Increase In Extracellular Dopamine Levels Research Articles

Neurotoxic effects of exposure to glyphosate in rat striatum: Effects and mechanisms of action on dopaminergic neurotransmission

The main objective of this study was to evaluate the effects and possible mechanisms of action of glyphosate and a glyphosate-based herbicide (GBH) on dopaminergic neurotransmission in the rat striatum. Acute exposure to glyphosate or GBH, administered by systemic (75 or 150 mg/kg, i.p.) or intrastriatal (1, 5, or 10 mM for 1 h) routes, produced significant concentration-dependent increases in dopamine release measured in vivo by cerebral microdialysis coupled to HPLC with electrochemical detection. Systemic administration of glyphosate also significantly impaired motor control and decreased striatal acetylcholinesterase activity and antioxidant capacity. At least two mechanisms can be proposed to explain the glyphosate-induced increases in extracellular dopamine levels: increased exocytotic dopamine release from synaptic vesicles or inhibition of dopamine transporter (DAT). Thus, we investigated the effects of intrastriatal administration of glyphosate (5 mM) in animals pretreated with tetrodotoxin (TTX) or reserpine. It was observed that TTX (10 or 20 μM) had no significant effect on glyphosate-induced dopamine release, while reserpine (10 mg/kg i.p) partially but significantly reduced the dopamine release. When glyphosate was coinfused with nomifensine (50 μM), the increase in dopamine levels was significantly higher than that observed with glyphosate or nomifensine alone. So, two possible hypotheses could explain this additive effect: both glyphosate and nomifensine act through different mechanisms at the dopaminergic terminals to increase dopamine levels; or both nomifensine and glyphosate act on DAT, with glyphosate simultaneously inhibiting reuptake and stimulating dopamine release by reversing the DAT function. Future research is needed to determine the effects of this pesticide at environmentally relevant doses.

Novel Allosteric Modulator Southern Research Institute-32743 Reverses HIV-1 Transactivator of Transcription-Induced Increase in Dopamine Release in the Caudate Putamen of Inducible Transactivator of Transcription Transgenic Mice.

Development of neurocognitive disorder in human immunodeficiency virus (HIV)-infected patients has been linked to dysregulation of dopamine by the HIV-1 transactivator of transcription (Tat) protein, a negative allosteric modulator of dopamine transporter (DAT). Using fast scan cyclic voltammetry, the present study determined the effects of in vivo Tat expression on dopamine release in the caudate putamen of inducible Tat transgenic (iTat-tg) mice and the impact of a novel DAT allosteric modulator, Southern Research Institute (SRI)-32743, on the Tat effect. We found that 7- or 14-day doxycycline (Dox)-induced Tat expression in iTat-tg mice resulted in a 2-fold increase in phasic but not tonic stimulated baseline dopamine release relative to saline control mice. To determine whether the Tat-induced increase in dopamine release is mediated by DAT regulation, we examined the effect of an in vitro applied DAT inhibitor, nomifensine, on the dopamine release. Nomifensine (1 nM-10 µM) concentration-dependently enhanced phasic stimulated dopamine release in both saline- and Dox-treated iTat-tg mice, while the magnitude of the nomifensine-mediated dopamine release was unchanged between saline and Dox treatment groups. A single systemic administration of SRI-32743 prior to animal sacrifice reversed the increased dopamine release in the baseline of phasic dopamine release and nomifensine-augmented dopamine levels in Dox-treated iTat-tg mice, while SRI-32743 alone did not alter baseline of dopamine release. These findings suggest that Tat expression induced an increase in extracellular dopamine levels by not only inhibiting DAT-mediated dopamine transport but also stimulating synaptic dopamine release. Thus, DAT allosteric modulators may serve as a potential therapeutic intervention for HIV infection-dysregulated dopamine system observed in HIV-1 positive individuals. SIGNIFICANCE STATEMENT: HIV infection-induced dysregulation of the dopaminergic system has been implicated in the development of neurocognitive impairments observed in HIV positive patients. Understanding the mechanisms underlying HIV-1 Tat protein-induced alteration of extracellular dopamine levels will provide insights into the development of molecules that can attenuate Tat interaction with targets in the dopaminergic system. Here, we determined whether Tat alters dopamine release and how the novel DAT allosteric modulator, SRI-32743, impacts dopamine neurotransmission to attenuate Tat-induced effects on extracellular dopamine dynamics.

Impairment of glutamate homeostasis in the nucleus accumbens core underpins cross-sensitization to cocaine following chronic restraint stress.

Though the facilitating influence of stress on drug abuse is well documented, the mechanisms underlying this interaction have yet to be fully elucidated. The present study explores the neurobiological mechanisms underpinning the sensitized response to the psychomotor-stimulating effects of cocaine following chronic restraint stress (CRS), emphasizing the differential contribution of both subcompartments of the nucleus accumbens (NA), the core (NAcore) and shell (NAshell), to this phenomenon. Adult male Wistar rats were restrained for 2 h/day for 7 days and, 2 weeks after the last stress exposure (day 21), all animals were randomly assigned to behavioral, biochemical or neurochemical tests. Our results demonstrated that the enduring CRS-induced increase in psychostimulant response to cocaine was paralleled by an increase of extracellular dopamine levels in the NAcore, but not the NAshell, greater than that observed in the non-stress group. Furthermore, we found that CRS induced an impairment of glutamate homeostasis in the NAcore, but not the NAshell. Its hallmarks were increased basal extracellular glutamate concentrations driven by a CRS-induced downregulation of GLT-1, blunted glutamate levels in response to cocaine and postsynaptic structural remodeling in pre-stressed animals. In addition, ceftriaxone, a known GLT-1 enhancer, prevented the CRS-induced GLT-1 downregulation, increased basal extracellular glutamate concentrations and changes in structural plasticity in the NAcore as well as behavioral cross-sensitization to cocaine, emphasizing the biological importance of GLT-1 in the comorbidity between chronic stress exposure and drug abuse. A future perspective concerning the paramount relevance of the stress-induced disruption of glutamate homeostasis as a vulnerability factor to the development of stress and substance use disorders during early life or adulthood of descendants is provided.

PKC inhibition decreases amphetamine-maintained responding under a progressive-ratio schedule of reinforcement.

Protein kinase C (PKC) is important for the mechanism of action of amphetamine (AMPH). Inhibiting PKC blocks AMPH-stimulated increases in extracellular dopamine levels and AMPH-stimulated locomotor activity. This study examined the effects of PKC inhibition on the reinforcing properties of AMPH. Male Sprague-Dawley rats were trained to respond for infusions of 0.032 mg/kg/infusion AMPH or for sucrose pellets under a progressive-ratio (PR) schedule of reinforcement. Number of infusions earned, breakpoints, and session duration were recorded over consecutive sessions. Once AMPH-maintained responding stabilized, rats were treated with 0, 10, or 30 pmol of enzastaurin, a PKCβ-selective inhibitor, or 6 mg/kg 6c, a brain-permeable PKC inhibitor, 18 hr prior to a self-administration session. Pretreatment with 30 pmol enzastaurin or 6 mg/kg 6c decreased the number of AMPH infusions earned and breakpoints without altering sucrose-maintained behaviors. These data suggest that PKC inhibition decreases motivation for AMPH and, therefore, is worth pursuing as a potential treatment for AMPH-use disorder. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Electrophysiological Effects of Transcranial Direct Current Stimulation on Neural Activity in the Rat Motor Cortex.

Transcranial direct current stimulation (tDCS) is a non-invasive technique that modulates the neuronal membrane potential. We have previously documented a sustainable increase in extracellular dopamine levels in the rat striatum of cathodal tDCS, suggesting that cathodal tDCS enhances the neuronal excitability of the cortex. In the present study, we investigated changes in neuronal activity in the cerebral cortex induced by tDCS at the point beneath the stimulus electrode in anesthetized rats in vivo. Multiunit recordings were performed to examine changes in neuronal activity before and after the application of tDCS. In the cathodal tDCS group, multiunit activity (indicating the collective firing rate of recorded neuronal populations) increased in the cerebral cortex. Both anodal and cathodal tDCS increased the firing rate of isolated single units in the cerebral cortex. Significant differences in activity were observed immediately following stimulation and persisted for more than an hour after stimulation. The primary finding of this study was that both anodal and cathodal tDCS increased in vivo neuronal activity in the rat cerebral cortex underneath the stimulus electrode.

The Role of Dopamine in the Stimulant Characteristics of Novel Psychoactive Substances (NPS)-Neurobiological and Computational Assessment Using the Case of Desoxypipradrol (2-DPMP).

Stimulant drugs, including novel psychoactive substances (NPS, formerly “legal highs”) have addictive potential which their users may not realize. Stimulants increase extracellular dopamine levels in the brain, including the reward and addiction pathways, through interacting with dopamine transporter (DAT). This work aimed to assess the molecular and atomistic mechanisms of stimulant NPS actions at DAT, which translate into biological outcomes such as dopamine release in the brain’s reward pathway. We applied combined in vitro, in vivo, and in silico methods and selected 2-diphenylmethylpiperidine (2-DPMP) as an example of stimulant NPS for this study. We measured in vitro binding of 2-DPMP to rat striatum and accumbens DAT by means of quantitative autoradiography with a selective DAT-radioligand [125I]RTI-121. We evaluated the effects of intravenously administered 2-DPMP on extracellular dopamine in the accumbens-shell and striatum using in vivo microdialysis in freely moving rats. We used dynamic modeling to investigate the interactions of 2-DPMP within DAT, in comparison with cocaine and amphetamine. 2-DPMP potently displaced the radioligand in the accumbens and striatum showing dose-dependence from 0.3 to 30 μM. IC50 values were: 5.65 × 10-7M for accumbens shell and 6.21 × 10-7M for dorsal striatum. Dose-dependent responses were also observed in accumbens-shell and striatum in vivo, with significant increases in extracellular dopamine levels. Molecular dynamics simulations identified contrasting conformational changes of DAT for inhibitors (cocaine) and releasers (amphetamine). 2-DPMP led to molecular rearrangements toward an outward-facing DAT conformation that suggested a cocaine-type effect. The present combination of molecular modeling with experimental neurobiological procedures allows for extensive characterization of the mechanisms of drug actions at DAT as the main molecular target of stimulants, and provides an insight into the role of dopamine in the molecular and neurobiological mechanisms of brain responses to stimulant NPS that have addictive potential. Such knowledge reveals the risk of addiction related to NPS use. The research presented here can be adapted for other psychostimulants that act at their membrane protein targets.

Triple reuptake inhibition of serotonin, norepinephrine, and dopamine increases the tonic activation of α2-adrenoceptors in the rat hippocampus and dopamine levels in the nucleus accumbens

Clinical studies have shown the therapeutic efficacy of an increase in dopamine (DA) transmission in treatment of major depressive disorder (MDD). In the present study, we investigated whether blockade of DA transporters in addition to serotonin (5-HT) and norepinephrine (NE) produced additional adaptations of monoaminergic systems. In vivo electrophysiological recordings were carried out in male anesthetized rats. Vehicle, the 5-HT reuptake inhibitor escitalopram, the NE/DA reuptake blocker nomifensine and their combination (triple reuptake inhibition; TRI) were delivered for 2 or 14 days. Firing activity of NE, 5-HT and DA neurons was assessed. Tonic activation of 5-HT1A receptors and α1- and α2-adrenoceptors was determined in the hippocampus and extracellular DA levels in the nucleus accumbens (NAc). Unlike escitalopram, nomifensine and TRI administration increased the tonic activation of α2-adrenoceptors in the hippocampus despite decreasing NE neuronal firing activity after 2 and 14 days of administration. The firing activity of 5-HT neurons was increased after prolonged nomifensine and TRI regimens, while addition of nomifensine to escitalopram prevented the early 2-day suppression of firing by 5-HT reuptake inhibition. The tonic activation of 5-HT1A receptors was enhanced only with escitalopram. Whereas escitalopram and nomifensine decreased firing activity of DA neurons after a 2-day administration, their combination normalized it to baseline level after 14 days; this was accompanied by a robust increase in extracellular DA levels in the NAc. In summary, these results indicate that TRI increases NE and DA but not 5-HT transmission, suggesting a differential efficacy profile in MDD patients.

Impaired alcohol-induced dopamine release in the nucleus accumbens in an inflammatory pain model: behavioral implications in male rats.

Recent studies have drawn the attention to the link between alcohol use disorder and the presence of pain. Indeed, the correct management of pain in patients with a previous history of alcohol use disorder has been reported to decrease the risk of relapse in alcohol drinking, suggesting that in this prone population, pain may increase the vulnerability to relapse. Previous data in male rats revealed that inflammatory pain desensitizes mu-opioid receptors in the ventral tegmental area and increases intake of high doses of heroin. Owing to the relevant role of mu-opioid receptors in alcohol effects, we hypothesize that pain may also alter alcohol reinforcing properties and therefore affect alcohol relapse in male rats. Our microdialysis studies show that the presence of inflammatory pain blunted the increase of extracellular dopamine levels in the nucleus accumbens induced by 1.5 g/kg of ethanol (s.c.). Moreover, we also revealed that the administration of 52 nmol of ethanol into the ventral tegmental area failed to induce place preference only in inflammatory pain-suffering animals, and a higher dose (70 nmol) was necessary to reverse this effect. Finally, we evaluated the effect of inflammatory pain on the alcohol deprivation effect in long-term ethanol-experienced male rats. After 4 cycles of free ethanol intake and abstinence periods, inflammatory pain induced alcohol deprivation effect without affecting its magnitude. These intriguing data reveal the impact of pain on neurochemical and behavioral effects after alcohol administration but also underscore the necessity of finding an appropriate paradigm to determine the long-term behavioral consequences.

Comparative neuropharmacological studies on three pyrrolidine-containing synthetic cathinones

Purpose3,4-Methylenedioxypyrovalerone (3,4-MDPV) is a prevalent member of α-pyrrolidinophenones, a group of new psychoactive substances, known for its strong psychostimulant effect resulting from potent stimulation of dopamine (DA) circuitry in the brain. As 3,4-MDPV and its derivatives are successively being scheduled, each year novel analogs appear on the market. This study aimed at examination and direct comparison of psychostimulant properties of structural isomer of 3,4-MDPV, namely 2,3-MDPV along with a model α-pyrrolidinophenone, pyrovalerone.MethodsOpen field spontaneous locomotor activity of mice was assessed as a measure of psychostimulant potency. To evaluate the in vivo pharmacological properties of the drugs, extracellular levels of DA and serotonin (5-HT) in the mouse striatum were measured using an in vivo microdialysis technique followed by high-performance liquid chromatography with electrochemical detection. Involvement of dopaminergic system in the behavioral effects of the tested α-pyrrolidinophenones was examined by pre-treatment with a selective D1-DA receptor antagonist, SCH 23390, before measurement of locomotor activity in response to the drugs.Results3,4-MDPV, 2,3-MDPV and pyrovalerone produced time- and dose-dependent stimulation of locomotor activity, with 3,4-MDPV being more potent than the other two compounds. Observed locomotor stimulation was mediated by elevated DA-ergic neurotransmission, as all compounds caused a significant increase of extracellular DA levels in the striatum, with 3,4-MDPV being the most potent, and psychostimulant effects were abolished by SCH 23390. Interestingly, the tested pyrovalerones caused in vivo elevation of extracellular 5-HT levels, which contrasted with their in vitro pharmacologic properties.ConclusionsPyrovalerone, 2,3-MDPV and 3,4-MDPV produced psychostimulant effects mediated by stimulation of dopaminergic neurotransmission. Additionally, all tested compounds elevated extracellular levels of 5-HT in vivo.

Mechanisms of action of paraoxon, an organophosphorus pesticide, on in vivo dopamine release in conscious and freely moving rats

Paraoxon is the active metabolite of parathion, an organophosphorus pesticide which can cause neurotoxic effects in animals and humans. In the present work, we investigated the effects of 5 mM paraoxon on striatal dopamine, DOPAC and HVA levels in conscious and freely moving rats, after treatment with TTX, reserpine, nomifensine, KCl, Ca++-free/EDTA medium, AP-5 or L-NAME. The intrastriatal administration of paraoxon for 60 min, through the microdialysis probe, significantly produced an increase of the dopamine to 1066 ± 120%, relative to basal levels. Administration of paraoxon to 20 μM TTX, 10 mg/kg reserpine or Ca++-free/EDTA medium-pretreated animals decreased the dopamine levels to 73%, 81%, and 70%, respectively, when compared with the effect of 5 mM paraoxon. Infusion of 50 μM nomifensine induced a maximal increase in extracellular dopamine levels to 1435 ± 387%, and when nomifensine was coadministered with paraoxon, striatal dopamine levels increased to 2429 ± 417%, an increase that was ∼230% higher that observed with the administration of the pesticide alone. Coinfusion of KCl and paraoxon produced an increase in extracellular dopamine to 1957 ± 445%, that was significantly higher than that observed with POX or KCl (1104 ± 220%) administered individually. Pretreatment with 650 μM AP-5 or 100 L-NAME reduced the effect of paraoxon on extracellular dopamine levels by 49.1% and 53.7%, respectively. Our results suggest that paraoxon induces dopamine release by a vesicular-, Ca++-, and deporalization-dependent mechanism, being independent of dopamine transporter. In addition, the paraoxon-induced dopamine release is mediated by glutamatergic and nitrergic neurotransmitter systems.

Protein Kinase Cβ Inhibitors Attenuate Amphetamine‐Stimulated Behaviors Through Direct and Indirect Mechanisms in Different Brain Regions

Amphetamines (AMPH) are a class of stimulants that elicit their behavioral effects through an increase in extracellular dopamine levels. Protein kinase Cβ (PKCβ) is necessary for the increase in extracellular dopamine levels and inhibition of PKCβ results in a decrease in AMPH‐stimulated dopamine release and AMPH‐stimulated locomotor activity. When PKCβ inhibitors are administered directly into the nucleus accumbens (NAc) of rats, the drugs act immediately to reduce AMPH's behavioral effects. However, when PKCβ inhibitors are administered into the lateral ventricles of rats, the drug must be administered 18 hrs prior to AMPH to alter AMPH's behavioral effects. In this study, we sought to assess the actions of the PKCβ inhibitors in distinct brain regions to better understand the mechanism of action and time course of the inhibitors. Male Sprague‐Dawley rats were administered 10 pmol of ruboxistaurin, a PKCβ‐selective inhibitor, directly into the NAc or the ventral tegmental area (VTA). Following a 30‐min or 18‐hr pretreatment time, rats were injected with 1 mg/kg AMPH (s.c.) and their locomotor activity was measured. In a separate group of rats, ventral striatal and VTA tissue were collected following an 18‐hr pretreatment of ruboxistaurin in the VTA and a 30‐min pretreatment of 1 mg/kg AMPH (s.c.). Levels of PKCβ and phosphorylated phosphoser41‐growth associated protein 43 (pGAP43), a PKC substrate, were evaluated via immunoblotting. An injection of ruboxistaurin into the NAc attenuated AMPH‐stimulated locomotor activity following a 30‐min pretreatment, but not an 18‐hr pretreatment. Conversely, an 18‐hr pretreatment of ruboxistaurin, but not a 30‐min pretreatment, in the VTA attenuated AMPH‐stimulated locomotor activity. PKCβ and pGAP43 levels were decreased in the VTA and pGAP43 levels were decreased in the ventral striatum. This suggests that an 18‐hr pretreatment of ruboxistaurin may result in a downregulation of PKCβ levels and a decrease in PKC activity, which could contribute to the decrease in AMPH‐stimulated locomotor activity at this timepoint. Furthermore, these studies demonstrate that PKCβ inhibitors act through at least two mechanisms; a direct and immediate mechanism at the NAc and an indirect mechanism at the VTA through regulation of PKCβ levels in the cell bodies. Future work will examine PKCβ levels and activity following a short or long pretreatment in the NAc.Support or Funding InformationFunded by NIH grant R01 DA11697, T32‐GM007767, Benedict and Diana Lucchesi Graduate Fellowship.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Reproductive and metabolic alterations in the progeny of obese rats: Role of sympathetic nervous system

Thiamethoxam (TMX) and clothianidin (CLO) are neonicotinoids insecticides. The main characteristic of these pesticides is their agonist action on nicotinic acetylcholine receptors (nAChRs). In the present work it was studied and characterized the effects of TMX and CLO, in different concentrations, on dopaminergic system of rat striatum using in vivo brain microdialysis coupled to HPLC-EC. Intrastriatal administration of 1 mM or 5 mM TMX has not produced significant increases on dopamine (DA) levels, nonetheless the infusion of 10 mM TMX increases the DA output to 841 ± 132%, when compared to basal levels. Infusion of 1 mM CLO has not induced a significant increase in DA levels, even so 2, 3.5 and 5 mM CLO have produced an increase of 438 ± 8%, 2778 ± 598% and 4604 ± 516%, respectively, every compared to basal levels. Mecamylamine (MEC), a non-competitive nAChRs antagonist, was used to investigate the role of nAChRs on DA release induced by TMX and CLO. The increases in extracellular DA levels induced by TMX and CLO when associated to MEC are 80% and 68% lower than the effect produced by CLO and TMX isolated. These results confirm that TMX and CLO appear to induce in vivo DA increased release in striatum of rats and it seems to be concentration dependent. Moreover, these results indicate that this effect might be related to nAChRs.

Reduced prefrontal dopaminergic activity in valproic acid-treated mouse autism model

Previous studies suggest that dysfunction of neurotransmitter systems is associated with the pathology of autism in humans and the disease model rodents, but the precise mechanism is not known. Rodent offspring exposed prenatally to VPA shows autism-related behavioral abnormalities. The present study examined the effect of prenatal VPA exposure on brain monoamine neurotransmitter systems in male and female mice. The prenatal VPA exposure did not affect the levels of dopamine (DA), noradrenaline (NA), serotonin (5-HT) and their metabolites in the prefrontal cortex and striatum, while it significantly reduced methamphetamine (METH) (1.0mg/kg)-induced hyperlocomotion in male offspring. In vivo microdialysis study demonstrated that prenatal VPA exposure attenuated METH-induced increases in extracellular DA levels in the prefrontal cortex, while it did not affect those in extracellular NA and 5-HT levels. Prenatal VPA exposure also decreased METH-induced c-Fos expression in the prefrontal cortex and the mRNA levels of DA D1 and D2 receptors in the prefrontal cortex. These effects of VPA were not observed in the striatum. In contrast to male offspring, prenatal VPA exposure did not affect METH-induced increases in locomotor activity and prefrontal DA levels and the D1 and D2 receptor mRNA levels in the prefrontal cortex in female offspring. These findings suggest that prenatal VPA exposure causes hypofunction of prefrontal DA system in a sex-dependent way.

The Role of Harmane and Norharmane in In Vitro Dopaminergic Function

Background. Nicotine is one of the most frequently used drugs of abuse. It has long been speculated that in addition to nicotine, tobacco contains other compounds that reinforce addiction. Two promising candidates are the tobacco smoke constituents, harmane and norharmane. Purpose. Our study evaluated the effects of exposure to harmane and norharmane alone and in combination with nicotine on midbrain neuronal cultures. Experimental design. After 24 h exposure, cytotoxicity, dopamine (DA) levels, pTH, TH, MAO-A, and MAO-B expression levels were investigated. Results. At low-to-moderate doses (1 μM–100 μM), harmane or norharmane exposure caused significant dose- dependent decreases in intracellular DA without causing high rate of cell death, mitochondrial dysfunction or generation of ROS. Harmane exposure also resulted in subsequent increase in extracellular DA levels. Conclusion. Along with downregulation of protein expression, notably MAO-B, our data indicate the potential of harmane and norharmane to act in synergy with nicotine in midbrain neurons.

Changes in dopamine transporter binding in nucleus accumbens following chronic self-administration cocaine: heroin combinations.

Concurrent use of cocaine and heroin (speedball) has been shown to exert synergistic effects on dopamine neurotransmission in the nucleus accumbens (NAc), as observed by significant increases in extracellular dopamine levels and compensatory elevations in the maximal reuptake rate of dopamine. The present studies were undertaken to determine whether chronic self-administration of cocaine, heroin or a combination of cocaine:heroin led to compensatory changes in the abundance and/or affinity of high- and low-affinity DAT binding sites. Saturation binding of the cocaine analog [(125) I] 3β-(4-iodophenyl)tropan-2β-carboxylic acid methyl ester ([(125) I]RTI-55) in rat NAc membranes resulted in binding curves that were best fit to two-site binding models, allowing calculation of dissociation constant (Kd ) and binding density (Bmax ) values corresponding to high- and low-affinity DAT binding sites. Scatchard analysis of the saturation binding curves clearly demonstrate the presence of high- and low- affinity binding sites in the NAc, with low-affinity sites comprising 85 to 94% of the binding sites. DAT binding analyses revealed that self-administration of cocaine and a cocaine:heroin combination increased the affinity of the low-affinity site for the cocaine congener RTI-55 compared to saline. These results indicate that the alterations observed following chronic speedball self-administration are likely due to the cocaine component alone; thus further studies are necessary to elaborate upon the synergistic effect of cocaine:heroin combinations on the dopamine system in the NAc.

A novel, potent, orally‐bioavailable nociceptin‐1 receptor antagonist reduces ethanol self‐administration and ethanol‐seeking in animal models of alcohol addiction (656.2)

Background: The receptor for nociceptin/orphanin‐FQ (NOP) is localized in the mesolimbic reward pathway and has been suggested to play a role in addiction and motivated behavior. We characterize here a novel, potent, orally‐bioavailable NOP antagonist in several ethanol self‐administration models.Methods: Compound 1 was evaluated in homecage ethanol self‐administration models, progressive ratio operant self‐administration, stress‐induced reinstatement to ethanol‐seeking, and in vivo microdialysis.Results: Compound 1 dose‐dependently reduced homecage ethanol self‐administration in both P and msP rats, without affecting food or water intake. Compound 1 also attenuated progressive ratio operant responding and breakpoints in P rats. Stress‐induced reinstatement of ethanol‐seeking in msP rats was completely blocked by compound 1. Furthermore, compound 1 blocked ethanol‐induced dopamine release in the nucleus accumbens.Conclusions: Compound 1 demonstrates efficacy in attenuating ethanol self‐administration and stress‐induced ethanol‐seeking in two different lines of rats that exhibit excessive ethanol consumption. Compound 1 also blocks the motivation to consume ethanol and ethanol‐stimulated increases in extracellular dopamine levels in the nucleus accumbens, suggesting potential therapeutic utility of NOP receptor antagonists in treating alcohol addiction.

Antipsychotic Drug-Like Effects of the Selective M4 Muscarinic Acetylcholine Receptor Positive Allosteric Modulator VU0152100

Accumulating evidence suggests that selective M4 muscarinic acetylcholine receptor (mAChR) activators may offer a novel strategy for the treatment of psychosis. However, previous efforts to develop selective M4 activators were unsuccessful because of the lack of M4 mAChR subtype specificity and off-target muscarinic adverse effects. We recently developed VU0152100, a highly selective M4 positive allosteric modulator (PAM) that exerts central effects after systemic administration. We now report that VU0152100 dose-dependently reverses amphetamine-induced hyperlocomotion in rats and wild-type mice, but not in M4 KO mice. VU0152100 also blocks amphetamine-induced disruption of the acquisition of contextual fear conditioning and prepulse inhibition of the acoustic startle reflex. These effects were observed at doses that do not produce catalepsy or peripheral adverse effects associated with non-selective mAChR agonists. To further understand the effects of selective potentiation of M4 on region-specific brain activation, VU0152100 alone and in combination with amphetamine were evaluated using pharmacologic magnetic resonance imaging (phMRI). Key neural substrates of M4-mediated modulation of the amphetamine response included the nucleus accumbens (NAS), caudate-putamen (CP), hippocampus, and medial thalamus. Functional connectivity analysis of phMRI data, specifically assessing correlations in activation between regions, revealed several brain networks involved in the M4 modulation of amphetamine-induced brain activation, including the NAS and retrosplenial cortex with motor cortex, hippocampus, and medial thalamus. Using in vivo microdialysis, we found that VU0152100 reversed amphetamine-induced increases in extracellular dopamine levels in NAS and CP. The present data are consistent with an antipsychotic drug-like profile of activity for VU0152100. Taken together, these data support the development of selective M4 PAMs as a new approach to the treatment of psychosis and cognitive impairments associated with psychiatric disorders such as schizophrenia.

Retrodialysis of N/OFQ into the nucleus accumbens shell blocks cocaine-induced increases in extracellular dopamine and locomotor activity

Nociceptin (N/OFQ) has been implicated in a variety of neurological disorders, most notably in reward processes and drug abuse. N/OFQ suppresses extracellular dopamine in the nucleus accumbens (NAc) after intracerebroventricular injection. This study sought to examine the effects of retrodialyzed N/OFQ on the cocaine-induced increase in extracellular dopamine levels in the NAc, as well as locomotor activity, in freely moving rats. 1.0μM, 10μM, and 1mM N/OFQ, in the NAc shell, significantly suppressed the cocaine-induced dopamine increase in the NAc, while N/OFQ alone had no significant effect on dopamine levels. Co-delivery of the selective NOP receptor antagonist SB612111 ([(–)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol] reversed the N/OFQ suppression of cocaine-induced dopamine in the NAc, suggesting that this is an NOP receptor-mediated effect. Using a novel system to assess locomotion, we measured various motor activities of the animals with simultaneous microdialysis from the home cage. Cocaine produced an expected increase in total activity, including horizontal movement and rearing behavior. Retrodialysis of N/OFQ with cocaine administration affected all motor activities, initially showing no effect on behavior, but over time inhibiting cocaine-induced motor behaviors. These results suggest that N/OFQ can act directly in the NAc shell to block cocaine-induced increases in extracellular dopamine levels. Extracellular dopamine and locomotor activity can be dissociated within the NAc and may reflect motor output differences in shell versus core regions of the NAc. These studies confirm the widespread involvement of NOP receptors in drug addiction and further validate the utility of an NOP receptor agonist as a medication for treatment of drug addiction.

Ginkgo bilobaleaf extract (EGb 761®) and its specific acylated flavonol constituents increase dopamine and acetylcholine levels in the rat medial prefrontal cortex: possible implications for the cognitive enhancing properties of EGb 761®

Experimental and clinical data suggest that the Ginkgo biloba standardized extract EGb 761® exerts beneficial effects in conditions which are associated with impaired cognitive function. However, the neurochemical correlates of these memory enhancing effects are not yet fully clarified. The aim of this study was to examine the effect of repeated oral administration of EGb 761® and some of its characteristic constituents on extracellular levels of dopamine (DA), noradrenaline (NA), serotonin (5-HT), acetylcholine (ACh) and the metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) in the medial prefrontal cortex (mPFC) of awake rats by use of in vivo microdialysis technique. Subacute (14 days, once daily), but not acute, oral treatment with EGb 761® (100 and 300 mg/kg) or the flavonoid fraction, which represents about 24% of the whole extract caused a significant and dose-dependent increase in extracellular DA levels in the mPFC. Repeated administration of EGb 761® also caused a modest but significant increase in the NA levels, whereas the concentrations of 5-HT and those of the metabolites DOPAC, HVA and 5-HIAA were not affected. The same treatment regimen was used in a subsequent study with the aim of investigating the effects of two Ginkgo-specific acylated flavonols, 3-O-(2''-O-(6'''-O-(p-hydroxy-trans-cinnamoyl)-β-D-glucosyl)-α-L-rhamnosyl)quercetin (Q-ag) and 3-O-(2''-O-(6'''-O-(p-hydroxy-trans-cinnamoyl)-β-D-glucosyl)-α-L-rhamnosyl)kaempferol (K-ag). Both compounds together represent about 4.5% of the whole extract. Repeated oral treatment with Q-ag (10 mg/kg) for 14 days caused a significant increase in extracellular DA levels of 159% and extracellular acetylcholine (ACh) levels of 151% compared to controls. Similarly, administration of K-ag (10 mg/kg) induced a significant rise of DA levels to 142% and ACh levels to 165% of controls, whereas treatment with isorhamnetin, an O-methylated aglycon component of EGb 761® flavonol glycosides had no effect. None of the tested flavonoids had a significant effect on extracellular DOPAC and HVA levels. The present findings provide evidence that the subacute treatment with EGb 761® and its flavonol constituents increases DA and ACh release in the rat mPFC, and suggest that the two Ginkgo-specific acylated flavonol glycosides Q-ag and K-ag are active constituents contributing to these effects. As seen for isorhamnetin, the effect on neurotransmitter levels seems not to be a general effect of flavonols but rather to be a specific action of acylated flavonol glycosides which are present in EGb 761®. The direct involvement of these two flavonol derivatives in the increase of dopaminergic and cholinergic neurotransmission in the prefrontal cortex may be one of the underlying mechanisms behind the reported effects of EGb 761® on the improvement of cognitive function.

Methamphetamine-induced dopamine terminal deficits in the nucleus accumbens are exacerbated by reward-associated cues and attenuated by CB1 receptor antagonism

Methamphetamine (METH) exposure is primarily associated with deleterious effects to dopaminergic neurons. While several studies have implicated the endocannabinoid system in METH's locomotor, rewarding and neurochemical effects, a role for this signaling system in METH's effects on dopamine terminal dynamics has not been elucidated. Given that CB1 receptor blockade reduces the acute potentiation of phasic extracellular dopamine release from other psychomotor stimulant drugs and that the degree of acute METH-induced increases in extracellular dopamine levels is related to the severity of dopamine depletion, we predicted that pretreatment with the CB1 receptor antagonist rimonabant would reduce METH-induced alterations at dopamine terminals. Furthermore, we hypothesized that administration of METH in environments where reward associated-cues were present would potentiate METH's acute effects on dopamine release in the nucleus accumbens and exacerbate changes in dopamine terminal activity. Fast-scan cyclic voltammetry was used to measure electrically-evoked dopamine release in the nucleus accumbens and revealed markers of compromised dopamine terminal integrity nine days after a single dose of METH. These were exacerbated in animals that received METH in the presence of reward-associated cues, and attenuated in rimonabant-pretreated animals. While these deficits in dopamine dynamics were associated with reduced operant responding on days following METH administration in animals treated with only METH, rimonabant-pretreated animals exhibited levels of operant responding comparable to control. Moreover, dopamine release correlated significantly with changes in lever pressing behavior that occurred on days following METH administration. Together these data suggest that the endocannabinoid system is involved in the subsecond dopaminergic response to METH.