Subpopulations of mesencephalic dopaminergic neurons express different levels of tyrosine hydroxylase messenger RNA

Quantitative whole-brain 3D imaging of tyrosine hydroxylase-labeled neuron architecture in the mouse MPTP model of Parkinson's disease

Parkinson's disease (PD) is a basal ganglia movement disorder characterized by progressive degeneration of the nigrostriatal dopaminergic system. Immunohistochemical methods have been widely used for characterization of dopaminergic neuronal injury in animal models of PD, including the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) mouse model. However, conventional immunohistochemical techniques applied to tissue sections have inherent limitations with respect to loss of 3D resolution, yielding insufficient information on the architecture of the dopaminergic system. To provide a more comprehensive and non-biased map of MPTP-induced changes in central dopaminergic pathways, we used iDISCO immunolabeling, light-sheet fluorescence microscopy (LSFM) and deep-learning computational methods for whole-brain three-dimensional visualization and automated quantitation of tyrosine hydroxylase (TH)-positive neurons in the adult mouse brain. Mice terminated 7 days after acute MPTP administration demonstrated widespread alterations in TH expression. Compared to vehicle controls, MPTP-dosed mice showed a significant loss of TH-positive neurons in the substantia nigra pars compacta and ventral tegmental area. Also, MPTP dosing reduced overall TH signal intensity in basal ganglia nuclei, i.e. the substantia nigra, caudate-putamen, globus pallidus and subthalamic nucleus. In contrast, increased TH signal intensity was predominantly observed in limbic regions, including several subdivisions of the amygdala and hypothalamus. In conclusion, mouse whole-brain 3D imaging is ideal for unbiased automated counting and densitometric analysis of TH-positive cells. The LSFM–deep learning pipeline tracked brain-wide changes in catecholaminergic pathways in the MPTP mouse model of PD, and may be applied for preclinical characterization of compounds targeting dopaminergic neurotransmission.

Summary: Whole-brain immunolabeling, mapping and absolute quantification of tyrosine hydroxylase neurons in the adult mouse brain provides a useful tool for studying changes in dopaminergic signaling in a mouse model of PD.

c-Fos marking of identified midbrain neurons coactive after nicotine administration in-vivo

Despite the reduced life expectancy and staggering financial burden of medical treatment associated with tobacco smoking, the molecular, cellular, and ensemble adaptations associated with chronic nicotine consumption remain poorly understood. Complex circuitry interconnecting dopaminergic and cholinergic regions of the midbrain and mesopontine tegmentum are critical for nicotine associated reward. Yet our knowledge of the nicotine activation of these regions is incomplete, in part due to their cell type diversity. We performed double immunohistochemistry for the immediate early gene and surrogate activity sensor, c-Fos, and markers for either cholinergic, dopaminergic or GABAergic cell types in mice treated with nicotine. Both acute (0.5 mg/kg) and chronic (0.5 mg/kg/day for 7 days) nicotine strongly activated GABAergic neurons of the interpeduncular nucleus and medial terminal nucleus of the accessory optic tract (MT). Acute but not chronic nicotine also activated small percentages of dopaminergic and other neurons in the ventral tegmental area (VTA) as well as noncholinergic neurons in the pedunculotegmental and laterodorsal tegmental nuclei (PTg/LDTg). Twenty four hours of nicotine withdrawal after chronic nicotine treatment suppressed c-Fos activation in the MT. In comparison to nicotine, a single dose of cocaine caused a similar activation in the PTg/LDTg but not the VTA where GABAergic cells were strongly activated but dopaminergic neurons were not affected. These results indicate the existence of drug of abuse specific ensembles. The loss of ensemble activation in the VTA and PTg/LDTg after chronic nicotine represents a molecular and cellular tolerance which may have implications for the mechanisms underlying nicotine dependence.

Considerations when using cre-driver rodent lines for studying ventral tegmental area circuitry

The use of Cre-driver rodent lines for targeting ventral tegmental area (VTA) cell types has generated important and novel insights into how precise neurocircuits regulate physiology and behavior. While this approach generally results in enhanced cellular specificity, an important issue has recently emerged related to the selectivity and penetrance of viral targeting of VTA neurons using several Cre-driver transgenic mouse lines. Here, we highlight several considerations when utilizing these tools to study the function of genetically defined neurocircuits. While VTA dopaminergic neurons have previously been targeted and defined by the expression of single genes important for aspects of dopamine neurotransmission, many VTA and neighboring cells display dynamic gene expression phenotypes that are partially consistent with both classically described dopaminergic and non-dopaminergic neurons. Thus, in addition to varying degrees of selectivity and penetrance, distinct Cre lines likely permit targeting of partially overlapping, but not identical VTA cell populations. This Matters Arising Response paper addresses the Lammel et al. (2015) Matters Arising paper, published concurrently in Neuron.

Phenotype, compartmental organization and differential vulnerability of nigral dopaminergic neurons

The degeneration of nigral dopaminergic (DA-) neurons is the histopathologic hallmark of Parkinson's disease (PD), but not all nigral DA-cells show the same susceptibility to degeneration. This starts in DA-cells in the ventrolateral and caudal regions of the susbtantia nigra (SN) and progresses to DA-cells in the dorsomedial and rostral regions of the SN and the ventral tegmental area, where many neurons remain intact until the final stages of the disease. This fact indicates a relationship between the topographic distribution of midbrain DA-cells and their differential vulnerability, and the possibility that this differential vulnerability is associated with phenotypic differences between different subpopulations of nigral DA-cells. Studies carried out during the last two decades have contributed to establishing the existence of different compartments of nigral DA-cells according to their neurochemical profile, and a possible relationship between the expression of some factors and the relative vulnerability or resistance of DA-cell subpopulations to degeneration. These aspects are reviewed and discussed here.

Sex and species differences in tyrosine hydroxylase-synthesizing cells of the rodent olfactory extended amygdala

The bed nucleus of the stria terminalis (BST) and the medial amygdala (MeA) are anatomically connected sites necessary for chemosensory regulation of social behaviors in rodents. Prairie voles (Microtus ochrogaster) are a valuable model for studying the neural regulation of social behaviors because, unlike many other rodents, they are gregarious, pair bond after copulating, and are biparental. We herein describe sex and species differences in immunoreactivity for tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine synthesis, in the BST and MeA. Virgin male prairie voles had a large number of TH-immunoreactive cells in areas analogous to the rat principal nucleus of the BST (pBST) and the posterodorsal medial amygdala (MeAPd). Virgin female prairie voles had far fewer TH-immunoreactive cells in these sites ( approximately 17% of the number of cells as males in the pBST, approximately 35% of the number of cells in the MeAPd). A few TH-immunoreactive cells were found in the BST of male and female hamsters and meadow voles, but not in rats. The MeApd also contained a few TH-immunoreactive cells in male and female hamsters and male meadow voles, but not rats. Castration greatly reduced the number of TH-immunoreactive cells in the male prairie vole pBST and MeAPd, an effect that could be reversed with testosterone. Furthermore, treating ovariectomized females with testosterone substantially increased TH-immunoreactive cells in both sites. Therefore, a species-specific sex difference in TH expression is found in a chemosensory pathway in prairie voles. Expression of TH in these sites is influenced by circulating gonadal hormones in adults, which may be related to changes in their display of social behaviors across the reproductive cycle.

Immediate early gene activity in song control nuclei and brain areas regulating motivation relates positively to singing behavior during, but not outside of, a breeding context

In some species, such as songbirds, much is known about how the brain regulates vocal learning, production, and perception. What remains a mystery is what regulates the motivation to communicate. European starlings (Sturnus vulgaris) sing throughout most of the year, but the social and environmental factors that motivate singing behavior differ seasonally. Male song is highly sexually motivated during, but not outside of, the breeding season. Brain areas outside the song control system, such as the medial preoptic nucleus (POM) and ventral tegmental area (VTA), have been implicated in regulating sexually motivated behaviors in birds, including song. The present study was designed to explore whether these regions, as well as three song control nuclei [area X, the high vocal center (HVC), and the robust nucleus of the arcopallium (RA)], might be involved differentially in song produced within compared to outside of a breeding context. We recorded the behavioral responses of breeding and nonbreeding condition male starlings to the introduction of a female conspecific. Males did not show context-dependent differences in the overall amount of song sung. However, immunocytochemistry for the protein product of the immediate early gene cFOS revealed a positive linear relationship between the total amount of songs sung and number of cFOS-labeled cells in POM, VTA, HVC, and RA for birds singing during, but not outside of, a breeding context. These results suggest that these regions differentially regulate male song production depending on reproductive context. Overall the data support the hypothesis that the POM and VTA interact with the song control system, specifically HVC and RA, to regulate sexually motivated vocal communication in songbirds.

Discovering novel phenotype-selective neurotrophic factors to treat neurodegenerative diseases

Astrocytes and neurons in the central nervous system (CNS) interact functionally to mediate processes as diverse as neuroprotection, neurogenesis and synaptogenesis. Moreover, the interaction can be homotypic, implying that astrocyte-derived secreted molecules affect their adjacent neurons optimally vs remote neurons. Astrocytes produce neurotrophic and extracellular matrix molecules that affect neuronal growth, development and survival, synaptic development, stabilization and functioning, and neurogenesis. This new knowledge offers the opportunity of developing astrocyte-derived, secreted proteins as a new class of therapeutics specifically to treat diseases of the CNS. However, primary astrocytes proliferate slowly in vitro, and when induced to immortalize by genetic manipulation, tend to lose their phenotype. These problems have limited the development of astrocytes as sources of potential drug candidates. We have successfully developed a method to induce spontaneous immortalization of astrocytes. Gene expression analysis, karyotyping and activity profiling data show that these spontaneously immortalized type-1 astrocyte cell lines retain the properties of their primary parents. The method is generic, such that cell lines can be prepared from any region of the CNS. To date, a library of 70 cell lines from four regions of the CNS: ventral mesencephalon, striatum, cerebral cortex and hippocampus, has been created. A phenotype-selective neurotrophic factor for dopaminergic neurons has been discovered from one of the cell lines (VMCL1). This mesencephalic astrocyte-derived neurotrophic factor (MANF) is a 20 kD, glycosylated, human secreted protein. Homologs of this protein have been identified in 16 other species including C. elegans. These new developments offer the opportunity of creating a library of astrocyte-derived molecules, and developing the ones with the best therapeutic indices for clinical use.

Tyrosine hydroxylase mRNA and protein are down-regulated by chronic clozapine in both the mesocorticolimbic and the nigrostriatal systems

The dopaminergic system is one of the most important targets for pharmacological treatment of schizophrenia. Despite substantial work on mechanisms of action, it is not clear which dopaminergic pathways mediate the therapeutic effects of antipsychotic drugs. It has been shown that chronic clozapine, an atypical antipsychotic, decreases dopamine levels in the mesocorticolimbic system but not in the nigrostriatal system. Because tyrosine hydroxylase is the rate-limiting enzyme in the biosynthesis of dopamine, we studied the effect of chronic clozapine in both dopaminergic systems. We demonstrated a decrease of tyrosine hydroxylase mRNA not only in the ventral tegmental area but also in the substantia nigra, the cell body areas of the mesocorticolimbic and the nigrostriatal systems, respectively. The reduced tyrosine hydroxylase mRNA level in these areas is accompanied by an ample reduction in the tyrosine hydroxylase protein level in their corresponding axonal terminal fields, the nucleus accumbens and the striatum. There was thus discordance between the clozapine-induced decrease of tyrosine hydroxylase mRNA and protein and the absence of an effect on dopamine levels in the nigrostriatal system. It has been suggested that reduced levels of dopamine in the mesocorticolimbic system are required for the antipsychotic effect of the drug. Therefore, the modulation of tyrosine hydroxylase gene expression by clozapine in the mesocorticolimbic system might be necessary for its antipsychotic effect; this effect might be of relevance when considering new atypical agents.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Differential regulation of tyrosine hydroxylase in the basal ganglia of mice lacking the dopamine transporter

Mice lacking the dopamine transporter (DAT) display biochemical and behavioural dopaminergic hyperactivity despite dramatic alteration in dopamine homeostasis. In order to determine the anatomical and functional integrity of the dopaminergic system, we examined the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis as well as DOPA decarboxylase and vesicular monoamine transporter. TH-positive neurons in the substantia nigra were only slightly decreased (-27.6 +/- 4.5%), which can not account for the dramatic decreases in the levels of TH and dopamine that we previously observed in the striatum. TH mRNA levels were decreased by 25% in the ventral midbrain with no modification in the ratio of TH mRNA levels per cell. However, TH protein levels were decreased by 90% in the striatum and 35% in the ventral midbrain. In the striatum, many dopaminergic projections had no detectable TH, while few projections maintained regular labelling as demonstrated using electron microscopy. DOPA decarboxylase levels were not modified and vesicular transporter levels were decreased by only 28.7% which suggests that the loss of TH labelling in the striatum is not due to loss of TH projections. Interestingly, we also observed sporadic TH-positive cell bodies using immunohistochemistry and in situ hybridization in the striatum of homozygote mice, and to some extent that of wild-type animals, which raises interesting possibilities as to their potential contribution to the dopamine hyperactivity and volume transmission previously reported in these animals. In conjunction with our previous findings, these results highlight the complex regulatory mechanisms controlling TH expression at the level of mRNA, protein, activity and distribution. The paradoxical hyperdopaminergia in the DAT KO mice despite a marked decrease in TH and dopamine levels suggests a parallel to Parkinson's disease implying that blockade of DAT may be beneficial in this condition.

An immortalized, type-1 astrocyte of mesencephalic origin source of a dopaminergic neurotrophic factor

Rat embryonic d 14 (E14) mesencephalic cells, 2.5% of which are glioblasts, were incubated in medium containing 10% of fetal bovine serum for 12 h and subsequently expanded in a serum-free medium using basic fibroblast growth factor (bFGF) as the mitogen. On a single occasion, after more than 15 d in culture, several islets of proliferating, glial-like cells were observed in one dish. The cells, when isolated and passaged, proliferated rapidly in either a serum-free or serum-containing growth medium. Subsequent immunocytochemical analysis showed that they stained positive for GFAP and vimentin, and negative for A2B5, O4, GalC, and MAP2. Serum-free conditioned medium (CM) prepared from these cells caused a fivefold increase in survival and promoted neuritic expansion of E14 mesencephalic dopaminergic neurons in culture. These actions are similar to those exerted by CM derived from primary, mesencephalic type-1 astrocytes. The pattern of expression of the region-selective genes; wnt-1, en-1, sis showed that 70% of the cells were heteroploid, and of these, 50% were tetraploid. No apparent decline in proliferative capacity has been observed after 25 passages. The properties of this cell line, named ventral mesencephalic cell line one (VMCL1), are consistent with those of an immortalized, type-1 astrocyte. The mesencephalic origin of the cell line, and the pattern and potency of the neurotrophic activity exerted by the CM, strongly suggest that the neurotrophic factor(s) identified are novel, and will likely be strong candidates with clinical utility for the treatment of Parkinson's disease.

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.

Haloperidol induces persistent down-regulation of tyrosine hydroxylase immunoreactivity in substantia nigra but not ventral tegmental area in the rat

The dopamine antagonist haloperidol can cause tardive side-effects that may persist after the drug is withdrawn. We studied the time course of changes in dopaminergic neurons of the substantia nigra and ventral tegmental area following withdrawal of haloperidol. Rats received daily intraperitoneal injections of saline or haloperidol for eight weeks and were killed at two, four or 12 weeks after the final injection. Sections of substantia nigra and ventral tegmental area were processed for tyrosine hydroxylase immunohistochemistry. Quantitative morphometric analysis was carried out blinded in order to determine the number, cell body size and topography of tyrosine hydroxylase-positive cells, and the immunoreactive area of the substantia nigra and ventral tegmental area. In haloperidol-treated rats, tyrosine hydroxylase-positive cell counts were normal in ventral tegmental area but were decreased in substantia nigra by 34% at two weeks withdrawal and by 52% at four weeks withdrawal; cell counts were almost fully recovered by 12 weeks withdrawal. Cross-sectional area of tyrosine hydroxylase immunoreactivity within the substantia nigra demonstrated a similar pattern of reduction, with full recovery by 12 weeks withdrawal. Mean cell size, by contrast, was essentially unchanged at two and four weeks withdrawal, but was significantly decreased in sub-regions of substantia nigra at 12 weeks withdrawal. These results indicate that haloperidol can produce selective changes in midbrain dopamine neurons that persist long after discontinuation of the drug. This decrease in tyrosine hydroxylase-immunoreactive cell counts may play a role in the neurobiology of the persistent tardive syndromes associated with the use of neuroleptics.

Plasticity of tyrosine hydroxylase gene expression within BALB/C and C57Black/6 mouse locus coeruleus

The plasticity of tyrosine hydroxylase (TH) phenotype in the locus coeruleus (LC) of two pure inbred strains of mice, Balb/C (C) and C57Black/6 (B6), was investigated at the molecular level by radioactive in situ hybridization. The results demonstrated that in basal conditions, C mouse LC contains less TH-mRNA-expressing cells than B6. After RU 24722-treatment, which induces long lasting TH gene expression in the LC, we previously reported an increase in TH-expressing cell number in C mouse LC only, equalizing TH phenotype between the two strains. Here, we demonstrate that strain specific plasticity of TH phenotype detected in spatially organized cells is associated with the regulation of TH-mRNA expression above a detectable level. These results suggest that interstrain differences and pharmacologically-induced phenotypic plasticity in TH phenotype may occur at the transcriptional level.

Identification of cell type-specific promoter elements associated with the rat tyrosine hydroxylase gene using transgenic founder analysis

Transcriptional regulatory elements capable of directing transgene expression to individual cells are powerful tools for manipulating a given CNS circuit. Delineating these elements via traditional transgenic analysis is both costly and labor intensive. Here we have used the rat tyrosine hydroxylase (TH) promoter as a model to describe and validate the use of founder animals for systematic promoter studies. No significant differences were found when data obtained from founder animals expressing a 6.0 kb TH promoter directing LacZ were compared with animals derived from an analogous transgenic line. Subsequent studies with founder animals expressing beta-galactosidase directed by various lengths of rat TH promoter revealed different patterns of expression. Specifically, a locus coeruleus regulatory domain was localized between 3.4 and 6.0 kb of the rat TH promoter, a hypothalamic regulatory domain between 2.5 and 3.4 kb and a brainstem regulatory domain between 0.8 and 6.0 kb. At least one element of a midbrain specific regulatory domain was within 2.5 kb of the transcriptional start site. Olfactory bulb specific elements however appeared to reside outside of the sequences tested. Specific patterns of ectopic gene expression were also observed suggesting the presence of negative regulatory elements. Thus, TH appears to be regulated in a complex modular fashion by both positive and negative regulatory elements. Taken together, this study demonstrates the feasibility and reliability of founder analysis for promoter studies of genes expressed in complex spatial and temporal patterns.

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.

Effect of the weaver mutation on the expression of dopamine membrane transporter, tyrosine hydroxylase and vesicular monoamine transporter in dopaminergic neurons of the substantia nigra and the ventral tegmental area

The adult homozygous weaver mutant mouse (wv/wv) is characterized by a loss of dopamine (DA) neurons in the nigrostriatal pathway. Quantitative in situ hybridization of three different dopaminergic markers: dopamine membrane transporter (DAT), tyrosine hydroxylase (TH), and vesicular monoamine transporter (VMAT2) was performed on individual dopaminergic cells of the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA) in 2-month-old wv/wv mice, in order to investigate the metabolic state of remaining dopaminergic cell bodies and gain further insight into modifications observed on dopaminergic nerve terminals in the striatum and the nucleus accumbens. Cellular expression of DAT mRNA in remaining dopaminergic cells of both the SNC and the VTA was decreased in the wv/wv mice compared to the wild-type mice (+/+). In contrast, the expression of TH and VMAT2 mRNA remained unchanged in the wv/wv mice. Furthermore, in 7-day-old wv/wv mice, before the onset of cell death in the midbrain. DAT mRNA levels were reduced in dopaminergic neurons in both the SNC and VTA. In these animals, the cellular expression of TH mRNA remained unchanged. These results taken together indicate that DAT expression is one of the first targets in the ventral mesencephalon of the wv mutation, inducing a specific decrease of DA uptake in the striatum and the nucleus accumbens. The alteration of the DA membrane transporter could play a role in the progression of DA neuronal death in the wv mice.

Tetrahydrobiopterin biosynthesis in the rat brain: heterogeneity of GTP cyclohydrolase I mRNA expression in monoamine-containing neurons

GTP cyclohydrolase I is the first and rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin. A quantitative in situ hybridization technique was used to study the expression of GTP cyclohydrolase I mRNA in the rat brain at the cellular level. Coronal sections between the diencephalon and myelencephalon were exposed to a 35S-labelled antisense GTP cyclohydrolase I cRNA probe. Sections serial to these were hybridized with a 35S-labelled antisense cRNA probe complementary to tyrosine hydroxylase mRNA. Tyrosine hydroxylase and GTP cyclohydrolase I mRNAs were found to colocalize within catecholamine neurons located throughout the brain. The overall distribution of neurons expressing GTP cyclohydrolase I mRNA was observed to correspond exactly to the known distribution of the dopamine, norepinephrine/epinephrine and serotonin-containing cell groups. Overall, a 30-fold range of GTP cyclohydrolase I mRNA expression was observed, with the transcript being significantly more abundant in serotonin than in dopamine or norepinephrine/epinephrine neurons. Comparisons across serotonin cell groups indicated that neurons of the median raphe nucleus, caudal linear nucleus raphe (B8) and the dorsal raphe (B6/B7) expressed the highest levels of GTP cyclohydrolase I mRNA. Comparisons across dopamine cell groups indicated that the transcript was more abundant in neurons of the ventral tegmental area (A10) than in neurons of the substantia nigra pars compacta (A9) and that both A9 and A10 dopamine neurons exhibited higher levels of expression than the DA neurons of the hypothalamus (A11-A14). Norepinephrine neurons of the locus coeruleus (A6) and subcoeruleus (A6v) exhibited significantly higher levels of GTP cyclohydrolase I mRNA than did neurons in other norepinephrine (A1 and A2) or epinephrine (C1 and C2) cell groups. GTP cyclohydrolase I mRNA could not be detected unequivocally in neurons known to contain nitric oxide synthase. Heterogeneity in the level of expression of GTP cyclohydrolase I mRNA by monoamine-containing neurons may play an important role in determining steady state levels of tetrahydrobiopterin and, ultimately, the regulation of monoamine biosynthesis.

Alterations in tyrosine hydroxylase expression following partial lesions of the nigrostriatal bundle

Effects of destruction of central dopaminergic neurons on tyrosine hydroxylase gene expression were investigated. Two weeks after the unilateral injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle, a 67% to 99% loss of striatal dopamine (DA) content was observed ipsilateral to the injection site. Measures of tyrosine hydroxylase (TH) protein levels revealed losses in striatal content proportional to DA content. Striatal dihydroxylphenylacetic acid (DOPAC) was somewhat less affected, resulting in 2- to 4-fold increases in the striatal DOPAC/DA ratio, depending on the severity of the lesion. Morphologically, surviving TH-positive substantia nigra pars compacta (SNc) neurons were more rounded than contralateral control cells, and exhibited decreases in cross-sectional area that were proportional to the loss of striatal DA. Measures of cytoplasmic TH mRNA levels in surviving neurons by in situ hybridization autoradiography revealed a significant 23% decrease in TH content per cell that could be correlated to lesion size. The decreases in cross-sectional area and TH mRNA content resulted in a small decrease in TH mRNA density of 6%. The determination of TH transcription rate by an intron-directed in situ hybridization assay found no significant change in TH transcriptional activity as a function of lesion. We conclude that the short-term effect of partial 6-OHDA-induced lesions of the nigrostriatal dopaminergic pathway is the selective loss or shrinkage of large DA neurons of the SNc, and that the associated down-regulation of TH mRNA expression in surviving neurons is due to a post-transcriptional mechanism related either to concomitant cellular hyperactivity or is secondary to the morphological alterations.

Regulation of tyrosine hydroxylase gene expression in mesencephalic dopamine neurons: effect of imipramine treatment

The effects of a chronic imipramine treatment on the mesoamygdaloid pathway of rats were examined. Using semiquantitative immunocytochemical techniques, it was observed that the level of TH mRNA was decreased in the ventral tegmental area (VTA). In contrast, the TH protein was increased in both the VTA and amygdala. The TH activity was decreased in the amygdala when assessed under normal conditions but increased after a preincubation to phosphorylate the enzyme, suggesting a lowering of the protein-specific activity in the terminals. These results show that TH protein turnover in the mesoamygdaloid neurons can be reduced by chronic imipramine treatments, thereby producing an accumulation of inactive TH protein in the neurons while also decreasing TH gene activity in the cell bodies.

Immunohistochemical study of catechol-O-methyltransferase in the human mesostriatal system

The cellular localization of catechol-O-methyltransferase was analysed in the mesostriatal system of human brain post mortem by means of immunohistochemistry. In the human nigral complex, catechol-O-methyltransferase immunostaining was not detected in melanized dopaminergic neurons, except in the ventral tegmental area and substantia nigra pars lateralis, where few neurons displayed intense immunolabelling. In the striatum, catechol-O-methyltransferase immunostaining was found in numerous cell bodies and in the neuropile. Observations at the electron microscope level revealed that catechol-O-methyltransferase immunoreactivity was present in the cell bodies of neurons and their processes, including the dendritic spines. No catechol-O-methyltransferase immunolabelling was observed in striatal nerve terminals in contact with dendritic spines, indicating that dopaminergic nerve terminals do not exhibit catechol-O-methyltransferase immunoreactivity. Catechol-O-methyltransferase-immunoreactive cell bodies and processes of glial cells were also detected in the striatum. The data suggest that catechol-O-methyltransferase is either not expressed or only slightly expressed by the dopaminergic nigrostriatal neurons, whereas it is clearly present in striatal neurons and glial cells. Thus, the catabolic degradation of striatal released dopamine by its O-methylation may involve postsynaptic neurons rather than dopaminergic presynaptic neurons. The presence of catechol-O-methyltransferase in some dopaminergic neurons of the ventral tegmental area and substantia nigra pars lateralis suggests that methylation of dopamine may occur in these neurons, which may consequently be better protected against dopamine auto-oxidation than those of the substantia nigra pars compacta.

DNA regulatory sequences of the rat tyrosine hydroxylase gene direct correct catecholaminergic cell-type specificity of a human growth hormone reporter in the CNS of transgenic mice causing a dwarf phenotype

Transgenic mice bearing 4.8 kilobases (kb) of upstream rat tyrosine hydroxylase (TH) sequences linked to a human growth hormone gene (hGH) exhibited cell-specific expression of hGH in all the appropriate catecholaminergic neurons in the central nervous system (CNS), although with different penetrance in two different mouse lineages. No ectopic expression was observed in any brain or peripheral region in one founder and its progeny. In another founder there was some ectopic expression in addition to appropriate and high levels of tissue-specific expression in all catecholaminergic areas. These results identify regulatory sequences that are sufficient for targeting expression to all catecholaminergic CNS neurons. Also, expression of exogenous hGH in the hypothalamus caused a dwarf phenotype, generating a novel genetic model for GH deficiency of hypothalamic origin.

Differential expression of tyrosine hydroxylase and membrane dopamine transporter genes in subpopulations of dopaminergic neurons of the rat mesencephalon

Dopaminergic (DA) cells of the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA) display differences in their topography, biochemistry and susceptibility to pathological processes. Neuronal dopamine concentration is regulated in large part by tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis, and by the dopamine reuptake system. In the present study, TH protein, TH mRNA and dopamine membrane transporter (DAT) mRNA were quantified at cellular level in 4 arbitrary subregions of the rat ventral mesencephalon (lateral, middle, medial SNC and VTA), using in situ hybridization and immunoautoradiography. The distribution of labelling for TH protein and TH mRNA was almost superimposable and close to that of DAT mRNA in mesencephalic neurons. Lower values of cellular expression in TH protein, TH mRNA and DAT mRNA were observed in the lateral part of the SNC compared to the other subregions. TH and DAT expression were correlated in SNC but not in VTA. Indeed DA cells in this region expressed low levels of DAT mRNA in comparison to the middle and medial SNC. These results suggest a heterogeneity of DA metabolism among populations of mesencephalic cells. The relative lower expression of the DAT gene in VTA neurons suggests a less efficient dopamine reuptake capacity, which may partly account for the relative sparing of the mesolimbic system reported in Parkinson's disease and MPTP-treated animals.

Dopaminergic and peptidergic mRNA levels in juvenile rat brain after prenatal cocaine treatment

The effects of prenatal cocaine treatment on gene expression in dopaminergic pathways of juvenile rats were investigated by in situ hybridization histochemistry. Pregnant rats from gestational day 8 to 20 were administered one of the following treatments: (A) 40 mg/kg cocaine hydrochloride/3 ml/day s.c.; (B) 0.9% saline/3 ml/day s.c. and pair fed to cocaine-exposed dams; (C) 0.9% saline/3 ml/day s.c. and placement on cellulose-diluted diet to match the caloric intake of the cocaine-treated group without explicit food restriction; (D) no injection and lab chow diet. Levels of mRNA for the dopamine transporter, tyrosine hydroxylase, cholecystokinin, D1 and D2 dopamine receptors and enkephalin were quantitated in relevant dopaminergic regions of forebrain and midbrain of offspring that were sacrificed on postnatal day 21. Quantitative analysis revealed no significant changes in mRNA levels in any of the brain regions examined. In the present animal model, cocaine exposure in utero had no significant effect on mRNA levels of the dopamine transporter, D1 or D2 dopamine receptors, enkephalin, tyrosine hydroxylase, or cholecystokinin in juvenile rats.

Haloperidol activates tyrosine hydroxylase gene-expression in the rat substantia nigra, pars reticulata

The cellular distribution of tyrosine hydroxylase (TH) and TH mRNA in the rat substantia nigra (SN) was investigated using immunohistochemistry (IMHC) and non-radioactive in situ hybridization histochemistry (ISH), respectively. Number and density of both TH immunoreactive and TH cRNA labeled cells were increased in the pars reticulata of the substantia nigra (SNr) 8 h after single administration of a dopamine antagonist haloperidol. At the same time number and density of TH positive cells remained unchanged in a ventro-medial, dorso-medial or lateral part of the pars compacta (SNc) and in the pars lateralis (SNl) of the substantia nigra. A D2 receptor-specific agonist, quinpirole, was without effect on either ISH or IMHC in any of these areas, including the SNr. These results reveal the existence of a population of TH-negative neurons in the SNr, in which TH gene-expression can be activated through a dopamine receptor-mediated mechanism, leading to detectable levels of both TH and TH mRNA. Furthermore they suggest that TH gene-expression in these neurons normally is inhibited by dopamine released from somata and dendrites in the SNr.

Acute nicotine injections induce c-fos mostly in non-dopaminergic neurons of the midbrain of the rat

Induction of c-fos gene is an immediate and early response in the cascade of molecular events that ultimately lead to long-term alterations in gene expression in neurons. The psychomotor stimulant and positive reinforcing effects of nicotine have been speculated to be mediated by the dopaminergic neurons of the ventral tegmental area (VTA). To identify the precise subsets of VTA neurons of the rat that mediate the acute nicotinergic effects, the pattern of expression of c-fos gene was mapped using immunocytochemical methods. Acute nicotine injections resulted in prominent Fos-like immunoreactivity (-LI) in the medial terminal nucleus of the accessory optic system, the interpeduncular nucleus, and in the caudal linear subnucleus of VTA. The neurons of other VTA subnuclei, viz., the rostral linear, paranigralis, nucleus parabrachialis pigmentosus, and nucleus interfascicularis or the substantia nigra pars compacta did not contain any cells with Fos-LI. Mecamylamine abolished Fos-LI in most of the VTA neurons. These results suggest that acute nicotine injections induce c-fos expression mostly in non-dopaminergic neurons of the ventral tegmental area of the rat and that nicotine induces c-fos most intensely in the interpeduncular nucleus, the superior colliculus, and several other subnuclei of the accessory optic system.

Immunocytochemical quantification of tyrosine hydroxylase at a cellular level in the mesencephalon of control subjects and patients with Parkinson's and Alzheimer's disease

Parkinson's disease is characterized by massive degeneration of the melanized dopaminergic neurons in the substantia nigra. The functional capacity of the surviving nigral neurons is affected, as indicated by the subnormal levels of tyrosine hydroxylase (TH) mRNA in these neurons and the presence in the parkinsonian mesencephalon of melanized neurons lacking TH immunoreactivity. This is apparently in contraction with the known overactivity of dopamine synthesis and release that occurs in the remaining dopaminergic terminals. To test the ability of the surviving neurons to express TH protein, a semiquantitative immunocytochemical method was developed. The relative amounts of TH were estimated with a computer-assisted image analysis system in the dopaminergic neurons of representative mesencephalic sections of control and parkinsonian brains and for comparison in brains from patients with Alzheimer's disease. In control brains, the mean TH content per neuron differed from one subject to another and between the different dopaminergic cell groups of the mesencephalon in the same subject. Within a given dopaminergic region, the level of TH was variable among neurons. In patients with Parkinson's disease, the ratio of TH protein content per neuron in the substantia nigra by reference to that of the central gray substance was reduced. In patients with Alzheimer's disease, the amount of TH was selectively reduced in the remaining dopaminergic neurons of the ventral tegmental area, a region characterized by a loss in dopaminergic neurons. The decrease in cellular TH content might therefore be related to the presence of the neurodegenerative process in the area considered. In patients with Parkinson's disease, the incapacity of the surviving neurons to express normal TH levels may reduce the efficiency of the hyperactivity mechanisms that develop in the remaining striatal dopaminergic terminals.

Cellular quantification of tyrosine hydroxylase in the rat brain by immunoautoradiography

We developed a rapid and sensitive radioimmunohistochemical method for the quantification of tyrosine hydroxylase (TH) at both the anatomical and cellular level. Coronal tissue sections from fresh-frozen rat brains were incubated in the presence of a TH monoclonal antibody. The reaction was revealed with a 35S-labeled secondary antibody. TH content was quantified in catecholaminergic brain areas by measuring optical density on autoradiographic films or silver grain density on autoradiographic emulsion-coated sections. Regional TH concentrations determined in the locus ceruleus (LC), substantia nigra pars compacta (SNC), and ventral tegmental area (VTA) were significantly increased by 45% after reserpine treatment in the LC but unchanged in the SNC and VTA. Microscopic examination of TH radioimmunolabeling showed a heavy accumulation of silver grains over catecholaminergic cell bodies. In the LC, grain density per cell was heterogeneous and higher in the ventral than in the dorsal part of the structure. After reserpine treatment, TH levels were significantly increased (57%) in the neurons of the LC but not in those of the SNC or VTA. The data support the validity of this radioimmunohistochemical method as a tool for quantifying TH protein at the cellular level and they confirm that TH protein content is differentially regulated in noradrenergic and dopaminergic neurons in response to reserpine.

GABA interneurons in the rat medial frontal cortex: characterization by quantitative in situ hybridization of the glutamic acid decarboxylase (GAD67) mRNA

In situ hybridization of mRNA encoding one isoform of glutamic acid decarboxylase (GAD67) was performed in the rat medial frontal cortex (MFC) to characterize GABA interneurons. Qualitatively, the labelling obtained with a [35S]cDNA probe was in register with neurons and was never associated with glial cells. No obvious differences in the density of labelled cells were observed between the different areas of the MFC examined (infralimbic, prelimbic, anterior cingulate and precentral medial) and between the various cortical layers. Grain counting was performed on single cells in the various layers of the prelimbic and the anterior cingulate area, two main areas of the MFC. According to their grain density, neurons were arbitrarily classified as low, high and very high GAD67 mRNA content. The neurons with the high GAD67 mRNA content corresponded to around 50% of the labelled cells in all the layers and in both areas. In the prelimbic area, the neuronal population with a low GAD67 mRNA content varied from 50% in layers I and II-III to 40% in layers V-VI whereas the very high GAD67 mRNA content neurons corresponded to around 5% of the labelled neurons in all layers. In the anterior cingulate area the neuronal population showing low GAD67 mRNA content varied from 35% in layers I and II-III to 20% in layers V-VI. In this area, neurons with a very high GAD67 mRNA content were more numerous than in the prelimbic area: they varied from 15% in layers I and II-III to 30% in layers V-VI. Parallel to the presence of very highly labelled cells, GAD enzymatic activity measured both in the presence and in the absence of pyridoxal 5'-phosphate was higher in the anterior cingulate area than in the prelimbic area. The heterogeneity of GAD67 mRNA content at the cellular level might underlie the existence of subpopulations of GABA interneurons in the MFC and suggests a higher GABAergic inhibitory control in the anterior cingulate area than in the prelimbic area.

Topography and collateralization of the dopaminergic projections to motor and lateral prefrontal cortex in owl monkeys

The sources and histochemical characteristics of dopaminergic projections to motor and premotor areas of cortex were investigated in owl monkeys in which information from related studies was used to subdivide cortex into motor fields. Brainstem projections to frontal cortex were identified by injections of different fluorescent dyes in the primary motor cortex (M1) and the supplementary motor area (SMA), first identified by microstimulation. Injections were also placed in dorsal premotor cortex and lateral prefrontal cortex. The distribution of retrogradely labeled neurons was related to the location of tyrosine hydroxylase immunolabeled neurons on the same or alternate brain sections to identify the dopamine (DA) neurons. All DA cortically projecting neurons were located in the A8-A10 complex, largely in its dorsal components, including the parabrachial pigmented n. of the ventral tegmental area (VTA), pars gamma of the substantia nigra compacta, and the dorsal part of the retrorubral area (A8). Fewer cells were in the midline groups of VTA (n. linearis rostralis and caudalis) and in the n. paranigralis. DA neurons projecting to M1, SMA, and prefrontal cortex were largely intermixed, and some of these neurons were double or triple labeled by the fluorescent dyes, indicating collateralization to two or three fields; DA cells projecting to M1 were more numerous than to the other locations. The dorsal components of the A8-A10 complex from which arose the DA mesocortical projection were also characterized by the presence of calbindin-immunoreactive neurons and by a dense neurotensin and noradrenergic terminal innervation. Compared to rodents or felines, the DA neurons projecting to the lateral frontal lobe of primates appear to be shifted dorsally and laterally in the nigral complex. The topographic overlap, partial collateralization, and common histochemical characteristics of the DA mesocortical neurons projecting to different fields of the lateral frontal lobe suggest that some degree of functional unity exists within this projection.

Differential expression of tyrosine hydroxylase mRNA in the developing rat mesencephalon

1. With respect to the mesostriatal projection, the mesencephalon is composed of two dopaminergic (DA) cell populations, called dorsal tier and ventral tier. Strong evidence suggests differences in both the spatial and the temporal sequence of the innervation of the striatum between the two groups, with the ventral tier neurons innervating striatal patches prenatally and dorsal tier cells innervating striatal matrix postnatally. 2. Using in situ hybridization, we have examined the expression of the gene coding for tyrosine hydroxylase (TH) in mesencephalic DA neurons with respect to their postnatal development. Two ontogenic patterns of expression were observed: (a) dorsal tier neurons of the medial mesencephalon exhibited a sharp increase in expression beginning after birth, peaking on day 14, then decreasing and, finally, stabilizing; and (b) ventral tier neurons and dorsal tier cells from the lateral and the medial-dorsal mesencephalon showed only a slight increase in TH mRNA, reaching a plateau at P10. 3. The time course of the observed increase in TH gene expression in the first group, generally parallels the innervation of their target cells in the striatal matrix, suggesting that TH gene expression in these cells may be influenced by their postsynaptic cells or by the innervation process.

Induction of nerve growth factor receptor (p75NGFr) mRNA within hypoglossal motoneurons following axonal injury

The hypoglossal nerve is a useful model system for analysis of gene expression in injured motoneurons. In particular, we sought to determine whether the increased appearance of the low affinity nerve growth factor receptor (p75NGFr) observed immunocytochemically following nerve injury can be directly correlated to increased levels of the p75NGFr mRNA. The present study also examined the relative effects of nerve crush versus nerve transection on the expression of p75NGFr mRNA. In sham-operated or intact animals, p75NGFr mRNA is detected rarely and then only at levels slightly higher than background. Following unilateral transection or crush of the rat hypoglossal nerve, the levels of p75NGFr mRNA increase in a time dependent fashion that parallels the appearance of the protein as reported previously. Moreover, this increase in p75NGFr mRNA following transection is dependent on a signal from the injured site, since blockage of axonal transport with vincristine also blocks the increased p75NGFr mRNA levels. When comparing the effect of nerve crush to nerve transection, we observed that the intensity of the response was greater in the crush paradigm versus that observed following transection. The duration of the response following nerve crush was shorter than that observed following transection of the nerve. The increase in p75NGFr mRNA after crush was most robust 4 days postlesion and appeared more robust primarily due to a 90-150% increased number of motoneurons expressing p75NGFr mRNA when compared to nerve transection. These data suggest that nerve crush is more effective than nerve transection in eliciting increased p75NGFr mRNA levels.

Lesions of nigrostriatal pathway reduce expression of tyrosine hydroxylase gene in residual dopaminergic neurons of substantia nigra

The effects of unilateral mechanical transection of the nigrostriatal bundle of rat brain on the level of tyrosine hydroxylase (TH) mRNA and on the activity of TH enzyme in the substantia nigra (SN) were examined. Lesions resulted, by 14 days, in reductions of TH mRNA level to 10% of control and of TH enzyme activity to 39% of control in the ipsilateral SN. The percentage of TH mRNA is lower than either the percentage of surviving dopaminergic neurons or the remaining TH enzyme activity. In situ hybridization analyses also demonstrated the reduction of TH mRNA concentration in surviving dopaminergic neurons in the ipsilateral SN.

A species-specific population of tyrosine hydroxylase-immunoreactive neurons in the medial amygdaloid nucleus of the Syrian hamster

The medial amygdaloid nucleus (Me) is part of a neural pathway that regulates sexual behavior in the male Syrian hamster. To characterize the neurochemical content of neurons in this nucleus, brains from colchicine-treated adult male and female hamsters were immunocytochemically labeled using antibodies that recognize the catecholamine-synthesizing enzymes, tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT), as well as dopamine. A large population of TH-immunoreactive (TH-IR) neurons was observed throughout Me of male and female hamsters, primarily concentrated in the midrostral and caudal portions of the nucleus. The somata were generally small to medium in size and bipolar. Brains from animals that did not receive colchicine contained a limited number of TH-IR neurons in Me as reported previously. The DBH and PNMT antisera did not label any cells in Me of colchicine-treated animals, and the dopamine antiserum labeled neurons in the same location as the caudal group of TH-IR cells. Therefore, these caudal TH-IR neurons are interpreted to be dopaminergic. The rostral group of TH-IR neurons, on the other hand, may be producing only the immediate precursor of dopamine, L-3,4-dihydroxyphenylalanine (L-DOPA). The TH-synthesizing neurons in Me of the Syrian hamster appear to be a species-specific group of cells located outside of the previously described catecholaminergic cell groups.