Dopamine neuronal transport kinetics and effects of amphetamine

MSI-1436 reduces acute food intake without affecting dopamine transporter activity

Many therapies designed to reduce food intake and body weight act, in part, by blocking the dopamine transporter (DAT) - a protein responsible for clearing extracellular dopamine (DA) after release thereby terminating its action. Here, we found that a single injection of the drug trodusquemine (MSI-1436) decreased food intake in rats. To assess the effects of MSI-1436 on DAT function, fast-scan cyclic voltammetry was used to measure DA concentration changes in the ventral striatum. DA release was evoked by electrical stimulation of the ventral tegmental area every 5 min. After 3 baseline measurements, rats were injected with MSI-1436 (10 mg/kg), the known DAT blocker bupropion (80 mg/kg) or saline and evoked DA release and reuptake were monitored for an additional hour. Neither saline nor MSI-1436 caused a significant change in the magnitude of evoked release from baseline values whereas bupropion caused a significant increase. In addition, neither saline nor MSI-1436 significantly increased DA decay rates while such an increase was observed with bupropion. Thus, over a time course when MSI-1436 suppresses food intake it does not affect DAT function. The results support MSI-1436 as an anti-obesity treatment which spares DAT.

Biogenic amines modulate pulse rate in the dorsal blood vessel of Lumbriculus variegatus

The biogenic amines are widespread regulators of physiological processes, and play an important role in regulating heart rate in diverse organisms. Here, we present the first pharmacological evidence for a role of the biogenic amines in the regulation of dorsal blood vessel pulse rate in an aquatic oligochaete, Lumbriculus variegatus (Müller, 1774). Bath application of octopamine to intact worms resulted in an acceleration of pulse rate, but not when co-applied with the adenylyl cyclase inhibitor MDL-12,330a. The phosphodiesterase inhibitor theophylline mimicked the effects of OA, but the polar adenosine receptor antagonist 8(p-sulphophenyl)theophylline was significantly less potent than theophylline. Pharmacologically blocking synaptic reuptake of the biogenic amines using the selective 5-HT reuptake blocker fluoxetine or various tricyclic antidepressants also accelerated heart rate. Depletion of the biogenic amines by treatment with the monoamine vesicular transporter blocker reserpine dramatically depressed pulse rate. Pulse rate was partially restored in amine-depleted worms after treatment with octopamine or dopamine, but fully restored following treatment with serotonin. This effect of 5-HT was weakly mimicked by 5-methoxytryptamine, but not by alpha-methylserotonin; it was completely blocked by clozapine and partially blocked by cyproheptadine. Because they are known to orchestrate a variety of adaptive behaviors in invertebrates, the biogenic amines may coordinate blood flow with behavioral state in L.variegatus.

The roles of dopamine transport inhibition and dopamine release facilitation in wake enhancement and rebound hypersomnolence induced by dopaminergic agents

STUDY OBJECTIVE

Rebound hypersomnolence (RHS: increased sleep following increased wake) is a limiting side-effect of many wake-promoting agents. In particular, RHS in the first few hours following wake appears to be associated with dopamine (DA)-releasing agents, e.g., amphetamine, but whether it can also be produced by DA transporter (DAT) inhibition alone is unknown. In these studies, DA-releasing and DAT-inhibiting agents and their interaction were systematically examined for their ability to increase wake and induce RHS.

DESIGN

Chronically implanted rats were evaluated in a blinded, pseudo-randomized design.

PARTICIPANTS

237 rats were used in these studies with 1 week between repeat tests.

INTERVENTIONS

Animals were habituated overnight and dosed the next day, 5 h after lights on, with test agents.

MEASUREMENTS AND RESULTS

Sleep/wake activityand RHS were evaluated using EEG/EMG recording up to 22 h post dosing. In vitro dopamine release was evaluated in rat synaptosomes. At doses that produced equal increases in wake, DA-releasing (amphetamine, methamphetamine, phentermine) and several DAT-inhibiting agents (cocaine, bupropion, and methylphenidate) produced RHS during the first few hours after the onset of sleep recovery. However, other DAT-inhibiting agents (mazindol, nomifensine, GBR-12909, and GBR-12935) did not produce RHS. Combination treatment with amphetamine and nomifensine produced waking activity greater than the sum of their individual activities alone while ameliorating the amphetamine-like RHS. In rat synaptosomes, nomifensine reduced the potency of amphetamine to induce DA release approximately 270-fold, potentially explaining its action in ameliorating amphetamine-induced RHS.

CONCLUSIONS

All DA releasing agents tested, and some DAT-inhibiting agents, produced RHS at equal wake-promoting doses. Thus amphetamine-like DA release appears sufficient for inducing RHS, but additional properties (pharmacologic and/or pharmacokinetic) evidently underlie RHS of other DAT inhibitors. Enhancing wake while mitigating RHS can be achieved by combining DAT-inhibiting and DA-releasing agents.

The reverse operation of Na(+)/Cl(-)-coupled neurotransmitter transporters--why amphetamines take two to tango

Sodium-chloride coupled neurotransmitter transporters achieve reuptake of their physiological substrate by exploiting the pre-existing sodium-gradient across the cellular membrane. This terminates the action of previously released substrate in the synaptic cleft. However, a change of the transmembrane ionic gradients or specific binding of some psychostimulant drugs to these proteins, like amphetamine and its derivatives, induce reverse operation of neurotransmitter:sodium symporters. This effect eventually leads to an increase in the synaptic concentration of non-exocytotically released neurotransmitters [and - in the case of the norepinephrine transporters, underlies the well-known indirect sympathomimetic activity]. While this action has long been appreciated, the underlying mechanistic details have been surprisingly difficult to understand. Some aspects can be resolved by incorporating insights into the oligomeric nature of transporters, into the nature of the accompanying ion fluxes, and changes in protein kinase activities.

Role of the dopamine transporter in the action of psychostimulants, nicotine, and other drugs of abuse

A number of studies over the last two decades have demonstrated the critical importance of dopamine (DA) in the behavioral pharmacology and addictive properties of abused drugs. The DA transporter (DAT) is a major target for drugs of abuse in the category of psychostimulants, and for methylphenidate (MPH), a drug used for treating attention deficit hyperactivity disorder (ADHD), which can also be a psychostimulant drug of abuse. Other drugs of abuse such as nicotine, ethanol, heroin and morphine interact with the DAT in more indirect ways. Despite the different ways in which drugs of abuse can affect DAT function, one evolving theme in all cases is regulation of the DAT at the level of surface expression. DAT function is dynamically regulated by multiple intracellular and extracellular signaling pathways and several protein-protein interactions. In addition, DAT expression is regulated through the removal (internalization) and recycling of the protein from the cell surface. Furthermore, recent studies have demonstrated that individual differences in response to novel environments and psychostimulants can be predicted based on individual basal functional DAT expression. Although current knowledge of multiple factors regulating DAT activity has greatly expanded, many aspects of this regulation remain to be elucidated; these data will enable efforts to identify drugs that might be used therapeutically for drug dependence therapeutics.

Phosphodiesterase 10A inhibition modulates the sensitivity of the mesolimbic dopaminergic system to D-amphetamine: involvement of the D1-regulated feedback control of midbrain dopamine neurons

Phosphodiesterase (PDE) 10A is highly expressed in medium spiny neurons of the striatum, at the confluence of the corticostriatal glutamatergic and the midbrain dopaminergic pathways, both believed to be involved in the physiopathology of schizophrenia. There is a growing body of evidence suggesting that targeting PDE10A may be beneficial for the treatment of positive symptoms in schizophrenia. The aim of the present study was to investigate how PDE10A inhibition modulates mesolimbic dopaminergic neurotransmission. We found that the selective PDE10A inhibitor, MP-10, blocked D-amphetamine-induced hyperactivity as well as D-amphetamine-induced dopamine efflux in the nucleus accumbens in a dose-dependent manner. We further investigated the mechanism by which PDE10A inhibition affects dopaminergic neurotransmission. We report that MP-10 potentiated the effect of a high but not a low dose of D-amphetamine on the mean firing rate of dopaminergic neurons recorded from the ventral tegmental area. Similarly, the effect of a high, but not a low dose of D-amphetamine, was completely reversed by the selective D(1) antagonist, SCH23390. These data suggest that the D(1)-regulated feedback control of midbrain dopamine neurons is a critical pathway involved in the modulation of the response of mesolimbic dopamine neurons to D-amphetamine by PDE10A inhibition.

Chronic methylphenidate treatment enhances striatal dopamine neurotransmission after experimental traumatic brain injury

Traumatic brain injury (TBI) results in functional deficits that often are effectively treated clinically with the neurostimulant, methylphenidate (MPH). We hypothesized that daily MPH administration would reverse striatal neurotransmission deficits observed in the controlled cortical impact (CCI) model of TBI. CCI or naïve rats received daily injections of MPH (5 mg/kg) or saline for 14 days and were assessed on day 15 using fast scan cyclic voltammetry. Dopamine (DA) transporter (DAT) localization, DA-related proteins, and transcription factor (c-fos) expression were also assessed. CCI resulted in reduced electrically evoked overflow of DA and maximal velocity of DA clearance (V(max)). In contrast, CCI was associated with a decrease in the apparent K(M) of DAT. Daily dose of MPH after CCI resulted in robust increases in evoked DA overflow and V(max) as well as increased apparent K(M). Reductions in total striatal DAT expression occurred after CCI and were not further affected by MPH. In contrast, membrane-bound striatal DAT levels were increased in both CCI groups. MPH post-CCI significantly increased striatal c-fos levels compared with saline. These results support the hypothesis that daily MPH improves striatal DA neurotransmission after CCI. DAT expression and transcriptional changes affecting DA protein function may underlie the injury and MPH-induced alterations in neurotransmission observed.

MsrA knockout mouse exhibits abnormal behavior and brain dopamine levels

Oxidative stress can cause methionine oxidation that has been implicated in various proteins malfunctions, if not adequately reduced by the methionine sulfoxide reductase system. Recent evidence has found oxidized methionine residues in neurodegenerative conditions. Previously, we have described elevated levels of brain pathologies and an abnormal walking pattern in the methionine sulfoxide reductase A knockout (MsrA(-/-)) mouse. Here we show that MsrA(-/-) mice have compromised complex task learning capabilities relative to wild-type mice. Likewise, MsrA(-/-) mice exhibit lower locomotor activity and altered gait that exacerbated with age. Furthermore, MsrA(-/-) mice were less responsive to amphetamine treatment. Consequently, brain dopamine levels were determined. Surprisingly, relative to wild-type mice, MsrA(-/-) brains contained significantly higher levels of dopamine up to 12 months of age, while lower levels of dopamine were observed at 16 months of age. Moreover, striatal regions of MsrA(-/-) mice showed an increase of dopamine release parallel to observed dopamine levels. Similarly, the expression pattern of tyrosine hydroxylase activating protein correlated with the age-dependent dopamine levels. Thus, it is suggested that dopamine regulation and signaling pathways are impaired in MsrA(-/-) mice, which may contribute to their abnormal behavior. These observations may be relevant to age-related neurological diseases associated with oxidative stress.

Short-acting cocaine and long-acting GBR-12909 both elicit rapid dopamine uptake inhibition following intravenous delivery

The rewarding effects of cocaine have been reported to occur within seconds of administration. Extensive evidence suggests that these actions involve the ability of cocaine to inhibit the dopamine (DA) transporter. We recently showed that 1.5 mg/kg i.v. cocaine inhibits DA uptake within 5 s. Despite this evidence, there remains a lack of consensus regarding how quickly i.v. cocaine and other DA uptake inhibitors elicit DA uptake inhibition. The current studies sought to better characterize the onset of cocaine-induced DA uptake inhibition and to compare these effects to those obtained with the high-affinity, long-acting DA transporter inhibitor, GBR-12909 (1-(2-bis(4-fluorphenyl)-methoxy)-ethyl)-4-(3-phenyl-propyl)piperazine). Using in vivo fast scan cyclic voltammetry, we showed that i.v. cocaine (0.75, 1.5, and 3.0 mg/kg) significantly inhibited DA uptake in the nucleus accumbens of anesthetized rats within 5 s. DA uptake inhibition peaked at 30 s and returned to baseline levels in approximately 1 h. The effects of cocaine were dose-dependent, with the 3.0 mg/kg dose producing greater uptake inhibition at the early time points and exhibiting a longer latency to return to baseline. Further, the blood-brain barrier impermeant cocaine-methiodide had no effect on DA uptake or peak height, indicating that the generalized peripheral effects of cocaine do not contribute to the CNS alterations measured here. Finally, we show that GBR-12909 (0.75, 1.5, and 3.0 mg/kg) also significantly inhibited DA uptake within 5 s post-injection, although the peak effect and return to baseline were markedly delayed compared with cocaine, particularly at the highest dose. Combined, these observations indicate that the central effects of dopamine uptake inhibitors occur extremely rapidly following i.v. drug delivery.

Mechanisms by which amphetamine redistributes dopamine out of vesicles: a computational study

Computer simulations of dopamine (DA) and amphetamine interactions associated with dopaminergic storage vesicles were developed in order to better explain how amphetamine causes redistribution of DA out of the vesicles. In the model, DA can be transported into vesicles via the vesicular monoamine transporter. Amphetamine competitively inhibits DA uptake either as a substrate for the transporter or by interference with DA binding to the transporter. Both of the amines can passively diffuse across the membrane in both directions, but only the neutral species can cross the membrane in this manner. The abundance of neutral and positive moieties of the amines is governed by the Henderson-Hasselbalch equation. The model reproduces experimentally observed steady-state DA levels in vesicles, vesicles emptying faster after a change of pH inside the vesicle than after a change in access of DA for the vesicular monoamine transporter, and the impact of amphetamine on DA uptake and efflux in a variety of experimental paradigms. The simulations indicate that a small percentage of DA is constantly diffusing out of vesicles and is replaced by DA entering the vesicle by the vesicular monoamine transporter. Low doses of amphetamine cause DA redistribution out of vesicles primarily by inhibiting DA storage while an enhancement of efflux rates as a result of a change in vesicle pH is added at higher concentrations of amphetamine.

Valproate and acetyl-L-carnitine prevent methamphetamine-induced behavioral sensitization in mice

This study deals with the possible inhibitory role played by acetyl-l-carnitine (ALC) against methamphetamine (METH)-induced behavioral sensitization. Because valproate (VAL) inhibits the behavioral sensitization exerted by different psychostimulants, we investigated ALC's potential to prevent the amplification of METH-mediated psychomotor effects. We therefore evaluated the locomotor effects of VAL or ALC alone or in combination with METH after acute (day 1) as well as repeated (day 7) drug challenge. Finally, to assess the induction of behavioral sensitization, we also recorded the METH-mediated locomotor response after 7 days of drug suspension (day 15). Results showed that both VAL and ALC prevented the METH-induced sensitization. Another interesting observation was the significantly higher METH-induced hyperactivity at day 15 (after a 7-day drug-free period), indicating that behavioral sensitization developed during the washout period. Results also showed that both the acute and repeated coadministration of METH with either VAL or ALC inhibited METH-induced hyperactivity. We present different hypotheses concerning similar but also peculiar mechanisms that might underlie the preventive action of VAL and ALC. These data add to a growing body of literature that illustrates the potential of ALC in protecting against the insult of dysfunctional mitochondrial metabolism and psychostimulant-mediated neurotoxicity. By demonstrating an in vivo action against one of the most abused drugs, these results raise the possibility of beneficial effects of ALC in abuse behavior.

Cellular and molecular mechanisms involved in the neurotoxicity of opioid and psychostimulant drugs

Substance abuse and addiction are the most costly of all the neuropsychiatric disorders. In the last decades, much progress has been achieved in understanding the effects of the drugs of abuse in the brain. However, efficient treatments that prevent relapse have not been developed. Drug addiction is now considered a brain disease, because the abuse of drugs affects several brain functions. Neurological impairments observed in drug addicts may reflect drug-induced neuronal dysfunction and neurotoxicity. The drugs of abuse directly or indirectly affect neurotransmitter systems, particularly dopaminergic and glutamatergic neurons. This review explores the literature reporting cellular and molecular alterations reflecting the cytotoxicity induced by amphetamines, cocaine and opiates in neuronal systems. The neurotoxic effects of drugs of abuse are often associated with oxidative stress, mitochondrial dysfunction, apoptosis and inhibition of neurogenesis, among other mechanisms. Understanding the mechanisms that underlie brain dysfunction observed in drug-addicted individuals may contribute to improve the treatment of drug addiction, which may have social and economic consequences.

A small dopamine permeability of storage vesicle membranes and end product inhibition of tyrosine hydroxylase are sufficient to explain changes occurring in dopamine synthesis and storage after inhibition of neuron firing

Computer simulations of dopamine (DA) regulation at a striatal varicosity were developed to determine basic principles that explain the pattern of changes in level of neurotransmitter and its rate of synthesis and metabolism when DA neuron firing is inhibited. The models suggest that DA synthesis is normally at a slower rate because of end-product inhibition of tyrosine hydroxylase (TH) by cytosolic DA. The vast majority of DA in the cytosol arrives there via "recycling"--DA that was released during an exocytotic event is moved into the cytosol via the dopamine transporter (DAT). When neuronal firing is inhibited, the amount of cytosolic DA markedly decreases as there is no recycling. The rate of DA synthesis then increases because of the loss of end-product inhibition of TH. The newly synthesized DA is stored in vesicles, thus increasing the total amount of DA in the vesicles. A small amount of DA is continually leaking out of vesicles, and the amount leaking out increases proportionally to the amount of DA in vesicles. When the amount of DA leaking out balances the amount being stored by the vesicular monoamine transporter, DA accumulates in the cytosol. The accumulating DA inhibits TH activity, and the system enters a steady state condition characterized by approximately double the normal amount of DA in vesicles and approximately normal rate of DA synthesis and metabolism.

Substrate binding and formation of an occluded state in the leucine transporter

Translocation through the extracellular vestibule and binding of leucine in the leucine transporter (LeuT) have been studied with molecular dynamics simulations. More than 0.1 mus of all-atom molecular dynamics simulations have been performed on different combinations of LeuT, bound substrate, and bound structural Na(+) ions to describe molecular events involved in substrate binding and in the formation of the occluded state and to investigate the dynamics of this state. Three structural features are found to be directly involved in the initial steps of leucine transport: a Na(+) ion directly coordinated to leucine (Na-1), two aromatic residues closing the binding site toward the extracellular vestibule (Tyr-108 and Phe-253), and a salt bridge in the extracellular vestibule (Arg-30 and Asp-404). These features account for observed differences between simulations of LeuT with and without bound substrate and for a possible pathway for leucine binding and thereby formation of the occluded LeuT binding site.

Vesicular monoamine transporter 2 regulates the sensitivity of rat dopaminergic neurons to disturbed cytosolic dopamine levels

An abnormal accumulation of cytosolic dopamine resulting in reactive oxygen species and dopamine-quinone products may play an important role in the rather selective degeneration of substantia nigra pars compacta (SNc) dopaminergic neurons in Parkinson's disease. The neuronal-specific vesicular monoamine transporter (VMAT2), responsible for uptake of dopamine into vesicles, has been shown to play a central role both in intracellular dopamine homeostasis and sequestration of dopaminergic neurotoxins. Direct or indirect enhancement of VMAT2 activity could therefore have neuroprotective effects by decreasing cytosolic dopamine levels. Here, we demonstrate that transfection of VMAT2 in the dopaminergic cell line, PC12, increases intracellular dopamine content, augments potassium-induced dopamine release and attenuates cell death induced by the cytosolic dopamine enhancer, methamphetamine, suggesting an enhancement in vesicular dopamine storage. In rat ventral mesencephalic cultures highly enriched for dopaminergic neurons, lentiviral delivery of recombinant VMAT2 using a neuronal-specific promoter also resulted in elevated intracellular dopamine content and neurotransmitter release after depolarization. The opposite was seen after downregulation of VMAT2 using virally delivered shRNAs. Furthermore, using this VMAT2 knockdown model, we are the first to report a direct link between enhanced cytoplasmic dopamine levels, measured following mild permeabilization of the plasma membrane using digitonin, and neurite degeneration in primary dopaminergic neurons. In conclusion, our data support the hypothesis that an increase in vesicular sequestration of dopamine by modulation of VMAT2 activity could restore neuronal function and enhance dopaminergic cell survival in conditions of dysregulated dopamine homeostasis such as Parkinson's disease.

Reduced vesicular storage of dopamine causes progressive nigrostriatal neurodegeneration

The vesicular monoamine transporter 2 (VMAT2; SLC18A2) is responsible for packaging dopamine into vesicles for subsequent release and has been suggested to serve a neuroprotective role in the dopamine system. Here, we show that mice that express approximately 5% of normal VMAT2 (VMAT2 LO) display age-associated nigrostriatal dopamine dysfunction that ultimately results in neurodegeneration. Elevated cysteinyl adducts to L-DOPA and DOPAC are seen early and are followed by increased striatal protein carbonyl and 3-nitrotyrosine formation. These changes were associated with decreased striatal dopamine and decreased expression of the dopamine transporter and tyrosine hydroxylase. Furthermore, we observed an increase in alpha-synuclein immunoreactivity and accumulation and neurodegeneration in the substantia nigra pars compacta in aged VMAT2 LO mice. Thus, VMAT2 LO animals display nigrostriatal degeneration that begins in the terminal fields and progresses to eventual loss of the cell bodies, alpha-synuclein accumulation, and an L-DOPA responsive behavioral deficit, replicating many of the key aspects of Parkinson's disease. These data suggest that mishandling of dopamine via reduced VMAT2 expression is, in and of itself, sufficient to cause dopamine-mediated toxicity and neurodegeneration in the nigrostriatal dopamine system. In addition, the altered dopamine homeostasis resulting from reduced VMAT2 function may be conducive to pathogenic mechanisms induced by genetic or environmental factors thought to be involved in Parkinson's disease.

Simple transporter trafficking model for amphetamine-induced dopamine efflux

Amphetamine and its derivatives are important drugs of abuse causing both short-term excitatory and long-term addictive effects. The short-term excitatory effects are linked to amphetamine's ability to maintain high levels of dopamine (DA) outside the cell both by inhibiting DA reuptake after synaptic transmission and by enhancing the efflux of DA from the dopaminergic cells. The molecular mechanisms by which amphetamine elicits the efflux of DA and similar monoamines are still unclear. Recent literature data suggest that trafficking of the monoamine transporters is a phenomenon that underlies observed changes in amphetamine-induced monoamine reuptake and efflux. We develop an ordinary differential equation model incorporating the diverse mechanistic details behind amphetamine-induced DA efflux and demonstrate its utility in describing our experimental data. We also demonstrate an experimental method to track the time-varying concentration of membrane-bound transporter molecules from the DA efflux data. The good fit between our model and the experimental data supports the hypothesis that amphetamine-induced transporter trafficking is necessary to produce extended efflux of DA. This model can explain the relative significance of different processes associated with DA efflux at different times and at different concentration ranges of amphetamine and DA.

Amphetamine and mCPP effects on dopamine and serotonin striatal in vivo microdialysates in an animal model of hyperactivity

In the neonatally 6-hydroxydopamine (6-OHDA)-lesioned rat hyperlocomotor activity, first described in the 1970s, was subsequently found to be increased by an additional lesion with 5,7-dihydroxytryptamine (5,7-DHT) (i.c.v.) in adulthood. The latter animal model (i.e., 134 microg 6-OHDA at 3 d postbirth plus 71 microg 5,7-DHT at 10 weeks; desipramine pretreatments) was used in this study, in an attempt to attribute hyperlocomotor attenuation by D,L-amphetamine sulfate (AMPH) and m-chlorophenylpiperazine di HCl (mCPP), to specific changes in extraneuronal (i.e., in vivo microdialysate) levels of dopamine (DA) and/or serotonin (5-HT). Despite the 98-99% reduction in striatal tissue content of DA, the baseline striatal microdialysate level of DA was reduced by 50% or less at 14 weeks, versus the intact control group. When challenged with AMPH (0.5 mg/kg), the microdialysate level of DA went either unchanged or was slightly reduced over the next 180 min (i.e., 20 min sampling), while in the vehicle group and 5,7-DHT (alone) lesioned group, the microdialysate level was maximally elevated by approximately 225% and approximately 450%, respectively--and over a span of nearly 2 h. Acute challenge with mCPP (1 mg/kg salt form) had little effect on microdialysate levels of DA, DOPAC and 5-HT. Moreover, there was no consistent change in the microdialysate levels of DA, DOPAC, and 5-HT between intact, 5-HT-lesioned rats, and DA-lesioned rats which might reasonably account for an attenuation of hyperlocomotor activity. These findings indicate that there are other important neurochemical changes produced by AMPH- and mCPP-attenuated hyperlocomotor activity, or perhaps a different brain region or multiple brain regional effects are involved in AMPH and mCPP behavioral actions.

Difference in magnitude of psychostimulant-induced extracellular norepinephrine in the ventral tegmental area contributes to discrepant prefrontal dopamine outflow

The dopamine (DA) efflux in the medial prefrontal cortex (mPFC) can be modulated by the interaction between afferent norepinephrine (NE) and somatodendritic DA in the ventral tegmental area (VTA). However, it is unclear how locally administered amphetamine (AMPH) or cocaine in the VTA results in discrepant response of DA efflux in the mPFC. In this study, intra-VTA infusion of AMPH (1000 microM) or cocaine (200 microM) in anesthetized rats was employed to study the concurrent profile of extracellular DA level in the VTA and mPFC. In addition, the extracellular NE levels during the intra-VTA infusion of these two psychostimulants were analyzed to compare their effects on prefrontal DA efflux. During the intra-VTA infusion of AMPH, both extracellular DA and NE increased significantly in the VTA (270 +/- 12% and 819 +/- 40%, respectively). Meanwhile, the DA efflux in the mPFC elevated significantly. During the intra-VTA infusion of cocaine, the extracellular DA and NE in the VTA also increased (271 +/- 21% and 150 +/- 15%, respectively). However, the DA efflux decreased significantly in the mPFC. Noteworthy, the increase of extracellular NE in the VTA was much more robust via AMPH infusion, as compared with cocaine. It is suggested that AMPH and cocaine enhance the extracellular NE concentrations in the VTA in different magnitudes, which in turn contribute to discrepant profiles of distal DA efflux in the mPFC.

Differential effects of Zn2+ on the kinetics and cocaine inhibition of dopamine transport by the human and rat dopamine transporters

Zn2+ may play a major role in the modulation of neurotransmission because it modulates membrane receptors and channels. Recent literature has shown Zn2+ inhibits dopamine transport by the dopamine transporter (DAT), the main target of cocaine and some other drugs of abuse. Cocaine inhibits DAT and modulation of the DAT by Zn2+ may alter effects of cocaine on dopamine neurotransmission. This study investigates how Zn2+ changes DAT kinetics and its inhibition by cocaine. Steady-state and pre-steady-state kinetics of DAT activity were investigated using rotating disk electrode voltammetry. Values of KM and Vmax in hDAT and effects of cocaine match those in the literature. Zn2+ allosterically inhibited transport in the human DAT (hDAT) with a KI=7.9+/-0.42 microM. Removal of endogenous Zn2+ with penicillamine in hDAT increased transport values. In contrast, Zn2+ did not alter transport by rat DAT (rDAT), with KM and Vmax values of 1.2+/-0.49 microM and 15.7+/-2.57 pmol/(sx10(6) cells), respectively, and removal of Zn2+ did not increase dopamine transport values. Zn2+ allosterically reduced the inhibition by cocaine in hDAT. Results of pre-steady-state studies demonstrated that Zn2+ increases the second order binding rate constant for dopamine to hDAT (3.5 fold to 19.2x10(6) M-1 s-1 for hDAT). In rat striatal homogenates Zn2+ increased initial dopamine transport velocity and decreased cocaine inhibition providing evidence for differences in sensitivity to Zn2+ between the three different preparations. Modulation of the DAT by Zn2+ needs to be assessed further in development of cocaine antagonists.

Neuropsychological function and delay discounting in methamphetamine-dependent individuals

RATIONALE

Methamphetamine (MA) dependence accounts for substantial neuropsychiatric morbidity. Furthermore, there is evidence in the literature of psychiatric and cognitive impairment in chronic users.

OBJECTIVES

This report compares the general psychiatric and cognitive functioning, including impulsive decision-making, of individuals dependent on MA and normal controls.

MATERIALS AND METHODS

Forty-one currently abstinent individuals in treatment for MA dependence and 41 controls participated. Controls were selected to minimize group differences in age and gender. MA users met DSM-IV criteria for MA dependence, had average daily use of 0.5 g/day (0.5-6 g/day), had been abstinent at least 2 weeks (2-24 weeks), and did not currently meet criteria for other Axis I psychiatric disorders. Psychiatric symptoms were rated on standardized scales. Cognitive function was assessed with a battery of standardized neuropsychological tests. Impulsivity was assessed using a delay discounting task, which measured preference for small, immediate, and large delayed rewards.

RESULTS

The MA group reported more psychiatric symptoms than controls, and was impaired relative to controls on the Babcock Story Recall-Delayed and the Rey Auditory Verbal Learning Test. MA-dependent subjects discounted delayed rewards more than controls, and this measure of impulsivity was correlated with memory impairment in the MA group but not in the controls.

CONCLUSIONS

MA-dependent individuals are more impulsive than controls, and this may be causally related to memory deficits but was unrelated to any other measure of psychiatric or cognitive impairment or any drug use history variable.

Nitrous oxide and xenon prevent amphetamine-induced carrier-mediated dopamine release in a memantine-like fashion and protect against behavioral sensitization

BACKGROUND

Amphetamine administration induces stimulation-independent dopamine release in the nucleus accumbens (NAcc) through reverse dopamine transport, a critical neurochemical event involved in its psychostimulant action, and furthermore decreases stimulation-dependent vesicular dopamine release. These effects may involve possible indirect glutamatergic mechanisms.

METHODS

We investigated the effects of nitrous oxide and xenon, which possess antagonistic action at the N-methyl-D-aspartate (NMDA) receptor, on brain slices ex vivo on amphetamine-induced changes in carrier-mediated and KCl-evoked dopamine release in the NAcc, and in vivo on amphetamine-induced locomotor sensitization.

RESULTS

Like the low-affinity NMDA receptor antagonist memantine, but not the prototypical compound MK-801, nitrous oxide and xenon at appropriate concentrations blocked both the increase in carrier-mediated dopamine release and locomotor sensitization produced by amphetamine.

CONCLUSIONS

In contrast to what has generally been found using prototypical NMDA receptor antagonists, these data regarding the effect of memantine, nitrous oxide, and xenon support the hypothesis that activation of certain NMDA receptors (possibly those containing the NR1a/NR2D subunit) in the NAcc is involved in the amphetamine-induced increase in carrier-mediated dopamine release and the development of behavioral sensitization to amphetamine. Nitrous oxide, xenon, and memantine may be of therapeutic interest for treating drug dependence.

Dopamine release is severely compromised in the R6/2 mouse model of Huntington's disease

Recently, alterations in dopamine signaling have been implicated in Huntington's disease. In this work, dopamine release and uptake was measured in striatal slices from the R6/2 transgenic mouse model of Huntington's disease using fast-scan cyclic voltammetry at carbon-fiber microelectrodes. Dopamine release in brain slices from 6-week-old R6/2 mice is substantially reduced (53% of wild type), while dopamine uptake is unaffected. In agreement with this, R6/2 mice injected with the dopamine uptake inhibitor cocaine exhibited a blunted motor activity response (54% of wild type). At 10 weeks of age, an even more dramatic motor activity decrease in response to cocaine injection (21% of wild type) was observed. Moreover, the pre-drug activity of 10-week-old R6/2 mice was significantly reduced (by 37%) compared with 6-week-old R6/2 mice. Striatal dopamine release decreased with age, indicating that progressive alterations in dopaminergic pathways may affect motor activity. The inhibition constants of cocaine and methamphetamine (METH) determined in brain slices differed little between genotype or age group, suggesting that the decreased responses to cocaine and METH arise from compromised dopamine release rather than differences in uptake or drug action. Collectively, these data demonstrate (i) a reduction in the ability of dopamine terminals to release dopamine and (ii) the importance of this attenuation of release on the motor symptoms of Huntington's disease.

The dopamine transporter: a vigilant border control for psychostimulant action

Neurotransmission within the mesocorticolimbic dopamine system has remained the central focus of investigation into the molecular, cellular and behavioral properties of psychostimulants for nearly three decades. The primary means by which dopamine transmission in the synapse is terminated is via the dopamine transporter (DAT), the presynaptic plasmalemmal protein that is responsible for the reuptake of released dopamine. Numerous abused as well as clinically important drugs have important pharmacological interactions with DAT. In general, these compounds fall into two categories: those that block dopamine transport (e.g., cocaine, methylphenidate) and those that serve as substrates for transport [e.g., dopamine, amphetamine and 3,4-methylenedioxymethamphetamine (MDMA or "ecstasy")]. Recent data from in vitro and in vivo studies have suggested that DAT, like other biogenic amine transporters, share several characteristics with classical ligand-gated ion channels. In addition, substrates for transport promote redistribution of DAT away from the plasma membrane, while transport inhibitors such as cocaine disrupt this process. In addition, presynaptic autoreceptors for dopamine have been implicated in the modulation of DAT surface expression and function. The present chapter summarizes some of the recent discoveries pertaining to the electrogenic properties of DAT and their potential relevance to the effects of amphetamine-like stimulants on DAT function. Although there are a number of intracellular and extracellular modulatory influences on dopamine clearance that may play particular roles in psychostimulant action, we specifically focus on the differential direct modulation of DAT function by transport substrates and inhibitors, and we also discusses the role of presynaptic D2 receptors in transport regulation.

ADHD and the dopamine transporter: are there reasons to pay attention?

The catecholamine dopamine (DA) plays an important role as a neurotransmitter in the brain in circuits linked to motor function, reward, and cognition. The presynaptic DA transporter (DAT) inactivates DA following release and provides a route for non-exocytotic DA release (efflux) triggered by amphetamines. The synaptic role of DATs first established through antagonist studies and more recently validated through mouse gene-knockout experiments, raises questions as to whether altered DAT structure or regulation support clinical disorders linked to compromised DA signaling, including drug abuse, schizophrenia, and attention deficit hyperactivity disorder (ADHD). As ADHD appears to have highly heritable components and the most commonly prescribed therapeutics for ADHD target DAT, studies ranging from brain imaging to genomic and genetic analyses have begun to probe the DAT gene and its protein for possible contributions to the disorder and/or its treatment. In this review, after a brief overview of ADHD prevalence and diagnostic criteria, we examine the rationale and experimental findings surrounding a role for human DAT in ADHD. Based on the available evidence from our lab and labs of workers in the field, we suggest that although a common variant within the human DAT (hDAT) gene (SLC6A3) is unlikely to play a major role in the ADHD, contributions of hDAT to risk maybe most evident in phenotypic subgroups. The in vitro and in vivo validation of functional variants, pursued for contributions to endophenotypes in a within family approach, may help elucidate DAT and DA contributions to ADHD and its treatment.

Voltammetric measurement of electrically evoked dopamine levels in the striatum of the anesthetized Syrian hamster

Microdialysis measurements in the Syrian hamster clearly demonstrate a role for accumbal dopamine (DA) in female sexual behavior. However, large probe size and slow sampling rate prevent associating specific behaviors with DA changes in subregions of the heterogeneous nucleus accumbens. Fast-scan cyclic voltammetry (FSCV) at a carbon-fiber microelectrode (CFM), which affords millisecond temporal resolution at a micron-sized probe, could address these important issues. Mostly used in other rodents, e.g. rats and mice, this technique has not been applied to hamsters. The goal of the present study was to establish the measurement of DA in the nucleus accumbens of the anesthetized male Syrian hamster using FSCV at a CFM. For comparison, DA was simultaneously measured in the caudate-putamen. Stimulation of the medial forebrain bundle was used to elicit DA. Electrically evoked DA levels in both striatal regions were sensitive to location of the stimulating electrode and CFM, stimulation frequency, inhibition of DA uptake by cocaine and DA autoreceptor blockade by raclopride. Regional differences were observed for DA release and uptake parameters, and the effects of cocaine. Taken together, these results establish the measurement of electrically evoked DA levels in the hamster striatum using FSCV at a CFM.

Mechanisms of neurotransmitter release by amphetamines: a review

Amphetamine and substituted amphetamines, including methamphetamine, methylphenidate (Ritalin), methylenedioxymethamphetamine (ecstasy), and the herbs khat and ephedra, encompass the only widely administered class of drugs that predominantly release neurotransmitter, in this case principally catecholamines, by a non-exocytic mechanism. These drugs play important medicinal and social roles in many cultures, exert profound effects on mental function and behavior, and can produce neurodegeneration and addiction. Numerous questions remain regarding the unusual molecular mechanisms by which these compounds induce catecholamine release. We review current issues on the two apparent primary mechanisms--the redistribution of catecholamines from synaptic vesicles to the cytosol, and induction of reverse transport of transmitter through plasma membrane uptake carriers--and on additional drug effects that affect extracellular catecholamine levels, including uptake inhibition, effects on exocytosis, neurotransmitter synthesis, and metabolism.

The role of 5-HT2A receptor antagonism in amphetamine-induced inhibition of A10 dopamine neurons in vitro

The role of the 5-HT(2A) receptor in modulating amphetamine-induced inhibition of dopamine neuronal firing in A9 and A10 was investigated in rat midbrain slices. The antipsychotic drugs olanzapine and clozapine more potently reversed the amphetamine-induced inhibition in A10 neurons compared to A9 neurons. Risperidone (0.03 and 0.1 microM) reversed amphetamine-induced inhibition of firing activity similarly in A9 and A10. The dopamine D2 receptor antagonist (-)sulpiride (0.05 and 1 microM) reversed the amphetamine (10 microM)-induced inhibition of firing activity in A9 and A10 neurons. The selective 5-HT(2A) receptor antagonist MDL 100907 (0.05 microM), strongly enhanced the reversal of amphetamine-induced inhibition by (-)sulpiride in A10, but its effectiveness was much smaller in A9 dopamine neurons. We conclude that 5-HT(2A) receptor antagonism enhanced reversal of amphetamine-induced inhibition by dopamine D2 antagonism in A10, suggesting that dopamine D(2) receptor antagonism combined with 5-HT(2A) receptor antagonism may play a role in antipsychotic drug atypicality.

Fast onset of dopamine uptake inhibition by intravenous cocaine

In vivo voltammetry in the nucleus accumbens of anesthetized rats was used to investigate the time of onset of dopamine uptake inhibition by intravenous cocaine. There is disagreement between behavioral and neurochemical studies concerning the time-course of cocaine effects. Because of the high temporal resolution of voltammetry, the processes of dopamine release and uptake could be temporally separated to make evaluation of cocaine effects on uptake easier to address. Within 4 s after intravenous cocaine administration (1.5 mg/kg) there was significant inhibition of dopamine uptake that reached a plateau in 20 s. The peak heights of electrically evoked dopamine signals were also rapidly increased by cocaine. The signals returned to baseline values within approximately 1 h. In parallel behavioral studies, locomotor activity was significantly increased within 5-6 s following intravenous infusion of cocaine. Here we demonstrate that intravenous cocaine administration begins inhibiting the uptake of dopamine within a few seconds. This is at least 10-fold faster than previous neurochemical estimates. The present findings may contribute to the understanding of the neurobiological mechanisms underlying the early behavioral responses to cocaine.

Neither non-contingent electric footshock nor administered corticosterone facilitate the acquisition of methamphetamine self-administration

Previous research has indicated a role for the hypothalamo-pituitary-adrenal (HPA) axis in the acquisition of intravenous cocaine self-administration since both exposure to stressors and exogenous injections of corticosterone facilitate this behavior. The present experiment was designed to determine whether electric footshock or pretreatment with corticosterone would produce similar effects on the acquisition of methamphetamine self-administration in male Wistar rats. Following initial food training, the rats were allowed to self-administer methamphetamine in ascending doses (0.0075-0.12 mg/kg/infusion) that were doubled weekly. Neither non-contingent electric footshock nor treatment with corticosterone (2.0 mg/kg, i.p.) affected the lowest dose at which the rats first acquired methamphetamine self-administration (0.015 mg/kg/infusion). The treatment groups all had similar inverted "U"-shaped acquisition curves typical of psychostimulants. Although these experiments do not indicate a major role for the HPA axis in the acquisition of methamphetamine self-administration, more studies need to be conducted to further evaluate the effects of the HPA axis on the acquisition of methamphetamine self-administration before a potential role can be ruled out.

Endobain E, a brain Na+, K+-ATPase inhibitor, decreases norepinephrine uptake in rat hypothalamus

The ability of an endogenous brain Na+, K+ -ATPase inhibitor, termed endobain E, to increase [3H]norepinephrine release in rat hypothalamus was previously reported. Endobain E effect on neurotransmitter uptake was studied by assaying [3H]norepinephrine uptake in rat hypothalamus preparations, to observe uptake inhibition, which reached 60% with endobain E equivalent to 100 mg fresh cerebral cortex, an effect achieved with 40 or 400 microM ouabain. Results support the proposal that endobain E behaves as an ouabain-like substance. Taken jointly results obtained on neurotransmitter release and uptake, the suggestion that endobain E may enhance norepinephrine availability in the synaptic gap and thus lead to an increase in noradrenergic activity is advanced.

Hyperforin depletes synaptic vesicles content and induces compartmental redistribution of nerve ending monoamines

Hyperforin, a phloroglucinol derivative found in Hypericum perforatum (St. John's wort) extracts has antidepressant properties in depressed patients. Hyperforin has a unique pharmacological profile and it inhibits uptake of biogenic monoamines as well as amino acid transmitters. We have recently showed that the monoamines uptake inhibition exerted by hyperforin is related to its ability to dissipate the pH gradient across the synaptic vesicle membrane thereby interfering with vesicular monoamines storage. In the present study we demonstrate that hyperforin induces dose-dependent efflux of preloaded [3H]5HT and [3H]DA from rat brain slices. Moreover, we show that hyperforin attenuates depolarization- dependent release of monoamines, while increasing monoamine release by amphetamine or fenfluramine. It is also demonstrated that preincubation of brain slices with reserpine is associated with dose- dependent blunting of efflux due to hyperforin. Our data indicate that hyperforin-induced efflux of [3H]5HT and [3H]DA reflect elevated cytoplasmic concentrations of the two monoamines secondary to the depletion of the synaptic vesicle content and the compartmental redistribution of nerve ending monoamines.

Effects of methamphetamine and lobeline on vesicular monoamine and dopamine transporter-mediated dopamine release in a cotransfected model system

Dopamine (DA) retention and drug-induced release kinetics were characterized in human embryonic kidney (HEK)-293 cells stably coexpressing the human DA transporter (hDAT) and human vesicular monoamine transporter (hVMAT2). Cofunction of hDAT and hVMAT2 caused greater retention of [3H]DA at 20 min (37 degrees C), or 45 min (22 degrees C) compared with cells that were treated with dihydrotetrabenazene (DHTB) to block the hVMAT2. In hDAT- and hVMAT2-coexpressing cells treated with DHTB during [3H]DA loading, methamphetamine (METH)-induced efflux was only 20% of preloaded [3H]DA, compared with 50 to 60% efflux in the absence of DHTB. Interestingly, the presence of DHTB (during release only) increased the potency and efficacy of METH at inducing [3H]DA release (without DHTB: EC50=33.8 microM, maximal release 51%; release with DHTB: EC50=3.2 microM, maximal release 61%), suggesting that the effects of METH and DHTB on vesicular storage are additive. High concentrations of lobeline induced a statistically significant release of [3H]DA from HEK-hDAT-hVMAT2 cells, but only in the absence of DHTB, suggesting an hVMAT2-mediated effect. Likewise, lobeline did not induce a significant release of [3H]DA from HEK-hDAT cells. The substrates DA and p-tyramine induced robust release of preloaded [3H]DA from cotransfected cells. Cocaine was somewhat effective at blocking substrate-induced [3H]DA efflux. These results suggest that coexpression of the hDAT and hVMAT2 can be used as a model system to distinguish functional pools of DA and to quantify differences in drug effects on DA disposition. In addition, cotransfected cells can be used to determine mechanisms of simultaneous drug interactions at multiple sites.

Environmental enrichment enhances sensitization to GBR 12935-induced activity and decreases dopamine transporter function in the medial prefrontal cortex

Rats raised in an enriched condition (EC) during development display increased hyperactivity to the effect of acute amphetamine compared to rats raised in an impoverished condition (IC). The present study determined whether environmental enrichment differentially alters the effects of GBR 12935 administration, a selective dopamine transporter (DAT) inhibitor. Acutely, EC rats showed a greater, dose-dependent GBR 12935-induced increase in activity compared to IC rats; however, basal activity for EC rats was lower than for IC rats. After repeated GBR 12935, only EC rats exhibited behavioral sensitization. Kinetic analysis of DAT function in medial prefrontal cortex (mPFC) revealed that the maximal velocity of [3H]dopamine ([3H]DA) uptake in EC rats was less than in IC rats (4.9 +/- 0.6 and 7.7 +/- 0.6 pmol/min/mg, respectively), but not in striatum or nucleus accumbens. Furthermore, GBR 12935-induced inhibition of DAT function, [3H]GBR 12935 binding density and DA content in mPFC, striatum and nucleus accumbens were not different between EC and IC rats. However, dihydroxyphenylacetic acid content in mPFC was lower in EC than IC rats, whereas no differences were found in striatum and nucleus accumbens. These results suggest that EC-induced changes in activity may be due to decreased DAT function and decreased DA metabolism in the mPFC.

Real-time decoding of dopamine concentration changes in the caudate-putamen during tonic and phasic firing

The fundamental process that underlies volume transmission in the brain is the extracellular diffusion of neurotransmitters from release sites to distal target cells. Dopaminergic neurons display a range of activity states, from low-frequency tonic firing to bursts of high-frequency action potentials (phasic firing). However, it is not clear how this activity affects volume transmission on a subsecond time scale. To evaluate this, we developed a finite-difference model that predicts the lifetime and diffusion of dopamine in brain tissue. We first used this model to decode in vivo amperometric measurements of electrically evoked dopamine, and obtained rate constants for release and uptake as well as the extent of diffusion. Accurate predictions were made under a variety of conditions including different regions, different stimulation parameters and with uptake inhibited. Second, we used the decoded rate constants to predict how heterogeneity of dopamine release and uptake sites would affect dopamine concentration fluctuations during different activity states in the absence of an electrode. These simulations show that synchronous phasic firing can produce spatially and temporally heterogeneous concentration profiles whereas asynchronous tonic firing elicits uniform, steady-state dopamine concentrations.

Distinct molecular recognition of psychostimulants by human and Drosophila serotonin transporters

In this study, human embryonic kidney (HEK)-293 cells stably expressing human, Drosophila, or a chimeric serotonin (5-hydroxytryptamine, 5-HT) transporter (hSERT, dSERT, and H(1-281)D(282-476)H(477-638), respectively) were used to explore the ability of two libraries of structurally distinct psychostimulants to inhibit 5-HT uptake. One library consisted of 3-phenyltropane analogs, whereas the second library consisted of several substituted amphetamines. hSERT exhibited a lower K(i) value for all the compounds in both libraries compared with dSERT, whereas the chimeric SERT exhibited properties more closely resembling those of dSERT. This species selectivity was explored using computer-generated comparative molecular field analysis to model the interactions of the cocaine analogs and substituted amphetamines at hSERT, dSERT, and the cross-species chimera. Models for the 3-phenyltropane analogs indicate that a region exists around the aromatic ring where decreased electron density is favored, particularly for hSERT. This finding may indicate pi-pi stacking with an aromatic amino acid residue in SERT. Also, electronegative substituents in the 4'-position provide favorable interactions. This structural feature was demonstrated by increased potency of analogs with electronegative substituents on the aromatic ring that withdraw electron density. For the substituted amphetamines, key areas for interaction exist around the amine, an electrostatic component surrounding the 3-position on the aromatic ring, and a steric component surrounding the 4-position.

The effect of phosphorylation on amphetamine-mediated outward transport

Amphetamine elicits its locomotor-activating and drug-reinforcing effects by releasing the catecholamines dopamine and norepinephrine into the synapse. Amphetamine is a substrate of the plasmalemmal transporters for both dopamine and norepinephrine. As such, it binds to the transporters in conjunction with Na+ and Cl-, facilitating a conformational change leading the transporter to face inward. The subsequent binding of intracellular catecholamine results in an outward transport and release of the catecholamine into the synapse. Both inward and outward transport through the catecholamine transporters are regulated by protein kinases, particularly protein kinase C, but the effect of the enzyme on the two processes appears to be asymmetric. The purpose of this review is to discuss the evidence showing that protein kinase C activation facilitates outward transport through the catecholamine plasmalemmal transporters which may mediate amphetamine action in intact tissue.

Presynaptic regulation of dopaminergic neurotransmission

The development of electrochemical recordings with small carbon-fiber electrodes has significantly advanced the understanding of the regulation of catecholamine transmission in various brain areas. Recordings in vivo or in slice preparations monitor diffusion of catecholamine following stimulated synaptic release into the surrounding tissue. This synaptic 'overflow' is defined by the amount of release, by the activity of reuptake, and by the diffusion parameters in brain tissue. Such studies have elucidated the complex regulation of catecholamine release and uptake, and how psychostimulants and anti-psychotic drugs interfere with it. Moreover, recordings with carbon-fiber electrodes from cultured neurons have provided analysis of catecholamine release and its plasticity at the quantal level.

Effect of amphetamine and phencyclidine on DNA-binding activities of serum response and dyad symmetry elements

Acute administration of D-amphetamine sulphate (AMPH) and (1-[1-phenylcyclohexyl]piperidine hydrochloride) (phencyclidine; PCP) produces a characteristic spatio-temporal distribution of c-Fos protein in the brain. As transcriptional mechanisms underlying the induction of c-fos gene expression may be regulated in a stimulus-specific manner, we have analyzed the binding activities of serum response element (SRE), dyad symmetry element (DSE) and calcium response element (CRE), the major regulatory sites of the c-fos promoter. Electrophoretic mobility shift showed that SRE binding activity was increased for 50-60%, 2-6h after AMPH, while treatment with PCP resulted in light decrease of SRE binding activity throughout the same time period. Co-administration of AMPH and PCP induced gradual increase of SRE binding activity, reaching maximum (86%) at 6h. Binding of nuclear proteins to DSE sequence was increased 1-2h after administration of AMPH (72-87%) and remained elevated till the end of the time window observed. PCP and AMPH/PCP caused different temporal profile of DSE binding with peak (40-54%) 4-6h after administration. In contrast, DNA-binding activity of the CRE sequences remained unchanged throughout the time period of 6h under all conditions. Finally, supershift analysis clearly demonstrated presence of SRF and c-Fos protein in the transcriptional complexes bound to SRE and DSE sequences irrespective to AMPH, PCP or combined treatment. These findings also showed that the presence of c-Fos protein in SRE and DSE nucleocomplex support the hypothesis concerning autoregulation of c-fos gene expression during psychostimulant action in vivo.

Birth insult interacts with stress at adulthood to alter dopaminergic function in animal models: possible implications for schizophrenia and other disorders

Altered subcortical dopaminergic activity is thought to be involved in the pathophysiology of several disorders including schizophrenia, substance abuse and attention deficit hyperactivity disorder. Epidemiological studies have implicated perinatal insults, particularly obstetric complications involving fetal or neonatal hypoxia, as etiological risk factors for schizophrenia. This suggests the possibility that perinatal hypoxia might have lasting effects on dopaminergic function. In animal models, dopaminergic systems appears to be particularly vulnerable to a wide range of perinatal insults, resulting in persistent alterations in function of mesolimbic and mesostriatal pathways. This review summarizes recent work characterizing long-term changes in dopaminergic function and biochemistry in models of Caesarean section (C-section) birth and of C-section birth with added global anoxia in the rat and guinea pig. C-section birth and C-section with anoxia appear to be two distinct hypoxic birth insults, with somewhat differing patterns of lasting effects on dopamine systems. In addition, birth insult alters the manner in which dopaminergic function is regulated by stress at adulthood. The possible relevance of these finding to effects of human birth procedures is discussed.

Dopamine efflux via wild-type and mutant dopamine transporters: alanine substitution for proline-572 enhances efflux and reduces dependence on extracellular dopamine, sodium and chloride concentrations

The dopamine transporter (DAT) can mediate not only inward uptake of dopamine, but also its outward efflux by mechanisms that have been only partially elucidated. DAT-dependent dopamine efflux can be studied kinetically and apparent substrate affinity and V(max) values determined. We now report that wild-type DAT displays apparent affinities for efflux more than 300-fold lower than those for uptake. Efflux rates are enhanced by increased extracellular concentrations of dopamine or amphetamine and by lowered extracellular concentrations of Na(+) or Cl(-). Alanine substitutions for six proline residues located in or near DAT transmembrane domains increase apparent affinity and decrease V(max) values for dopamine efflux mediated by these mutant transporters. Mutant 12P572A displays increased DAT efflux with reduced dependence on ion or dopamine concentrations. These data add to evidence for the specificity of transporter-mediated efflux processes and begin to elucidate DAT candidate domains that may be preferentially involved with efflux activities.

Methamphetamine-induced degeneration of dopaminergic neurons involves autophagy and upregulation of dopamine synthesis

Methamphetamine (METH) selectively injures the neurites of dopamine (DA) neurons, generally without inducing cell death. It has been proposed that METH-induced redistribution of DA from the vesicular storage pool to the cytoplasm, where DA can oxidize to produce quinones and additional reactive oxygen species, may account for this selective neurotoxicity. To test this hypothesis, we used mice heterozygous (+/-) or homozygous (-/-) for the brain vesicular monoamine uptake transporter VMAT2, which mediates the accumulation of cytosolic DA into synaptic vesicles. In postnatal ventral midbrain neuronal cultures derived from these mice, METH-induced degeneration of DA neurites and accumulation of oxyradicals, including metabolites of oxidized DA, varied inversely with VMAT2 expression. METH administration also promoted the synthesis of DA via upregulation of tyrosine hydroxylase activity, resulting in an elevation of cytosolic DA even in the absence of vesicular sequestration. Electron microscopy and fluorescent labeling confirmed that METH promoted the formation of autophagic granules, particularly in neuronal varicosities and, ultimately, within cell bodies of dopaminergic neurons. Therefore, we propose that METH neurotoxicity results from the induction of a specific cellular pathway that is activated when DA cannot be effectively sequestered in synaptic vesicles, thereby producing oxyradical stress, autophagy, and neurite degeneration.

Food restriction modulates amphetamine-conditioned place preference and nucleus accumbens dopamine release in the rat

Food restriction has been shown to increase self-administration of psychostimulants, including cocaine and amphetamine (AMPH). Consistent with this, food-restricted rats are more sensitized to the rewarding effects of cocaine as measured by conditioned place preference (CPP). This study investigated whether moderate food restriction in rats (15 g/day) results in an increased CPP, relative to ad libitum-fed controls, to a second psychostimulant, AMPH. Conditioning trials consisted of six alternating injections of i.p. AMPH (0.425-6.8 mg/kg) and i.p. saline, paired with distinct environments. On Day 7, a drug-free 20-min choice test for environment was carried out to assess CPP. 0.85 mg/kg AMPH significantly increased CPP in food-restricted vs. ad libitum-fed rats. At 1.7 and 3.4 mg/kg AMPH, food-restricted rats showed decreased CPP, but increased locomotor activity, relative to ad libitum fed controls. To evaluate whether an alteration in extracellular fluid DA levels in the nucleus accumbens (NAc) core could account for these behavioral alterations, DA release was measured by microdialysis. DA release to a single acute i.p. injection of either 0.85 or 1.7 mg/kg AMPH was comparable in food-restricted and ad libitum fed rats. However, ad libitum fed rats demonstrated conditioned DA release after an AMPH conditioning paradigm analogous to the CPP paradigm, whereas food-restricted rats demonstrated no conditioned DA release. In conclusion, altered DA release in the nucleus accumbens core is not a primary effect of moderate food restriction and cannot completely account for either the altered CPP behavior or enhanced locomotor activity observed in this study.

Dopamine autoreceptor regulation of release and uptake in mouse brain slices in the absence of D(3) receptors

The effects of the dopamine D(3) receptor, a putative autoreceptor, have been investigated by comparing behavioral and neurochemical properties of wild-type mice and mice with a genetic deletion of the D(3) receptor. The D(3) knock-out mice were modestly hyper-responsive to a novel environment relative to wild-type mice, and, consistent with this, quantitative in vivo microdialysis revealed elevated striatal dopamine extracellular levels. The dynamic actions of autoreceptors on electrically evoked dopamine release were examined in striatal brain slices from these animals and monitored with fast scan cyclic voltammetry at carbon-fiber microelectrodes. Quinpirole, a dopamine receptor agonist with potency at both D(2) and D(3) receptors, inhibited evoked dopamine in a dose-dependent manner with a slightly higher dose required in the knock-out animals (EC(50) of 60+/-10 nM in wild-type animals and 130+/-40 in D(3) knock-out animals; both curves had a Hill slope near 2). Dopamine synthesis inhibition with alpha-methyl-p-tyrosine caused released dopamine levels to decrease in each genotype. However, regulation of secretion by autoreceptors was still operant. Dose-response curves to quinpirole were unchanged in D(3) knock-out tissue, but secretion-regulated release exhibited a Hill slope decreased to 1 in the wild-type animals. In both genotypes, similar quinpirole-evoked increases in uptake rate were evident following synthesis inhibition. These data are consistent with the D(3) receptor having a small but significant role as a dopamine autoreceptor that partially regulates secretion, but not synthesis, in the caudate-putamen.

Release and uptake of catecholamines in the bed nucleus of the stria terminalis measured in the mouse brain slice

The release and clearance of electrically evoked catecholamine (CA) in the ventral portion of the bed nucleus of the stria terminalis (BSTV) in mouse brain slices was evaluated with fast-scan cyclic voltammetry at carbon-fiber microelectrodes (CFME). Uptake in this region was observed to be markedly slower than in the caudate putamen (CPu). Clearance rates were reduced in the BSTV in both norepinephrine transporter knockout (NET KO) and dopamine transporter knockout (DAT KO) mice when compared to results in wild-type (WT) mice. However, uptake was faster in the BSTV in both the DAT and NET KO mice than in the CPu of DAT KO mice. This indicates that both transporters play a role in CA clearance in the BSTV. The transporters restrict extracellular CA to the general area of the BSTV, as revealed by the diminished signal as the CFME is moved sequentially further and further from the site where CA release is evoked. However, in slices from the DAT KOs and NET KOs, CA release could be observed outside of the BSTV region during such experiments. These results show that the low rate uptake in the BSTV facilitates extrasynaptic diffusion of catecholamine, but that uptake still regulates and limits the range of the transmitter to the region. Slower clearance from the extracellular fluid allows the released CA to act as a volume transmitter and diffuse to distant sites within the region to exert its neurochemical action.

Determination of release and uptake parameters from electrically evoked dopamine dynamics measured by real-time voltammetry

Quantifying mechanisms underlying extracellular signaling by the neurotransmitter dopamine (DA) is a difficult task, particularly in the complex extracellular microenvironment of the intact brain. In this study, two methods for evaluating release and uptake from DA dynamics monitored by real-time voltammetry are described. Both are based on a neurochemical model characterizing electrically evoked levels of DA as a balance between these opposing mechanisms. The theoretical basis of what is called here nonlinear regression and single curve analyses is given. Fitting simulated data tests the reliability of the methods. The two analyses are also compared with an experimental data set describing the effects of pharmacologically inhibiting the DA transporter in the caudate-putamen (CP) and nucleus accumbens (NAc). The results indicate that nonlinear regression and single curve analyses are suitable for quantifying release and uptake mechanisms underlying DA neurotransmission. Additionally, the most important experimental finding of this technical study was the independent confirmation of high affinity (approximately 0.2 microM) DA uptake in the intact striatum.

Amphetamine distorts stimulation-dependent dopamine overflow: effects on D2 autoreceptors, transporters, and synaptic vesicle stores

Amphetamine (AMPH) is known to raise extracellular dopamine (DA) levels by inducing stimulation-independent DA efflux via reverse transport through the DA transporter and by inhibiting DA re-uptake. In contrast, recent studies indicate that AMPH decreases stimulation-dependent vesicular DA release. One candidate mechanism for this effect is the AMPH-mediated redistribution of DA from vesicles to the cytosol. In addition, the inhibition of stimulation-dependent release may occur because of D2 autoreceptor activation by DA that is released via reverse transport. We used the D2 receptor antagonist sulpiride and mice lacking the D2 receptor to address this issue. To evaluate carefully AMPH effects on release and uptake, we recorded stimulated DA overflow in striatal slices by using continuous amperometry and cyclic voltammetry. Recordings were fit by a random walk simulation of DA diffusion, including uptake with Michaelis-Menten kinetics, that provided estimates of DA concentration and uptake parameters. AMPH (10 microm) promoted the overflow of synaptically released DA by decreasing the apparent affinity for DA uptake (K(m) increase from 0.8 to 32 microm). The amount of DA released per pulse, however, was decreased by 82%. This release inhibition was prevented partly by superfusion with sulpiride (47% inhibition) and was reduced in D2 mutant mice (23% inhibition). When D2 autoreceptor activation was minimal, the combined effects of AMPH on DA release and uptake resulted in an enhanced overflow of exocytically released DA. Such enhancement of stimulation-dependent DA overflow may occur under conditions of low D2 receptor activity or expression, for example as a result of AMPH sensitization.