Differential time-course profiles of dopamine release and uptake changes induced by three dopamine uptake inhibitors

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

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

Characterization of dopamine releasable and reserve pools in Drosophila larvae using ATP/P2X2 -mediated stimulation

Dopaminergic signaling pathways are conserved between mammals and Drosophila, but the factors important for maintaining the functional pool of synaptic dopamine are not fully understood in Drosophila. In this study, we characterized the releasable and reserve dopamine pools in Drosophila larvae using ATP/P2X2 -mediated stimulation. Dopamine release was stable with stimulations performed at least every 5 min but decayed with stimulations performed 2 min apart or less, indicating the replenishment of the releasable pool occurred on a time scale between 2 and 5 min. Dopamine synthesis or uptake was pharmacologically inhibited with 3-iodotyrosine and cocaine, respectively, to evaluate their contributions to maintain the releasable dopamine pool. We found that both synthesis and uptake were needed to maintain the releasable dopamine pool, with synthesis playing a major part in long-term replenishment and uptake being more important for short-term replenishment. These effects of synthesis and uptake on different time scales in Drosophila are analogous to mammals. However, unlike in mammals, cocaine did not activate a reserve pool of dopamine in Drosophila when using P2X2 stimulations. Our study shows that both synthesis and uptake replenish the releasable pool, providing a better understanding of dopamine regulation in Drosophila. The maintenance of the releasable dopamine pool was examined in Drosophila larva. Both synthesis and uptake were needed to maintain the releasable dopamine pool, with synthesis being most important on a longer time scale and uptake on a shorter time scale. Dopamine release was stimulated by applying ATP which activated P2X2 channels specifically expressed in dopaminergic neurons.

Persistent conditioned place preference to cocaine and withdrawal hypo-locomotion to mephedrone in the flatworm planaria

The purpose of the present study was to determine the effects of exposure to cocaine and mephedrone on conditioned place preference (CPP) and locomotion in the flatworm planaria. Planaria were treated with either cocaine or mephedrone at 1 or 10 μM. Planaria were exposed to 15 min of drug in their non-preferred place (either a rough- or smooth-floored petri dish) on alternate days, and were exposed to normal water in their preferred place on the following day. There were 5 days of conditioning to drug. Planaria were then tested for CPP on day 2, 6 and 13 after withdrawal. We found that animals exhibited CPP to cocaine at both 1 and 10 μM, but not to mephedrone. When examining locomotor activity we found that neither cocaine nor mephedrone treatment showed any evidence of evoking increased motility or locomotor sensitisation. Hypo-motility was seen on the first day of conditioning at concentrations of 10 μM for both cocaine and mephedrone, but had disappeared by the last day of conditioning. Examining chronic withdrawal, only 10 μM mephedrone had a significant effect on motility, decreasing locomotion on day 2 of withdrawal. Taken together we have shown that cocaine evoked CPP in planaria. We have also shown withdrawal depressing effects of mephedrone on motility.

Stimulant mechanisms of cathinones - effects of mephedrone and other cathinones on basal and electrically evoked dopamine efflux in rat accumbens brain slices

Mephedrone, an erstwhile "legal high", and some non-abused cathinones (ethcathinone, diethylpropion and bupropion) were tested for stimulant effects in vitro, through assessing their abilities to increase basal and electrically evoked dopamine efflux in rat accumbens brain slices, and compared with cocaine and amphetamine. We also tested mephedrone against cocaine in a dopamine transporter binding study. Dopamine efflux was electrically evoked and recorded using voltammetry in the rat accumbens core. We constructed concentration response curves for these cathinones for effects on basal dopamine levels; peak efflux after local electrical stimulation and the time-constant of the dopamine decay phase, an index of dopamine reuptake. We also examined competition between mephedrone or cocaine and [(125)I]RTI121 at the dopamine transporter. Mephedrone was less potent than cocaine at displacing [(125)I]RTI121. Mephedrone and amphetamine increased basal levels of dopamine in the absence of electrical stimulation. Cocaine, bupropion, diethylpropion and ethcathinone all increased the peak dopamine efflux after electrical stimulation and slowed dopamine reuptake. Cocaine was more potent than bupropion and ethcathinone, while diethylpropion was least potent. Notably, cocaine had the fastest onset of action. These data suggest that, with respect to dopamine efflux, mephedrone is more similar to amphetamine than cocaine. These findings also show that cocaine was more potent than bupropion and ethcathinone while diethylpropion was least potent. Mephedrone's binding to the dopamine transporter is consistent with stimulant effects but its potency was lower than that of cocaine. These findings confirm and further characterize stimulant properties of mephedrone and other cathinones in adolescent rat brain.

Glutamate and dopamine transmission from midbrain dopamine neurons share similar release properties but are differentially affected by cocaine

Synaptic transmission between ventral tegmental area and nucleus accumbens (NAc) is critically involved in reward-motivated behaviors and thought to be altered in addiction. In addition to dopamine (DA), glutamate is packaged and released by a subset of mesolimbic DA neurons, eliciting EPSCs onto medium spiny neurons in NAc. Little is known about the properties and modulation of glutamate release from DA midbrain terminals and the effect of cocaine. Using an optogenetic approach to selectively activate midbrain DA fibers, we compared the properties and modulation of DA transients and EPSCs measured using fast-scan cyclic voltammetry and whole-cell recordings in mouse brain slices. DA transients and EPSCs were inhibited by DA receptor D2R agonist and showed a marked paired-pulse depression that required 2 min for full recovery. Cocaine depressed EPSCs amplitude by 50% but enhanced the overall DA transmission from midbrain DA neurons. AMPA and NMDA receptor-mediated EPSCs were equally inhibited by cocaine, suggesting a presynaptic mechanism of action. Pharmacological blockage and genetic deletion of D2R in DA neurons prevented the cocaine-induced inhibition of EPSCs and caused a larger increase in DA transient peak, confirming the involvement of presynaptic D2R. These findings demonstrate that acute cocaine inhibits DA and glutamate release from midbrain DA neurons via presynaptic D2R but has differential overall effects on their transmissions in the NAc. We postulate that cocaine, by blocking DA reuptake, prolongs DA transients and facilitates the feedback inhibition of DA and glutamate release from these terminals.

The effects of benzofury (5-APB) on the dopamine transporter and 5-HT2-dependent vasoconstriction in the rat

5-APB, commonly marketed as 'benzofury' is a new psychoactive substance and erstwhile 'legal high' which has been implicated in 10 recent drug-related deaths in the UK. This drug was available on the internet and in 'head shops' and was one of the most commonly sold legal highs up until its recent UK temporary ban (UK Home Office). Despite its prominence, very little is known about its pharmacology. This study was undertaken to examine the pharmacology of 5-APB in vitro. We hypothesised that 5-APB would activate the dopamine and 5-HT systems which may underlie its putative stimulant and hallucinogenic effects. Autoradiographic studies showed that 5-APB displaced both [(125)I] RTI-121 and [(3)H] ketanserin from rat brain tissue suggesting affinity at the dopamine transporter and 5-HT2 receptor sites respectively. Voltammetric studies in rat accumbens brain slices revealed that 5-APB slowed dopamine reuptake, and at high concentrations caused reverse transport of dopamine. 5-APB also caused vasoconstriction of rat aorta, an effect antagonised by the 5-HT2A receptor antagonist ketanserin, and caused contraction of rat stomach fundus, which was reversed by the 5-HT2B receptor antagonist RS-127445. These data show that 5-APB interacts with the dopamine transporter and is an agonist at the 5-HT2A and 5-HT2B receptors in the rat. Thus 5-APB's pharmacology is consistent with it having both stimulant and hallucinogenic properties. In addition, 5-APB's activity at the 5-HT2B receptor may cause cardiotoxicity.

Amphetamine elicits opposing actions on readily releasable and reserve pools for dopamine

Amphetamine, a highly addictive drug with therapeutic efficacy, exerts paradoxical effects on the fundamental communication modes employed by dopamine neurons in modulating behavior. While amphetamine elevates tonic dopamine signaling by depleting vesicular stores and driving non-exocytotic release through reverse transport, this psychostimulant also activates phasic dopamine signaling by up-regulating vesicular dopamine release. We hypothesized that these seemingly incongruent effects arise from amphetamine depleting the reserve pool and enhancing the readily releasable pool. This novel hypothesis was tested using in vivo voltammetry and stimulus trains of varying duration to access different vesicular stores. We show that amphetamine actions are stimulus dependent in the dorsal striatum. Specifically, amphetamine up-regulated vesicular dopamine release elicited by a short-duration train, which interrogates the readily releasable pool, but depleted release elicited by a long-duration train, which interrogates the reserve pool. These opposing actions of vesicular dopamine release were associated with concurrent increases in tonic and phasic dopamine responses. A link between vesicular depletion and tonic signaling was supported by results obtained for amphetamine in the ventral striatum and cocaine in both striatal sub-regions, which demonstrated augmented vesicular release and phasic signals only. We submit that amphetamine differentially targeting dopamine stores reconciles the paradoxical activation of tonic and phasic dopamine signaling. Overall, these results further highlight the unique and region-distinct cellular mechanisms of amphetamine and may have important implications for its addictive and therapeutic properties.

Amphetamine augments vesicular dopamine release in the dorsal and ventral striatum through different mechanisms

Amphetamine has well-established actions on pre-synaptic dopamine signaling, such as inhibiting uptake and degradation, activating synthesis, depleting vesicular stores, and promoting dopamine-transporter reversal and non-exocytotic release. Recent in vivo studies have identified an additional mechanism: augmenting vesicular release. In this study, we investigated how amphetamine elicits this effect. Our hypothesis was that amphetamine enhances vesicular dopamine release in dorsal and ventral striata by differentially targeting dopamine synthesis and degradation. In urethane-anesthetized rats, we employed voltammetry to monitor dopamine, electrical stimulation to deplete stores or assess vesicular release and uptake, and pharmacology to isolate degradation and synthesis. While amphetamine increased electrically evoked dopamine levels, inhibited uptake, and up-regulated vesicular release in both striatal sub-regions in controls, this psychostimulant elicited region-specific effects on evoked levels and vesicular release but not uptake in drug treatments. Evoked levels better correlated with vesicular release compared with uptake, supporting enhanced vesicular release as an important amphetamine mechanism. Taken together, these results suggested that amphetamine enhances vesicular release in the dorsal striatum by activating dopamine synthesis and inhibiting dopamine degradation, but targeting an alternative mechanism in the ventral striatum. Region-distinct activation of vesicular dopamine release highlights complex cellular actions of amphetamine and may have implications for its behavioral effects.

Amphetamine paradoxically augments exocytotic dopamine release and phasic dopamine signals

Drugs of abuse hijack brain-reward circuitry during the addiction process by augmenting action potential-dependent phasic dopamine release events associated with learning and goal-directed behavior. One prominent exception to this notion would appear to be amphetamine (AMPH) and related analogs, which are proposed instead to disrupt normal patterns of dopamine neurotransmission by depleting vesicular stores and promoting nonexocytotic dopamine efflux via reverse transport. This mechanism of AMPH action, though, is inconsistent with its therapeutic effects and addictive properties, which are thought to be reliant on phasic dopamine signaling. Here we used fast-scan cyclic voltammetry in freely moving rats to interrogate principal neurochemical responses to AMPH in the striatum and relate these changes to behavior. First, we showed that AMPH dose-dependently enhanced evoked dopamine responses to phasic-like current pulse trains for up to 2 h. Modeling the data revealed that AMPH inhibited dopamine uptake but also unexpectedly potentiated vesicular dopamine release. Second, we found that AMPH increased the amplitude, duration, and frequency of spontaneous dopamine transients, the naturally occurring, nonelectrically evoked, phasic increases in extracellular dopamine. Finally, using an operant sugar reward paradigm, we showed that low-dose AMPH augmented dopamine transients elicited by sugar-predictive cues. However, operant behavior failed at high-dose AMPH, which was due to phasic dopamine hyperactivity and the decoupling of dopamine transients from the reward predictive cue. These findings identify upregulation of exocytotic dopamine release as a key AMPH action in behaving animals and support a unified mechanism of abused drugs to activate phasic dopamine signaling.

Harmine augments electrically evoked dopamine efflux in the nucleus accumbens shell

Harmine is a β-carboline alkaloid and major component of ayahuasca, a traditional South American psychoactive tea with anecdotal efficacy for treatment of cocaine dependence. Harmine is an inhibitor of monoamine oxidase A (MAO-A) and interacts in vitro with several pharmacological targets which modulate dopamine (DA) neurotransmission. In vivo studies have demonstrated dopaminergic effects of harmine, attributed to monoamine oxidase inhibitor (MAOI) activity, however none have directly demonstrated a pharmacological mechanism. This study investigated the acute effects, and pharmacological mechanism(s), of harmine on electrically evoked DA efflux parameters in the nucleus accumbens both in the absence and presence of cocaine. Fast cyclic voltammetry in rat brain slices was used to measure electrically evoked DA efflux in accumbens core and shell. Harmine (300 nM) significantly augmented DA efflux (148±8% of baseline) in the accumbens shell. Cocaine augmented efflux in shell additive to harmine (260±35%). Harmine had no effect on efflux in the accumbens core or on reuptake in either sub-region. The effect of harmine in the shell was attenuated by the 5-HT(2A/2C) antagonist ketanserin. The MAOI moclobemide (10 µM) had no effect on DA efflux. These data suggest that harmine augments DA efflux via a novel, shell-specific, presynaptic 5-HT(2A) receptor-dependent mechanism, independent of MAOI activity. A DA-releasing 'agonist therapy' mechanism may thus contribute to the putative therapeutic efficacy of ayahuasca for cocaine dependence.

Desoxypipradrol is more potent than cocaine on evoked dopamine efflux in the nucleus accumbens

Desoxypipradrol is a methylphenidate-like drug that has been recently found in a number of 'legal highs'. Evidence from emergency room toxicology reports suggests that this drug might have led to a number of psychotic events in drug abusers in the UK and elsewhere. However, very little research has been done on the effects of this drug on the brain. Here we used rat brain slices from the nucleus accumbens core, which were exposed to either cocaine (1, 3 or 10 µM) or desoxypipradrol (1, 3 or 10 µM) for 60 min. Dopamine efflux was electrically evoked and recorded using fast cyclic voltammetry. Both drugs increased the peak dopamine efflux and also slowed dopamine re-uptake. Desoxypipradrol was more potent than cocaine causing a sevenfold increase in peak dopamine levels (versus a threefold increase for cocaine) and increasing dopamine re-uptake half-life 15-fold (versus fivefold for cocaine). These data suggest that desoxypipradrol is more potent than cocaine at dopamine terminals, and this could account for its psychotogenic effects.

High doses of amphetamine augment, rather than disrupt, exocytotic dopamine release in the dorsal and ventral striatum of the anesthetized rat

High doses of amphetamine (AMPH) are thought to disrupt normal patterns of action potential-dependent dopaminergic neurotransmission by depleting vesicular stores of dopamine (DA) and inducing robust non-exocytotic DA release or efflux via dopamine transporter (DAT) reversal. However, these cardinal AMPH actions have been difficult to establish definitively in vivo. Here, we use fast-scan cyclic voltammetry (FSCV) in the urethane-anesthetized rat to evaluate the effects of 10 and 20 mg/kg AMPH on vesicular DA release and DAT function in dorsal and ventral striata. An equivalent high dose of cocaine (40 mg/kg) was also examined for comparison to psychostimulants acting preferentially by DAT inhibition. Parameters describing exocytotic DA release and neuronal DA uptake were determined from dynamic DA signals evoked by mild electrical stimulation previously established to be reinforcing. High-sensitivity FSCV with nanomolar detection was used to monitor changes in the background voltammetric signal as an index of DA efflux. Both doses of AMPH and cocaine markedly elevated evoked DA levels over the entire 2-h time course in the dorsal and ventral striatum. These increases were mediated by augmented vesicular DA release and diminished DA uptake typically acting concurrently. AMPH, but not cocaine, induced a slow, DA-like rise in some baseline recordings. However, this effect was highly variable in amplitude and duration, modest, and generally not present at all. These data thus describe a mechanistically similar activation of action potential-dependent dopaminergic neurotransmission by AMPH and cocaine in vivo. Moreover, DA efflux appears to be a unique, but secondary, AMPH action.

Reserpine differentially affects cocaine-induced behavior in low and high responders to novelty

Individuals are known to differ in their sensitivity to cocaine. Cocaine is known to inhibit the re-uptake of monoamines. The response to cocaine has also been found to depend on monoamines inside reserpine-sensitive storage vesicles. The present study examined the effects of reserpine (1-2 mg/kg) on cocaine-induced behavior (10-15 mg/kg) in Low Responders (LR) and High Responders (HR) to novelty rats. LR displayed less cocaine-induced walking, wall rearing, free rearing and stereotyped behavior than HR did. The dose of 1 mg/kg of reserpine decreased cocaine-induced walking, wall rearing, free rearing and stereotyped behavior in LR, but not in HR. A dose of 2 mg/kg of reserpine was required to inhibit cocaine-induced behavior in HR. Combining these behavioral findings with our previously reported neurochemical finding that a higher dose of reserpine was required to inhibit the accumbal dopamine response to cocaine in HR than in LR (Verheij et al., 2008), suggests that HR are more sensitive to the behavioral effects of cocaine than LR because cocaine can release more monoamines from storage vesicles in HR than in LR. Our behavioral data also demonstrate that the individual differences in sensitivity to reserpine are not only limited to the dopaminergic system of the nucleus accumbens.

Synapsins differentially control dopamine and serotonin release

Synapsins are a family of synaptic vesicle proteins that are important for neurotransmitter release. Here we have used triple knock-out (TKO) mice lacking all three synapsin genes to determine the roles of synapsins in the release of two monoamine neurotransmitters, dopamine and serotonin. Serotonin release evoked by electrical stimulation was identical in substantia nigra pars reticulata slices prepared from TKO and wild-type mice. In contrast, release of dopamine in response to electrical stimulation was approximately doubled in striatum of TKO mice, both in vivo and in striatal slices, in comparison to wild-type controls. This was due to loss of synapsin III, because deletion of synapsin III alone was sufficient to increase dopamine release. Deletion of synapsins also increased the sensitivity of dopamine release to extracellular calcium ions. Although cocaine did not affect the release of serotonin from nigral tissue, this drug did enhance dopamine release. Cocaine-induced facilitation of dopamine release was a function of external calcium, an effect that was reduced in TKO mice. We conclude that synapsins play different roles in the control of release of dopamine and serotonin, with release of dopamine being negatively regulated by synapsins, specifically synapsin III, while serotonin release appears to be relatively independent of synapsins. These results provide further support for the concept that synapsin function in presynaptic terminals varies according to the neurotransmitter being released.

Rats that differentially respond to cocaine differ in their dopaminergic storage capacity of the nucleus accumbens

Cocaine (COC) inhibits the re-uptake of dopamine. However, the dopamine response to COC also depends on dopamine inside storage vesicles. The aim of this study was to investigate whether rats that differentially respond to COC differ in their dopaminergic storage capacity of the nucleus accumbens. Total and vesicular levels of accumbal dopamine as well as accumbal vesicular monoamine transporter-2 levels were established in high (HR) and low responders (LR) to novelty rats. Moreover, the effects of reserpine (RES) on the COC-induced increase of extracellular accumbal dopamine were investigated. HR displayed higher accumbal levels of total and vesicular dopamine than LR. Moreover, HR displayed more accumbal vesicular monoamine transporters-2 than LR. COC increased extracellular accumbal dopamine more strongly in HR than in LR. A low dose of RES prevented the COC-induced increase of accumbal dopamine in LR, but not in HR. A higher dose of RES was required to inhibit the COC-induced increase of accumbal dopamine in HR. These data demonstrate that HR were marked by a larger accumbal dopaminergic storage pool than LR. It is hypothesized that HR are more sensitive to COC than LR, because COC can release more dopamine from accumbal storage vesicles in HR than in LR.

J. Neurochem. (2008) 105, 2122–2133.

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.

Effects of ginseng saponin on acute cocaine-induced alterations in evoked dopamine release and uptake in rat brain nucleus accumbens

In traditional medicine, Panax ginseng has been used to treat various behavioral effects of psychostimulants (e.g., cocaine) and other drugs of abuse and to ameliorate withdrawal symptoms. The neurochemical bases for this efficacy, however, remain to be elucidated. We previously used the real-time fast-scan cyclic voltammetry in rat nucleus accumbens slices to demonstrate that cocaine not only enhances DA release evoked by single-pulse electrical stimulation and inhibits DA uptake during application but also further increases the release upon washout (termed a "rebound" release enhancement). In the present study, we determined whether co-application and washout of ginseng total saponin (GTS), the active ingredient of Panax ginseng, with cocaine attenuate cocaine-induced enhancement of evoked DA release, DA uptake inhibition and/or withdrawal-associated rebound enhancement. Cocaine rapidly potentiated the DA release within the first 10 min of application, and acute cocaine withdrawal caused a rebound increase. Co-application of GTS with cocaine inhibited the release enhancement and subsequently prevented the rebound increase during acute withdrawal. The effect of GTS was concentration-dependent. In contrast, GTS had no significant effects on the cocaine-mediated DA uptake inhibition. These results suggest that the attenuation of the cocaine-induced enhancement of impulse-dependent DA release, rather than uptake inhibition, might be one of the pharmacological bases for attenuation of behavioral effects of cocaine and amelioration of acute withdrawal symptoms by ginseng.

Differential contribution of storage pools to the extracellular amount of accumbal dopamine in high and low responders to novelty: effects of reserpine

The present study examined the effects of reserpine on the extracellular concentration of accumbal dopamine in high responders (HR) and low responders (LR) to novelty rats. Reserpine reduced the baseline concentration of extracellular accumbal dopamine more in HR than in LR, indicating that the dopamine release is more dependent on reserpine-sensitive storage vesicles in non-challenged HR than in non-challenged LR. In addition, reserpine reduced the novelty-induced increase of the extracellular concentration of accumbal dopamine in LR, but not in HR, indicating that the dopamine release in response to novelty depends on reserpine-sensitive storage vesicles only in LR, not in HR. Our data clearly demonstrate that HR and LR differ in the characteristics of those monoaminergic storage vesicles that mediate accumbal dopamine release.

The role of peripheral and central sodium channels in mediating brain temperature fluctuations induced by intravenous cocaine

While cocaine's interaction with the dopamine (DA) transporter and subsequent increase in DA transmission are usually considered key factors responsible for its locomotor stimulatory and reinforcing properties, many centrally mediated physiological and psychoemotional effects of cocaine are resistant to DA receptor blockade, suggesting the importance of other non-DA mechanisms. To explore the role of cocaine's interaction with Na+ channels, rats were used to compare locomotor stimulatory and temperature (NAcc, temporal muscle and skin) effects of repeated iv injections of cocaine (1 mg/kg) with those induced by procaine (PRO 5 mg/kg), a short-acting local anesthetic with negligible effect on the DA transporter, and cocaine methiodide (COC-MET 1.31 mg/kg), a quaternary cocaine derivative that is unable to cross the blood-brain barrier. While PRO, unlike cocaine, did not induce locomotor activation, it mimicked cocaine in its ability to increase brain temperature following the initial injection and to induce biphasic, down-up fluctuations following repeated injections. This similarity suggests that both these effects of cocaine may be driven by its action on Na+ channels, a common action of both drugs. While COC-MET also did not affect locomotor activity, it shared with cocaine and PRO their ability to increase brain temperature but failed to induce temperature decreases after repeated injections. These findings point toward activation of peripheral Na+ channels as the primary mechanism of rapid excitatory effects of cocaine and inhibition of centrally located Na+ channels as the primary mechanism for transient inhibitory effects of cocaine. DA receptor blockade (SCH23390+eticlopride) fully eliminated locomotor stimulatory and temperature-increasing effects of cocaine, but its temperature-decreasing effects remained intact. Surprisingly, DA receptor blockade also altered the temperature fluctuations caused by PRO and COC-MET, suggesting that some of the central effects triggered via Na+ channels are in fact DA-dependent. Finally, repeated administration of PRO to animals that had previous cocaine experience led to conditioned locomotion and potentiated temperature-increasing effects of this drug. It appears, therefore, that, in addition to the central effects of cocaine mediated via interaction with the DA transporter and potentiation of DA uptake, interaction with peripheral and central Na+ channels is important for the initial physiological and, perhaps, affective effects of cocaine, likely contributing to the unique abuse potential of this drug.

Reduced psychostimulant effects on dopamine dynamics in the nucleus accumbens of mu-opioid receptor knockout mice

Dopamine neurotransmission in the nucleus accumbens plays a pivotal role in the reinforcing properties of drugs of abuse. Two interacting processes regulate nucleus accumbens dopamine overflow: release of dopamine from presynaptic terminals and the subsequent reuptake by dopamine transporters. Opioid neurotransmission, primarily through mu-opioid receptors has also been strongly implicated in drug reward. We have previously shown that mice lacking the mu-opioid receptor display decreased cocaine self-administration. In addition, we found decreased impulse activity of midbrain dopaminergic neurons and an increased GABAergic input to these neurons in mu-opioid receptor knockout mice. In the present study we investigated whether these changes in dopaminergic cell bodies are accompanied by altered dopamine dynamics at the terminal level. To that aim, we measured nucleus accumbens dopamine overflow using fast scan cyclic voltammetry. Our data demonstrate that in mu-opioid receptor knockout mice 1) the reuptake of dopamine in the nucleus accumbens is slower, and 2) the relative effect of cocaine and amphetamine on the reuptake of dopamine is smaller compared with wild type mice. These data provide a mechanism for the decreased reinforcing properties of cocaine observed in mu-opioid receptor knockout mice.

Cocaine increases dopamine release by mobilization of a synapsin-dependent reserve pool

Cocaine primarily exerts its behavioral effects by enhancing dopaminergic neurotransmission, amplifying dopamine-encoded sensorimotor integration. The presumed mechanism for this effect is inhibition of the dopamine transporter, which blocks dopamine uptake and prolongs the duration of dopamine in the extracellular space. However, there is growing evidence that cocaine can also augment dopamine release. Here, we directly monitored the actions of cocaine on dopamine release by using electrochemical detection to measure extracellular dopamine in the striatum of anesthetized mice. Cocaine enhanced the levels of striatal dopamine produced by electrical stimulation of dopaminergic neurons. Even after pretreatment with alpha-methyl-p-tyrosine, which depletes the readily releasable pool of dopamine, cocaine was still capable of elevating dopamine levels. This suggests that cocaine enhances dopamine release by mobilizing a reserve pool of dopamine-containing synaptic vesicles. To test this hypothesis, we examined electrically evoked dopamine release in synapsin I/II/III triple knock-out mice, which have impaired synaptic vesicle reserve pools. Knock-out of synapsins greatly reduced the ability of cocaine to enhance dopamine release with long stimulus trains or after depletion of the newly synthesized pool. We therefore conclude that cocaine enhances dopamine release and does so by mobilizing a synapsin-dependent reserve pool of dopamine-containing synaptic vesicles. This capacity to enhance exocytotic release of dopamine may be important for the psychostimulant actions of cocaine.

Acute and chronic continuous methamphetamine have different long-term behavioral and neurochemical consequences

We compared two different methamphetamine dosing regimens and found distinct long-term behavioral and neurochemical changes. Adult rats were treated with 1-day methamphetamine injection (3x5 mg/kg s.c., 3 h apart) or 7-day methamphetamine minipump (20 mg/kg/day s.c.). The minipump regimen models the sustained methamphetamine plasma levels in some human bingers whereas the 1-day regimen models a naive user overdose. On withdrawal days 7 and 28, rats were acutely challenged with cocaine to test for behavioral sensitization and subsequently sacrificed for caudate and accumbens dopamine tissue content. Other rats were analyzed on withdrawal days 3, 7 or 28 using voltammetry in caudate slices. On withdrawal days 7 and 28, the methamphetamine injection but not the minipump rats showed behavioral cross-sensitization to cocaine. There was no change in baseline dopamine release, reuptake or sensitivity to quinpirole in any treatment group on either withdrawal day. However, consistent with the behavioral sensitization, cocaine had a greater effect in potentiating dopamine release and in blocking dopamine reuptake in methamphetamine injection versus saline irrespective of withdrawal day. The minipump group showed tolerance to the dopamine releasing effect of cocaine on withdrawal day 28 and had lower dopamine tissue content in the caudate versus the methamphetamine injection group. Dopamine turnover as measured by the DOPAC/dopamine ratio tended to be higher in the minipump-treated rats. These data suggest that the behavioral cross-sensitization seen in the methamphetamine injection rats could be in part due to the increased potency of cocaine in blocking dopamine reuptake and in increasing dopamine release. The decreased potency of cocaine in the caudate slices from the minipump-treated group may be related to decreased dopamine tissue content.

In vivo interaction of cocaine with the dopamine transporter as measured by voltammetry

The goal of this review is to describe what the voltammetry technique tells us about cocaine-dopamine transporter (DAT) interactions and the subsequent changes in extracellular dopamine levels in the brain. The primary advantage of voltammetry, in this regard, is the capability for kinetic analysis in situ. Analysis of electrically evoked dynamics suggests that cocaine competitively inhibits dopamine uptake in the caudate-putamen and nucleus accumbens with a similar efficacy. The preferential increase in accumbal dopamine following systemic cocaine administration was found to be related not to a unique cocaine-DAT interaction, but rather to a unique combination of dopamine release and uptake rates. Similar enhancement occurs in sub-regions of the caudate-putamen exhibiting this release and uptake combination. Other factors such as diffusion and whether dopaminergic signaling is tonic or phasic also determine the effects of cocaine on striatal dopamine levels.

Kinetic analysis of striatal clearance of exogenous dopamine recorded by chronoamperometry in freely-moving rats

Previously, we developed technology that coupled high-speed chronoamperometry with microejections of dopamine (DA) to measure DA clearance in the brains of freely-behaving rats. Here, by varying the ejection volumes of DA across a 200-fold difference, the kinetics of striatal clearance were analyzed as a function of time and DA volume from 289 chronoamperometric signals (n=20 rats). Each DA clearance trace was fitted to a first-order exponential decay function to determine the rate constant for DA clearance (k). Additionally, the apparent Michaelis-Menten V(max) and K(m) kinetic constants were determined in freely-moving rats, enabling quantitative comparison of our values with other models of reuptake. The first-order rate constant for DA clearance, which reflects the V(max)/K(m) ratio or clearance efficiency, did not vary significantly when small volumes of DA were ejected resulting in peak DA signal amplitudes (A(max)) of <5 microM. However, following nomifensine-induced DAT inhibition, A(max) was increased and k was attenuated simultaneously with behavioral activation; and A(max) and behavior remained elevated beyond the initial period. Our results indicate that the analysis of kinetic parameters from chronoamperometric DA signals may be useful for investigating drug-induced regulation of DAT kinetics in relation to the behavior of freely-moving rats.