Dopamine Neuronal Systems Research Articles

Long-term intrathecal infusion of low-dose morphine effectively relieves symptoms of severe restless legs syndrome/Willis-Ekbom disease without inducing opioid tolerance.

Restless legs syndrome/Willis-Ekbom disease (RLS/WED) causes a strong urge to move legs while resting. Restless legs syndrome/WED is an often-inherited disease occurring in 3% to 10% of adult populations, increasing with age. Severity varies from mild disturbance of sleep to painful restless legs and arms, loss of sleep, fatigue, and risk of suicide. Dopaminergic drugs relieve symptoms, but cause augmentation, ie, initially helpful but later increase the burden of symptoms. Oral gabapentinoids and opioids are often added, but opioid tolerance and adverse effects are common. With the high prevalence and incomplete help from oral drugs, significant unmet needs exist for effective therapy for severe RLS/WED. Ongoing spinal intrathecal infusion of low-dose morphine is effective, but not generally recognized, as only 12 cases have been published since 2002. We report 7 patients suffering from severe RLS/WED, who had no relief from oral dopaminergic, gabapentinoid, or opioid drugs; they all had excellent relief during ongoing spinal intrathecal infusion of morphine at only 1 to 5 μg/h, ongoing for 1 to 21 years without need of higher doses of morphine.. We suggest that morphine may be transported with the cerebrospinal fluid reaching and readjusting malfunctioning dopamine neuronal systems in the brain and spinal cord. The effects last only as long as the infusion continues. A patient with RLS/WED and persistent genital arousal disorder (PGAD) was relieved of both RLS/WED and PGAD symptoms. These case reports suggest that intrathecal infusion of low-dose morphine is an effective treatment of severe RLS.

Music rhythm perception and production relate to treatment response in schizophrenia

Accumulating evidence indicates that pathophysiology of schizophrenia involves abnormalities in the dopamine and glutamatergic neuronal systems. Antipsychotic medications are currently used to normalize dopaminergic function for schizophrenia. However, approximately 30 % of the patients have no response to antipsychotic medications, which is classified as treatment-resistant schizophrenia (TRS). Furthermore, dopamine and glutamate levels in the neural basis have been reported to differ between TRS and non-TRS. In this study, we assumed that these differences may affect music rhythm perception and production abilities between the two groups. We examined fifty-seven schizophrenia (26 TRS, 31 non-TRS) and thirty-one healthy controls (HCs) by using the Harvard Beat Assessment Test (H-BAT). As a result, we found that rhythm production was worse in patients with TRS compared to patients with non-TRS and HCs, while no difference was observed between patients with non-TRS and HCs. In addition, rhythm perception and production abilities were impaired in the whole patient group compared with HCs. Furthermore, in the patient group, the deficits were correlated with cognitive impairments. Collectively, these results suggest that patients with schizophrenia may have rhythm processing deficits, with particular a rhythm production problem in the TRS group.

Effects of (−)-sesamin on motor and memory deficits in an MPTP-lesioned mouse model of Parkinson’s disease treated with l-DOPA

The present study investigated the effects of (−)-sesamin on motor and memory deficits in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mouse model of Parkinson’s disease (PD) with l-3,4-dihydroxyphenylalanine (l-DOPA). MPTP-lesioned (30mg/kg/day, 5days) mice showed deficits in memory including habit learning memory and spatial memory, which were further aggravated by daily treatment with 25mg/kg l-DOPA for 21days. However, daily treatment with (−)-sesamin (25 and 50mg/kg) for 21days ameliorated memory deficits in an MPTP-lesioned mouse model of PD treated with l-DOPA (25mg/kg). Both (−)-sesamin doses reduced decreases in the retention latency time in the passive avoidance test, latency to fall of rotarod test and distance traveled in the open field test, and attenuated decreases in tyrosine hydroxylase (TH)-immunopositive cells, dopamine, and its metabolites in the substantia nigra-striatum. (−)-Sesamin reduced increases in the retention transfer latency time in the elevated plus-maze test and N-methyl-d-aspartate receptor (NMDAR) expression and reduced decreases in the phosphorylation of extracellular signal-regulated kinase (ERK1/2) and cyclic AMP-response element binding protein (CREB) in the hippocampus. In contrast, daily treatment with 10mg/kg l-DOPA for 21days ameliorated memory deficits in MPTP-lesioned mice, and this effect was further improved by treatment with (−)-sesamin (25 and 50mg/kg). These results suggest that (−)-sesamin protects against habit learning memory deficits by activating the dopamine neuronal system, while spatial memory deficits are decreased by its modulatory effects on the NMDAR-ERK1/2-CREB system. Accordingly, (−)-sesamin may act as an adjuvant phytonutrient for motor and memory deficits in patients with PD receiving l-DOPA.

Multiple D2 heteroreceptor complexes: new targets for treatment of schizophrenia.

The dopamine (DA) neuron system most relevant for schizophrenia is the meso-limbic-cortical DA system inter alia densely innervating subcortical limbic regions. The field of dopamine D2 receptors and schizophrenia changed markedly with the discovery of many types of D2 heteroreceptor complexes in subcortical limbic areas as well as the dorsal striatum. The results indicate that the D2 is a hub receptor which interacts not only with many other G protein-coupled receptors (GPCRs) including DA isoreceptors but also with ion-channel receptors, receptor tyrosine kinases, scaffolding proteins and DA transporters. Disturbances in several of these D2 heteroreceptor complexes may contribute to the development of schizophrenia through changes in the balance of diverse D2 homo- and heteroreceptor complexes mediating the DA signal, especially to the ventral striato-pallidal γ-aminobutyric acid (GABA) pathway. This will have consequences for the control of this pathway of the glutamate drive to the prefrontal cortex via the mediodorsal thalamic nucleus which can contribute to psychotic processes. Agonist activation of the A2A protomer in the A2A-D2 heteroreceptor complex inhibits D2 Gi/o mediated signaling but increases the D2 β-arrestin2 mediated signaling. Through this allosteric receptor-receptor interaction, the A2A agonist becomes a biased inhibitory modulator of the Gi/o mediated D2 signaling, which may the main mechanism for its atypical antipsychotic properties especially linked to the limbic A2A-D2 heterocomplexes. The DA and glutamate hypotheses of schizophrenia come together in the signal integration in D2-N-methyl-d-aspartate (NMDA) and A2A-D2-metabotropic glutamate receptor 5 (mGlu5) heteroreceptor complexes, especially in the ventral striatum. 5-Hydroxytryptamine 2A (5-HT2A)-D2 heteroreceptor complexes are special targets for atypical antipsychotics with high potency to block their 5-HT2A protomer signaling in view of the potential development of pathological allosteric facilitatory 5-HT2A-D2 interaction increasing D2 protomer signaling. Neurotensin (NTS1)-D2 heterocomplexes also exist in the ventral and dorsal striatum, and likely also in midbrain DA nerve cells as NTS1-D2 autoreceptor complexes where neurotensin produces antipsychotic and propsychotic actions, respectively.

MiR-126 contributes to Parkinson's disease by dysregulating the insulin-like growth factor/phosphoinositide 3-kinase signaling

Dopamine (DA) neurons in sporadic Parkinson's disease (PD) display dysregulated gene expression networks and signaling pathways that are implicated in PD pathogenesis. Micro (mi)RNAs are regulators of gene expression, which could be involved in neurodegenerative diseases. We determined the miRNA profiles in laser microdissected DA neurons from postmortem sporadic PD patients' brains and age-matched controls. DA neurons had a distinctive miRNA signature and a set of miRNAs was dysregulated in PD. Bioinformatics analysis provided evidence for correlations of miRNAs with signaling pathways relevant to PD, including an association of miR-126 with insulin/IGF-1/PI3K signaling. In DA neuronal cell systems, enhanced expression of miR-126 impaired IGF-1 signaling and increased vulnerability to the neurotoxin 6-OHDA by downregulating factors in IGF-1/PI3K signaling, including its targets p85β, IRS-1, and SPRED1. Blocking of miR-126 function increased IGF-1 trophism and neuroprotection to 6-OHDA. Our data imply that elevated levels of miR-126 may play a functional role in DA neurons and in PD pathogenesis by downregulating IGF-1/PI3K/AKT signaling and that its inhibition could be a mechanism of neuroprotection.

Functional integrity and modification of retinal dopaminergic neurons in the rd1 mutant mouse: roles of melanopsin and GABA

The progressive loss of rod and cone photoreceptors in human subjects with retinitis pigmentosa causes a gradual decline in vision and can result in blindness. Current treatment strategies for the disease rely on the integrity of inner retinal neurons, such as amacrine cells, that are postsynaptic to photoreceptors. Previous work has demonstrated that a specialized subclass of retinal amacrine cell that synthesizes and releases the key neurotransmitter dopamine remains morphologically intact during the disease; however, the pathophysiological function of these neurons remains poorly understood. Here we examined spontaneous and light-evoked spike activity of genetically labeled dopamine neurons from the retinas of retinal degeneration 1 (rd1) mice. Our results indicated that rd1 dopamine neurons remained functionally intact with preserved spontaneous spiking activity and light-evoked responses. The light responses were mediated exclusively by melanopsin phototransduction, not by surviving cones. Our data also suggested that dopamine neurons were altered during photoreceptor loss, as evidenced by less spontaneous bursting activity and increased light-evoked responses with age. Further evidence showed that these alterations were attributed to enhanced GABA/melanopsin signaling to dopamine neurons during disease progression. Taken together, our studies provide valuable information regarding the preservation and functional modification of the retinal dopamine neuronal system in rd1; this information should be considered when designing treatment strategies for retinitis pigmentosa.

Effects of 3-<i>O</i>-Methyldopa, <small>L</small>-3,4-Dihydroxyphenylalanine Metabolite, on Locomotor Activity and Dopamine Turnover in Rats

It has been well known that 3-O-methyldopa (3-OMD) is a metabolite of L-3,4-dihydroxyphenylalanine (L-DOPA) formed by catechol O-methyltransferase (COMT), and 3-OMD blood level often reaches higher than physiological level in Parkinson's disease (PD) patients receiving long term L-DOPA therapy. However, the physiological role of 3-OMD has not been well understood. Therefore, in order to clarify the effects of 3-OMD on physiological function, we examined the behavioral alteration in rats based on locomotor activity, and measured dopamine (DA) and its metabolites levels in rats at the same time after 3-OMD subchronic administration. The study results showed that repeated administrations of 3-OMD increased its blood and the striatum tissue levels in those rats, and decreased locomotor activity in a dose dependent manner. Although 3-OMD subchronic administration showed no significant change in DA level in the striatum, DA metabolite levels, such as 3,4-dihydroxyphenylacetic acid (DOPAC), 3-methoxytyramine (3-MT), and homovanillic acid (HVA) were significantly decreased. After 3-OMD washout period (7 d), locomotor activity and DA turnover in those rats returned to normal levels. Furthermore, locomotor activity and DA turnover decreased by 3-OMD administration were recovered to normal level by acute L-DOPA administration. These results suggested that 3-OMD affect to locomotor activity via DA neuron system. In conclusion, 3-OMD itself may have a disadvantage in PD patients receiving L-DOPA therapy.

Effect of fetal exposure to titanium dioxide nanoparticle on brain development − brain region information

The production of man-made nanoparticles is increasing in nanotechnology, and health effect of nanomaterials is of concern. We previously reported that fetal exposure to titanium dioxide (TiO(2)) affects the brain of offspring during the perinatal period. The aim of this study was to extract candidate brain regions of interest using a specific group of Medical Subject Headings (MeSH) from a microarray dataset of the whole brain of mice prenatally exposed to TiO(2) nanoparticle. After subcutaneous injection of TiO(2) (total 0.4 mg) into pregnant mice on gestational days 6-15, brain tissues were collected from male fetuses on embryonic day 16 and from male pups on postnatal days 2, 7, 14 and 21. Gene expression changes were determined by microarray and analyzed with MeSH indicating brain regions. As a result, a total of twenty-one MeSH were significantly enriched from gene expression data. The results provide data to support the hypothesis that prenatal TiO(2) exposure results in alteration to the cerebral cortex, olfactory bulb and some regions intimately related to dopamine systems of offspring mice. The genes associated with the striatum were differentially expressed during the perinatal period, and those associated with the regions related to dopamine neuron system and the prefrontal region were dysregulated in the later infantile period. The anatomical information gave us clues as to the mechanisms that underlie alteration of cerebral gene expression and phenotypes induced by fetal TiO(2) exposure.

Nucleus Accumbens Invulnerability to Methamphetamine Neurotoxicity

Methamphetamine (Meth) is a neurotoxic drug of abuse that damages neurons and nerve endings throughout the central nervous system. Emerging studies of human Meth addicts using both postmortem analyses of brain tissue and noninvasive imaging studies of intact brains have confirmed that Meth causes persistent structural abnormalities. Animal and human studies have also defined a number of significant functional problems and comorbid psychiatric disorders associated with long-term Meth abuse. This review summarizes the salient features of Meth-induced neurotoxicity with a focus on the dopamine (DA) neuronal system. DA nerve endings in the caudate-putamen (CPu) are damaged by Meth in a highly delimited manner. Even within the CPu, damage is remarkably heterogeneous, with ventral and lateral aspects showing the greatest deficits. The nucleus accumbens (NAc) is largely spared the damage that accompanies binge Meth intoxication, but relatively subtle changes in the disposition of DA in its nerve endings can lead to dramatic increases in Meth-induced toxicity in the CPu and overcome the normal resistance of the NAc to damage. In contrast to the CPu, where DA neuronal deficiencies are persistent, alterations in the NAc show a partial recovery. Animal models have been indispensable in studies of the causes and consequences of Meth neurotoxicity and in the development of new therapies. This research has shown that increases in cytoplasmic DA dramatically broaden the neurotoxic profile of Meth to include brain structures not normally targeted for damage. The resistance of the NAc to Meth-induced neurotoxicity and its ability to recover reveal a fundamentally different neuroplasticity by comparison to the CPu. Recruitment of the NAc as a target of Meth neurotoxicity by alterations in DA homeostasis is significant in light of the numerous important roles played by this brain structure.

Differential effects of stress on [11C]raclopride and [11C]MNPA binding to striatal D2/D3 dopamine receptors: A PET study in conscious monkeys

It has been reported that stress and facilitation of dopamine neuronal system are closely related. In the present study, the effects of stress on the binding of antagonist-based [¹¹C]raclopride and agonist-based (R)-2-CH3O-N-n- propylnorapomorphine ([¹¹C]MNPA) to D₂/D₃ receptors were evaluated in the striatum of conscious monkey brain. The stress state assessed from plasma cortisol level was negatively correlated with [¹¹C]raclopride binding as expected. It was noteworthy that [¹¹C]MNPA binding exhibited a positive correlation with stress state; thus, the animals with higher cortisol levels showed higher binding to D₂/D₃ receptors.

The role of endogenous serotonin in methamphetamine‐induced neurotoxicity to dopamine nerve endings of the striatum

Methamphetamine (METH) is a neurotoxic drug of abuse that damages the dopamine (DA) neuronal system in a highly delimited manner. The brain structure most affected by METH is the striatum where long-term DA depletion and microglial activation are maximal. Endogenous DA has been implicated as a critical participant in METH-induced neurotoxicity, most likely as a substrate for non-enzymatic oxidation by METH-generated reactive oxygen species. The striatum is also extensively innervated by serotonin (5HT) nerve endings and this neurochemical system is modified by METH in much the same manner as seen in DA nerve endings (i.e., increased release of 5HT, loss of function in tryptophan hydroxylase and the serotonin transporter, long-term depletion of 5HT stores). 5HT can also be modified by reactive oxygen species to form highly reactive species that damage neurons but its role in METH neurotoxicity has not been assessed. Increases in 5HT levels with 5-hydroxytryptophan do not change METH-induced neurotoxicity to the DA nerve endings as revealed by reductions in DA, tyrosine hydroxylase and dopamine transporter levels. Partial reductions in 5HT with p-chlorophenylalanine are without effect on METH toxicity, despite the fact that p-chlorophenylalanine largely prevents METH-induced hyperthermia. Mice lacking the gene for brain tryptophan hydroxylase 2 are devoid of brain 5HT and respond to METH in the same manner as wild-type controls, despite showing enhanced drug-induced hyperthermia. Taken together, the present results indicate that endogenous 5HT does not appear to play a role in METH-induced damage to DA nerve endings of the striatum.

Ondansetron Augmentation in Treatment-Resistant Obsessive-Compulsive Disorder

Serotonin and dopamine neuronal systems have been implicated in the modulation of obsessive-compulsive disorder (OCD) symptoms. About 40% of OCD patients do not respond to first-line selective serotonin reuptake inhibitor (SSRI) treatment; among those, dopamine blocker augmentation has been reported to improve the rate of response by an additional one-third. Given that serotonin 5-HT(3) receptors are indirect inhibitors of cortico-mesolimbic dopamine release, augmentation with the 5-HT(3) receptor antagonist ondansetron in combination with SSRIs and antipsychotics has potential efficacy in treatment-resistant OCD patients. To assess the efficacy and tolerability of ondansetron in combination with SSRIs and antipsychotics in patients with treatment-resistant OCD. In total, 14 patients with a DSM-IV diagnosis of OCD, who were treatment resistant and receiving stable treatment with SSRIs and antipsychotic augmentation, entered a 12-week, single-blind trial of ondansetron. The drug was initiated at a dosage of 0.25 mg twice daily for 6 weeks and was then titrated to 0.5 mg twice daily for 6 weeks. Of the 14 patients, nine (64.3%) experienced a treatment response (> or =25% reduction in the Yale-Brown Obsessive Compulsive Scale [YBOCS] score and a Clinical Global Impressions-Improvement [CGI-I] score of 1 or 2) at 12 weeks. The average reduction in YBOCS-rated symptoms for the whole group was 23.2%. None of the treated patients experienced symptom exacerbation or significant adverse effects. These results suggest that low-dose ondansetron may have promise as an augmentation strategy for some patients with OCD resistant to SSRIs and antipsychotic augmentation, but further controlled trials are required. Trial registration number (ClinicalTrials.gov): NCT00796497.

Learning Substrates in the Primate Prefrontal Cortex and Striatum: Sustained Activity Related to Successful Actions

Learning from experience requires knowing whether a past action resulted in a desired outcome. The prefrontal cortex and basal ganglia are thought to play key roles in such learning of arbitrary stimulus-response associations. Previous studies have found neural activity in these areas, similar to dopaminergic neurons' signals, that transiently reflect whether a response is correct or incorrect. However, it is unclear how this transient activity, which fades in under a second, influences actions that occur much later. Here, we report that single neurons in both areas show sustained, persistent outcome-related responses. Moreover, single behavioral outcomes influence future neural activity and behavior: behavioral responses are more often correct and single neurons more accurately discriminate between the possible responses when the previous response was correct. These long-lasting signals about trial outcome provide a way to link one action to the next and may allow reward signals to be combined over time to implement successful learning.

Increases in cytoplasmic dopamine compromise the normal resistance of the nucleus accumbens to methamphetamine neurotoxicity

Methamphetamine (METH) is a neurotoxic drug of abuse that damages the dopamine (DA) neuronal system in a highly delimited manner. The brain structure most affected by METH is the caudate-putamen (CPu) where long-term DA depletion and microglial activation are most evident. Even damage within the CPu is remarkably heterogenous with lateral and ventral aspects showing the greatest deficits. The nucleus accumbens (NAc) is largely spared of the damage that accompanies binge METH intoxication. Increases in cytoplasmic DA produced by reserpine, L-DOPA or clorgyline prior to METH uncover damage in the NAc as evidenced by microglial activation and depletion of DA, tyrosine hydroxylase (TH), and the DA transporter. These effects do not occur in the NAc after treatment with METH alone. In contrast to the CPu where DA, TH, and DA transporter levels remain depleted chronically, DA nerve ending alterations in the NAc show a partial recovery over time. None of the treatments that enhance METH toxicity in the NAc and CPu lead to losses of TH protein or DA cell bodies in the substantia nigra or the ventral tegmentum. These data show that increases in cytoplasmic DA dramatically broaden the neurotoxic profile of METH to include brain structures not normally targeted for damage by METH alone. The resistance of the NAc to METH-induced neurotoxicity and its ability to recover reveal a fundamentally different neuroplasticity by comparison to the CPu. Recruitment of the NAc as a target of METH neurotoxicity by alterations in DA homeostasis is significant in light of the important roles played by this brain structure.

Expression of transgenes in midbrain dopamine neurons using the tyrosine hydroxylase promoter

Billions of neurons are interconnected in the central nervous system (CNS). Identification of specific neuronal circuit is indispensable for understanding the relationship between structure and function in the CNS. The midbrain dopamine (DA) neuron system consists of the retrorubral area (A8), the substantia nigra (SN; A9) and the ventral tegmental area (VTA; A10). We hypothesized that genetic methods using cell-type-specific promoters may offer the possibility to express tracer molecules in DA neurons to facilitate neuronal tracing. To address this, we used the 2.5 kb rat tyrosine hydroxylase (TH) promoter in adenovirus or adeno-associated virus (AAV) to express tracers specifically in DA neurons. We found that stereotaxic injection of TH promoter containing adenoviral construct resulted in cell-type-specific transgene expression in the noradrenaline (NA) neurons of the locus coeruleus (LC). However, it caused a significant toxicity to DA neurons in the SN. In contrast, stereotaxic injection of TH promoter containing AAV to the SN resulted in cell-type-specific transgene expression in DA neurons with no detectable toxicity. Taken together, our results demonstrate that it is possible to selectively trace DA neuronal circuits in rodent brains using the TH promoter in the context of AAV.

CRF Facilitates Calcium Release from Intracellular Stores in Midbrain Dopamine Neurons

Changes in cytosolic calcium are crucial for numerous processes including neuronal plasticity. This study investigates the regulation of cytosolic calcium by corticotropin-releasing factor (CRF) in midbrain dopamine neurons. The results demonstrate that CRF stimulates the release of intracellular calcium from stores through activation of adenylyl cyclase and PKA. Imaging and photolysis experiments showed that the calcium originated from dendrites and required both functional IP3 and ryanodine receptor channels. The elevation in cytosolic calcium potentiated calcium-sensitive potassium channels (sK) activated by action potentials and metabotropic Gq-coupled receptors for glutamate and acetylcholine. This increase in cytosolic calcium activated by postsynaptic Gs-coupled CRF receptors may represent a fundamental mechanism by which stress peptides and hormones can shape Gq-coupled receptor-mediated regulation of neuronal excitability and synaptic plasticity in dopamine neurons.

Poor evidence for depolarization block but uncoupling of nigral from striatal dopamine metabolism after chronic haloperidol treatment in the rat

Chronic haloperidol treatment induces depolarization block in midbrain dopamine neuronal systems. We studied the effect of this treatment on nigrostriatal dopamine catabolism using microwave fixation in situ of the brain to prevent post-mortem changes. Male Sprague-Dawley rats were given haloperidol (0.4 mg/kg/day, i.p.) or vehicle for 21 days. On day 22, some rats in each group received a haloperidol challenge (0.4 mg/kg, i.p.), and the remaining rats were given the vehicle. Dopamine metabolite levels 60 min after the challenge were assayed by combined gas chromatography-mass fragmentography. Haloperidol pretreatment significantly modified haloperidol challenge effect on regional dopamine metabolite contents. The challenge elevated all striatal metabolites studied similarly in the chronic vehicle- or chronic haloperidol-pretreated rats. In contrast, it did not significantly affect nigral dopamine metabolites except it elevated 3,4-dihydroxyphenylacetic acid in the haloperidol-pretreated rats. A linear correlation between the nigral and striatal contents of 3-methoxytyramine (R = 0.72, p = 0.03), and a trend for correlation (R = 0.65, p = 0.06) between the respective 3,4-dihydroxyphenylacetic acid contents were found after the haloperidol challenge in the vehicle-pretreated rats only. These results suggest that chronic haloperidol treatment uncouples somatodendritic dopamine turnover and release from those in the axon terminals of nigrostriatal dopamine neurons.

Neurochemical investigations of dopamine neuronal systems in iron-regulatory protein 2 (IRP-2) knockout mice

Abnormal iron accumulations are frequently observed in the brains of patients with Parkinson's disease and in normal aging. Iron metabolism is regulated in the CNS by iron regulatory proteins (IRP-1 and IRP-2). Mice engineered to lack IRP-2 develop abnormal motoric behaviors including tremors at rest, abnormal gait, and bradykinesia at middle to late age (18 to 24 months). To further characterize the dopamine (DA) systems of IRP-2 −/− mice, we harvested CNS tissue from age-matched wild type and IRP-2 −/− (16–19 months) and analyzed the protein levels of tyrosine hydroxylase (TH), dopamine transporter (DAT), vesicular monoamine transporter (VMAT2), and DA levels in dorsal striatum, ventral striatum (including the core and shell of nucleus accumbens), and midbrain. We further analyzed the phosphorylation of TH in striatum at serine 40, serine 31, and serine 19. In both dorsal and ventral striatum of IRP-2 knockout mice, there was a 20–25% loss of TH protein and accompanied by a ∼50% increase in serine 40 phosphorylation above wild-type levels. No change in serine 31 phosphorylation was observed. In the ventral striatum, there was also a significant loss (∼40%) of DAT and VMAT2. Levels of DA were decreased (∼20%) in dorsal striatum, but turnover of DA was also elevated (∼30%) in dorsal striatum of IRP-2 −/− mice. We conclude that iron misregulation associated with the loss of IRP-2 protein affects DA regulation in the striatum. However, the modest loss of DA and DA-regulating proteins does not reflect the pathology of PD or animal models of PD. Instead, these observations support that the IRP-2 −/− genotype may enable neurobiological events associated with aging.

Nitrotyrosine as a marker for peroxynitrite‐induced neurotoxicity: the beginning or the end of the end of dopamine neurons?

This review examines the involvement of nitrotyrosine as a marker for peroxynitrite-mediated damage in the dopamine neuronal system. We propose that the dopamine neuronal phenotype can influence the cytotoxic signature of peroxynitrite. Dopamine and tetrahydrobiopterin are concentrated in dopamine neurons, and both are essential for their proper neurochemical function. It is not well appreciated that dopamine and tetrahydrobiopterin are also powerful blockers of peroxynitrite-induced tyrosine nitration. What is more, the reaction of peroxynitrite with either dopamine or tetrahydrobiopterin forms chemical species (i.e. o-quinones and pterin radicals, respectively) whose cytotoxic effects may be manifested far earlier than nitrotyrosine formation in the course of dopamine neuronal damage. A better understanding of how the dopamine neuronal phenotype modulates the effects of reactive nitrogen species could reveal early steps in drug- and disease-induced damage to the dopamine neuron and form the basis for rational, protective therapies.

Aberrant trajectory of ascending dopaminergic pathway in mice lacking Nkx2.1

In the embryonic brain, the transcription factor Nkx2.1 is localized in the medial ganglionic eminence and the ventromedial part of the hypothalamus. In the present study, we examined the development of mesencephalic dopamine (DA) neuron system in mice lacking Nkx2.1. In normal mice, tyrosine hydroxylase-immunoreactive axons from mesencephalic DA cells extended bilaterally in the lateral hypothalamus at embryonic day 12.5 (E12.5) and project to the ipsilateral striatum by E14.5. In the mutant brain, mesencephalic DA cell groups appeared to develop normally, but the majority of their ascending axons were observed to cross the ventral midline of the caudal hypothalamus and project to the contralateral striatum. DiI, a fluorescent dye, placed in the ventrolateral mesencephalon of E14.5 mutant mice, further revealed that majority of DiI-labeled axons projected to the contralateral striatum, while a minor ipsilateral projection was also observed. In the ventromedial hypothalamus of mutants, the neuroepithelium of third ventricle was missing, and immunoreactivity of semaphorin 3A, a soluble type of axon repellent, which was normally localized in the neuroepithelium, was remarkably reduced. Together with the recent evidence that the expression of slit2, another axon-repellent diffusible factor, is also eliminated in the hypothalamic neuroepithelium of Nkx2.1-deficient mice, the abnormal crossing of ascending DA axons observed may be attributed to the elimination of these chemorepulsive signals in the medial part of the mutant hypothalamus.