter with an intruder since the duration of this behavior was significantly increased at 15 min after METH but not at 20 h post drug. These results indicate that repeated injections of METH can increase fighting behaviors and also alter social interactions in mice. Thus, intermittent administration of METH might be useful as a pharmacological model to study the biochemical and molecular bases of aggressiveness.

Methamphetamine-induced increase in striatal p53 DNA-binding activity is attenuated in Cu,Zn-superoxide dismutase transgenic mice

The striatal DNA-binding activities of p53 as a transcription factor were gradually increased at several days after a single methamphetamine (METH) injection, while they were more rapidly increased within several hours after repeated METH injections (×4 with a 2 h interval). The elevation of striatal p53 DNA-binding after repeated METH injections was markedly attenuated in Cu,Zn-superoxide dismutase transgenic mice, but not affected by treatments with N-methyl- d-aspartate or D1 receptor antagonists. The present results suggest that METH-induced production of reactive oxygen species activates striatal p53 DNA-binding activity; this, in turn, may activate other downstream pathways that are responsible for chronic neurotoxicity of METH.

Cholecystokinin injection-induced vague patch of c-fos mRNA expression in the rat cerebellum: blocking by repeated methamphetamine injections

Cholecystokinin injection-induced vague patch of c-fos mRNA expression in the rat cerebellum: blocking by repeated methamphetamine injections

Differential decreases in c- fos and aldolase C mRNA expression in the rat cerebellum after repeated administration of methamphetamine

The effects of repeated methamphetamine administration on c- fos mRNA and aldolase C (Zebrin) mRNA expression in the rat cerebellum were investigated. A single dose of methamphetamine induced c- fos mRNA expression in granule and Purkinje cells of both anterior and posterior lobes. In the posterior lobe, in particular, c- fos mRNA signals were distributed in a parasagittal organization, like Zebrin bands. Repeated methamphetamine injections reduced methamphetamine-induced c- fos mRNA signals in the anterior hemisphere and in part of the posterior vermis (lobule VII) and posterior hemisphere. Aldolase C mRNA signals in Purkinje cells decreased only in lobules where methamphetamine-induced c- fos signals were not reduced (lobules VI and IX). Therefore, differential decreases in c- fos mRNA and aldolase C mRNA expression after repeated methamphetamine administration depend upon the localization of Purkinje cells in the cerebellum. Since c- fos mRNA and aldolase C mRNA expressions are markers of excitability and the metabolic state of Purkinje cells, respectively, hypofunction of inhibitory Purkinje cells could be induced if methamphetamine is repeatedly injected. Since repeated methamphetamine administration in this experimental paradigm increased horizontal movement and the rearing activity of rats, the hemisphere of the cerebellum may be involved in development of methamphetamine-induced motor behavioral sensitization in addition to the striatum and the nucleus accumbens.

Characterizing cortical neuron injury with Fluoro-Jade labeling after a neurotoxic regimen of methamphetamine.

We used Fluoro-Jade, a recently-developed fluorescent indicator of neuronal damage, to identify neurons injured 1-21 days after repeated injections of methamphetamine (m-AMPH) or saline. The m-AMPH-treated rats showed Fluoro-Jade positive neurons in parietal cortex (layers III and IV) and had less striatal tyrosine hydroxylase immunoreactivity than did saline-injected controls. Fluoro-Jade positive neurons were greatest in number 3 days post-treatment; some fluorescent neurons displayed bud-like surface protrusions. These observations support the hypothesis that certain neocortical neurons degenerate after m-AMPH.

Characterizing cortical neuron injury with fluoro‐jade labeling after a neurotoxic regimen of methamphetamine

We used Fluoro-Jade, a recently-developed fluorescent indicator of neuronal damage, to identify neurons injured 1–21 days after repeated injections of methamphetamine (m-AMPH) or saline. The m-AMPH-treated rats showed Fluoro-Jade positive neurons in parietal cortex (layers III and IV) and had less striatal tyrosine hydroxylase immunoreactivity than did saline-injected controls. Fluoro-Jade positive neurons were greatest in number 3 days post-treatment; some fluorescent neurons displayed bud-like surface protrusions. These observations support the hypothesis that certain neocortical neurons degenerate after m-AMPH. Synapse 30:329–333, 1998. © 1998 Wiley-Liss, Inc.

Uridine reduces rotation induced by l-Dopa and methamphetamine in 6-OHDA-treated rats

The pyrimidine nucleoside uridine may reduce side effects associated with antipsychotic medication by interacting with dopamine or GABA neurotransmission. Male Sprague-Dawley rats were used to investigate coadministration of uridine with agents that alter food intake (amphetamine, haloperidol, and chlordiazepoxide) and locomotor activity (methamphetamine and l-dopa). Results indicated that chronic uridine [32.0 mg/kg, intraperitoneally (IP)] alone did not alter milk intake or reduction of milk intake induced by amphetamine (dose range 0.5–2.0 mg/kg, IP) or haloperidol (0.125–1.0 mg/kg, IP), nor did it alter the biphasic response induced by chlordiazepoxide (5.0–40.0 mg/kg, IP). However, uridine-treated animals with unilateral striatal lesions exhibited no rotational behavior in the absence of drug challenge, but showed decreased rotation induced by the dopamine agonist, l-dopa (50.0–200.0 mg/kg, IP) compared with controls. In addition, uridine-treated rats exhibited reduced rotation after repeated injections of methamphetamine (4.0 mg/kg, IP) in contrast to increasingly greater rotation observed in control animals. These results are further evidence that chronic uridine may alter drug-induced dopaminergic activity without exerting effects itself.

Suppressive effect of cycloheximide on behavioral sensitization to methamphetamine in mice

The effect of a protein synthesis inhibitor, cycloheximide, on behavioral sensitization to methamphetamine was investigated in mice. As indicated by the sensitization tests, repeated injection of methamphetamine (2 mg/kg i.p.) at intervals of 3 and 4 days resulted in a progressive augmentation of the locomotor-stimulating effect of methamphetamine. This phenomenon, called locomotor sensitization, was attenuated by simultaneous treatment with cycloheximide (120 mg/kg i.p.) at the time of stimulant injection. In contrast, when mice were treated with cycloheximide 4 h after stimulant injection, locomotor activity was progressively augmented in the same way as observed in mice receiving repeated injections of methamphetamine alone. On challenge, it was noted that locomotor activity was significantly higher in mice injected repeatedly with the stimulant alone and in those mice treated with the inhibitor 4 h after the stimulant injection compared to the saline-treated control mice. However, mice that had been simultaneously treated with cycloheximide and methamphetamine showed almost the same locomotor activity as the saline-treated control mice. These observations indicated that the locomotor sensitization to methamphetamine was possibly suppressed by simultaneous treatment with cycloheximide. We then examined the dose- and time-dependent nature of the effect of cycloheximide on locomotor sensitization. The stimulation of locomotion observed after repeated injection of the stimulant at a dose of 1.5 mg/kg was significantly attenuated by simultaneous treatment with 120 or 240 mg/kg of cycloheximide, but not by treatment with 60 mg/kg of the inhibitor. However, all the treatments failed to suppress the development of locomotor sensitization elicited by 3 mg/kg of methamphetamine. The locomotor stimulation observed after repeated injection of 1.5 mg/kg methamphetamine was effectively suppressed by treatment with 120 mg/kg of cycloheximide given either simultaneously or 2 h after the stimulant injection. Treatment 4 h after methamphetamine did not suppress the stimulation of locomotion. These results suggest that the protein synthesis inhibitor has a suppressive effect on the development of behavioral sensitization to repeated injections of stimulant drugs in a dose- and time-dependent manner.