Stimulants and the developing brain

Methylphenidate and atomoxetine inhibit social play behavior through prefrontal and subcortical limbic mechanisms in rats

Positive social interactions during the juvenile and adolescent phases of life, in the form of social play behavior, are important for social and cognitive development. However, the neural mechanisms of social play behavior remain incompletely understood. We have previously shown that methylphenidate and atomoxetine, drugs widely used for the treatment of attention-deficit hyperactivity disorder (ADHD), suppress social play in rats through a noradrenergic mechanism of action. Here, we aimed to identify the neural substrates of the play-suppressant effects of these drugs. Methylphenidate is thought to exert its effects on cognition and emotion through limbic corticostriatal systems. Therefore, methylphenidate was infused into prefrontal and orbitofrontal cortical regions as well as into several subcortical limbic areas implicated in social play. Infusion of methylphenidate into the anterior cingulate cortex, infralimbic cortex, basolateral amygdala, and habenula inhibited social play, but not social exploratory behavior or locomotor activity. Consistent with a noradrenergic mechanism of action of methylphenidate, infusion of the noradrenaline reuptake inhibitor atomoxetine into these same regions also reduced social play. Methylphenidate administration into the prelimbic, medial/ventral orbitofrontal, and ventrolateral orbitofrontal cortex, mediodorsal thalamus, or nucleus accumbens shell was ineffective. Our data show that the inhibitory effects of methylphenidate and atomoxetine on social play are mediated through a distributed network of prefrontal and limbic subcortical regions implicated in cognitive control and emotional processes. These findings increase our understanding of the neural underpinnings of this developmentally important social behavior, as well as the mechanism of action of two widely used treatments for ADHD.

The Ethics of Neuroenhancement

According to several recent studies, a big chunk of college students in North America and Europe uses so called ‘smart drugs' to enhance their cognitive capacities aiming at improving their academic performance. With these practices, there comes a certain moral unease. This unease is shared by many, yet it is difficult to pinpoint and in need of justification. Other than simply pointing to the medical risks coming along with using non-prescribed medication, the salient moral question is whether these practices are troubling in and of themselves. In due consideration of empirical insights into the concrete effects of smart drugs on brain and behavior, our attempt is to articulate wherein this moral unease consists and to argue for why the authors believe cognitive enhancement to be morally objectionable. The authors will contend that the moral problem with these practices lies less in the end it seeks, than in the underlying human disposition it expresses and promotes. Some might ask, what is wrong with molding our cognitive capacities to achieve excellence, get a competitive edge, or, as the whim takes us? In all of these occasions, the usage of smart drugs serves a certain goal, a telos. The goal is, broadly speaking, this: outsmarting opponents in an arms race for limited resources and thereby yielding a competitive edge. In plain words: competition is valued higher than cooperation or solidarity. What is wrong with striving for this goal? The authors submit that the question whether people really want to live in a society that promotes the mentality ‘individual competition over societal cooperation' deserves serious consideration. In developing their answer, the authors draw on an ‘Ethics of Constraint' framework, arguing that widespread off-label use of smart drugs bears the risk of negative neural/behavioral consequences for the individual that might, in the long run, be accompanied by changing social value orientations for the worse.

Monoamine-sensitive developmental periods impacting adult emotional and cognitive behaviors

Development passes through sensitive periods, during which plasticity allows for genetic and environmental factors to exert indelible influence on the maturation of the organism. In the context of central nervous system development, such sensitive periods shape the formation of neurocircuits that mediate, regulate, and control behavior. This general mechanism allows for development to be guided by both the genetic blueprint as well as the environmental context. While allowing for adaptation, such sensitive periods are also vulnerability windows during which external and internal factors can confer risk to disorders by derailing otherwise resilient developmental programs. Here we review developmental periods that are sensitive to monoamine signaling and impact adult behaviors of relevance to psychiatry. Specifically, we review (1) a serotonin-sensitive period that impacts sensory system development, (2) a serotonin-sensitive period that impacts cognition, anxiety- and depression-related behaviors, and (3) a dopamine- and serotonin-sensitive period affecting aggression, impulsivity and behavioral response to psychostimulants. We discuss preclinical data to provide mechanistic insight, as well as epidemiological and clinical data to point out translational relevance. The field of translational developmental neuroscience has progressed exponentially providing solid conceptual advances and unprecedented mechanistic insight. With such knowledge at hand and important methodological innovation ongoing, the field is poised for breakthroughs elucidating the developmental origins of neuropsychiatric disorders, and thus understanding pathophysiology. Such knowledge of sensitive periods that determine the developmental trajectory of complex behaviors is a necessary step towards improving prevention and treatment approaches for neuropsychiatric disorders.

Potentiated gene regulation by methylphenidate plus fluoxetine treatment: Long-term gene blunting (Zif268, Homer1a) and behavioral correlates

Use of psychostimulants such as methylphenidate (Ritalin) in medical treatments and as cognitive enhancers in the healthy is increasing. Methylphenidate produces some addiction-related gene regulation in animal models. Recent findings show that combining selective serotonin reuptake inhibitor (SSRI) antidepressants such as fluoxetine with methylphenidate potentiates methylphenidate-induced gene regulation. We investigated the endurance of such abnormal gene regulation by assessing an established marker for altered gene regulation after drug treatments - blunting (repression) of immediate-early gene (IEG) inducibility - 14 days after repeated methylphenidate+fluoxetine treatment in adolescent rats. Thus, we measured the effects of a 6-day repeated treatment with methylphenidate (5 mg/kg), fluoxetine (5 mg/kg) or their combination on the inducibility (by cocaine) of neuroplasticity-related IEGs (Zif268, Homer1a) in the striatum, by in situ hybridization histochemistry. Repeated methylphenidate treatment alone produced modest gene blunting, while fluoxetine alone had no effect. In contrast, fluoxetine given in conjunction with methylphenidate produced pronounced potentiation of methylphenidate-induced blunting for both genes. This potentiation was seen in many functional domains of the striatum, but was most robust in the lateral, sensorimotor striatum. These enduring molecular changes were associated with potentiated induction of behavioral stereotypies in an open-field test. For illicit psychostimulants, blunting of gene expression is considered part of the molecular basis of addiction. Our results thus suggest that SSRIs such as fluoxetine may increase the addiction liability of methylphenidate. Key words: cognitive enhancer, dopamine, serotonin, gene expression, psychostimulant, SSRI antidepressant, striatum.

Acoustic noise improves motor learning in spontaneously hypertensive rats, a rat model of attention deficit hyperactivity disorder

The spontaneously hypertensive (SH) rat model of ADHD displays impaired motor learning. We used this characteristic to study if the recently described acoustic noise benefit in learning in children with ADHD is also observed in the SH rat model. SH rats and a Wistar control strain were trained in skilled reach and rotarod running under either ambient noise or in 75 dBA white noise. In other animals the effect of methylphenidate (MPH) on motor learning was assessed with the same paradigms. To determine if acoustic noise influenced spontaneous motor activity, the effect of acoustic noise was also determined in the open field activity paradigm. We confirm impaired motor learning in the SH rat compared to Wistar SCA controls. Acoustic noise restored motor learning in SH rats learning the Montoya reach test and the rotarod test, but had no influence on learning in Wistar rats. Noise had no effect on open field activity in SH rats, but increased corner time in Wistar. MPH completely restored rotarod learning and performance but did not improve skilled reach in the SH rat. It is suggested that the acoustic noise benefit previously reported in children with ADHD is shared by the SH rat model of ADHD, and the effect is in the same range as that of stimulant treatment. Acoustic noise may be useful as a non-pharmacological alternative to stimulant medication in the treatment of ADHD.

Selective serotonin re-uptake inhibitors potentiate gene blunting induced by repeated methylphenidate treatment: Zif268 versus Homer1a

There is a growing use of psychostimulants, such as methylphenidate (Ritalin; dopamine re-uptake inhibitor), for medical treatments and as cognitive enhancers in the healthy. Methylphenidate is known to produce some addiction-related gene regulation. Recent findings in animal models show that selective serotonin re-uptake inhibitors (SSRIs), including fluoxetine, can potentiate acute induction of gene expression by methylphenidate, thus indicating an acute facilitatory role for serotonin in dopamine-induced gene regulation. We investigated whether repeated exposure to fluoxetine, in conjunction with methylphenidate, in adolescent rats facilitated a gene regulation effect well established for repeated exposure to illicit psychostimulants such as cocaine-blunting (repression) of gene inducibility. We measured, by in situ hybridization histochemistry, the effects of a 5-day repeated treatment with methylphenidate (5 mg/kg), fluoxetine (5 mg/kg) or a combination on the inducibility (by cocaine) of neuroplasticity-related genes (Zif268, Homer1a) in the striatum. Repeated methylphenidate treatment alone produced minimal gene blunting, while fluoxetine alone had no effect. In contrast, fluoxetine added to methylphenidate robustly potentiated methylphenidate-induced blunting for both genes. This potentiation was widespread throughout the striatum, but was most robust in the lateral, sensorimotor striatum, thus mimicking cocaine effects. For illicit psychostimulants, blunting of gene expression is considered part of the molecular basis of addiction. Our results thus suggest that SSRIs, such as fluoxetine, may increase the addiction liability of methylphenidate.

Effects of developmental methylphenidate (MPH) treatment on monoamine neurochemistry of male and female rats

Attention Deficit Hyperactivity Disorder (ADHD) is estimated to affect 4-5% of the adult human population (Kessler et al., 2006; Willcutt, 2012). Often prescribed to attenuate ADHD symptoms (Nair and Moss, 2009), methylphenidate hydrochloride (MPH) can have substantial positive effects. However, there is a paucity of literature regarding its use during pregnancy. Thus, adult women with ADHD face a difficult decision when contemplating pregnancy. In this study, pregnant Sprague-Dawley rats were orally treated a total of 0 (water), 6 (low), 18 (medium), or 42 (high) mg MPH/kg body weight/day (divided into three doses) on gestational days 6-21 (i.e., the low dose received 2 mg MPH/kg body weight 3×/day). Offspring were orally treated with the same daily dose as their dam (divided into two doses) on postnatal days (PNDs) 1-21. One offspring/sex/litter was sacrificed at PND 22 or PND 104 (n=6-7/age/sex/treatment group) and the striatum was quickly dissected and frozen. High Performance Liquid Chromatography (HPLC) coupled to a Photo Diode Array detector (PDA) was used to analyze monoamine content in the striatum of one side while a sandwich ELISA was used to analyze tyrosine hydroxylase (TH) from the other side. Age significantly affected monoamine and metabolite content as well as turnover ratios (i.e., DA, DOPAC, HVA, DOPAC/DA, HVA/DA, 5-HT and 5-HIAA); however, there were no significant effects of sex. Adult rats of the low MPH group had higher DA levels than control adults (p<0.05). At both ages, subjects of the low MPH group had higher TH levels than controls (p<0.05), although neither effect (i.e., higher DA or TH levels) exhibited an apparent dose-response. PND 22 subjects of the high MPH treatment group had higher ratios of HVA/DA and DOPAC/DA than same-age control subjects (p<0.05). The increased TH levels of the low MPH group may be related to the increased DA levels of adult rats. While developmental MPH treatment appears to have some effects on monoamine system development, further studies are required to determine if these alterations manifest as functional changes in behavior.

A study on fear memory retrieval and REM sleep in maternal separation and isolation stressed rats

As rapid brain development occurs during the neonatal period, environmental manipulation during this period may have a significant impact on sleep and memory functions. Moreover, rapid eye movement (REM) sleep plays an important role in integrating new information with the previously stored emotional experience. Hence, the impact of early maternal separation and isolation stress (MS) during the stress hyporesponsive period (SHRP) on fear memory retention and sleep in rats were studied. The neonatal rats were subjected to maternal separation and isolation stress during postnatal days 5-7 (6h daily/3d). Polysomnographic recordings and differential fear conditioning was carried out in two different sets of rats aged 2 months. The neuronal replay during REM sleep was analyzed using different parameters. MS rats showed increased time in REM stage and total sleep period also increased. MS rats showed fear generalization with increased fear memory retention than normal control (NC). The detailed analysis of the local field potentials across different time periods of REM sleep showed increased theta oscillations in the hippocampus, amygdala and cortical circuits. Our findings suggest that stress during SHRP has sensitized the hippocampus-amygdala-cortical loops which could be due to increased release of corticosterone that generally occurs during REM sleep. These rats when subjected to fear conditioning exhibit increased fear memory and increased fear generalization. The development of helplessness, anxiety and sleep changes in human patients, thus, could be related to the reduced thermal, tactile and social stimulation during SHRP on brain plasticity and fear memory functions.

Cocaine-seeking behavior in a genetic model of attention-deficit/hyperactivity disorder following adolescent methylphenidate or atomoxetine treatments

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is often comorbid with cocaine abuse. Controversy exists regarding long-term consequences of ADHD medications on cocaine abuse liability. Whereas childhood methylphenidate treatment may be preventative, methylphenidate in teens appears to further increase later cocaine abuse risk. In rodents, adolescent methylphenidate treatment further increases adult cocaine self-administration in the Spontaneously Hypertensive Rat (SHR) model of ADHD, whereas adolescent atomoxetine treatment does not. Effects of ADHD medications on cocaine cue reactivity, a critical component of addiction, are unknown.

METHODS

To investigate this, SHR, Wistar-Kyoto (inbred control) and Wistar (outbred control) rats received therapeutically relevant doses of methylphenidate (1.5 mg/kg, oral) and atomoxetine (0.3 mg/kg, intraperitoneal), or respective vehicles from post-natal day 28-55. Cocaine seeking, reflecting cue reactivity, was measured in adulthood during self-administration maintenance and cue-induced reinstatement tests conducted under a second-order schedule.

RESULTS

Compared to control strains, SHR earned more cocaine infusions, emitted more cocaine-seeking responses during maintenance and reinstatement testing, and required more sessions to reach the extinction criterion. Compared to vehicle, adolescent methylphenidate, but not atomoxetine, further increased cocaine intake during maintenance testing in SHR. Adolescent atomoxetine, but not methylphenidate, decreased cocaine seeking during reinstatement testing in SHR. Neither medication had effects on cocaine intake or cue reactivity in control strains.

CONCLUSIONS

The SHR successfully model ADHD and cocaine abuse comorbidity and show differential effects of adolescent ADHD medications on cocaine intake and cue reactivity during adulthood. Thus, SHR have heuristic value for assessing neurobiology underlying the ADHD phenotype and for evaluating pharmacotherapeutics for ADHD.

Withdrawal effect of chronic amphetamine exposure during adolescence on complex maze performance

National survey data suggest a steady increase in the diagnosis and treatment of mental disorders in children, particularly Attention Deficit/Hyperactivity Disorder (ADHD). As nearly all children diagnosed with ADHD are prescribed stimulant drugs, rationale exists to quantitatively characterize behavioral responses following withdrawal from chronic stimulant dosing. These rodent experiments involved chronic administration of 7.5 mg/kg, s.c. amphetamine to subjects throughout adolescence followed by cognitive tests to gauge learning and performance during the withdrawal stage 7 to 14 days past withdrawal. Tests used a complex Stone 14-unit multiple T-maze, which is a robust paradigm for demonstrating age-related differences in rodent models when behavioral cognitive endpoints are used. Results reveal that amphetamine-treated subjects committed fewer major and retracing errors with increased minor errors and a significantly lower mean completion time. These findings suggest that pharmacotherapy aimed at adolescent-phase treatment of ADHD does not provoke spatial memory deficits at times proximal to drug withdrawal and lends support to amphetamine use in the treatment of ADHD children.

Repeated aripiprazole treatment causes dopamine D2 receptor up-regulation and dopamine supersensitivity in young rats

Aripiprazole is a second-generation antipsychotic that is increasingly being prescribed to children and adolescents. Despite this trend, little preclinical research has been done on the neural and behavioral actions of aripiprazole during early development. In the present study, young male and female Sprague-Dawley rats were pretreated with vehicle, haloperidol (1 mg/kg), or aripiprazole (10 mg/kg) once daily on postnatal days (PD) 10-20. After 1, 4, or 8 days (i.e. on PD 21, PD 24, or PD 28), amphetamine-induced locomotor activity and stereotypy, as well as dorsal striatal D2 receptor levels, were measured in separate groups of rats. Pretreating young rats with aripiprazole or haloperidol increased D2 binding sites in the dorsal striatum. Consistent with these results, dopamine supersensitivity was apparent when aripiprazole- and haloperidol-pretreated rats were given a test day injection of amphetamine (2 or 4 mg/kg). Increased D2 receptor levels and altered behavioral responding persisted for at least 8 days after conclusion of the pretreatment regimen. Contrary to what has been reported in adults, repeated aripiprazole treatment caused D2 receptor up-regulation and persistent alterations of amphetamine-induced behavior in young rats. These findings are consistent with human clinical studies showing that children and adolescents are more prone than adults to aripiprazole-induced side effects, including extrapyramidal symptoms.

Reducing substance use during adolescence: a translational framework for prevention

RATIONALE

Most substance use is initiated during adolescence when substantial development of relevant brain circuitry is still rapidly maturing. Developmental differences in reward processing, behavioral flexibility, and self-regulation lead to changes in resilience or vulnerability to drugs of abuse depending on exposure to risk factors. Intervention and prevention approaches to reducing addiction in teens may be able to capitalize on malleable brain systems in a predictable manner.

OBJECTIVE

This review will highlight what is known about how factors that increase vulnerability to addiction, including developmental stage, exposure to early life adversity (ranging from abuse, neglect, and bullying), drug exposure, and genetic predisposition, impact the development of relevant systems.

RESULTS AND CONCLUSIONS

Appropriate, early intervention may restore the normal course of an abnormal trajectory and reduce the likelihood of developing a substance use disorder (SUD) later in life. A considerable amount is known about the functional neuroanatomy and/or pharmacology of risky behaviors based on clinical and preclinical studies, but relatively little has been directly translated to reduce their impact on addiction in high-risk children or teenagers. An opportunity exists to effectively intervene before adolescence when substance use is likely to emerge.

Protracted maturation of forebrain afferent connections of the ventral tegmental area in the rat

The mesocorticolimbic dopamine system has long attracted the interest of researchers concerned with the unique gamut of behavioral and mental health vulnerabilities associated with adolescence. Accordingly, the development of the mesocorticolimbic system has been studied extensively, but almost exclusively with regard to dopaminergic output, particularly in the nucleus accumbens and medial prefrontal cortex. To the contrary, the ontogeny of inputs to the ventral tegmental area (VTA), the source of mesocorticolimbic dopamine, has been neglected. This is not a trivial oversight, as the activity of VTA neurons, which reflects their capacity to transmit information about salient events, is sensitively modulated by inputs. Here, we assessed the development of VTA afferent connections using the β subunit of cholera toxin (Ctβ) as a retrograde axonal tracer in adolescent (postnatal day 39) and early adult (8-9-week-old) rats. After intra-VTA injections of Ctβ, adolescent and early adult animals exhibited qualitatively similar distributions of retrogradely labeled neurons in the sense that VTA-projecting neurons were present at all of the same rostrocaudal levels in all of the same structures in both age groups. However, quantitation of retrogradely labeled neurons revealed that adolescent brains, compared with early adult brains, had significantly fewer VTA-projecting neurons preferentially within an interconnected network of cortical and striatopallidal forebrain structures. These findings provide a novel perspective on the development of the mesocorticolimbic dopamine system and may have important implications for age-dependent specificity in the function of this system, particularly with regard to adolescent impulsivity and mental health vulnerabilities.

Animal models of obsessive-compulsive spectrum disorders

Obsessive-compulsive disorder (OCD) and related conditions (trichotillomania, pathological skin-picking, pathological nail-biting) are common and disabling. Current treatment approaches fail to help a significant proportion of patients. Multiple tiers of evidence link these conditions with underlying dysregulation of particular cortico-subcortical circuitry and monoamine systems, which represent targets for treatment. Animal models designed to capture aspects of these conditions are critical for several reasons. First, they help in furthering our understanding of neuroanatomical and neurochemical underpinnings of the obsessive-compulsive (OC) spectrum. Second, they help to account for the brain mechanisms by which existing treatments (pharmacotherapy, psychotherapy, deep brain stimulation) exert their beneficial effects on patients. Third, they inform the search for novel treatments. This article provides a critique of key animal models for selected OC spectrum disorders, beginning with initial work relating to anxiety, but moving on to recent developments in domains of genetic, pharmacological, cognitive, and ethological models. We find that there is a burgeoning literature in these areas with important ramifications, which are considered, along with salient future lines of research.

Juvenile methylphenidate reduces prefrontal cortex plasticity via D3 receptor and BDNF in adulthood

BACKGROUND

Early drug intervention in childhood disorders aims to maximize individual potential in the short- and long-term. Consistently, juvenile exposure to psychostimulants, such as methylphenidate (MPH), reduces risk for substance use in animals and sub-populations of individuals with attention deficit hyperactivity disorder (ADHD). We investigated the effects of MPH on brain plasticity via dopamine receptor D3 (D3R) and brain-derived neurotrophic factor (BDNF) expression in developing rats.

METHODS

Between postnatal days 20-35, rat pups were administered saline vehicle (Veh) or MPH (2 mg/kg), the D3R-preferring agonist ±7-OHDPAT, or the antagonist nafadotride (0.05 mg/kg) alone, or in combination with MPH twice a day. In adulthood, subjects were challenged to Veh or cocaine (10 mg/kg for two days). The prefrontal cortex was analyzed for protein and mRNA levels of total BDNF, its splice variants I, IIc, III/IV, and IV/VI, and D3 receptors. A separate group of subjects was assessed for splice variants at 20, 35, 40, and 60 days of age.

RESULTS

Across age strong correlations were evident between Drd3 and Bdnf mRNA levels (r = 0.65) and a negative relationship between Drd3 and exon IIc after MPH treatment (r = -0.73). BDNF protein levels did not differ between Veh- and MPH subjects at baseline, but were significantly lower in MPH-treated and cocaine challenged subjects (30.3 ± 9.7%). Bdnf mRNA was significantly higher in MPH-treated subjects, and reversed upon exposure to cocaine. This effect was blocked by nafadotride. Furthermore, Bdnf total and Bdnf splice variants I, IIc, III/IV, and IV/VI changed across the transitions between juvenility and late adolescence.

CONCLUSIONS

These data suggest a sensitive window of vulnerability to modulation of BDNF expression around adolescence, and that compared to normal animals, juvenile exposure to MPH permanently reduces prefrontal BDNF transcription and translation upon cocaine exposure in adulthood by a D3R-mediated mechanism.

Simple shelter-style environmental enrichment alters behavior in mice

Environmental enrichment aims to improve the well-being of laboratory animals and provides an opportunity to improve experimental reliability and validity. Animals raised in more stimulating environments have improved learning and memory as well as more complex brain architecture. However, the effects of environmental enrichment on motor performance, anxiety and emotional development have been poorly studied. Moreover, most investigators studying the effects of enrichment provide extremely large and complex housing conditions to maximize the likelihood of finding effects. These situations are difficult to replicate across animal facilities and are not operationally practical. In this experiment, we investigated how simple, inexpensive disposable shelterstyle enrichment items alter behavior in C57Bl/6 and 129S6 mice. Breeding pairs were established in the presence of a Ketchum “Refuge”, Shepherd Shack “Dome”, or no enrichment. Offspring were assessed neurobehaviorally, either just after weaning (pre-adolescent, P22–P25), or as young adults (P60–P90). Major strain differences were observed in open field activity, elevated maze exploration, and Y-maze activity levels. The presence of the Refuge and/or Dome enrichment shelters significantly altered motor activity, coordination and some measures of anxiety. Mice housed in the presence of shelters were also less dominant than control mice in a tube test assay. Our experiments provide a detailed analysis of the effects of inexpensive and practical methods of housing enrichment on biobehavioral phenotypes in these two commonly used strains of laboratory mice, and suggest that the effects of these shelters on mouse neurobiology and behavior need to be rigorously analyzed before being adopted within vivariums.

Life-long consequences of juvenile exposure to psychotropic drugs on brain and behavior

Psychostimulants such as methylphenidate (MPH) and antidepressants such as fluoxetine (FLX) are widely used in the treatment of various mental disorders or as cognitive enhancers. These medications are often combined, for example, to treat comorbid disorders. There is a considerable body of evidence from animal models indicating that individually these psychotropic medications can have detrimental effects on the brain and behavior, especially when given during sensitive periods of brain development. However, almost no studies investigate possible interactions between these drugs. This is surprising given that their combined neurochemical effects (enhanced dopamine and serotonin neurotransmission) mimic some effects of illicit drugs such as cocaine and amphetamine. Here, we summarize recent studies in juvenile rats on the molecular effects in the mid- and forebrain and associated behavioral changes, after such combination treatments. Our findings indicate that these combined MPH+FLX treatments can produce similar molecular changes as seen after cocaine exposure while inducing behavioral changes indicative of dysregulated mood and motivation, effects that often endure into adulthood.

Repeated ketamine exposure induces an enduring resilient phenotype in adolescent and adult rats

BACKGROUND

Major depressive disorder afflicts up to 10% of adolescents. However, nearly 50% of those afflicted are considered nonresponsive to available treatments. Ketamine, a noncompetitive N-methyl-D-aspartate receptor antagonist has shown potential as a rapid-acting and long-lasting treatment for major depressive disorder in adults. Thus, the effectiveness and functional consequences of ketamine exposure during adolescence were explored.

METHODS

Adolescent male rats (postnatal day [PD] 35) received two ketamine (0, 5, 10, or 20 mg/kg) injections, 4 hours apart, after exposure to day 1 of the forced swim test (FST). The next day, rats were reexposed to the FST to assess ketamine-induced antidepressant-like responses. Separate groups were exposed to chronic unpredictable stress to confirm findings from the FST. After these initial experiments, adolescent naive rats were exposed to either 1 or 15 consecutive days (PD35-49) of ketamine (20 mg/kg) twice daily. Ketamine's influence on behavioral reactivity to rewarding (i.e., sucrose preference) and aversive (i.e., elevated plus-maze, FST) circumstances was then assessed 2 months after treatment. To control for age-dependent effects, adult rats (PD75-89) were exposed to identical experimental conditions.

RESULTS

Ketamine (20 mg/kg) reversed the chronic unpredictable stress-induced depression-like behaviors in the FST. Repeated ketamine exposure resulted in anxiolytic- and antidepressant-like responses 2 months after drug exposure. None of the ketamine doses used were capable of inducing drug-seeking behaviors as measured by place preference conditioning.

CONCLUSIONS

Repeated ketamine exposure induces enduring resilient-like responses regardless of age of exposure. These findings point to ketamine, and its repeated exposure, as a potentially useful antidepressant during adolescence.

Developmental disruption of gamma-aminobutyric acid function in the medial prefrontal cortex by noncontingent cocaine exposure during early adolescence

BACKGROUND

Drug experimentation during adolescence is associated with increased risk of drug addiction relative to any other age group. To further understand the neurobiology underlying such liability, we investigate how early adolescent cocaine experience impacts medial prefrontal cortex (mPFC) network function in adulthood.

METHODS

A noncontingent administration paradigm was used to assess the impact of early adolescent cocaine treatment (rats; postnatal days [PD] 35-40) on the overall inhibitory regulation of mPFC activity in adulthood (PD 65-75) by means of histochemical and in vivo electrophysiological measures combined with pharmacologic manipulations.

RESULTS

Cocaine exposure during early adolescence yields a distinctive hypermetabolic prefrontal cortex state that was not observed in adult-treated rats (PD 75-80). Local field potential recordings revealed that early adolescent cocaine exposure is associated with an attenuation of mPFC gamma-aminobutyric acid (GABA)ergic inhibition evoked by ventral hippocampal stimulation at beta and gamma frequencies that endures throughout adulthood. Such cocaine-induced mPFC disinhibition was not observed in adult-exposed animals. Furthermore, the normal developmental upregulation of parvalbumin immunoreactivity observed in the mPFC from PD 35 to PD 65 is lacking following early adolescent cocaine treatment.

CONCLUSIONS

Our data indicate that repeated cocaine exposure during early adolescence can elicit a state of mPFC disinhibition resulting from a functional impairment of the local prefrontal GABAergic network that endures through adulthood. A lack of acquisition of prefrontal GABAergic function during adolescence could trigger long-term deficits in the mPFC that may increase the susceptibility for the onset of substance abuse and related psychiatric disorders.

MR imaging of the effects of methylphenidate on brain structure and function in attention-deficit/hyperactivity disorder

Methylphenidate is the first-choice pharmacological intervention for the treatment of Attention-Deficit/Hyperactivity Disorder (ADHD). The pharmacological and behavioral effects of methylphenidate are well described, but less is known about neurochemical brain changes induced by methylphenidate. This level of analysis may be informative on how the behavioral effects of methylphenidate are established. This paper reviews structural and functional MRI studies that have investigated effects of methylphenidate in children with ADHD. Structural MRI studies provide evidence that long-term stimulant treatment may normalize structural brain changes found in the white matter, the anterior cingulate cortex, the thalamus, and the cerebellum in ADHD. Moreover, preliminary evidence suggests that methylphenidate treatment may normalize the trajectory of cortical development in ADHD. Functional MRI has provided evidence that methylphenidate administration has acute effects on brain functioning, and even suggests that methylphenidate may normalize brain activation patterns as well as functional connectivity in children with ADHD during cognitive control, attention, and during rest. The effects of methylphenidate on the developing brain appear highly specific and dependent on numerous factors, including biological factors such as genetic predispositions, subject-related factors such as age and symptom severity, and task-related factors such as task difficulty. Future studies on structural and functional brain changes in ADHD may benefit from inclusion strategies guided by current medication status and medication history. Further studies on the effects of methylphenidate treatment on structural and functional MRI parameters are needed to address unresolved issues of the long-term effects of treatment, as well as the mechanism through which medication-induced brain changes bring about clinical improvement.

What influences clinicians' decisions about ADHD medication? Initial data from the Influences on Prescribing for ADHD Questionnaire (IPAQ)

Despite evidence for its efficacy and effectiveness, the use of medication for the treatment of ADHD remains controversial. Little is known about the factors that influence clinicians' decisions to use medication for ADHD. Here, we present initial data on the attitudes of prescribing clinicians from the Influences on Prescribing for ADHD Questionnaire (IPAQ)-a new clinician-completed, 40-item scale. The eight IPAQ subscales cover attitudes towards (1) treatment outcome optimisation, (2) the use of rule based over more informal approaches, (3) side effects, (4) symptoms control as the primary goal of treatment, (5) the influence of external pressure on medication-related decisions, (6) the value of taking the child's views into account, (7) long-term medication use and (8) the value of psychosocial approaches for the treatment of ADHD. Sixty-eight clinicians from Belgium and the UK took part. All subscales had acceptable levels of internal reliability (Chronbach's alpha = 0.62-0.78). Overall, clinicians reported taking a rule-based approach to prescribing with a focus on treatment optimisation, taking the child's view into account and valuing psycho-social approaches. They focused on treating broader patterns of impairment, but were wary of the potential side effects and long-term treatment. Psychiatrists scored high on their focus on symptom control and preference for long-term medication use, while paediatricians reported using more rule-based approaches. We identified four distinctive response profiles: (1) pro-psychosocial; (2) medication focused; (3) unsystematic; and (4) response optimizers. Future larger scale studies are required to replicate these profiles and to explore their relationship with prescribing behaviour and treatment outcomes.

Co-segregation of hyperactivity, active coping styles, and cognitive dysfunction in mice selectively bred for low levels of anxiety

We established mouse models of extremes in trait anxiety, which are based on selective breeding for low vs. normal vs. high open-arm exploration on the elevated plus-maze. Genetically selected low anxiety-related behavior (LAB) coincided with hyperactivity in the home cage. Given the fact that several psychiatric disorders such as schizophrenia, mania, and attention deficit hyperactivity disorder (ADHD) share hyperactivity symptom, we systematically examined LAB mice with respect to unique and overlapping endophenotypes of the three diseases. To this end Venn diagrams were used as an instrument for discrimination of possible models. We arranged the endophenotypes in Venn diagrams and translated them into different behavioral tests. LAB mice showed elevated levels of locomotion in the open field (OF) test with deficits in habituation, compared to mice bred for normal (NAB) and high anxiety-related behavior (HAB). Cross-breeding of hypoactive HAB and hyperactive LAB mice resulted in offspring showing a low level of locomotion comparable to HAB mice, indicating that the HAB alleles are dominant over LAB alleles in determining the level of locomotion. In a holeboard test, LAB mice spent less time in hole exploration, as shown in patients with schizophrenia and ADHD; however, LAB mice displayed no impairments in social interaction and prepulse inhibition (PPI), implying a unlikelihood of LAB as an animal model of schizophrenia. Although LAB mice displayed hyperarousal, active coping styles, and cognitive deficits, symptoms shared by mania and ADHD, they failed to reveal the classic manic endophenotypes, such as increased hedonia and object interaction. The neuroleptic haloperidol reduced locomotor activity in all mouse lines. The mood stabilizer lithium and the psychostimulant amphetamine, in contrast, selectively reduced hyperactivity in LAB mice. Based on the behavioral and pharmacological profiles, LAB mice are suggested as a novel rodent model of ADHD-like symptoms.

Effects of withdrawal from repeated amphetamine exposure in peri-puberty on neuroplasticity-related genes in mice

Although extensive evidence demonstrates that repeated administration of amphetamine (AMPH) induces behavioral and neurochemical sensitization, the influence of the developmental timing of AMPH administration is unknown. This is an important issue to address because it could help clarify the influence of early drug exposure on neuronal plasticity and the involvement of dopaminergic sensitization in the etiopathology of neuropsychiatric disorders. Thus, we decided to investigate the molecular alterations induced by the administration of AMPH during adolescence, when repeated exposure to the psychostimulant may interfere with developmental neuroplasticity. We investigated the expression of the neurotrophin brain-derived neurotrophic factor (BDNF) and of two inducible-early genes (arc and cfos) that bridge neuronal activity with long-lasting functional alterations. We found that peri-pubertal treatment with AMPH induces long-lasting changes in the expression of bdnf and of activity-regulated genes in the hippocampus and in the prefrontal/frontal cortex, and leads to alterations of their short-term modulation in response to a subsequent acute AMPH challenge. These data suggest that AMPH exposure in peri-puberty may negatively affect the maturation of brain structures, such as the prefrontal cortex, which facilitate the development of dopamine sensitization and may contribute to dopamine-dependent behavioral dysfunctions and molecular alterations in adulthood.

Early ontogeny of D-amphetamine-induced one-trial behavioral sensitization

The early ontogeny of D-amphetamine-induced one-trial behavioral sensitization was characterized using male and female preweanling and preadolescent rats. In Experiment 1, rats were injected with saline or D-amphetamine (1, 4, or 8mg/kg) in activity chambers or the home cage on postnatal day (PD) 12, PD 16, PD 20, or PD 24. One day later, rats were challenged with either 0.5 or 2mg/kg D-amphetamine and distance traveled was measured in activity chambers for 120min. In Experiment 2, saline or D-amphetamine was administered in activity chambers on PD 24, while a challenge injection of D-amphetamine (0.25-4mg/kg) was given on PD 25. At younger ages (PD 13 and PD 17), a strong sensitized response was evident on the test day regardless of whether rats were pretreated with D-amphetamine (4 or 8mg/kg) before being placed in the activity chamber or 30min after being returned to the home cage. Rats did not display D-amphetamine-induced behavioral sensitization on PD 21, nor was context-dependent sensitization apparent on PD 25 even when a broad dose range of D-amphetamine was used. When low doses of D-amphetamine were administered on the pretreatment and test days (1 and 0.5mg/kg, respectively), sensitized responding was not evident at any age. In summary, D-amphetamine-induced one-trial behavioral sensitization was only apparent within a narrow developmental window during early ontogeny. This ontogenetic pattern of sensitized responding is similar to the one produced by methamphetamine and distinct from the pattern produced by cocaine. The unique sensitization profiles resulting from repeated D-amphetamine and cocaine treatment may be a consequence of their different mechanisms of action.

Altered gene expression in the prefrontal cortex of young rats induced by the ADHD drug atomoxetine

Atomoxetine (ATX), a selective norepinephrine reuptake inhibitor, is a non-stimulant approved for the treatment of attention deficit/hyperactivity disorder (ADHD). Little is known about the molecular basis for its therapeutic effect. The objective of this animal study was to determine alterations in gene expression patterns in the prefrontal cortex after long-term administration of atomoxetine. Rats were treated for 21 days during childhood and early adolescent stages of development with a once-daily oral application of 0.05 g/kg atomoxetine, which resulted in plasma levels similar to those described in children. A whole genome RNA-microarray of rat prefrontal cortical gene expression after administration of atomoxetine versus sterile water revealed an mRNA increase in 114 genes (≥2-fold) while 11 genes were down-regulated (≤0.5-fold). By applying quantitative real-time PCR (qRT-PCR) and Western Blot we confirmed a significant increase in the expression of GABA A receptor subunits as well as ubiquinol-cytochrome c reductase complex core protein 2 (Uqcrc2). SNAP-25 (synaptosomal-associated protein of 25 kDa), which is an ADHD candidate gene and an important vesicle protein involved in axonal growth, synaptic plasticity and regulation of neurotransmitter release was also significantly upregulated on RNA- and protein level after atomoxetine treatment. In summary, we could show that long-term treatment with the ADHD drug atomoxetine induces the regulation of several genes in the prefrontal cortex of young rats. Especially the increased expression of SNAP-25 and GABA-A receptor subunits may indicate additional active therapeutic mechanisms for atomoxetine.

Maternal caffeine administration leads to adverse effects on adult mice offspring

PURPOSE

This study aimed to evaluate the role of caffeine chronic administration during gestation of C57BL/6 mice on cardiac remodeling and the expression of components of the renin-angiotensin system (RAS) in male offspring as adults.

METHODS

Pregnant C57BL/6 female mice were divided into two groups (n = 10): Control group (C), dams were injected with the vehicle only (saline 0.9% NaCl); Caffeine group (CF), dams received daily a subcutaneous injection of 20 mg/kg of caffeine/day (1 mg/mL saline). Pups had free access to standard chow since weaning to 3 months of age, when they were killed.

RESULTS

CF group showed increased energy expenditure (+7%) with consequent reduction in body mass (BM) gain (-18%), increased blood pressure (+48%), and higher heart rate (+10%) than C group. The ratio between LV mass/BM was greater (+10%), with bigger cardiomyocytes (+40%), and reduced vascularization (-25%) in CF group than in C group. In the LV, the expression of angiotensin-converting enzyme (+30%), Angiotensin II (AngII) (+60%), AngII receptor (ATR)-1 (+77%) were higher, and the expression of ATR-2 was lower (-46%; P < 0.05) in CF group than in C group. In the kidney, the expressions of renin (+128%) and ATR-1 (+88%) were higher in CF group than in C group.

CONCLUSIONS

Chronic administration of caffeine to pregnant dams led to persistent activation of local RAS in the kidney and heart of the offspring, which, in turn, leads to high BP and adverse cardiac remodeling. These findings highlight the urge to encourage pregnant women to avoid food or medicines containing caffeine.

Ontogeny of methamphetamine-induced and cocaine-induced one-trial behavioral sensitization in preweanling and adolescent rats

The ontogenetic profile of psychostimulant-induced one-trial behavioral sensitization has not been determined. The purpose of this study was to systematically assess the ontogeny of methamphetamine-induced and cocaine-induced behavioral sensitization across the preweanling and adolescent periods. To this end, rats were injected with methamphetamine, cocaine, or saline in either an activity chamber or home cage during the preweanling [postnatal day (PD) 12, PD 16, or PD 20], preadolescent (PD 24), or adolescent (PD 34) periods. One day later, rats were challenged with the same psychostimulant and locomotion was measured in an activity chamber. The results showed that methamphetamine produced one-trial locomotor sensitization on PD 13 and PD 17; whereas, cocaine-induced behavioral sensitization was only evident on PD 21. The sensitized responding of preweanling rats was not influenced by environmental context. Interestingly, preadolescent and adolescent rats did not exhibit locomotor sensitization. The latter result is generally consistent with past studies showing that rats from the middle and late adolescent periods do not exhibit cocaine-induced one-trial behavioral sensitization. The present results show that methamphetamine, as well as cocaine, can produce one-trial context-independent behavioral sensitization during early ontogeny, but sensitized responding is only apparent within a narrow developmental window.

The effects of ecstasy (MDMA) on brain serotonin transporters are dependent on age-of-first exposure in recreational users and animals

Rationale and Objective

Little is known on the effects of ecstasy (MDMA, a potent 5-HT-releaser and neurotoxin) exposure on brain development in teenagers. The objective of this study was to investigate whether in humans, like previous observations made in animals, the effects of MDMA on the 5-HT system are dependent on age-of-first exposure.

Methods

5-HT transporter (SERT) densities in the frontal cortex and midbrain were assessed with [¹²³I]β-CIT single photon emission computed tomography in 33 users of ecstasy. Subjects were stratified for early-exposed users (age-at-first exposure 14–18 years; developing brain), and late-exposed users (age-at-first exposure 18–36 years; mature brain). In parallel, we investigated the effects of age experimentally with MDMA in early-exposed (adolescent) rats and late-exposed (adult) rats using the same radioligand.

Results

On average, five years after first exposure, we found a strong inverse relationship, wherein age-at-first exposure predicted 79% of the midbrain SERT variability in early (developing brain) exposed ecstasy users, whereas this was only 0.3% in late (mature brain) exposed users (p = 0.007). No such effect was observed in the frontal cortex. In rats, a significant age-BY-treatment effect (p<0.01) was observed as well, however only in the frontal cortex.

Conclusions

These age-related effects most likely reflect differences in the maturational stage of the 5-HT projection fields at age-at-first exposure and enhanced outgrowth of the 5-HT system due to 5-HT’s neurotrophic effects. Ultimately, our findings stress the need for more knowledge on the effects of pharmacotherapies that alter brain 5-HT levels in the pediatric population.

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.

Dopamine receptor inactivation in the caudate-putamen differentially affects the behavior of preweanling and adult rats

The irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) has been used to study the ontogeny of dopamine (DA) receptor functioning in young and adult rats. Most notably, systemic administration of EEDQ blocks the DA agonist-induced behaviors of adult rats, while leaving the behavior of preweanling rats unaffected. The purpose of the present study was to: (a) determine whether the age-dependent actions of EEDQ involve receptors located in the dorsal caudate-putamen (CPu) and (b) confirm that EEDQ's behavioral effects result from the inactivation of DA receptors rather than some other receptor type. In Experiment 1, EEDQ or DMSO was bilaterally infused into the CPu on PD 17 or PD 84. After 24h, rats were given bilateral microinjections of the full DA agonist R(-)-propylnorapomorphine (NPA) or vehicle into the dorsal CPu and behavior was assessed for 40 min. In Experiment 2, preweanling rats were treated as just described, except that DA receptors were protected from EEDQ-induced alkylation by administering systemic injections of D1 (SCH23390) and D2 (sulpiride) receptor antagonists. As predicted, microinjecting EEDQ into the dorsal CPu attenuated the NPA-induced locomotor activity and stereotypy of adult rats. In contrast, rats given bilateral EEDQ infusions on PD 17 exhibited a potentiated locomotor response when treated with NPA. Experiment 2 showed that DA receptor inactivation was responsible for NPA's actions. A likely explanation for these results is that EEDQ inactivates a sizable percentage of DA receptors on PD 17, but leaves the remaining receptors in a supersensitive state. This receptor supersensitivity, which probably involves alterations in G protein coupling, could account for NPA-induced locomotor potentiation. It is likely that adult rats to not show a similar EEDQ-induced change in receptor dynamics or DA receptor inactivation was more complete in older animals and effectively eliminated the expression of DA agonist-induced behaviors.

Evidence for higher reaction time variability for children with ADHD on a range of cognitive tasks including reward and event rate manipulations

OBJECTIVE

The purpose of the research study was to examine the manifestation of variability in reaction times (RT) in children with attention deficit hyperactivity disorder (ADHD) and to examine whether RT variability presented differently across a variety of neuropsychological tasks, was present across the two most common ADHD subtypes, and whether it was affected by reward and event rate (ER) manipulations.

METHOD

Children with ADHD-combined type (n = 51), ADHD-predominantly inattentive type (n = 53), and 47 controls completed five neuropsychological tasks (Choice Discrimination Task, Child Attentional Network Task, Go/No-Go task, Stop Signal Task, and N-back task), each allowing trial-by-trial assessment of RTs. Multiple indicators of RT variability including RT standard deviation, coefficient of variation and ex-Gaussian tau were used.

RESULTS

Children with ADHD demonstrated greater RT variability than controls across all five tasks as measured by the ex-Gaussian indicator tau. There were minimal differences in RT variability across the ADHD subtypes. Children with ADHD also had poorer task accuracy than controls across all tasks except the Choice Discrimination task. Although ER and reward manipulations did affect children's RT variability and task accuracy, these manipulations largely did not differentially affect children with ADHD compared to controls. RT variability and task accuracy were highly correlated across tasks. Removing variance attributable to RT variability from task accuracy did not appreciably affect between-groups differences in task accuracy.

CONCLUSIONS

High RT variability is a ubiquitous and robust phenomenon in children with ADHD.

A review of psychostimulant-induced neuroadaptation in developing animals

The effects of clinically relevant doses of commonly prescribed stimulants methylphenidate (MPH), d-amphetamine (d-AMPH), and dl-AMPH or mixed amphetamine salts (MAS) such as Adderall, on short- and long-term gene neuroadaptations in developing animals have not been widely investigated. In the present review, the effects of oral stimulant administration were compared with those of the subcutaneous or intra-peritoneal route. A selective set of studies between 1979 and 2010, which incorporated in their design developmental period, clinically relevant doses of stimulants, and repeated daily doses were reviewed. These studies indicate that neuroadaptation to chronic stimulants includes blunting of stimulated immediate early gene expression, sensitivity of younger (prepubertal) brain to smaller dosages of stimulants, and the persistence of some effects, especially behavioral neuroadaptations, into adulthood. In addition, oral amphetamines (MAS) have more profound effects than does oral MPH. Further animal developmental studies are required to understand potential long-term neuroadaptations to low, daily oral doses of stimulants. Implications for clinical practice were also discussed.

One-trial behavioral sensitization in preweanling rats: differential effects of cocaine, methamphetamine, methylphenidate, and D-amphetamine

RATIONALE

Preweanling rats exhibit robust one-trial cocaine-induced behavioral sensitization; however, it is uncertain whether other psychostimulants can also induce sensitization in young rats using the one-trial procedure.

OBJECTIVE

The purpose of this study was to determine whether methamphetamine, methylphenidate, and D: -amphetamine are capable of inducing one-trial locomotor sensitization in preweanling rats.

METHODS

In a series of four experiments, rats were pretreated with cocaine (30 mg/kg), methamphetamine (2-12 mg/kg), methylphenidate (5-20 mg/kg), or amphetamine (5 mg/kg) before being placed in a novel activity chamber or the home cage on PD 19. Rats were then challenged with the same psychostimulant (20 mg/kg cocaine, 1-8 mg/kg methamphetamine, 2.5-7.5 mg/kg methylphenidate, or 1-2 mg/kg amphetamine) on PD 21, with distance traveled being measured for 180 min. In a separate experiment, rats were pretreated with methamphetamine on PD 16-19 and challenged with methamphetamine on PD 21.

RESULTS

Only cocaine, but not various dose combinations of other psychostimulants, was able to produce one-trial behavioral sensitization in preweanling rats. Context-dependent locomotor sensitization was also evident if rats were pretreated with methamphetamine on PD 16-19 and tested on PD 21.

CONCLUSIONS

It is uncertain why only cocaine was able to induce one-trial locomotor sensitization in preweanling rats, but it is possible that: (a) the neural circuitry mediating sensitization differs according to psychostimulant, (b) cocaine is more readily associated with environmental contexts than other psychostimulants, or (c) affinity and pharmacokinetic factors may underlie cocaine's ability to induce one-trial behavioral sensitization in preweanling rats.

Activity-dependent reduction of dopamine D2 receptors during a postnatal critical period of plasticity in rat striatum is not affected by prenatal haloperidol treatment

Motor activity induced in the Circling Training test (CT) during a postnatal (PN) critical period of plasticity (PN30-37) produces a long-lasting decrease in the number of binding sites and mRNA expression levels of the dopamine D2 receptor (D2R) in rat striatum. Prenatal exposure to the antipsychotic haloperidol also decreases postnatal levels of the striatal D2R in the offspring. We examined whether such fetal exposure to haloperidol could affect the activity-dependent reduction of the D2R system during the critical period. Half of the male offspring exposed to either haloperidol (2.5 mg/kg/day), i.p.) or saline during gestational days 5-18 were subjected to the CT during the critical period, while the remaining represented CT control animals. The adult number of binding sites and mRNA expression levels of the striatal D2R at PN90 were not changed by prenatal haloperidol treatment alone. On the other hand, only pups subjected to the CT during the critical period showed decreases in both studied parameters, regardless the prenatal treatment. These findings indicated that the postnatal reduction of the striatal D2R binding induced prenatally by haloperidol does not affect long-lasting activity-dependent plastic changes on the same receptor system elicited by motor activity in an ontogenetic critical period of plasticity in rat striatum.

Lasting effects of chronic fluoxetine treatment on the late developing rat brain: age-dependent changes in the serotonergic neurotransmitter system assessed by pharmacological MRI

RATIONALE

With the growing prevalence of psychotropic drug prescriptions among children and adolescents, the need for studies on lasting effects of drug exposure on the developing brain rises. Fluoxetine is the only selective serotonin reuptake inhibitor (SSRI) officially registered to treat major depressive disorder in children. Although various (pre)clinical studies have assessed the (long-term) effects of fluoxetine exposure in the perinatal period and in adulthood, limited data is available on its effects on the developing brain later in life, i.e. during adolescence.

OBJECTIVE

The present study aimed at investigating the effects of age following chronic SSRI treatment on the central serotonin (5-HT) system. To this end, pharmacological MRI (phMRI) was performed in chronic fluoxetine-treated (5 mg/kg, oral gavage for 3 weeks) juvenile (PND25) and adult rats (PND65) after a 1-week washout period, using an acute fluoxetine challenge (5 mg/kg, i.v.) to trigger the 5-HT system.

RESULTS

We observed a diminished brain response to the acute challenge in adult treated animals when compared to control animals, whereas this response was increased in juvenile treated rats. As a result, a significant age by treatment interaction effect was seen in several (subcortical) 5-HT related brain regions.

CONCLUSION

An opposite effect of chronic fluoxetine treatment was seen in the developing brain compared to that in matured brain, as assessed non-invasively using phMRI. These findings most likely reflect neuronal imprinting effects of juvenile SSRI treatment and may underlie emotional disturbances seen in animals and children treated with this drug. Also, our findings suggest that phMRI might be ideally suited to study this important issue in the pediatric population.

Developmental trajectories during adolescence in males and females: a cross-species understanding of underlying brain changes

Adolescence is a transitional period between childhood and adulthood that encompasses vast changes within brain systems that parallel some, but not all, behavioral changes. Elevations in emotional reactivity and reward processing follow an inverted U shape in terms of onset and remission, with the peak occurring during adolescence. However, cognitive processing follows a more linear course of development. This review will focus on changes within key structures and will highlight the relationships between brain changes and behavior, with evidence spanning from functional magnetic resonance imaging (fMRI) in humans to molecular studies of receptor and signaling factors in animals. Adolescent changes in neuronal substrates will be used to understand how typical and atypical behaviors arise during adolescence. We draw upon clinical and preclinical studies to provide a neural framework for defining adolescence and its role in the transition to adulthood.

Ontogeny and regulation of the serotonin transporter: providing insights into human disorders

Serotonin (5-hydroxytryptamine, 5-HT) was one of the first neurotransmitters for which a role in development was identified. Pharmacological and gene knockout studies have revealed a critical role for 5-HT in numerous processes, including cell division, neuronal migration, differentiation and synaptogenesis. An excess in brain 5-HT appears to be mechanistically linked to abnormal brain development, which in turn is associated with neurological disorders. Ambient levels of 5-HT are controlled by a vast orchestra of proteins, including a multiplicity of pre- and post-synaptic 5-HT receptors, heteroreceptors, enzymes and transporters. The 5-HT transporter (SERT, 5-HTT) is arguably the most powerful regulator of ambient extracellular 5-HT. SERT is the high-affinity uptake mechanism for 5-HT and exerts tight control over the strength and duration of serotonergic neurotransmission. Perturbation of its expression level or function has been implicated in many diseases, prominent among them are psychiatric disorders. This review synthesizes existing information on the ontogeny of SERT during embryonic and early postnatal development though adolescence, along with factors that influence its expression and function during these critical developmental windows. We integrate this knowledge to emphasize how inappropriate SERT expression or its dysregulation may be linked to the pathophysiology of psychiatric, cardiovascular and gastrointestinal diseases.

Effects of stimulant medication, incentives, and event rate on reaction time variability in children with ADHD

This study examined the effects of methylphenidate (MPH) on reaction time (RT) variability in children with attention deficit hyperactivity disorder (ADHD). Using a broad battery of computerized tasks, and both conventional and ex-Gaussian indicators of RT variability, in addition to within-task manipulations of incentive and event rate (ER), this study comprehensively examined the breadth, specificity, and possible moderators of effects of MPH on RT variability. A total of 93 children with ADHD completed a 4-week within-subject, randomized, double-blind, placebo-controlled crossover trial of MPH to identify an optimal dosage. Children were then randomly assigned to receive either their optimal MPH dose or placebo after which they completed five neuropsychological tasks, each allowing trial-by-trial assessment of RTs. Stimulant effects on RT variability were observed on both measures of the total RT distribution (ie, coefficient of variation) as well as on an ex-Gaussian measure examining the exponential portion of the RT distribution (ie, τ). There was minimal, if any, effect of MPH on performance accuracy or RT speed. Within-task incentive and ER manipulations did not appreciably affect stimulant effects across the tasks. The pattern of significant and pervasive effects of MPH on RT variability, and few effects of MPH on accuracy and RT speed suggest that MPH primarily affects RT variability. Given the magnitude and breadth of effects of MPH on RT variability as well as the apparent specificity of these effects of MPH on RT variability indicators, future research should focus on neurophysiological correlates of effects of MPH on RT variability in an effort to better define MPH pharmacodynamics.

Annual Research Review: New frontiers in developmental neuropharmacology: can long-term therapeutic effects of drugs be optimized through carefully timed early intervention?

Our aim is to present a working model that may serve as a valuable heuristic to predict enduring effects of drugs when administered during development. Our primary tenet is that a greater understanding of neurodevelopment can lead to improved treatment that intervenes early in the progression of a given disorder and prevents symptoms from manifesting. The immature brain undergoes significant changes during the transitions between childhood, adolescence, and adulthood. Such changes in innervation, neurotransmitter levels, and their respective signaling mechanisms have profound and observable changes on typical behavior, but also increase vulnerability to psychiatric disorders when the maturational process goes awry. Given the remarkable plasticity of the immature brain to adapt to its external milieu, preventive interventions may be possible. We intend for this review to initiate a discussion of how currently used psychotropic agents can influence brain development. Drug exposure during sensitive periods may have beneficial long-term effects, but harmful delayed consequences may be possible as well. Regardless of the outcome, this information needs to be used to improve or develop alternative approaches for the treatment of childhood disorders. With this framework in mind, we present what is known about the effects of stimulants, antidepressants, and antipsychotics on brain maturation (including animal studies that use more clinically-relevant dosing paradigms or relevant animal models). We endeavor to provocatively set the stage for altering treatment approaches for improving mental health in non-adult populations.

Methamphetamine treatment during development attenuates the dopaminergic deficits caused by subsequent high-dose methamphetamine administration

Administration of high doses of methamphetamine (METH) causes persistent dopaminergic deficits in both nonhuman preclinical models and METH-dependent persons. Noteworthy, adolescent [i.e., postnatal day (PND) 40] rats are less susceptible to this damage than young adult (PND90) rats. In addition, biweekly treatment with METH, beginning at PND40 and continuing throughout development, prevents the persistent dopaminergic deficits caused by a "challenge" high-dose METH regimen when administered at PND90. Mechanisms underlying this "resistance" were thus investigated. Results revealed that biweekly METH treatment throughout development attenuated both the acute and persistent deficits in VMAT2 function, as well as the acute hyperthermia, caused by a challenge METH treatment. Pharmacokinetic alterations did not appear to contribute to the protection afforded by the biweekly treatment. Maintenance of METH-induced hyperthermia abolished the protection against both the acute and persistent VMAT2-associated deficits suggesting that alterations in thermoregulation were caused by exposure of rats to METH during development. These findings suggest METH during development prevents METH-induced hyperthermia and the consequent METH-related neurotoxicity.

Early methylphenidate exposure enhances cocaine self-administration but not cocaine-induced conditioned place preference in young adult rats

RATIONALE

Previous studies in rodents show that early exposure to methylphenidate alters later responsiveness to drugs of abuse. An interesting feature of these studies is that early methylphenidate treatment decreases the rewarding value of cocaine when measured by conditioned place preference (CPP), but the same treatment increases cocaine self-administration.

OBJECTIVE

The goal of the present study was to examine the effects of early methylphenidate exposure on cocaine-induced responding using both reward paradigms.

METHODS

Rats were treated with methylphenidate (0, 2, or 5 mg/kg) from postnatal days (PDs) 11 to 20, and then cocaine-induced CPP or cocaine self-administration was measured in separate groups of rats in adulthood. The CPP procedure included 8 days of acquisition training, 8 days of extinction training, and a reinstatement test. Rats were conditioned with 0, 10, or 20 mg/kg cocaine. Reinstatement was assessed after a priming dose of cocaine (10 mg/kg). For the self-administration experiment, a jugular catheter was implanted and rats were trained to press a lever reinforced with cocaine (0.25 or 0.75 mg/kg/infusion) on a fixed ratio (FR) one schedule. Rats were gradually moved from an FR1 to an FR10 schedule and, after criterion was reached, rats were placed on a progressive ratio schedule for 5 days.

RESULTS

Cocaine produced robust rewarding effects as determined by both the CPP and self-administration experiments; however, early methylphenidate exposure only enhanced the reinforcing effects of cocaine on the self-administration paradigm. Interestingly, this methylphenidate enhancement was only seen in male rats.

CONCLUSIONS

These data suggest that in males, methylphenidate enhances the reinforcing value of cocaine, but not cocaine-associated cues.

Methylxanthines during pregnancy and early postnatal life

World-wide, many fetuses and infants are exposed to methylxanthines via maternal consumption of coffee and other beverages containing these substances. Methylxanthines (caffeine, theophylline and aminophylline) are also commonly used as a medication for apnea of prematurity.The metabolism of methylxanthines is impaired in pregnant women, fetuses and neonates, leading to accumulating levels thereof. Methylxanthines readily passes the placenta barrier and enters all tissues and thus may affect the fetus/newborn at any time during pregnancy or postnatal life, given that the effector systems are mature.At clinically relevant doses, the major effector system for methylxanthines is adenosine receptors. Animal studies suggest that adenosine receptors in the cardiovascular, respiratory and immune system are developed at birth, but that cerebral adenosine receptors are not fully functional. Furthermore animal studies have shown protective positive effects of methylxanthines in situations of hypoxia/ischemia in neonates. Similarly, a positive long-term effect on lung function and CNS development was found in human preterm infants treated with high doses of caffeine for apneas. There is now evidence that the overall benefits from methylxanthine therapy for apnea of prematurity outweigh potential short-term risks.On the other hand it is important to note that experimental studies have indicated that long-term effects of caffeine during pregnancy and postnatally may include altered behavior and altered respiratory control in the offspring, although there is currently no human data to support this.Some epidemiology studies have reported negative effects on pregnancy and perinatal outcomes related to maternal ingestion of high doses of caffeine, but the results are inconclusive. The evidence base for adverse effects of caffeine in first third of pregnancy are stronger than for later parts of pregnancy and there is currently insufficient evidence to advise women to restrict caffeine intake after the first trimester.

A novel, multiple symptom model of obsessive-compulsive-like behaviors in animals

BACKGROUND

Current animal models of obsessive-compulsive disorder (OCD) typically involve acute, drug-induced symptom provocation or a genetic association with stereotypies or anxiety. None of these current models demonstrate multiple OCD-like behaviors.

METHODS

Neonatal rats were treated with the tricyclic antidepressant clomipramine or vehicle between days 9 and 16 twice daily and behaviorally tested in adulthood.

RESULTS

Clomipramine exposure in immature rats produced significant behavioral and biochemical changes that include enhanced anxiety (elevated plus maze and marble burying), behavioral inflexibility (perseveration in the spontaneous alternation task and impaired reversal learning), working memory impairment (e.g., win-shift paradigm), hoarding, and corticostriatal dysfunction. Dopamine D2 receptors were elevated in the striatum, whereas serotonin 2C, but not serotonin 1A, receptors were elevated in the orbital frontal cortex.

CONCLUSIONS

This is the first demonstration of multiple symptoms consistent with an OCD-like profile in animals. Moreover, these behaviors are accompanied by biochemical changes in brain regions previously identified as relevant to OCD. This novel model of OCD demonstrates that drug exposure during a sensitive period can program disease-like systems permanently, which could have implications for current and future therapeutic strategies for this and other psychiatric disorders.

Selective serotonin reuptake inhibitor antidepressants potentiate methylphenidate (Ritalin)-induced gene regulation in the adolescent striatum

The psychostimulant methylphenidate (Ritalin) is used in conjunction with selective serotonin reuptake inhibitors (SSRIs) in the treatment of medical conditions such as attention-deficit hyperactivity disorder with anxiety/depression comorbidity and major depression. Co-exposure also occurs in patients on SSRIs who use psychostimulant 'cognitive enhancers'. Methylphenidate is a dopamine/norepinephrine reuptake inhibitor that produces altered gene expression in the forebrain; these effects partly mimic gene regulation by cocaine (dopamine/norepinephrine/serotonin reuptake inhibitor). We investigated whether the addition of SSRIs (fluoxetine or citalopram; 5 mg/kg) modified gene regulation by methylphenidate (2-5 mg/kg) in the striatum and cortex of adolescent rats. Our results show that SSRIs potentiate methylphenidate-induced expression of the transcription factor genes zif268 and c-fos in the striatum, rendering these molecular changes more cocaine-like. Present throughout most of the striatum, this potentiation was most robust in its sensorimotor parts. The methylphenidate + SSRI combination also enhanced behavioral stereotypies, consistent with dysfunction in sensorimotor striatal circuits. In so far as such gene regulation is implicated in psychostimulant addiction, our findings suggest that SSRIs may enhance the addiction potential of methylphenidate.

Chronic low dose Adderall XR down-regulates cfos expression in infantile and prepubertal rat striatum and cortex

We previously reported that treatment of prepubertal male rats with low, injected or oral, doses of methylphenidate stimulated cfos, fosB and arc expression in many areas of the developing brain. In the present study our objective was to determine whether the widely prescribed psychostimulant Adderall XR (ADD) exerted similar effects in infantile and prepubertal rat brain. We report here, for the first time, that low threshold doses of oral ADD, an extended-release mixture of amphetamine salts, now routinely used for the treatment of Attention Deficit Hyperactivity Disorder (ADHD), also increased cfos expression in infantile (postnatal day 10; PD10) and prepubertal (PD24) rat brain. These threshold doses were correlated with blood levels of amphetamine determined by liquid chromatography-mass spectrometry. Moreover, we observed that chronic treatment with oral ADD (1.6 mg/kg; x 14 days) not only significantly down-regulated cfos expression following a final challenge dose of ADD in prepubertal (PD24) rat striatum and cortex, quantified in terms of FOS immunoreactivity (FOS-ir), but did so at a daily dose that was without effect with methylphenidate (MPH); that is a much higher oral dose of MPH (7.5 mg/kg; x 14 days) failed to induce down-regulation of cfos expression. Similar experiments in infantile rats (PD10), but using a threshold injected dose of ADD (1.25 mg/kg sc) also significantly reduced striatal and cingulate cortical FOS-ir. An additional finding in the prepubertal rats was that oral ADD-induced FOS-ir was observed in the cerebral cortex following doses lower than the threshold dose necessary to increase FOS-ir in the striatum. This was not the case in the PD10 rats. In conclusion, our efforts to calibrate biological responses, such as immediate early gene expression, to clinically relevant blood levels of stimulants confirmed that expression of cfos is very sensitive to repeated low doses of Adderall XR. It is now feasible to examine whether other genes are also affected in these young rats and if the changes we report are reversible. The implications of such studies should be relevant to the putative effects of psychostimulant treatment of very young children.

Effects of repeated and acute aripiprazole or haloperidol treatment on dopamine synthesis in the dorsal striatum of young rats: comparison to adult rats

The purpose of the present study was to determine whether repeated treatment with the D2 partial agonist aripiprazole or the D2 antagonist haloperidol alters dopamine (DA) synthesis characteristics in the dorsal striatum of young rats. To this end, rats received a daily pretreatment regimen of aripiprazole or haloperidol on postnatal days (PD) 10-20 and were tested 24 or 72 h later after an acute injection of vehicle, aripiprazole, haloperidol, or quinpirole (a D2 agonist). For comparison purposes, adult rats were pretreated with an 11-day regimen of saline or haloperidol on PD 70-80 and DA synthesis was measured after acute drug treatment on PD 83. Dorsal striatal DA synthesis was determined by measuring L-dihydroxyphenylalanine accumulation after NSD-1015 treatment. In a separate experiment, the ability of repeated drug treatment to up-regulate dorsal striatal D2 receptors was assessed in young and adult rats 72 h after drug discontinuation. The major findings of this study were that: (a) acute treatment with haloperidol and aripiprazole increased DA synthesis while quinpirole reduced it; (b) pretreatment with haloperidol and aripiprazole blunted the synthesis-modulating effects of acutely administered dopaminergic drugs; and (c) DA synthesis of young and adult rats was affected in a qualitatively similar manner by DA agonist, antagonist, and partial agonist drugs. In conclusion, results from the present study suggest that synthesis-modulating autoreceptors in the dorsal striatum are functionally mature by the end of the preweanling period and DA synthesis declines to near basal levels during the course of repeated aripiprazole treatment.

Fluoxetine potentiates methylphenidate-induced gene regulation in addiction-related brain regions: concerns for use of cognitive enhancers?

BACKGROUND

There is growing use of psychostimulant cognitive enhancers such as methylphenidate (Ritalin). Methylphenidate differs from the psychostimulant cocaine because it does not enhance synaptic levels of serotonin. We investigated whether exposure to methylphenidate combined with a serotonin-enhancing medication, the prototypical selective serotonin reuptake inhibitor (SSRI) fluoxetine (Prozac), would produce more "cocaine-like" molecular and behavioral changes.

METHODS

We measured the effects of fluoxetine on gene expression induced by the cognitive enhancer methylphenidate in the striatum and nucleus accumbens of rats, by in situ hybridization histochemistry. We also determined whether fluoxetine modified behavioral effects of methylphenidate.

RESULTS

Fluoxetine robustly potentiated methylphenidate-induced expression of the transcription factors c-fos and zif 268 throughout the striatum and to some degree in the nucleus accumbens. Fluoxetine also enhanced methylphenidate-induced stereotypical behavior.

CONCLUSIONS

Both potentiated gene regulation in the striatum and the behavioral effects indicate that combining the SSRI fluoxetine with the cognitive enhancer methylphenidate mimics cocaine effects, consistent with an increased risk for substance use disorder.