Single and Repeated Administration of Methylphenidate Modulates Synaptic Plasticity in Opposite Directions via Insertion of AMPA Receptors in Rat Hippocampal Neurons

Memory loss induced by lisdexamfetamine in the rat: A behavioral, electrophysiological, and histopathological Study

Lisdexamfetamine (LDX) is one of the drugs commonly used to treat attention deficit hyperactivity disorder (ADHD). However, its neurological side effects, particularly on cognition, are not fully understood. The present study focused on memory in rats treated with four weeks of LDX injection. We compared LDX-treated rats with control ones, using several methods to evaluate the behavioral responses and electrophysiological, molecular, and histological properties in the hippocampus. Our findings demonstrated that subchronic administration of LDX impaired behavioral performance in all memory assessment tests (Y maze, Morris Water Maze, and Shuttle box). Although LDX did not alter population spike (PS) amplitude, it increased the field excitatory postsynaptic potential (fEPSP) slope of evoked potentials of LTP components. Also, in addition to an increase in expression of caspase-3 in the hippocampus, which indicates the susceptibility to apoptosis in LDX-treated rats, the number of microglia and astrocytes went up significantly in the LDX group. Moreover, Sholl's analysis showed an increase in the soma size and total process length in both hippocampal astrocytes and microglia. Overall, because of these destructive effects of LDX on the hippocampus, which is one of the critical memory-related areas of the brain, the findings of this investigation provide evidence to show the disruption of memory-related variables following the LDX. However, more research is needed to clarify it.

Attention Deficit-Hyperactivity Disorder (ADHD): From Abnormal Behavior to Impairment in Synaptic Plasticity

Attention deficit-hyperactivity disorder (ADHD) is a neurodevelopmental disorder with high incidence in children and adolescents characterized by motor hyperactivity, impulsivity, and inattention. Magnetic resonance imaging (MRI) has revealed that neuroanatomical abnormalities such as the volume reduction in the neocortex and hippocampus are shared by several neuropsychiatric diseases such as schizophrenia, autism spectrum disorder and ADHD. Furthermore, the abnormal development and postnatal pruning of dendritic spines of neocortical neurons in schizophrenia, autism spectrum disorder and intellectual disability are well documented. Dendritic spines are dynamic structures exhibiting Hebbian and homeostatic plasticity that triggers intracellular cascades involving glutamate receptors, calcium influx and remodeling of the F-actin network. The long-term potentiation (LTP)-induced insertion of postsynaptic glutamate receptors is associated with the enlargement of spine heads and long-term depression (LTD) with spine shrinkage. Using a murine model of ADHD, a delay in dendritic spines' maturation in CA1 hippocampal neurons correlated with impaired working memory and hippocampal LTP has recently reported. The aim of this review is to summarize recent evidence that has emerged from studies focused on the neuroanatomical and genetic features found in ADHD patients as well as reports from animal models describing the molecular structure and remodeling of dendritic spines.

Endocytosis of AMPA receptors: Role in neurological conditions

AMPA receptors are glutamate-gated ion channels, present in a wide range of neuron types and in glial cells. Their main role is to mediate fast excitatory synaptic transmission, and therefore, they are critical for normal brain function. In neurons, AMPA receptors undergo constitutive and activity-dependent trafficking between the synaptic, extrasynaptic and intracellular pools. The kinetics of AMPA receptor trafficking is crucial for the precise functioning of both individual neurons and neural networks involved in information processing and learning. Many of the neurological diseases evoked by neurodevelopmental and neurodegenerative malfunctions or traumatic injuries are caused by impaired synaptic function in the central nervous system. For example, attention-deficit/hyperactivity disorder (ADHD), Alzheimer's disease (AD), tumors, seizures, ischemic strokes, and traumatic brain injury are all characterized by impaired glutamate homeostasis and associated neuronal death, typically caused by excitotoxicity. Given the important role of AMPA receptors in neuronal function, it is not surprising that perturbations in AMPA receptor trafficking are associated with these neurological disorders. In this book chapter, we will first introduce the structure, physiology and synthesis of AMPA receptors, followed by an in-depth description of the molecular mechanisms that control AMPA receptor endocytosis and surface levels under basal conditions or synaptic plasticity. Finally, we will discuss how impairments in AMPA receptor trafficking, particularly endocytosis, contribute to the pathophysiology of various neurological disorders and what efforts are being made to therapeutically target this process.

Methylphenidate Restores Behavioral and Neuroplasticity Impairments in the Prenatal Nicotine Exposure Mouse Model of ADHD: Evidence for Involvement of AMPA Receptor Subunit Composition and Synaptic Spine Morphology in the Hippocampus

In ADHD treatment, methylphenidate (MPH) is the most frequently used medication. The present work provides evidence that MPH restored behavioral impairments and neuroplasticity due to changes in AMPAR subunit composition and distribution, as well as maturation of dendritic spines, in a prenatal nicotine exposure (PNE) ADHD mouse model. PNE animals and controls were given a single oral dose of MPH (1 mg/kg), and their behavior was tested for attention, hyperactivity, and working memory. Long-term potentiation (LTP) was induced and analyzed at the CA3/CA1 synapse in hippocampal slices taken from the same animals tested behaviorally, measuring fEPSPs and whole-cell patch-clamp EPSCs. By applying crosslinking and Western blots, we estimated the LTP effects on AMPAR subunit composition and distribution. The density and types of dendritic spines were quantified by using the Golgi staining method. MPH completely restored the behavioral impairments of PNE mice. Reduced LTP and AMPA-receptor-mediated EPSCs were also restored. EPSC amplitudes were tightly correlated with numbers of GluA1/GluA1 AMPA receptors at the cell surface. Finally, we found a lower density of dendritic spines in hippocampal pyramidal neurons in PNE mice, with a higher fraction of thin-type immature spines and a lower fraction of mushroom mature spines; the latter effect was also reversed by MPH.

Differential effects of memory enhancing and impairing doses of methylphenidate on serotonin metabolism and 5-HT1A, GABA, glutamate receptor expression in the rat prefrontal cortex

Methylphenidate (MPD), a psychostimulant, is a prescription medicine for treating attention deficit hyperactivity disorder (ADHD). Previously we have shown that moderate doses of MPD enhanced learning and memory while higher doses impaired it. To understand neurochemical mechanisms and receptors involved in memory enhancing and impairing effects of MPD, the present study concerns the effects of these doses of MPD on serotonin, 5-HT1A, GABA, and NMDA receptor mRNA expression in the prefrontal cortex (PFC). We found that low doses (2.5 mg/kg) of MPD improved performance in the water-maze test but higher doses (5 mg/kg) impaired memory retention. Animals showing improved performance had high 5-HT metabolism in the PFC while these levels were not affected in the group treated with higher MPD doses and exhibiting impaired memory. There was downregulation of 5-HT1A receptors in the PFC of rats treated with higher dose MPD, which didn't occur in low dose of MPD treated animals. Further, a decrease in GABA_Areceptor mRNA expression occurred in low doses of MPD treated animals and GluN2A expression was reduced in higher doses of MPD treated animals. The findings suggest that memory enhancing doses of MPD increase 5-HT and reduce GABA_A receptor mRNA expression in the PFC to release excitatory glutamate neurons from the inhibitory influence of GABA. Conversely, higher dose of MPD downregulates 5-HT1A receptor mRNA expression to enhance inhibitory GABA influence on glutamate neurons and impair cognitive performance. The findings show an important role of 5-HT1A heteroreceptors in the PFC for improving therapeutic use of MPD and developing novel cognitive enhancers.

Cumulative exposure to ADHD medication is inversely related to hippocampus subregional volume in children

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Cumulative exposure to ADHD medication characterized as the product of lifetime duration and dose.

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Medication effects investigated on 51 subregional volumes.

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Smaller hippocampus CA1 volumes associated with higher medication exposure.

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Effects remained when correcting for age and ADHD symptom severity.

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No global effects of medication on cortical thickness or surface area detected.

Background

Although there is some evidence for a normalization of brain structure following exposure to ADHD medication, literature on the effects of duration and dose of continued use on the brain is scarce. Here, we investigated the association between cumulative exposure to medication (range 1 week to 4.69 years) and cortical structures and subcortical volumes in a clinical sample of children with ADHD taking medication (n = 109). To the best of our knowledge, this is the first structural MRI study investigating the effects of cumulative exposure to medication on subregional volumes in children treated for ADHD.

Methods

Cumulative exposure to ADHD medication (CEM) was defined as the product of duration on medication (days) and dose (mg/day), yielding the area under the curve (total mg). Cortical thickness and surface area measurements (CIVET-1.1.12), and subcortical volumes in 51 regions (MAGeT-Brain) were analyzed using general linear modelling.

Results

Significant effects of CEM were found in two subregions of the left hippocampus, the CA1 (df = 95; q = 0.003) and the strata radiatum/lacunosum/moleculare (df = 95; q = 0.003). Specifically, higher CEM was associated with smaller volumes within these subregions. No effects of medication exposure were detected on cortical thickness or surface area.

Conclusions

Although this study is cross-sectional, the results found within this sample of children show that prolonged ADHD medication use at higher doses is significantly associated with smaller hippocampus volumes in specific subregions. More research is required to determine whether these results are reproduced in other samples of children of ADHD, and further, whether these are beneficial or off-target effects of the medication.