Induced Locomotor Sensitization Research Articles

Cocaine induces locomotor sensitization through a dopamine-dependent VTA-MPFC-FrA cortico-cortical pathway in male mice

Cocaine induces locomotor sensitization through a dopamine-dependent VTA-MPFC-FrA cortico-cortical pathway in male mice

Cocaine induces locomotor sensitization through a dopamine-dependent VTA-mPFC-FrA cortico-cortical pathway in male mice

As a central part of the mammalian brain, the prefrontal cortex (PFC) has been implicated in regulating cocaine-induced behaviors including compulsive seeking and reinstatement. Although dysfunction of the PFC has been reported in animal and human users with chronic cocaine abuse, less is known about how the PFC is involved in cocaine-induced behaviors. By using two-photon Ca2+ imaging to simultaneously record tens of intact individual networking neurons in the frontal association cortex (FrA) in awake male mice, here we report that a systematic acute cocaine exposure decreased the FrA neural activity in mice, while the chemogenetic intervention blocked the cocaine-induced locomotor sensitization. The hypoactivity of FrA neurons was critically dependent on both dopamine transporters and dopamine transmission in the ventromedial PFC (vmPFC). Both dopamine D1R and D2R neurons in the vmPFC projected to and innervated FrA neurons, the manipulation of which changed the cocaine-induced hypoactivity of the FrA and locomotor sensitization. Together, this work demonstrates acute cocaine-induced hypoactivity of FrA neurons in awake mice, which defines a cortico-cortical projection bridging dopamine transmission and cocaine sensitization.

Alpha-pyrrolidinopentiothiophenone (α-PVT) activates the TLR–NF-κB–MAPK signaling pathway and proinflammatory cytokine production and induces behavioral sensitization in mice

Synthetic cathinones are chemical derivatives of cathinone, a structural analog to amphetamine. It has been shown that synthetic cathinones have abuse potentials similar to psychomotor stimulants such as amphetamine and induce neuroinflammation. Among the novel synthetic cathinones, α-pyrrolidinopentiothiophenone (α-PVT) has been known to produce rewarding and reinforcing effects in rodent models. However, it has not yet been determined whether α-PVT induces neuroinflammation in vivo. In the present study, mice were exposed to repeated saline or α-PVT (20 mg/kg, intraperitoneally) for 7 days to test changes in locomotor activity and neuroinflammation-related factors in the striatum of mice. Repeated administration of α-PVT significantly induced locomotor sensitization. In addition, repeated α-PVT administration significantly increased the number of microglial cells, accompanied by marked increases in TLR1, TLR4, TLR6, and TLR7 mRNA levels in the striatum of mice. Furthermore, acute or repeated α-PVT administration increased the levels of phosphorylated NF-κB, ERK, p38, and JNK MAPK activation and repeated α-PVT, but not acute, increased the levels of TNF-α and IL-6 mRNA in the striatum of mice. Finally, systemic administration of TAK-242 (5 mg/kg, i.p.) or MPLA (50 μg/kg, i.p.), each an inhibitor or activator of TLR4, did not change α-PVT-induced behavioral sensitization in mice. These results suggest that the activation of TLR4 by repeated α-PVT administration may lead to neuroinflammation via TLR-mediated NF-κB and MAPK signaling pathways and the production of TNF-α and IL-6 in the striatum of mice, at least without the regulation of behavioral sensitization.

N-Ethylnorketamine has anesthetic and analgesic effects with abuse liability

Arylcyclohexylamines is an ever-growing class of new psychoactive substances, including an increasing number of ketamine analogs. N-Ethylnorketamine (NENK) is a new synthetic ketamine analog that has emerged as an abused drug, but little is known about the pharmacological profile of NENK. In this study, we investigated the anesthetic and analgesic activity, abuse liability of NENK compared with ketamine. The ED50 values of anesthetic activity for NENK and ketamine were 96.9, 69.4 mg/kg, respectively. The ED50 values of analgesic activity for NENK and ketamine were 45.9 and 23.6 mg/kg, respectively. NENK induced significant conditioned place preference at a minimum dose of 10.0 mg/kg in mice, an effect comparable to that of ketamine (3.0 mg/kg). Acute injections of NENK or ketamine at 30.0 mg/kg enhanced locomotor activity, and repeated treatments with this dose induced locomotor sensitization after withdrawal. Taken together, these results clearly demonstrated that NENK has lower anesthetic and analgesic activity compared to ketamine, but has significant abuse liability.

Prenatal Morphine Exposure Differentially Alters Addictive and Emotional Behavior in Adolescent and Adult Rats in a Sex-Specific Manner.

The effects of prenatal opioid exposure in adult animals has been widely studied, but little is known about the effects of prenatal opioid on adolescents. Most of the risk behaviors associated with drug abuse are initiated during adolescence. The developmental state of the adolescent brain makes it vulnerable to initiate drug use and susceptible to drug-induced brain changes. In this study, pregnant rats were subcutaneously injected with an increasing dose of morphine (5mg/kg, 7mg/kg, 10mg/kg) for 9 days since the gestation day 11. The effects of prenatal morphine (PNM) on learning and memory, anxiety- and depressive- like behavior, morphine induced conditioned place preference (CPP) as well as locomotor sensitization were tested in both adolescent and adult rats. The results showed that: (1) PNM decreased anxiety-like behavior in both adolescent and adult female rats, but not males; (2) PNM decreased depressive-like behavior in adolescent but increased depressive -like behavior in adult females; (3) PNM increased low dose morphine induced locomotor sensitization in females; (4) PNM decreased tyrosine hydroxylase (TH) expression in the prefrontal cortex but decreased dopamine D1 receptor expression in the nucleus-accumbens (NAc) in female rats. These results suggested that PNM altered the emotional and addictive behavior mainly in female rats, with female rats being less anxiety and depressive during adolescence, but more depressive in adult, and more sensitive to low dose morphine induced locomotor activity sensitization, which might be mediated in part by the differential expression of the TH, dopamine D1 receptors in the female brain.

Methamphetamine Disturbs Gut Homeostasis and Reshapes Serum Metabolome, Inducing Neurotoxicity and Abnormal Behaviors in Mice.

As an illicit psychostimulant, repeated methamphetamine (MA) exposure results in addiction and causes severe neurotoxicity. Studies have revealed complex interactions among gut homeostasis, metabolism, and the central nervous system (CNS). To investigate the disturbance of gut homeostasis and metabolism in MA-induced neurotoxicity, 2 mg/kg MA or equal volume saline was intraperitoneally (i.p.) injected into C57BL/6 mice. Behavioral tests and western blotting were used to evaluate neurotoxicity. To determine alterations of colonic dysbiosis, 16s rRNA gene sequencing was performed to analyze the status of gut microbiota, while RNA-sequencing (RNA-seq) and Western Blot analysis were performed to detect colonic damage. Serum metabolome was profiled by LC–MS analysis. We found that MA induced locomotor sensitization, depression-, and anxiety-like behaviors in mice, along with dysfunction of the dopaminergic system and stimulation of autophagy as well as apoptosis in the striatum. Notably, MA significantly decreased microbial diversity and altered the component of microbiota. Moreover, findings from RNA-seq implied stimulation of the inflammation-related pathway after MA treatment. Western blotting confirmed that MA mediated colonic inflammation by activating the TLR4-MyD88-NF-κB pathway and impaired colonic barrier. In addition, serum metabolome was reshaped after MA treatment. Specifically, bacteroides-derived sphingolipids and serotonin were obviously altered, which were closely correlated with locomotor sensitization, depression-, and anxiety-like behaviors. These findings suggest that MA disrupts gut homeostasis by altering its microbiome and arousing inflammation, and reshapes serum metabolome, which provide new insights into understanding the interactions between gut homeostasis and MA-induced neurotoxicity.

2-Fluorodeschloroketamine has similar abuse potential as ketamine.

2-Fluorodeschloroketamine (2-FDCK) as a substitute for ketamine has emerged among drug abusers in recent years. However, 2-FDCK has not been controlled or regulated in many countries, which may be partly related to the lack of evidence on its abuse potential. In this study, we evaluated the abuse potential of 2-FDCK via the tests of the conditioned place preference (CPP), locomotor sensitization, drug self-administration and drug discrimination using ketamine as a reference. 2-FDCK induced significant CPP at a minimum dose of 3mg/kg in mice, an effect comparable with that of ketamine (3mg/kg). Acute injections of 2-FDCK or ketamine at 30 mg/kg enhanced locomotor activity. Repeated treatments with this dose of 2-FDCK and ketamine induced locomotor sensitization after withdrawal. 2-FDCK readily induced self-administration with 0.5mg/kg/infusion, the same dose for ketamine, and induced the highest seeking response at 1mg/kg. Drug discrimination test showed that 2-FDCK dose-dependently substitute for ketamine with comparable ED50 to ketamine in substitution testing. Taken together, these results strongly suggested that 2-FDCK has an abuse potential comparable with ketamine.

Captopril and losartan attenuate behavioural sensitization in mice chronically exposed to toluene

Inhalants are consumed worldwide for recreational purposes. The main component found in many inhalants is toluene. One of the most deleterious behavioural effects caused by chronic exposure to inhalants is addiction. This response has been associated with activation of the mesolimbic dopaminergic pathway, and it is known that the renin angiotensin system plays a role in the modulation of this dopaminergic system. In the present work, we hypothesize that blockade of the RAS with angiotensin converting enzyme inhibitors or angiotensin II type 1 receptor blockers is able to attenuate the addictive response induced by toluene. We exposed mice to toluene for four weeks to induce locomotor sensitization. In the second phase of the work, captopril or losartan were administered for 20 days. Subsequently, the expression of behavioural sensitization was evaluated with a toluene challenge. To exclude false associations between the observed responses and treatments, motor coordination and blood pressure were analysed in animals treated with captopril or losartan. At the end of the behavioural studies, animal brains were harvested and Ang II/Ang-(1–7) and Ang-(1–7)/Ang II ratios were analysed in the nucleus accumbens (NAc) and prefrontal cortex (PFCx). The results showed that toluene induced behavioural sensitization, while captopril or losartan treatment attenuated the expression of this response. No significant differences were observed in motor coordination or blood pressure. Repeated toluene administration decreased Ang-(1–7)/Ang II ratio in the PFCx. On the other hand, treatment with captopril or losartan decreased the Ang II/Ang-(1–7) ratio and enhanced the Ang-(1–7)/Ang II ratio in the NAc. This work suggests that blockade of RAS attenuates the toluene-induced behavioural sensitization.

Morphine reward effects and morphine behavioral sensitization: The adventitious association of morphine activation of brain reward effects with ongoing spontaneous activity

The development of sensitization is one of the hallmarks of addictive drugs such as morphine. We administered morphine (10 mg/kg; MOR) to induce locomotor sensitization and ERK activation in the VTA and NAc. In the first experiment, four groups of rats received five daily 30 min sessions in an open-field, and locomotion was measured. For the first four sessions, one group received MOR pre-test (MOR-P); a second group received vehicle pre-test (MOR-UP) and MOR 30 min post-test; the remaining 2 groups received vehicle (VEH) pre-test. On the fifth session, the MOR-P, MOR-UP, and one VEH group received MOR pre-test and the remaining VEH group received VEH. Sensitization emerged in the first 5 min and progressed over to the second and third 5 min blocks only in the MOR-P group. For the second experiment, 4 groups received MOR and 4 groups VEH, and were then returned to their home cage and after 5, 15, 30 or 60 min post-injection, were euthanized for ERK measurements in VTA and NAc. ERK activation increased and peaked at 5 min post injection in the MOR group and then declined to VEH levels by 30 min. Another two groups received either MOR or VEH immediately before a 5 min arena test and ERK was measured immediately post-test. MOR had no effect on locomotion but increased ERK in the VTA and NAc. The peak ERK activation in VTA reflected activation of reward systems by morphine that reinforced locomotor behavior and with repeated treatments, induced a sensitization effect.

Cocaine Induces Sex-Associated Changes in Lipid Profiles of Brain Extracellular Vesicles.

Cocaine is a highly addictive stimulant with diverse effects on physiology. Recent studies indicate the involvement of extracellular vesicles (EVs) secreted by neural cells in the cocaine addiction process. It is hypothesized that cocaine affects secretion levels of EVs and their cargos, resulting in modulation of synaptic transmission and plasticity related to addiction physiology and pathology. Lipids present in EVs are important for EV formation and for intercellular lipid exchange that may trigger physiological and pathological responses, including neuroplasticity, neurotoxicity, and neuroinflammation. Specific lipids are highly enriched in EVs compared to parent cells, and recent studies suggest the involvement of various lipids in drug-induced synaptic plasticity during the development and maintenance of addiction processes. Therefore, we examined interstitial small EVs isolated from the brain of mice treated with either saline or cocaine, focusing on the effects of cocaine on the lipid composition of EVs. We demonstrate that 12days of noncontingent repeated cocaine (10mg/kg) injections to mice, which induce locomotor sensitization, cause lipid composition changes in brain EVs of male mice as compared with saline-injected controls. The most prominent change is the elevation of GD1a ganglioside in brain EVs of males. However, cocaine does not affect the EV lipid profiles of the brain in female mice. Understanding the relationship between lipid composition in EVs and vulnerability to cocaine addiction may provide insight into novel targets for therapies for addiction.

Propofol but not dexmedetomidine produce locomotor sensitization via nitric oxide in rats

The abused potential of some anesthetics has been debated. Measurement of locomotor sensitization is a better way to detect the neurobehavioral plasticity of addiction. The present study aims to explore whether propofol and dexmedetomidine are capable of inducing locomotor sensitization. Male Wistar rats (250-300g) were the subjects (n= 8 for each group). Propofol (20 and 40mg/kg) and dexmedetomidine (2.5-20μg/kg) or saline were injected to rats intraperitoneally (IP), and their locomotor activities were recorded for 15min. Consequently, L-NAME (30 and 60mg/kg)-a nitric oxide (NO) inhibitory agent-was injected to rats 30min before propofol (40mg/kg) or saline injections, and the locomotor activity was recorded. The process was carried out for 13days, with 7 sessions applied every other day. Dexmedetomidine did not produce any significant locomotor sensitization. While propofol (20mg/kg) produced a significant locomotor sensitization in the last treatment session (day 13), at the higher dose, it prompted a significant locomotor sensitization from the 3rd treatment session. L-NAME blocked propofol-induced locomotor hyperactivity and sensitization significantly without producing any noteworthy changes on the locomotor activity during the testing period of 13days when administered alone. Our results suggest that propofol but not dexmedetomidine produced a significant locomotor sensitization via central nitrergic system. Dexmedetomidine may have a lesser psychostimulant type addictive potential than propofol. Sensitization development by propofol implies that this drug might be effective on the neuroadaptive processes associated with a stimulant type of dependence.

The potential rewarding and reinforcing effects of the substituted benzofurans 2-EAPB and 5-EAPB in rodents

Accounts regarding the use of novel psychoactive substances continue to escalate annually. These include reports on substituted benzofurans (SBs), such as 1-(1-benzofuran-2-yl)-N-ethylpropan-2-amine (2-EAPB) and 1-(1-benzofuran-5-yl)-N-ethylpropan-2-amine (5-EAPB). Reports on the deaths and adverse consequences from the use of SBs warrant the investigation of their mechanism, possibly predicting the effects of similar compounds. Accordingly, we investigated the possible rewarding and reinforcing effects of 2-EAPB and 5-EAPB through conditioned place preference (CPP), self-administration, and locomotor sensitization tests. We also determined the possible influence of 2-EAPB and 5-EAPB administration on dopamine- and plasticity-related proteins in the nucleus accumbens and ventral tegmental area. 2-EAPB and 5-EAPB induced CPP at different doses and were self-administered by rats. Only 5-EAPB induced locomotor sensitization in mice. 2-EAPB and 5-EAPB did not alter the expressions of dopamine D1 and D2 receptors in the nucleus accumbens, nor changed tyrosine hydroxylase and dopamine transporter expressions in the ventral tegmental area. Both 2-EAPB and 5-EAPB enhanced deltaFosB, but not transcription factor cyclic AMP-response-element binding protein and brain-derived neurotrophic factor in the nucleus accumbens. Hence, the potential rewarding and reinforcing effects on rodents induced by 2-EAPB and 5-EAPB may possibly be associated with alterations in other neurotransmitter systems (besides mesolimbic) and/or neuro-plastic modifications.

Repeated restraint stress potentiates methylphenidate and modafinil-induced behavioral sensitization in rats.

Stress increases the susceptibility of drug abuse and drugs of abuse impair behavioral tolerance. It has been shown that stress exposure enhances the sensitivity to the reinforcing properties of drugs, augments locomotor sensitization effects of drugs of abuse and impairs behavioral tolerance. Previously, it has been shown that long-term administration of psychostimulants (Methylphenidate and Modafinil) induced locomotor sensitization effect that was more pronounced after 13days of drug administration and was greater at high dose. The present study is designed to investigate the relationship between restraint stress and psychostimulants (Methylphenidate and Modafinil) that induced sensitization. Methylphenidate (10mg/kg/day twice a day), modafinil (75mg/kg/day once daily), and saline (0.9% NaCl; 1ml/kg/day) were administered orally to treated and control animals. Rats were exposed to immobilization stress for 30days (until locomotor sensitization produced) to monitor any change in drug-induced behavioral sensitization. The motor activity was compared daily by using familiar environment of home cage and weekly by novel environment of open field. The results show that the methylphenidate and modafinil-induced locomotor sensitization is enhanced and impaired behavioral tolerance in repeated restrained rats. It shows that the psychostimulants like methylphenidate and modafinil produce greater locomotor sensitization in stressful environment, suggesting addictive effects of stress and psychostimulants (methylphenidate/modafinil) on dopaminergic neurotransmission. These finding may be helpful to develop potential pharmacotherapies for the patients with co-occurring depression and substance abuse/dependence disorder.

The role of the nucleus accumbens OXR1 in cocaine-induced locomotor sensitization

Re-exposure to drug or drug-associated cues after withdrawal can induce behavioral sensitization expression in animals or increase in the expected effect to drug in humans, which mean an enhanced drug seeking/taking motivation to trigger relapse after abstinence. The Nucleus accumbens (NAc) is known to play a key role in mediating this motivation. Recently, it has been shown that systemic administration of orexin receptor 1 (OXR1) antagonist attenuates animals’ motivation behavior to take drug by self-administration paradigm, which is more effectively than orexin receptor 2 (OXR2) antagonist. However, the effect of OXR1 in the NAc on drug-induced locomotor sensitization remains elusive. The present study was designed to investigate the effect of OXR1 in the NAc on chronic cocaine-induced locomotor sensitization. Rats were given 10 mg/kg cocaine intraperitoneal injection (i.p.) for five consecutive days, followed by 10 mg/kg cocaine re-exposure (challenge) on the 14th day of withdrawal. Results showed that re-exposure to cocaine after withdrawal could induce locomotor sensitization expression in cocaine-sensitized rats. Simultaneously, the number of OXR1 positive neurons and OXR1 membrane protein level in the NAc core but not the shell were significantly increased following the cocaine re-exposure. Further, micro-infusion of SB-334867, an OXR1 selective antagonist, into the NAc core but not the shell before cocaine re-exposure, significantly attenuated the expression of locomotor sensitization in rats. The findings demonstrate that OXR1 in the NAc core partially mediates the expression of chronic cocaine-induced locomotor sensitization.

Locomotor sensitization in male Sprague-Dawley rats following repeated concurrent treatment with 4-methylmethcathinone and 3,4-methylenedioxymethamphetamine.

Recreational abuse of illicit synthetic cathinones is an ongoing public health concern. Recent studies indicate that the methcathinone derivative 4-methylmethcathinone (4-MMC) produces behavioral and neurochemical effects similar to the entactogen 3,4-methylenedioxymethamphetamine (MDMA). Whereas polysubstance abuse is common, most preclinical studies of drug abuse liability only evaluate the effects of single drugs. Utilizing the locomotor sensitization paradigm, the present study assessed the combined locomotor stimulant effects of 4-MMC and MDMA for induction of sensitization following repeated administration and for expression of sensitization to a challenge dose of either substance alone after a 10-day period of drug abstinence. Male Sprague-Dawley rats received once daily intraperitoneal injections of saline, 4-MMC (1.0 mg/kg or 5.0 mg/kg), MDMA (3.0 mg/kg), or a mixture containing 4-MMC (1.0 mg/kg or 5.0 mg/kg) + MDMA (3.0 mg/kg) for 7 consecutive days. Following a 10-day drug-free period, rats were given a single intraperitoneal injection of either saline, 4-MMC (1.0 or 5.0 mg/kg), or 3.0 mg/kg MDMA. Activity was recorded for 1 h immediately before and 1 h immediately after injections on days 1, 7, and 17. 4-MMC treatment failed to induce locomotor sensitization, but, when combined with MDMA, sensitization was induced to a greater extent than with MDMA alone. Furthermore, the expression of sensitization to a subsequent challenge dose of MDMA was observed only in animals previously exposed to MDMA or a 5.0 mg/kg 4-MMC + MDMA mixture. In consideration of these findings along with the fact that 4-MMC has similar neurochemical actions to MDMA, further research may be warranted to determine the abuse liability of drug mixtures including 4-MMC and MDMA.

Further pharmacological comparison of d-methamphetamine and l-methamphetamine in rats: abuse-related behavioral and physiological indices

Previous preclinical research suggests that L-methamphetamine (L -MA) has potential therapeutic utility to treat psychostimulant abuse. This study examined potential abuse-related and adverse physiological effects of D -MA and L -MA alone and in combination in rats, as these effects had not been previously characterized. Potential abuse-related effects were examined in locomotor sensitization and conditioned place preference paradigms. Body temperature was monitored to assess the physiological effects of these drugs or drug combinations. In the locomotor study, D-MA induced locomotor sensitization to both D-MA and L -MA. L -MA induced locomotor sensitization only to D-MA. Responses to a combination of L-MA and D -MA were not differentially affected by L-MA or D-MA conditioning. In the conditioned place preference study, D-MA and L -MA each induced significant place preference. L -MA did not attenuate D-MA-induced place preference. In the body temperature study, D-MA induced hyperthermia and L -MA induced hypothermia. In combination, L -MA did not affect D-MA-induced hyperthermia. These data suggest that L -MA alone produces less abuse-related and adverse physiological effects than D-MA, but modulates and is modulated by concurrent and subsequent D-MA exposure, which may enhance the abuse liability of both drugs. These findings should be considered when L -MA is proposed for replacement therapy.

Chronic adenosine A2A receptor blockade induces locomotor sensitization and potentiates striatal LTD IN GPR37-deficient mice.

Adenosine A2A receptors (A2A R) play a key role in modulating dopamine-dependent locomotor activity, as heralded by the sensitization of locomotor activity upon chronic A2A R blockade, which is associated with elevated dopamine levels and altered corticostriatal synaptic plasticity. Since the orphan receptor GPR37 has been shown to modulate A2A R function invivo, we aimed to test whether the A2A R-mediated sensitization of locomotor activity is GPR37-dependent and involves adaptations of synaptic plasticity. To this end, we administered a selective A2A R antagonist, SCH58261 (1mg/kg, i.p.), daily for 14days, and the locomotor sensitization, striatum-dependent cued learning, and corticostriatal synaptic plasticity (i.e., long-term depression) were compared in wild-type and GPR37-/- mice. Notably, GPR37 deletion promoted A2A R-associated locomotor sensitization but not striatum-dependent cued learning revealed upon chronic SCH58261 treatment of mice. Furthermore, chronic A2A R blockade potentiated striatal long-term depression in corticostriatal synapses of GPR37-/- but not of wild-type mice, thus correlating well with neurochemical alterations of the adenosinergic system. Overall, these results revealed the importance of GPR37 regulating A2A R-dependent locomotor sensitization and synaptic plasticity in the basal ganglia circuitry. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

Regulation of microRNA-29c in the nucleus accumbens modulates methamphetamine -induced locomotor sensitization in mice

Changes in microRNA (miRNA)-mediated gene expression in the nucleus accumbens (NAc) may play important roles in regulating drug addiction. MiR-29c is a highly expressed miRNA in the human and rodent nervous systems where it plays a broad regulatory role. As the first step towards investigating potential functions of miR-29c in methamphetamine (METH) addiction, we used C57BL/6 mice in a model of METH-induced locomotor sensitization. We measured miR-29c expression changes in the NAc of the mice after repeated-intermittent METH exposure and acute METH administration respectively by using quantitative real-time PCR (qPCR). We found that miR-29c expression was significantly down-regulated in the NAc of METH-sensitized mice but not in the acute METH-treated mice. Then, we tested the respective effects of miR-29c over-expression and inhibition in the NAc on METH-induced locomotor sensitization. To reach this goal, we constructed adeno-associated virus (AAV)-expressing miR-29c (AAV-miR-29c) and its corresponding inhibitor - tough decoy (AAV-anti-miR-29c TuD) to over-express and inhibit miR-29c, respectively. We found that AAV-miR-29c over-expression in the NAc enhanced METH-induced locomotor sensitization, whereas AAV inhibition of miR-29c expression in the NAc attenuated the effects of METH. Moreover, we observed the participation of Dnmt3a, Dnmt3b, and Meg3 in the effects of miR-29c on METH sensitization. Our results suggest that miR-29c is an important epigenetic regulator of METH-induced behavioural sensitization and changes in gene expression. These data further suggest a potential role of miR-29c in regulating long-term METH-induced adaptation in the brain.

Repeated pre-trial and post-trial low and high dose apomorphine treatments induce comparable inhibitory/excitatory sensitization and conditioned drug effects

This investigation was undertaken to compare the sensitization/conditioned effects induced by apomorphine given pre-trial versus administered immediately post-trial or 15 min post-trial. We measured the effects on locomotor activity of 5 daily apomorphine treatments induced by an inhibitory low auto-receptor dose (0.05 mg/kg) and a stimulatory high postsynaptic dose (2.0 mg/kg). Three sets of four groups were used and each set of four groups was comprised of two vehicle and two apomorphine groups (0.05/2.0 apomorphine). The only difference among the three sets of four groups was when the treatments were administered relative to placement in the novel environment. One set received the treatment pre-test, another set was injected immediately after and the third set injected 15 min after 5 min test sessions in a novel environment. The repeated pre and immediate post-test apomorphine treatments induced locomotor sensitization over the 5 days of treatment. The low dose pre and immediate post-test treatments progressively decreased locomotion and the high dose pre and immediate post-test progressively increased locomotion. Critically, the tests for the immediate post-test groups were non-drug and for both the pre-test and immediate post-test groups, sensitization effects did not occur until the second test day. To control for non-associative apomorphine effects, the same apomorphine treatments were given post-test after a 15 minute delay and were found to be equivalent to vehicle. In a subsequent conditioning test, both the pre and immediate post-test low dose apomorphine groups showed conditioned behavioral inhibition and the pre and immediate post-test high dose apomorphine groups showed conditioned behavioral stimulation. We propose that the inhibitory low dose apomorphine decreased the salience/incentive of the novel environment association and thereby decreased the behavioral response and conversely that the high dose excitatory apomorphine treatment increased the salience/incentive value of the novel environment association and potentiated the behavioral response.

5-Methoxy-α-methyltryptamine (5-MeO-AMT), a tryptamine derivative, induces head-twitch responses in mice through the activation of serotonin receptor 2a in the prefrontal cortex

5-Methoxy-α-methyltryptamine (5-MeO-AMT) is a tryptamine derivative that is used recreationally because of its reported hallucinogenic and mood elevating effects. Studies suggest that the psychopharmacological effects of tryptamines involve serotonin receptor 2a (5-HTR2a) activation in the brain. The head-twitch response (HTR) is widely used as a behavioral correlate for assessing 5-HTR2a agonist activity of a drug. Thus, we investigated whether 5-MeO-AMT induces HTR in mice and explored its mechanism of action. 5-MeO-AMT (0.3, 1, 3, 10 mg/kg) was administered once a day for 7 days, and the HTR was measured after 1 day (acute) and 7 days (repeated) of administration. Another cohort of mice was treated with 5-HTR2a antagonist ketanserin (KS) before 5-MeO-AMT administration. We measured 5-HTR2a and 5-HTR2c mRNA levels in the prefrontal cortex of the mice treated acutely or repeatedly with 5-MeO-AMT. We performed western blotting to determine the effects of the drug on the expression of G protein (Gq/11), protein kinase C gamma (PKC-γ), and extracellular signal-regulated kinases 1/2 (ERK1/2), in addition to PKC-γ and ERK1/2 phosphorylation. Additionally, we evaluated potential rewarding and reinforcing effects of 5-MeO-AMT using locomotor sensitization, conditioned place preference (CPP), and self-administration (SA) paradigms. Acute 5-MeO-AMT administration elicited the HTR, while repeated administration resulted in tolerance. KS blocked the 5-MeO-AMT-induced HTR. 5-MeO-AMT increased 5-HTR2a mRNA levels and induced PKC-γ phosphorylation in the prefrontal cortex. 5-MeO-AMT did not induce locomotor sensitization, CPP, or SA. This study shows that 5-MeO-AMT induces HTR through 5-HTR2a activation in the prefrontal cortex, and may have low potential for abuse.