Early appearance of tyrosine hydroxylase immunoreactive cells in the mesencephalon of mouse embryos

Dopamine Neurons That Cotransmit Glutamate, From Synapses to Circuits to Behavior

Discovered just over 20 years ago, dopamine neurons have the ability to cotransmit both dopamine and glutamate. Yet, the functional roles of dopamine neuron glutamate cotransmission and their implications for therapeutic use are just emerging. This review article encompasses the current body of evidence investigating the functions of dopamine neurons of the ventral midbrain that cotransmit glutamate. Since its discovery in dopamine neuron cultures, further work in vivo confirmed dopamine neuron glutamate cotransmission across species. From there, growing interest has led to research related to neural functioning including roles in synaptic signaling, development, and behavior. Functional connectome mapping reveals robust connections in multiple forebrain regions to various cell types, most notably to cholinergic interneurons in both the medial shell of the nucleus accumbens and the lateral dorsal striatum. Glutamate markers in dopamine neurons reach peak levels during embryonic development and increase in response to various toxins, suggesting dopamine neuron glutamate cotransmission may serve neuroprotective roles. Findings from behavioral analyses reveal prominent roles for dopamine neuron glutamate cotransmission in responses to psychostimulants, in positive valence and cognitive systems and for subtle roles in negative valence systems. Insight into dopamine neuron glutamate cotransmission informs the pathophysiology of neuropsychiatric disorders such as addiction, schizophrenia and Parkinson Disease, with therapeutic implications.

Current Status of Stem Cell-Derived Therapies for Parkinson's Disease: From Cell Assessment and Imaging Modalities to Clinical Trials

Curative therapies or treatments reversing the progression of Parkinson’s disease (PD) have attracted considerable interest in the last few decades. PD is characterized by the gradual loss of dopaminergic (DA) neurons and decreased striatal dopamine levels. Current challenges include optimizing neuroprotective strategies, developing personalized drug therapy, and minimizing side effects from the long-term prescription of pharmacological drugs used to relieve short-term motor symptoms. Transplantation of DA cells into PD patients’ brains to replace degenerated DA has the potential to change the treatment paradigm. Herein, we provide updates on current progress in stem cell-derived DA neuron transplantation as a therapeutic alternative for PD. We briefly highlight cell sources for transplantation and focus on cell assessment methods such as identification of genetic markers, single-cell sequencing, and imaging modalities used to access cell survival and function. More importantly, we summarize clinical reports of patients who have undergone cell-derived transplantation in PD to better perceive lessons that can be drawn from past and present clinical outcomes. Modifying factors include (1) source of the stem cells, (2) quality of the stem cells, (3) age of the patient, (4) stage of disease progression at the time of cell therapy, (5) surgical technique/practices, and (6) the use of immunosuppression. We await the outcomes of joint efforts in clinical trials around the world such as NYSTEM and CiRA to further guide us in the selection of the most suitable parameters for cell-based neurotransplantation in PD.

Molecular Regulation in Dopaminergic Neuron Development. Cues to Unveil Molecular Pathogenesis and Pharmacological Targets of Neurodegeneration

The relatively few dopaminergic neurons in the mammalian brain are mostly located in the midbrain and regulate many important neural functions, including motor integration, cognition, emotive behaviors and reward. Therefore, alteration of their function or degeneration leads to severe neurological and neuropsychiatric diseases. Unraveling the mechanisms of midbrain dopaminergic (mDA) phenotype induction and maturation and elucidating the role of the gene network involved in the development and maintenance of these neurons is of pivotal importance to rescue or substitute these cells in order to restore dopaminergic functions. Recently, in addition to morphogens and transcription factors, microRNAs have been identified as critical players to confer mDA identity. The elucidation of the gene network involved in mDA neuron development and function will be crucial to identify early changes of mDA neurons that occur in pre-symptomatic pathological conditions, such as Parkinson’s disease. In addition, it can help to identify targets for new therapies and for cell reprogramming into mDA neurons. In this essay, we review the cascade of transcriptional and posttranscriptional regulation that confers mDA identity and regulates their functions. Additionally, we highlight certain mechanisms that offer important clues to unveil molecular pathogenesis of mDA neuron dysfunction and potential pharmacological targets for the treatment of mDA neuron dysfunction.

Dopaminergic modulation of olfactory-evoked motor output in sea lampreys (Petromyzon marinus L.)

Detection of chemical cues is important to guide locomotion in association with feeding and sexual behavior. Two neural pathways responsible for odor-evoked locomotion have been characterized in the sea lamprey (Petromyzon marinus L.), a basal vertebrate. There is a medial pathway originating in the medial olfactory bulb (OB) and a lateral pathway originating from the rest of the OB. These olfactomotor pathways are present throughout the life cycle of lampreys, but olfactory-driven behaviors differ according to the developmental stage. Among possible mechanisms, dopaminergic (DA) modulation in the OB might explain the behavioral changes. Here, we examined DA modulation of olfactory transmission in lampreys. Immunofluorescence against DA revealed immunoreactivity in the OB that was denser in the medial part (medOB), where processes were observed close to primary olfactory afferents and projection neurons. Dopaminergic neurons labeled by tracer injections in the medOB were located in the OB, the posterior tuberculum, and the dorsal hypothalamic nucleus, suggesting the presence of both intrinsic and extrinsic DA innervation. Electrical stimulation of the olfactory nerve in an in vitro whole-brain preparation elicited synaptic responses in reticulospinal cells that were modulated by DA. Local injection of DA agonists in the medOB decreased the reticulospinal cell responses whereas the D2 receptor antagonist raclopride increased the response amplitude. These observations suggest that DA in the medOB could modulate odor-evoked locomotion. Altogether, these results show the presence of a DA innervation within the medOB that may play a role in modulating olfactory inputs to the motor command system of lampreys.

Embryonic development of selectively vulnerable neurons in Parkinson's disease

A specific set of brainstem nuclei are susceptible to degeneration in Parkinson's disease. We hypothesise that neuronal vulnerability reflects shared phenotypic characteristics that confer selective vulnerability to degeneration. Neuronal phenotypic specification is mainly the cumulative result of a transcriptional regulatory program that is active during the development. By manual curation of the developmental biology literature, we comprehensively reconstructed an anatomically resolved cellular developmental lineage for the adult neurons in five brainstem regions that are selectively vulnerable to degeneration in prodromal or early Parkinson's disease. We synthesised the literature on transcription factors that are required to be active, or required to be inactive, in the development of each of these five brainstem regions, and at least two differentially vulnerable nuclei within each region. Certain transcription factors, e.g., Ascl1 and Lmx1b, seem to be required for specification of many brainstem regions that are susceptible to degeneration in early Parkinson's disease. Some transcription factors can even distinguish between differentially vulnerable nuclei within the same brain region, e.g., Pitx3 is required for specification of the substantia nigra pars compacta, but not the ventral tegmental area. We do not suggest that Parkinson's disease is a developmental disorder. In contrast, we consider identification of shared developmental trajectories as part of a broader effort to identify the molecular mechanisms that underlie the phenotypic features that are shared by selectively vulnerable neurons. Systematic in vivo assessment of fate determining transcription factors should be completed for all neuronal populations vulnerable to degeneration in early Parkinson's disease.

An Efficient and Versatile System for Visualization and Genetic Modification of Dopaminergic Neurons in Transgenic Mice

Background & Aims

The brain dopaminergic (DA) system is involved in fine tuning many behaviors and several human diseases are associated with pathological alterations of the DA system such as Parkinson’s disease (PD) and drug addiction. Because of its complex network integration, detailed analyses of physiological and pathophysiological conditions are only possible in a whole organism with a sophisticated tool box for visualization and functional modification.

Methods & Results

Here, we have generated transgenic mice expressing the tetracycline-regulated transactivator (tTA) or the reverse tetracycline-regulated transactivator (rtTA) under control of the tyrosine hydroxylase (TH) promoter, TH-tTA (tet-OFF) and TH-rtTA (tet-ON) mice, to visualize and genetically modify DA neurons. We show their tight regulation and efficient use to overexpress proteins under the control of tet-responsive elements or to delete genes of interest with tet-responsive Cre. In combination with mice encoding tet-responsive luciferase, we visualized the DA system in living mice progressively over time.

Conclusion

These experiments establish TH-tTA and TH-rtTA mice as a powerful tool to generate and monitor mouse models for DA system diseases.

Expression of early developmental markers predicts the efficiency of embryonic stem cell differentiation into midbrain dopaminergic neurons

Dopaminergic neurons derived from pluripotent stem cells are among the best investigated products of in vitro stem cell differentiation owing to their potential use for neurorestorative therapy of Parkinson's disease. However, the classical differentiation protocols for both mouse and human pluripotent stem cells generate a limited percentage of dopaminergic neurons and yield a considerable cellular heterogeneity comprising numerous scarcely characterized cell populations. To improve pluripotent stem cell differentiation protocols for midbrain dopaminergic neurons, we established extensive and strictly quantitative gene expression profiles, including markers for pluripotent cells, neural progenitors, non-neural cells, pan-neuronal and glial cells, neurotransmitter phenotypes, midbrain and nonmidbrain populations, floor plate and basal plate populations, as well as for Hedgehog, Fgf, and Wnt signaling pathways. The profiles were applied to discrete stages of in vitro differentiation of mouse embryonic stem cells toward the dopaminergic lineage and after transplantation into the striatum of 6-hydroxy-dopamine-lesioned rats. The comparison of gene expression in vitro with stages in the developing ventral midbrain between embryonic day 11.5 and 13.5 ex vivo revealed dynamic changes in the expression of transcription factors and signaling molecules. Based on these profiles, we propose quantitative gene expression milestones that predict the efficiency of dopaminergic differentiation achieved at the end point of the protocol, already at earlier stages of differentiation.

Prenatal exposure to methanol as a dopamine system sensitization model in C57BL/6J mice

AIMS

In this study, the effects of prenatal exposure to methanol (MeOH) on the nigrostriatal dopamine (NSDA) system were examined to determine if the interaction could sensitize this system, and serve as an underpinning for Parkinson's disease (PD) like changes that occur later in life. Methanol was studied because its toxicity resembles the symptoms of PD and the symptoms are relieved by L-dopa meaning that MeOH targets the NSDA system. Since fermentation and wood combustion are major sources for MeOH, the incidence of human encounters with MeOH is high. As a superior solvent and the precursor for formaldehyde, MeOH has a powerful and sometimes, irreversible impact on chemical processes, such as cross-linking proteins and nucleic acids. It may cause subthreshold changes that sensitizes the NSDA system to PD, that occur during aging.

MAIN METHODS

To study the prenatal effects of MeOH, pregnant C57BL/6J mice were administered 40 mg/kg MeOH by oral gavage during gestation days 8-12, twice daily. Twelve weeks after birth, behavior impairments were recorded. The striatum was dissected for the determination of tyrosine hydroxylase (TH), L-aromatic amino acid decarboxylase (LAAD), α-synuclein and levels of dopamine (DA) and its metabolites.

KEY FINDINGS

MeOH reduced striatal TH and LAAD protein by 47% and 57% respectively and DA by 32%.

SIGNIFICANCE

The results mean that in utero exposure to toxins similar to MeOH could sensitize the striatal system to changes that cause PD. This study may help identify strategies to block this type of in utero toxicity.

En1 and Wnt signaling in midbrain dopaminergic neuronal development

Dopaminergic neurons of the ventral mesodiencephalon are affected in significant health disorders such as Parkinson's disease, schizophrenia, and addiction. The ultimate goal of current research endeavors is to improve the clinical treatment of such disorders, such as providing a protocol for cell replacement therapy in Parkinson's disease that will successfully promote the specific differentiation of a stem cell into a dopaminergic neuronal phenotype. Decades of research on the developmental mechanisms of the mesodiencephalic dopaminergic (mdDA) system have led to the identification of many signaling pathways and transcription factors critical in its development. The unraveling of these pathways will help fill in the pieces of the puzzle that today dominates neurodevelopment research: how to make and maintain a mdDA neuron. In the present review, we provide an overview of the mdDA system, the processes and signaling molecules involved in its genesis, with a focus on the transcription factor En1 and the canonical Wnt pathway, highlighting recent findings on their relevance--and interplay--in the development and maintenance of the mdDA system.

Expression and subcellular location of alpha-synuclein during mouse-embryonic development

Alpha-synuclein (alpha-SYN) is one of the major components of intracellular fibrillary aggregates in the brains of a subset of neurodegenerative disorders. Although alpha-SYN expression has been found in developing mouse brain, a detailed distribution during mouse-embryonic development has not been made. Here we describe the expression pattern of alpha-SYN during the development of mice from E9.5 to P0 by immunohistochemistry (IHC). As a result, alpha-SYN was detected as early as E9.5. During the embryonic stages, alpha-SYN was dynamically expressed in several regions of the brain. In the neocortex, expression was detected in the marginal zone (MZ) in the early stages and was later condensed in the MZ and in the subplate (SP); in the cerebellum, expression was initially detected in the deep cerebellar nuclei (DCN) and was later condensed in the Purkinje cells. These spatio-temporal expression patterns matched the neuronal migratory pathways and the formation of the synapse connections. Additionally, alpha-SYN was detected in the sensory systems, including the nasal mucosa, the optic cup, the sensory ganglia, and their dominating nerve fibers. Furthermore, the nuclear location of alpha-SYN protein was found in developing neurons in the early stages, and later it was mostly found in the non-nuclear compartments. This finding was further confirmed by Western blot analysis. These results suggest that alpha-SYN may be involved not only in the migration of neurons and in the synaptogenesis of the central nervous system (CNS) but also in the establishment of the sensory systems. The nuclear location of alpha-SYN may hint at an important function in these events.

Comparison of gene expression profile in embryonic mesencephalon and neuronal primary cultures

In the mammalian central nervous system (CNS) an important contingent of dopaminergic neurons are localized in the substantia nigra and in the ventral tegmental area of the ventral midbrain. They constitute an anatomically and functionally heterogeneous group of cells involved in a variety of regulatory mechanisms, from locomotion to emotional/motivational behavior. Midbrain dopaminergic neuron (mDA) primary cultures represent a useful tool to study molecular mechanisms involved in their development and maintenance. Considerable information has been gathered on the mDA neurons development and maturation in vivo, as well as on the molecular features of mDA primary cultures. Here we investigated in detail the gene expression differences between the tissue of origin and ventral midbrain primary cultures enriched in mDA neurons, using microarray technique. We integrated the results based on different re-annotations of the microarray probes. By using knowledge-based gene network techniques and promoter sequence analysis, we also uncovered mechanisms that might regulate the expression of CNS genes involved in the definition of the identity of specific cell types in the ventral midbrain. We integrate bioinformatics and functional genomics, together with developmental neurobiology. Moreover, we propose guidelines for the computational analysis of microarray gene expression data. Our findings help to clarify some molecular aspects of the development and differentiation of DA neurons within the midbrain.

Bdnf gene is a downstream target of Nurr1 transcription factor in rat midbrain neurons in vitro

The transcription factor Nurr1 is essential for the generation of midbrain dopaminergic neurons (mDA). Only a few Nurr1-regulated genes have so far been identified and it remains unclear how Nurr1 influences the development and function of dopaminergic neurons. To identify novel Nurr1 target genes we have used genome-wide expression profiling in rat midbrain primary cultures, enriched in dopaminergic neurons, following up-regulation of Nurr1 expression by depolarization. In this study we demonstrate that following depolarization the hyperexpression of Nurr1 and the brain derived neurotrophic factor (BDNF) are phospholipase C- and protein kinase C-dependent. We show that Bdnf, which encodes a neurotrophin involved also in the phenotypic maturation of mDA neurons, is a novel Nurr1 target gene. By RNA interference experiments we show that a decreased Nurr1 expression is followed by tyrosine hydroxylase and BDNF mRNA and protein down-regulation. Reporter gene assay experiments performed on midbrain primary cultures using four Bdnf promoter constructs show that Bdnf is a direct target gene of Nurr1. Taken together, our findings suggest that Nurr1 might also influence the development and the function of midbrain dopaminergic neurons via direct regulation of Bdnf expression.

Neurogenin 2 is required for the development of ventral midbrain dopaminergic neurons

Proneural genes are crucial regulators of neurogenesis and subtype specification in many areas of the nervous system; however, their function in dopaminergic neuron development is unknown. We report that proneural genes have an intricate pattern of expression in the ventricular zone of the ventral midbrain, where mesencephalic dopaminergic neurons are generated. Neurogenin 2(Ngn2) and Mash1 are expressed in the ventral midline, while Ngn1, Ngn2 and Mash1 are co-localized more laterally in the ventricular zone. Ngn2 is also expressed in an intermediate zone immediately adjacent to the ventricular zone at the ventral midline. To examine the function of these genes, we analyzed mutant mice in which one or two of these genes were deleted (Ngn1, Ngn2 and Mash1) or substituted (Mash1 in the Ngn2 locus). Our results demonstrate that Ngn2 is required for the differentiation of Sox2+ ventricular zone progenitors into Nurr1+postmitotic dopaminergic neuron precursors in the intermediate zone, and that it is also likely to be required for their subsequent differentiation into tyrosine hydroxylase-positive dopaminergic neurons in the marginal zone. Although Mash1 normally has no detectable function in dopaminergic neuron development, it could partially rescue the generation of dopaminergic neuron precursors in the absence of Ngn2. These results demonstrate that Ngn2 is uniquely required for the development of midbrain dopaminergic neurons.

Ontogeny of tyrosine hydroxylase mRNA expression in mid- and forebrain: neuromeric pattern and novel positive regions

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamines and, thus, critical in determining the catecholaminergic phenotype. In this study, we have examined the expression of TH mRNA by in situ hybridization in the embryonic mouse forebrain and midbrain and have mapped its localization according to the neuromeric pattern. We find that early in embryonic development, 10 to 12 days post coitum (dpc), TH mRNA is expressed in ample continuous regions of the neuroepithelium, extending across several neuromeres. However, from 12.5 dpc onward, the expression becomes restricted to discrete regions, which correspond to the dopaminergic nuclei (A8 to A15). In addition to these nuclei previously described, TH mRNA is also observed in regions that do not express this enzyme according to immunohistochemical studies. This difference in relation to protein expression pattern is consequent with the known posttranscriptional regulation of TH expression. The most representative example of a novel positive region is the conspicuous mRNA expression in both medial and lateral ganglionic eminences. This result agrees with reports describing the capacity of striatal stem cells (that is, located at the lateral ganglionic eminence) to become dopaminergic in vitro. Other regions include the isthmic mantle layer and the early floor plate of the midbrain-caudal forebrain. On the whole, the expression map we have obtained opens new perspectives for evolutionary/comparative studies, as well as for therapeutic approaches looking for potentially dopaminergic cells. Developmental Dynamics 234:709-717, 2005. (c) 2005 Wiley-Liss, Inc.

Development of A9/A10 dopamine neurons during the second and third trimesters in the African green monkey

Disruption in the development of dopamine-containing neurons has been postulated to underlie several CNS disorders. However, there have been no quantitative studies on the normal development of primate dopamine neurons. Thus, the fetal maturation of primate midbrain dopamine neurons was examined to establish changes that occur in the A9/A10 groups during the second and third trimesters. Eleven fetal African green monkey midbrains were immunostained for tyrosine hydroxylase (TH-ir) as a marker for dopamine neurons and quantified using stereological techniques (nucleator method). The number and size of defined dopamine neurons and the volume occupied by A9/A10 neurons increased in near linear fashion throughout the term. The estimated number of defined dopamine neurons in each hemisphere rose from approximately 50,000 at embryonic day (E) 70 to 225,000 at birth (E165), similar to the adult population. The size and the area occupied by them at birth were, however, well below the estimated adult levels. Additionally, the younger fetal midbrains had far less diversity in dopamine cell volumes compared with older fetuses and adult brains. Until midway through gestation (E81), clusters of apparently immature midbrain TH-ir cells were observed, but could not be counted. Even though the majority of cells destined to become dopamine neurons are generated in the first trimester, phenotypical maturation of A9 and A10 cell bodies continues steadily throughout gestation and extends well into the postnatal period. These data have relevance to transplantation studies that employ fetal dopaminergic grafts, and to disorders hypothesized to result from damage to developing midbrain dopamine neurons.

Midbrain dopaminergic neurons: control of their cell fate by the engrailed transcription factors

As for any other cell population, the development, cell fate, and properties of mesencephalic dopaminergic (mesDA) neurons are ultimately controlled at the transcriptional level. The genes for two transcription factors Engrailed-1 ( En1) and Engrailed-2 ( En2) play an essential role in the development and maintenance of these cells. They belong to a family of genes that have been investigated in Drosophila for more than half a century. The products of these genes are all characterized by homeotic tissue transformation and a highly conserved protein sequence, the homeobox. En1 and En2 act upon at least two steps of the differentiation of mesDA neurons. They take part in the regionalization event, which gives rise to the neuroepithelium that provides the precursor cells in the ventral midbrain with the fibroblast growth factor 8 signal necessary for their induction. Additionally, these genes are required in postmitotic mesDA neurons in which they are expressed from embryonic day 12 continuously into adulthood. In mutant mice homozygous null for En1 and En2, the neurons are generated in the ventral midbrain, become postmitotic, and begin to express their neurotransmitter phenotype. However, thereafter, they rapidly die by apoptosis. Cell mixing experiments in vitro and in vivo have demonstrated that the engrailed requirement for the survival of mesDA neurons is cell-autonomous. The inactivation of engrailed by RNA interference induces apoptosis in less than 24 h. These data suggest that the engrailed genes control an essential mechanism for the survival of mesDA neurons.

Engrailed genes are cell-autonomously required to prevent apoptosis in mesencephalic dopaminergic neurons

The neuropathological hallmark of Parkinson's disease is the loss of dopaminergic neurons in the substantia nigra pars compacta, presumably mediated by apoptosis. The homeobox transcription factors engrailed 1 and engrailed 2 are expressed by this neuronal population from early in development to adulthood. Despite a large mid-hindbrain deletion in double mutants null for both genes, mesencephalic dopaminergic (mDA) neurons are induced, become postmitotic and acquire their neurotransmitter phenotype. However, at birth, no mDA neurons are left. We show that the entire population of these neurons is lost by E14 in the mutant animals, earlier than in any other described genetic model system for Parkinson's disease. This disappearance is caused by apoptosis revealed by the presence of activated caspase 3 in the dying tyrosine hydroxylase-positive mutant cells. Furthermore, using in vitro cell mixing experiments and RNA interference on primary cell culture of ventral midbrain we were able to show that the demise of mDA neurons in the mutant mice is due to a cell-autonomously requirement of the engrailed genes and not a result of the missing mid-hindbrain tissue. Gene silencing in the postmitotic neurons by RNA interference activates caspase 3 and induces apoptosis in less than 24 hours. This rapid induction of cell death in mDA neurons suggests that the engrailed genes participate directly in the regulation of apoptosis, a proposed mechanism for Parkinson's disease.

Making connections: the development of mesencephalic dopaminergic neurons

The disorders of two adjacent sets of mesencephalic dopaminergic (MDNs) are associated with two significant health problems: Parkinson's disease and drug addiction. Because of this, a great deal of research has focused on understanding the growth, development and maintenance of MDNs. Many transcription factors and signaling pathways are known to be required for normal MDNs formation, but a unified model of MDN development is still unclear. The long-term goal is to design therapeutic strategies to: (i) nurture and/or heal endogenous MDNs, (ii) replace the affected tissue with exogenous MDNs from in vitro cultivated stem cells and (iii) restore normal connectivity. Recent developmental biology studies show great promise in understanding how MDNs develop both in vivo and in vitro. This information has great therapeutic value and may provide insight into how environmental and genetic factors increase vulnerability to addiction.

Region-specific effects of glia on neuronal induction and differentiation with a focus on dopaminergic neurons

Radial glia (RG) are the first glial cell type to appear in the nervous system. Their broad distribution and apparent similarity hide important brain region-specific differences that are likely to be essential for development. However, recent evidence supports the stimulating concept that in addition to their classical function as neuroblast guides, RG are neuronal precursors (Malatesta et al. Development 127:5253-5263, 2000; Miyata et al. Neuron 31:727-741, 2001; Noctor et al. Nature 409:714-720, 2001; Skogh et al. Mol Cell Neurosci 17:811-820, 2001). We propose that RG not only generate and guide newborn neurons, but could also instruct their own neuronal progeny to adopt appropriate region-specific phenotypes.

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.

Optimal conditions for in vivo induction of dopaminergic neurons from embryonic stem cells through stromal cell-derived inducing activity

A method of inducing dopamine (DA) neurons from mouse embryonic stem (ES) cells by stromal cell-derived inducing activity (SDIA) was previously reported. When transplanted, SDIA-induced DA neurons integrate into the mouse striatum and remain positive for tyrosine hydroxylase (TH) expression. In the present study, to optimize the transplantation efficiency, we treated mouse ES cells with SDIA for various numbers of days (8-14 days). SDIA-treated ES cell colonies were isolated by papain treatment and then grafted into the 6-hydroxydopamine (6-OHDA)-lesioned mouse striatum. The ratio of the number of surviving TH-positive cells to the total number of grafted cells was highest when ES cells were treated with SDIA for 12 days before transplantation. This ratio revealed that grafting cell colonies was more efficient for obtaining TH-positive cells in vivo than grafting cell suspensions. When we grafted a cell suspension of 2 x 10(5), 2 x 10(4), or 2 x 10(3) cells into the 6-OHDA-lesioned mouse striatum, we observed only a few surviving TH-positive cells. In conclusion, inducing DA neurons from mouse ES cells by SDIA for 12 days and grafting cell colonies into mouse striatum was the most effective method for the survival of TH-positive neurons in vivo.

Specification of catecholaminergic and serotonergic neurons

The specification of neurotransmitter phenotype is an important aspect of neuronal fate determination. Substantial progress has been made in uncovering key extracellular signals and transcriptional regulators that control the mode of neurotransmission in several model systems, among which catecholaminergic and serotonergic neurons feature prominently. Here, we review our current knowledge of the regulatory circuits that direct neurotransmitter choice, and discuss the development of well-studied types of catecholaminergic and serotonergic neurons. One emerging concept is that different types of neuron use a similar core programme to control shared modes of neurotransmission, but recruit different factors that are specific for each neuronal type. Another is that most factors that specify neurotransmitter identity also control other features of the neuronal phenotype.

Development of tyrosine hydroxylase-immunoreactive systems in the brain of the larval lamprey Lampetra fluviatilis

The development of the catecholaminergic system of the brain of the lamprey (Lampetra fluviatilis) was studied with immunocytochemistry in a series of larvae of different sizes by using two different antibodies directed against tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine synthesis. In group 1 larvae (length: 29-54 mm, ages: 8 months to 1.5 years), the only TH-immunoreactive somata observed were located in the caudal wall of the recessus praeopticus (RP) and in the nucleus tuberculi posterioris (NTP). In group 2 larvae (length: 55-80 mm, ages: 1.5-2.5 years), the somata of immunolabeled cells of the NTP give rise to fibers, most of which are ascending and terminate in the corpus striatum. Additional immunoreactive cells are observed in the nucleus praeopticus (NP), which has differentiated, and in the spinal cord. In group 3 larvae (length: 81-110 mm, ages: 2.5-4 years), the spatial distribution of TH-immunoreactive elements (somata, fibers, and terminals) bears many resemblances to that seen in the adult. Immunolabeled cells may be observed in the olfactory bulb, in the nucleus commissurae postopticae (NCP), and in the nucleus dorsalis hypothalami (NDH). Nevertheless, some groups of TH-immunoreactive cells found in the adult are not observed in group 3 larvae; these may appear during the metamorphic phase. By comparative analysis, we show that, in spite of several differences, the spatiotemporal sequence of appearance of TH-immunoreactive cell bodies and fibers in the lamprey presents many similarities to that described in gnathostomes.

Expression and developmental regulation of gap junction connexins cx26, cx32, cx43 and cx45 in the rat midbrain-floor

Connexins (cx) constitute a family of transmembrane proteins that form gap junction channels allowing metabolic and electrical coupling of cellular networks. Initial studies on the expression of cx in the developing brain have suggested that cx may undergo dynamic changes and may possibly be implicated in synchronizing development and differentiation of neural progenitor cells and young neurons. We have investigated expression of cx26, cx32, cx43, and cx45 in the midbrain floor, where nigrostriatal dopaminergic neurons originate and differentiate. This neuron population is of major importance in regulating motor-functions. Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) revealed low levels of cx26-mRNA in the midbrain floor at E12, which gradually increased during pre- and postnatal development, reaching a maximum in the adult. Cx32-mRNA-levels reached a first peak at E16, and showed highest levels in adulthood. Cx43 was highly expressed at E12, decreased until E18, and subsequently increased again until adulthood. Cx45 mRNA was prominent at all developmental ages, but slightly decreased after the first postnatal week. Double-labeling for the dopaminergic neuronal marker tyrosine hydroxylase (TH), and cx-immunoreactivities (ir) evaluated by quantitative confocal laser microscopy revealed both distinct and similar developmental patterns for the individual cx investigated. Cx26 was highest at E14, decreased towards birth, and subsequently increased again reaching about 50% of the E14 level in the adult. Cx32-ir peaked at E16 and dropped to low levels after birth. Cx43-ir was highest at E12, decreased sharply at E14, reached its lowest levels at birth, but modestly increased again afterwards. Cx45-ir showed a biphasic pattern, with two prominent peaks at E12 and E18, followed by a massive postnatal decrease. Taken together, our results reveal that expression and ir of cx in the midbrain floor and dopaminergic neurons, respectively, follow cx-type specific patterns that temporally coincide with important steps of midbrain morphogenesis, as e.g. progenitor cell formation and migration (E12), early differentiation (E14-16), target encounter (E16-18) and postnatal functional maturation of the nigrostriatal system.

Fetal exposure to methamphetamine in utero stimulates development of serotonergic neurons in three-dimensional reaggregate tissue culture

Methamphetamine is a potent psychomotor stimulant with neurotoxic potential which is widely abused by females of childbearing age, raising serious public health concerns in terms of exposure of the fetus to the drug. The current study was conducted to determine the effect of maternal administration of methamphetamine on developing monoaminergic neurons using three-dimensional reaggregate tissue cultures prepared from fetal mesencephalic and striatal cells. In this culture system, the dopaminergic and serotonergic mesencephalic-striatal projections are reconstructed and develop with a time course similar to that observed in vivo. Pregnant C57Bl/6J mice were injected twice daily with 40 mg/kg methamphetamine or saline from gestational days 6-13. On gestational day 14, cells from methamphetamine and saline-exposed embryos were used to prepare reaggregate cultures. Levels of neurotransmitters and their metabolites in the reaggregates and culture medium were monitored at 14, 29, 43, and 64 days of culture. Reaggregates prepared from methamphetamine-exposed embryos showed a significant elevation in serotonin levels at all culture ages compared to reaggregates prepared from saline-treated embryos. Levels of 5-HIAA in reaggregates and culture medium were also elevated in 14- and 29-day-old cultures derived from drug-exposed embryos. The development of the dopaminergic nigrostriatal projection was resistant to repeated in utero exposure to methamphetamine. In contrast, exposure of the fetus to methamphetamine, during early to midgestation, produced a long-lasting stimulatory effect on serotonergic development in culture.

Methamphetamine concentrations in fetal and maternal brain following prenatal exposure

Levels of methamphetamine in maternal striatum and whole fetal mouse brain were assessed at 0.5, 1, 2, and 4 h postinjection on gestational day 14 (GD14) following a single, subcutaneous injection of 40 mg/kg (+)-methamphetamine hydrochloride to pregnant mice. In the dams, striatal concentrations of methamphetamine peaked at 1 h postinjection, reaching levels of approximately 510 ng/mg protein. Amphetamine, the primary metabolite of methamphetamine, increased to 77 ng/mg protein at 2 h and remained elevated by 4 h postinjection. In the fetal brain, peak methamphetamine concentrations of approximately 122 ng/mg protein were attained at 1 h. Amphetamine was only detectable in fetal brain at 2 and 4 h postinjection. Regional analysis of methamphetamine levels in fetal striatum, cortex, and brainstem revealed that the drug was not uniformly distributed. Maternal administration of methamphetamine results in fetal brain drug concentrations, which approximate those reported in human infants whose mother abused methamphetamine. This dosage regimen, therefore, serves as an appropriate animal model for assessing the potential risks to human offspring exposed to methamphetamine in utero.

Structure of longitudinal brain zones that provide the origin for the substantia nigra and ventral tegmental area in human embryos, as revealed by cytoarchitecture and tyrosine hydroxylase, calretinin, calbindin, and GABA immunoreactions

In a previous work, mapping early tyrosine hydroxylase (TH) expressing primordia in human embryos, the tegmental origin of the substantia nigra (SN) and ventral tegmental area (VTA) was located across several neuromeric domains: prosomeres 1-3, midbrain, and isthmus (Puelles and Verney, [1998] J. Comp. Neurol. 394:283-308). The present study examines in detail the architecture of the neural wall along this tegmental continuum in 6-7 week human embryos, to better define the development of the SN and VTA. TH-immunoreactive (TH-IR) structures were mapped relative to longitudinal subdivisions (floor plate, basal plate, alar plate), as well as to radially superposed strata of the neural wall (periventricular, intermediate, and superficial strata). These morphologic entities were delineated at each relevant segmental level by using Nissl-stained sections and immunocytochemical mapping of calbindin, calretinin, and GABA in adjacent sagittal or frontal sections. A numerous and varied neuronal population originates in the floor plate area, and some of its derivatives become related through lateral tangential migration with other neuronal populations born in distinct medial and lateral portions of the basal plate and in a transition zone at the border with the alar plate. Some structural differences characterize each segmental domain within this common schema. The TH-IR neuroblasts arise predominantly within the ventricular zone of the floor plate and, more sparsely, within the adjacent medial part of the basal plate. They first migrate radially from the ventricular zone to the pia and then apparently move laterally and slightly rostralward, crossing the superficial stratum of the basal plate. Several GABA-IR cell populations are present in this region. One of them, which might represent the anlage of the SN pars reticulata, is generated in the lateral part of the basal plate.

Ontogeny of the dopamine D2 receptor mRNA expressing cells in the human hippocampal formation and temporal neocortex

The study details the cellular expression of the dopamine D2 receptor mRNA in the human temporal lobe during prenatal development. At 13 embryonic weeks (E13) D2 mRNA was widely expressed in the temporal lobe. At this time point in the dentate gyrus D2 mRNA positive cells first appeared at the outer border of the granular layer and their number increased with development. The CA1 exhibited the highest level of D2 mRNA expression. By E19-25 the hippocampal formation underwent rapid morphological maturation. D2 mRNA expression became more uniform and dense in the ammonic subfield. At all ages the subiculum appeared more mature morphologically but less intensely stained for D2 mRNA than the ammonic fields. In the entorhinal cortex D2 mRNA expression was most conspicuous in the future layer II at all ages. In the temporal neocortex D2 mRNA-positive cells were detected in the subplate and cortical plate. Differentiation of the cortical plate was accompanied by concentration of D2 mRNA-positive cells in layer V. The most conspicuous cells expressing D2 mRNA were found in the marginal zone of all regions and resembled Cajal-Retzius cells in morphology and location. Density of putative Cajal-Retzius cells expressing D2 mRNA decreased with development. They all but disappeared from the hippocampal areas by mid gestation, but in the temporal neocortex occasional cells were seen even at term. Early and widespread but region and cell type specific expression of D2 receptor mRNA suggests an important role of this DA receptor subtype in prenatal development of the human temporal lobe.

Dopamine control of striatal gene expression during development: relevance to knockout mice for the dopamine transporter

The aim of this study was to determine at which developmental stage and how dopamine regulates the expression of striatal dopamine receptor and neuropeptide mRNAs. For this, we studied the expression of these mRNAs, in relation to dopamine innervation, in normal mice from gestational day 13 (G13) to adult. Particularly, we investigated the adaptive changes in the expression of these markers in mice lacking the dopamine transporter during development. We detected tyrosine hydroxylase, by immunohistochemistry, in the ventral mesencephalon and the striatal anlage in both genotypes at G13, whereas the dopamine transporter appeared in the striatum of normal mice at G14. By in situ hybridization, we detected striatal dopamine D1, D2, D3 receptor, and substance P mRNAs at G13, preproenkephalin A mRNA at G14 and dynorphin mRNA at G17 in normal mice. Although the time of initial detection and the distribution were not affected in mutant mice, quantitative changes were observed. Indeed, D1 and D2 receptor as well as preproenkephalin A mRNA levels were decreased from G14 on, and dynorphin mRNA level was increased from G17 on. In contrast, substance P mRNA level was unaffected. Our data demonstrate that the influence of dopamine on striatal neurons occurs early during the development of the mesostriatal system as quantitative changes appeared in mutant mice as soon as G14. These findings bring new insights to the critical influence of dopamine on the expression of striatal dopamine receptor and neuropeptide mRNAs during development, and suggest that mesostriatal dopamine transmission functions from G14 on.

Epigenetic cues in midbrain dopaminergic neuron development

Midbrain dopaminergic (DA) neurons subserve complex and varied neural functions in vertebrate CNS. Their progenitors give rise to DA neurons by the action of two extracellular inducers, Sonic Hedgehog and FGF8. After this first commitment, the function of selectively activated transcription factors, like the orphan steroid nuclear receptor Nurr1, is required for DA final determination. Subsequently, DA function is selectively modulated by specific interaction with the developing striatal target tissue. Committed and determined DA neurons express the key genes involved in DA neurotransmission at different times in development. Synthesis and intracellular accumulation of DA is achieved shortly after expression of Nurr1, while high affinity uptake, responsible for ending the neurotransmission, takes place after a few days. Cell contacts between the presynaptic DA neurons and target striatal neurons are apparently necessary for the fine modulation of DA function, in vivo and in vitro.

Neuroprotection and neuronal differentiation studies using substantia nigra dopaminergic cells derived from transgenic mouse embryos

The major pathological lesion of Parkinson's disease (PD) is the selective cell death of dopaminergic (DA) neurons in substantia nigra (SN). Although the initial cause and subsequent molecular signaling mechanisms leading to DA cell death underlying the PD process remain elusive, brain-derived neurotrophic factor (BDNF) is thought to exert neuroprotective as well as neurotrophic roles for the survival and differentiation of DA neurons in SN. Addressing molecular mechanisms of BDNF action in both primary embryonic mesencephalic cultures and in vivo animal models has been technically difficult because DA neurons in SN are relatively rare and present with many heterogeneous cell populations in midbrain. We have developed and characterized a DA neuronal cell line of embryonic SN origin that is more accessible to molecular analysis and can be used as an in vitro model system for studying SN DA neurons. A clonal SN DA neuronal progenitor cell line SN4741, arrested at an early DA developmental stage, was established from transgenic mouse embryos containing the targeted expression of the thermolabile SV40Tag in SN DA neurons. The phenotypic and morphological differentiation of the SN4741 cells could be manipulated by environmental cues in vitro. Exogenous BDNF treatment produced significant neuroprotection against 1-methyl-4-phenylpyridinium, glutamate, and nitric oxide-induced neurotoxicity in the SN4741 cells. Simultaneous phosphorylation of receptor tyrosine kinase B accompanied the neuroprotection. This SN DA neuronal cell line provides a unique model system to circumvent the limitations associated with primary mesencephalic cultures for the elucidation of molecular mechanisms of BDNF action on DA neurons of the SN.

Gli2 is required for induction of floor plate and adjacent cells, but not most ventral neurons in the mouse central nervous system

Induction of the floor plate at the ventral midline of the neural tube is one of the earliest events in the establishment of dorsoventral (d/v) polarity in the vertebrate central nervous system (CNS). The secreted molecule, Sonic hedgehog, has been shown to be both necessary and sufficient for this induction. In vertebrates, several downstream components of this signalling pathway have been identified, including members of the Gli transcription factor family. In this study, we have examined d/v patterning of the CNS in Gli2 mouse mutants. We have found that the floor plate throughout the midbrain, hindbrain and spinal cord does not form in Gli2 homozygotes. Despite this, motoneurons and ventral interneurons form in their normal d/v positions at 9.5 to 12.5 days postcoitum (dpc). However, cells that are generated in the region flanking the floor plate, including dopaminergic and serotonergic neurons, were greatly reduced in number or absent in Gli2 homozygous embryos. These results suggest that early signals derived from the notochord can be sufficient for establishing the basic d/v domains of cell differentiation in the ventral spinal cord and hindbrain. Interestingly, the notochord in Gli2 mutants does not regress ventrally after 10.5 dpc, as in normal embryos. Finally, the spinal cord of Gli1/Gli2 zinc-finger-deletion double homozygous mutants appeared similar to Gli2 homozygotes, indicating that neither gene is required downstream of Shh for the early development of ventral cell fates outside the ventral midline.

Coordinate expression of L1 and 6B4 proteoglycan/phosphacan is correlated with the migration of mesencephalic dopaminergic neurons in mice

Mesencephalic dopaminergic (DA) neurons of mice are generated from embryonic day 10 to 12 (E10-12) in the ventricular zone of the mesencephalon. They first migrate toward the ventral mesencephalon, and then turn laterally, or tangentially, in the basal part of the mesencephalon. With immunohistochemical analysis of E10-E15 ICR mice, we found that cell adhesion molecule L1 was transiently expressed on the median part of tangential fibers coincident with the lateral migration of DA neurons from E11 to E13, when neurons move along the tangential fibers toward their final destinations: the reticular formation, the substantia nigra pars compact, and the ventral tegmental area. While L1 expression was not observed in DA neurons, they expressed a chondroitin sulfate proteoglycan, 6B4 proteoglycan/phosphacan, which has been shown to bind to L1/Ng-CAM in vitro. These results suggest that the heterophilic interaction between 6B4 proteoglycan on the neurons and L1 on the fibers is involved in the lateral migration of mesencephalic DA neurons in mice.

Early neuromeric distribution of tyrosine-hydroxylase-immunoreactive neurons in human embryos

A segmental mapping of brain tyrosine-hydroxylase-immunoreactive (TH-IR) neurons in human embryos between 4.5 and 6 weeks of gestation locates with novel precision the dorsoventral and anteroposterior topography of the catecholamine-synthetizing primordia relative to neuromeric units. The data support the following conclusions. (1) All transverse sectors of the brain (prosomeres in the forebrain, midbrain, rhombomeres in the hindbrain, spinal cord) produce TH-IR neuronal populations. (2) Each segment shows peculiarities in its contribution to the catecholamine system, but there are some overall regularities, which reflect that some TH-IR populations develop similarly in different segments. (3) Dorsoventral topology of the TH-IR neurons indicates that at least four separate longitudinal zones (in the floor and basal plates and twice in the alar plate) found across most segments are capable of producing the TH-IR phenotype. (4) Basal plate TH-IR neurons tend to migrate intrasegmentally to a ventrolateral superficial position, although some remain periventricular; those in the brainstem are related to motoneurons of the oculomotor and branchiomotor nuclei. (5) Some alar TH-IR populations migrate superficially within the segmental boundaries. (6) Most catecholaminergic anatomical entities are formed as fusions of smaller segmental components, each of which show similar histogenetic patterns. A nomenclature is proposed that partly adheres to previous terminology but introduces the distinction of embryologically different cell populations and unifies longitudinally analogous entities. Such a model, as presented in the present study, is convenient for resolving problems of homology of the catecholamine system across the diversity of vertebrate forms.

Morphometric characteristics of cryopreserved mesencephalic dopamine neurons in culture

Blocks of embryonic rat mesencephalon were freeze-stored for 1-2 years in liquid nitrogen at -196 degrees C with 7.5% dimethyl sulfoxide (DMSO) as cryoprotectant. After thawing, pooled mesencephalic tissues were mechanically dissociated. The cells, plated at two different densities (4.10[5] and 2.10[5]/cm2) were cultured in a serum-supplemented medium for at least 2 weeks before immunocytochemical staining with highly specific antidopamine (DA) antibodies. The cryopreserved DA-immunoreactive (IR) neurons were compared, by means of computerized morphometry, to the fresh ones plated at the same densities. A separate analysis of the dendritic and axonal morphometric parameters revealed that the cryopreserved DA-IR cells, whatever the experimental conditions, had significantly larger dendritic fields and, less significantly, larger axonal fields than their fresh counterparts. A principal component analysis, mainly based on the dendritic morphometric parameters, allowed to individualize only two populations (cryopreserved and fresh) among the four groups studied. These findings underline the role of dendrites as potential sites of release and/or re-uptake of dopamine and their possible implications in functionally effective cryopreserved nigral grafts.

Epigenetic factors and midbrain dopaminergic neurone development

In the mammalian brain dopamine systems play a central role in the control of movement, hormone release, emotional balance and reward. Alteration of dopaminergic neurotransmission is involved in Parkinson's disease and other movement disorders, as well as in some psychotic syndromes. This review summarises recent findings, which shed some light on signals and cellular interactions involved in the specification and maturation of the dopaminergic function during neurogenesis. In particular we will focus on three major issues: (1) the differentiation of dopaminergic neurones triggered by direct contact with the midbrain floor plate cells through the action of sonic hedgehog; (2) the neurotrophic factors acting on dopaminergic neurones; and (3) the role of target striatal cells on the survival and the axonal growth of developing or grafted dopaminergic neurones.

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

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

Early postnatal changes of the dopaminergic mesencephalic neurons in the weaver mutant mouse

In adult weaver (wv) mutant mice up to 70% of the mesostriatal dopaminergic neurons are lost and major alterations of the dopaminergic dendrites of the substantia nigra have been described. We sought to determine the time of onset of these alterations. Cell counts of the main dopaminergic (DA) mesencephalic cell groups (A8, A9, A10), as labeled with tyrosine hydroxylase immunocytochemistry were done in wild-type and homozygous wv/wv pups. No loss of the DA neurons, was detectable at postnatal day 7 (P7), while reductions in substantia nigra (and retrorubral area) amounted to 35% at P14 and 47% by P21. On the other hand, the severe reduction of dopaminergic dendrites, particularly of their distal compartments was already visible from P3 on. During the first postnatal week, this was associated to abnormal clustering of the dopaminergic neurons. These early neuritic alterations were present, though to a milder degree, in heterozygous (wv/+) mice.

Migration of dopaminergic neurons in the embryonic mesencephalon of mice

Migration of dopamine (DA)-containing neurons and its guiding cues were histologically examined in the embryonic mesencephalon of normal mice. Cells immunoreactive (ir) for tyrosine hydroxylase (TH), a DA-synthesizing enzyme, were first detected on embryonic day 10 (E10) in the medio-basal part of the mesencephalon and were distributed throughout the entire length of the ventral mesencephalic wall at E12. By E14, TH-ir cells were located laterally along the ventral pial surface to form the primordia of the substantia nigra. Experiments with a single injection of bromodeoxyuridine, a thymidine analog, demonstrated that cells generated in the ventricular surface of the ventral mesencephalon at E11 migrated ventrally and then moved laterally to form the substantia nigra and the ventral tegmental area. Electron microscopic examination of the ventral mesencephalon of E12 mice disclosed that in the dorsal part ventrally migrating immature neurons made close contacts with the processes of radial glial cells. The expression of tenascin was transiently seen on radial glial processes between E10 and E13 coincident with the period of the ventral migration of mesencephalic DA neurons. By double immunostaining of E13 mesencephalon, ventrally migrating TH-ir cells were seen to be apposed to tenascin-bearing radial glial processes. On the other hand, laterally migrating neurons in the basal part of the mesencephalon were observed by electron microscopy to contact with tangentially arranged nerve fibers which were immunopositive for the 160 kDa neurofilament polypeptide at the light microscopic level from E10. Double immunostaining of E13 mesencephalon demonstrated that laterally migrating TH-ir cells were intermingled among neurofilament-ir fiber bundles. The cells of origin of the tangential nerve fibers were detected in the lateral part of the mesencephalon, when a fluorescent dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) was injected into the basal part of the mesencephalon of fixed E12 mice. The present results suggest that guiding cues of the radial migration of mesencephalic DA neurons represent processes of radial glial cells which express tenascin. On the other hand, tangentially arranged nerve fibers originating from the lateral part of the mesencephalon may provide a scaffolding along which the mesencephalic DA neurons subsequently migrate laterally to form the ventral tegmental area and the substantia nigra.

Development of the catecholamine neurons in human embryos and fetuses, with special emphasis on the innervation of the cerebral cortex

The cathecholaminergic (CA) systems have been described as appearing early in the development of the mammalian central nervous system (CNS), but their exact distribution in humans has been studied only following gestational week (g.w.) 13. Furthermore, it is not known when CA fibers initially penetrate the developing cerebral cortex. In this study, the CA cells groups and fibers are described in the human central nervous system from 6 to 13 g.w. as revealed with immunocytochemical techniques, with antibodies raised against three synthetic enzymes of the catecholamine (CA) pathway: tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT). At 6 g.w., TH-like immunoreactive (TH-IR) cell groups were widespread through the caudorostral extension of the CNS corresponding to the different dopaminergic mesencephalic and hypothalamic groups. Noradrenergic groups also were labeled in the medulla oblongata and in the locus coeruleus as well as in other areas in the pons. Additional TH-IR cell groups might represent a transient developmental expression of TH similar to that observed in the rat. DBH immunoreactivity labeled primarily the noradrenergic pontic cell groups and, to a lesser extent, groups located in the medulla oblongata. Rare PNMT-IR neurons were detected in the medulla oblongata only at 13 g.w. The main CA bundles described in the adult were also observed in human embryos and fetuses. At 6 g.w., TH-IR pathways extended caudorostrally within the central tegmental tract and the dorsal tegmental bundle, the latter merging with the dopaminergic mesotelencephalic pathway giving rise to the medial forebrain bundle in the basal forebrain. At 7-8 g.w., TH-IR fibers extended to the basal ganglia and the telencephalic wall. The first TH-IR and, to a much lesser extent, DBH-IR fibers penetrated the frontal lateral cortical anlage through the intermediate zone and sparsely through the marginal zone but not through the thin cortical plate. A second stream entered the telencephalic anlage frontomedially, ventral to the septal area. At 11 g.w., numerous TH-IR fibers invaded the subplate layer, but they penetrated the cortical plate only at 13 g.w. At that time, TH-IR and DBH-IR fibers had reached the occipital cortex in a rostrocaudal gradient. The appearance of well-organized CA system already in embryonic stages in humans could be of great importance for normal shaping of the nervous system as well as for development of cortical circuitry.

Development of catecholamine systems in the brain of the lizard Gallotia galloti

For a better insight into general and derived traits of developmental aspects of catecholaminergic (CA) systems in amniotes, we have studied the development of these systems in the brain of a lizard, Gallotia galloti, with tyrosine hydroxylase (TH)- and dopamine (DA) immunohistochemical techniques. Two main groups of TH-immunoreactive (THi) perikarya appear very early in development: one group in the midbrain which gives rise to the future ventral tegmental area, substantia nigra and retrorubral cell groups, and another group in the tuberomammillary hypothalamus. Somewhat later in development, TH/DA-immunoreactive cells are observed in the thalamus, rostrodorsal hypothalamus and spinal cord, and, with another delay, in the suprachiasmatic nucleus, the periventricular organ, and the pretectal posterodorsal nucleus. CA cell groups that appear rather late in development include the cells in the olfactory bulb, the locus coeruleus and the caudal brainstem. As expected, the development of immunoreactive fibers stays behind that of the cell bodies, but reaches the adult-like pattern just prior to hatching. The present study revealed considerable variation in the relation between the state of cytodifferentiation and first expression of TH/DA immunoreactivity between CA cell groups. Catecholamine cells in the midbrain and tuberomammillary hypothalamus are still migrating, immature (absence of dendrites) and express only TH immunoreactivity at the time of first detection. Cells which appear at later developmental stages lie already further away from the ventricle, possess two or more dendritic processes, and generally express both TH- and DA immunoreactivity.

Raphé neural cells immortalized with a temperature-sensitive oncogene: differentiation under basal conditions down an APUD cell lineage

Dividing cells from the midline of the ventral rhombencephalon and medulla oblongata have been transduced with a modulatable oncogene, (ts)SV40-T, using retroviral gene transfer. At the permissive temperature of the oncogene (33 degrees C), cells replicated and were isolated as individual, homogeneous clones. The effects of simply raising the temperature to the oncogene's non-permissive value, namely 39 degrees C, were analyzed by immunohistochemical methods. In one clone in particular (921202-6), cells ceased replication and started to differentiate. Certain neuronal characteristics became apparent: neurone-specific enolase-like immunoreactivity developed, as did the ability to take up exogenously applied 5-hydroxytryptamine (5HT). In addition, the cells took up exogenous 5-hydroxytryptophan (5HTP), and subsequently decarboxylated it to 5HT. However, they were unable to synthesize immunohistochemically detectable amounts of 5HT using L-tryptophan as a precursor. No 5HT uptake was found either in mitotic cells of this clone held at 33 degrees C, or in several other neuronal clones differentiating at 39 degrees C. Neither the neuronal nor the serotoninergic characteristics of clone 921202-6 developed in the presence of retinoic acid. It is concluded that 921202-6 cells differentiate under basal conditions down a neuronal pathway typical of an APUD cell, and that the choice of this pathway is made prior to the end of cell cycling. Furthermore, predisposition of the precursor cells to the neuronal/APUD phenotype can be overridden by extraneous epigenetic factors.

Mitogenic effect of basic fibroblast growth factor on embryonic ventral mesencephalic dopaminergic neurone precursors

The effects of embryonic age and the presence of basic fibroblast growth factor (bFGF) have been examined on the survival and rate of cell division of dopaminergic neurones of the ventral mesencephalon. Cultures were produced from 7.5 mm and 11 mm rat embryos, pulsed with [3H]thymidine during the first 12 h, and the survival and labelling of cells measured after 3 and 7 days in vitro. bFGF largely prevented the decline in numbers of tyrosine hydroxylase (TH)-positive neurones that occurred in control cultures between 3 days and 1 week. In cultures derived from the younger 7.5 mm embryos there were more TH-positive neurones in the presence of exogenous bFGF than under control conditions after 3 days in vitro. No similar effect was seen in the cultures derived from the older 11 mm embryos. Combined [3H]thymidine labelling and TH immunocytochemistry suggested that this effect was attributable, at least in part, to a bFGF-associated increase in the proliferation of TH-positive neurone progenitors during the first day or so, which was seen in cultures from 7.5 mm but not 11 mm embryos. The effect of bFGF on cultures from older embryos is therefore purely on neuronal survival, while the effect on cultures from younger embryos is a mixture of survival and mitogenic actions.

Specific Reinnervation of Lesioned Mouse Striatum by Grafted Mesencephalic Dopaminergic Neurons

Selective lesions of the dopaminergic nigrostriatal system and embryonic neuron grafts were used to study the mechanism by which exogenous neurons can restore transmitter function and to examine CNS development and plasticity. C57BL mice treated with acetaldehyde/1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine show irreversible loss of substantia nigra dopaminergic neurons. Implants of embryonic mesencephalic dopaminergic neurons functionally reinnervate the striatum and form a dense network of fibres; approximately 20% of the implanted dopaminergic cells survive for several months. However, dopaminergic fibre outgrowth and mesencephalic graft development appear lower in control, non-lesioned, animals. Moreover, implants of embryonic hypothalamic dopaminergic neurons show little or no survival. These results indicate that interactions between embryonic and adult neurons are selective. We suggest that this specificity may be sustained by the action of still unknown trophic and/or tropic factors, possibly produced by the lesioned striatum and by putative inhibitory mechanisms of cell migration and neuritic outgrowth.

Early evidence of catecholaminergic cell groups in 5- and 6-week-old human embryos using tyrosine hydroxylase and dopamine-beta-hydroxylase immunocytochemistry

Catecholaminergic systems were visualized in the CNS of human embryos from stage 15-16 (5 gestational week, g.w.) to 18 (6 g.w.) using tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) as immunocytochemical markers. At 5 g.w., several TH-like immunoreactive (TH-IR) cell groups were identified in the medulla oblongata, pons, mesencephalon and the anlage of the hypothalamic area. DBH immunoreactivity was restricted to the locus coeruleus and to rare neurons in the medulla oblongata. At 6 g.w., the density of TH-IR neurons was strikingly increased in these different areas--especially in the prospective substantia nigra and ventral tegmental area--and two main bundles of catecholaminergic axons extended from the medulla oblongata until the basal forebrain and from the mesencephalic tegmentum to the anlage of the striatum. These pathways were mainly TH-IR but DBH-IR was also observed in the former. No TH-IR fibers reached the telencephalon at 6 g.w.

Dopamine synthesis precedes dopamine uptake in embryonic rat mesencephalic neurons

We have measured [3H]dopamine ([3H]DA) uptake and tyrosine hydroxylase-immunopositive immunostaining in cells acutely dissociated from the embryonic ventral mesencephalon (MSC). DA and its metabolites as well as catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO) activities were determined in homogenates taken from the MSC and striatum (STR). In the embryonic ventral MSC measurable DA and tyrosine hydroxylase (TH) immunostaining were present as early as embryonic day (E) 12.5. At E14 the number of TH+ neurons was about 50% of the values at E18. In the MSC, DA concentration increased sharply at E16 and reached a plateau before birth that was 10-fold lower than adult values. In the STR, DA was first detected at E16, suggesting that DA fibers reach the STR at this embryonic stage. High-affinity DA uptake appeared in the MSC only at E16, concomitantly with the arrival of DA fibers in the STR, increased sharply between E16 and E18, and reached a plateau before birth. This uptake mechanism was not selective for catecholamine uptake inhibitors. Thus, DA synthesis in the MSC preceded the onset of high-affinity uptake mechanism, which could be correlated to the beginning of striatal DA innervation. Measurable MAO and COMT activities were detected as early as E13 (MSC) and E15 (STR), but not DA metabolites, which appeared later. We conclude that the high-affinity DA uptake mechanism in MSC DA neurons develops coincident with the arrival of DA fibers to the STR. The sharp increase of DA uptake between E16 and E18 is due only in part to an increase in the number of TH+ cells. These results support the hypothesis that in vivo the target STR neurons regulate the maturation of MSC DA cells.