Amine weak bases disrupt vesicular storage and promote exocytosis in chromaffin cells

Amphetamine-induced reverse transport of dopamine does not require cytosolic Ca2

Amphetamines (AMPHs) are substrates of the dopamine transporter (DAT) and reverse the direction of dopamine (DA) transport. This has been suggested to depend on activation of Ca2+-dependent pathways, but the mechanism underlying reverse transport via endogenously expressed DAT is still unclear. Here, to enable concurrent visualization by live imaging of extracellular DA dynamics and cytosolic Ca2+ levels, we employ the fluorescent Ca2+ sensor jRGECO1a expressed in cultured dopaminergic neurons together with the fluorescent DA sensor GRABDA1H expressed in cocultured "sniffer" cells. In the presence of the Na+-channel blocker tetrodotoxin to prevent exocytotic DA release, AMPH induced in the cultured neurons a profound dose-dependent efflux of DA that was blocked both by inhibition of DAT with cocaine and by inhibition of the vesicular monoamine transporter-2 with Ro-4-1284 or reserpine. However, the AMPH-induced DA efflux was not accompanied by an increase in cytosolic Ca2+ and was unaffected by blockade of voltage-gated calcium channels or chelation of cytosolic Ca2+. The independence of cytosolic Ca2+ was further supported by activation of N-methyl-D-aspartate-type ionotropic glutamate receptors leading to a marked increase in cytosolic Ca2+ without affecting AMPH-induced DA efflux. Curiously, AMPH elicited spontaneous Ca2+ spikes upon blockade of the D2 receptor, suggesting that AMPH can regulate intracellular Ca2+ in an autoreceptor-dependent manner regardless of the apparent independence of Ca2+ for AMPH-induced efflux. We conclude that AMPH-induced DA efflux in dopaminergic neurons does not require cytosolic Ca2+ but is strictly dependent on the concerted action of AMPH on both vesicular monoamine transporter-2 and DAT.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

A microfluidic platform for continuous monitoring of dopamine homeostasis in dopaminergic cells

Homeostasis of dopamine, a classical neurotransmitter, is a key indicator of neuronal health. Dysfunction in the regulation of dopamine is implicated in a long list of neurological disorders, including addiction, depression, and neurodegeneration. The existing methods used to evaluate dopamine homeostasis in vitro are inconvenient and do not allow for continuous non-destructive measurement. In response to this challenge, we introduce an integrated microfluidic system that combines dopaminergic cell culture and differentiation with electroanalytical measurements of extracellular dopamine in real-time at any point during an assay. We used the system to examine the behavior of differentiated SH-SY5Y cells upon exposure to four dopamine transporter ant/agonists (cocaine, ketamine, epigallocatechin gallate, and amphetamine) and study their pharmacokinetics. The IC₅₀ values of cocaine, ketamine, and epigallocatechin gallate were determined to be (average ± standard deviation) 3.7 ± 1.1 µM, 51.4 ± 17.9 µM, and 2.6 ± 0.8 µM, respectively. Furthermore, we used the new system to study amphetamine-mediated dopamine release to probe the related phenomena of dopamine transporter-mediated reverse-transport and dopamine release from vesicles. We propose that this platform, which is the first platform to simultaneously evaluate uptake and release, could be useful to screen for drugs and other agents that target dopaminergic neurons and the function of the dopamine transporter. More broadly, this platform should be adaptable for any application that could benefit from high-temporal resolution electroanalysis combined with multi-day cell culture using small numbers of cells.

Molecular Mechanisms of Amphetamines

There is a plethora of amphetamine derivatives exerting stimulant, euphoric, anti-fatigue, and hallucinogenic effects; all structural properties allowing these effects are contained within the amphetamine structure. In the first part of this review, the interaction of amphetamine with the dopamine transporter (DAT), crucially involved in its behavioral effects, is covered, as well as the role of dopamine synthesis, the vesicular monoamine transporter VMAT2, and organic cation 3 transporter (OCT3). The second part deals with requirements in amphetamine's effect on the kinases PKC, CaMKII, and ERK, whereas the third part focuses on where we are in developing anti-amphetamine therapeutics. Thus, treatments are discussed that target DAT, VMAT2, PKC, CaMKII, and OCT3. As is generally true for the development of therapeutics for substance use disorder, there are multiple preclinically promising specific compounds against (meth)amphetamine, for which further development and clinical trials are badly needed.

Striatal Tyrosine Hydroxylase Is Stimulated via TAAR1 by 3-Iodothyronamine, But Not by Tyramine or β-Phenylethylamine

The trace amine-associated receptor 1 (TAAR1) is expressed by dopaminergic neurons, but the precise influence of trace amines upon their functional activity remains to be fully characterized. Here, we examined the regulation of tyrosine hydroxylase (TH) by tyramine and beta-phenylethylamine (β-PEA) compared to 3-iodothyronamine (T₁AM). Immunoblotting and amperometry were performed in dorsal striatal slices from wild-type (WT) and TAAR1 knockout (KO) mice. T₁AM increased TH phosphorylation at both Ser¹⁹ and Ser⁴⁰, actions that should promote functional activity of TH. Indeed, HPLC data revealed higher rates of L-dihydroxyphenylalanine (DOPA) accumulation in WT animals treated with T₁AM after the administration of a DOPA decarboxylase inhibitor. These effects were abolished both in TAAR1 KO mice and by the TAAR1 antagonist, EPPTB. Further, they were specific inasmuch as Ser⁸⁴⁵ phosphorylation of the post-synaptic GluA1 AMPAR subunit was unaffected. The effects of T₁AM on TH phosphorylation at both Ser¹⁹ (CamKII-targeted), and Ser⁴⁰ (PKA-phosphorylated) were inhibited by KN-92 and H-89, inhibitors of CamKII and PKA respectively. Conversely, there was no effect of an EPAC analog, 8-CPT-2Me-cAMP, on TH phosphorylation. In line with these data, T₁AM increased evoked striatal dopamine release in TAAR1 WT mice, an action blunted in TAAR1 KO mice and by EPPTB. Mass spectrometry imaging revealed no endogenous T₁AM in the brain, but detected T₁AM in several brain areas upon systemic administration in both WT and TAAR1 KO mice. In contrast to T₁AM, tyramine decreased the phosphorylation of Ser⁴⁰-TH, while increasing Ser⁸⁴⁵-GluA1 phosphorylation, actions that were not blocked in TAAR1 KO mice. Likewise, β-PEA reduced Ser⁴⁰-TH and tended to promote Ser⁸⁴⁵-GluA1 phosphorylation. The D₁ receptor antagonist SCH23390 blocked tyramine-induced Ser⁸⁴⁵-GluA1 phosphorylation, but had no effect on tyramine- or β-PEA-induced Ser⁴⁰-TH phosphorylation. In conclusion, by intracellular cascades involving CaMKII and PKA, T₁AM, but not tyramine and β-PEA, acts via TAAR1 to promote the phosphorylation and functional activity of TH in the dorsal striatum, supporting a modulatory influence on dopamine transmission.

A mouse model of autism implicates endosome pH in the regulation of presynaptic calcium entry

Psychoactive compounds such as chloroquine and amphetamine act by dissipating the pH gradient across intracellular membranes, but the physiological mechanisms that normally regulate organelle pH remain poorly understood. Interestingly, recent human genetic studies have implicated the endosomal Na⁺/H⁺ exchanger NHE9 in both autism spectrum disorders (ASD) and attention deficit hyperactivity disorder (ADHD). Plasma membrane NHEs regulate cytosolic pH, but the role of intracellular isoforms has remained unclear. We now find that inactivation of NHE9 in mice reproduces behavioral features of ASD including impaired social interaction, repetitive behaviors, and altered sensory processing. Physiological characterization reveals hyperacidic endosomes, a cell-autonomous defect in glutamate receptor expression and impaired neurotransmitter release due to a defect in presynaptic Ca²⁺ entry. Acute inhibition of synaptic vesicle acidification rescues release but without affecting the primary defect due to loss of NHE9.

The Na⁺/H⁺ exchanger NHE9 is proposed to regulate the H⁺ electrochemical gradient across endosomal membranes. Here, the authors find that NHE9 knockout mice show autism spectrum disorder-like behaviors and disrupted synaptic vesicle exocytosis due to impaired presynaptic calcium entry.

The role of striatal Gαq/11 protein in methamphetamine-induced behavioral sensitization in mice

Gα_q/11 protein transduces signals from neurotransmitter receptors and has been implicated in several functions of the central nervous system. In this study, the role of Gα_q/11 protein in methamphetamine (METH)-induced behavioral sensitization was investigated using neurochemical and behavioral approaches. Repeated treatment with METH (2mg/kg, intraperitoneally) significantly increased behavioral sensitization as well as Gα_q/11 protein expression and Gα protein activity in the striata of mice, while a single treatment of METH at the same dose did not affect these parameters. Repeated intrastriatal injections of a Gα_q/11 inhibitor, [D-Trp7,9,10]-substance P, significantly reduced behavioral sensitization and striatal dopamine (DA) level in response to METH, with no effect on striatal tyrosine hydroxylase expression. These results suggest that Gα_q/11 protein facilitates METH-induced behavioral sensitization by modulating DA release in the mouse striatum.

Pharmacological profile of methylphenidate-based designer drugs

BACKGROUND

Methylphenidate-based designer drugs are new psychoactive substances (NPS) that are used outside medical settings and their pharmacology is largely unexplored. The aim of the present study was to characterize the pharmacology of methylphenidate-based substances in vitro.

METHODS

We determined the potencies of the methylphenidate-based NPS N-benzylethylphenidate, 3,4-dichloroethylphenidate, 3,4-dichloromethylphenidate, ethylnaphthidate, ethylphenidate, 4-fluoromethylphenidate, isopropylphenidate, 4-methylmethylphenidate, methylmorphenate, and propylphenidate and the potencies of the related compounds cocaine and modafinil with respect to norepinephrine, dopamine, and serotonin transporter inhibition in transporter-transfected human embryonic kidney 293 cells. We also investigated monoamine efflux and monoamine receptor and transporter binding affinities. Furthermore, we assessed the cell integrity under assay conditions.

RESULTS

All methylphenidate-based substances inhibited the norepinephrine and dopamine transporters 4 to >1000-fold more potently than the serotonin transporter. Similar to methylphenidate and cocaine, methylphenidate-based NPS did not elicit transporter-mediated efflux of monoamines. Besides binding to monoamine transporters, several test drugs had affinity for adrenergic, serotonergic, and rat trace amine-associated receptors but not for dopaminergic or mouse trace amine-associated receptors. No cytotoxicity was observed after drug treatment at assay concentrations.

CONCLUSION

Methylphenidate-based substances had pharmacological profiles similar to methylphenidate and cocaine. The predominant actions on dopamine transporters vs. serotonin transporters may be relevant when considering abuse liability. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

How intravesicular composition affects exocytosis

Large dense core vesicles and chromaffin granules accumulate solutes at large concentrations (for instance, catecholamines, 0.5-1 M; ATP, 120-300 mM; or Ca²⁺, 40 mM (12)). Solutes seem to aggregate to a condensed protein matrix, which is mainly composed of chromogranins, to elude osmotic lysis. This association is also responsible for the delayed release of catecholamines during exocytosis. Here, we compile experimental evidence, obtained since the inception of single-cell amperometry, demonstrating how the alteration of intravesicular composition promotes changes in the quantum characteristics of exocytosis. As chromaffin cells are large and their vesicles contain a high concentration of electrochemically detectable species, most experimental data comes from this cell model.

The hemostatic effect study of Cirsium setosum on regulating α1-ARs via mediating norepinephrine synthesis by enzyme catalysis

BACKGROUND

Cirsium setosum (CS) is the aboveground part of Cephalanoplos segetum Kitam. Although it has been used as a hemostatic treatment for thousands of years and is still in use today, the mechanism of CS on regulating ARs is still not clear.

PURPOSE

In this study, we aimed to clarify the mechanism of CS on regulating ARs.

METHODS

We developed a simple method based on UPLC/Q-TOF MS combined adrenergic receptor dual-luciferase reporter assay systems for the rapid determination of active constituents in CS. The mechanism of tyramine, the main active component for regulating ARs, was further investigated by an in vitro norepinephrine biotransformation test and in vivo vaso activity tests.

RESULTS

Two phenethylamine ARs regulators (tyramine and N-methyltyramine) in CS were characterized, and it was found that tyramine could induce vasoconstriction via regulation of α1-ARs by mediating norepinephrine synthesis.

CONCLUSION

The hemostatic effect of CS is associated with tyramine and N-methyltyramine, via regulation of α1-ARs, and the mechanism of tyramine is related to mediating norepinephrine synthesis by enzyme catalysis.

Oral treatment with Lactobacillus rhamnosus attenuates behavioural deficits and immune changes in chronic social stress

BACKGROUND

Stress-related disorders involve systemic alterations, including disruption of the intestinal microbial community. Given the putative connections between the microbiota, immunity, neural function, and behaviour, we investigated the potential for microbe-induced gut-to-brain signalling to modulate the impact of stress on host behaviour and immunoregulation.

METHODS

Male C57BL/6 mice treated orally over 28 days with either Lactobacillus rhamnosus (JB-1) ™ or vehicle were subjected to chronic social defeat and assessed for alterations in behaviour and immune cell phenotype. 16S rRNA sequencing and mass spectrometry were employed to analyse the faecal microbial community and metabolite profile.

RESULTS

Treatment with JB-1 decreased stress-induced anxiety-like behaviour and prevented deficits in social interaction with conspecifics. However, JB-1 did not alter development of aggressor avoidance following social defeat. Microbial treatment attenuated stress-related activation of dendritic cells while increasing IL-10+ regulatory T cells. Furthermore, JB-1 modulated the effect of stress on faecal metabolites with neuroactive and immunomodulatory properties. Exposure to social defeat altered faecal microbial community composition and reduced species richness and diversity, none of which was prevented by JB-1. Stress-related microbiota disruptions persisted in vehicle-treated mice for 3 weeks following stressor cessation.

CONCLUSIONS

These data demonstrate that despite the complexity of the gut microbiota, exposure to a single microbial strain can protect against certain stress-induced behaviours and systemic immune alterations without preventing dysbiosis. This work supports microbe-based interventions for stress-related disorders.

Illicit dopamine transients: reconciling actions of abused drugs

Phasic increases in brain dopamine are required for cue-directed reward seeking. Although compelling within the framework of appetitive behavior, the view that illicit drugs hijack reward circuits by hyperactivating these dopamine transients is inconsistent with established psychostimulant pharmacology. However, recent work reclassifying amphetamine (AMPH), cocaine, and other addictive dopamine-transporter inhibitors (DAT-Is) supports transient hyperactivation as a unifying hypothesis of abused drugs. We argue here that reclassification also identifies generating burst firing by dopamine neurons as a keystone action. Unlike natural rewards, which are processed by sensory systems, drugs act directly on the brain. Consequently, to mimic natural rewards and exploit reward circuits, dopamine transients must be elicited de novo. Of available drug targets, only burst firing achieves this essential outcome.

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.

Vesicular Ca(2+) mediates granule motion and exocytosis

Secretory vesicles of chromaffin cells are acidic organelles that maintain an increasing pH gradient towards the cytosol (5.5 vs. 7.3) that is mediated by V-ATPase activity. This gradient is primarily responsible for the accumulation of large concentrations of amines and Ca(2+), although the mechanisms mediating Ca(2+) uptake and release from granules, and the physiological relevance of these processes, remain unclear. The presence of a vesicular matrix appears to create a bi-compartmentalised medium in which the major fractions of solutes, including catecholamines, nucleotides and Ca(2+), are strongly associated with vesicle proteins, particularly chromogranins. This association appears to be favoured at acidic pH values. It has been demonstrated that disrupting the pH gradient of secretory vesicles reduces their rate of exocytosis and promotes the leakage of vesicular amines and Ca(2+), dramatically increasing the movement of secretory vesicles and triggering exocytosis. In this short review, we will discuss the data available that highlights the importance of pH in regulating the association between chromogranins, vesicular amines and Ca(2+). We will also address the potential role of vesicular Ca(2+) in two major processes in secretory cells, vesicle movement and exocytosis.

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.

Tyramine reduces glycinergic transmission by inhibiting presynaptic Ca(2+) channels in the rat trigeminal subnucleus caudalis

We have recently reported that tyramine acts on putative presynaptic trace amine receptors to inhibit glycinergic transmission in substantia gelatinosa (SG) neurons of the rat trigeminal subnucleus caudalis. However, it is still unknown how tyramine elicits presynaptic inhibition of glycine release. In the present study, therefore, we investigated cellular mechanisms underlying the tyramine-induced inhibition of glycinergic transmission in SG neurons using a conventional whole-cell patch clamp technique. Tyramine (100 μM) reversibly and repetitively decreased the amplitude of action potential-dependent glycinergic inhibitory postsynaptic currents (IPSCs), and increased the paired-pulse ratio. Pharmacological data suggest that the tyramine-induced decrease in glycinergic IPSCs was not mediated by the modulation of adenylyl cyclase, protein kinase A and C, or G-protein coupled inwardly rectifying K(+) channels. On the other hand, glycinergic IPSCs were mainly mediated by the Ca(2+) influx passing through presynaptic N-type and P/Q-type Ca(2+) channels. The tyramine-induced decrease in glycinergic IPSCs was completely blocked by ω-conotoxin GVIA, an N-type Ca(2+) channel blocker, but not ω-agatoxin IVA, a P/Q-type Ca(2+) channel blocker. The results suggest that tyramine acts presynaptically to decrease action potential-dependent glycine release onto SG neurons via the selective inhibition of presynaptic N-type Ca(2+) channels. This tyramine-induced inhibition of glycinergic transmission in SG neurons might affect the process of orofacial nociceptive signals.

On the role of intravesicular calcium in the motion and exocytosis of secretory organelles

Secretory vesicles of sympathetic neurons and chromaffin granules maintain a pH gradient towards the cytosol (5.5 vs. 7.2) promoted by the V-ATPase activity. This gradient of pH is mainly responsible for the accumulation of amines. The secretory vesicles contain large amounts of total Ca(2+), but the free intragranular [Ca(2+)], the mechanisms for Ca(2+) uptake and release from the granules and their physiological relevance regarding exocytosis are still matters of debate.We have recently shown that disruption of the pH gradient of secretory vesicles slowed down exocytosis. Fluorimetric measurements, using the dye Oregon green BAPTA-2, showed that the V-ATPase inhibitor bafilomycin A1 directly released Ca(2+) from freshly isolated vesicles. Accordingly, vesicle alkalinization released Ca(2+) from the granules to the cytosol, measured with fura-2 in intact chromaffin cells. Using TIRFM in cells overexpressing the EGFP-labeled synaptobrevin (VAMP2-EGFP) protein, we have then shown that the Ca(2+) released from the vesicles to the cytosol in the presence of bafilomycin, dramatically increased the granule motion of chromaffin- or PC12-derived granules, and triggered exocytosis (measured by amperometry).We conclude that the gradient of pH of secretory vesicles might be involved in the homeostatic regulation of the local cytosolic Ca(2+) around the vesicles and in two of the major functions of secretory cells, vesicle motion and exocytosis.1.

Fluorescent β-Blockers as Tools to Study Presynaptic Mechanisms of Neurosecretion

Several, if not all adrenergic β-blockers (β-Bs), accumulate progressively inside secretory vesicles in a time- and concentration-dependent manner, and could be considered to be false neurotransmitters. This transmitter effect is most likely unrelated to their ability to block adrenergic receptors, but it could explain the delay in lowering arterial pressure in hypertensive patients. We have developed a new drug to monitor the accumulation of β-Bs inside living cells, RCTM-3, which fluoresces in the visible spectrum. Here we describe the procedure to synthesize this new compound, as well as its fluorescent properties, pharmacological profile and its accumulation inside the secretory vesicles of PC12 cells.

How addictive drugs disrupt presynaptic dopamine neurotransmission

The fundamental principle that unites addictive drugs appears to be that each enhances synaptic dopamine by means that dissociate it from normal behavioral control, so that they act to reinforce their own acquisition. This occurs via the modulation of synaptic mechanisms that can be involved in learning, including enhanced excitation or disinhibition of dopamine neuron activity, blockade of dopamine reuptake, and altering the state of the presynaptic terminal to enhance evoked over basal transmission. Amphetamines offer an exception to such modulation in that they combine multiple effects to produce nonexocytic stimulation-independent release of neurotransmitter via reverse transport independent from normal presynaptic function. Questions about the molecular actions of addictive drugs, prominently including the actions of alcohol and solvents, remain unresolved, but their ability to co-opt normal presynaptic functions helps to explain why treatment for addiction has been challenging.

Role of secretory granules in inositol 1,4,5-trisphosphate-dependent Ca(2+) signaling: from phytoplankton to mammals

The majority of secretory cell calcium is stored in secretory granules that serve as the major IP(3)-dependent intracellular Ca(2+) store. Even in unicellular phytoplankton secretory granules are responsible for the IP(3)-induced Ca(2+) release that triggers exocytosis. The number of secretory granules in the cell is directly related not only to the magnitude of IP(3)-induced Ca(2+) release, which accounts for the majority of the IP(3)-induced cytoplasmic Ca(2+) release in neuroendocrine cells, but also to the IP(3) sensitivity of the cytoplasmic IP(3) receptor (IP(3)R)/Ca(2+) channels. Moreover, secretory granules contain the highest IP(3)R concentrations and the largest amounts of IP(3)Rs in any subcellular organelles in neuroendocrine cells. Secretory granules from phytoplankton to mammals contain large amounts of polyanionic molecules, chromogranins being the major molecules in mammals, in addition to acidic intragranular pH and high Ca(2+) concentrations. The polyanionic molecules undergo pH- and Ca(2+)-dependent conformational changes that serve as a molecular basis for condensation-decondensation phase transitions of the intragranular matrix. Likewise, chromogranins undergo pH- and Ca(2+)-dependent conformational changes with increased exposure of the structure and increased interactions with Ca(2+) and other granule components at acidic pH. The unique physico-chemical properties of polyanionic molecules appear to be at the center of biogenesis, and physiological functions of secretory granules in living organisms from primitive to advanced species.

Inositol 1,4,5-trisphosphate receptor in chromaffin secretory granules and its relation to chromogranins

The inositol 1,4,5-trisphosphate (IP(3))-mediated intracellular Ca(2+) releases in secretory cells play vital roles in controlling not only the intracellular Ca(2+) concentrations but also the Ca(2+)-dependent exocytotic processes. Of intracellular organelles that release Ca(2+) in response to IP(3), secretory granules stand out as the most prominent organelle and are responsible for the majority of IP(3)-dependent Ca(2+) releases in the cytoplasm of chromaffin cells. Bovine chromaffin granules were the first granules that demonstrated the IP(3)-mediated Ca(2+) release as well as the presence of the IP(3) receptor (IP(3)R) in granule membranes. Secretory granules contain all three (type 1, 2, and 3) IP(3)R isoforms, and 58-69% of total cellular IP(3)R isoforms are expressed in bovine chromaffin granules. Moreover, secretory granules contain large amounts (2-4 mM) of chromogranins and secretogranins; chromogranins A and B, and secretogranin II being the major species. Chromogranins A and B, and secretogranin II are high-capacity, low-affinity Ca(2+) binding proteins, binding 30-93 mol of Ca(2+)/mol of protein with dissociation constants of 1.5-4.0 mM. Due to this high Ca(2+) storage properties of chromogranins secretory granules contain ~40 mM Ca(2+). Furthermore, chromogranins A and B directly interact with the IP(3)Rs and modulate the IP(3)R/Ca(2+) channels, i.e., increasing the open probability and the mean open time of the channels 8- to 16-fold and 9- to 42-fold, respectively. Coupled chromogranins change the IP(3)R/Ca(2+) channels to a more ordered, release-ready state, whereby making the IP(3)R/Ca(2+) channels significantly more sensitive to IP(3).

Intravesicular factors controlling exocytosis in chromaffin cells

Chromaffin granules are similar organelles to the large dense core vesicles (LDCV) present in many secretory cell types including neurons. LDCV accumulate solutes at high concentrations (catecholamines, 0.5-1 M; ATP, 120-300 mM; or Ca(2+), 40 mM (Bulenda and Gratzl Biochemistry 24:7760-7765, 1985). Solutes seem to aggregate to a condensed matrix to elude osmotic lysis. The affinity of solutes for LDCV matrix is responsible for the delayed release of catecholamines during exocytosis. The aggregation of solutes occurs due to a specific H(+) pump denominated V-ATPase that maintains an inner acidic media (pH ≈5.5). This pH gradient against cytosol is also responsible for the vesicular accumulation of amines and Ca(2+). When this gradient is reduced by modulation of the V-ATPase activity, catecholamines and Ca(2+) are moved toward the cytosol. In addition, some drugs largely accumulate inside LDCV and not only impair the accumulation of natural solutes, but also act as false neurotransmitters when they are co-released with catecholamines. There is much experimental evidence to conclude that the physiological modulation of vesicle pH and the manipulation of intravesicular media with drugs affect the LDCV cargo and change the kinetics of exocytosis. Here, we will present some experimental data demonstrating the participation of drugs in the kinetics of exocytosis through changes in the composition of vesicular media. We also offer a model to explain the regulation of exocytosis by the intravesicular media that conciliate the experimentally obtained data.

Chromogranins as regulators of exocytosis

Chromogranins (Cgs) constitute the main protein component in the vesicular matrix of large dense core vesicles (LDCV). These acidic proteins have been implicated in several physiological processes such as vesicle sorting, the generation of bioactive peptides and the accumulation of soluble species inside LDCV. This latter feature of Cgs accounts for the ability of vesicles to concentrate catecholamines and Ca(2+). Indeed, the low affinity and high capacity of Cgs to bind solutes at the low pH of the LDCV lumen seems to be behind the delay in the neurotransmitter exit towards the extracellular milieu after vesicle fusion. The availability of new mouse strains lacking Cgs in combination with the arrival of several techniques for the direct monitoring of exocytosis (like amperometry, patch-amperometry and intracellular electrochemistry), have helped advance our understanding of how these granins concentrate catecholamines and Ca(2+) in LDCV, and how they influence the kinetics of exocytosis. In this review, we will discuss the roles of Cgs A and B in maintaining the intravesicular environment of secretory vesicles and in exocytosis, bringing together the most recent findings from adrenal chromaffin cells.

Intravesicular calcium release mediates the motion and exocytosis of secretory organelles: a study with adrenal chromaffin cells

Secretory vesicles of sympathetic neurons and chromaffin granules maintain a pH gradient toward the cytosol (pH 5.5 versus 7.2) promoted by the V-ATPase activity. This gradient of pH is also responsible for the accumulation of amines and Ca2+ because their transporters use H+ as the counter ion. We have recently shown that alkalinization of secretory vesicles slowed down exocytosis, whereas acidification caused the opposite effect. In this paper, we measure the alkalinization of vesicular pH, caused by the V-ATPase inhibitor bafilomycin A1, by total internal reflection fluorescence microscopy in cells overexpressing the enhanced green fluorescent protein-labeled synaptobrevin (VAMP2-EGFP) protein. The disruption of the vesicular gradient of pH caused the leak of Ca2+, measured with fura-2. Fluorimetric measurements, using the dye Oregon green BAPTA-2, showed that bafilomycin directly released Ca2+ from freshly isolated vesicles. The Ca2+ released from vesicles to the cytosol dramatically increased the granule motion of chromaffin- or PC12-derived granules and triggered exocytosis (measured by amperometry). We conclude that the gradient of pH of secretory vesicles might be involved in the homeostatic regulation of cytosolic Ca2+ and in two of the major functions of secretory cells, vesicle motion and exocytosis.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

Our present understanding of exocytosis of catecholamines has benefited tremendously from the arrival of single-cell electrochemical methods (amperometry and voltammetry), electrophysiological techniques (whole-cell and patch capacitance) and from the combination of both techniques (patch amperometry). In this brief review, we will outline the strengths and limitations of amperometric and electrophysiological methods and highlight the major contribution obtained with the use of these techniques in chromaffin cells.

Secretory vesicle rebound hyperacidification and increased quantal size resulting from prolonged methamphetamine exposure

Acute exposure to amphetamines (AMPHs) collapses secretory vesicle pH gradients, which increases cytosolic catecholamine levels while decreasing the quantal size of catecholamine release during fusion events. AMPH and methamphetamine (METH), however, are retained in tissues over long durations. We used optical and electron microscopic probes to measure the effects of long-term METH exposure on secretory vesicle pH, and amperometry and intracellular patch electrochemistry to observe the effects on neurosecretion and cytosolic catecholamines in cultured rat chromaffin cells. In contrast to acute METH effects, exposure to the drug for 6-48 h at 10 microM and higher concentrations produced a concentration-dependent rebound hyperacidification of secretory vesicles. At 5-10 microM levels, prolonged METH increased the quantal size and reinstated exocytotic catecholamine release, although very high (> 100 microM) levels of the drug, while continuing to produce rebound hyperacidification, did not increase quantal size. Secretory vesicle rebound hyperacidification was temperature dependent with optimal response at approximately 37 degrees C, was not blocked by the transcription inhibitor, puromycin, and appears to be a general compensatory response to prolonged exposure with membranophilic weak bases, including AMPHs, methylphenidate, cocaine, and ammonia. Thus, under some conditions of prolonged exposure, AMPHs and other weak bases can enhance, rather than deplete, the vesicular release of catecholamines via a compensatory response resulting in vesicle acidification.

Trace amine-associated receptor 1-Family archetype or iconoclast?

Interest has recently been rekindled in receptors that are activated by low molecular weight, noncatecholic, biogenic amines that are typically found as trace constituents of various vertebrate and invertebrate tissues and fluids. The timing of this resurgent focus on receptors activated by the "trace amines" (TA) beta-phenylethylamine (PEA), tyramine (TYR), octopamine (OCT), synephrine (SYN), and tryptamine (TRYP) is the direct result of 2 publications that appeared in 2001 describing the cloning of a novel G protein-coupled receptor (GPCR) referred to by their discoverers Borowsky et al. as TA1 and Bunzow et al. as TA receptor 1 (TAR1). When heterologously expressed in Xenopus laevis oocytes and various eukaryotic cell lines, recombinant rodent and human TAR dose-dependently couple to the stimulation of adenosine 3',5'-monophosphate (cAMP) production. Structure-activity profiling based on this functional response has revealed that in addition to the TA, other biologically active compounds containing a 2-carbon aliphatic side chain linking an amino group to at least 1 benzene ring are potent and efficacious TA receptor agonists with amphetamine (AMPH), methamphetamine, 3-iodothyronamine, thyronamine, and dopamine (DA) among the most notable. Almost 100 years after the search for TAR began, numerous TA1/TAR1-related sequences, now called TA-associated receptors (TAAR), have been identified in the genome of every species of vertebrate examined to date. Consequently, even though heterologously expressed TAAR1 fits the pharmacological criteria established for a bona fide TAR, a major challenge for those working in the field is to discern the in vivo pharmacology and physiology of each purported member of this extended family of GPCR. Only then will it be possible to establish whether TAAR1 is the family archetype or an iconoclast.

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

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

Methylamine-dependent release of nitric oxide and dopamine in the CNS modulates food intake in fasting rats

BACKGROUND AND PURPOSE

Methylamine is an endogenous aliphatic amine exhibiting anorexigenic properties in mice. The aim of this work was to show whether methylamine also modifies feeding behaviour in rats and, if so, to identify the mediator(s) responsible for such effects.

EXPERIMENTAL APPROACH

Microdialysis experiments with the probe inserted in the periventricular hypothalamic nucleus were carried out in 12 h starved, freely moving rats. Collected perfusate samples following methylamine injection (i.c.v.) were analysed for nitric oxide by chemiluminescence and for dopamine and 5-hydroxytryptamine content by HPLC. Kv1.6 potassium channel expression was reduced by antisense strategy and this decrease quantified by semi-quantitative RT-PCR analysis.

KEY RESULTS

Methylamine showed biphasic dose-related effects on rat feeding. At doses of 15-30 microg per rat, it was hyperphagic whereas higher doses (60-80 microg) were hypophagic. Methylamine stimulated central nitric oxide (+115% vs. basal) following hyperphagic and dopamine release (60% over basal values) at hypophagic doses, respectively. Treatment with L-N(G)-nitro-L-arginine-methyl ester (i.c.v. 2 microg 10 microl(-1)) or with alpha-methyl-p-tyrosine (i.p. 100 mg kg(-1)) before methylamine injection, reduced nitric oxide output and hyperphagia, or dopamine release and hypophagia respectively. Moreover, hypophagia and hyperphagia, as well as nitric oxide and dopamine release were significantly reduced by down-regulating brain Kv1.6 potassium channel expression.

CONCLUSIONS AND IMPLICATIONS

The effects of methylamine on feeding depend on the hypothalamic release of nitric oxide and dopamine as a result of interaction at the Kv1.6 channels. The study of methylamine levels in the CNS may provide new perspectives on the physiopathology of alimentary behaviour.

4-methyl benzylamine stimulates food consumption and counteracts the hypophagic effects of amphetamine acting on brain Shaker-like Kv1.1 channels

1.--4-methyl benzylamine (4-MBZ; 28 microg, 231 nmol) elicits a hyperphagic response in starved mice in contrast to the hypophagia induced by the parent compound benzylamine (BZ; 33 microg, 231 nmol) or by amphetamine (AMPH, 2 mug). 2.--In mice starved for only 4 h, and therefore with little stimulation to eat, the maximal increase in food consumption induced by intracerebroventricular (i.c.v.)-injected 4-MBZ was 190% over that of the controls (ED(50) 8.3+/-2.7 microg mouse(-1); 68+/-22 nmol mouse(-1)), whereas after i.p. administration, these values were 160% and approximately 129 mg kg(-1), respectively. 3.--The hyperphagic effect of 4-MBZ was reduced by more than 60% in mice pretreated with antisense oligodeoxyribonucleotide (aODN(1)) previously found to selectively inhibit (over 50%) the expression of Shaker-like Kv1.1 channels. 4.--In mice highly stimulated to eat after 12-h fasting, 4-MBZ (28 microg) significantly reduced (to about 70%) the hypophagic response by AMPH (2 microg) or BZ (33 microg). Conversely, these two compounds reduced (respectively, by 69 and 44%) the hyperphagic response of 4-MBZ in 4-h fasting mice. 5.--4-MBZ (28 microg) also reduced the hypermotility and the stimulation of inspection activity elicited by AMPH in mice and the release of DA stimulated by AMPH (2 microg) from the nucleus accumbens of rats. We hypothesize that 4-MBZ elicits hyperphagic effects probably by opening Shaker-like Kv1.1 subtypes in the brain, whereas AMPH and BZ are hypophagic by blocking these channels.

On the effects of psychostimulants, antidepressants, and the antiparkinsonian drug levodopa on dopamine neurons

The dopaminergic system constitutes the principal target of many psychostimulants, antidepressant, and antiparkinsonian drugs. The effects caused by these compounds are partly associated with an increased dopamine (DA) levels within the terminal areas of DA neurons and in the ventral midbrain. Therefore, several substances of abuse, antidepressants, and endogenous compounds (levodopa and trace amines [TAs]) regulate the activity of DA cells by activating D2 autoreceptors located on the terminals, soma, and dendrites. Considering our past and recent experimental studies on this issue, here we will briefly reexamine the mechanisms of action of several psychoactive drugs on DA neurons. In particular, we propose three different modalities by which the mesencephalic DA neurons can be regulated by drugs: amphetamine/TAs-like, cocaine-like, and levodopa-like. We, therefore, discuss the potential therapeutic and addictive properties of the psychoactive substances.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Extracellular ATP stimulates exocytosis via localized Ca(2+) release from acidic stores in rat pancreatic beta cells

Three different methods, membrane capacitance (C(m)) measurement, amperometry and FM dye labeling were used to investigate the role of extracellular ATP in insulin secretion from rat pancreatic beta cells. We found that extracellular application of ATP mobilized intracellular Ca(2+) stores and synchronously triggered vigorous exocytosis. No influence of ATP on the readily releasable pool of vesicles was observed, which argues against a direct modulation of the secretory machinery at a level downstream of Ca(2+) elevation. The stimulatory effects of ATP were greatly reduced by intracellular perfusion of BAPTA but not EGTA, suggesting a close spatial association of fusion sites with intracellular Ca(2+) releasing sites. ATP-induced Ca(2+) transients and exocytosis were not blocked by thapsigargin (TG), by a ryanodine receptor antagonist or by dissipation of pH in acidic stores by monensin alone, but they were greatly attenuated by IP(3) receptor inhibition as well as ionomycin plus monensin, suggesting involvement of IP(3)-sensitive acidic Ca(2+) stores. Taken together, our data suggest that extracellular ATP triggers exocytosis by mobilizing spatially limited acidic Ca(2+) stores through IP(3) receptors. This mechanism may explain how insulin secretion from the pancreas is coordinated through diffusible ATP that is co-released with insulin.

Vesicular Ca(2+) -induced secretion promoted by intracellular pH-gradient disruption

The actions of the protonophore CCCP on intracellular Ca2+ regulation and exocytosis in chromaffin cells have been examined. Simultaneous fura-2 imaging and amperometry reveal that exposure to CCCP not only perturbs mitochondrial function but that it also alters vesicular storage of Ca2+ and catecholamines. By disrupting the pH gradient of the secretory vesicle membrane, the protonophore allows both Ca(2+) and catecholamine to leak into the cytosol. Unlike the high cytosolic Ca2+ concentrations resulting from mitochondrial membrane disruption, Ca2+ leakage from secretory vesicles may initiate exocytotic release. In conjunction with previous studies, this work reveals that catalytic and self-sustained vesicular Ca(2+) -induced exocytosis occurs with extended exposure to weak acid or base protonophores.

Intragranular pH rapidly modulates exocytosis in adrenal chromaffin cells

Several drugs produce rapid changes in the kinetics of exocytosis of catecholamines, as measured at the single event level with amperometry. This study is intended to unveil whether the mechanism(s) responsible for these effects involve changes in the intravesicular pH. Cell incubation with bafilomycin A1, a blocker of the vesicular proton pump, caused both a deceleration in the kinetics of exocytosis and a reduction in the catecholamine content of vesicle. These effects were also observed upon reduction of proton gradient by nigericin or NH4Cl. pH measurements using fluorescent probes (acridine orange, quinacrine or enhanced green fluorescent protein-synaptobrevin) showed a strong correlation between vesicular pH and the kinetics of exocytosis. Hence, all maneuvers tested that decelerated exocytosis also alkalinized secretory vesicles and vice versa. On the other hand, calcium entry caused a transient acidification of granules. We therefore propose that the regulation of vesicular pH is, at least partially, a necessary step in the modulation of the kinetics of exocytosis and quantal size operated by some cell signals.

Mechanisms of neurotransmitter release by amphetamines: a review

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

Calcium dynamics in catecholamine-containing secretory vesicles

We have used an aequorin chimera targeted to the membrane of the secretory granules to monitor the free [Ca(2+)] inside them in neurosecretory PC12 cells. More than 95% of the probe was located in a compartment with an homogeneous [Ca(2+)] around 40 microM. Cell stimulation with either ATP, caffeine or high-K(+) depolarization increased cytosolic [Ca(2+)] and decreased secretory granule [Ca(2+)] ([Ca(2+)](SG)). Inositol-(1,4,5)-trisphosphate, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate were all ineffective to release Ca(2+) from the granules. Changes in cytosolic [Na(+)] (0-140 mM) or [Ca(2+)] (0-10 microM) did not modify either ([Ca(2+)](SG)). Instead, [Ca(2+)](SG) was highly sensitive to changes in the pH gradient between the cytosol and the granules. Both carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) and nigericin, as well as cytosolic acidification, reversibly decreased [Ca(2+)](SG), while cytosolic alcalinization reversibly increased [Ca(2+)](SG). These results are consistent with the operation of a H(+)/Ca(2+) antiporter in the vesicular membrane. This antiporter could also mediate the effects of ATP, caffeine and high-K(+) on [Ca(2+)](SG), because all of them induced a transient cytosolic acidification. The FCCP-induced decrease in [Ca(2+)](SG) was reversible in 10-15 min even in the absence of cytosolic Ca(2+) or ATP, suggesting that most of the calcium content of the vesicles is bound to a slowly exchanging Ca(2+) buffer. This large store buffers [Ca(2+)](SG) changes in the long-term but allows highly dynamic free [Ca(2+)](SG) changes to occur in seconds or minutes.

Trace Amines Depress GABAB Response in Dopaminergic Neurons by Inhibiting G-βγ-Gated Inwardly Rectifying Potassium Channels

Trace amines (TAs) are present in the central nervous system in which they up-regulate catecholamine release and are implicated in the pathogenesis of addiction, attention-deficit/hyper-activity disorder, Parkinson's disease, and schizophrenia. By using intracellular and patch-clamp recordings from dopaminergic cells in the rat midbrain slices, we report a depressant postsynaptic action of two TAs, beta-phenylethylamine (beta-PEA) and tyramine (TYR) on the GABA(B)-mediated slow inhibitory postsynaptic potential and baclofen-activated outward currents. beta-PEA and TYR activated G-proteins, interfering with the coupling between GABA(B) receptors and G-betagamma-gated inwardly rectifying potassium channels. This is the first demonstration that beta-PEA and TYR depress inhibitory synaptic potentials in neurons of the central nervous system, supporting their emerging role as neuromodulators.

Intracellular Ca2+ regulates amphetamine-induced dopamine efflux and currents mediated by the human dopamine transporter

Although it is clear that amphetamine-induced dopamine (DA) release mediated by the dopamine transporter (DAT) is integral to the behavioral actions of this psychostimulant, the mechanism of this release is not clear. In this study, we explored the requirement for intracellular Ca(2+) in amphetamine-induced DA efflux and currents mediated by the human DAT. The patch-clamp technique in the whole-cell configuration was used on Na(+) and DA-preloaded human embryonic kidney 293 cells stably transfected with the human DAT (hDAT cells). Chelation of intracellular Ca(2+) by inclusion of 50 microM BAPTA in the whole-cell pipette reduced the voltage-dependent amphetamine-induced hDAT current, with the greatest effect seen at positive voltages. Likewise, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) reduced amphetamine-induced DA efflux as measured by amperometry. Furthermore, preincubation of the cells with 50 microM BAPTA acetoxy methyl ester (AM) or thapsigargin also blocked amphetamine-induced release of preloaded N-methyl-4-[(3)H]phenylpyridinium from superfused hDAT cells. BAPTA-AM also reduced DA release from striatal synaptosomes. Amphetamine also led to an increase in intracellular Ca(2+) that was blocked by prior treatment with 5 microM thapsigargin or 10 microM cocaine. These studies demonstrate that amphetamine-induced DAT-mediated currents and substrate efflux require internal Ca(2+) and that amphetamine can stimulate dopamine efflux by regulating cytoplasmic Ca(2+) levels through its interaction with DAT.

Inhibitory effects of trace amines on rat midbrain dopaminergic neurons

Trace amines are biological compounds that are still awaiting identification of their role in neuronal function. Using intracellular electrophysiological recordings, we investigated the depressant action of two trace amines (beta-phenylethylamine and tyramine) on the firing activity of dopaminergic neurons of the substantia nigra pars compacta and ventral tegmental area. This inhibition was due to a membrane hyperpolarisation that was blocked by the D2 dopamine receptor antagonist sulpiride and was not potentiated by the dopamine-uptake blocker, cocaine. Inhibition of the dopamine transporter did not mediate the effects of trace amines, because unlike cocaine, trace amines did not potentiate the inhibitory responses to exogenously applied dopamine. The inhibitory actions of beta-phenylethylamine and tyramine were present in reserpine-treated animals but were abolished when the dopamine-synthesis inhibitor carbidopa was applied. Our data suggest that trace amines cause an indirect activation of dopamine autoreceptors, by an increased efflux of newly synthesised dopamine. The inhibition of dopaminergic activity by trace amines may relate to their involvement in neuronal processes linked to drug addiction, schizophrenia, attention deficit hyperactive disorders and Parkinson's disease.

Methylamine, but not ammonia, is hypophagic in mouse by interaction with brain Kv1.6 channel subtype

Ammonia and methylamine (MET) are endogenous compounds increased during liver and renal failure, Alzheimer's disease, vascular dementia and diabetes, where they alter some neurobehavioural functions probably acting as potassium channel blockers. We have already described that potassium channel blockers including tetraethylammonium (TEA), ammonia and MET are hypophagic in mice. Antisense oligonucleotides (aODNs) against Shaker-like Kv1.1 gene abolished the effect of TEA but not of ammonia and MET. The central effects elicited in fasted mice by ammonia and MET were further studied. For MET, an ED(50) value 71.4+/-1.8 nmol mouse(-1) was calculated. The slope of the dose-response curves for these two compounds and the partial hypophagic effect elicited by ammonia indicated a different action mechanism for these amines. The aODNs pretreatments capable of temporarily reducing the expression of all seven known subtypes of Shaker-like gene or to inactivate specifically the Kv1.6 subtype abolished the hypophagic effect of MET but not that of ammonia. Reverse transcription-polymerase chain reaction, Western blot and immunohistochemical results indicate that a full expression in the brain of Kv1.6 is required only for the activity of MET, and confirms the different action mechanism of ammonia and MET.

The effect of phosphorylation on amphetamine-mediated outward transport

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

Intracellular patch electrochemistry: regulation of cytosolic catecholamines in chromaffin cells

Alterations in the cytosolic pool directly affect neurotransmitter synthesis and release and are suggested to be key factors in various neurodegenerative disorders. Although this cytosolic pool is the most metabolically active, it is miniscule compared with the amount of vesicular transmitter and has never been quantified separately. Here, we introduce intracellular patch electrochemistry (IPE), a technique that for the first time provides direct measurements of cytosolic oxidizable molecules in single mammalian cells. In amperometric mode, IPE detects total catechols, whereas in cyclic voltammetric mode, it preferentially measures catecholamines. In cultured chromaffin cells, the total cytosolic catechol concentration was 50-500 microm, of which approximately 10% were catecholamines. Reserpine, a vesicular monoamine transporter inhibitor, had no effect on the catecholamine pool but increased total catechols by fourfold to fivefold. Combined with pargyline, a monoamine oxidase inhibitor, reserpine increased catecholamine levels in the cytosol by approximately sixfold. Amphetamine induced a transient approximately fivefold accumulation of cytosolic catecholamines and a slow increase of total catechols. In cells incubated with 3,4-dihydroxy-L-phenylalanine (L-DOPA), catecholamines increased by approximately 2.5-fold and total catechols increased by approximately fourfold. Cytosolic catecholamines returned to control levels <or=10 min after L-DOPA withdrawal, whereas total catechols remained approximately twofold elevated even after a 1.5 hr incubation in L-DOPA-free media. Our data indicate that cytosolic catecholamines are strictly maintained at a defined level, and drug-induced increases in their concentrations lead to the accumulation of other catecholamine derivatives, such as DOPAC and 3,4-dihydroxyphenylethyleneglycol. These derivatives reside in the cytosol for hours after treatment and may be an underlying cause of drug-related cytotoxicity.