Dopaminergic neurons intrinsic to the striatum

Postnatal development of tyrosine hydroxylase mRNA-expressing neurons in mouse neostriatum

The striatum harbors a small number of tyrosine hydroxylase (TH) mRNA-containing GABAergic neurons that express TH immunoreactivity after dopamine depletion, some of which reportedly resembled striatal medium spiny projection neurons (MSNs). To clarify whether the TH mRNA-expressing neurons were a subset of MSNs, we characterized their postnatal development of electrophysiological and morphological properties using a transgenic mouse strain expressing enhanced green fluorescent protein (EGFP) under the control of the rat TH gene promoter. At postnatal day (P)1, EGFP-TH+ neurons were present as clusters in the striatum and, thereafter, gradually scattered ventromedially by P18 without regard to the striatal compartments. They were immunonegative for calbindin, but immunopositive for enkephalin (54.5%) and dynorphin (80.0%). Whole-cell patch-clamp recordings revealed at least two distinct neuronal types, termed EGFP-TH+ Type A and B. Whereas Type B neurons were aspiny and negative for the MSN marker dopamine- and cyclic AMP-regulated phosphoprotein of 32 kDa (DARPP-32), Type A neurons constituted 75% of the EGFP+ cells, had dendritic spines (24.6%), contained DARPP-32 (73.6%) and a proportion acquired TH immunoreactivity after injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 3-nitropropionic acid. The membrane properties and N-methyl-d-aspartate : non-N-methyl-d-aspartate excitatory postsynaptic current ratio of Type A neurons were very similar to MSNs at P18. However, their resting membrane potentials and spike widths were statistically different from those of MSNs. In addition, the calbindin-like, DARPP-32-like and dynorphin B-like immunoreactivity of Type A neurons developed differently from that of MSNs in the matrix. Thus, Type A neurons closely resemble MSNs, but constitute a cell type distinct from classical MSNs.

Neuroprotection effects of retained acupuncture in neurotoxin-induced Parkinson's disease mice

The aim of this study was to investigate the role of retained acupuncture (RA) in neurotoxin-induced Parkinson's disease (PD) mice. Male C57BL/6 mice were injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to induce the PD model. The mice were divided into four groups, namely, (1) normal; (2) MPTP+retained acupuncture (RA); (3) MPTP+electroacupuncture (EA); (4) MPTP+sham acupuncture (SA). After mice being manipulated with/without acupuncture at acupoints (Daling, PC 7), groups 2-4 were injected with MPTP (15 mg/kg/d). The mice were evaluated for behavioral changes, in terms of time of landing, after acupuncture treatment. The animals were sacrificed and their brains assayed for dopamine and its metabolites and tyrosine hydroxylase (TH) expression by using HPLC and immunohistochemistry/Western blotting, respectively. [(123)I] IBZM-SPECT imaging between SA and RA groups were compared. The results showed that the time of landing of the three groups with treatment was significant longer than group 1 (normal) (4.33±0.15 s). Nonetheless, group 2 (RA) (7.13±0.20 s) had a shorter time of landing than group 4 (SA) (7.89±0.46 s). The number of TH (+) neurons and the expression of TH proteins were significantly higher in the RA group than in the SA/EA groups. RA also increased the uptake of [(123)I] IBZM into the triatum compared to the SA group. We conclude that RA possibly attenuates neuronal damage in MPTP-induced PD mice, which suggests RA may be useful as a complementary strategy when treating human PD.

Unilateral nigrostriatal 6-hydroxydopamine lesions in mice II: predicting l-DOPA-induced dyskinesia

In the 6-hydroxydopamine (6-OHDA) lesioned rodent the location of the lesion produces significantly different behavioural phenotypes, responses to the dopamine precursor l-3,4-dihydroxyphenylalanine (l-DOPA) and neuropathology. Lesion extent is commonly determined by a series of motor tests, but whether any of these tests have a relationship to the development and predictability of dyskinesia is unknown. We used mice with 6-OHDA lesions of the striatum, medial forebrain bundle and substantia nigra to examine the relationship between a range of tests used to determine motor function in the absence of l-DOPA: rotarod, cylinder, corridor, the balance beam, locomotor activity, psycho-stimulant and spontaneous rotational behaviour. The mice were subsequently treated with l-DOPA in progressively increasing doses and the development of l-DOPA-induced dyskinesia assessed. Most of these tests predict dopamine depletion but only rotarod, spontaneous rotations, apomorphine-induced rotations and locomotor activities were significantly correlated with the development of dyskinesia at 6mg/kg and 25mg/kg l-DOPA. The losses of dopaminergic neurons and serotonergic density in the ventral and dorsal striatum were dependent upon lesion type and were also correlated with l-DOPA-induced dyskinesia. The expression of FosB/ΔFosB was differentially affected in the striatum and nucleus accumbens regions in dyskinetic mice according to lesion type.

Neurochemical characterization of the tree shrew dorsal striatum

The striatum is a major component of the basal ganglia and is associated with motor and cognitive functions. Striatal pathologies have been linked to several disorders, including Huntington’s, Tourette’s syndrome, obsessive–compulsive disorders, and schizophrenia. For the study of these striatal pathologies different animal models have been used, including rodents and non-human primates. Rodents lack on morphological complexity (for example, the lack of well defined caudate and putamen nuclei), which makes it difficult to translate data to the human paradigm. Primates, and especially higher primates, are the closest model to humans, but there are ever-increasing restrictions to the use of these animals for research. In our search for a non-primate animal model with a striatum that anatomically (and perhaps functionally) can resemble that of humans, we turned our attention to the tree shrew. Evolutionary genetic studies have provided strong data supporting that the tree shrews (Scadentia) are one of the closest groups to primates, although their brain anatomy has only been studied in detail for specific brain areas. Morphologically, the tree shrew striatum resembles the primate striatum with the presence of an internal capsule separating the caudate and putamen, but little is known about its neurochemical composition. Here we analyzed the expression of calcium-binding proteins, the presence and distribution of the striosome and matrix compartments (by the use of calbindin, tyrosine hydroxylase, and acetylcholinesterase immunohistochemistry), and the GABAergic system by immunohistochemistry against glutamic acid decarboxylase and Golgi impregnation. In summary, our results show that when compared to primates, the tree shrew dorsal striatum presents striking similarities in the distribution of most of the markers studied, while presenting some marked divergences when compared to the rodent striatum.

Serotonin and dopamine striatal innervation in Parkinson's disease and Huntington's chorea

In contrast to our vast knowledge of the dopamine (DA) system, much less is known about the involvement of serotonin (5-HT) in neurodegenerative diseases affecting the basal ganglia. Therefore, we designed a study that aimed at characterizing the status of the striatal DA and 5-HT systems in patients who suffered from either Parkinson's (PD) or Huntington's disease (HD), compared to age-matched controls. Antibodies against tyrosine hydroxylase (TH) and 5-HT transporter (SERT) were used as markers of DA and 5-HT axonal profiles, respectively. The density and pattern of TH+ and SERT + innervation were determined by optical density measurements as well as by direct stereological estimates of labeled axon varicosities. The results reveal a significant decrease in TH immunoreactivity and TH + axon terminals throughout the striatum in both PD and HD, whereas the intensity of SERT immunostaining and the density of SERT + axon varicosities were found to be slightly increased in the striatum of PD and HD patients compared to controls. These findings reveal that the nigrostriatal DA system is significantly impaired in both PD and HD compared to the striatal 5-HT innervation, which is slightly increased in these two conditions. The striatal 5-HT augmentation observed in PD might be the result of a neural mechanism designed to compensate for DA denervation, whereas the marked atrophy of the striatum might explain the increase in the 5-HT innervation noted in HD. These findings underline the importance of the complex interplay between DA and 5-HT striatal afferents in the elaboration of appropriate motor behaviour.

Distribution of tyrosine hydroxylase-expressing interneurons with respect to anatomical organization of the neostriatum

We have recently shown in vitro that striatal tyrosine hydroxylase-expressing interneurons identified in transgenic mice by expression of enhanced green fluorescent protein (TH-eGFP) display electrophysiological profiles that are distinct from those of other striatal interneurons. Furthermore, striatal TH-eGFP interneurons show marked diversity in their electrophysiological properties and have been divided into four distinct subtypes. One question that arises from these observations is whether striatal TH-eGFP interneurons are distributed randomly, or obey some sort of organizational plan as has been shown to be the case with other striatal interneurons. An understanding of the striatal TH-eGFP interneuronal patterning is a vital step in understanding the role of these neurons in striatal functioning. Therefore, in the present set of studies the location of electrophysiologically identified striatal TH-eGFP interneurons was mapped. In addition, the distribution of TH-eGFP interneurons with respect to the striatal striosome–matrix compartmental organization was determined using μ-opioid receptor (MOR) immunofluorescence or intrinsic TH-eGFP fluorescence to delineate striosome and matrix compartments. Overall, the distribution of the different TH-eGFP interneuronal subtypes did not differ in dorsal versus ventral striatum. However, striatal TH-eGFP interneurons were found to be mostly in the matrix in the dorsal striatum whereas a significantly higher proportion of these neurons was located in MOR-enriched domains of the ventral striatum. Further, the majority of striatal TH-eGFP interneurons was found to be located within 100 μm of a striosome–matrix boundary. Taken together, the current results suggest that TH-eGFP interneurons obey different organizational principles in dorsal versus ventral striatum, and may play a role in communication between striatal striosome and matrix compartments.

The impact of maternal separation on the number of tyrosine hydroxylase-expressing midbrain neurons during different stages of ontogenesis

Early life stressors have life-long functional and anatomical consequences. Though many neurotransmitters are involved in the functional impact of early life stress, dopamine seems to be important because of its roles in motor control, adaptation to stressful conditions, mood, cognition, attention and reward. Thus, in the present study, we investigated the way that early life stress, in the form of maternal separation (MS), affects the populations of tyrosine hydroxylase-immunoreactive (TH-IR) dopaminergic neurons in rat midbrain structures during ontogenesis. We included in the study the sub-regions of the substantia nigra (SN) and the ventral tegmental area (VTA). In both the control and MS rats, we found that the estimated total number of TH-expressing neurons fluctuated during ontogenesis. Moreover, MS influenced the number of TH-IR cells, especially in the SN pars reticulata (SNr) and VTA. Shortly after the termination of MS, on postnatal day (PND) 15, a decrease in the estimated total number of TH-IR neurons was observed in the SNr and VTA (in both males and females). On PND 35, MS caused a transient increase in the number of TH-IR cells only in the SNr of female rats. On PND 70, MS affected the number of TH-IR neurons in the VTA of females; specifically, an increase in the number of these cells was observed. Additionally, MS did not alter TH-IR cell sizes or the total levels of TH (measured by Western blot analysis) in the SN and VTA for all stages of ontogenesis in both males and females. The results from the study herein indicate that early life stress has enduring effects on the populations of midbrain TH-expressing dopaminergic neurons (especially in female rats), which are critically important for dopamine-regulated brain function throughout ontogenesis.

Impact of the lesion procedure on the profiles of motor impairment and molecular responsiveness to L-DOPA in the 6-hydroxydopamine mouse model of Parkinson's disease

6-Hydroxydopamine (6-OHDA) lesions are being used in the mouse for basic research on Parkinson's disease and L-DOPA-induced dyskinesia. We set out to compare unilateral lesion models produced by intrastriatal or intramesencephalic injections of a fixed 6-OHDA concentration (3.2 μg/μl) in C57BL/6 mice. In the first experiment, toxin injections were performed either at two striatal coordinates (1 or 2 μl per site, termed "striatum(2 × 1 μl)" and "striatum(2 × 2 μl)" models), in the medial forebrain bundle (MFB), or in the substantia nigra pars compacta (SN) (1 μl per site). All the four lesion models produced significant forelimb use asymmetry, but spontaneous turning asymmetry only occurred in the MFB and striatum(2 × 2 μl) models. After the behavioral studies, the induction of phosphorylated extracellular signal-regulated kinases 1 and 2 (pERK1/2) by acute L-DOPA (30 mg/kg) was used as a marker of post-synaptic supersensitivity. Striatal pERK1/2 expression was sparse in the SN and striatum(2 × 1 μl) groups, but pronounced in the striatum(2 × 2 μl) and MFB-lesioned mice. In further experiments, mice with MFB and striatal(2 × 2 μl) lesions were used to compare behavioral and molecular responses to chronic L-DOPA treatment (12 days at 3 and 6 mg/kg/day). Maximally severe abnormal involuntary movements (AIMs) occurred in all MFB-lesioned mice, whereas only 35% of the mice with striatal lesions developed dyskinesia. Striatal tissue levels of dopamine were significantly lower in the dyskinetic animals (both MFB and striatum(2 × 2 μl) groups) in comparison with the non-dyskinetic ones. Noradrenaline levels were significantly reduced only in MFB lesioned animals and did not differ among the dyskinetic and non-dyskinetic cases with striatal lesions. In all groups, the L-DOPA-induced AIM scores correlated closely with the number of cells immunoreactive for tyrosine hydroxylase or FosB/∆FosB in the striatum. In conclusion, among the four lesion procedures examined here, only the MFB and striatum(2 × 2 μl) models yielded a degree of dopamine denervation sufficient to produce spontaneous postural asymmetry and molecular supersensitivity to L-DOPA. Both lesion models are suitable to reproduce L-DOPA-induced dyskinesia, although only MFB lesions yield a pronounced and widespread expression of post-synaptic supersensitivity markers in the striatum.

17β-estradiol protects dopaminergic neurons in organotypic slice of mesencephalon by MAPK-mediated activation of anti-apoptosis gene BCL2

Both clinical and experimental studies provide growing evidences that marked sex differences in certain neurological disorders or disease models are largely attributed to the neuroprotective effects of estrogen. The purposes of this study were to assess the neuroprotective effect of 17β-estradiol (E2) on dopaminergic neurons against 6-hydroxydopamine (6-OHDA) in organotypic mesencephalic slice culture and to elucidate the possible mechanism underlying neuroprotection. It was found that long-term exposure to E2 exerted marked effects on restoring the number of dopaminergic neurons, maintaining normal morphology of dopaminergic neurons, and preserving their ability to release dopamine at the presence of 6-OHDA. The neuroprotective effect of E2 could be dramatically blocked by an estrogen receptor antagonist ICI 182, 780 (ICI). The expression of GFAP, TLR4, and anti-apoptosis gene BCL2 were elevated at the presence of E2, whereas only BCL2 activation was blocked by ICI, dominantly responsible for E2-induced neuroprotection. Furthermore, activation of BCL2 was speculated to be mainly mediated through mitogen-activated protein kinase (MAPK) pathways, yet phosphatidylinositol-3-kinase signaling contributed largely to GFAP and TLR4 upregulation. Taken together, MAPK pathway-mediated BCL2 expression accounted for one of the key mechanisms involved in E2 neuroprotective effect on dopaminergic neurons against 6-OHDA insult. This finding provides new insight into controversial estrogen replacement therapy.

Contexts for dopamine specification by calcium spike activity in the CNS

Calcium-dependent electrical activity plays a significant role in neurotransmitter specification at early stages of development. To test the hypothesis that activity-dependent differentiation depends on molecular context, we investigated the development of dopaminergic neurons in the CNS of larval Xenopus laevis. We find that different dopaminergic nuclei respond to manipulation of this early electrical activity by ion channel misexpression with different increases and decreases in numbers of dopaminergic neurons. Focusing on the ventral suprachiasmatic nucleus and the spinal cord to gain insight into these differences, we identify distinct subpopulations of neurons that express characteristic combinations of GABA and neuropeptide Y as cotransmitters and Lim1,2 and Nurr1 transcription factors. We demonstrate that the developmental state of neurons identified by their spatial location and expression of these molecular markers is correlated with characteristic spontaneous calcium spike activity. Different subpopulations of dopaminergic neurons respond differently to manipulation of this early electrical activity. Moreover, retinohypothalamic circuit activation of the ventral suprachiasmatic nucleus recruits expression of dopamine selectively in reserve pool neurons that already express GABA and neuropeptide Y. The results are consistent with the hypothesis that spontaneously active neurons expressing GABA are most susceptible to activity-dependent expression of dopamine in both the spinal cord and brain. Because loss of dopaminergic neurons plays a role in neurological disorders such as Parkinson's disease, understanding how subpopulations of neurons become dopaminergic may lead to protocols for differentiation of neurons in vitro to replace those that have been lost in vivo.

Heterogeneity and diversity of striatal GABAergic interneurons

The canonical view of striatal GABAergic interneurons has evolved over several decades of neuroanatomical/neurochemical and electrophysiological studies. From the anatomical studies, three distinct GABAergic interneuronal subtypes are generally recognized. The best-studied subtype expresses the calcium-binding protein, parvalbumin. The second best known interneuron type expresses a number of neuropeptides and enzymes, including neuropeptide Y, somatostatin, and nitric oxide synthase. The last GABAergic interneuron subtype expresses the calcium binding protein, calretinin. There is no overlap or co-localization of these three different sets of markers. The parvalbumin-immunoreactive GABAergic interneurons have been recorded in vitro and shown to exhibit a fast-spiking phenotype characterized by short duration action potentials with large and rapid spike AHPs. They often fire in a stuttering pattern of high frequency firing interrupted by periods of silence. They are capable of sustained firing rates of over 200 Hz. The NPY/SOM/NOS interneurons have been identified as PLTS cells, exhibiting very high input resistances, low threshold spike and prolonged plateau potentials in response to intracellular depolarization or excitatory synaptic stimulation. Thus far, no recordings from identified CR interneurons have been obtained. Recent advances in technological approaches, most notably the generation of several BAC transgenic mouse strains which express a fluorescent marker, enhanced green fluorescent protein, specifically and selectively only in neurons of a certain genetic makeup (e.g., parvalbumin-, neuropeptide Y-, or tyrosine hydroxylase-expressing neurons etc.) have led to the ability of electrophysiologists to visualize and patch specific neuron types in brain slices with epifluorescence illumination. This has led to a rapid expansion of the number of neurochemically and/or electrophysiologically identified interneuronal cell types in the striatum and elsewhere. This article will review the anatomy, neurochemistry, electrophysiology, synaptic connections, and function of the three “classic” striatal GABAergic interneurons as well as more recent data derived from in vitro recordings from BAC transgenic mice as well as recent in vivo data.

Electrophysiological and morphological characteristics and synaptic connectivity of tyrosine hydroxylase-expressing neurons in adult mouse striatum

Whole-cell recordings were obtained from tyrosine hydroxylase-expressing (TH(+)) neurons in striatal slices from bacterial artificial chromosome transgenic mice that synthesize enhanced green fluorescent protein (EGFP) selectively in neurons expressing TH transcriptional regulatory sequences. Stereological cell counting indicated that there were approximately 2700 EGFP-TH(+) neurons/striatum. Whole-cell recordings in striatal slices demonstrated that EGFP-TH(+) neurons comprise four electrophysiologically distinct neuron types whose electrophysiological properties have not been reported previously in striatum. EGFP-TH(+) neurons were identified in retrograde tracing studies as interneurons. Recordings from synaptically connected pairs of EGFP-TH(+) interneurons and spiny neurons showed that the interneurons elicited GABAergic IPSPs/IPSCs in spiny neurons powerful enough to significantly delay evoked spiking. EGFP-TH(+) interneurons responded to local or cortical stimulation with glutamatergic EPSPs. Local stimulation also elicited GABA(A) IPSPs, at least some of which arose from identified spiny neurons. Single-cell reverse transcription-PCR showed expression of VMAT1 in EGFP-TH(+) interneurons, consistent with previous suggestions that these interneurons may be dopaminergic as well as GABAergic. All four classes of interneurons were medium sized with modestly branching, varicose dendrites, and dense, highly varicose axon collateral fields. These data show for the first time that there exists in the normal rodent striatum a substantial population of TH(+)/GABAergic interneurons comprising four electrophysiologically distinct subtypes whose electrophysiological properties differ significantly from those of previously described striatal GABAergic interneurons. These interneurons are likely to play an important role in striatal function through fast GABAergic synaptic transmission in addition to, and independent of, their potential role in compensation for dopamine loss in experimental or idiopathic Parkinson's disease.

Striatal interneurons in dissociated cell culture

In addition to the well-characterized direct and indirect projection neurons there are four major interneuron types in the striatum. Three contain GABA and either parvalbumin, calretinin or NOS/NPY/somatostatin. The fourth is cholinergic. It might be assumed that dissociated cell cultures of striatum (typically from embryonic day E18.5 in rat and E14.5 for mouse) contain each of these neuronal types. However, in dissociated rat striatal (caudate/putamen, CPu) cultures arguably the most important interneuron, the giant aspiny cholinergic neuron, is not present. When dissociated striatal neurons from E14.5 Sprague–Dawley rats were mixed with those from E18.5 rats, combined cultures from these two gestational periods yielded surviving cholinergic interneurons and representative populations of the other interneuron types at 5 weeks in vitro. Neurons from E12.5 CD-1 mice were combined with CPu neurons from E14.5 mice and the characteristics of striatal interneurons after 5 weeks in vitro were determined. All four major classes of interneurons were identified in these cultures as well as rare tyrosine hydroxylase positive interneurons. However, E14.5 mouse CPu cultures contained relatively few cholinergic interneurons rather than the nearly total absence seen in the rat. A later dissection day (E16.5) was required to obtain mouse CPu cultures totally lacking the cholinergic interneuron. We show that these cultures generated from two gestational age cells have much more nearly normal proportions of interneurons than the more common organotypic cultures of striatum. Interneurons are generated from both ages of embryos except for the cholinergic interneurons that originate from the medial ganglionic eminence of younger embryos. Study of these cultures should more accurately reflect neuronal processing as it occurs in the striatum in vivo. Furthermore, these results reveal a procedure for parallel culture of striatum and cholinergic depleted striatum that can be used to examine the function of the cholinergic interneuron in striatal networks.

Pharmacological therapy in Parkinson's disease: focus on neuroprotection

Although the number of available therapeutic approaches in Parkinson's disease (PD) is steadily increasing the search for effective neuroprotective agent is continuing. Such research is directed at influencing the key steps in the pathomechanism: the mitochondrial dysfunction, the oxidative stress, the neuroinflammatory processes and the final common apoptotic pathway. Earlier-developed symptomatic therapies were implicated to be neuroprotective, and promising novel disease modifying approaches were brought into the focus of interest. The current review presents a survey of our current knowledge relating to the pathomechanism of PD and discusses the putative neuroprotective therapy.

The sea lamprey tyrosine hydroxylase: cDNA cloning and in situ hybridization study in the brain

Lampreys belong to the oldest group of extant vertebrates, the agnathans or cyclostomes. Thus, they occupy a key phylogenetic position near the root of the vertebrate tree, which makes them important to the study of nervous system evolution. Tyrosine hydroxylase is the rate-limiting enzyme of catecholamine biosynthesis and is considered a marker of catecholaminergic neurons. In the present study, we report partial cloning of the sea lamprey tyrosine hydroxylase (TH) cDNA and the pattern of TH transcript expression in the adult brain by means of in situ hybridization. Sea lamprey TH mRNA is characterized by the presence of a large untranslated sequence in the 3' end that contains a typical polyadenylation signal (ATTAAA). The deduced partial TH protein sequence presents a conserved domain with two His residues coordinating Fe(2+) binding and a conserved cofactor binding site. Neurons expressing the TH transcript were observed in the preoptic, postoptic commissure, dorsal hypothalamic, ventral hypothalamic, mammillary and paratubercular nuclei of the prosencephalon. In situ hybridization experiments also confirmed the existence of a catecholaminergic (dopaminergic) striatal population in the brain of the adult sea lamprey. A few granule-like cells in the olfactory bulbs also showed weak TH transcript expression. No cells showing TH transcript expression were observed in the rostral rhombencephalon, which suggests the absence of a locus coeruleus in the sea lamprey. Comparison of the pattern of TH mRNA expression in the prosencephalon between lampreys and teleost fishes revealed both similarities and differences. Our results suggest that the duplication of the TH gene might have occurred before the separation of agnathans and gnathostomes.

Androgen decreases dopamine neurone survival in rat midbrain

Clinical studies show that men are more likely to develop disorders affecting midbrain dopaminergic pathways, such as drug addiction and Parkinson's disease (PD). Although a great deal of focus has been given to the role of oestrogen in the maintenance of midbrain dopaminergic pathways, little is known about how testosterone influences these pathways. In the present study, we used stereological analysis of tyrosine hydroxylase-immunoreactive (TH-IR) cell bodies to determine how testosterone influences the dopaminergic cell bodies of the substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA). Rats and mice were castrated at postnatal day (PN) 60, and these midbrain cell populations were counted on PN 90. One month after castration, TH-IR cell number had increased in the SNpc and VTA of rats and mice. Replacement with testosterone or the non-aromatisable analogue dihydrotestosterone (DHT) in castrated animals reduced TH-IR cell number in the SNpc and VTA in rats. In mice, the decrease of TH-IR cell number with testosterone or DHT replacement was observed only in the SNpc. The apparent increase in TH-IR neurone number after castration is not explained by an increase in TH expression because the number of nondopaminergic cells (TH-immunonegative, TH-IN) did not decrease proportionally after castration. TH-IN cell number did not change after castration or hormone replacement in rat or mouse SNpc or VTA. These findings suggest that testosterone may play a suppressive role in midbrain dopaminergic pathways.

Convergent evidence for abnormal striatal synaptic plasticity in dystonia

Dystonia is a functionally disabling movement disorder characterized by abnormal movements and postures. Although substantial recent progress has been made in identifying genetic factors, the pathophysiology of the disease remains a mystery. A provocative suggestion gaining broader acceptance is that some aspect of neural plasticity may be abnormal. There is also evidence that, at least in some forms of dystonia, sensorimotor "use" may be a contributing factor. Most empirical evidence of abnormal plasticity in dystonia comes from measures of sensorimotor cortical organization and physiology. However, the basal ganglia also play a critical role in sensorimotor function. Furthermore, the basal ganglia are prominently implicated in traditional models of dystonia, are the primary targets of stereotactic neurosurgical interventions, and provide a neural substrate for sensorimotor learning influenced by neuromodulators. Our working hypothesis is that abnormal plasticity in the basal ganglia is a critical link between the etiology and pathophysiology of dystonia. In this review we set up the background for this hypothesis by integrating a large body of disparate indirect evidence that dystonia may involve abnormalities in synaptic plasticity in the striatum. After reviewing evidence implicating the striatum in dystonia, we focus on the influence of two neuromodulatory systems: dopamine and acetylcholine. For both of these neuromodulators, we first describe the evidence for abnormalities in dystonia and then the means by which it may influence striatal synaptic plasticity. Collectively, the evidence suggests that many different forms of dystonia may involve abnormal plasticity in the striatum. An improved understanding of these altered plastic processes would help inform our understanding of the pathophysiology of dystonia, and, given the role of the striatum in sensorimotor learning, provide a principled basis for designing therapies aimed at the dynamic processes linking etiology to pathophysiology of the disease.

How useful are stem cells in PD therapy?

Parkinson's disease (PD) is one of the most common neurodegenerative disorders. The onset of PD is usually after the age of 50. Clinical symptoms of PD are not manifested until 60-80% of dopaminergic neurons in the midbrain have been affected. Cell replacement has been a promising approach for the treatment of PD. Fetal mesencephalic dopaminergic neurons seemed to improve the motor disability in patients in some early studies. However, the clinical application of this approach may be limited by ethical and logistic concerns, as well as by side effects. On the other hand, embryonic stem (ES) cells are promising candidates because of their ability to provide an unlimited supply of specific cell types, their accessibility to genetic modifications, and their broad developmental potentials. Transplants of undifferentiated ES cells were able to proliferate and fully differentiate into dopaminergic neurons in a rodent PD model. One of the concerns though is the risk of tumor formation. The tumorigenic potential of ES cells seems to be greatly reduced when cells are predifferentiated into dopaminergic neurons in vitro before implantation. Recent developments in the induction of pluripotent stem cells from somatic adult cells provide a tremendous opportunity for this field. Initial success has been reported in a rodent PD model using iPS cells (induced pluripotent stem cells). However, whether this initial result can be successfully translated into human clinical studies still needs to be determined.

Nurr1 is required for maintenance of maturing and adult midbrain dopamine neurons

Transcription factors involved in the specification and differentiation of neurons often continue to be expressed in the adult brain, but remarkably little is known about their late functions. Nurr1, one such transcription factor, is essential for early differentiation of midbrain dopamine (mDA) neurons but continues to be expressed into adulthood. In Parkinson's disease, Nurr1 expression is diminished and mutations in the Nurr1 gene have been identified in rare cases of disease; however, the significance of these observations remains unclear. Here, a mouse strain for conditional targeting of the Nurr1 gene was generated, and Nurr1 was ablated either at late stages of mDA neuron development by crossing with mice carrying Cre under control of the dopamine transporter locus or in the adult brain by transduction of adeno-associated virus Cre-encoding vectors. Nurr1 deficiency in maturing mDA neurons resulted in rapid loss of striatal DA, loss of mDA neuron markers, and neuron degeneration. In contrast, a more slowly progressing loss of striatal DA and mDA neuron markers was observed after ablation in the adult brain. As in Parkinson's disease, neurons of the substantia nigra compacta were more vulnerable than cells in the ventral tegmental area when Nurr1 was ablated at late embryogenesis. The results show that developmental pathways play key roles for the maintenance of terminally differentiated neurons and suggest that disrupted function of Nurr1 and other developmental transcription factors may contribute to neurodegenerative disease.

Basal ganglia pathology in schizophrenia: dopamine connections and anomalies

Schizophrenia is a severe mental illness that affects 1% of the world population. The disease usually manifests itself in early adulthood with hallucinations, delusions, cognitive and emotional disturbances and disorganized thought and behavior. Dopamine was the first neurotransmitter to be implicated in the disease, and though no longer the only suspect in schizophrenia pathophysiology, it obviously plays an important role. The basal ganglia are the site of most of the dopamine neurons in the brain and the target of anti-psychotic drugs. In this review, we will start with an overview of basal ganglia anatomy emphasizing dopamine circuitry. Then, we will review the major deficits in dopamine function in schizophrenia, emphasizing the role of excessive dopamine in the basal ganglia and the link to psychosis.

Dopamine and gamma-aminobutyric acid are colocalized in restricted groups of neurons in the sea lamprey brain: insights into the early evolution of neurotransmitter colocalization in vertebrates

Since its discovery, the possible corelease of classic neurotransmitters from neurons has received much attention. Colocalization of monoamines and amino acidergic neurotransmitters [mainly glutamate and dopamine (DA) or serotonin] in mammalian neurons has been reported. However, few studies have dealt with the colocalization of DA and gamma-aminobutyric acid (GABA) in neurons. With the aim of providing some insight into the colocalization of neurotransmitters during early vertebrate phylogeny, we studied GABA expression in dopaminergic neurons in the sea lamprey brain by using double-immunofluorescence methods with anti-DA and anti-GABA antibodies. Different degrees of colocalization of DA and GABA were observed in different dopaminergic brain nuclei. A high degree of colocalization (GABA in at least 25% of DA-immunoreactive neurons) was observed in populations of the caudal rhombencephalon, ventral isthmus, postoptic commissure nucleus, preoptic nucleus and in granule-like cells of the olfactory bulb. A new DA-immunoreactive striatal population that showed colocalization with GABA in about a quarter of its neurons was observed. In the periventricular hypothalamus, colocalization was observed in only a few cells, despite the abundance of DA- and GABA-immunoreactive neurons, and no double-labelled cells were observed in the paratubercular nucleus. The frequent colocalization of DA and GABA reveals that the dopaminergic populations of lampreys are more complex than previously reported. Double-labelled fibres or terminals were observed in different brain regions, suggesting possible corelease of DA and GABA by these lamprey neurons. The present results suggest that colocalization of DA and GABA in neurons appeared early in vertebrate evolution.

Effect of estradiol on striatal dopamine activity of female hemiparkinsonian monkeys

A higher prevalence and incidence of Parkinson's disease is observed in men, and beneficial motor effects of estrogens are observed in parkinsonian women. In rodents, an effect of estradiol on dopamine systems is documented, whereas much less is known in monkeys. Enkephalin was shown to exert a compensatory modulatory effect on the denervated dopamine nigrostriatal pathway in monkeys and in humans. Moreover in rodents, striatal preproenkephalin mRNA is increased by estrogen treatment. Hence, we investigated the responsiveness of striatal dopamine to estradiol in long-term ovariectomized monkeys bearing a unilateral lesion with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to mimic parkinsonian postmenopausal women. Seven ovariectomized female monkeys received a unilateral MPTP lesion; 4 years after ovariectomy, three received 1-month treatment with 17beta-estradiol and the others received vehicle. The lesioned striata showed extensive denervation in all monkeys as measured with dopamine and metabolite concentrations assayed by high-performance liquid chromatography and by autoradiography of the dopamine transporter. The lesioned and intact striata of estradiol-treated monkeys had increased 3-methoxytyramine, and lesioned putamen increased dopamine concentrations compared with vehicle-treated monkeys. Estradiol treatment increased the dopamine transporter in subregions of the intact caudate and putamen compared with the intact striata of vehicle-treated monkeys, but not in the lesioned striata. Preproenkephalin mRNA levels measured by in situ hybridization were increased in the lesioned striata of vehicle treated monkeys and were not further enhanced in estradiol-treated monkeys. These results show that long after ovariectomy, modeling postmenopausal hormonal conditions, brain dopamine metabolism, and transporter are still responsive to estradiol.

L-dopa therapy for Parkinson's disease: past, present, and future

Dopamine (DA) supplementation therapy by l-dopa for Parkinson's disease (PD) was established around 1970. The dose of l-dopa can be reduced by the combined administration of inhibitors of peripheral l-amino acid decarboxylase (AADC), catechol O-methyltransferase (COMT), or monoamine oxidase B (MAO B). DA in the striatum may be produced from exogenously administered l-dopa by various AADC-containing cells, such as serotonin neurons. The long-term administration of l-dopa in PD patients may produce l-dopa-induced dyskinesia (LID), which may be due to chronic overstimulation of supersensitive DA D1 receptors. l-dopa may be used in combination with various new strategies such as gene therapy or transplantation in the future.

Dopamine-dependent periadolescent maturation of corticostriatal functional connectivity in mouse

Altered corticostriatal information processing associated with early dopamine systems dysfunction may contribute to attention deficit/hyperactivity disorder (ADHD). Mice with neonatal dopamine-depleting lesions exhibit hyperactivity that wanes after puberty and is reduced by psychostimulants, reminiscent of some aspects of ADHD. To assess whether the maturation of corticostriatal functional connectivity is altered by early dopamine depletion, we examined preadolescent and postadolescent urethane-anesthetized mice with or without dopamine-depleting lesions. Specifically, we assessed (1) synchronization between striatal neuron discharges and oscillations in frontal cortex field potentials and (2) striatal neuron responses to frontal cortex stimulation. In adult control mice striatal neurons were less spontaneously active, less responsive to cortical stimulation, and more temporally tuned to cortical rhythms than in infants. Striatal neurons from hyperlocomotor mice required more current to respond to cortical input and were less phase locked to ongoing oscillations, resulting in fewer neurons responding to refined cortical commands. By adulthood some electrophysiological deficits waned together with hyperlocomotion, but striatal spontaneous activity remained substantially elevated. Moreover, dopamine-depleted animals showing normal locomotor scores exhibited normal corticostriatal synchronization, suggesting that the lesion allows, but is not sufficient, for the emergence of corticostriatal changes and hyperactivity. Although amphetamine normalized corticostriatal tuning in hyperlocomotor mice, it reduced horizontal activity in dopamine-depleted animals regardless of their locomotor phenotype, suggesting that amphetamine modified locomotion through a parallel mechanism, rather than that modified by dopamine depletion. In summary, functional maturation of striatal activity continues after infancy, and early dopamine depletion delays the maturation of core functional capacities of the corticostriatal system.

Pre-synaptic dopaminergic compensation after moderate nigrostriatal damage in non-human primates

Despite a dramatic loss of nigrostriatal dopaminergic neurons in Parkinson's disease, clinical symptoms only arise with 70-80% reduction of striatal dopamine. The mechanisms responsible for this functional compensation are currently under debate. Although initial studies showed an enhanced pre-synaptic dopaminergic function with nigrostriatal degeneration, more recent work suggests that functional compensation is not dopamine-mediated. To address this issue, we used cyclic voltammetry to directly measure endogenous dopamine release from striatal slices of control monkeys and animals with a moderate or severe MPTP-induced dopaminergic lesion. The moderately lesioned monkeys were asymptomatic, while the severely lesioned animals were parkinsonian. In monkeys with a moderate lesion, a 300% increase was obtained in endogenous striatal dopamine release. In contrast, in striatal slices from severely lesioned animals, a small % of evoked dopamine signals were similar in amplitude to control while the greater majority were undetectable. These findings suggest that pre-synaptic dopaminergic compensation develops in residual dopaminergic terminals with moderate lesioning, but that this response is lost with severe nigrostriatal damage. Such an interpretation is supported by the results of dopamine turnover studies. This enhanced pre-synaptic dopaminergic activity may be important in maintaining normal motor function during the initial stages of Parkinson's disease.

The number of dopaminergic cells is increased in the olfactory bulb of monkeys chronically exposed to MPTP

We investigated the impact of the nigrostriatal lesion on the olfactory tyrosine hydroxylase-immunoreactive (TH-ir) cells in monkeys. The majority of these TH-ir cells appeared in the glomerular layer of the olfactory bulb and many were immature but functional dopaminergic neurons. In parkinsonian monkeys the number of olfactory dopaminergic neurons increased up to 100% as compared to controls, but their phenotype did not change. This increased TH-ir cell population might be a direct consequence of the nigral cell loss and contribute to the hyposmia reported by Parkinson's disease patients.

Beneficial effects of dietary omega-3 polyunsaturated fatty acid on toxin-induced neuronal degeneration in an animal model of Parkinson's disease

In this study, we examined whether omega-3 (n-3) polyunsaturated fatty acids (PUFAs) may exert neuroprotective action in Parkinson's disease, as previously shown in Alzheimer's disease. We exposed mice to either a control or a high n-3 PUFA diet from 2 to 12 months of age and then treated them with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP; 140 mg/kg in 5 days). High n-3 PUFA dietary consumption completely prevented the MPTP-induced decrease of tyrosine hydroxylase (TH)-labeled nigral cells (P<0.01 vs. MPTP mice on control diet), Nurr1 mRNA (P<0.01 vs. MPTP mice on control diet), and dopamine transporter mRNA levels (P<0.05 vs. MPTP mice on control diet) in the substantia nigra. Although n-3 PUFA dietary treatment had no effect on striatal dopaminergic terminals, the high n-3 PUFA diet protected against the MPTP-induced decrease in dopamine (P<0.05 vs. MPTP mice on control diet) and its metabolite dihydroxyphenylacetic acid (P<0.05 vs. MPTP mice on control diet) in the striatum. Taken together, these data suggest that a high n-3 PUFA dietary intake exerts neuroprotective actions in an animal model of Parkinsonism.

L-Dopa treatment abolishes the numerical increase in striatal dopaminergic neurons in parkinsonian monkeys

The striatum harbors a population of dopaminergic interneurons that increases in number in animal models of Parkinson's disease (PD), presumably to compensate for dopamine (DA) depletion. The purpose of the present study was to determine the fate of striatal dopaminergic neurons in parkinsonian monkeys in which striatal DA depletion had been alleviated by systemic administration of l-dopa. The number of striatal dopaminergic neurons, visualized with tyrosine hydroxylase (TH) immunohistochemistry, was measured in three groups of cynomolgus (Macaca fascicularis) monkeys: (1) normal untreated monkeys; (2) monkeys rendered parkinsonian following systemic injection of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), but otherwise untreated; and (3) MPTP-intoxicated monkeys that received oral l-dopa on a chronic basis. In agreement with previous studies, the number of striatal TH-positive (TH+) neurons in l-dopa-free parkinsonian monkeys was significantly higher (p<0.05) than in normal (non-parkinsonian) monkeys. However, this increase was abolished in parkinsonian monkeys that received l-dopa treatment. In fact, the number of striatal TH+ neurons in l-dopa-treated parkinsonian monkeys was not significantly different (p>0.05) from values obtained in normal monkeys. These findings suggest that the DA concentration regulates the numerical density of this ectopic neuronal population, a phenomenon that is more likely the result of a shift in the phenotype of preexistent striatal interneurons rather than the recruitment of newborn neurons that would eventually develop a DA phenotype. Our data also reinforce the hypothesis that striatal TH+ neurons act as local DA source and, as such, are part of a compensatory mechanism that could be artificially enhanced to alleviate or delay PD symptoms.