Administration of oral methylphenidate during adolescence prevents suppressive development of dopamine projections into prefrontal cortex and amygdala after an early pharmacological challenge in gerbils

Developmental Manipulation-Induced Changes in Cognitive Functioning

Schizophrenia is a complex neurodevelopmental disorder with as-yet no identified cause. The use of animals has been critical to teasing apart the potential individual and intersecting roles of genetic and environmental risk factors in the development of schizophrenia. One way to recreate in animals the cognitive impairments seen in people with schizophrenia is to disrupt the prenatal or neonatal environment of laboratory rodent offspring. This approach can result in congruent perturbations in brain physiology, learning, memory, attention, and sensorimotor domains. Experimental designs utilizing such animal models have led to a greatly improved understanding of the biological mechanisms that could underlie the etiology and symptomology of schizophrenia, although there is still more to be discovered. The implementation of the Research and Domain Criterion (RDoC) has been critical in taking a more comprehensive approach to determining neural mechanisms underlying abnormal behavior in people with schizophrenia through its transdiagnostic approach toward targeting mechanisms rather than focusing on symptoms. Here, we describe several neurodevelopmental animal models of schizophrenia using an RDoC perspective approach. The implementation of animal models, combined with an RDoC framework, will bolster schizophrenia research leading to more targeted and likely effective therapeutic interventions resulting in better patient outcomes.

Sex differences, learning flexibility, and striatal dopamine D1 and D2 following adolescent drug exposure in rats

Corticostriatal circuitry supports flexible reward learning and emotional behavior from the critical neurodevelopmental stage of adolescence through adulthood. It is still poorly understood how prescription drug exposure in adolescence may impact these outcomes in the long-term. We studied adolescent methylphenidate (MPH) and fluoxetine (FLX) exposure in rats and their impact on learning and emotion in adulthood. In Experiment 1, male and female rats were administered MPH, FLX, or saline (SAL), and compared with methamphetamine (mAMPH) treatment beginning in postnatal day (PND) 37. The rats were then tested on discrimination and reversal learning in adulthood. In Experiment 2, animals were administered MPH or SAL also beginning in PND 37 and later tested in adulthood for anxiety levels. In Experiment 3, we analyzed striatal dopamine D1 and D2 receptor expression in adulthood following either extensive learning (after Experiment 1) or more brief emotional measures (after Experiment 2). We found sex differences in discrimination learning and attenuated reversal learning after MPH and only sex differences in adulthood anxiety. In learners, there was enhanced striatal D1, but not D2, after either adolescent MPH or mAMPH. Lastly, also in learners, there was a sex x treatment group interaction for D2, but not D1, driven by the MPH-pretreated females, who expressed significantly higher D2 levels compared to SAL. These results show enduring effects of adolescent MPH on reversal learning in rats. Developmental psychostimulant exposure may interact with learning to enhance D1 expression in adulthood, and affect D2 expression in a sex-dependent manner.

Long-term oral methylphenidate treatment in adolescent and adult rats: differential effects on brain morphology and function

Methylphenidate is a widely prescribed psychostimulant for treatment of attention deficit hyperactivity disorder (ADHD) in children and adolescents, which raises questions regarding its potential interference with the developing brain. In the present study, we investigated effects of 3 weeks oral methylphenidate (5 mg/kg) vs vehicle treatment on brain structure and function in adolescent (post-natal day [P]25) and adult (P65) rats. Following a 1-week washout period, we used multimodal magnetic resonance imaging (MRI) to assess effects of age and treatment on independent component analysis-based functional connectivity (resting-state functional MRI), D-amphetamine-induced neural activation responses (pharmacological MRI), gray and white matter tissue volumes and cortical thickness (postmortem structural MRI), and white matter structural integrity (postmortem diffusion tensor imaging (DTI)). Many age-related differences were found, including cortical thinning, white matter development, larger dopamine-mediated activation responses and increased striatal functional connectivity. Methylphenidate reduced anterior cingulate cortical network strength in both adolescents and adults. In contrast to clinical observations from ADHD patient studies, methylphenidate did not increase white matter tissue volume or cortical thickness in rat. Nevertheless, DTI-based fractional anisotropy was higher in the anterior part of the corpus callosum following adolescent treatment. Furthermore, methylphenidate differentially affected adolescents and adults as evidenced by reduced striatal volume and myelination upon adolescent treatment, although we did not observe adverse treatment effects on striatal functional activity. Our findings of small but significant age-dependent effects of psychostimulant treatment in the striatum of healthy rats highlights the importance of further research in children and adolescents that are exposed to methylphenidate.

Is the treatment with psychostimulants in children and adolescents with attention deficit hyperactivity disorder harmful for the dopaminergic system?

A major concern regarding psychostimulant medication (amphetamine and methylphenidate) in the treatment of children and adolescents with attention deficit/hyperactivity disorder (ADHD) are the potential adverse effects to the developing brain, particularly in regard to dopaminergic brain function. The present review focuses on the pharmacology of these psychostimulants, their mode of action in the human brain and their potential neurotoxic effects to the developing brain in animals, particularly concerning DA brain function. The potential clinical significance of these findings for the treatment of ADHD in children and adolescents is discussed. Studies on sensitization to psychostimulants' rewarding effects, which is a process expected to increase the risk of substance abuse in humans, are not included. The available findings in non-human primates support the notion that the administration of amphetamine and methylphenidate with procedures simulating clinical treatment conditions does not lead to long-term adverse effects in regard to development, neurobiology or behaviour as related to the central dopaminergic system.

Effects of adolescent social defeat on adult amphetamine-induced locomotion and corticoaccumbal dopamine release in male rats

Maturation of mesocorticolimbic dopamine systems occurs during adolescence, and exposure to social stress during this period results in behavioral dysfunction including substance abuse disorders. Adult male rats exposed to repeated social defeat in adolescence exhibit reduced basal dopamine tissue content in the medial prefrontal cortex, altered dopamine tissue content in corticoaccumbal dopamine regions following acute amphetamine, and increased amphetamine conditioned place preference following repeated amphetamine treatment. Such changes may reflect altered amphetamine-induced extracellular dopamine release in the corticoaccumbal regions. Therefore, we used in vivo microdialysis to measure extracellular dopamine simultaneously within the medial prefrontal cortex and nucleus accumbens core of previously defeated rats and controls, in response to either acute or repeated (7 daily injections) of amphetamine (1.0 mg/kg). Locomotion responses to acute/repeated amphetamine were also assessed the day prior to taking dopamine measurements. Adolescent defeat potentiated adult locomotion responses to acute amphetamine, which was negatively correlated with attenuated amphetamine-induced dopamine release in the medial prefrontal cortex, but there was no difference in amphetamine-induced accumbal dopamine release. However, both locomotion and corticoaccumbal dopamine responses to repeated amphetamine were equivalent between previously defeated rats and controls. These data suggest adolescent defeat enhances behavioral responses to initial amphetamine exposure as a function of diminished prefrontal cortex dopamine activity, which may be sufficient to promote subsequently enhanced seeking of drug-associated cues. Interestingly, repeated amphetamine treatment appears to normalize amphetamine-elicited locomotion and cortical dopamine responses observed in adult rats exposed to adolescent social defeat, providing implications for treating stress-induced dopamine dysfunction.

Developmental chlorpyrifos and methyl parathion exposure alters radial-arm maze performance in juvenile and adult rats

Although the use of organophosphate (OP) insecticides has been restricted, sufficient exposure can occur to induce detrimental neurobehavioral effects. In this study, we measured physical and reflex development and spatial learning and memory in rats repeatedly exposed to incremental doses of chlorpyrifos (CPS) and methyl parathion (MPS) from postnatal day (PND) 1 to PND21. Other than decreased body weight in the higher dosage groups, no effects on physical or reflex development were observed. Significant hippocampal cholinesterase inhibition was induced in all treatment groups for up to 19 days following exposure. Beginning on PND36, working and reference memory was tested using a 12-arm radial maze, with subject animals trained and tested 4 days a week for 4 weeks. In males, working memory was decreased with the medium and high dosage of MPS but only the high dosage of CPS; while in females, no deficits were observed. For reference memory, errors were significantly increased in males exposed to the high dosage of CPS and all dosages of MPS. In females, enhanced performance was observed within the medium and high dosages of CPS but not with MPS. These data show that repeated developmental exposure to OP insecticides can induce sex-selective alterations and long-lasting changes in spatial learning and memory formation when measured using a radial arm maze and that MPS and CPS induce different neurobehavioral outcomes.

Environmental enrichment has no effect on the development of dopaminergic and GABAergic fibers during methylphenidate treatment of early traumatized gerbils

It is widely believed, that environmental factors play a crucial role in the etiology and outcome of psychiatric diseases such as Attention-Deficit/Hyperactivity Disorder (ADHD). A former study from our laboratory has shown that both methylphenidate (MP) and handling have a positive effect on the dopaminergic fiber density in the prefrontal cortex (PFC) of early traumatized gerbils (Meriones unguiculatus). The current study was performed to investigate if enriched environment during MP application has an additional influence on the dopaminergic and GABAergic fiber densities in the PFC and amygdala in this animal model.

Animals received a single early dose of methamphetamine (MA; 50 mg/kg; i.p.) on postnatal day (PD) 14, which is known to cause multiple changes in the subsequent development of several neurotransmitter systems including the dopaminergic systems, and were then treated with oral daily applications of MP (5 mg/kg) from PD30–60. Animals treated this way were either transferred to an enriched environment after weaning (on PD30) or were kept under impoverished rearing conditions.

There was no effect of an enriched environment on the dopaminergic or GABAergic fiber density neither in the PFC nor in the amygdala. With regard to former studies these results underline the particular impact of MP in the treatment of ADHD.

Influence of methylphenidate on brain development--an update of recent animal experiments

Methylphenidate (MPH) is the most commonly used drug to treat attention deficit/hyperactivity disorder (ADHD) in children effectively and safely. In spite of its widespread application throughout one of the most plastic and sensitive phases of brain development, very little is known to date about its long-term effects on brain structure and function. Hence, this short review updates the influence of MPH on brain development, since recent human and animal studies suggest that MPH alters the dopaminergic system with long-term effects beyond the termination of treatment.

Animal studies imply that the effects of MPH may depend on the neural responder system: Whereas structural and functional parameters are improved by MPH in animals with psychomotor impairments, they remain unaltered or get worse in healthy controls. While recent behavioural studies do not fully support such a differential effect of MPH in ADHD, the animal studies certainly prompt for further investigation of this issue. Furthermore, the abuse of MPH, when (rarely) intravenously applied, may even impair the maturation of dopaminergic fibres in subcortical brain areas. This argues for careful clinical assessment and diagnostics of ADHD symptomatology not only in conjunction with the prescription of MPH. Hence, one should be assured that MPH is only given to children with clear ADHD symptomatology leading to psychosocial impairment. The animal data suggest that under these conditions MPH is supportive for brain development and the related behaviour in children with ADHD.