Amphetamine and Other Weak Bases Act to Promote Reverse Transport of Dopamine in Ventral Midbrain Neurons

Abstract: Amphetamine-like psychostimulants are thought to produce rewarding effects by increasing dopamine levels at mesolimbic synapses. Paradoxically, dopamine uptake blockers, which generally increase extracellular dopamine, inhibit amphetamine-induced dopamine overflow. This effect could be due to either inhibition of amphetamine uptake or inhibition of dopamine efflux through the transporter (reverse transport). We used weak bases and dopamine uptake blockers in ventral midbrain neuron cultures to separate the effects on blockade of amphetamine uptake from reverse transport of dopamine. Amphetamine, ammonium chloride, tributylamine, and monensin, at concentrations that produce similar reductions in acidic pH gradients, increased dopamine release. This effect was inhibited by uptake blockers. Although in the case of amphetamine the inhibition of release could have been due to blockade of amphetamine uptake, inhibition also occurred with weak bases that are not transporter substrates. This suggests that reduction of vesicular pH gradients increases cytoplasmic dopamine which in turn promotes reverse transport. Consistent with this model, extracellular 3,4-dihydroxyphenylacetic acid was increased by ammonium chloride and monensin, as would be expected with elevated cytoplasmic dopamine levels. These findings extend the weak base mechanism of amphetamine action, in which amphetamine reduces vesicular pH gradients resulting in increased cytoplasmic dopamine that promotes reverse transport.     

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.     

Carrier-Mediated Release, Transport Rates, and Charge Transfer Induced by Amphetamine, Tyramine, and Dopamine in Mammalian Cells Transfected with the Human Dopamine Transporter

Abstract: Amphetamine and related substances induce dopamine release. According to a traditional explanation, this dopamine release occurs in exchange for amphetamine by means of the dopamine transporter (DAT). We tested this hypothesis in human embryonic kidney 293 cells stably transfected with the human DAT by measuring the uptake of dopamine, tyramine, and d - and l -amphetamine as well as substrate-induced release of preloaded N -methyl-4-[³H]phenylpyridinium ([³H]MPP⁺). The uptake of substrates was sodium-dependent and was inhibited by ouabain and cocaine, which also prevented substrate-induced release of MPP⁺. Patch-clamp recordings revealed that all four substrates elicited voltage-dependent inward currents (on top of constitutive leak currents) that were prevented by cocaine. Whereas individual substrates had similar affinities in release, uptake, and patch-clamp experiments, maximal effects displayed remarkable differences. Hence, maximal effects in release and current induction were ∼25% higher for d -amphetamine as compared with the other substrates. By contrast, dopamine was the most efficacious substrate in uptake experiments, with its maximal initial uptake rate exceeding those of amphetamine and tyramine by factors of 20 and 4, respectively. Our experiments indicate a poor correlation between substrate-induced release and the transport of substrates, whereas the ability of substrates to induce currents correlates well with their releasing action.     

Mechanism of the inhibiting effect of atypical antidepressants and psychostimulants on the synaptosomal uptake of monoamines

The effects of viloxazine and zimelidine on reverse monoamine neurotransmitter uptake by crude synaptosomal fraction of the rat brain were studied and compared to those of imipramine, amphetamine and cocaine. Imipramine noncompetitively inhibited the uptake of all the monoamines under study, with a greater specificity as regards serotonin. Viloxazine and zimelidine strongly inhibited the transport of noradrenaline, dopamine and serotonine, the transport of the latter being inhibited to a greater degree. Kinetic analysis showed the active centers of noradrenaline and dopamine carriers to be very much alike, those of catecholamine and serotonin carriers to be less alike. The data obtained made it possible to describe (at least partly) the structure of the active centers of monoamine carriers and to specify the action modes of antidepressants and psychostimulants.     

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.     

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.     

Dopamine neuronal transport kinetics and effects of amphetamine

The dopamine (DA) transporter (DAT) regulates DA neurotransmission by recycling DA back into neurons. Drugs that interfere with DAT function, e.g., cocaine and amphetamine, can have profound behavioral effects. The kinetics of DA transport by DAT in isolated synaptosomal or single cell preparations have been previously studied. To investigate how DA transport is regulated in intact tissue and to examine how amphetamine affects the DAT, the kinetics of DA uptake by the DAT were examined in tissue slices of the mouse caudate-putamen with fast-scan cyclic voltammetry. The data demonstrate that inward DA transport is saturable and sodium-dependent. Elevated levels of cytoplasmic DA resulting from disruption of vesicular storage by incubation with 10 microM Ro 4-1284 did not generate DA efflux or decrease its uptake rate. However, incubation with 10 microM amphetamine reduced the net DA uptake rate and increased extracellular DA levels due to DA efflux through the DAT. In addition, a new, elevated steady-state level of extracellular DA was established after electrically stimulated DA release in the presence of amphetamine, norepinephrine, and exogenous DA. These results from intact tissue are consistent with a kinetic model of the DAT established in more purified preparations in which amphetamine and other transported substances make the inwardly facing DAT available for outward transport of intracellular DA.     

Dopamine modulates acute responses to cocaine, nicotine and ethanol in Drosophila

BACKGROUND: Drugs of abuse have a common property in mammals, which is their ability to facilitate the release of the neurotransmitter and neuromodulator dopamine in specific brain regions involved in reward and motivation. This increase in synaptic dopamine levels is believed to act as a positive reinforcer and to mediate some of the acute responses to drugs. The mechanisms by which dopamine regulates acute drug responses and addiction remain unknown. RESULTS: We present evidence that dopamine plays a role in the responses of Drosophila to cocaine, nicotine or ethanol. We used a startle-induced negative geotaxis assay and a locomotor tracking system to measure the effect of psychostimulants on fly behavior. Using these assays, we show that acute responses to cocaine and nicotine are blunted by pharmacologically induced reductions in dopamine levels. Cocaine and nicotine showed a high degree of synergy in their effects, which is consistent with an action through convergent pathways. In addition, we found that dopamine is involved in the acute locomotor-activating effect, but not the sedating effect, of ethanol. CONCLUSIONS: We show that in Drosophila, as in mammals, dopaminergic pathways play a role in modulating specific behavioral responses to cocaine, nicotine or ethanol. We therefore suggest that Drosophila can be used as a genetically tractable model system in which to study the mechanisms underlying behavioral responses to multiple drugs of abuse.     

Food reward and cocaine increase extracellular dopamine in the nucleus accumbens as measured by microdialysis

Dopamine was measured by microdialysis in the nucleus accumbens of freely moving rats while they experienced rewarding food, brain stimulation and drugs. Extracellular dopamine increased 37% when the animals pressed a lever for food reward. Electrical stimulation of a lateral hypothalamic feeding-reward (self-stimulation) site caused a similar increase in dopamine, with or without food. At the site in the nucleus accumbens where rats will administer amphetamine to themselves, injections of amphetamine or cocaine increased extracellular dopamine five-fold. Thus amphetamine and cocaine increase dopamine in a behavior reinforcement system which is normally activated by eating. Conversely, the release of dopamine by eating could be a factor in addiction to food.     

Energy dependence and functional reconstitution of the gamma-aminobutyric acid carrier from synaptic vesicles.

The energy dependence of gamma-aminobutyric acid (GABA) uptake was characterized in rat brain synaptic vesicles and in proteoliposomes reconstituted with a new procedure from vesicular detergent extracts. The proteoliposomes displayed high ATP-dependent GABA uptake activity with properties virtually identical to those of intact vesicles. GABA uptake was similar at chloride concentrations of 0 and 150 mM, i.e. conditions under which either the membrane potential (delta psi) or the pH difference (delta pH) predominates. Delta psi was gradually dissipated by increasing the concentration of SCN-. GABA uptake was reduced by 10 mM SCN-, showing less sensitivity to delta psi reduction than glutamate uptake but more than dopamine uptake. Dissipation of delta pH with NH+4 abolished GABA uptake at pH 7.3, whereas no significant inhibition occurred at pH 6.5. In contrast, dopamine uptake was inhibited more strongly, even at pH 6.5, and glutamate uptake was not reduced in either condition. We conclude that GABA uptake is driven by both components of the proton electrochemical gradient, delta pH and delta psi, and that this is different from the uptake of both dopamine and glutamate, which is more strongly dependent on delta pH and delta psi, respectively. Thus, our data suggest that GABA uptake is electrogenic and occurs in exchange for protons.     

Distinct temperature-dependent dopamine-releasing effect of drugs of abuse in the olfactory bulb

It was recently shown in the olfactory bulb (OB) that the response to olfactory stimulation might be related to local reinforcement mechanisms involved in discrimination of different odors. Therefore, it seemed interesting to study the effects of several drugs of abuse on the release of dopamine (DA) in the OB. Nicotine, amphetamine, 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy"), and cocaine at 37 degrees C increased the release of [3H] DA from olfactory bulb slice preparations of the rats. While nicotine, amphetamine, and MDMA directly evoked DA release, cocaine, by inhibiting the reuptake processes, enhanced the electrical stimulation-evoked release. At low temperature (17 degrees C), a condition in which the transmitter uptake carriers of the plasma membrane in both the normal and reverse mode of operation are inhibited, the nicotine-evoked [3H] DA release was potentiated, whereas those evoked by amphetamine and MDMA were inhibited. At low temperature the field stimulation-evoked [3H] DA release was potentiated, but under this condition cocaine failed to increase the release. Our results show that low temperature (a) increases the concentration of extracellular DA released by Ca(2+)-dependent vesicular exocytosis elicited by nicotine, (b) inhibits the extracellular Ca(2+)-independent amphetamine- and MDMA-induced release of DA that occurs by the reverse operation of membrane carriers transporting DA from the cytoplasm of presynaptic terminals to the extraneuronal space, and (c) does not alter the inhibitory effect of cocaine on DA uptake that increases the concentration of extracellular DA released by field stimulation. The findings that the drugs of abuse tested all enhanced the release of DA in the olfactory bulb suggest that local reinforcing mechanisms may also exist in this brain area. In addition, we also show that lowering the temperature in in vitro experiments is an easy and straightforward method for separating vesicular and cytoplasmic release of transmitters, and is suitable for studying the mechanism of catecholamine release evoked by drugs of abuse. This technique may be applicable in other neurochemical studies that need inhibition of the uptake carriers without the blockade of the ligand-gated ion channels caused by reuptake inhibitor drugs.     

Augmented behavioral response and enhanced synaptosomal calcium transport induced by repeated cocaine administration are decreased by calcium channel blockers

Recent studies suggest that calcium influx via L-type calcium channels is necessary for psychostimulant-induced behavioral sensitization. In addition, chronic amphetamine upregulates subtype Cav1.2-containing L-type calcium channels. In the present studies, we assessed the effect of calcium channel blockers (CCBs) on cocaine-induced behavioral sensitization and determined whether the functional activity of L-type calcium channels is altered after repeated cocaine administration. Rats were administered daily intraperitoneal injections of either flunarizine (40 mg/kg), diltiazem (40 mg/kg) or cocaine (20 mg/kg) and the combination of the CCBs and cocaine for 30 days. Motor activities were monitored on Day 1, and every 6th day during the 30-day treatment period. Daily cocaine administration produced increased locomotor activity. Maximal augmentation of behavioral response to repeated cocaine administration was observed on Day 18. Flunarizine pretreatment abolished the augmented behavioral response to repeated cocaine administration while diltiazem was less effective. Measurement of tissue monoamine levels on Day 18 revealed cocaine-induced increases in DA and 5-HT in the nucleus accumbens. By contrast to behavioral response, diltiazem was more effective in attenuating increases in monoamine levels than flunarizine. Cocaine administration for 18 days produced increases in calcium uptake in synaptosomes prepared from the nucleus accumbens and frontal cortex. Increases in calcium uptake were abolished by flunarizine and diltiazem pretreatment. Taken together, the augmented cocaine-induced behavioral response on Day 18 may be due to increased calcium uptake in the nucleus accumbens leading to increased dopamine (DA) and serotonin (5-HT) release. Flunarizine and diltiazem attenuated the behavioral response by decreasing calcium uptake and decreasing neurochemical release.     

Effects of Daily Cocaine and Morphine Treatment on Somatodendritic and Terminal Field Dopamine Release

Daily injections of cocaine or morphine into rodents produces behavioral sensitization such that the last daily injection results in a greater motor stimulant effect than the first injection. To evaluate a role for brain dopamine in behavioral sensitization to cocaine and morphine, tissue slices from the ventromedial mesencephalon (containing dopamine cell bodies), the nucleus accumbens, and striatum (dopamine terminal fields) were obtained from rats pretreated with daily cocaine, morphine, or saline 2-3 weeks earlier. When the tissue slices were depolarized by increasing potassium concentration in the superfusate, the release of endogenous dopamine from the ventromedial mesencephalon of cocaine- and morphine-pretreated rats was significantly decreased. In contrast, the release of dopamine from the nucleus accumbens and striatum was either unaltered or slightly enhanced in rats pretreated with cocaine and morphine. When dopamine was released by amphetamine, a significant decrease in dopamine release from the ventromedial mesencephalon of cocaine-pretreated rats was measured. No other significant changes were measured after amphetamine-induced release. It is postulated that the decrease in dopamine release from the ventromedial mesencephalon of cocaine- and morphine-sensitized rats results in less somatodendritic autoreceptor stimulation, and thereby produces an increase in dopamine neuronal activity.     

The Role of the Dopamine Transporter in the Effects of Amphetamine on Sleep and Sleep Architecture in Drosophila

The dopamine transporter (DAT) mediates the inactivation of released dopamine (DA) through its reuptake, and thereby plays an important homeostatic role in dopaminergic neurotransmission. Amphetamines exert their stimulant effects by targeting DAT and inducing the reverse transport of DA, leading to a dramatic increase of extracellular DA. Animal models have proven critical to investigating the molecular and cellular mechanisms underlying transporter function and its modulation by psychostimulants such as amphetamine. Here we establish a behavioral model for amphetamine action using adult Drosophila melanogaster. We use it to characterize the effects of amphetamine on sleep and sleep architecture. Our data show that amphetamine induces hyperactivity and disrupts sleep in a DA-dependent manner. Flies that do not express a functional DAT (dDAT null mutants) have been shown to be hyperactive and to exhibit significantly reduced sleep at baseline. Our data show that, in contrast to its action in control flies, amphetamine decreases the locomotor activity of dDAT null mutants and restores their sleep by modulating distinct aspects of sleep structure. To begin to explore the circuitry involved in the actions of amphetamine on sleep, we also describe the localization of dDAT throughout the fly brain, particularly in neuropils known to regulate sleep. Together, our data establish Drosophila as a robust model for studying the regulatory mechanisms that govern DAT function and psychostimulant action.

     

In Vitro Release of Endogenous Dopamine from the Striatum of the Weaver Mutant Mouse

The weaver mutant mouse has a genetically determined defect in the nigrostriatal dopaminergic system. The present study was undertaken to test the hypothesis that in the weaver mutant mouse, striatal nerve terminals undergo compensatory changes in response to this deficiency. To test this hypothesis, we studied the basal and stimulated release of dopamine from striatal slices of weaver mutant mice and matched controls. By using a superfusion system and concentrating the superfusate by passage over alumina, resting dopamine release could be determined in the weaver mutant despite the fact that striatal tissue content of dopamine in these mice is reduced by greater than 75% compared with control mice. Fractional resting release of dopamine in weaver striatal slices was significantly elevated compared with that in controls, suggesting that the release mechanisms in the weaver may be adapting to overcome the dopamine deficit. Potassium-evoked release (24 and 48 mM potassium) was not significantly different between the two genotypes. In contrast, amphetamine-evoked release (1 microM) was significantly greater in the weaver mice than in controls. In both genotypes, release evoked by amphetamine was completely inhibited by cocaine, implicating the dopamine uptake carrier in this release process. These findings suggest that fundamental differences in dopamine release mechanisms exist between weaver and control mice and support the hypothesis that compensatory mechanisms may develop in neurons in response to dopamine deficits.     

Paradoxical striatal cellular signaling responses to psychostimulants in hyperactive mice

Recent investigations have shown that three major striatal-signaling pathways (protein kinase A/DARPP-32, Akt/glycogen synthase kinase 3, and ERK) are involved in the regulation of locomotor activity by the monoaminergic neurotransmitter dopamine. Here we used dopamine transporter knock-out mice to examine which particular changes in the regulation of these cell signaling mechanisms are associated with distinct behavioral responses to psychostimulants. In normal animals, amphetamine and methylphenidate increase extracellular levels of dopamine, leading to an enhancement of locomotor activity. However, in dopamine transporter knock-out mice that display a hyperactivity phenotype resulting from a persistent hyperdopaminergic state, these drugs antagonize hyperactivity. Under basal conditions, dopamine transporter knock-out mice show enhanced striatal DARPP-32 phosphorylation, activation of ERK, and inactivation of Akt as compared with wild-type littermates. However, administration of amphetamine or methylphenidate to these mice reveals that inhibition of ERK signaling is a common determinant for the ability of these drugs to antagonize hyperactivity. In contrast, psychostimulants activate ERK and induce hyperactivity in normal animals. In hyperactive mice psychostimulant-mediated behavioral inhibition and ERK regulation are also mimicked by the serotonergic drugs fluoxetine and 5-carboxamidotryptamine, thereby revealing the involvement of serotonin-dependent inhibition of striatal ERK signaling. Furthermore, direct inhibition of the ERK signaling cascade in vivo using the MEK inhibitor SL327 recapitulates the actions of psychostimulants in hyperactive mice and prevents the locomotor-enhancing effects of amphetamine in normal animals. These data suggest that the inhibitory action of psychostimulants on dopamine-dependent hyperactivity results from altered regulation of striatal ERK signaling. In addition, these results illustrate how altered homeostatic state of neurotransmission can influence in vivo signaling responses and biological actions of pharmacological agents used to manage psychiatric conditions such as Attention Deficit Hyperactivity Disorder (ADHD).     

Changes in function and ultrastructure of striatal dopaminergic terminals that regenerate following partial lesions of the SNpc

Following partial substantia nigra lesions, remaining dopaminergic neurones sprout, returning terminal density in the dorsal striatum to normal by 16 weeks. This suggests regeneration and maintenance of terminal density is regulated to release appropriate levels of dopamine. This study examined the structure and function of these reinnervated terminals, defining characteristics of dopamine uptake and release, density and affinity of the dopamine transporter (DAT) and ultrastructural morphology of dopamine terminals in the reinnervated dorsal striatum. Finally, rotational behaviour of animals in response to amphetamine was examined 4 and 16 weeks after substantia nigra pars compacta (SNpc) lesions. Dopamine transport was markedly reduced 16 weeks after lesioning along with reduced density and affinity of DAT. Rate of dopamine release and peak concentration, measured electrochemically, was similar in lesioned and control animals, while clearance was prolonged after lesioning. Ultrastructurally, terminals after lesioning were morphologically distinct, having increased bouton size, vesicle number and mitochondria, and more proximal contacts on post-synaptic cells. After 4 weeks, tendency to rotate in response to amphetamine was proportional to lesion size. By 16 weeks, rotational behaviour returned to near normal in animals where lesions were less than 70%, although some animals demonstrated unusual rotational patterns at the beginning and end of the amphetamine effect. Together, these changes indicate that sprouted terminals are well compensated for dopamine release but that transport mechanisms are functionally impaired. We discuss these results in terms of implications for dyskinesia and other behavioural states.     

Proteins interacting with monoamine transporters: current state and future challenges

Plasma membrane and vesicular transporters for the biogenic amines, dopamine, norepinephrine, and serotonin, represent a group of proteins that play a crucial role in the regulation of neurotransmission. Clinically, mono amine transporters are the primary targets for the actions of many therapeutic agents used to treat mood disorders, as well as the site of action for highly addictive psychostimulants such as cocaine, amphetamine, methamphetamine, and 3,4-methylenedioxymethamphetamine. Over the past decade, the use of approaches such as yeast two-hybrid and proteomics has identified a multitude of transporter interacting proteins, suggesting that the function and regulation of these transporters are more complex than previously anticipated. With the increasing number of interacting proteins, the rules dictating transporter synthesis, assembly, targeting, trafficking, and function are beginning to be deciphered. Although many of these protein interactions have yet to be fully characterized, current knowledge is beginning to shed light on novel transporter mechanisms involved in monoamine homeostasis, the molecular actions of psychostimulants, and potential disease mechanisms. While future studies resolving the spatial and temporal resolution of these, and yet unknown, interactions will be needed, the realization that monoamine transporters do not work alone opens the path to a plethora of possible pharmacological interventions.     

Intranasal Administration of the Dopaminergic Agonists l-DOPA, Amphetamine, and Cocaine Increases Dopamine Activity in the Neostriatum: A Microdialysis Study in the Rat

Abstract: The effectiveness of intranasal drug administration to stimulate central neuronal systems is well known from drug addiction and has also been considered as an alternative pharmacokinetic approach to treat brain disorders such as Parkinson's disease. In the present study, the possible neurochemical effects of intranasal administration of the psychostimulants cocaine and amphetamine and of the antiparkinsonian drug l -DOPA were analyzed. By using in vivo microdialysis in the urethane-anesthetized rat, it was found that unilateral intranasal administration of either of the psychostimulants led to huge and rapid increases of extracellular dopamine levels in the neostriatum followed by decreases of its metabolites dihydroxyphenylacetic acid and homovanillic acid. Furthermore, intranasal administration of l -DOPA, but not of the saline vehicle, also led to increased extracellular levels of neostriatal dopamine and to increases of its metabolites. Because the effect of intranasal l -DOPA on neostriatal dopamine was observed only ipsilaterally but not contralaterally to the side of intranasal drug administration, it can be hypothesized that l -DOPA was not effective via passage through the circulation but may have acted through a neuronal or an extraneuronal route. These data provide neurochemical evidence that the intranasal route may not only be efficient in drug abuse, but may also be useful to target the brain therapeutically, as in the case of neurodegenerative brain disorders.