Activation of metabotropic glutamate receptor 5 is associated with effect of amphetamine on brain neurons

The pharmacology of amphetamine and methylphenidate: Relevance to the neurobiology of attention-deficit/hyperactivity disorder and other psychiatric comorbidities

Psychostimulants, including amphetamines and methylphenidate, are first-line pharmacotherapies for individuals with attention-deficit/hyperactivity disorder (ADHD). This review aims to educate physicians regarding differences in pharmacology and mechanisms of action between amphetamine and methylphenidate, thus enhancing physician understanding of psychostimulants and their use in managing individuals with ADHD who may have comorbid psychiatric conditions. A systematic literature review of PubMed was conducted in April 2017, focusing on cellular- and brain system-level effects of amphetamine and methylphenidate. The primary pharmacologic effect of both amphetamine and methylphenidate is to increase central dopamine and norepinephrine activity, which impacts executive and attentional function. Amphetamine actions include dopamine and norepinephrine transporter inhibition, vesicular monoamine transporter 2 (VMAT-2) inhibition, and monoamine oxidase activity inhibition. Methylphenidate actions include dopamine and norepinephrine transporter inhibition, agonist activity at the serotonin type 1A receptor, and redistribution of the VMAT-2. There is also evidence for interactions with glutamate and opioid systems. Clinical implications of these actions in individuals with ADHD with comorbid depression, anxiety, substance use disorder, and sleep disturbances are discussed.

Acute dextro-amphetamine administration does not alter brain myo-inositol levels in humans and animals: MRS investigations at 3 and 18.8 T

The pathophysiological underpinnings of bipolar disorder are not fully understood. However, they may be due in part to changes in the phosphatidylinositol second messenger system (PI-cycle) generally, or changes in myo-inositol concentrations more specifically. Dextro-amphetamine has been used as a model for mania in several human studies as it causes similar subjective and physiological symptoms. We wanted to determine if dextro-amphetamine altered myo-inositol concentrations in vivo as it would clearly define a mechanism linking putative changes in the PI-cycle to the subjective psychological changes seen with dextro-amphetamine administration. Fifteen healthy human volunteers received a baseline scan, followed by second scan 75 min after receiving a 25 mg oral dose of dextro-amphetamine. Stimulated echo proton magnetic resonance spectroscopy (MRS) scans were preformed at 3.0 Tesla (T) in the dorsal medial prefrontal cortex (DMPFC). Metabolite data were adjusted for tissue composition and analyzed using LCModel. Twelve adult male rats were treated acutely with a 5-mg/kg intraperitoneal dose of dextro-amphetamine. After 1 h rats were decapitated and the brains were rapidly removed and frozen until dissection. Rat brains were dissected into frontal, temporal, and occipital cortical areas, as well as hippocampus. Tissue was analyzed using a Varian 18.8 T spectrometer. Metabolites were identified and quantified using Chenomx Profiler software. The main finding in the present study was that myo-inositol concentrations in the DMPFC of human volunteers and in the four rat brain regions were not altered by acute dextro-amphetamine. While it remains possible that the PI-cycle may be involved in the pathophysiology of bipolar disorder, it is not likely that the subjective and physiological of dextro-amphetamine are mediated, directly or indirectly, via alternations in myo-inositol concentrations.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Effects of amphetamine on serotoninergic and GABAergic expression of developing brain

Roles of age and withdrawal were explored in mechanisms underlying the action of amphetamine (Amph), by monitoring the serotonergic and GABAergic expression in key brain regions of the rat. Postnatal 21 and 60 day-old male rats were intraperitoneally injected with D-Amph, 5 mg/kg, or saline, three times daily for 14 days and then withdrawn from Amph for 0 or 14 days; these animals received single injections on day 15 (W0d) or day 29 (W14d). Following Amph injections, though both age groups exhibited hyperlocomotion, stereotypy and behavioral sensitization, the juvenile showed 100-300% longer latencies to reach and 30%-42% shorter duration of maximal behavioral scores than the adult from day 2-29. Immunocytochemical analysis revealed down-regulation of 42-76% in 5-hydroxytryptamine (HT) immunoreactive processes in motor and somatosensory cortices, and hippocampus of both ages after Amph exposure at W0d. At W14d, the 5-HT resembled saline-control in the Amph-treated juvenile, whereas remained weakened in the adult. By contrast, densities of GAD67 (glutamic acid decarboxylase)-boutons were up-regulated by 35-545% in the neocortical areas, nucleus accumbens, caudate-putamen and hippocampus of all Amph-administered rats. After 14 days withdrawal, the juvenile recovered the decreased 5-HT fibers, but not the increased GABAergic, indicating unique roles of the two systems in response to Amph.

Effects of group I metabotropic glutamate receptor antagonists on the behavioral sensitization to motor effects of cocaine in rats

RATIONALE

Metabotropic glutamate receptors (mGluRs) were reported to regulate various behavioral effects of addictive drugs.

OBJECTIVE

The present study evaluated the role of group I mGluRs in the progressive augmentation ("sensitization") of the behavioral effects observed after repeated, intermittent cocaine exposure.

MATERIALS AND METHODS

After habituation to handling and baseline activity measurement (days 1-2), rats received eight injections of cocaine (10 mg/kg) or saline on days 3-6, 8-11, and then, were tested twice with acute saline and cocaine given in a counterbalanced manner on days 13 and 15. Before the test sessions, subjects were pretreated with mGluR1 antagonist EMQMCM (JNJ16567083, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) and mGluR5 antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine).

RESULTS

Pretreatment with EMQMCM (2.5-10 mg/kg) but not MTEP (2.5-10 mg/kg) significantly reduced expression of the sensitized ambulatory motor activity of the cocaine-experienced animals acutely challenged with cocaine. Both EMQMCM and MTEP significantly reduced vertical motor activity across all cocaine/saline treatment conditions.

CONCLUSIONS

These findings indicate that the expression of behavioral sensitization to cocaine-induced stimulation of locomotor activity may be modulated by group I mGluR antagonists (mGluR1 rather than mGluR5), but these effects occur at the dose levels that attenuate vertical activity.

Valproate attenuates dextroamphetamine-induced subjective changes more than lithium

Dextroamphetamine administration in healthy controls produces a range of subjective and physiological effects, which have been likened to those occurring during mania. However, it is uncertain if these can be attenuated by lithium since conflicting results have been reported. To date there have been no previous studies examining the effects of valproate on dextroamphetamine-induced mood and physiological changes. The current study was a double-blind, placebo-controlled, study in which volunteers received either 1000 mg sodium valproate (n=12), 900 mg lithium (n=9), or placebo (n=12) pre-treatment for 14 days. Subjective and physiological measures were then obtained prior to administration of a 25 mg dose of dextroamphetamine, and at two time points after administration. Differences in the response to dextroamphetamine were assessed between the three treatment groups. The results of this study show that pre-treatment with lithium only significantly attenuated dextroamphetamine-induced change in happiness, while valproate pre-treatment significantly attenuated the effects of dextroamphetamine on happiness, energy, alertness and on the diastolic blood pressure. These results suggest that lithium and valproate do not have the same mechanism of action on dextroamphetamine-induced changes, and this finding may relate to differences in their mechanism of action in mood disorders.

Effects of acute amphetamine administration on AMPA-mediated synaptic activity and expression of AMPA receptor subunit 2 of brain neurons

We investigated the role of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor (AMPAR) in mechanisms underlying the action of amphetamine (Amph) on brain neurons, for AMPAR has been proposed to participate in psychotic and neurodegenerative disorders. In the cultured rat brain cortical neurons pretreated with 1 microM Amph for 1 h, the accumulation of 45Ca2+ driven by 10 min incubation with 100 microM AMPA was reduced by about 36%. This Amph-induced decrease seems to involve L-type voltage-gated Ca2+ channels, because the AMPA-induced 45Ca2+ uptake was blocked by 70% and 80%, respectively, for untreated and Amph-treated neurons in the presence of nifedipine (1 microM), an antagonist to L-type calcium channels. Whole-cell, patch-clamp recording revealed that AMPA-elicited current amplitude became 26% lower than the control in Amph-treated cultured neurons. Moreover, Amph treatment down-regulated the level of flip-form glutamate receptor 2 (GluR2) mRNA by 27% in cultured neurons but did not change the expression of GluR2 proteins and flop-form mRNA, as detected by quantitative immunocytochemistry and in situ hybridization. In contrast, in postnatal day 4 rats at 1 h after receiving one intraperitoneal injection of 5 mg/kg of Amph, levels of flip GluR2 mRNA were up-regulated by 13% and 18% in neurons of motor cortex layer 5 and pyramidal neurons of hippocampal CA3, respectively. The data suggest that acute action of Amph on brain neurons is possibly associated with decreased AMPA-mediated Ca2+ influx and current amplitude, as well as modified expression of the GluR2 mRNA.

Lithium and valproate attenuate dextroamphetamine-induced changes in brain activation

BACKGROUND

Previous studies have suggested that both lithium and valproate may decrease phosphoinositol second messenger system (PI-cycle) activity. There is also evidence that dextroamphetamine may increase PI cycle activity. It was previously demonstrated that dextroamphetamine administration in volunteers causes a region and task dependent decrease in brain activation in healthy volunteers. The current study assessed the effect of 14 days pretreatment with lithium and valproate on these dextroamphetamine-induced changes in regional brain activity in healthy volunteers.

METHODS

This was a double-blind, placebo-controlled, study in which volunteers received either 1000 mg sodium valproate (n = 12), 900 mg lithium (n = 9) or placebo (n = 12). Functional images were acquired using functional magnetic resonance imaging (fMRI) while subjects performed three cognitive tasks, a word generation paradigm, a spatial attention task and a working memory task. fMRI was carried out both before and after administration of dextroamphetamine (25 mg). Changes in the number of activated pixels and changes in the magnitude of the blood-oxygen-level-dependent (BOLD) signal after dextroamphetamine administration were then determined.

RESULTS

In keeping with previous findings dextroamphetamine administration decreased regional brain activation in all three tasks. Pretreatment with lithium attenuated changes in the word generation paradigm and the spatial attention task, while pretreatment with valproate attenuated the changes in the working memory task.

CONCLUSIONS

These results suggest that both lithium and valproate can significantly attenuate dextroamphetamine-induced changes in brain activity in a task dependent and region specific manner. This is the first human evidence to suggest that both lithium and valproate may have a similar effect on regional brain activation, conceivably via similar effects on PI-cycle activity.

Bipolar disorder and myo-inositol: a review of the magnetic resonance spectroscopy findings

OBJECTIVES

Myo-inositol is an important component of the phosphatidylinositol second messenger system (PI-cycle). Alterations in PI-cycle activity have been suggested to be involved in the pathophysiology and/or treatment of bipolar disorder. More specifically, lithium has been suggested to act primarily by lowering myo-inositol concentrations, the so-called inositol-depletion hypothesis. myo-Inositol concentrations can be measured in vivo with magnetic resonance spectroscopy (MRS).

METHODS

The current review primarily examines animal and human MRS studies that evaluated the role of myo-inositol in bipolar illness and treatment.

RESULTS

Studies have been carried out in patients who are manic, depressed, and euthymic, both on and off treatment. However, there are several limitations of these studies.

CONCLUSIONS

The preclinical and clinical MRS findings were generally supportive of the involvement of myo-inositol in bipolar disorder and its treatment. Overall, in bipolar patients who are manic or depressed there are abnormalities in brain myo-inositol concentrations, with changes in frontal and temporal lobes, as well as the cingulate gyrus and basal ganglia. These abnormalities are not seen in either euthymic patients or healthy controls, possibly due to a normalizing effect of treatment with either lithium or sodium valproate. There is also increasing evidence that sodium valproate may also act upon the PI-cycle. Nonetheless, it remains uncertain if these changes in myo-inositol concentration are primary or secondary. Findings regarding the specific inositol-depletion hypothesis are also generally supportive in acutely ill patients, although it is not yet possible to definitively confirm or refute this hypothesis based on the current MRS evidence.

The ups and downs of addiction: role of metabotropic glutamate receptors

Drug addiction is characterized by drug-induced positive affect, followed by withdrawal-associated negative affect. Such drug-induced positive and negative affective states provide crucial sources of motivation that drive compulsive drug consumption. Metabotropic glutamate (mGlu) receptors, which are responsible for slow glutamate-mediated neurotransmission, are located throughout limbic and cortical brain regions that are implicated in drug addiction. Emerging evidence indicates that mGlu receptors regulate many behavioral actions of addictive drugs. In particular, group I mGlu receptors play an important role in regulating the reinforcing effects of drugs of abuse. Furthermore, group II mGlu receptors have been implicated in the synaptic adaptations that occur in response to chronic drug exposure and contribute to the aversive behavioral syndrome observed during withdrawal. These findings increase our understanding of the pathological processes that are associated with the development of drug addiction, and might ultimately lead to new therapies for the treatment of this disorder.