Biomedical subjects

Trent J Volz

Publications and source records attributed to Trent J Volz.

10 recordsLinked to original sources

PubMed2007

New insights into the mechanism of action of amphetamines.

Amphetamine is a psychostimulant commonly used to treat several disorders, including attention deficit, narcolepsy, and obesity. Plasmalemmal and vesicular monoamine transporters, such as the neuronal dopamine transporter and the vesicular monoamine transporter-2, are two of its principal targets. This review focuses on new insights, obtained from both in vivo and in vitro studies, into the molecular mechanisms whereby amphetamine, and the closely related compounds methamphetamine and methylenedioxymethamphetamine, cause monoamine, and particularly dopamine, release. These mechanisms include amphetamine-induced exchange diffusion, reverse transport, and channel-like transport phenomena as well as the weak base properties of amphetamine. Additionally, amphetamine analogs may affect monoamine transporters through phosphorylation, transporter trafficking, and the production of reactive oxygen and nitrogen species. All of these mechanisms have potential implications for both amphetamine- and methamphetamine-induced neurotoxicity, as well as dopaminergic neurodegenerative diseases.

Amphetamines↗

PubMed2006

Methamphetamine administration reduces hippocampal vesicular monoamine transporter-2 uptake.

Repeated high-dose injections of methamphetamine (METH) rapidly decrease dopamine uptake by the vesicular monoamine transporter-2 (VMAT-2) associated with dopaminergic nerve terminals, as assessed in nonmembrane-associated vesicles purified from striata of treated rats. The purpose of this study was to determine whether METH similarly affects vesicular uptake in the hippocampus; a region innervated by both serotonergic and noradrenergic neurons and profoundly affected by METH treatment. Results revealed that repeated high-dose METH administrations rapidly (within 1 h) reduced hippocampal vesicular dopamine uptake, as assessed in vesicles purified from treated rats. This reduction was likely associated with serotonergic nerve terminals because METH did not further reduce vesicular monoamine uptake in para-chloroamphetamine-lesioned animals. Pretreatment with the serotonin transporter inhibitor fluoxetine blocked both this acute effect on VMAT-2 and the decrease in serotonin content observed 7 days after METH treatment. In contrast, there was no conclusive evidence that METH affected vesicular dopamine uptake in noradrenergic neurons or caused persistent noradrenergic deficits. These findings suggest a link between METH-induced alterations in serotonergic hippocampal vesicular uptake and the persistent hippocampal serotonergic deficits induced by the stimulant.

Animals↗

PubMed2006

Measurement of kinetically resolved vesicular dopamine uptake and efflux using rotating disk electrode voltammetry.

The vesicular monoamine transporter-2 (VMAT-2) sequesters cytoplasmic dopamine (DA) into vesicles for storage and subsequent release. VMAT-2 activity has traditionally been measured in small synaptic vesicles isolated from rat striatum by monitoring [3H] DA uptake and in cellular expression systems using fast scan cyclic voltammetry. This is the first report using rotating disk electrode (RDE) voltammetry to measure VMAT-2 DA uptake and efflux in small synaptic vesicles. DA uptake profiles followed mixed order kinetics with apparent zero order kinetics for the first 25 s and apparent first order kinetics thereafter. Vesicular DA uptake was temperature- and ATP-dependent and was blocked by the VMAT-2 inhibitor tetrabenazine. Initial velocities of DA uptake were kinetically resolved and displayed Michaelis-Menten kinetics with a Km and Vmax of 289 +/- 59 nM and 1.9 +/- 0.2 fmol/(s microg protein), respectively. Methamphetamine-induced DA efflux was blocked by tetrabenazine and kinetically resolved with an initial velocity of 0.54 +/- 0.08 fmol/(s microg protein). These results suggest that RDE voltammetry can be used to make kinetically resolved measurements of vesicular DA uptake and efflux and will allow the design of experiments that could reveal important information about the kinetics of VMAT-2 activity and its inhibition.

Adenosine Triphosphate↗

PubMed2006

Kinetic analysis of developmental changes in vesicular monoamine transporter-2 function.

The vesicular monoamine transporter-2 (VMAT-2) sequesters cytoplasmic dopamine (DA) into vesicles and may thus protect neurons from DA-associated oxidative damage. VMAT-2 function, as assessed by measuring [(3)H]DA uptake, is less in adolescent when compared with young adult rats and VMAT-2 susceptibility to methamphetamine also changes with age. Thus, developmental alterations in VMAT-2 function warrant further investigation. The current study extends these findings by showing that the initial velocities of both DA uptake and methamphetamine-induced DA efflux are less in adolescent postnatal day (PND 38-42) vs. young adult (PND 88-92) rats as assessed in nonmembrane associated (presumably cytoplasmic) vesicles purified from rat striatal synaptosomes. The decrease in DA uptake velocities is due to a decrease in the V(max) of DA uptake with no change in the K(m). The density of kinetically active VMAT-2 and VMAT-2 immunoreactivity are less in adolescent vs. young adult rats while both the turnover number (2.4-2.8 s(-) (1)) and rate constant for the association of DA with VMAT-2 ( 1 x 10(7) M(-) (1) s(-) (1)) are similar in these age groups. These results suggest that the kinetics of DA binding and translocation across the membrane are unaltered in the vesicles of PND 38-42 vs. PND 88-92 rats. However, decreased VMAT-2 density in PND 38-42 rats reduces V(max), which in turn lowers DA uptake.

Aging↗

PubMed2005

Methylphenidate analogs with behavioral differences interact differently with arginine residues on the dopamine transporter in rat striatum.

The methylphenidate analogs N-methyl-4-methyl-methylphenidate and N-benzylmethylphenidate are believed to interact differently with the dopamine transporter (DAT) in vitro and in vivo. Herein, we report that methylphenidate and N-methyl-4-methyl-methylphenidate, but not N-benzylmethylphenidate, protect the rat striatal DAT from the arginine-selective chemical modifying agent, phenylglyoxal. This suggests that methylphenidate and N-methyl-4-methyl-methylphenidate, but not N-benzylmethylphenidate, interact with the guanidine groups of arginine residues in the DAT of rat striatum. This differential interaction may, at least in part, explain the in vitro and in vivo differences between N-methyl-4-methyl-methylphenidate and N-benzylmethylphenidate.

1-Methyl-4-phenylpyridinium↗

PubMed2005

A comprehensive atlas of the topography of functional groups of the dopamine transporter.

The neuronal dopamine transporter (DAT) is a transmembrane transporter that clears DA from the synaptic cleft. Knowledge of DAT functional group topography is a prerequisite for understanding the molecular basis of transporter function, the actions of psychostimulant drugs, and mechanisms of dopaminergic neurodegeneration. Information concerning the molecular interactions of drugs of abuse (such as cocaine, amphetamine, and methamphetamine) with the DAT at the functional group level may also aid in the development of compounds useful as therapeutic agents for the treatment of drug abuse. This review will provide a cumulative and comprehensive focus on the amino acid functional group topography of the rat and human DATs, as revealed by protein chemical modification and the techniques of site-directed mutagenesis. The results from these studies, represented mostly by site-directed mutagenesis, can be classified into several main categories: modifications without substantial affects on substrate transport, DAT membrane expression, or cocaine analog binding; those modifications which alter both substrate transport and cocaine analog binding; and those that affect DAT membrane expression. Finally, some modifications can selectively affect either substrate transport or cocaine analog binding. Taken together, these literature results show that domains for substrates and cocaine analogs are formed by interactions with multiple and sometimes distinct DAT functional groups.

Amino Acid Sequence↗

PubMed2005

Acute ethanol decreases dopamine transporter velocity in rat striatum: in vivo and in vitro electrochemical measurements.

BACKGROUND: Ethanol increases dopamine transporter (DAT) velocity when measured in cell expression systems, but its effects in vivo are mixed. The present experiments examined the effect of acute ethanol on dopamine transmission, particularly DAT velocity, in anesthetized animals as well as rat striatal suspensions. METHODS: To determine the effect of acute ethanol on DAT function in vivo, we measured dopamine uptake in real time using fast-scan cyclic voltammetry and constant potential amperometry in the olfactory tubercle of anesthetized rats. Dopamine fibers were electrically stimulated, and the resulting transient dopamine signals were analyzed to describe the release and uptake kinetics. We also measured the effect of ethanol on DAT velocity in vitro in striatal tissue suspensions using rotating disk electrode voltammetry. RESULTS: Ethanol (2.5 and 4 g/kg, intraperitoneally) decreased the electrically stimulated dopamine signal in the olfactory tubercle by 35-55%. The slope of the clearance curve of dopamine was 40% shallower after both doses of ethanol, indicating slower uptake. Modeling the data using Michaelis-Menten uptake kinetics showed that the slower uptake was due to a decrease in DAT V(max). These results were confirmed in vitro, because ethanol decreased the velocity of dopamine uptake by 35% in striatal tissue suspensions. CONCLUSIONS: These results indicate that acute ethanol decreases DAT function in rat dorsal and ventral striatum in anesthetized rats and tissue suspensions, in contrast to its effects on human DAT expressed in single cells. Given the variety of molecular targets of ethanol in the brain, including the DAT itself, it is likely that several mechanisms converge to produce a net effect on DAT regulation and function that could very well be different in intact tissue versus single cells.

Algorithms↗

PubMed2004

L-arginine increases dopamine transporter activity in rat striatum via a nitric oxide synthase-dependent mechanism.

Literature reports suggest that nitric oxide (NO) participates in the regulation of dopaminergic neurotransmission, possibly through interaction with cysteine residues of the dopamine transporter (DAT). Rotating disk electrode voltammetry was used to measure dopamine (DA) transport in rat striatum to determine if 1) the nitric oxide synthase (NOS) substrate, L-arginine (L-Arg), could affect DAT activity; 2) L-Arg-dependent effects on DAT activity could be blocked by NOS and guanylate cyclase inhibitors, a NO scavenger, DA, and cocaine; 3) a NO donor could affect DAT activity; and 4) L-Arg could protect the DAT from a sulfhydryl agent. L-Arg increased DAT activity by increasing V(max). NOS inhibitors (S-ethylisothiourea and S-isopropylisothiourea), a NO scavenger (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), DA, and cocaine blocked the L-Arg effect. The guanylate cyclase inhibitor, 1H-(1,2,4)-oxadiazolo[4,3a]quinoxalin-1-one, did not. The NO donor, S-nitroso-N-acetylpenicillamine, decreased DAT activity and L-Arg protected the DAT from the effects of the sulfhydryl agent N-ethylmaleimide. These results suggest that L-Arg, via NO, may play a role in regulating DAT activity in rat striatum by increasing the V(max) of DA transport. Furthermore, it is suggested that the effects of L-Arg on DAT activity may be due to modification of the DAT itself, possibly via the NO-mediated modification of DAT cysteine residues. Finally, NO produced from L-Arg may affect the DAT differently than NO from NO donors. These results further the notion that dopaminergic neurotransmission may be regulated by changes in DAT activity caused by L-Arg and NOS.

Animals↗

PubMed2004

Covalent and noncovalent chemical modifications of arginine residues decrease dopamine transporter activity.

Rotating disk electrode voltammetry was used to measure dopamine (DA) transport in rat striatum and in human embryonic kidney cells expressing the rat dopamine transporter (DAT). The goals of this study were to determine 1) if arginine (Arg) selective agents could alter DA transport, and 2) if DA analogs and DAT inhibitors could attenuate the effects of these agents on the DAT. Phenylglyoxal (PG), Hill coefficient 2.5, and other Arg selective agents decreased DA transport velocities. DA, Hill coefficient 1.0, and its analogs 3-hydroxyphenethylamine and 4-hydroxyphenethylamine attenuated the effects of PG on the DAT while phenethylamine did not. The tropane-based DAT inhibitors cocaine, WIN 35065-2, and WIN 35428 also attenuated the effects of PG. Benztropine, GBR 12935, and GBR 12909 did not. Thus, Arg residues are important for DAT activity and the results suggest that DA and cocaine both interact with Arg residues. Structure-activity studies suggest that DA interacts with Arg through its catechol hydroxyl groups and cocaine through the ester linkage attached to carbon 2 of the tropane ring. The results that 1). DA and cocaine may interact with the same functionally important Arg residue at the DAT, and 2). some members of the tropane and 1,4-dialkylpiperazine classes of DAT inhibitors may interact differently with DAT-derived Arg residue(s) furthers the notion that DAT activity sparing antagonists of cocaine can be designed.

Animals↗

PubMed2002

Evidence for two different active oxygen species in cytochrome P450 BM3 mediated sulfoxidation and N-dealkylation reactions.

Herein, we report the results from two experiments that are consistent with sulfoxidation and N-dealkylation involving two different enzyme substrate complexes and thus two different active oxygen species that do not interchange. The first experiment involves the use of a mutant that may increase the amount of the hydroperoxy-iron species (FeIIIO2H).1 This mutant increases the amount of sulfoxidation relative to the amount of N-dealkylation by 4-fold. In a second experiment, deuterium substitution on the N-methyl groups of substrate does not result in an increase in sulfoxidation. This later result is consistent with N-dealkylation and sulfoxidation being mediated by two different active oxygen species. While the data indicate two active oxygen species, they do not distinguish between the different possibilities for the active oxygen species.

Alkylation↗