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

Methamphetamine compromises gap junctional communication in astrocytes and neurons

Methamphetamine (meth) is a central nervous system (CNS) stimulant that results in psychological and physical dependency. The long-term effects of meth within the CNS include neuronal plasticity changes, blood-brain barrier compromise, inflammation, electrical dysfunction, neuronal/glial toxicity, and an increased risk to infectious diseases including HIV. Most of the reported meth effects in the CNS are related to dysregulation of chemical synapses by altering the release and uptake of neurotransmitters, especially dopamine, norepinephrine, and epinephrine. However, little is known about the effects of meth on connexin (Cx) containing channels, such as gap junctions (GJ) and hemichannels (HC). We examined the effects of meth on Cx expression, function, and its role in NeuroAIDS. We found that meth altered Cx expression and localization, decreased GJ communication between neurons and astrocytes, and induced the opening of Cx43/Cx36 HC. Furthermore, we found that these changes in GJ and HC induced by meth treatment were mediated by activation of dopamine receptors, suggesting that dysregulation of dopamine signaling induced by meth is essential for GJ and HC compromise. Meth-induced changes in GJ and HC contributed to amplified CNS toxicity by dysregulating glutamate metabolism and increasing the susceptibility of neurons and astrocytes to bystander apoptosis induced by HIV. Together, our results indicate that connexin containing channels, GJ and HC, are essential in the pathogenesis of meth and increase the sensitivity of the CNS to HIV CNS disease. Methamphetamine (meth) is an extremely addictive central nervous system stimulant. Meth reduced gap junctional (GJ) communication by inducing internalization of connexin-43 (Cx43) in astrocytes and reducing expression of Cx36 in neurons by a mechanism involving activation of dopamine receptors (see cartoon). Meth-induced changes in Cx containing channels increased extracellular levels of glutamate and resulted in higher sensitivity of neurons and astrocytes to apoptosis in response to HIV infection.

The effects of locus coeruleus and norepinephrine in methamphetamine toxicity

The activity of locus coeruleus (LC) neurons has been extensively investigated in a variety of behavioural states. In fact this norepinephrine (NE)-containing nucleus modulates many physiological and pathological conditions including the sleep-waking cycle, movement disorders, mood alterations, convulsive seizures, and the effects of drugs such as psychostimulants and opioids. This review focuses on the modulation exerted by central NE pathways on the behavioural and neurotoxic effects produced by the psychostimulant methamphetamine, essentially the modulation of the activity of mesencephalic dopamine (DA) neurons. In fact, although NE in itself mediates some behavioural effects induced by methamphetamine, NE modulation of DA release is pivotal for methamphetamine-induced behavioural states and neurotoxicity. These interactions are discussed on the basis of the state of the art of the functional neuroanatomy of central NE- and DA systems. Emphasis is given to those brain sites possessing a remarkable overlapping of both neurotransmitters.

Effects and Trimethyltin on Hippocampal Dopaminergic Markers and Cognitive Behaviour

The triorganotin compound trimethyltin (TMT) is a highly toxic molecule which has a great impact on human health. The aim of this study was to investigate the specific alteration of dopamine receptors and transporters in the hippocampus of TMT-treated rats. The TMT-treated group showed impaired spatial reference memory in a Morris water maze task compared to the control group, whereas memory consolidation tested 24 hours after the last training session was preserved. In the open field, TMT-treated rats showed a decrease in time spent in rearing episodes reflecting a lower interest to explore a novel environment. In the hippocampal area of the TMT-treated group, we observed a reduction in neuronal viability accompanied by a significant decrease in the expression of the dopamine receptors (D1 and D2), and dopamine transporters (DAT, VMAT1 and VMAT2). A less pronounced reduction was observed for D3 and D5 while D4 did not change. These data were confirmed by RT-PCR analysis. The present study on TMT-induced neurodegeneration highlights the link between hippocampal asset of dopamine receptors and transporters and the impaired performance of rats in a spatial reference memory task.

Mathematical insights into the effects of levodopa

Parkinson’s disease has been traditionally thought of as a dopaminergic disease in which cells of the substantia nigra pars compacta (SNc) die. However, accumulating evidence implies an important role for the serotonergic system in Parkinson’s disease in general and in physiological responses to levodopa therapy, the first line of treatment. We use a mathematical model to investigate the consequences of levodopa therapy on the serotonergic system and on the pulsatile release of dopamine (DA) from dopaminergic and serotonergic terminals in the striatum. Levodopa competes with tyrosine and tryptophan at the blood-brain barrier and is taken up by serotonin neurons in which it competes for aromatic amino acid decarboxylase. The DA produced competes with serotonin (5HT) for packaging into vesicles. We predict the time courses of LD, cytosolic DA, and vesicular DA in 5HT neurons during an LD dose. We predict the time courses of DA and 5HT release from 5HT cell bodies and 5HT terminals as well as the changes in 5HT firing rate due to lower 5HT release. We compute the time course of DA release in the striatum from both 5HT and DA neurons and show how the time course changes as more and more SNc cells die. This enables us to explain the shortening of the therapeutic time window for the efficacy of levodopa as Parkinson’s disease progresses. Finally, we study the effects 5HT1a and 5HT1b autoreceptor agonists and explain why they have a synergistic effect and why they lengthen the therapeutic time window for LD therapy. Our results are consistent with and help explain results in the experimental literature and provide new predictions that can be tested experimentally.

Analysis of synaptic proteins in the cerebrospinal fluid as a new tool in the study of inborn errors of neurotransmission

In a few rare diseases, specialised studies in cerebrospinal fluid (CSF) are required to identify the underlying metabolic disorder. We aimed to explore the possibility of detecting key synaptic proteins in the CSF, in particular dopaminergic and gabaergic, as new procedures that could be useful for both pathophysiological and diagnostic purposes in investigation of inherited disorders of neurotransmission. Dopamine receptor type 2 (D2R), dopamine transporter (DAT) and vesicular monoamine transporter type 2 (VMAT2) were analysed in CSF samples from 30 healthy controls (11 days to 17 years) by western blot analysis. Because VMAT2 was the only protein with intracellular localisation, and in order to compare results, GABA vesicular transporter, which is another intracellular protein, was also studied. Spearman's correlation and Student's t tests were applied to compare optical density signals between different proteins. All these synaptic proteins could be easily detected and quantified in the CSF. DAT, D2R and GABA VT expression decrease with age, particularly in the first months of life, reflecting the expected intense synaptic activity and neuronal circuitry formation. A statistically significant relationship was found between D2R and DAT expression, reinforcing the previous evidence of DAT regulation by D2R. To our knowledge, there are no previous studies on human CSF reporting a reliable analysis of these proteins. These kinds of studies could help elucidate new causes of disturbed dopaminergic and gabaergic transmission as well as understanding different responses to L-dopa in inherited disorders affecting dopamine metabolism. Moreover, this approach to synaptic activity in vivo can be extended to different groups of proteins and diseases.

Rotating disk electrode voltammetric measurements of serotonin transporter kinetics in synaptosomes

Altered serotonin (5-HT) signaling is implicated in several neuropsychiatric disorders, including depression, anxiety, obsessive-compulsive disorder, and autism. The 5-HT transporter (SERT) modulates 5-HT neurotransmission strength and duration. This is the first study using rotating disk electrode voltammetry (RDEV) to measure 5-HT clearance. SERT kinetics were measured in whole brain synaptosomes. Uptake kinetics of exogenous 5-HT were measured using glassy carbon electrodes rotated in 500 μL glass chambers containing synaptosomes from SERT-knockout (-/-), heterozygous (+/-), or wild-type (+/+) mice. RDEV detected 5-HT concentrations of 5nM and higher. Initial velocities were kinetically resolved with K(m) and V(max) values of 99±35 standard error of regression (SER) nM and 181±11 SER fmol/(s×mg protein), respectively in wild-type synaptosomes. The method enables control over drug and chemical concentrations, facilitating interpretation of results. Results are compared in detail to other techniques used to measure SERT kinetics, including tritium labeled assays, chronoamperometry, and fast scan cyclic voltammetry. RDEV exhibits decreased 5-HT detection limits, decreased vulnerability to 5-HT oxidation products that reduce electrode sensitivity, and also overcomes diffusion limitations via forced convection by providing a continuous, kinetically resolved signal. Finally, RDEV distinguishes functional differences between genotypes, notably, between wild-type and heterozygous mice, an experimental problem with other experimental approaches.

Neuropharmacological mechanisms underlying the neuroprotective effects of methylphenidate

Methylphenidate is a psychostimulant that inhibits the neuronal dopamine transporter. In addition, methylphenidate has the intriguing ability to provide neuroprotection from the neurotoxic effects of methamphetamine and perhaps also Parkinson’s disease; both of which may likely involve the abnormal accumulation of cytoplasmic dopamine inside dopaminergic neurons and the resulting formation of dopamine-associated reactive oxygen species. As delineated in this review, the neuroprotective effects of methylphenidate are due, at least in part, to its ability to attenuate or prevent this abnormal cytoplasmic dopamine accumulation through several possible neuropharmacological mechanisms. These may include 1) direct interactions between methylphenidate and the neuronal dopamine transporter which may attenuate or prevent the entry of methamphetamine into dopaminergic neurons and may also decrease the synthesis of cytoplasmic dopamine through a D2 receptor-mediated signal cascade process, and 2) indirect effects upon the functioning of the vesicular monoamine transporter-2 which may increase vesicular dopamine sequestration through both vesicle trafficking and the kinetic upregulation of the vesicular monoamine transporter-2 protein. Understanding these neuropharmacological mechanisms of methylphenidate neuroprotection may provide important insights into the physiologic regulation of dopaminergic systems as well as the pathophysiology of a variety of disorders involving abnormal dopamine disposition ranging from substance abuse to neurodegenerative diseases such as Parkinson’s disease.

Homeostatic mechanisms in dopamine synthesis and release: a mathematical model

BACKGROUND

Dopamine is a catecholamine that is used as a neurotransmitter both in the periphery and in the central nervous system. Dysfunction in various dopaminergic systems is known to be associated with various disorders, including schizophrenia, Parkinson's disease, and Tourette's syndrome. Furthermore, microdialysis studies have shown that addictive drugs increase extracellular dopamine and brain imaging has shown a correlation between euphoria and psycho-stimulant-induced increases in extracellular dopamine 1. These consequences of dopamine dysfunction indicate the importance of maintaining dopamine functionality through homeostatic mechanisms that have been attributed to the delicate balance between synthesis, storage, release, metabolism, and reuptake.

METHODS

We construct a mathematical model of dopamine synthesis, release, and reuptake and use it to study homeostasis in single dopaminergic neuron terminals. We investigate the substrate inhibition of tyrosine hydroxylase by tyrosine, the consequences of the rapid uptake of extracellular dopamine by the dopamine transporters, and the effects of the autoreceoptors on dopaminergic function. The main focus is to understand the regulation and control of synthesis and release and to explicate and interpret experimental findings.

RESULTS

We show that the substrate inhibition of tyrosine hydroxylase by tyrosine stabilizes cytosolic and vesicular dopamine against changes in tyrosine availability due to meals. We find that the autoreceptors dampen the fluctuations in extracellular dopamine caused by changes in tyrosine hydroxylase expression and changes in the rate of firing. We show that short bursts of action potentials create significant dopamine signals against the background of tonic firing. We explain the observed time courses of extracellular dopamine responses to stimulation in wild type mice and mice that have genetically altered dopamine transporter densities and the observed half-lives of extracellular dopamine under various treatment protocols.

CONCLUSION

Dopaminergic systems must respond robustly to important biological signals such as bursts, while at the same time maintaining homeostasis in the face of normal biological fluctuations in inputs, expression levels, and firing rates. This is accomplished through the cooperative effect of many different homeostatic mechanisms including special properties of tyrosine hydroxylase, the dopamine transporters, and the dopamine autoreceptors.

Age-dependent differences in dopamine transporter and vesicular monoamine transporter-2 function and their implications for methamphetamine neurotoxicity

The abuse of methamphetamine (METH) is a serious public health problem because METH can cause persistent dopaminergic deficits in the brains of both animal models and humans. Surprisingly, adolescent postnatal day (PND)40 rats are resistant to these METH-induced deficits, whereas young adult PND90 rats are not. Studies described in this report used rotating disk electrode voltammetry and western blotting techniques to investigate whether there are age-dependent differences in monoamine transporter function in PND38-42 and PND88-92 rats that could contribute to this phenomenon. The initial velocities of dopamine (DA) transport into, METH-induced DA efflux from, and DA transporter (DAT) immunoreactivity in striatal suspensions are greater in PND38-42 rats than in PND88-92 rats. DA transport velocities into vesicles that cofractionate with synaptosomal membranes after osmotic lysis are also greater in PND38-42 rats. However, there is no difference in vesicular monoamine transporter-2 (VMAT-2) immunoreactivity between the two age groups in this fraction. This suggests that younger rats have a greater capacity to sequester cytoplasmic DA into membrane-associated vesicles due to kinetically upregulated VMAT-2 and also have increased levels of functionally active DAT. In the presence of METH, these may provide additional routes of cellular efflux for DA that is released from vesicles into the cytoplasm and thereby prevent cytoplasmic DA concentrations in younger rats from rising to neurotoxic levels after drug administration. These findings provide novel insight into the age-dependent physiological regulation of neuronal DA sequestration and may advance the treatment of disorders involving abnormal DA disposition including substance abuse and Parkinson's disease.

Methylphenidate-induced alterations in synaptic vesicle trafficking and activity

The psychostimulant, methylphenidate (MPD), is commonly prescribed to treat attention-deficit hyperactivity disorder. MPD binds to the neuronal dopamine (DA) transporter, where it blocks the inward transport of DA. The present study expands upon these findings by examining the effects of in vivo MPD administration on the vesicular monoamine transporter-2 (VMAT-2) in membrane-associated vesicle and cytoplasmic vesicle subcellular fractions (i.e., those vesicles that do and do not co-fractionate with synaptosomal membranes after osmotic lysis, respectively) isolated from lysates of rat striatal synaptosomes. The results indicate that a single MPD administration redistributes VMAT-2 and associated vesicles within nerve terminals away from the synaptosomal membranes and into the cytoplasm, as assessed 1 hour after treatment. DA transport is also increased by MPD in both vesicle fractions (on account of vesicle trafficking in the cytoplasmic vesicles and to kinetic upregulation of the VMAT-2 in the membrane-associated vesicles). This, in turn, leads to an increase in the DA content of both vesicle fractions as well as an increase in the velocity and magnitude of K(+)-stimulated DA release from striatal suspensions. Taken together, these data show that the trafficking, DA sequestration function, DA content, and exocytotic DA release function of synaptic vesicles can all be pharmacologically manipulated by in vivo MPD treatment. These findings may provide important insights useful for understanding and treating disorders involving abnormal DA transmission including drug abuse, Parkinson's disease, and attention-deficit hyperactivity disorder.

Dopamine, vesicular transporters and dopamine receptor expression and localization in rat thymus and spleen

The localization of dopamine stores and the expression and localization of vesicular monoamine transporter (VMAT) type-1 and 2 and of dopamine D1-like and D2-like receptor subtypes were investigated in rat thymus and spleen by immunohistochemical, immunochemical techniques and by RT-PCR. In the thymus dopamine immunoreactivity was developed in the cortico-medullary junction and in the medulla, but not in the thymic cortex. In the spleen, dopamine stores were found in reticular structures in the white pulp border and in the white pulp, but not in the red one. Both thymus and spleen expressed VMAT-1 and VMAT-2 immunoreactivity as well as dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity. Immunohistochemistry revealed VMAT-1, VMAT-2 and dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity primarily in the thymic cortical-medulla transitional zone and to a lesser extent in the medulla but not in the cortex. In the spleen, VMAT-1, VMAT-2 and dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity was located primarily in the white pulp border and to a lesser extent in the white pulp. These findings indicate that both thymus and spleen express a dopaminergic system characterized by the presence of dopamine, vesicular monoamine transporters and the five subtypes of dopamine receptors. The presence of these dopaminergic markers suggests that dopamine likely originating from immune cells and/or from sympathetic neuroeffector plexus is released in the lymphoid microenvironment. Based on the microanatomical localization of dopaminergic markers investigated, a role of dopamine in maturation and selection of lymphocytes and activation of immune responses is suggested.

Differential sensitivity to SSRI and tricyclic antidepressants in juvenile and adult mice of three strains

Clinical studies have shown differential efficacy of several antidepressants in children and adolescents compared to adults, yet few animal studies have sought to characterize this phenomenon. We compared effects of fluoxetine and imipramine in two common behavioral assays that hold high predictive validity for antidepressant activity, tail suspension and forced swim test, using juvenile (5 weeks) and adult (12 weeks) mice from 3 strains. C57BL/6J-Tyr(c-Brd) (C57), hybrid C57BL/6J-Tyr(c-Brd)x129S5/SvEvBrd (F2), and Balb/cAnNTac (Balb/C) mice were tested in forced swim test and tail suspension after i.p. dosing with either fluoxetine or imipramine. Brain tissues were analyzed to evaluate levels of VMAT2, a possible modulator of age-dependent sensitivity to antidepressants. Imipramine had more consistent antidepressant effect across age groups and strains. Imipramine increased struggle in mice of both ages. Fluoxetine did not have an effect on immobility in Balb/C of both ages in tail suspension. Fluoxetine also did not increase forced swim struggle behavior in juvenile mice of all strains, but was effective in increasing struggle in adults. Juvenile mice had higher immobility and lower struggle than adults in forced swim, and juveniles also had higher immobility in tail suspension test for Balb/C and C57. In addition, VMAT2 levels were increased in juveniles. These results confirm that standard antidepressants produce effects in both juveniles and adults but age-related differences were evident in both tests. Further examination of these effects is needed to determine whether it may be related to age-dependent difference in the clinical response to antidepressants of these classes.

Psychostimulant-induced alterations in vesicular monoamine transporter-2 function: neurotoxic and therapeutic implications

The vesicular monoamine transporter-2 (VMAT-2) is an important regulator of intraneuronal monoamine concentrations and disposition as this protein sequesters critical cytoplasmic monoaminergic transmitters and contributes to their subsequent exocytotic release. This review primarily discusses the impact of psychoactive drugs (including those with abuse potential) on dopamine (DA)-related VMAT-2 and its function. In particular, the different responses by DA-related VMAT-2 and associated vesicles to plasmalemmal uptake blockers like methylphenidate and releasers like methamphetamine are presented. Recent preclinical findings suggest that vesicular transporter systems are highly regulatable, both by changes in localization as well as alterations in the kinetics of the VMAT-2 protein. The capacity for such shifts in VMAT-2 functions suggests the presence of physiological regulation that likely influences the activity of DA systems. In addition, these findings may contribute to our understanding of the pathogenesis of a variety of DA-related disorders such as substance abuse and Parkinson's disease and also suggest new therapeutic targets for treating such diseases.

Methylphenidate administration alters vesicular monoamine transporter-2 function in cytoplasmic and membrane-associated vesicles

In vivo methylphenidate (MPD) administration increases vesicular monoamine transporter-2 (VMAT-2) immunoreactivity, VMAT-2-mediated dopamine (DA) transport, and DA content in a nonmembrane-associated (referred to herein as cytoplasmic) vesicular subcellular fraction purified from rat striatum: a phenomenon attributed to a redistribution of VMAT-2-associated vesicles within nerve terminals. In contrast, the present study elucidated the nature of, and the impact of MPD on, VMAT-2-associated vesicles that cofractionate with synaptosomal membranes after osmotic lysis (referred to herein as membrane-associated vesicles). Results revealed that, in striking contrast to the cytoplasmic vesicles, DA transport velocity versus substrate concentration curves in the membrane-associated vesicles were sigmoidal, suggesting positive cooperativity with respect to DA transport. Additionally, DA transport into membrane-associated vesicles was greater in total capacity in the presence of high DA concentrations than transport into cytoplasmic vesicles. Of potential therapeutic relevance, MPD increased DA transport into the membrane-associated vesicles despite rapidly decreasing (presumably by redistributing) VMAT-2 immunoreactivity in this fraction. Functional relevance was suggested by findings that MPD treatment increased both the DA content of the membrane-associated vesicle fraction and K(+)-stimulated DA release from striatal suspensions. In summary, the present data demonstrate the existence of a previously uncharacterized pool of membrane-associated VMAT-2-containing vesicles that displays novel transport kinetics, has a large sequestration capacity, and responds to in vivo pharmacological manipulation. These findings provide insight into both the regulation of vesicular DA sequestration and the mechanism of action of MPD, and they may have implications regarding treatment of disorders involving abnormal DA disposition, including Parkinson's disease and substance abuse.

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.

Methamphetamine-induced alterations in monoamine transport: implications for neurotoxicity, neuroprotection and treatment

AIM

To review studies delineating the neurotoxic effects of methamphetamine on monoamine transport in dopaminergic neurons of the striatum and nucleus accumbens.

METHOD

The scope of this review includes the English language dopamine transporter and vesicular monoamine transporter-2 primary literature to April 2006 identified by Pubmed, Science Citation Index and SciFinder Scholar literature searches.

RESULTS

Changes in the function of the plasmalemmal dopamine transporter and the vesicular monoamine transporter-2 are key components of methamphetamine-induced persistent dopaminergic deficits. These deficits include persistent reductions in dopamine content, dopamine transporter density and tyrosine hydroxylase activity. The striatum is susceptible to these effects of methamphetamine while the nucleus accumbens is resistant. Differences in dopamine transporter density and activity, extracellular dopamine levels and antioxidant levels in these two brain regions may, in part, account for the resistance of the nucleus accumbens.

CONCLUSION

These findings concerning the nature of methamphetamine-induced changes in plasmalemmal and vesicular dopamine transport have very important implications for drug targets and for understanding the etiology of dopaminergic neurodegenerative processes, such as those associated with methamphetamine addiction and Parkinson's disease.

The role of the plasmalemmal dopamine and vesicular monoamine transporters in methamphetamine‐induced dopaminergic deficits

Amphetamine (AMPH) and methamphetamine (METH) are members of a collection of phenethylamine psychostimulants that are commonly referred to collectively as "amphetamines." Amphetamines exert their effects, in part, by affecting neuronal dopamine transport. This review thus focuses on the effects of AMPH and METH on the plasmalemmal dopamine transporter and the vesicular monoamine transporter-2 in animal models with a particular emphasis on how these effects, which may vary for the different stereoisomers, contribute to persistent dopaminergic deficits.