Biomedical subjects

James O Schenk

Publications and source records attributed to James O Schenk.

9 recordsLinked to original sources

PubMed2006

Repeated cocaine effects on learning, memory and extinction in the pond snail Lymnaea stagnalis.

The persistence of drug addiction suggests that drugs of abuse enhance learning and/or impair extinction of the drug memory. We studied the effects of repeated cocaine on learning, memory and reinstatement in the pond snail, Lymnaea stagnalis. Respiratory behavior can be operantly conditioned and extinguished in Lymnaea, and this behavior is dependent on a critical dopamine neuron. We tested the hypothesis that repeated cocaine exposure promotes learning and memory or attenuates the ability to extinguish the memory of respiratory behavior that relies on this dopaminergic neuron. Rotating disk electrode voltammetry revealed a K(m) and V(max) of dopamine uptake in snail brain of 0.9 micromol l(-1) and 558 pmol s(-1) g(-1) respectively, and the IC(50) of cocaine for dopamine was approximately 0.03 micromol l(-1). For operant conditioning, snails were given 5 days of 1 h day(-1) immersion in water (control) or 0.1 micromol l(-1) cocaine, which was the lowest dose that maximally inhibited dopamine uptake, and snails were trained 3 days later. No changes were found between the two groups for learning or memory of the operant behavior. However, snails treated with 0.1 micromol l(-1) cocaine demonstrated impairment of extinction memory during reinstatement of the behavior compared with controls. Our findings suggest that repeated exposure to cocaine modifies the interaction between the original memory trace and active inhibition of this trace through extinction training. An understanding of these basic processes in a simple model system may have important implications for treatment strategies in cocaine addiction.

Animals↗

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

Unraveling neuronal dopamine transporter mechanisms with rotating disk electrode voltammetry.

Herein we describe how the rotating disk electrode voltammetric technique can be used to examine the mechanism(s) of the inward transport of dopamine by the neuronal transporter for dopamine (DAT). The usefulness of making measurements kinetically resolving dopamine transport, interpretations of changes in Km and Vmax, approaches to defining pre-steady-state binding of dopamine to DAT, interactions between competing inhibitors, chemical modification of functional groups within DAT, and a presentation of a hypothetical multi-state model of dopamine transport are presented and discussed.

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↗

PubMed2003

What can be learned from studies of multisubstrate mechanisms of neuronal dopamine transport?

The dopamine transporter (DAT) is a Na+- and Cl--dependent transporter and, with respect to its three apparent substrates, both partially random sequential as well as ordered mechanisms have been reported. Here we describe some of the features of DAT, such as the coupling of energy to concentrate dopamine and the properties of slippage and leakage. Further, in considering the regulation of transport velocities by DAT few have considered issues related to substrate regulation of DAT activity. Specifically, what effect do changes in the constants (K) for the participation of Na+ and Cl- have on dopamine transport velocity? It is shown that DAT may possess properties of slippage, an argument is made that leakage may be important in neuronal systems containing DAT, and the influence of changing values of K for the participation of Na+ and Cl- in transport is shown to produce large effects on DAT activity depending on the multisubstrate kinetic mechanism.

Animals↗

PubMed2002

The functioning neuronal transporter for dopamine: kinetic mechanisms and effects of amphetamines, cocaine and methylphenidate.

The dopamine transporter (DAT) is a transmembrane spanning protein that catalyzes the transport of dopamine across the neuronal membrane to concentrate the neurotransmitter inside the cell. Although the uptake of dopamine has been studied since the 1960s, more recent advances in knowledge of the protein itself and in making kinetically resolved measurements of its action have led to more insights into its mechanism and pharmacology. The literature of the kinetics of transporters and kinetic measurements of DAT activity is reviewed to provide an overview of the multisubstrate mechanism of DAT activity, its pharmacology with regard to amphetamine, cocaine and methylphenidate, and correlations of DAT activity with some behavioral outputs.

Amphetamines↗