Effect of methylphenidate on dopamine/DARPP signalling in adult, but not young, mice

Adult ADHD: it is old and new at the same time - what is it?

Even though the number of studies aiming to improve comprehension of ADHD pathology has increased in recent years, there still is an urgent need for more effective studies, particularly in understanding adult ADHD, both at preclinical and clinical levels, due to the increasing evidence that adult ADHD is highly distinct and a different entity from childhood ADHD. This review paper outlines the symptoms, diagnostics, and neurobiological mechanisms of ADHD, with emphasis on how adult ADHD could be different from childhood-onset. Data show a difference in the environmental, genetic, epigenetic, and brain structural changes, when combined, could greatly impact the behavioral presentations and the severity of ADHD in adults. Furthermore, a crucial aspect in the quest to fully understand this disorder could be through longitudinal analysis. In this way, we will determine if and how the pathology and pharmacology of ADHD change with age. This goal could revolutionize our understanding of the disorder and address the weaknesses in the current clinical classification systems, improving the characterization and validity of ADHD diagnosis, specifically those in adults.

Neuroprotection in late life attention-deficit/hyperactivity disorder: A review of pharmacotherapy and phenotype across the lifespan

For decades, psychostimulants have been the gold standard pharmaceutical treatment for attention-deficit/hyperactivity disorder (ADHD). In the United States, an astounding 9% of all boys and 4% of girls will be prescribed stimulant drugs at some point during their childhood. Recent meta-analyses have revealed that individuals with ADHD have reduced brain volume loss later in life (>60 y.o.) compared to the normal aging brain, which suggests that either ADHD or its treatment may be neuroprotective. Crucially, these neuroprotective effects were significant in brain regions (e.g., hippocampus, amygdala) where severe volume loss is linked to cognitive impairment and Alzheimer's disease. Historically, the ADHD diagnosis and its pharmacotherapy came about nearly simultaneously, making it difficult to evaluate their effects in isolation. Certain evidence suggests that psychostimulants may normalize structural brain changes typically observed in the ADHD brain. If ADHD itself is neuroprotective, perhaps exercising the brain, then psychostimulants may not be recommended across the lifespan. Alternatively, if stimulant drugs are neuroprotective, then this class of medications may warrant further investigation for their therapeutic effects. Here, we take a bottom-up holistic approach to review the psychopharmacology of ADHD in the context of recent models of attention. We suggest that future studies are greatly needed to better appreciate the interactions amongst an ADHD diagnosis, stimulant treatment across the lifespan, and structure-function alterations in the aging brain.

Adolescent rats respond differently to methylphenidate as compared to adult rats- concomitant VTA neuronal and behavioral Recordings

Methylphenidate (MPD) is the most widely prescribed psychostimulant used in adolescents and adults to treat attention-deficit/hyperactivity disorder (ADHD). The recreational use of MPD is becoming more prevalent because of its ability to improve cognitive enhancement. The ventral tegmental area (VTA) of the brain is highly associated with reward, cognition and addiction to drugs including psychostimulants like MPD. The VTA neuronal activity was recorded alongside the horizontal behavioral activity from freely behaving non-anesthetized rats. Four adolescent and four adult groups were treated with either saline, 0.6, 2.5 or 10.0 mg/kg MPD. In both adolescent and adult animals, the animals responded to MPD in a dose-dependent manner, such that as the dose of MPD increased, more animals and more VTA unit responded to the drug. The same doses of MPD elicited in some animals behavioral and neuronal sensitization and in other animals behavioral and neuronal tolerance. In the 0.6 and 10.0 mg/kg MPD dose groups there were significant differences between the age groups for how many animals expressed behavioral sensitization and behavioral tolerance to chronic MPD exposure. Additionally, the animal's behavioral response to MPD by excitation or attenuation of activity did not always correlate to the VTA neuronal response, and the age group with significantly higher behavioral responses did not always correlate to the age group with significantly higher VTA neuronal responses for a given MPD dose. These findings differ from similar studies recorded from the prefrontal cortex (PFC), which exhibited behavioral responses continuously directly correlated to PFC responses for increasing MPD doses. This demonstrates that unlike other areas of the brain, there is not a direct relationship between VTA firing and behavioral activity, suggesting that there is input or modulation of this area from elsewhere in the brain. Further investigation is needed to clearly understand the relationship between VTA firing rates and behavioral responses to different MPD doses, especially given the significant differences in response between young and adult animals and the increasing use of the drug in adolescent populations.

Dopaminergic loss of cyclin-dependent kinase-like 5 recapitulates methylphenidate-remediable hyperlocomotion in mouse model of CDKL5 deficiency disorder

Cyclin-dependent kinase-like 5 (CDKL5), a serine-threonine kinase encoded by an X-linked gene, is highly expressed in the mammalian forebrain. Mutations in this gene cause CDKL5 deficiency disorder, a neurodevelopmental encephalopathy characterized by early-onset seizures, motor dysfunction, and intellectual disability. We previously found that mice lacking CDKL5 exhibit hyperlocomotion and increased impulsivity, resembling the core symptoms in attention-deficit hyperactivity disorder (ADHD). Here, we report the potential neural mechanisms and treatment for hyperlocomotion induced by CDKL5 deficiency. Our results showed that loss of CDKL5 decreases the proportion of phosphorylated dopamine transporter (DAT) in the rostral striatum, leading to increased levels of extracellular dopamine and hyperlocomotion. Administration of methylphenidate (MPH), a DAT inhibitor clinically effective to improve symptoms in ADHD, significantly alleviated the hyperlocomotion phenotype in Cdkl5 null mice. In addition, the improved behavioral effects of MPH were accompanied by a region-specific restoration of phosphorylated dopamine- and cAMP-regulated phosphoprotein Mr 32 kDa, a key signaling protein for striatal motor output. Finally, mice carrying a Cdkl5 deletion selectively in DAT-expressing dopaminergic neurons, but not dopamine receptive neurons, recapitulated the hyperlocomotion phenotype found in Cdkl5 null mice. Our findings suggest that CDKL5 is essential to control locomotor behavior by regulating region-specific dopamine content and phosphorylation of dopamine signaling proteins in the striatum. The direct, as well as indirect, target proteins regulated by CDKL5 may play a key role in movement control and the therapeutic development for hyperactivity disorders.

Concomitant behavioral and prefrontal cortex neuronal responses following acute and chronic methylphenidate exposure in adolescent and adult rats

There is growing concern that the psychostimulant Methylphenidate (MPD) is being abused for cognitive enhancement and recreation by healthy adults and adolescents seeking to improve their work or academic performance. This study concomitantly recorded the behavioral and prefrontal cortex (PFC) neuronal activity in freely behaving animals exposed to acute and chronic MPD doses (0.6, 2.5, and 10.0 mg/kg MPD) in order to compare MPD effects on adult and adolescent rats. The PFC is one of the primary brain areas affected by MPD and the drug of choice for treating ADHD. Moreover, the PFC is one of the last brain areas to complete development, suggesting that the behavioral and neurophysiological response to MPD may differ in adolescents and adults. In both adult and adolescent animals, it was observed that the same repetitive (chronic) dose of either 0.6, 2.5, or 10.0 mg/kg MPD elicited behavioral sensitization in some animals and tolerance in others, experimental biomarkers indicating drug of abuse symptoms, and the majority of PFC units recorded in animals expressing behavioral sensitization or tolerance to chronic MPD exposure responded by increasing and decreasing their neuronal firing rate, respectively. Further, it was shown that high doses of 10.0 mg/kg MPD significantly modified adolescent behavioral activity but did not impact adults suggesting that adolescents may be more receptive to chronic MPD exposure. These findings raise concerns regarding the use and abuse of MPD in normal, healthy individuals and support the notion that the adolescent PFC is more susceptible than the adult PFC to neuromodulation from chronic MPD use.

Topiramate via NMDA, AMPA/kainate, GABAA and Alpha2 receptors and by modulation of CREB/BDNF and Akt/GSK3 signaling pathway exerts neuroprotective effects against methylphenidate-induced neurotoxicity in rats

Chronic abuse of methylphenidate (MPH) often causes neuronal cell death. Topiramate (TPM) carries neuroprotective effects, but its exact mechanism of action remains unclear. In the present study, the role of various doses of TPM and its possible mechanisms, receptors and signaling pathways involved against MPH-induced hippocampal neurodegeneration were evaluated in vivo. Thus, domoic acid (DOM) was used as AMPA/kainate receptor agonist, bicuculline (BIC) as GABA_A receptor antagonist, ketamine (KET) as NMDA receptor antagonist, yohimbine (YOH) as α₂ adrenergic receptor antagonist and haloperidol (HAL) was used as dopamine D₂ receptor antagonist. Open field test (OFT) was used to investigate the disturbances in motor activity. Hippocampal neurodegenerative parameters were evaluated. Protein expressions of CREB/BDNF and Akt/GSK3 signaling pathways were also evaluated. Cresyl violet staining was performed to show and confirm the changes in the shape of the cells. TPM (70 and 100 mg/kg) reduced MPH-induced rise in lipid peroxidation, oxidized form of glutathione (GSSG), IL-1β and TNF-α levels, Bax expression and motor activity disturbances. In addition, TPM treatment increased Bcl-2 expression, the level of reduced form of glutathione (GSH) and the levels and activities of superoxide dismutase, glutathione peroxidase and glutathione reductase enzymes. TPM also inhibited MPH-induced hippocampal degeneration. Pretreatment of animals with DOM, BIC, KET and YOH inhibited TPM-induced neuroprotection and increased oxidative stress, neuroinflammation, neuroapoptosis and neurodegeneration while reducing CREB, BDNF and Akt protein expressions. Also pretreatment with DOM, BIC, KET and YOH inhibited TPM-induced decreases in GSK3. It can be concluded that the mentioned receptors by modulation of CREB/BDNF and Akt/GSK3 pathways, are involved in neuroprotection of TPM against MPH-induced neurodegeneration.

Methamphetamine-enhanced female sexual motivation is dependent on dopamine and progesterone signaling in the medial amygdala

Methamphetamine (METH) is a psychomotor stimulant strongly associated with increases in sexual drive and impulsive sexual behaviors that often lead to unsafe sexual practices. In women METH users, such practices have been associated with increases in unplanned pregnancies and sexually transmitted diseases. Despite this significant heath concern, the neural mechanisms underlying this drug-sex association are not known. We previously established a rodent model of METH-facilitated female sexual behavior in which estradiol and progesterone interact with METH to increase motivational components of female behavior and neuronal activation in the posterodorsal medial amygdala (MePD) (Holder et al., 2010; Holder and Mong, 2010). The current study more directly examines the mechanisms underlying the drug-sex interaction. Here, we hypothesize that METH-induced increases in MePD dopamine signaling bridge the METH-hormone interaction. In support of this hypothesis, we found that excitotoxic lesions targeted to the MePD attenuated the METH-induced increases in proceptive behavior. Furthermore, infusion of a D1 agonist into the MePD increased proceptive behavior, while infusion of a D1 antagonist blocked the ability of METH to increase proceptive behaviors. Additionally, we found that METH-treatment increased progesterone receptor (PR) immunoreactivity in the MePD, suggesting an interaction between dopamine and progesterone signaling. Indeed, infusions of the PR antagonist, RU486, prevented METH-induced increases in sexual behavior. Thus, taken together, the current findings suggest that dopamine in the MePD modulates enhanced sexual motivation via an amplification of progesterone signaling and contributes to a better understanding of the neurobiology of drug-enhanced sexual behaviors.

Methylphenidate restores novel object recognition in DARPP-32 knockout mice

Previously, we have shown that Dopamine- and cAMP-regulated phosphoprotein of 32kDa (DARPP-32) knockout mice required significantly more trials to reach criterion than wild-type mice in an operant reversal-learning task. The present study was conducted to examine adult male and female DARPP-32 knockout mice and wild-type controls in a novel object recognition test. Wild-type and knockout mice exhibited comparable behavior during the initial exploration trials. As expected, wild-type mice exhibited preferential exploration of the novel object during the substitution test, demonstrating recognition memory. In contrast, knockout mice did not show preferential exploration of the novel object, instead exhibiting an increase in exploration of all objects during the test trial. Given that the removal of DARPP-32 is an intracellular manipulation, it seemed possible to pharmacologically restore some cellular activity and behavior by stimulating dopamine receptors. Therefore, a second experiment was conducted examining the effect of methylphenidate. The results show that methylphenidate increased horizontal activity in both wild-type and knockout mice, though this increase was blunted in knockout mice. Pretreatment with methylphenidate significantly impaired novel object recognition in wild-type mice. In contrast, pretreatment with methylphenidate restored the behavior of DARPP-32 knockout mice to that observed in wild-type mice given saline. These results provide additional evidence for a functional role of DARPP-32 in the mediation of processes underlying learning and memory. These results also indicate that the behavioral deficits in DARPP-32 knockout mice may be restored by the administration of methylphenidate.

Acute effects of resveratrol to enhance cocaine-induced dopamine neurotransmission in the striatum

Resveratrol is known as an activator of SIRT1, which leads to the deacetylation of histone and non-histone protein substrates, but also has other pharmacological profiles such as the inhibition of monoamine oxidase (MAO)-A and MAO-B. Resveratrol was previously demonstrated to potentiate the rewarding effects of chronic cocaine via activation of SIRT1. However, the role of resveratrol in cocaine responses in the acute phase remains unexplored. Therefore, we investigated the acute effects of resveratrol on cocaine-stimulated dopamine neurotransmission by analyzing protein phosphorylation in neostriatal slices. Treatment with resveratrol (50μM for 30min) enhanced cocaine-induced increases in the phosphorylation of DARPP-32 at Thr34 and GluA1 at Ser845, postsynaptic substrates for dopamine/D1 receptor/PKA signaling, and a cocaine-induced decrease in the phosphorylation of tyrosine hydroxylase at Ser40, a presynaptic substrate for dopamine/D2 receptor signaling. The inhibition of both MAO-A and MAO-B by clorgyline and pargyline, respectively, enhanced the effects of cocaine on DARPP-32 phosphorylation. The acute effect of resveratrol on cocaine-induced DARPP-32 phosphorylation was occluded with inhibition of MAO-A and MAO-B. In behavioral studies, resveratrol (40mg/kg, s.c.) enhanced the increase in locomotor activity induced by acute cocaine administration (10mg/kg, i.p.). Thus, this study provides pharmacological evidence that acute resveratrol enhances cocaine-induced dopamine neurotransmission and behavioral responses, presumably via mechanisms involving the inhibition of dopamine catabolism by MAO-A and MAO-B. Resveratrol may be useful to treat dysregulated dopamine neurotransmission, but it may enhance the risk of developing drug addiction.

Stimulant drugs trigger transient volumetric changes in the human ventral striatum

The ventral striatum (VStr) integrates mesolimbic dopaminergic and corticolimbic glutamatergic afferents and forms an essential component of the neural circuitry regulating impulsive behaviour. This structure represents a primary target of psychostimulant medication, the first-choice treatment for attention-deficit/hyperactivity disorder (ADHD), and is biochemically modified by these drugs in animals. However, the effects of stimulants on the human VStr remain to be determined. We acquired anatomical brain MRI scans from 23 never-medicated adult patients with ADHD, 31 adult patients with a history of stimulant treatment and 32 control subjects, and VStr volumes were determined using individual rater-blinded region of interest delineation on high-resolution neuroanatomical scans. Furthermore, we also extracted VStr volumes before and after methylphenidate treatment in a subsample of the medication-naïve adult patients as well as in 20 never-medicated children with ADHD. We observed smaller VStr volumes in adult patients with a history of stimulant treatment in comparison to never-medicated patients. Moreover, our longitudinal analyses uncovered a reduction of grey matter volume in the bilateral VStr in adult patients after exposure to methylphenidate, which was followed by volumetric recovery to control level. In children, the same pattern of VStr volume changes was observed after treatment with methylphenidate. These findings suggest that the altered VStr volumes previously observed in patients with ADHD may represent a transitory effect of stimulant exposure rather than an intrinsic feature of the disorder. More generally, these data show that stimulant drugs can render plastic volume changes in human VStr neuroanatomy.

Addiction-related gene regulation: risks of exposure to cognitive enhancers vs. other psychostimulants

The psychostimulants methylphenidate (Ritalin, Concerta), amphetamine (Adderall), and modafinil (Provigil) are widely used in the treatment of medical conditions such as attention-deficit hyperactivity disorder and narcolepsy and, increasingly, as "cognitive enhancers" by healthy people. The long-term neuronal effects of these drugs, however, are poorly understood. A substantial amount of research over the past two decades has investigated the effects of psychostimulants such as cocaine and amphetamines on gene regulation in the brain because these molecular changes are considered critical for psychostimulant addiction. This work has determined in some detail the neurochemical and cellular mechanisms that mediate psychostimulant-induced gene regulation and has also identified the neuronal systems altered by these drugs. Among the most affected brain systems are corticostriatal circuits, which are part of cortico-basal ganglia-cortical loops that mediate motivated behavior. The neurotransmitters critical for such gene regulation are dopamine in interaction with glutamate, while other neurotransmitters (e.g., serotonin) play modulatory roles. This review presents (1) an overview of the main findings on cocaine- and amphetamine-induced gene regulation in corticostriatal circuits in an effort to provide a cellular framework for (2) an assessment of the molecular changes produced by methylphenidate, medical amphetamine (Adderall), and modafinil. The findings lead to the conclusion that protracted exposure to these cognitive enhancers can induce gene regulation effects in corticostriatal circuits that are qualitatively similar to those of cocaine and other amphetamines. These neuronal changes may contribute to the addiction liability of the psychostimulant cognitive enhancers.

Effects of ADHD therapeutic agents, methylphenidate and atomoxetine, on hippocampal neurogenesis in the adolescent mouse dentate gyrus

Methylphenidate (MPH) and atomoxetine (ATX) are commonly used as attention-deficit/hyperactivity disorder (ADHD) therapeutic agents. In the present study, we investigated the effects of MPH and ATX on cell proliferation and neuronal differentiation in the dentate gyrus (DG) of the adolescent mouse by 5-bromo-2'-deoxyuridine (BrdU) and doublecortin (DCX) immunohistochemistry. BrdU-positive ((+)) cells, DCX(+) cells and BrdU(+)/NeuN(+) neurons (BrdU(+) cells with NeuN immunoreaction) were easily detected in the subgranular zone (SGZ) of the DG in the vehicle-, MPH- and ATX-treated groups. Among them, only in the 10mg/kg MPH-treated group, the numbers of BrdU(+), DCX(+) and BrdU(+)/NeuN(+) cells were significantly increased compared to those in the vehicle-treated group. In addition, brain-derived neurotrophic factor (BDNF) level was significantly increased in 10mg/kg MPH-treated group, not in the other experimental groups, compared to the vehicle-treated group. These results indicate that MPH, not ATX, can enhance cell proliferation and neuroblast differentiation in the SGZ of the DG via increasing BDNF level.

Diacylglycerol kinase β knockout mice exhibit attention-deficit behavior and an abnormal response on methylphenidate-induced hyperactivity

Background

Diacylglycerol kinase (DGK) is an enzyme that phosphorylates diacylglycerol to produce phosphatidic acid. DGKβ is one of the subtypes of the DGK family and regulates many intracellular signaling pathways in the central nervous system. Previously, we demonstrated that DGKβ knockout (KO) mice showed various dysfunctions of higher brain function, such as cognitive impairment (with lower spine density), hyperactivity, reduced anxiety, and careless behavior. In the present study, we conducted further tests on DGKβ KO mice in order to investigate the function of DGKβ in the central nervous system, especially in the pathophysiology of attention deficit hyperactivity disorder (ADHD).

Methodology/Principal Findings

DGKβ KO mice showed attention-deficit behavior in the object-based attention test and it was ameliorated by methylphenidate (MPH, 30 mg/kg, i.p.). In the open field test, DGKβ KO mice displayed a decreased response to the locomotor stimulating effects of MPH (30 mg/kg, i.p.), but showed a similar response to an N-methyl-d-aspartate (NMDA) receptor antagonist, MK-801 (0.3 mg/kg, i.p.), when compared to WT mice. Examination of the phosphorylation of extracellular signal-regulated kinase (ERK), which is involved in regulation of locomotor activity, indicated that ERK1/2 activation induced by MPH treatment was defective in the striatum of DGKβ KO mice.

Conclusions/Significance

These findings suggest that DGKβ KO mice showed attention-deficit and hyperactive phenotype, similar to ADHD. Furthermore, the hyporesponsiveness of DGKβ KO mice to MPH was due to dysregulation of ERK phosphorylation, and that DGKβ has a pivotal involvement in ERK regulation in the striatum.

Melanocortin 4 receptor signaling in dopamine 1 receptor neurons is required for procedural memory learning

It is now widely recognized that exposure to palatable foods engages reward circuits that promote over-eating and facilitate the development of obesity. While the melanocortin 4 receptor (MC4R) has previously been shown to regulate food intake and energy expenditure, little is known about its role in food reward. We demonstrate that MC4R is co-expressed with the dopamine 1 receptor (D1R) in the ventral striatum. While MC4R-null mice are hyperphagic and obese, they exhibit impairments in acquisition of operant responding for a high fat reinforcement. Restoration of MC4R signaling in D1R neurons normalizes procedural learning without affecting motivation to obtain high fat diet. MC4R signaling in D1R neurons is also required for learning in a non-food-reinforced version of the cued water maze. Finally, MC4R signaling in neostriatal slices increases phosphorylation of the Thr34 residue of DARPP-32, a protein phosphatase-1 inhibitor that regulates synaptic plasticity. These data identify a novel requirement for MC4R signaling in procedural memory learning.

Attention-deficit/hyperactivity phenotype in mice lacking the cyclin-dependent kinase 5 cofactor p35

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) may result from delayed establishment of corticolimbic circuitry or perturbed dopamine (DA) neurotransmission. Despite the widespread use of stimulants to treat ADHD, little is known regarding their long-term effects on neurotransmitter levels and metabolism. Cyclin-dependent kinase 5 (Cdk5) regulates DA signaling through control of synthesis, postsynaptic responses, and vesicle release. Mice lacking the Cdk5-activating cofactor p35 are deficient in cortical lamination, suggesting altered motor/reward circuitry.

METHODS

We employed mice lacking p35 to study the effect of altered circuitry in vivo. Positron emission tomography measured glucose metabolism in the cerebral cortex using 2-deoxy-2-[¹⁸F] fluoro-d-glucose as the radiotracer. Retrograde dye tracing and tyrosine hydroxylase immunostains assessed the effect of p35 knockout on the medial prefrontal cortex (PFC), especially in relation to mesolimbic circuit formation. We defined the influence of Cdk5/p35 activity on catecholaminergic neurotransmission and motor activity via examination of locomotor responses to psychostimulants, monoamine neurotransmitter levels, and DA signal transduction.

RESULTS

Here, we report that mice deficient in p35 display increased glucose uptake in the cerebral cortex, basal hyperactivity, and paradoxical decreased locomotion in response to chronic injection of cocaine or methylphenidate. Knockout mice also exhibited an increased susceptibility to changes in PFC neurotransmitter content after chronic methylphenidate exposure and altered basal DAergic activity in acute striatal and PFC slices.

CONCLUSIONS

Our findings suggest that dysregulation of Cdk5/p35 activity during development may contribute to ADHD pathology, as indicated by the behavioral phenotype, improperly established mesolimbic circuitry, and aberrations in striatal and PFC catecholaminergic signaling in p35 knockout mice.

Phosphorylation on the PPP2R5D B regulatory subunit modulates the biochemical properties of protein phosphatase 2A

To characterize the biochemical properties of the PP2A regulatory B subunit, PPP2R5D, we analyzed its phosphorylation sites, stoichiometry and effect on holoenzyme activity. PPP2R5D was phosphorylated on Ser-53, Ser-68, Ser-81, and Ser-566 by protein kinase A, and mutations at all four of these sites abolished any significant phosphorylation in vitro. In HEK293 cells, however, the Ser-566 was the major phosphorylation site after PKA activation by forskolin, with marginal phosphorylation on Ser-81. Inhibitory tyrosine phosphorylation on Tyr-307 of the PP2A catalytic C subunit was decreased after forskolin treatment. Kinetic analysis showed that overall PP2A activity was increased with phosphorylation by PPP2R5D phosphorylation. The apparent Km was reduced from 11.25 microM to 1.175 microM with PPP2R5D phosphorylation, resulting in an increase in catalytic activity. These data suggest that PKAmediated activation of PP2A is enabled by PPP2R5D phosphorylation, which modulates the affinity of the PP2A holoenzyme to its physiological substrates.

"Sex, drugs and the brain": the interaction between drugs of abuse and sexual behavior in the female rat

Preclinical and clinical research investigating female sexual motivation has lagged behind research on male sexual function. The present review summarizes recent advances in our understanding of the specific roles of various brain areas, as well as our understanding of the role of dopaminergic neurotransmission in sexual motivation of the female rat. A number of behavioral paradigms that can be used to thoroughly evaluate sexual behavior in the female rat are first discussed. Although traditional assessment of the reflexive, lordosis posture has been useful in understanding the neuroanatomical and neurochemical systems that contribute to copulatory behavior, the additional behavioral paradigms described in this review have helped us expand our understanding of appetitive and consumatory behavioral patterns that better assess sexual motivation - the equivalent of "desire" in humans. A summary of numerous lesion studies indicates that different areas of the brain, including forebrain and midbrain structures, work together to produce the complex repertoire of female sexual behavior. In addition, by investigating the effects of commonly addictive drugs, we are beginning to elucidate the role of dopaminergic neurotransmission in female sexual motivation. Consequently, research in this area may contribute to meaningful advances in the treatment of human female sexual dysfunction.

Methamphetamine facilitates female sexual behavior and enhances neuronal activation in the medial amygdala and ventromedial nucleus of the hypothalamus

Methamphetamine (MA) abuse has reached epidemic proportions in the United States. Users of MA report dramatic increases in sexual drive that have been associated with increased engagement in risky sexual behavior leading to higher rates of sexually transmitted diseases and unplanned pregnancies. The ability of MA to enhance sexual drive in females is enigmatic since related psychostimulants like amphetamine and cocaine appear not to affect sexual drive in women, and in rodents models, amphetamine has been reported to be inhibitory to female sexual behavior. Examination of MA's effects on female sexual behavior in an animal model is lacking. Here, using a rodent model, we have demonstrated that MA enhanced female sexual behavior. MA (5mg/kg) or saline vehicle was administered once daily for 3 days to adult ovariectomized rats primed with ovarian steroids. MA treatment significantly increased the number of proceptive events and the lordosis response compared to hormonally primed, saline controls. The effect of MA on the neural circuitry underlying the motivation for sexual behavior was examined using Fos immunoreactivity. In the medial amygdala and the ventromedial nucleus of the hypothalamus, nuclei implicated in motivated behaviors, ovarian hormones and MA independently enhance the neuronal activation, but more striking was the significantly greater activation induced by their combined administration. Increases in dopamine neurotransmission may underlie the MA/hormone mediated increase in neuronal activation. In support of this possibility, ovarian hormones significantly increased tyrosine hydroxylase (the rate limiting enzyme in dopamine synthesis) immunoreactivity in the medial amygdala. Thus our present data suggest that the interactions of MA and ovarian hormones leads to changes in the neural substrate of key nuclei involved in mediating female sexual behaviors, and these changes may underlie MA's ability to enhance these behaviors.

Social and physical environment alter cocaine conditioned place preference and dopaminergic markers in adolescent male rats

This study was done to determine whether social and environmental factors alter cocaine reward and proteins implicated in mediating drug reward in rats during early adolescence. On postnatal day (PND) 23, rats were housed under conditions where both social (number of rats per cage) and environmental (availability of toys) factors were manipulated. Socially isolated rats were housed alone impoverished with no toys (II) or enriched with toys (IE). Social rats were housed two rats/cage with no toys (SI2) or with toys (SE2), or three/cage with (SE3) or without (SI3) toys. On PND 43, cocaine conditioned place preference (CPP) sessions began with the post-test done on PND 47. Cocaine CPP was established in response to 5 or 10 mg/kg cocaine in II rats, and CPP was decreased with the addition of cage mates or toys. No CPP was seen to any dose in SI3 or SE3 rats. Enriched housing (SE3) increased dopamine transporter (DAT) protein in the nucleus accumbens compared to II. There also were differential effects of cocaine on tyrosine hydroxylase and DAT depending on housing, with both increased by cocaine in II but not SE3 rats. DARPP-32 was unchanged by housing or cocaine, while phospho-Thr(34)-DARPP-32 was increased by cocaine treatment across conditions. Thus, both social and environmental enrichment decrease cocaine CPP during adolescence and different housing alters proteins that regulate dopaminergic neurotransmission in a manner that may account for the observed differences in cocaine-induced reward.

Narcolepsy and depression and the neurobiology of gammahydroxybutyrate

A voluminous literature describes the relationship between disturbed sleep and depression. The breakdown of sleep is one of the cardinal features of depression and often also heralds its onset. Frequent arousals, periods of wakefulness and a short sleep onset REM latency are typical polysomnographic features of depression. The short latency to REM sleep has been attributed to the combination of a monoaminergic deficiency and cholinergic supersensitivity and these irregularities have been proposed to form the biological basis of the disorder. A similar imbalance between monoaminergic and cholinergic neurotransmission has been found in narcolepsy, a condition in which frequent awakenings, periods of wakefulness and short sleep onset REM latencies are also characteristic findings during sleep. In many cases of narcolepsy, this imbalance appears to result from a deficiency of hypocretin but once established, whether in depression or narcolepsy, this disequilibrium sets the stage for the dissociation or premature appearance of REM sleep and for the dissociation of the motor inhibitory component of REM sleep or cataplexy. In the presence of this monoaminergic/cholinergic imbalance, gammahydroxybutyrate (GHB) may acutely further reduce the latency of REM sleep and induce cataplexy, in both patients with narcolepsy or depression. On the other hand, the repeated nocturnal application of GHB in patients with narcolepsy improves the continuity of sleep, prolongs the latency to REM sleep and prevents cataplexy. Evidence to date suggests that GHB may restore the normal balance between monoaminergic and cholinergic neurotransmission. As such, the repeated use of GHB at night and the stabilization of sleep over time makes GHB an effective treatment for narcolepsy and a potentially effective treatment for depression.

METHYLPHENIDATE (RITALIN): BEHAVIORAL STUDIES IN THE RAT

Attention Deficit Hyperactivity Disorder (ADHD) is a neuropsychiatric syndrome with an onset in childhood characterized by an inability to remain focused or to concentrate for prolonged periods of time. Children suffering from this disease are many times described as either inattentive or as hyperactive-impulsive depending on what form of the disease they manifest. Methylphenidate is the preferred treatment for this behavioral disorder and is used for long term disease management. Much still remains unknown concerning this stimulant and its effects on behavior and future abuse potential are pertinent questions. Since animal models are used to study the mechanism of drug action and rats are used often in drug studies, the objective of this review is to summarize the research reports that mainly have used rats as the model to investigate the action of methylphenidate. Topics discussed in this review include: (1) What effect does a single dose of methylphenidate have on locomotion activity; (2) Does repeated administration of methylphenidate result in tolerance or sensitization; and (3) Does methylphenidate have rewarding properties as measured by the self-administration and condition placed preference paradigms.

Intermittent Dopaminergic stimulation causes behavioral sensitization in the addicted brain and parkinsonism

The gold standard therapy for Parkinson's disease (PD) consists in chronic administration of pulses of the dopamine (DA) precursor l-dihydroxyphenylalanine (l-DOPA). Although the main brain area which is DA-deficient is the dorsal striatum (more the putamen than the caudate nucleus), other DA-innervated brain regions (i.e., the ventral striatum and other limbic areas) are affected by systemic administration of l-DOPA. While such a therapy produces an increase in synaptic and nonsynaptic DA, which replace the neurotransmitter deficiency, peaks of extracellular DA in the course of disease progression produce abnormal involuntary movements related to behavioral sensitization. Methamphetamine (METH), a widely abused drug, is known to produce behavioral sensitization, related to DA release (more in the ventral than dorsal striatum as well as other limbic regions). The present review discusses the overlapping between these treatments, based on pulses of DA stimulation with an emphasis on the class of DA receptors; signal transduction pathways; rearranged expression of neurotransmitters, cotransmitters, and their receptors coupled with ultrastructural changes. In fact, all these levels of synaptic plasticity show a surprising homology following these treatments, posing the mechanisms of behavioral sensitization during DA-replacement therapy in PD very close to the neurobiological mechanisms operating during METH abuse. In line with this view is the growing evidence of addictive behaviors in PD patients during the course of DA-replacement therapy.

Methylphenidate increases creatine kinase activity in the brain of young and adult rats

AIMS

The high prevalence of Attention Deficit/Hyperactivity Disorder (ADHD) and the increased therapeutic use of methylphenidate (MPH) raise some concerns regarding its long-term side effects and safety profile. Considering that MPH effects on brain metabolism are poorly known and that creatine kinase (CK) plays an important role in cell energy homeostasis, we evaluated CK activity in the brain of young and adult rats following acute (one injection) or chronic (28 days) administration of MPH.

MAIN METHODS

MPH was acutely or chronically administered to young and adult rats. For acute administration, a single injection of MPH was given to rats on postnatal day (PD) 25 or PD 60, in the young and adult groups, respectively. For chronic administration, MPH injections were given to young rats starting at PD 25 once daily for 28 days (last injection at PD 53). In adult rats, the same regimen was performed starting at PD 60 (last injection at PD 88). CK activity was measured in brain homogenates.

KEY FINDINGS

Our results showed that MPH acute administration increased the enzyme in prefrontal cortex, hippocampus, striatum and cerebral cortex, but not cerebellum of young and adult rats. Chronic administration of MPH also increased CK activity in these brain regions, as well as the cerebellum, in young and adult rats. The highest dose (10.0 mg/kg) presented more pronouncing effects.

SIGNIFICANCE

The present findings suggest that acute or chronic exposure to MPH increased CK activity, an enzyme involved in energy homeostasis, in the brain of young and adult rats.

The effects of acute dopamine reuptake inhibition on performance

INTRODUCTION

Acute bupropion (dopamine/noradrenaline reuptake inhibitor) administration significantly improved time trial performance and increased core temperature in the heat (30 degrees C).

PURPOSE

The present study was performed to examine the effect of a dopaminergic reuptake inhibitor on exercise capacity and thermoregulation during prolonged exercise in temperate and warm conditions.

METHODS

Eight healthy well-trained male cyclists participated in this study. Subjects ingested either placebo (PLA; 20 mg) or methylphenidate (MPH; Ritalin; 20 mg) 1 h before the start of exercise in temperate (18 degrees C) or warm (30 degrees C) conditions and cycled for 60 min at 55% Wmax, immediately followed by a time trial (TT; PLA18 and MPH18; PLA30 and MPH30) to measure exercise performance.

RESULTS

MPH did not influence TT performance at 18 degrees C (P = 0.397). TT was completed 16% faster in MPH30 (38.1 +/- 6.4 min) than in PLA30 (45.4 +/- 7.3 min; P = 0.049). In the heat Tcore was significantly higher at rest (P = 0.009), and throughout the TT in MPH30 (P < 0.018), reaching values above 40 degrees C. Throughout MPH30, heart rates were significantly higher (P < 0.05).

CONCLUSIONS

These results show that MPH has a clear ergogenic effect that was not apparent in 18 degrees C. The combination of a dopamine reuptake inhibitor and exercise in the heat clearly improved performance and caused hyperthermia without any change in the perception of effort or thermal stress compared with the PLA trial. This response may potentially increase the risk of developing heat illness during exercise in individuals taking drugs of this nature.

Methylphenidate alters NCS-1 expression in rat brain

Methylphenidate has been used as an effective treatment for attention deficit hyperactivity disorder (ADHD). Methylphenidate (MPH) blocks dopamine and norepinephrine transporters causing an increase in extracellular levels. The use of psychomotor stimulants continues to rise due to both the treatment of ADHD and illicit abuse. Methylphenidate sensitization mechanism has still poor knowledge. Neuronal calcium sensor 1 was identified as a dopaminergic receptor interacting protein. When expressed in mammalian cells, neuronal calcium sensor 1 attenuates dopamine-induced D2 receptor internalization by a mechanism that involves a reduction in D2 receptor phosphorylation. Neuronal calcium sensor 1 appears to play a pivotal role in regulating D2 receptor function, it will be important to determine if there are alterations in neuronal calcium sensor 1 in neuropathologies associated with deregulation in dopaminergic signaling. Then, we investigated if methylphenidate could alter neuronal calcium sensor 1 expression in five brain regions (striatum, hippocampus, prefrontal cortex, cortex and cerebellum) in young and adult rats. These regions were chosen because some are located in brain circuits related with attention deficit hyperactivity disorder. Our results showed changes in neuronal calcium sensor 1 expression in hippocampus, prefrontal cortex and cerebellum mainly in adult rats. The demonstration that methylphenidate induces changes in neuronal calcium sensor 1 levels in rat brain may help to understand sensitization mechanisms as well as methylphenidate therapeutic effects to improve attention deficit hyperactivity disorder symptoms.

Trk: a neuromodulator of age-specific behavioral and neurochemical responses to cocaine in mice

Responses to psychostimulants vary with age, but the molecular etiologies of these differences are largely unknown. The goal of the present research was to identify age-specific behavioral and molecular adaptations to cocaine and to elucidate the mechanisms involved therein. Postweanling, periadolescent, and adult male CD-1 mice were exposed to cocaine (20 mg/kg) for 7 d. The rewarding effects of cocaine were assessed, as were the response to a Trk antagonist and the regulation of dopamine and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32). Cocaine was rewarding in both periadolescent and adult mice using a conditioned place preference procedure. In contrast, postweanling mice failed to demonstrate significant cocaine-induced place preference. Because components of the neurotrophin system including brain-derived neurotrophic factor and TrkB are developmentally regulated, their role in the age-specific effects of cocaine was determined using the Trk receptor antagonist K252a. Postweanling mice that received K252a before daily cocaine showed a significant place preference to the cocaine-paired environment that was not seen in the absence of K252a. DARPP-32 protein levels were significantly upregulated in the lateral region of the caudate-putamen exclusively in postweanling mice after chronic cocaine. Daily pretreatment with K252a attenuated the induction of DARPP-32 in the postweanling striatum. These data indicate that Trk neurotransmission plays a role in age-specific behavioral and molecular responses to cocaine and concurrently modulates DARPP-32 levels.

Phasic dysfunction of dopamine transmission in Tourette's syndrome evaluated with 99mTc TRODAT-1 imaging

This study investigated the complex dysregulation of the dopaminergic neurotransmitter system in Tourette's syndrome (TS) patients challenged with methylphenidate (MPH). Eight drug-naïve male patients (aged 21-25 years) who met DSM-IV criteria for TS and had a mean disease severity of 25 on the Yale Global Tic Severity Scale were recruited. Brain (99m)TC TRODAT-1 dopamine transporter (DAT) single photon emission computed tomography (SPECT) was performed 5 days before, and 2 h after 10 mg of orally administered MPH. Eight age-matched healthy males served as controls. Repeated measures analysis of variance was used to measure differences in DAT-binding ratios before and after MPH challenge between the TS patients and controls. The DAT-binding ratios decreased significantly after MPH treatment in both groups. However, a significant interaction between group and MPH effects was found only in the right caudate, which was mainly due to a smaller decline of the DAT-binding ratio after MPH in the TS group than in the controls. Such a distinction was not found in the other striatal sub-regions in the two groups. No correlation, however, was observed between the tic severity score and DAT-binding ratio measured from the whole striatum or its sub-regions. The observed change in the DAT-binding ratio might indicate a functional abnormality of the dopaminergic system in the right caudate nucleus of TS patients. Future studies exploring dopamine transmission are thus needed to understand the pathophysiology of TS.

Sensitization and cross-sensitization after chronic treatment with methylphenidate in adolescent Wistar rats

An increasing debate exists about the potential of early exposure to methylphenidate to increase the risk for drug abuse. In addition, little is known about the neurobiological effects of early exposure to methylphenidate. This study was designed to investigate whether chronic treatment with methylphenidate induces behavioral sensitization to subsequent methylphenidate and D-amphetamine challenge in adolescent Wistar rats. Young Wistar rats (P25) were treated with either methylphenidate (1, 2, or 10 mg/kg, intraperitoneally) or saline for 28 days. After 14 days of washout, animals were challenged with methylphenidate 2.5 mg/kg intraperitoneally or D-amphetamine 2 mg/kg intraperitoneally (P67). Locomotor behavior was assessed using the open field test. Rats chronically treated with methylphenidate in the adolescent period showed augmented locomotor sensitization to D-amphetamine but not to methylphenidate in the adult phase. These findings suggest that early exposure do methylphenidate might increase the risk for subsequent D-amphetamine abuse. Further studies focusing on the neurobiological effects of early exposure to methylphenidate are warranted.

Chronic administration of methylphenidate activates mitochondrial respiratory chain in brain of young rats

Methylphenidate is frequently prescribed for the treatment of attention deficit/hyperactivity disorder. Psychostimulants can cause long-lasting neurochemical and behavioral adaptations. The exact mechanisms underlying its therapeutic and adverse effects are still not well understood. In this context, it was previously demonstrated that methylphenidate altered brain metabolic activity, evaluated by glucose consumption. Most cell energy is obtained through oxidative phosphorylation, in the mitochondrial respiratory chain. Tissues with high energy demands, such as the brain, contain a large number of mitochondria. In this work, our aim was to measure the activities of mitochondrial respiratory chain complexes II and IV and succinate dehydrogenase in cerebellum, prefrontal cortex, hippocampus, striatum, and cerebral cortex of young rats (starting on 25th post-natal day and finishing on 53rd post-natal day) chronically treated with methylphenidate. Our results showed that mitochondrial respiratory chain enzymes activities were increased by chronic administration of this drug. Succinate dehydrogenase was activated in cerebellum, prefrontal cortex and striatum, but did not change in hippocampus and brain cortex. Complex II activity was increased in cerebellum and prefrontal cortex and was not affected in hippocampus, striatum and brain cortex. Finally, complex IV activity was increased in cerebellum, hippocampus, striatum and brain cortex, and was not affected in prefrontal cortex. These findings suggest that chronic exposure to methylphenidate in young rats increases mitochondrial enzymes involved in brain metabolism. Further research is being carried out in order to better understand the effects of this drug on developing nervous system and the potential consequences in adulthood resulting from early-life drug exposure.

Preadolescent methylphenidate versus cocaine treatment differ in the expression of cocaine-induced locomotor sensitization during adolescence and adulthood

BACKGROUND

Methylphenidate (MPH), the most commonly prescribed medication for childhood attention-deficit/hyperactivity disorder (ADHD), shares chemical and mechanistic similarities to cocaine which has stimulated research to address the addiction liability following treatment.

METHODS

Utilizing locomotor sensitization we examined the consequences of recurrent MPH versus cocaine treatment during preadolescence in altering cocaine-induced locomotor behavior in adolescent and adult mice. Black Swiss Webster mice were treated with MPH, cocaine, or saline during preadolescence. To test whether MPH pretreatment during preadolescence contributed to an altered sensitivity to cocaine during adolescence, these mice were treated with recurrent cocaine or saline during adolescence. All mice were challenged with cocaine as adults.

RESULTS

Recurrent MPH treatment, unlike cocaine treatment in preadolescent mice, had no effect on locomotor sensitization to cocaine during adolescence or adulthood, as compared with saline controls. Furthermore, unlike cocaine, administration of MPH in adolescence did not augment the response to cocaine challenge.

CONCLUSIONS

MPH treatment during preadolescence does not increase subsequent sensitivity to cocaine, whereas cocaine treatment does. Thus, MPH treatment during preadolescence does not appear to persistently induce long-term adaptations, which may underlie an enhanced liability for subsequent drug abuse.

Paradoxical Striatal Cellular Signaling Responses to Psychostimulants in Hyperactive Mice

Recent investigations have shown that three major striatal-signaling pathways (protein kinase A/DARPP-32, Akt/glycogen synthase kinase 3, and ERK) are involved in the regulation of locomotor activity by the monoaminergic neurotransmitter dopamine. Here we used dopamine transporter knock-out mice to examine which particular changes in the regulation of these cell signaling mechanisms are associated with distinct behavioral responses to psychostimulants. In normal animals, amphetamine and methylphenidate increase extracellular levels of dopamine, leading to an enhancement of locomotor activity. However, in dopamine transporter knock-out mice that display a hyperactivity phenotype resulting from a persistent hyperdopaminergic state, these drugs antagonize hyperactivity. Under basal conditions, dopamine transporter knock-out mice show enhanced striatal DARPP-32 phosphorylation, activation of ERK, and inactivation of Akt as compared with wild-type littermates. However, administration of amphetamine or methylphenidate to these mice reveals that inhibition of ERK signaling is a common determinant for the ability of these drugs to antagonize hyperactivity. In contrast, psychostimulants activate ERK and induce hyperactivity in normal animals. In hyperactive mice psychostimulant-mediated behavioral inhibition and ERK regulation are also mimicked by the serotonergic drugs fluoxetine and 5-carboxamidotryptamine, thereby revealing the involvement of serotonin-dependent inhibition of striatal ERK signaling. Furthermore, direct inhibition of the ERK signaling cascade in vivo using the MEK inhibitor SL327 recapitulates the actions of psychostimulants in hyperactive mice and prevents the locomotor-enhancing effects of amphetamine in normal animals. These data suggest that the inhibitory action of psychostimulants on dopamine-dependent hyperactivity results from altered regulation of striatal ERK signaling. In addition, these results illustrate how altered homeostatic state of neurotransmission can influence in vivo signaling responses and biological actions of pharmacological agents used to manage psychiatric conditions such as Attention Deficit Hyperactivity Disorder (ADHD).

Methylphenidate treatment induces oxidative stress in young rat brain

Methylphenidate (MPH) is frequently prescribed for the treatment of attention deficit/hyperactivity disorder. Psychostimulants can cause long-lasting neurochemical and behavioral adaptations. Here, we evaluated oxidative damage in the rat brain and the differential age-dependent response to MPH after acute and chronic exposure. We investigated the oxidative damage, assessed by the thiobarbituric acid reactive species (TBARS), and the protein carbonyl assays in cerebellum, prefrontal cortex, hippocampus, striatum, and cerebral cortex of young (25 days old) and adult (60 days old) male Wistar rats after acute and chronic exposure to MPH. Chronic MPH-treated young rats presented a dose-dependent increase in TBARS content and protein carbonyls formation in specific rat brain regions. In the acute exposure, only MPH highest dose increased lipid peroxidation in the hippocampus. No difference in protein carbonylation was observed among groups in all structures analyzed. In adult rats, we did not find oxidative damage in both acute and chronic treatment. Chronic exposure to MPH in induces oxidative damage in young rat brain, differentially from chronic exposure during adulthood. These findings highlight the need for further research to improve understanding of MPH effects on developing nervous system and the potential consequences in adulthood resulting from early-life drug exposure.

Age-specific behavioral responses to psychostimulants in mice

This study investigated the influence of age on the behavioral responses elicited by psychostimulants in male CD-1 mice. Behavioral activity including locomotion and stereotypy was measured following acute or repeated administration of cocaine, methylphenidate, amphetamine or saline to postweanling (24 days old), periadolescent (33 days old) and adult (60 days old) mice. Postweanling mice exhibited less total and ambulatory activity than periadolescent mice following a single acute injection of cocaine (20 or 30 and 30 mg/kg, respectively). Further, postweanling mice showed less total activity than both periadolescent and adult mice at a dose of 10 mg/kg methylphenidate. Less stereotypy was also seen in postweanling mice when compared to adolescent mice after 30 mg/kg amphetamine. Seven daily injections of cocaine resulted in a heightened behavioral response on day 7 as compared to day 1, indicative of behavioral sensitization in adult and periadolescent, but not postweanling mice. Repeated methylphenidate resulted in increased total activity in adult, but not periadolescent or postweanling mice. None of the animals were sensitized to the behavioral activating effects of amphetamine. The magnitude of behavioral response and the development of sensitization were dependent upon the age of the animal and the agent tested.

Blink rate in boys with fragile X syndrome: preliminary evidence for altered dopamine function

BACKGROUND

Dopamine, a neurotransmitter involved in motor and cognitive functioning, can be non-invasively measured via observation of spontaneous blink rates. Blink rates have been studied in a number of clinical conditions including schizophrenia, autism, Parkinsons, and attention deficit/hyperactivity disorder with results implicating either hyper or hypo dopaminergic states.

METHODS

This study examined spontaneous blink rate in boys with fragile X syndrome (FXS). Blink rates of boys (4-8 years old) with FXS (n = 6) were compared with those of age-matched typically developing boys (n = 6) during active and passive tasks. Blink rates (blinks per minute) for each task were compared between the two groups. Then, the relation between blink measures and core FXS-related features [problem behaviours, arousal, fmr 1 protein (FMRP)] were examined within the group of boys with FXS.

RESULTS

Blink rate in boys with FXS was significantly higher than typically developing boys during passive tasks. Within the FXS group, there were significant correlations between blink rate and problem behaviours and physiological arousal (i.e. heart activity) but not with FMRP.

CONCLUSIONS

Observed differences in spontaneous blink rate between boys with and without FXS and the relation between blink rate and physiological and behavioural measures in boys with FXS suggests that further work examining dopamine dysfunction as a factor in the pathophysiology of FXS may be warranted.

Methylphenidate differentially regulates c-fos and fosB expression in the developing rat striatum

Methylphenidate (MPH, Ritalin) is a psychostimulant drug used in very young children with attention deficit hyperactivity disorder (ADHD). To explore the central effects of MPH, we compared repeated MPH treatments on c-fos and fosB expression in the striatum of immature and adult rats. Prepubertal (PD25-38) or adult (PD53-66) male rats were treated once daily for: (a) 14 days with either saline or MPH (2 or 10 mg/kg) or (b) 13 days with saline followed by a single dose of MPH (2 or 10 mg/kg) on day 14. To determine long-term effects of MPH, another group of prepubertal rats was allowed a drug-free period of 4 weeks following the initial 14 days of treatment, and received a challenge dose of MPH at adulthood. All rats were sacrificed 2 h post-injection on the final day. Expression of c-fos and fosB was quantified by densitometric analysis of cFOS and FOSB-immunoreactivity (-ir). We demonstrated that FOSB-ir was increased by a single dose of MPH in the prepubertal and adult striatum, and this effect was further elevated by chronic MPH in prepubertal rats, in contrast to the inhibitory effect of MPH (2 and 10 mg/kg) on cFOS-ir. In adult rats, repeated MPH down-regulated cFOS-ir only at the higher dose (10 mg/kg), while fosB expression remained at levels comparable to acute MPH. The reduction in cFOS-ir observed in prepubertal rats given repeated MPH (10 mg/kg) persisted in the adult striatum following MPH challenge at adulthood. Our results suggest that (1) repeated MPH treatment differentially regulates c-fos and fosB expression in the immature and adult brain; (2) MPH-induced changes in gene expression may be enduring, and (3) the immature brain is more sensitive to the stimulant effects of MPH than the adult. Thus, our findings have implications for the long-term use of MPH in ADHD.

Methylphenidate regulates c-fos and fosB expression in multiple regions of the immature rat brain

Methylphenidate (Ritalin, MPH) is a common psychostimulant used to treat childhood attention-deficit hyperactivity disorder (ADHD). Little is known about the long-term developmental effects on gene expression and behavior, which may occur with extended MPH use. We reported previously that the striatum is a major target of MPH, consistent with human MRI studies. In the present study, we tested the hypothesis that MPH is likely to have widespread effects in extra-striatal regions of the brain. We used the expression of two immediate early genes, c-fos and fosB, as probes to map the response of the immature rat brain to single (1 day) versus repeated (14 days) MPH treatment (2 or 10 mg/kg; s.c.) from postnatal day 25 to 38. Consistent with previous reports, the striatum is a major target of acute MPH action, as indicated by elevated levels of cFOS-immunoreactivity (-ir). Increases in c-fos expression were also seen in the nucleus accumbens, cingulate/frontal cortex and piriform cortex, and Islands of Calleja. FosB expression was elevated only in the striatum following a single stimulation. Chronic MPH treatment (10 mg/kg/day for 14 days) resulted in an attenuation of c-fos expression in the striatum and Islands of Calleja. However, levels of cFOS-ir remained elevated in the nucleus accumbens and frontal cortex. In contrast to the inhibitory effect of repeated MPH exposure on c-fos expression, FOSB-ir was further elevated in the striatum, and an increase was observed in the cingulate/frontal and piriform cortices. Thus, chronic MPH differentially regulated expression of c-fos and fosB in several brain regions. Our data suggest that MPH may exert its stimulant effects at multiple sites in the immature brain, which has implications for long-term treatment in children.

Identification and characterization of Xenopus OMP25

This study describes the isolation of mitochondrial outer membrane protein 25 (OMP25) from Xenopus laevis and an analysis of its role in early development. X. laevis OMP25 (xOMP25) is a transmembrane protein of the mitochondrial outer membrane with a PDZ domain in the cytoplasmic tail, and an approximate molecular size of 25 kDa. We isolated xOMP25 from a cDNA library of X. laevis tailbud embryos. Amino acid sequence analysis of xOMP25 showed 57% identity to mouse OMP25, with 73% identity in the PDZ domains. XOMP25 mRNA is expressed maternally, and at a constant level throughout early development. The transcript is localized to eye, otic vesicle, branchial arch and neural tube. Mitochondrial targeting of an EGFP-fusion protein of xOMP25 was visualized using a mitochondria-specific fluorescent dye. Overexpression of xOMP25 in embryos caused curved axes, small eyes and disorganized head structures. Knockdown of xOMP25 protein using antisense morpholino oligonucleotides resulted in slightly shortened axes and decreased neural tissue. Although the mechanism remains unclear, our results implicate xOMP25 protein in the formation of the intact neural tube.