Evidence that amphetamine and Na+ gradient reversal increase striatal synaptosomal dopamine synthesis through carrier-mediated efflux of dopamine

Bath salts and synthetic cathinones: an emerging designer drug phenomenon

Synthetic cathinones are an emerging class of designer drugs abused for psychostimulant and hallucinogenic effects similar to cocaine, methylenedioxymethamphetamine (MDMA), or other amphetamines. Abuse of synthetic cathinones, frequently included in products sold as 'bath salts', became prevalent in early 2009, leading to legislative classification throughout Europe in 2010 and schedule I classification within the United States in 2011. Recent pre-clinical and clinical studies indicate that dysregulation of central monoamine systems is a principal mechanism of synthetic cathinone action and presumably underlie the behavioral effects and abuse liability associated with these drugs. This review provides insight into the development of synthetic cathinones as substances of abuse, current patterns of their abuse, known mechanisms of their action and toxicology, and the benefits and drawbacks of their classification.

Amphetamine and methamphetamine reduce striatal dopamine transporter function without concurrent dopamine transporter relocalization

Amphetamine (AMPH) and methamphetamine (METH) alter dopamine transporter (DAT) function. In vitro heterologous cell line and synaptosome studies demonstrate AMPH-induced DAT internalization, implicating relocalization in reduced DAT uptake following drug exposure. However, few studies have evaluated DAT localization following in vivo AMPH/METH administration. To determine DAT subcellular localization following drug administration, a centrifugation technique was developed to isolate striatal synaptosomal membrane and vesicle fractions. DAT was distributed between the synaptosomal membrane (60%) and endosomal vesicles (40%), and in vitro application of the protein kinase C activator phorbol 12-myristate 13-acetate to striatal synaptosomes caused DAT internalization into the vesicle fractions. In contrast, neither single nor repeated in vivo AMPH and/or METH administrations altered DAT localization 5, 15, 30, or 60 min post-treatment, despite reduced DAT uptake. Importantly, repeated METH injections uniformly decreased total DAT immunoreactivity within all fractions 7 days post-treatment. These findings suggest that factors other than internalization can contribute to the observed acute and persistent DAT dysfunction and dopaminergic deficits following in vivo AMPH or METH administration.

Characterization of interactions between phencyclidine and amphetamine in rodent prefrontal cortex and striatum: Implications in NMDA/glycine-site-mediated dopaminergic dysregulation and dopamine transporter function

N-Methyl-D-aspartate (NMDA) antagonists induced behavioral and neurochemical changes in rodents that serve as animal models of schizophrenia. Chronic phencyclidine (PCP, 15 mg/(kg day) for 3 weeks via Alzet osmotic pump) administration enhances the amphetamine (AMPH)-induced dopamine (DA) efflux in prefrontal cortex (PFC), similar to that observed in schizophrenia. NMDA/glycine-site agonists, such as glycine (GLY), administered via dietary supplementation, reverse the enhanced effect. The present study investigated mechanisms of glycine-induced reversal of PCP-induced stimulation of AMPH-induced DA release, using simultaneous measurement of DA and AMPH in brain microdialysate, as well as peripheral and tissue AMPH levels. PCP treatment, by itself, increased peripheral and central AMPH levels, presumably via interaction with hepatic enzymes (e.g. cytochrome P450 CYP2C11). GLY (16% diet) had no effect on peripheral AMPH levels in the presence of PCP. Nevertheless, GLY significantly reduced extracellular/tissue AMPH ratios in both PFC and striatum (STR), especially following PCP administration, suggesting a feedback mediated effect on the dopamine transporter. GLY also inhibited acute AMPH (5 mg/kg)-induced DA release in PFC, but not STR. These findings suggest that GLY may modulate DA release in brain by producing feedback regulation of dopamine transporter function, possibly via potentiation of NMDA-stimulated GABA release and presynaptic GABAB receptor activation. The present studies also demonstrate pharmacokinetic interaction between AMPH and PCP, which may be of both clinical and research relevance.

Age-specific behavioral responses to psychostimulants in mice

This study investigated the influence of age on the behavioral responses elicited by psychostimulants in male CD-1 mice. Behavioral activity including locomotion and stereotypy was measured following acute or repeated administration of cocaine, methylphenidate, amphetamine or saline to postweanling (24 days old), periadolescent (33 days old) and adult (60 days old) mice. Postweanling mice exhibited less total and ambulatory activity than periadolescent mice following a single acute injection of cocaine (20 or 30 and 30 mg/kg, respectively). Further, postweanling mice showed less total activity than both periadolescent and adult mice at a dose of 10 mg/kg methylphenidate. Less stereotypy was also seen in postweanling mice when compared to adolescent mice after 30 mg/kg amphetamine. Seven daily injections of cocaine resulted in a heightened behavioral response on day 7 as compared to day 1, indicative of behavioral sensitization in adult and periadolescent, but not postweanling mice. Repeated methylphenidate resulted in increased total activity in adult, but not periadolescent or postweanling mice. None of the animals were sensitized to the behavioral activating effects of amphetamine. The magnitude of behavioral response and the development of sensitization were dependent upon the age of the animal and the agent tested.

Changes in the striatal extracellular levels of dopamine and dihydroxyphenylacetic acid evoked by ammonia and N-methyl-D-aspartate: modulation by taurine

Acute hyperammonemia is associated with motor disturbances that are thought to involve striatal dopaminergic dysfunction. Discharge of striatal dopaminergic neurons is controlled by N-methyl-D-aspartate (NMDA) receptors, the excessive activation of which contributes to ammonia neurotoxicity. Here we show that ammonium chloride ("ammonia", extracellular concentration 5 mM) or NMDA (1 mM), when directly administered to the rat striatum via a microdialysis probe, evoke a prompt accumulation of dopamine (DA) in the microdialysates. However, while ammonia increases, NMDA decreases, the extracellular dihydroxyphenylacetate (DOPAC) level. The results point to the NMDA receptor-mediated enhancement of DA release and increased DA metabolism as two independent ways by which ammonia affects the striatal dopaminergic system. Taurine (extracellular concentration 10 mM) attenuated the NMDA- and ammonia-evoked DA release and ammonia-induced accumulation of DOPAC, reflecting two different neuroprotective mechanisms of this amino acid.

Direct effects of 3,4-methylenedioxymethamphetamine (MDMA) on serotonin or dopamine release and uptake in the caudate putamen, nucleus accumbens, substantia nigra pars reticulata, and the dorsal raphé nucleus slices

We examined the effects of pressure ejected 3, 4-methylenedioxymethamphetamine (MDMA) from a micropipette on direct chemically stimulated release, and on electrically stimulated serotonin (5-HT) or dopamine (DA) release in the caudate putamen (CPu), nucleus accumbens (NAc), substantia nigra pars reticulata (SNr), and the dorsal raphé nucleus (DRN) brain slices of rat, using fast cyclic voltammetry (FCV). MDMA is electroactive, oxidising at +1100 mV. When the anodic input waveform was reduced from +1.4 to +1.0 volt, MDMA was not electroactive. Using this waveform, pressure ejection of MDMA did not release 5-HT or DA in brain slices prepared from any of the nuclei studied. MDMA significantly potentiated electrically stimulated 5-HT release in the SNr and DA release in CPu. In the DRN or in the NAc, MDMA was without effect on peak electrically stimulated 5-HT or DA release. The rates of neurotransmitter uptake, expressed as t(1/2), were in all cases significantly decreased after MDMA. The results indicate that MDMA, unlike (+)amphetamine, is not as a releaser of DA or 5-HT, it is a potent inhibitor of both DA and 5-HT uptake.

Imaging synaptic neurotransmission with in vivo binding competition techniques: a critical review

Several groups have provided evidence that positron emission tomography (PET) and single-photon emission computed tomography (SPECT) neuroreceptor imaging techniques might be applied to measure acute fluctuations in dopamine (DA) synaptic concentration in the living human brain. Competition between DA and radioligands for binding to D2 receptor is the principle underlying this approach. This new application of neuroreceptor imaging provides a dynamic measurement of neurotransmission that is likely to be informative to our understanding of neuropsychiatric conditions. This article reviews and discusses the body of data supporting the feasibility and potential of this imaging paradigm. Endogenous competition studies performed in rodents, nonhuman primates, and humans are first summarized. After this overview, the validity of the model underlying the interpretation of these imaging data is critically assessed. The current reference model is defined as the occupancy model, since changes in radiotracer binding potential (BP) are assumed to be directly caused by changes in occupancy of D2 receptors by DA. Experimental data supporting this model are presented. The evidence that manipulation of DA synaptic levels induces change in the BP of several D2 radiotracers (catecholamines and benzamides) is unequivocal. The fact that these changes in BP are mediated by changes in DA synaptic concentration is well documented. The relationship between the magnitude of BP changes measured with PET or SPECT and the magnitude of changes in DA concentration measured by microdialysis supports the use of these noninvasive techniques to measure changes in neurotransmission. On the other hand, several observations remain unexplained. First, the amphetamine-induced changes in the BP of D2 receptor antagonists [123I]IBZM and [11C]raclopride last longer than amphetamine-induced changes in DA extracellular concentration. Second, nonbenzamide D2 receptor antagonists, such as spiperone and pimozide, are not affected by changes in DA release, or are affected in a direction opposite to that predicted by the occupancy model. Similar observations are reported with D1 radiotracers. These results suggest that the changes in BP following changes in DA concentration might not be fully accounted by a simple occupancy model. Specifically, the data are reviewed supporting that agonist-mediated receptor internalization might play an important role in characterizing receptor-ligand interactions. Finally, it is proposed that a better understanding of the mechanism underlying the effects observed with benzamides is essential to develop this imaging technique to other receptor systems.

DOI, a 5-HT2A/2C receptor agonist, potentiates amphetamine-induced dopamine release in rat striatum

The effects of (+-)-DOI (1-(2,5-dimethoxy-4-iodophenyl)-aminopropane) hydrochloride, a mixed 5-HT2A/2C receptor agonist, on the release of dopamine (DA) following D-amphetamine sulfate (AMP) or a DA D2 autoreceptor selective dose of (-)-apomorphine hydrochloride (APO), were investigated in rat striatum (STR) and nucleus accumbens (NAC), using in vivo microdialysis. AMP (1.0 mg/kg, s.c.) produced marked increases in extracellular DA levels in both the STR and the NAC whereas DOI (2.5 mg/kg, i.p.) alone had no significant effect on extracellular DA levels in either region. Pretreatment with DOI 30 min prior to AMP, further enhanced the AMP-induced increase in striatal extracellular DA levels. On the other hand, DOI pretreatment attenuated the APO (50 micrograms/kg, s.c.)-induced decrease in extracellular DA levels in the STR. Pretreatment with DOI did not affect the ability of either AMP or APO to modulate extracellular DA levels in the NAC. These results provide further evidence that 5-HT2A/2C receptors modulate the release mechanisms of DA in the STR. Possible mechanisms are discussed.

The role of 5-HT2A receptors in antipsychotic activity

The correlation between the clinical activity of antipsychotic agents and their affinity for the D2 dopamine receptor has been the mainstay of the hypothesis that schizophrenia is due to excessive dopaminergic function. More recently, the unique clinical profile of the atypical antipsychotic clozapine has been proposed to involve actions on additional receptor systems. In particular, the high affinity of clozapine for the 5HT2A receptor subtype has been suggested to contribute to its reduced side-effect liability, greater efficacy and its activity in therapy-resistant schizophrenia. We have used the highly selective 5-HT2A antagonist MDL 100,907 to explore the contribution of 5-HT2A receptor blockade to antipsychotic activity. Biochemical, electrophysiological and behavioral studies reveal that selective 5HT2A receptor antagonists have the preclinical profile of an atypical antipsychotic. The limited clinical evidence available also suggests that compounds producing 5-HT2A receptor blockade are effective, in particular, against the negative symptoms of schizophrenia.

Differential action of (+)-amphetamine on electrically evoked dopamine overflow in rat brain slices containing corpus striatum and nucleus accumbens

1. The effects of (+)-amphetamine on electrically evoked dopamine overflow were examined in the rat brain slice containing either anterior caudate putamen (aCPu) or nucleus accumbens (NAc), by fast cyclic voltammetry. 2. (+)-Amphetamine (1 microM) caused a time-dependent increase in the extracellular concentration of dopamine ([dopamine]ex) due to displacement of dopamine from terminal sites. After a 40 min superfusion, [dopamine]ex in the aCPu was 0.617 +/- 0.117 microM and in the NAc was 0.270 +/- 0.04 microM. Pretreatment with (-)-sulpiride (1 microM) did not affect this action of (+)-amphetamine. 3. (+)-Amphetamine (1 microM) exhibited a complex and time-dependent effect on electrically stimulated dopamine overflow, evoked by 1p, 4p/10 Hz and 20p/20 Hz. 4. In the aCPu, (+)-amphetamine (1 microM) resulted in attenuation of dopamine overflow due to 1p and 4p/10 Hz but potentiation of dopamine overflow to 20p/20 Hz. (-)-Sulpiride (1 microM) prior to (+)-amphetamine (1 microM) reversed the attenuation of dopamine overflow evoked by 1p and 4p/10 Hz but had no significant effect on dopamine overflow evoked by 20p/20 Hz. 5. (+)-Amphetamine (1 microM) potentiated dopamine overflow in the NAc to all three stimuli. (-)-Sulpiride (1 microM) prior to (+)-amphetamine (1 microM), resulted initially, in a further potentiation of overflow, followed by a time-dependent attenuation of dopamine overflow to all three stimuli. 6. t1/2, the rate of removal of [dopamine]ex following electrical stimulation was not significantly different in the aCPu and NAc for any of the stimulation conditions. After a 40 min superfusion with (+)-amphetamine (1 MicroM), t1/2 for ip, 4p/10 Hz and 20p/20 Hz was significantly increased in both the aCPuand NAc, the increase in t1/2 being significantly greater in the aCPu than in the NAc.7. In conclusion, this study indicates that the dopamine displacement and uptake inhibitory actions of(+ )-amphetamine result in complex and differential effects on electrically evoked dopamine overflow in the aCPu and NAc.

5-HT2 receptors exert a state-dependent regulation of dopaminergic function: studies with MDL 100,907 and the amphetamine analogue, 3,4-methylenedioxymethamphetamine

The highly selective 5-HT2 receptor antagonist, MDL 100,907, was used to explore the role of serotonin in the stimulation of dopaminergic function produced by the amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA). MDL 100,907 blocked MDMA-stimulated dopamine synthesis in vivo without affecting basal synthesis. The long-term deficits in 5-HT concentrations believed to be a consequence of MDMA-induced dopamine release were also blocked by MDL 100,907 over the same dose range. In vivo microdialysis confirmed that 5-HT2 receptor blockade with MDL 100,907 attenuated MDMA-induced increases in extracellular concentrations of striatal dopamine. In contrast to its effect on MDMA-induced synthesis, MDL 100,907 did not alter dopamine synthesis stimulated by haloperidol or reserpine. In vivo dopamine release produced by haloperidol was also unaffected by MDL 100,907. The results suggest a permissive role for 5-HT2 receptors in the activation of the dopamine system which occurs during states of high serotonergic activity or during conditions of elevated dopamine efflux with high D2 receptor occupancy.

Dopamine transmission in the initiation and expression of drug- and stress-induced sensitization of motor activity

Progress has been made over the last 10 years in determining the neural mechanisms of sensitization induced by amphetamine-like psychostimulants, opioids and stressors. Changes in dopamine transmission in axon terminal fields such as the nucleus accumbens appear to underlie the expression of sensitization, but the actions of drugs and stressors in the somatodendritic regions of the A10/A9 dopamine neurons seem critical for the initiation of sensitization. Manipulations that increase somatodendritic dopamine release and permit the stimulation of D1 dopamine receptors in this region induce changes in the dopamine system that lead to the development of long-term sensitization. However, it is not known exactly how the changes in the A10/A9 region are encoded to permit augmented dopamine transmission in the terminal field. One possibility is that the dopamine neurons of sensitized animals have become increasingly sensitive to excitatory pharmacological and environmental stimuli or desensitized to inhibitory regulation. Alternatively, changes in cellular activity or protein synthesis may result in a change in the presynaptic regulation of axon terminal dopamine release.

Effects of acute and chronic administration of cocaine on striatal uptake, compartmentalization and release of [3H]dopamine

The effects of acute and repetitive administration of cocaine were studied on several parameters associated with the uptake and release of [3H]dopamine ([3H]DA) in the striatum. It was found that repetitive administration of cocaine followed 7 days later by acute challenge with cocaine, produced an increase in the Vmax with no change in the affinity of the uptake carrier for either dopamine (DA) or cocaine. The intracellular compartmentalization of [3H]DA in synaptosomes was not altered by either acute or repeated treatment with cocaine. However, chronic administration of cocaine abolished the stimulatory effect that 1 microM amphetamine normally has on the efflux of [3H]DA from the fast pool in untreated synaptosomes. The K(+)-stimulated release of [3H]DA from slices of striatum was not affected by acute or chronically administered cocaine; however, chronically administered cocaine, plus acute challenge with cocaine potentiated the effect of amphetamine on the K(+)-induced release of [3H]DA. This was accompanied by a reduction of the effect of amphetamine on the spontaneous release of DA. In addition, chronically administered cocaine plus acute challenge with cocaine increased K(+)-stimulated release of [14C]acetylcholine [( 14C]ACh). These data suggest that repetitive administration of cocaine, in a regimen that elicits behavioral sensitization, alters the substrates through which amphetamine exerts its effects on the subcellular distribution and release of [3H]DA, and further, that challenge with cocaine of sensitized rats produces a compensatory increase in the uptake of [3H]DA that is correlated with increased depolarization-induced release of [14C]ACh.

Apomorphine does not alter amphetamine-induced dopamine release measured in striatal dialysates

Amphetamine facilitates the release of dopamine from nerve terminals, but the mechanisms underlying this effect have not been fully delineated. The present experiments were designed to test the extent to which amphetamine-induced dopamine release is dependent on impulse flow and autoreceptor function in dopaminergic neurons. Rats were pretreated with a low dose of apomorphine (0.05 mg/kg) to inhibit dopamine neuronal activity, and the striatal dopaminergic response to amphetamine (0.5 mg/kg) was assessed by in vivo dialysis in freely moving animals. Consistent with previous results, apomorphine alone substantially decreased, whereas amphetamine increased, striatal dialysate dopamine concentrations. However, whereas apomorphine pretreatment decreased the locomotor response to amphetamine, the amphetamine-induced increase in dialysate dopamine was unaffected. These results indicate that amphetamine-facilitated dopamine release is independent of neuronal firing and autoreceptor regulation, consistent with the putative accelerative exchange-diffusion mechanism of amphetamine-induced dopamine release. Other possible mechanisms underlying the inhibitory effects of apomorphine on amphetamine locomotor activation are discussed.

Ammonia-induced release of neurotransmitters from rat brain synaptosomes: differences between the effects on amines and amino acids

The effect of NH4Cl on release of amine and amino acid transmitters from rat brain synaptosomes was investigated. Ammonia (0.1-10 mM) stimulated the secretion of dopamine and 5-hydroxytryptamine in a dose-dependent manner, in a process which was additive with the effect of 40 mM K+, almost unaffected by withdrawal of Ca2+, and markedly decreased by increasing [H+] in the medium. The NH4Cl-induced dopamine efflux, in contrast to that caused by high [K+]e, was inhibited by benztropine. The release of gamma-aminobutyric acid, aspartate, and glutamate was unaltered by [NH4Cl] less than 5 mM, but somewhat stimulated at higher levels. Transmembrane pH gradient, acid inside, was dissipated by NH4Cl in a concentration-dependent manner and the internal alkalinization correlated with the stimulation of the rate of dopamine efflux. Transmembrane electrical potential was unaffected by [ammonia] less than 5 mM, but a small depolarization was observed at higher levels. It is postulated that ammonia-induced alkalinization of the intrasynaptic storage granules causes extrusion of amines into the cytoplasm and their subsequent leakage into the medium through a reversal of the plasma membrane transporters. A lack of correlation between the release of amino acid neurotransmitters and the dissipation of the delta pH suggests that in rat brain intrasynaptic vesicles, acidic inside, are unlikely to store substantial amounts of gamma-aminobutyric acid, aspartate, or glutamate.