Restorative effects of GDNF on striatal dopamine release in rats treated with neurotoxic doses of methamphetamine

Molecular, Behavioral, and Physiological Consequences of Methamphetamine Neurotoxicity: Implications for Treatment

Understanding the relationship between the molecular mechanisms underlying neurotoxicity of high-dose methamphetamine (METH) and related clinical manifestations is imperative for providing more effective treatments for human METH users. This article provides an overview of clinical manifestations of METH neurotoxicity to the central nervous system and neurobiology underlying the consequences of administration of neurotoxic METH doses, and discusses implications of METH neurotoxicity for treatment of human abusers of the drug.

Epothilone D prevents binge methamphetamine-mediated loss of striatal dopaminergic markers

Exposure to binge methamphetamine (METH) can result in a permanent or transient loss of dopaminergic (DAergic) markers such as dopamine (DA), dopamine transporter, and tyrosine hydroxylase (TH) in the striatum. We hypothesized that the METH-induced loss of striatal DAergic markers was, in part, due to a destabilization of microtubules (MTs) in the nigrostriatal DA pathway that ultimately impedes anterograde axonal transport of these markers. To test this hypothesis, adult male Sprague-Dawley rats were treated with binge METH or saline in the presence or absence of epothilone D (EpoD), a MT-stabilizing compound, and assessed 3 days after the treatments for the levels of several DAergic markers as well as for the levels of tubulins and their post-translational modifications (PMTs). Binge METH induced a loss of stable long-lived MTs within the striatum but not within the substantia nigra pars compacta (SNpc). Treatment with a low dose of EpoD increased the levels of markers of stable MTs and prevented METH-mediated deficits in several DAergic markers in the striatum. In contrast, administration of a high dose of EpoD appeared to destabilize MTs and potentiated the METH-induced deficits in several DAergic markers. The low-dose EpoD also prevented the METH-induced increase in striatal DA turnover and increased behavioral stereotypy during METH treatment. Together, these results demonstrate that MT dynamics plays a role in the development of METH-induced losses of several DAergic markers in the striatum and may mediate METH-induced degeneration of terminals in the nigrostriatal DA pathway. Our study also demonstrates that MT-stabilizing drugs such as EpoD have a potential to serve as useful therapeutic agents to restore function of DAergic nerve terminals following METH exposure when administered at low doses. Administration of binge methamphetamine (METH) negatively impacts neurotransmission in the nigrostriatal dopamine (DA) system. The effects of METH include decreasing the levels of DAergic markers in the striatum. We have determined that high-dose METH destabilizes microtubules in this pathway, which is manifested by decreased levels of acetylated (Acetyl) and detyrosinated (Detyr) α-tubulin (I). A microtubule stabilizing agent epothilone D protects striatal microtubules form the METH-induced loss of DAergic markers (II). These findings provide a new strategy for protection form METH - restoration of proper axonal transport.

Running wheel exercise before a binge regimen of methamphetamine does not protect against striatal dopaminergic damage

Repeated administration of methamphetamine (mAMPH) to rodents in a single-day "binge" dosing regimen produces long-lasting damage to forebrain dopaminergic nerve terminals as measured by decreases in tissue dopamine (DA) content and levels of the plasmalemmal DA transporter (DAT). However, the midbrain cell bodies from which the DA terminals arise survive, and previous reports show that striatal DA markers return to control levels by 12 months post-mAMPH, suggesting long-term repair or regrowth of damaged DA terminals. We previously showed that when rats engaged in voluntary aerobic exercise for 3 weeks before and 3 weeks after a binge regimen of mAMPH, exercise significantly ameliorated mAMPH-induced decreases in striatal DAT. However, these data left unresolved the question of whether exercise protected against the initial neurotoxicity from the mAMPH binge or accelerated the repair of the damaged DA terminals. The present experiments were designed to test whether exercise protects against the mAMPH-induced injury. Adult male Sprague-Dawley rats were allowed to run in wheels for 3 weeks before an acute binge regimen of mAMPH or saline, then placed into nonwheel cages for an additional week before autoradiographic determination of striatal DAT binding. The autoradiographic findings showed that prior exercise provided no protection against mAMPH-induced damage to striatal DA terminals. These results, together with analyses from our previous experiments, suggest that voluntary exercise may accelerate the repair of mAMPH-damaged DA terminals and that voluntary exercise may be useful as therapeutic adjunct in the treatment mAMPH addicts.

Methamphetamine influences on brain and behavior: unsafe at any speed?

Methamphetamine damages monoamine-containing nerve terminals in the brains of both animals and human drug abusers, and the cellular mechanisms underlying this injury have been extensively studied. More recently, the growing evidence for methamphetamine influences on memory and executive function of human users has prompted studies of cognitive impairments in methamphetamine-exposed animals. After summarizing current knowledge about the cellular mechanisms of methamphetamine-induced brain injury, this review emphasizes research into the brain changes that underlie the cognitive deficits that accompany repeated methamphetamine exposure. Novel approaches to mitigating or reversing methamphetamine-induced brain and behavioral changes are described, and it is argued that the slow spontaneous reversibility of the injury produced by this drug may offer opportunities for novel treatment development.

Enhanced dopamine transporter activity in middle-aged Gdnf heterozygous mice

Glial cell line-derived neurotrophic factor (GDNF) supports the viability of midbrain dopamine (DA) neurons that degenerate in Parkinson's disease. Middle-aged, 12 month old, Gdnf heterozygous (Gdnf(+/-)) mice have diminished spontaneous locomotor activity and enhanced synaptosomal DA uptake compared with wild type mice. In this study, dopamine transporter (DAT) function in middle-aged, 12 month old Gdnf(+/-) mice was more thoroughly investigated using in vivo electrochemistry. Gdnf(+/-) mice injected with the DAT inhibitor, nomifensine, exhibited significantly more locomotor activity than wild type mice. In vivo electrochemistry with carbon fiber microelectrodes demonstrated enhanced clearance of DA in the striatum of Gdnf(+/-) mice, suggesting greater surface expression of DAT than in wild type littermates. Additionally, 12 month old Gdnf(+/-) mice expressed greater D(2) receptor mRNA and protein in the striatum than wild type mice. Neurochemical analyses of striatal tissue samples indicated significant reductions in DA and a faster DA metabolic rate in Gdnf(+/-) mice than in wild type mice. Altogether, these data support an important role for GDNF in the regulation of uptake, synthesis, and metabolism of DA during aging.

Evoked dopamine overflow is augmented in the striatum of calcitriol treated rats

Calcitriol, the active metabolite of vitamin D, has been shown to have significant effects on the brain. These actions include reducing the severity of some central nervous system lesions, possibly by upregulating trophic factors such as glial cell line-derived neurotrophic factor (GDNF). GDNF has substantial effects on the nigrostriatal dopamine (DA) system of young adult, aged and lesioned animals. Thus, the administration of calcitriol may lead to significant effects on nigrostriatal DA neuron functioning. The present experiments were designed to examine the ability of calcitriol to alter striatal DA release, and striatal and nigral tissue levels of DA. Male Fischer-344 rats were administered vehicle or calcitriol (0.3, 1.0, or 3.0 μg/kg, s.c.) once daily for eight consecutive days. Three weeks later in vivo microdialysis experiments were conducted to measure basal and stimulus evoked overflow of DA from the striatum. Basal levels of extracellular DA were not significantly affected by the calcitriol treatments. However, the 1.0 and 3.0 μg/kg doses of calcitriol led to increases in both potassium and amphetamine evoked overflow of striatal DA. Although post-mortem tissue levels of striatal DA were not altered by the calcitriol injections, nigral tissue levels of DA and its main metabolites were increased by both the 1.0 and 3.0 μg/kg doses of calcitriol. In a separate group of animals GDNF levels were augmented in the striatum and substantia nigra after eight consecutive daily injections of calcitriol. These results suggest that systemically administered calcitriol can upregulate dopaminergic release processes in the striatum and DA levels in the substantia nigra. Increases in the levels of endogenous GDNF following calcitriol treatment may in part be responsible for these changes. The ability of calcitriol to lead to augmented DA release in the striatum suggests that calcitriol may be beneficial in disease processes involving dopaminergic dysfunction.

Impact of methamphetamine on dopamine neurons in primates is dependent on age: implications for development of Parkinson's disease

Methamphetamine is a CNS stimulant with limited therapeutic indications, but is widely abused. Short-term exposure to higher doses, or long-term exposure to lower doses, of methamphetamine induces lasting damage to nigrostriatal dopamine neurons in man and animals. Strong evidence indicates that the mechanism for this detrimental effect on dopamine neurons involves oxidative stress exerted by reactive oxygen species. This study investigates the relative susceptibility of dopamine neurons in mid-gestation, young, and adult (not aged) monkeys to four treatments with methamphetamine over 2 days. Primate dopamine neurons undergo natural cell death at mid-gestation, and we hypothesized that during this event they are particularly vulnerable to oxidative stress. The results indicated that at mid-gestation and in adults, dopamine neurons were susceptible to methamphetamine-induced damage, as indicated by loss of striatal tyrosine hydroxylase (TH) immunoreactivity and dopamine concentration. However, dopamine neurons in young animals appeared totally resistant to the treatment, despite this group having higher brain levels of methamphetamine 3 h after administration than the adults. As a possible explanation for the protection, striatal glial-derived neurotrophic factor (GDNF) levels were elevated in young animals 1 week after treatment, but not in adults following methamphetamine treatment. Implications of these primate studies are: (1) the susceptibility of dopamine neurons at mid-gestation to methamphetamine warns against the risk of exposing pregnant women to the drug or oxidative stressors, and supports the hypothesis of Parkinson's disease being associated with oxidative stress during development, (2) elucidation of the mechanism of resistance of dopamine neurons in the young animals to methamphetamine-induced oxidative stress may provide targets for slowing or preventing age- or disease-related loss of adult nigrostriatal dopamine (DA) neurons, and (3) the increased striatal production of GDNF in young animals, but not in adults, in response to methamphetamine, suggests the possibility of an age-related change in the neurotrophic capacity of the striatal dopamine system.

The glial cell modulator and phosphodiesterase inhibitor, AV411 (ibudilast), attenuates prime- and stress-induced methamphetamine relapse

Stress and renewed contact with drug (a "slip") have been linked to persisting relapse of methamphetamine abuse. Human brain microglial activation has been linked with methamphetamine abuse, and inhibitors of glial cell activation, certain phosphodiesterase (PDE) inhibitors, and glial cell derived neurotrophic factor (GDNF) have been reported to modulate drug abuse effects. Our objective was to determine whether the glial cell attenuator, 3-isobutyryl-2-isopropylpyrazolo-[1,5-a]pyridine (AV411, ibudilast), a non-selective PDE inhibitor and promoter of GDNF, could reduce stress- and methamphetamine prime-induced reinstatement of methamphetamine-seeking behavior. Male Long-Evans hooded rats were trained to lever press reinforced with 0.1 mg/kg i.v. methamphetamine infusion according to fixed-ratio 1 (FR1) reinforcement schedules during daily, 2-hour experimental sessions. After performance had stabilized, lever pressing was extinguished for 12 consecutive sessions and doses of 0 (vehicle), 2.5 and 7.5 mg/kg AV411 were then administered intraperitoneally b.i.d. on the last 2 days of extinction and then once on the testday to separate groups of 12 rats. During testing, the rats were given 15 min of intermittent footshock or a 1 mg/kg i.p. methamphetamine prime followed by a 2-hour reinstatement test session. AV411 significantly reduced response levels of footshock-induced (2.5 and 7.5 mg/kg) and prime-induced (7.5 mg/kg) reinstatement of extinguished methamphetamine-maintained responding. AV411 has properties consistent with the ability to attenuate relapse precipitated by stress and methamphetamine "slips" during abstinence. These results thus reinforce interest in atypical neurobiological mechanisms which could be exploited for developing novel medications for treating drug abuse disorders.

Amphetamine toxicities: classical and emerging mechanisms

The drugs of abuse, methamphetamine and MDMA, produce long-term decreases in markers of biogenic amine neurotransmission. These decreases have been traditionally linked to nerve terminals and are evident in a variety of species, including rodents, nonhuman primates, and humans. Recent studies indicate that the damage produced by these drugs may be more widespread than originally believed. Changes indicative of damage to cell bodies of biogenic and nonbiogenic amine-containing neurons in several brain areas and endothelial cells that make up the blood-brain barrier have been reported. The processes that mediate this damage involve not only oxidative stress but also include excitotoxic mechanisms, neuroinflammation, the ubiquitin proteasome system, as well as mitochondrial and neurotrophic factor dysfunction. These mechanisms also underlie the toxicity associated with chronic stress and human immunodeficiency virus (HIV) infection, both of which have been shown to augment the toxicity to methamphetamine. Overall, multiple mechanisms are involved and interact to promote neurotoxicity to methamphetamine and MDMA. Moreover, the high coincidence of substituted amphetamine abuse by humans with HIV and/or chronic stress exposure suggests a potential enhanced vulnerability of these individuals to the neurotoxic actions of the amphetamines.

Molecular bases of methamphetamine-induced neurodegeneration

Methamphetamine (METH) is a highly addictive psychostimulant drug, whose abuse has reached epidemic proportions worldwide. The addiction to METH is a major public concern because its chronic abuse is associated with serious health complications including deficits in attention, memory, and executive functions in humans. These neuropsychiatric complications might, in part, be related to drug-induced neurotoxic effects, which include damage to dopaminergic and serotonergic terminals, neuronal apoptosis, as well as activated astroglial and microglial cells in the brain. Thus, the purpose of the present paper is to review cellular and molecular mechanisms that might be responsible for METH neurotoxicity. These include oxidative stress, activation of transcription factors, DNA damage, excitotoxicity, blood-brain barrier breakdown, microglial activation, and various apoptotic pathways. Several approaches that allow protection against METH-induced neurotoxic effects are also discussed. Better understanding of the cellular and molecular mechanisms involved in METH toxicity should help to generate modern therapeutic approaches to prevent or attenuate the long-term consequences of psychostimulant use disorders in humans.

Calcitriol protects against the dopamine- and serotonin-depleting effects of neurotoxic doses of methamphetamine

Repeated methamphetamine (METH) administration to animals can result in long-lasting decreases in brain dopamine (DA) and serotonin (5-HT) content. Calcitriol, the active metabolite of vitamin D, has potent effects on brain cells, both in vitro and in vivo, including the ability to upregulate trophic factors and protect against various lesions. The present experiments were designed to examine the ability of calcitriol to protect against METH-induced reductions in striatal and nucleus accumbens levels of DA and 5-HT. Male Fischer-344 rats were administered vehicle or calcitriol (1 microg/kg, s.c.) once a day for eight consecutive days. After the seventh day of treatment the animals were given METH (5 mg/kg, s.c.) or saline four times in 1 day at 2-h intervals. Seven days later the striata and accumbens were harvested from the animals for high-performance liquid chromatography (HPLC) analysis of monoamines and metabolites. In animals treated with vehicle and METH, there were significant reductions in DA, 5-HT, and their metabolites in both the striatum and accumbens. In animals treated with calcitriol and METH, the magnitude of the METH-induced reductions in DA, 5-HT, and metabolites was substantially and significantly attenuated. The calcitriol treatments did not reduce the hyperthermia associated with multiple injections of METH, indicating that the neuroprotective effects of calcitriol are not due to the prevention of increases in body temperature. These results suggest that calcitriol can provide significant protection against the DA- and 5-HT-depleting effects of neurotoxic doses of METH.

Speed kills: cellular and molecular bases of methamphetamine‐induced nerve terminal degeneration and neuronal apoptosis

Methamphetamine (METH) is a drug of abuse that has long been known to damage monoaminergic systems in the mammalian brain. Recent reports have provided conclusive evidence that METH can cause neuropathological changes in the rodent brain via apoptotic mechanisms akin to those reported in various models of neuronal death. The purpose of this review is to provide an interim account for a role of oxygen-based radicals and the participation of transcription factors and the involvement of cell death genes in METH-induced neurodegeneration. We discuss data suggesting the participation of endoplasmic reticulum and mitochondria-mediated activation of caspase-dependent and -independent cascades in the manifestation of METH-induced apoptosis. Studies that use more comprehensive approaches to gene expression profiling should allow us to draw more instructive molecular portraits of the complex plastic and degenerative effects of this drug.

Dopaminergic neuroprotection and regeneration by neurturin assessed by using behavioral, biochemical and histochemical measurements in a model of progressive Parkinson's disease

Trophic effects of neurturin, a member of the glial cell line-derived neurotrophic factor-family, have been demonstrated on mesencephalic dopaminergic neurons, suggesting its therapeutic potential for Parkinson's disease. This study was designed to test the neuroprotective and regenerative effects of an intrastriatal injection of neurturin based on behavioral, neurochemical and histochemical changes in a rat model of progressive Parkinson's disease. An extensive and progressive dopaminergic lesion was unilaterally made by intrastriatal convection-enhanced delivery of 6-hydroxydopamine (6-OHDA), in which 20 microg of 6-OHDA dissolved in 20 microl of vehicle was infused at a rate of 0.2 microl/min. For neuroprotection study, recombinant human neurturin (5 microg in 5 microl of vehicle) was stereotaxically injected into the unilateral striatum. The 6-OHDA lesion was made on the ipsilateral side 3 days after the neurturin treatment. Tyrosine hydroxylase (TH)-immunoreactive neurons of the substantia nigra were protected from progressive degeneration in the neurturin-treated animals compared with the vehicle-treated animals 2 and 8 weeks after the 6-OHDA lesion. Eight weeks after the 6-OHDA lesion, dopamine concentration significantly increased in the striatum of neurturin-treated animals with improvement of methamphetamine-induced rotation behavior. For neuroregeneration study, 5 microg of neurturin was injected into the striatum 12 weeks after the 6-OHDA lesion. Four weeks after neurturin or vehicle injection, there were no significant differences in the survival of nigral TH-immunoreactive neurons between the groups. However, TH-immunoreactive fibers were thicker and more abundant in the striatum of the neurturin-treated rats compared to those of the control group, suggesting neurturin-induced growth of the dopaminergic axons. Striatal dopamine levels also significantly increased in the neurturin-treated rats compared with those in the control group of rats, accompanied by the recovery of methamphetamine-induced rotation in the neurturin-treated rats. In conclusion, an intrastriatal injection of neurturin is a useful method to protect nigral dopaminergic neurons from extensive cell death in a model of progressive Parkinson's disease, as well as to promote the axonal regeneration and dopaminergic function.