Repeated exposure to methamphetamine causes long-lasting presynaptic corticostriatal depression that is renormalized with drug readministration

Addiction-associated behaviors such as drug craving and relapse are hypothesized to result from synaptic changes that persist long after withdrawal and are renormalized by drug reinstatement, although such chronic synaptic effects have not been identified. We report that exposure to the dopamine releaser methamphetamine for 10 days elicits a long-lasting (>4 month) depression at corticostriatal terminals that is reversed by methamphetamine readministration. Both methamphetamine-induced chronic presynaptic depression and the drug's selective renormalization in drug-experienced animals are independent of corresponding long-term changes in synaptic dopamine release but are due to alterations in D1 dopamine and cholinergic receptor systems. These mechanisms might provide a synaptic basis that underlies addiction and habit learning and their long-term maintenance.     

Dopamine Release Dynamics Change during Adolescence and after Voluntary Alcohol Intake

Adolescence is associated with high impulsivity and risk taking, making adolescent individuals more inclined to use drugs. Early drug use is correlated to increased risk for substance use disorders later in life but the neurobiological basis is unclear. The brain undergoes extensive development during adolescence and disturbances at this time are hypothesized to contribute to increased vulnerability. The transition from controlled to compulsive drug use and addiction involve long-lasting changes in neural networks including a shift from the nucleus accumbens, mediating acute reinforcing effects, to recruitment of the dorsal striatum and habit formation. This study aimed to test the hypothesis of increased dopamine release after a pharmacological challenge in adolescent rats. Potassium-evoked dopamine release and uptake was investigated using chronoamperometric dopamine recordings in combination with a challenge by amphetamine in early and late adolescent rats and in adult rats. In addition, the consequences of voluntary alcohol intake during adolescence on these effects were investigated. The data show a gradual increase of evoked dopamine release with age, supporting previous studies suggesting that the pool of releasable dopamine increases with age. In contrast, a gradual decrease in evoked release with age was seen in response to amphetamine, supporting a proportionally larger storage pool of dopamine in younger animals. Dopamine measures after voluntary alcohol intake resulted in lower release amplitudes in response to potassium-chloride, indicating that alcohol affects the releasable pool of dopamine and this may have implications for vulnerability to addiction and other psychiatric diagnoses involving dopamine in the dorsal striatum.     

Action of Thymoxamine on Mydriasis induced by Levodopa and Dopamine

Eye-drops of levodopa and dopamine induce pupillary dilatation which is inhibited by thymoxamine, an alpha-adrenergic blocking drug. This indicates that the mydriatic action of levodopa and dopamine involves excitation of alpha-adrenergic receptors of the dilator pupillae muscle. Such a conclusion is in accord with the previously expressed suggestion that levodopa is rapidly converted to dopamine, which displaces noradrenaline from adrenergic nerve endings.The findings that dopamine exerts alpha-adrenergic effects at the periphery may be construed as evidence in support of the view that the hypotensive action of levodopa is mediated via the central nervous system.     

Differential regulation of somatodendritic and nerve terminal dopamine release by serotonergic innervation of substantia nigra

Nigrostriatal dopaminergic neurons release dopamine from dendrites in substantia nigra and axon terminals in striatum. The cellular mechanisms for somatodendritic and axonal dopamine release are similar, but somatodendritic and nerve terminal dopamine release may not always occur in parallel. The current studies used in vivo microdialysis to simultaneously measure changes in dendritic and nerve terminal dopamine efflux in substantia nigra and ipsilateral striatum respectively, following intranigral application of various drugs by reverse dialysis through the nigral probe. The serotonin releasers (+/-)-fenfluramine (100 micro m) and (+)-fenfluramine (100 micro m) significantly increased dendritic dopamine efflux without affecting extracellular dopamine in striatum. The non-selective serotonin receptor agonist 1-(m-chlorophenyl)-piperazine (100 micro m) elicited a similar pattern of dopamine release in substantia nigra and striatum. NMDA (33 micro m) produced an increase in nigral dopamine of a similar magnitude to mCPP or either fenfluramine drug. However, NMDA also induced a concurrent increase in striatal dopamine. The D2 agonist quinpirole (100 micro m) had a parallel inhibitory effect on dopamine release from dendritic and terminal sites as well. Taken together, these data suggest that serotonergic afferents to substantia nigra may evoke dendritic dopamine release through a mechanism that is uncoupled from the impulse-dependent control of nerve terminal dopamine release.     

Differential Effects of Locally Administered Clozapine and Haloperidol on Dopamine Efflux in the Rat Prefrontal Cortex and Caudate-Putamen

Abstract: Previous research has shown that systemically administered antipsychotic drugs enhance dopamine release from the nigrostriatal and mesocortical dopamine pathways. However, the degree of enhancement differs as a function of the drug used (atypical versus typical antipsychotic) and the dopamine pathway examined. The present studies examined whether these differences result from differential actions of these drugs on dopamine terminal regions. Clozapine or haloperidol was infused locally into the caudate-putamen or prefrontal cortex through reverse microdialysis. Although both drugs increased extracellular dopamine levels, clozapine produced greater effects than haloperidol in the prefrontal cortex, whereas haloperidol produced greater effects in the caudate-putamen. These results suggest that neurochemical differences within dopamine terminal regions may explain the differential actions of antipsychotic drugs on striatal and cortical dopamine release.     

Ethanol‐ and cocaine‐induced locomotion are genetically related to increases in accumbal dopamine

Neuroanatomical research suggests that interactions between dopamine and glutamate within the mesolimbic dopamine system are involved in both drug‐induced locomotor stimulation and addiction. Therefore, genetically determined differences in the locomotor responses to ethanol and cocaine may be related to differences in the effects of these drugs on this system. To test this, we measured drug‐induced changes in dopamine and glutamate within the nucleus accumbens (NAcc), a major target of mesolimbic dopamine neurons, using in vivo microdialysis in selectively bred FAST and SLOW mouse lines, which were bred for extreme sensitivity (FAST) and insensitivity (SLOW) to the locomotor stimulant effects of ethanol. These mice also show a genetically correlated difference in stimulant response to cocaine (FAST > SLOW). Single injections of ethanol (2 g/kg) or cocaine (40 mg/kg) resulted in larger increases in dopamine within the NAcc in FAST compared with SLOW mice. There was no effect of either drug on NAcc glutamate levels. These experiments indicate that response of the mesolimbic dopamine system is genetically correlated with sensitivity to ethanol‐ and cocaine‐induced locomotion. Because increased sensitivity to the stimulating effects of ethanol appears to be associated with greater risk for alcohol abuse, genetically determined differences in the mesolimbic dopamine response to ethanol may represent a critical underlying mechanism for increased genetic risk for alcoholism.     

Molecular cloning of the mouse dopamine transporter and pharmacological comparison with the human homologue.

Drug abuse is a serious problem in the United States and in the world. Cocaine and amphetamines, widely used drugs of abuse, bind to dopamine (DA), serotonin, and norepinephrine transporters with high affinity and block their functions. It is believed that the dopamine transporter plays a key role in the mechanism of cocaine addiction. Because a good portion of our knowledge about drug addiction is derived from studying mouse as an animal model, it is essential to compare the properties of dopamine transporter from human and mouse. We report here the cloning of the mouse dopamine transporter (mDAT) cDNA and its expression and comparison with the human DAT. The 3.4 kilobase (kb) cDNA encodes a polypeptide that is 93.5% identical to the hDAT, with 619 amino acid residues and a calculated molecular weight of 68.8kDa. Dopamine transporters from mouse and human were stably expressed in the same parental MDCK cells and their properties were compared. The Michaelis-Menten constant Km values are 2.0 microM for mDAT and 2.4 microM for hDAT. Mouse and human DAT were also compared for drug inhibition profiles. Dopamine transporters from the two species have the same sensitivity to amphetamine (Kd: 0.75 microM) and bupropion (Kd: 1.5 microM). However, hDAT is more sensitive than mDAT to cocaine (Kd: 0.14 microM and 0. 29 microM respectively) and to ritalin (Kd: 0.038 microM and 0. 12 microM respectively). The cloning of mDAT cDNA provides an important tool for further study of the mechanism of drug addiction using mouse as an animal model.     

In vivo electrochemical studies of rat striatal dopamine and serotonin release after morphine

The effect of the reference opiate, morphine (d-morphine-sulfate), on endogenously released striatal dopamine and serotonin was studied in male, adult, anesthetized Sprague-Dawley rats. The intraperitoneal administration of morphine produced a biphasic effect on striatal dopamine release. A significant increase in the dopamine signal was seen in the first hour after drug administration; a significant decrease in the dopamine signal was seen in the second and third hour after drug administration. On the other hand, the effect of morphine on striatal serotonin release was monophasic. Morphine significantly increased serotonin release from rat striatum. The effect lasted three hours after morphine administration, i.e., the effect persisted significantly throughout the study. These data show a simultaneous opiate-dopaminergic and opiate-serotonergic interaction in rat striatum. These data further extend studies which have suggested that the pharmacological mechanism of action of morphine may have its etiology in the concurrent modulation of more than one neurotransmitter.     

SELECTIVE INCREASE OF BRAIN DOPAMINE SYNTHESIS BY SULPIRIDE

—Sulpiride (5–200 mg/kg) increases brain HVA and DOPAC levels, causes no change in dopamine concentration, does not interfere with the outflow of HVA from the CNS and enhances the disappearance of brain dopamine after inhibition of tyrosine hydroxylase. The compound influences neither 5-HT nor NE metabolism. The central action of sulpiride differs from that of classic neuroleptics in that this drug stimulates dopamine turnover without producing catalepsy.     

Methamphetamine Self-Administration Is Associated with Persistent Biochemical Alterations in Striatal and Cortical Dopaminergic Terminals in the Rat

Methamphetamine (meth) is an illicit psychostimulant that is abused throughout the world. Repeated passive injections of the drug given in a single day or over a few days cause significant and long-term depletion of dopamine and serotonin in the mammalian brain. Because meth self-administration may better mimic some aspects of human drug-taking behaviors, we examined to what extent this pattern of drug treatment might also result in damage to monoaminergic systems in the brain. Rats were allowed to intravenously self-administer meth (yoked control rats received vehicle) 15 hours per day for 8 days before being euthanized at either 24 hours or at 7 and 14 days after cessation of drug taking. Meth self-administration by the rats was associated with a progressive escalation of daily drug intake to 14 mg/kg per day. Animals that self-administered meth exhibited dose-dependent decreases in striatal dopamine levels during the period of observation. In addition, there were significant reductions in the levels of striatal dopamine transporter and tyrosine hydroxylase proteins. There were also significant decreases in the levels of dopamine, dopamine transporter, and tyrosine hydroxylase in the cortex. In contrast, meth self-administration caused only transient decreases in norepinephrine and serotonin levels in the two brain regions, with these values returning to normal at seven days after cessation of drug taking. Importantly, meth self-administration was associated with significant dose-dependent increases in glial fibrillary acidic protein in both striatum and cortex, with these changes being of greater magnitude in the striatum. These results suggest that meth self-administration by rats is associated with long-term biochemical changes that are reminiscent of those observed in post-mortem brain tissues of chronic meth abusers.     

Imbalanced Decision Hierarchy in Addicts Emerging from Drug-Hijacked Dopamine Spiraling Circuit

Despite explicitly wanting to quit, long-term addicts find themselves powerless to resist drugs, despite knowing that drug-taking may be a harmful course of action. Such inconsistency between the explicit knowledge of negative consequences and the compulsive behavioral patterns represents a cognitive/behavioral conflict that is a central characteristic of addiction. Neurobiologically, differential cue-induced activity in distinct striatal subregions, as well as the dopamine connectivity spiraling from ventral striatal regions to the dorsal regions, play critical roles in compulsive drug seeking. However, the functional mechanism that integrates these neuropharmacological observations with the above-mentioned cognitive/behavioral conflict is unknown. Here we provide a formal computational explanation for the drug-induced cognitive inconsistency that is apparent in the addicts'' “self-described mistake”. We show that addictive drugs gradually produce a motivational bias toward drug-seeking at low-level habitual decision processes, despite the low abstract cognitive valuation of this behavior. This pathology emerges within the hierarchical reinforcement learning framework when chronic exposure to the drug pharmacologically produces pathologicaly persistent phasic dopamine signals. Thereby the drug hijacks the dopaminergic spirals that cascade the reinforcement signals down the ventro-dorsal cortico-striatal hierarchy. Neurobiologically, our theory accounts for rapid development of drug cue-elicited dopamine efflux in the ventral striatum and a delayed response in the dorsal striatum. Our theory also shows how this response pattern depends critically on the dopamine spiraling circuitry. Behaviorally, our framework explains gradual insensitivity of drug-seeking to drug-associated punishments, the blocking phenomenon for drug outcomes, and the persistent preference for drugs over natural rewards by addicts. The model suggests testable predictions and beyond that, sets the stage for a view of addiction as a pathology of hierarchical decision-making processes. This view is complementary to the traditional interpretation of addiction as interaction between habitual and goal-directed decision systems.     

ntPET: a new application of PET imaging for characterizing the kinetics of endogenous neurotransmitter release

We present a new application of positron emission tomography ("ntPET" or "neurotransmitter PET") designed to recover temporal patterns of neurotransmitter release from dynamic data. Our approach employs an enhanced tracer kinetic model that describes uptake of a labeled dopamine D2/D3 receptor ligand in the presence of a time-varying rise and fall in endogenous dopamine. Data must be acquired during both baseline and stimulus (transient dopamine release) conditions. Data from a reference region in both conditions are used as an input function, which alleviates the need for any arterial blood sampling. We use simulation studies to demonstrate the ability of the method to recover the temporal characteristics of an increase in dopamine concentration that might be expected following a drug treatment. The accuracy and precision of the method-as well as its potential for false-positive responses due to noise or changes in blood flow-were examined. Finally, we applied the ntPET method to small-animal imaging data in order to produce the first noninvasive assay of the time-varying release of dopamine in the rat striatum following alcohol.     

Effect of methylphenidate on dopamine/DARPP signalling in adult, but not young, mice

Methylphenidate (MPH), a dopamine uptake inhibitor, is the most commonly prescribed drug for the treatment of attention-deficit/hyperactivity disorder (ADHD) in children. We examined the effect of MPH on dopamine- and cAMP-regulated phosphoprotein, Mr 32 kDa (DARPP-32) phosphorylation at Thr34 (PKA-site) and Thr75 (Cdk5-site) using neostriatal slices from young (14-15- and 21-22-day-old) and adult (6-8-week-old) mice. MPH increased DARPP-32 Thr34 phosphorylation and decreased Thr75 phosphorylation in slices from adult mice. The effect of MPH was blocked by a dopamine D1 antagonist, SCH23390. In slices from young mice, MPH did not affect DARPP-32 phosphorylation. As with MPH, cocaine stimulated DARPP-32 Thr34 phosphorylation in slices from adult, but not from young mice. In contrast, a dopamine D1 agonist, SKF81297, regulated DARPP-32 phosphorylation comparably in slices from young and adult mice, as did methamphetamine, a dopamine releaser. The results suggest that dopamine synthesis and the dopamine transporter are functional at dopaminergic terminals in young mice. In contrast, the lack of effect of MPH in young mice is likely attributable to immature development of the machinery that regulates vesicular dopamine release.     

Dopamine‐loaded poly(d,l‐lactic‐co‐glycolic acid) microspheres: New strategy for encapsulating small hydrophilic drugs with high efficiency

The effective controlled release of small hydrophilic drugs from poly(d ,l ‐lactic‐co‐glycolic acid) (PLGA) microspheres has remained a challenge, largely due to the difficulty of loading a large amount of the drug inside the microspheres, owing to the hydrophilicity of the drugs. This study provides a new strategy for increasing encapsulation of small hydrophilic drugs inside PLGA microspheres by utilizing noncovalent, physical adsorption between hydrophilic drugs and emulsifying polymers of poly(vinyl alcohol) and pluronic. An order of magnitude increase in drug loading efficiency from 2.7 to 18.6% for dopamine, a model small hydrophilic drug, was achieved. The large amount of dopamine‐loaded PLGA formulation herein could be useful for the treatment of Parkinson's disease. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 30:215–223, 2014     

Evidence on extracellular dopamine level in rat striatum: implications for the validity of quantitative microdialysis

Microdialysis zero-net-flux (ZNF) method is commonly used to monitor drug-induced changes in neurotransmitter baseline and release/uptake processes. Recent studies in this field suggest that microdialysis ZNF method seriously underestimates the resting concentration of extracellular dopamine in the rat neostriatum because probe implantation preferentially damages nearby dopamine release sites and that dopamine uptake inhibition increases the relative recovery of dopamine by microdialysis. This study assessed the validity of these claims by examining current data on extracellular dopamine levels at rest and after drug application obtained by voltammetry, a technique thought to induce less tissue disruption than microdialysis. To obtain the extracellular baseline value for dopamine from the evoked overflow data, we modified the existing dopamine kinetic model to suit both the resting and stimulated circumstances. It was found that dopamine uptake inhibition did in fact decrease the microdialysis relative recovery of dopamine, implying that the average basal extracellular dopamine level is within the range of 7-20 nm in rat striatum. This study concludes that the microdialysis ZNF method indeed underestimates the extracellular dopamine concentration, although not by as much as had been thought. Chronic microdialysis damages both neurotransmitter release and uptake, but it does so in a somewhat relative and proportional way for both processes. Thus the validity of the microdialysis ZNF method is not seriously undermined.     

Development and function of the midbrain dopamine system: what we know and what we need to

The past two decades have seen an explosion in our understanding of the origin and development of the midbrain dopamine system. Much of this work has been focused on the aspects of dopamine neuron development related to the onset of movement disorders such as Parkinson's disease, with the intent of hopefully delaying, preventing or fixing symptoms. While midbrain dopamine degeneration is a major focus for treatment and research, many other human disorders are impacted by abnormal dopamine, including drug addiction, autism and schizophrenia. Understanding dopamine neuron ontogeny and how dopamine connections and circuitry develops may provide us with key insights into potentially important avenues of research for other dopamine‐related disorders. This review will provide a brief overview of the major molecular and genetic players throughout the development of midbrain dopamine neurons and what we know about the behavioral‐ and disease‐related implications associated with perturbations to midbrain dopamine neuron development. We intend to combine the knowledge of two broad fields of neuroscience, both developmental and behavioral, with the intent on fostering greater discussion between branches of neuroscience in the service of addressing complex cognitive questions from a developmental perspective and identifying important gaps in our knowledge for future study.     

Dopamine Receptor Activation Increases HIV Entry into Primary Human Macrophages

Macrophages are the primary cell type infected with HIV in the central nervous system, and infection of these cells is a major component in the development of neuropathogenesis and HIV-associated neurocognitive disorders. Within the brains of drug abusers, macrophages are exposed to increased levels of dopamine, a neurotransmitter that mediates the addictive and reinforcing effects of drugs of abuse such as cocaine and methamphetamine. In this study we examined the effects of dopamine on HIV entry into primary human macrophages. Exposure to dopamine during infection increased the entry of R5 tropic HIV into macrophages, irrespective of the concentration of the viral inoculum. The entry pathway affected was CCR5 dependent, as antagonizing CCR5 with the small molecule inhibitor TAK779 completely blocked entry. The effect was dose-dependent and had a steep threshold, only occurring above 10⁸ M dopamine. The dopamine-mediated increase in entry required dopamine receptor activation, as it was abrogated by the pan-dopamine receptor antagonist flupenthixol, and could be mediated through both subtypes of dopamine receptors. These findings indicate that the effects of dopamine on macrophages may have a significant impact on HIV pathogenesis. They also suggest that drug-induced increases in CNS dopamine may be a common mechanism by which drugs of abuse with distinct modes of action exacerbate neuroinflammation and contribute to HIV-associated neurocognitive disorders in infected drug abusers.     

Hub and switches: endocannabinoid signalling in midbrain dopamine neurons

The last decade has provided a wealth of experimental data on the role played by lipids belonging to the endocannabinoid family in several facets of physiopathology of dopamine neurons. We currently suggest that these molecules, being intimately connected with diverse metabolic and signalling pathways, might differently affect various functions of dopamine neurons through activation not only of surface receptors, but also of nuclear receptors. It is now emerging how dopamine neurons can regulate their constituent biomolecules to compensate for changes in either internal functions or external conditions. Consequently, dopamine neurons use these lipid molecules as metabolic and homeostatic signal detectors, which can dynamically impact cell function and fitness. Because dysfunctions of the dopamine system underlie diverse neuropsychiatric disorders, including schizophrenia and drug addiction, the importance of better understanding the correlation between an unbalanced endocannabinoid signal and the dopamine system is even greater. Particularly, because dopamine neurons are critical in controlling incentive-motivated behaviours, the involvement of endocannabinoid molecules in fine-tuning dopamine cell activity opened new avenues in both understanding and treating drug addiction. Here, we review recent advances that have shed new light on the understanding of differential roles of endocannabinoids and their cognate molecules in the regulation of the reward circuit, and discuss their anti-addicting properties, particularly with a focus on their potential engagement in the prevention of relapse.     

Coordinated accumbal dopamine release and neural activity drive goal-directed behavior

Intracranial self-stimulation (ICSS) activates the neural pathways that mediate reward, including dopaminergic terminal areas such as the nucleus accumbens (NAc). However, a direct role of dopamine in ICSS-mediated reward has been questioned. Here, simultaneous voltammetric and electrophysiological recordings from the same electrode reveal that, at certain sites, the onset of anticipatory dopamine surges and changes in neuronal firing patterns during ICSS are coincident, whereas sites lacking dopamine changes also lack patterned firing. Intrashell microinfusion of a D1, but not a D2 receptor antagonist, blocks ICSS. An iontophoresis approach was implemented to explore the effect of dopamine antagonists on firing patterns without altering behavior. Similar to the microinfusion experiments, ICSS-related firing is selectively attenuated following D1 receptor blockade. This work establishes a temporal link between anticipatory rises of dopamine and firing patterns in the NAc shell during ICSS and suggests that they may play a similar role with natural rewards and during drug self-administration.     

125I-Spiperone: a novel ligand for D2 dopamine receptors

125I-Spiperone binds with high affinity (KD 0.3 nM) to a single specific site (Bmax 34 pmol/g wet weight) in homogenates of rat corpus striatum. Specific binding is about 40-60 percent of total binding and is displaced stereo-specifically by butaclamol and clopenthixol. Neuroleptic drugs of various classes are potent inhibitors of 125I-spiperone binding (Ki's 1-10 nM). Selective dopamine antagonists such as sulpiride (Ki 50 nM) and dopamine agonists such as apomorphine (Ki 200 nM) are also potent inhibitors. The drug specificity of 125I-spiperone binding correlates well with that of 3H-spiperone binding, providing good evidence that 125I-spiperone labels D2 dopamine receptors in striatal membranes. 125I-Spiperone, with its high specific activity (2200 Ci/mmol) may prove to be a useful ligand in studies examining D2 dopamine receptors in soluble preparations and by autoradiography. Furthermore iodinated spiperone may be useful in radioreceptor assays of neuroleptic drug levels and, in a 123I-labeled form, for imaging of dopamine receptors, in vivo, using single photon tomography.