Loss of the sustained antidepressant-like effect of (2R,6R)-hydroxynorketamine in NMDA receptor GluN2D subunit knockout mice.

Ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, has attracted attention for its acute and sustained antidepressant effects in patients with depression. Hydroxynorketamine (HNK), a metabolite of ketamine, exerts antidepressant effects without exerting ketamine's side effects and has attracted much attention in recent years. However, the detailed pharmacological mechanism of action of HNK remains unclear. We previously showed that the GluN2D NMDA receptor subunit is important for sustained antidepressant-like effects of (R)-ketamine. Therefore, we investigated whether the GluN2D subunit is involved in antidepressant-like effects of (2R,6R)-HNK and (2S,6S)-HNK. Treatment with (2R,6R)-HNK but not (2S,6S)-HNK exerted acute and sustained antidepressant-like effects in the tail-suspension test in wildtype mice. Interestingly, sustained antidepressant-like effects of (2R,6R)-HNK were abolished in GluN2D-knockout mice, whereas acute antidepressant-like effects were maintained in GluN2D-knockout mice. When expression levels of GluN2A and GluN2B subunits were evaluated, a decrease in GluN2B protein expression in the nucleus accumbens was found in stressed wildtype mice but not in stressed GluN2D-knockout mice. These results suggest that the GluN2D subunit and possibly the GluN2B subunit are involved in the sustained antidepressant-like effect of (2R,6R)-HNK.

Understanding the role of the NMDA receptor subunit, GluN2D, in mediating NMDA receptor antagonist-induced behavioral disruptions in male and female mice.

Noncompetitive NMDA receptor (NMDAR) antagonists like phencyclidine (PCP) and ketamine cause psychosis-like symptoms in healthy humans, exacerbate schizophrenia symptoms in people with the disorder, and disrupt a range of schizophrenia-relevant behaviors in rodents, including hyperlocomotion. This is negated in mice lacking the GluN2D subunit of the NMDAR, suggesting the GluN2D subunit mediates the hyperlocomotor effects of these drugs. However, the role of GluN2D in mediating other schizophrenia-relevant NMDAR antagonist-induced behavioral disturbances, and in both sexes, is unclear. This study aimed to investigate the role of the GluN2D subunit in mediating schizophrenia-relevant behaviors induced by a range of NMDA receptor antagonists. Using both male and female GluN2D knockout (KO) mice, we examined the effects of the NMDAR antagonist's PCP, the S-ketamine enantiomer (S-ket), and the ketamine metabolite R-norketamine (R-norket) on locomotor activity, anxiety-related behavior, and recognition and short-term spatial memory. GluN2D-KO mice showed a blunted locomotor response to R-norket, S-ket, and PCP, a phenotype present in both sexes. GluN2D-KO mice of both sexes showed an anxious phenotype and S-ket, R-norket, and PCP showed anxiolytic effects that were dependent on sex and genotype. S-ket disrupted spatial recognition memory in females and novel object recognition memory in both sexes, independent of genotype. This datum identifies a role for the GluN2D subunit in sex-specific effects of NMDAR antagonists and on the differential effects of the R- and S-ket enantiomers.

Effects of the novel selective κ-opioid receptor agonist NP-5497-KA on morphine-induced reward-related behaviors.

Opioid addiction and the opioid overdose epidemic are becoming more serious, and the development of therapeutic agents is essential for the pharmacological treatment of substance use disorders. The κ-opioid receptor (KOP) is a member of the opioid receptor system that has been gaining attention as a promising molecular target for the treatment of numerous human disorders, including pain, depression, anxiety, and drug addiction. Here, we biologically and pharmacologically evaluated a novel azepane-derived ligand, NP-5497-KA, as a selective KOP agonist. NP-5497-KA had 1000-fold higher selectivity for the KOP over the µ-opioid receptor (MOP), which was higher than nalfurafine (KOP/MOP: 65-fold), and acted as a selective KOP full agonist in the 3',5'-cyclic adenosine monophosphate assay. The oral administration of NP-5497-KA (1-10 mg/kg) dose-dependently suppressed morphine-induced conditioned place preference in C57BL/6 J mice, and its effects were comparable to an intraperitoneal injection of nalfurafine (1-10 µg/kg). Nalfurafine (10 µg/kg) significantly inhibited rotarod performance, whereas NP-5497-KA (10 mg/kg) exerted no effect on rotarod performance. These results indicate that NP-5497-KA may be a novel option for the treatment of opioid use disorder with fewer side effects.

Role for µ-opioid receptor in antidepressant effects of δ-opioid receptor agonist KNT-127.

Previous pharmacological data have shown the possible existence of functional interactions between µ- (MOP), κ- (KOP), and δ-opioid receptors (DOP) in pain and mood disorders. We previously reported that MOP knockout (KO) mice exhibit a lower stress response compared with wildtype (WT) mice. Moreover, DOP agonists have been shown to exert antidepressant-like effects in numerous animal models. In the present study, the tail suspension test (TST) and forced swim test (FST) were used to examine the roles of MOP and DOP in behavioral despair. MOP-KO mice and WT mice were treated with KNT-127 (10 mg/kg), a selective DOP agonist. The results indicated a significant decrease in immobility time in the KNT-127 group compared with the saline group in all genotypes in both tests. In the saline groups, immobility time significantly decreased in MOP-KO mice compared with WT mice in both tests. In female MOP-KO mice, KNT-127 significantly decreased immobility time in the TST compared with WT mice. In male MOP-KO mice, however, no genotypic differences were found in the TST after either KNT-127 or saline treatment. Thus, at least in the FST and TST, the activation of DOP and absence of MOP had additive effects in reducing measures of behavioral despair, suggesting that effects on this behavior by DOP activation occur independently of MOP.

Inhibitory Effects of a Novel µ-Opioid Receptor Nonpeptide Antagonist, UD-030, on Morphine-Induced Conditioned Place Preference.

Although opioids are widely used to treat moderate to severe pain, opioid addiction and the opioid overdose epidemic are becoming more serious. Although opioid receptor antagonists/partial agonists, such as naltrexone and buprenorphine, have relatively low selectivity for the µ-opioid receptor (MOP), they have been used for the management of opioid use disorder. The utility of highly selective MOP antagonists remains to be evaluated. Here, we biologically and pharmacologically evaluated a novel nonpeptide ligand, UD-030, as a selective MOP antagonist. UD-030 had more than 100-fold higher binding affinity for the human MOP (Ki = 3.1 nM) than for δ-opioid, κ-opioid, and nociceptin receptors (Ki = 1800, 460, and 1800 nM, respectively) in competitive binding assays. The [35S]-GTPγS binding assay showed that UD-030 acts as a selective MOP full antagonist. The oral administration of UD-030 dose-dependently suppressed the acquisition and expression of morphine-induced conditioned place preference in C57BL/6J mice, and its effects were comparable to naltrexone. These results indicate the UD-030 may be a new candidate for the treatment of opioid use disorder, with characteristics that differ from traditional medications that are in clinical use.

Overexpression of tyrosine hydroxylase in dopaminergic neurons increased sensitivity to methamphetamine.

Drug abuse is one of the great social problems in the world and a major healthcare challenge. It is supposed that sensitivity and reactivity to abuse drugs may vary from person to person, while its molecular basis is largely unknown. Dopaminergic neurons are deeply involved in addiction, and tyrosine hydroxylase (TH) catalyzes the first and rate-limiting step of the biosynthesis of dopamine (DA). We investigated the effects of increased TH expression on the metabolism of DA and reactivity to methamphetamine (METH), a drug of abuse, in mice. Wild-type TH (WT-TH) or the S40E mutant of TH (S40E-TH), which is an active form of TH mimicking phosphorylated TH at the 40th serine, was expressed in midbrain dopaminergic neurons using an adeno-associated virus (AAV) vector. The biochemical analysis showed that the turnover rates of DA in the nerve terminals were increased by the expression of WT-TH and S40E-TH, while there were few changes in the DA contents. Next, we administered METH to TH-overexpressing mice. We found that the S40E-TH-expressing mice responded to lower doses of METH than the control mice and WT-TH mice. The stereotyped behaviors appeared first in S40E-TH mice and then in WT-TH and control mice in this order. These data showed that the TH activity and expression level differentially affect DA metabolism in the nerve terminals from that in the cell bodies and that the TH activity and expression level are one of the determining factors for sensitivity and reactivity to METH. We suggest that TH may be a drug target for ameliorating sensitivity to drugs of abuse.

GIRK Channels as Candidate Targets for the Treatment of Substance Use Disorders.

Substance use disorders (SUDs) are chronic, lifelong disorders that have serious consequences. Repeated substance use alters brain function. G-protein-activated inwardly rectifying potassium (GIRK) channels are expressed widely in the brain, including the reward system, and regulate neuronal excitability. Functional GIRK channels are identified as heterotetramers of GIRK subunits (GIRK1-4). The GIRK1, GIRK2, and GIRK3 subunits are mainly expressed in rodent brain regions, and various addictive substances act on the brain through GIRK channels. Studies with animals (knockout and missense mutation animals) and humans have demonstrated the involvement of GIRK channels in the effects of addictive substances. Additionally, GIRK channel blockers affect behavioral responses to addictive substances. Thus, GIRK channels play a key role in SUDs, and GIRK channel modulators may be candidate medications. Ifenprodil is a GIRK channel blocker that does not have serious side effects. Two clinical trials were conducted to investigate the effects of ifenprodil in patients with alcohol or methamphetamine use disorder. Although the number of participants was relatively low, evidence of its safety and efficacy was found. The present review discusses the potential of GIRK channel modulators as possible medications for addiction. Therapeutic agents that target GIRK channels may be promising for the treatment of SUDs.

Discovery of Benzylpiperazine Derivatives as CNS-Penetrant and Selective Histone Deacetylase 6 Inhibitors.

Inhibition of histone deacetylase 6 (HDAC6) in the brain is a highly attractive therapeutic target for the treatment of neurodegenerative diseases. The low blood-brain barrier permeability of most known HDAC6 inhibitors, however, prevents their application as central nervous system (CNS) drugs. To overcome this problem, we designed and synthesized benzylpiperazine derivatives using a hybrid strategy of combining HDAC6 inhibitors and brain-penetrant histamine H1 receptor antagonists. Introducing the benzylpiperazine units to the cap region of hydroxamate-type HDAC6 inhibitors led us to identify isozyme-selective and CNS-penetrant HDAC6 inhibitor KH-259 (1) with the appropriate pharmacokinetic and safety properties. Intraperitoneal administration of KH-259 (10 mg/kg) had antidepressant activity and increased acetylated α-tubulin in the brain without promoting acetylated histone H3K9. These findings indicate that our hybrid strategy of combining HDAC6 inhibitors and histamine H1 receptor antagonists is an effective methodology for designing CNS-penetrant HDAC6 inhibitors.

Interaction between social behavior and paternal age in offspring of the same paternal mice.

AIM: Previous studies reported that advanced paternal age (APA) may increase the risk of autism spectrum disorder (ASD) in offspring. However, effects of APA on behaviors have not been investigated in offspring of the same paternal mice. The present study sought to identify behavioral differences in mouse offspring of the same fathers at different paternal ages. METHODS: We assessed locomotor activity, anxiety-like behavior, and social behavior in male mouse offspring that were born from the same fathers at three different paternal ages (3, 12, and 15 months old). RESULTS: No differences in locomotor activity or anxiety-like behavior were observed among any of the offspring groups. In the three-chamber test, although the control group (3-month-old paternal age) exhibited significantly higher approach behavior toward the novel mouse compared with the novel object, the APA groups (12- and 15-month-old paternal ages) did not exhibit significant approach toward the novel mouse. CONCLUSION: Offspring of 3-month-old fathers but not 12- or 15-month-old APA fathers exhibited social preference behavior. Although the present study was only exploratory, it demonstrated an interaction between social behavior and paternal age in offspring of the same paternal mice.

Abolished ketamine effects on the spontaneous excitatory postsynaptic current of medial prefrontal cortex neurons in GluN2D knockout mice.

Ketamine, a non-competitive antagonist of the N-methyl-D-aspartate receptor (NMDAR), generates a rapidly-acting antidepressant effect. It exerts psychomimetic effects, yet demands a further investigation of its mechanism. Previous research showed that ketamine did no longer promote hyperlocomotion in GluN2D knockout (KO) mice, which is a subunit of NMDAR. In the present study, we tested whether GluN2D-containing NMDARs participate in the physiological changes in the medial prefrontal cortex (mPFC) triggered by ketamine. Sub-anesthetic dose of ketamine (25 mg/kg) elevated the frequency of spontaneous excitatory postsynaptic currents (sEPSC) in wild-type (WT) mice, but not in GluN2D KO mice, 1 h after the injection. The amplitude of sEPSC and paired-pulse ratio (PPR) were unaltered by ketamine in both WT and GluN2D KO mice. These findings suggest that GluN2D-containing NMDARs might play a role in the ketamine-mediated changes in glutamatergic neurons in mPFC and, presumably, in ketamine-induced hyperlocomotion.

Future direction of addiction research-An expert questionnaire survey in Japan.

Addiction has become a major worldwide medical, public health, and social problem. Because the prevalence of addiction varies widely geographically, due to differences in ethnicity, culture, education, social environment, and regulation, each country or region needs to understand its current state of addiction and to take appropriate measures, in multidisciplinary collaboration. In order to understand the direction of addiction research in Japan, we analyzed 50 research and development topics and their characteristics, based on an expert questionnaire survey. The topics were placed in five categories, as follows. Category 1: Basic science; all 10 topics were of the Long-term project and International cooperation types. Category 2: Translational and clinical research; 6 out of 10 topics were of the Long-term project. Category 3: Fact-finding surveys; 8 out of 10 topics were of the Japan-specific type. Category 4: Health system and service; 8 out of 10 topics were of the Japan-specific type and Short-term project. Category 5: Study on society, culture, environment, education, and regulation; 7 out of 10 topics were of the Short-term project (similar to Category 4). As far as we know, this is the first systematic questionnaire survey on the direction of addiction research. The results of this study might support developing a strategy for addiction research, not only in Japan, but also in other countries.

Adolescent Problem Gaming and Loot Box Purchasing in Video Games: Cross-sectional Observational Study Using Population-Based Cohort Data.

BACKGROUND: Video game loot boxes, which can typically be purchased by players or are given as reward, contain random virtual items, or loot, ranging from simple customization options for a player's avatar or character, to game-changing equipment such as weapons and armor. Loot boxes have drawn concern, as purchasing loot boxes might lead to the development of problematic gambling for adolescents. Although parental problem gambling is associated with adolescent problem gambling, no studies have evaluated the prevalence of loot box purchases in adolescents' parents. OBJECTIVE: This study investigated the association between loot box purchasing among adolescents and parents, and problem online gaming in population-based samples. METHODS: In total, 1615 adolescent (aged 14 years) gamers from Japan responded to a questionnaire regarding their loot box purchasing and problem online gaming behaviors. Problem online gaming was defined as four or more of the nine addictive behaviors from the Diagnostic and Statistical Manual of Mental Disorders. The adolescents' primary caregivers were asked about their loot box purchasing. RESULTS: Of the 1615 participants, 57 (3.5%) reported loot box purchasing. This prevalence did not differ according to primary caregivers' loot box purchasing, but adolescents who purchased loot boxes were significantly more likely to exhibit problem online gaming (odds ratio 3.75, 95% CI 2.17-6.48). CONCLUSIONS: Adolescent loot box purchasing is linked to problem online gaming, but not with parents' loot box purchasing. Measures to reduce these behaviors should target reducing addictive symptoms in young video gamers.

Caenorhabditis Elegans Exhibits Morphine Addiction-like Behavior via the Opioid-like Receptor NPR-17.

Addiction has become a profound societal problem worldwide, and few effective treatments are available. The nematode Caenorhabditis elegans (C. elegans) is an excellent invertebrate model to study neurobiological disease states. C. elegans reportedly developed a preference for cues that had previously been paired with addictive drugs, similar to place conditioning findings in rodents. Moreover, several recent studies discovered and reported the existence of an opioid-like system in C. elegans. Still unclear, however, is whether C. elegans exhibits addictive-like behaviors for opioids, such as morphine. In the present study, we found that C. elegans exhibited dose-dependent preference for morphine using the conditioned chemosensory-cue preference (CCP) test. This preference was blocked by co-treatment with the opioid receptor antagonist naloxone. C. elegans also exhibited aversion to naloxone-precipitated withdrawal from chronic morphine exposure. The expression of morphine-induced CCP and morphine withdrawal were abolished in worms that lacked the opioid-like receptor NPR-17. Dopamine-deficient mutant (cat-2 (e1112)) worms also did not exhibit morphine-induced CCP. These results indicate that the addictive function of the opioid system exists in C. elegans, which may serve as a useful model of opioid addiction.

Absence of methamphetamine-induced conditioned place preference in weaver mutant mice.

AIMS: G protein-activated inwardly rectifying potassium (GIRK) channels are related to rewarding effects of addictive drugs. The GIRK2 subunit is thought to play key roles in the reward system. Weaver mutant mice exhibit abnormal GIRK2 function and different behaviors that are caused by several addictive substances compared with wild-type mice. However, mechanisms of reward-related alterations in weaver mutant mice remain unclear. The present study investigated changes in the rewarding effects of methamphetamine (METH) in weaver mutant mice. METHODS: The rewarding effects of METH (4.0 mg/kg) were investigated using the conditioned place preference (CPP) paradigm. Extracellular dopamine level in the nucleus accumbens (NAc) was measured by in vivo microdialysis. To identify brain regions that were associated with these changes in rewarding effects, METH-induced alterations of Fos expression were investigated by immunohistochemical analysis. RESULTS: Weaver mutant mice exhibited a significant decrease in METH-induced CPP and dopamine release in the NAc. Methamphetamine significantly increased Fos expression in the posterior NAc (pNAc) shell in wild-type but not in weaver mutant mice. CONCLUSIONS: Methamphetamine did not induce rewarding effects in weaver mutant mice. The pNAc shell exhibited a significant difference in neuronal activity between wild-type and weaver mutant mice. The present results suggest that the absence of METH-induced CPP in weaver mutant mice is probably related to an innate reduction of dopamine and decreased neural activity in the pNAc shell that is partially attributable to the change of GIRK channel function. GIRK channels, especially those containing the GIRK2 subunit, appear to be involved in METH dependence.

Cognitive Impairment That Is Induced by (R)-Ketamine Is Abolished in NMDA GluN2D Receptor Subunit Knockout Mice.

Although the N-methyl-D-aspartate receptor antagonist ketamine has attracted attention because of its rapid and sustained antidepressant effects in depressed patients, its side effects have raised some concerns. Ketamine is a racemic mixture of equal amounts of the enantiomers (R)-ketamine and (S)-ketamine. The neural mechanisms that underlie the differential effects of these enantiomers remain unclear. We investigated cognitive impairment that was induced by ketamine and its enantiomers in N-methyl-D-aspartate GluN2D receptor subunit knockout (GluN2D-KO) mice. In the novel object recognition test, (RS)-ketamine and (S)-ketamine caused cognitive impairment in both wild-type and GluN2D-KO mice, whereas (R)-ketamine induced such cognitive impairment only in wild-type mice. The present results suggest that the GluN2D subunit plays an important role in cognitive impairment that is induced by (R)-ketamine, whereas this subunit does not appear to be involved in cognitive impairment that is induced by (RS)-ketamine or (S)-ketamine.

Three-dimensional alteration of neurites in schizophrenia.

Psychiatric symptoms of schizophrenia suggest alteration of cerebral neurons. However, the physical basis of the schizophrenia symptoms has not been delineated at the cellular level. Here, we report nanometer-scale three-dimensional analysis of brain tissues of schizophrenia and control cases. Structures of cerebral tissues of the anterior cingulate cortex were visualized with synchrotron radiation nanotomography. Tissue constituents visualized in the three-dimensional images were traced to build Cartesian coordinate models of tissue constituents, such as neurons and blood vessels. The obtained Cartesian coordinates were used for calculating curvature and torsion of neurites in order to analyze their geometry. Results of the geometric analyses indicated that the curvature of neurites is significantly different between schizophrenia and control cases. The mean curvature of distal neurites of the schizophrenia cases was ~1.5 times higher than that of the controls. The schizophrenia case with the highest neurite curvature carried a frame shift mutation in the GLO1 gene, suggesting that oxidative stress due to the GLO1 mutation caused the structural alteration of the neurites. The differences in the neurite curvature result in differences in the spatial trajectory and hence alter neuronal circuits. It has been shown that the anterior cingulate cortex analyzed in this study has emotional and cognitive functions. We suggest that the structural alteration of neurons in the schizophrenia cases should reflect psychiatric symptoms of schizophrenia.

Opioid and nondopamine reward circuitry and state-dependent mechanisms.

A common notion is that essentially all addictive drugs, including opioids, activate dopaminergic pathways in the brain reward system, and the inappropriate use of such drugs induces drug dependence. However, an opioid reward response is reportedly still observed in several models of dopamine depletion, including in animals that are treated with dopamine blockers, animals that are subjected to dopaminergic neuron lesions, and dopamine-deficient mice. The intracranial self-stimulation response is enhanced by stimulants but reduced by morphine. These findings suggest that dopaminergic neurotransmission may not always be required for opioid reward responses. Previous findings also indicate the possibility that dopamine-independent opioid reward may be observed in opioid-naive states but not in opioid-dependent states. Therefore, a history of opioid use should be considered when evaluating the dopamine dependency of opioid reward.

Roles of noradrenergic transmission within the ventral part of the bed nucleus of the stria terminalis in bidirectional brain-intestine interactions.

AIMS: The bed nucleus of the stria terminalis (BNST) is a limbic structure mediating autonomic and neuroendocrine responses and negative affective states such as anxiety and fear. We previously demonstrated that noradrenergic transmission via ß-adrenoceptors within the ventral part of BNST (vBNST) is involved in bidirectional interactions between the brain and the upper gastrointestinal (GI) tract. The present study aimed to examine the roles of intra-vBNST noradrenergic transmission via ß-adrenoceptors in bidirectional interactions between the brain and lower GI tract. METHODS: In vivo microdialysis experiments were performed to examine colorectal distention (CRD)-induced noradrenaline release within the vBNST of freely moving male Sprague-Dawley rats. Colonic transit and abdominal pain perception were examined following intra-vBNST injections of isoproterenol, a ß-adrenoceptor agonist, with and without co-administration of timolol, a ß-adrenoceptor antagonist. RESULTS: CRD increased extracellular noradrenaline levels within the vBNST and evoked abdominal contractions in a pressure-dependent manner (30-60 mm Hg). Bilateral intra-vBNST injections of isoproterenol (30 nmol/side) significantly increased CRD (30 mm Hg)-induced abdominal contractions. Intra-vBNST injections of isoproterenol (30 nmol/side) significantly increased colonic transit, which was reversed by co-administration of timolol (30 nmol/side). CONCLUSION: The results of this study suggest (a) the existence of a positive feedback loop between intra-vBNST noradrenaline release and abdominal pain perception, and (b) the modulation of colonic motility by intra-vBNST noradrenergic transmission via ß-adrenoceptors. Dysfunction of the lower GI tract may increase noradrenaline release within the vBNST, which, in turn, may exacerbate impairment of its motility and pain perception.

Reward-enhancing effect of methylphenidate is abolished in dopamine transporter knockout mice: A model of attention-deficit/hyperactivity disorder.

AIM: Attention-deficit/hyperactivity disorder is a heterogeneous neurobiological disorder that is characterized by inattention, impulsivity, and an increase in motor activity. Although methylphenidate has been used as a medication for decades, unknown is whether methylphenidate treatment can cause drug dependence in patients with attention-deficit/hyperactivity disorder. This study investigated the reward-enhancing effects of methylphenidate using intracranial self-stimulation in an animal model of attention-deficit/hyperactivity disorder, dopamine transporter knockout mice. METHODS: For the intracranial self-stimulation procedures, the mice were trained to nosepoke to receive direct electrical stimulation via an electrode that was implanted in the lateral hypothalamus. After the acquisition of nosepoke responding for intracranial self-stimulation, the effects of methylphenidate on intracranial self-stimulation were investigated. RESULTS: In the progressive-ratio procedure, dopamine transporter knockout mice exhibited an increase in intracranial self-stimulation compared with wild-type mice. Treatment with 5 and 10 mg/kg methylphenidate increased intracranial self-stimulation responding in wild-type mice. Methylphenidate at the same doses did not affect intracranial self-stimulation responding in dopamine transporter knockout mice. We then investigated the effects of high-dose methylphenidate (60 mg/kg) in a rate-frequency procedure. High-dose methylphenidate significantly decreased intracranial self-stimulation responding in both wild-type and dopamine transporter knockout mice. CONCLUSIONS: These results suggest that low-dose methylphenidate alters the reward system (ie, increases intracranial self-stimulation responding) in wild-type mice via dopamine transporter inhibition, whereas dopamine transporter knockout mice do not exhibit such alterations. High-dose methylphenidate appears to suppress intracranial self-stimulation responding not through dopamine transporter inhibition but rather through other mechanisms. These results support the possibility that methylphenidate treatment for attention-deficit/hyperactivity disorder does not increase the risk of drug dependence, in attention-deficit/hyperactivity disorder patients with dopamine transporter dysfunction.