1-Methyl-4-phenylpyridinium uptake by human and rat striatal synaptosomes

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

The dopamine transporter is responsible for recycling dopamine after release. Inhibitors of the dopamine transporter, such as cocaine, will stop the reuptake of dopamine and allow it to stay extracellularly, causing prominent changes at the molecular, cellular, and behavioral levels. There is much left to be known about the mechanism and site(s) of binding, as well as the effect that cocaine administration does to dopamine transporter-cocaine binding sites and gene expression which also plays a strong role in cocaine abusers and their behavioral characteristics. Thus, if more light is shed on the dopamine transporter-cocaine interaction, treatments for addiction and even other diseases of the dopaminergic system may not be too far ahead. As today's ongoing research expands on the shoulders of classic research done in the 1990s and 2000s, the foundation of core research done in that time period will be reviewed, which forms the basis of today's work and tomorrow's therapies.

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.

MPP+ toxicity and plasma membrane dopamine transporter: study using cell lines expressing the wild-type and mutant rat dopamine transporters

The Parkinsonism-inducing neurotoxin 1-methyl-4-phenylpyridinium (MPP+) causes specific cell death in dopaminergic neurons after accumulation by the dopamine transporter (DAT). COS cells, a non-neuronal cell line insensitive to high doses of MPP+, becomes sensitive to MPP+ when transfected with the rat DAT cDNA. We analyzed the bi-directional transport of MPP+ and its toxicity in several cell lines expressing wild or mutant DATs. Cell death in COS cells expressing wild DAT by exposure to MPP+ was concentration-dependent and cocaine-reversible. Increased wild DAT expression caused higher sensitivities to the toxin in HeLa cells. Although several mutant DATs demonstrated greater transport activity than the wild-type, they displayed similar or lower sensitivity to MPP+ toxicity. Reverse transport of preloaded [3H]MPP+ through DAT was facilitated in COS cells expressing certain mutant DATs, which consistently displayed less sensitivity to MPP+ toxicity. These results suggest that re-distribution of MPP+ due to influx/efflux turnover through the transporter is a key factor in MPP+ toxicity.

Tyrosine-533 of rat dopamine transporter: involvement in interactions with 1-methyl-4-phenylpyridinium and cocaine

To improve our understanding of structure-function relationships for neurotransmitter transporters, we performed site-directed mutagenesis of the rat dopamine transporter (DAT) and assessed the functions of the mutants in transiently-expressing COS cells. Tyrosine-533 of rat DAT lies in the 11th transmembrane region, where the corresponding amino acid of human DAT is phenylalanine. Alanine substitution of tyrosine-533 (Y533A) conferred an increased affinity for 1-methyl-4-phenylpyridinium (MPP+). Phenylalanine substitution of tyrosine-533 (Y533F) increased the velocity of MPP+ uptake but decreased DAT's affinity for MPP+. Cocaine's potency in inhibiting dopamine uptake was unchanged with Y533A, but increased with Y533F. Differences in the uptake kinetics and inhibitory potency of cocaine between rat and human DATs were similar to the differences observed between the wild-type and Y533F mutants DATs. Tyrosine-533 may be important for the DAT function and for species differences in transporter functions, including differential sensitivities to cocaine and 1-methyl-1,2,3,6-tetrahydropyridine (MPTP) in humans and rats.

Dopamine transporter gene expression in rat mesencephalic dopaminergic neurons is increased by direct interaction with target striatal cells in vitro

By using a semi-quantitative reverse transcriptase-PCR assay (RT-PCR) we have analyzed dopamine transporter (DAT), tyrosine hydroxylase (TH) and synaptic vesicle monoamine transporter (VMAT2) gene expression in rat mesencephalic (MES) primary cultures. Consistent with previous data obtained during rat MES ontogeny, the onset of DAT transcription in vitro is delayed in embryonic day (E)13, but not in E16, MES neurons when compared to that of TH and VMAT2. In co-culture, the addition of target striatal cells (STR) to E13 MES selectively increases DAT mRNA level in DA neurons during the first 3 days in vitro; cortical cells are ineffective. On the contrary, DAT gene does not appear up-regulated in E16 MES co-cultured with target STR cells, indicating that MES DA neurons respond to STR stimulation only at defined developmental stages. Up-regulation of DAT mRNA level by STR in E13 MES seems to require direct cell interactions since target cells do not exert their effect on DAT transcription when are separated from MES cells by a porous barrier, which only allows diffusion of soluble molecules. Thus maturation of DA neurotransmission in vitro appears to follow a developmental program which can be specifically modulated by their target STR cells.

Alteration of corticotropin-releasing factor immunoreactivity in MPTP-treated rats

A decrease of corticotropin-releasing factor (CRF) concentration has been reported in patients with Parkinson's disease (PD). The present study further examined the role of CRF in an animal model of parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Results indicated that both subchronic (2 days) and chronic (7 days) MPTP treatments decreased the number of CRF immunoreactive neurons in both the paraventricular nucleus (PVN) of the hypothalamus and the central nuelcus of the amygdala (ACN). This effect lasted for almost a month after withdrawal of chronic MPTP injections. In addition, nomifensine pretreatment protected against MPTP's toxicity on DA neurons, as assessed by tyrosine hydroxylase immunoreactivity in the substantia nigra. However, the same treatment did not prevent the toxicity of MPTP on CRF neurons. Further, no significant difference was notable in the number of CRF immunoreactive neurons between normal young adult and normal middle-aged rats in both the PVN and the ACN. These results suggest that MPTP also produces a neurotoxicity on CRF neurons, and this effect is not secondary to MPTP's effect on DA neurons. Besides, altered CRF neuronal activity is involved in the process of pathological ageing, but not physiological ageing. Further, reduced CRF immunoreactivity in the PVN and ACN may imply alterations of neuroendocrine, autonomic as well as central functions caused by MPTP.

Differential interactive effects of gliotoxin and MPTP in the substantia nigra and the locus coeruleus in BALB/c mice

We have previously demonstrated that chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) infusion to the substantia nigra (SN) and the locus coeruleus (LC) both produce a long-lasting neurotoxicity on dopamine (DA) and norepinephrine (NE) neurons in these two areas, respectively. In the present study, we further examined the toxicity of MPTP in these two areas by using the immunohistochemical method. We have also assessed the role of glia cells in the SN and LC in mediating the toxicity of MPTP. Immunohistochemical results have confirmed the direct toxicity of MPTP in the SN, as revealed by significant decreases of tyrosine hydroxylase (TH)-positive cells in the SN and TH-positive fibers in the striatum. The specific gliotoxin alpha-aminoadipic acid (alpha-AA), when administered to the SN at 48 h interval, partially antagonized DA depletions and behavioral deficits produced by chronic MPTP treatment. When alpha-AA was administered to the SN every 24 h, it completely abolished the toxicity of MPTP. On the other hand, chronic MPTP infusions to the LC significantly decreased DA-beta-hydroxylase-positive cells in this area. When alpha-AA was injected into the LC at 48 h intervals, it did not prevent depletions of NE in the LC and the hippocampus caused by chronic MPTP infusions. It did not protect against the behavioral deficits produced by MPTP, either. When alpha-AA was injected into the LC every 24 h, it only partially prevented the toxicity of MPTP on NE in the LC. It also partially prevented the motor-impairing effect of MPTP; however, it barely protected against MPTP's toxicity on NE in the hippocampus and it did not antagonize the stereotypy deficit produced by chronic MPTP, either. Phasic tremor and rigidity were observed following MPTP infusions to the SN and the LC every day, but these symptoms were less frequently observed during the later experimental stage. Serotonin measures were not significantly altered by these treatments throughout these experiments. Immunoblotting results of glial fibrillary acidic protein (GFAP), a marker protein of astrocytes, have confirmed proper lesions of astrocytes by alpha-AA. These results together suggest that chronic MPTP treatment exerts a direct and long-lasting toxicity on DA neurons along the nigrostriatal pathway and NE neurons along the coeruleus-hippocampal pathway. The neurotoxicity of MPTP is probably mediated through astrocytes in the SN, and may be partly mediated through astrocytes in the LC also. These results imply a role for dendritic uptake of DA and NE in these cell body regions. However, these findings also suggest the possibility of differential mechanisms of MPTP's toxicity in these two areas.

Dopamine transporter mRNA content in human substantia nigra decreases precipitously with age

The dopamine transporter is the primary means of inactivating synaptic dopamine as well as a major site of action for psychostimulants (such as cocaine and amphetamine) and for neurotoxins that induce parkinsonism. In the present study, a human dopamine transporter partial cDNA clone obtained by polymerase chain reaction exhibited 87% and 89% identity at the nucleic acid and amino acid levels, respectively, with transmembrane domains 3-5 of the rat homolog. This clone was used to quantitate human dopamine transporter mRNA by nuclease protection assay. The postmortem content of dopamine transporter mRNA in the substantia nigrae of 18- to 57-yr-old subjects was relatively constant, while in subjects greater than 57 yr old, a precipitous (greater than 95%) decline in substantia nigra dopamine transporter mRNA was evident. In contrast, tyrosine hydroxylase mRNA in the same samples declined in a linear manner with increasing age. In situ hybridization experiments confirmed the profound loss of dopamine transporter gene expression in melanin-positive (presumptive dopamine) nigral neurons. These data may begin to shed light on compensatory changes occurring in human dopamine neurons during normal aging.