Quantitative longitudinal imaging of activated microglia as a marker of inflammation in the pilocarpine rat model of epilepsy using [11C]-( R)-PK11195 PET and MRI.

Inflammation may play a role in the development of epilepsy after brain insults. [11C]-( R)-PK11195 binds to TSPO, expressed by activated microglia. We quantified [11C]-( R)-PK11195 binding during epileptogenesis after pilocarpine-induced status epilepticus (SE), a model of temporal lobe epilepsy. Nine male rats were studied thrice (D0-1, D0 + 6, D0 + 35, D0 = SE induction). In the same session, 7T T2-weighted images and DTI for mean diffusivity (MD) and fractional anisotropy (FA) maps were acquired, followed by dynamic PET/CT. On D0 + 35, femoral arterial blood was sampled for rat-specific metabolite-corrected arterial plasma input functions (AIFs). In multiple MR-derived ROIs, we assessed four kinetic models (two with AIFs; two using a reference region), standard uptake values (SUVs), and a model with a mean AIF. All models showed large (up to two-fold) and significant TSPO binding increases in regions expected to be affected, and comparatively little change in the brainstem, at D0 + 6. Some individuals showed increases at D0 + 35. AIF models yielded more consistent increases at D0 + 6. FA values were decreased at D0 + 6 and had recovered by D0 + 35. MD was increased at D0 + 6 and more so at D0 + 35. [11C]-( R)-PK11195 PET binding and MR biomarker changes could be detected with only nine rats, highlighting the potential of longitudinal imaging studies.

BDNF/ TrkB interaction regulates migration of SVZ precursor cells via PI3-K and MAP-K signalling pathways.

Neuroblasts born in the subventricular zone (SVZ) migrate along the rostral migratory stream, reaching the olfactory bulb (OB) where they differentiate into local interneurons. Several extracellular factors have been suggested to control specific steps of this process. The brain-derived neurotrophic factor (BDNF) has been demonstrated to promote morphological differentiation and survival of OB interneurons. Here we show that BDNF and its receptor TrkB are expressed in vivo throughout the migratory pathway, implying that BDNF might also mediate migratory signals. By using in vitro models we demonstrate that BDNF promotes migration of SVZ neuroblasts, acting both as inducer and attractant through TrkB activation. We show that BDNF induces cAMP response element-binding protein (CREB) activation in migrating neuroblasts via phosphatidylinositol 3-kinase (PI3-K) and mitogen-activated protein kinase (MAP-K) signalling. Pharmacological blockade of these pathways on SVZ explants significantly reduces CREB activation and impairs neuronal migration. This study identifies a function of BDNF in the SVZ system, which involves multiple protein kinase pathways leading to neuroblast migration.

Regulation of expression and enzymatic activities of tyrosine and tryptophan hydroxylases in rat brain after acute electroconvulsive shock.

Acute electroconvulsive shock (ECS) causes a significant increase of protein synthesis in depressive patients and such an increase raises the possibility that the regulation of specific proteins and enzymatic activities in the brain might be one of the mechanisms required for the induction of long-term adaptive neurochemical changes after electroconvulsive therapy. In current studies, we investigated and compared simultaneously the short- and long-term effects of an acute ECS on the expression and enzymatic activities of both tyrosine and tryptophan hydroxylases (TH and TpOH, respectively) in different rat brain areas. Our results demonstrated that an acute ECS produced: (1) a long-lasting decrease in TH and TpOH protein levels in locus ceruleus (LC), ventral tegmental area (VTA) and in TpOH protein level in the raphe centralis (RC), maximal at 72 h, with concomitant changes in mRNA levels and enzymatic activities in the LC only; (2) large increase of TpOH protein levels in the frontal cortex (Cxf) (+145%) and increase of TH protein levels in the hippocampus (Hip) (+207%), maximal at 72 h and 7 days which was not accompanied by corresponding increase of in vivo enzymatic activities. Furthermore, a second ECS increased in vivo TpOH activity in the Cxf (+19%) while decreasing K(m) value (-50%) for tetrahydrobiopterin cofactor. A stability of the observed findings on TpOH activity in the Cxf after repeated ECS might be one of the mechanisms for the antidepressant effects of electroconvulsive therapy.

Bax, Bcl-2, and cyclin expression and apoptosis in rat substantia nigra during development.

Naturally occurring cell death via apoptosis has been reported in the substantia nigra of rats during development, culminating during the perinatal period. Cellular pathways leading to apoptotic death of developing nigral dopamine neurons remain unknown, although the apoptotic mediator, caspase 3, has been shown to be activated during this process. Our previous results demonstrated the inability of antioxidants to rescue the nigral dopamine neurons that undergo apoptosis during development. In the present study, we investigated using immunohistochemistry the expression of cyclins D1, D3, and E in the substantia nigra during pre- and postnatal development, since their re-expression in postmitotic neurons has been proposed to contribute to developmental apoptosis. We also investigated by Western blot analysis of nigral tissue isolated during the first postnatal week the expression of the anti- and pro-apoptotic proteins, Bcl-2 and Bax, respectively, since altered Bcl-2 expression during developmental apoptosis has been described. During apoptotic death of nigral dopamine neurons in development, we detected a significant increase in the Bax:Bcl-2 ratio, which is consistent with enhanced apoptosis. There were no changes in the expression of the cyclins during the same apoptotic period. These novel findings suggest that nigral dopamine neurons undergo developmental apoptotic death through a Bax:Bcl-2-sensitive pathway that does not involve cyclin mediation.

Lipid peroxidation-mediated oxidative stress and dopamine neuronal apoptosis in the substantia nigra during development.

The cellular pathways underlying naturally occurring neuronal apoptosis in the rat substantia nigra (SN) during the perinatal period remain largely unknown. Determining the mediators of this process in development may shed light on causes of premature neuronal death in adult neurodegenerative disorders, including the loss of dopamine neurons in Parkinson's disease. In the present study, we investigated whether lipid peroxidation-mediated oxidative stress mediates developmental death of nigral neurons by (1) establishing the profile of lipid peroxidation and other oxidative stress markers throughout the postnatal period both in the SN and striatum, and (2) examining whether the inhibitor of lipid peroxidation, alpha-tocopherol, protects these neurons from death. In addition to monitoring, the level of lipid peroxidation throughout development, we also measured the activities of three antioxidant enzymes, namely superoxide dismutase (SOD), catalase and glutathione peroxidase (GPx). We have shown that lipid peroxidation and SOD activity progressively increased from postnatal day (PND) 3 to PND 42 in both SN and striatum. During this period, GPx activity remained stable, while catalase activity transiently increased at PND 8 only in the SN. Furthermore, alpha-tocopherol treatment from embryonic day 18 to PND 2 did not reduce the number of apoptotic neurons at PND 3. These results do not support the hypothesis that lipid peroxidation-mediated oxidative stress is the major mediator of nigral dopamine neuronal apoptosis during the perinatal period.

Tetrahydrobiopterin enhances apoptotic PC12 cell death following withdrawal of trophic support.

(6R)-Tetrahydro-l-biopterin (BH(4)) is the rate-limiting cofactor in the production of catecholamine and indoleamine neurotransmitters and is also essential for the synthesis of nitric oxide by nitric-oxide synthase. We have previously reported that BH(4) administration induces PC12 cell proliferation and that nerve growth factor- or epidermal growth factor-induced PC12 cell proliferation requires the elevation of intracellular BH(4) levels. We show here that BH(4) accelerates apoptosis in undifferentiated PC12 cells deprived of serum and in differentiated neuron-like PC12 cells after nerve growth factor withdrawal. Increased production of catecholamines or nitric oxide cannot account for the enhancement of apoptosis by BH(4). Furthermore, increased calcium influx by exogenous BH(4) administration is not involved in the BH(4) proapoptotic effect. Our data also argue against the possibility that increased oxidative stress, due to BH(4) autoxidation, is responsible for the observed BH(4) effects. Instead, they are consistent with the hypothesis that BH(4) induces apoptosis by increasing cell cycle progression. Elevation of intracellular BH(4) during serum withdrawal increased c-Myc (and especially Myc S) expression earlier than serum withdrawal alone. Furthermore, N-acetylcysteine and the cyclin-dependent kinase inhibitor olomoucine ameliorated the BH(4) proapoptotic effect. These data suggest that BH(4) affects c-Myc expression and cell cycle-dependent events, possibly accounting for its effects on promoting cell cycle progression or apoptosis.

Singular subsets of locus coeruleus neurons may recover tyrosine hydroxylase phenotype transiently expressed during development.

The number of tyrosine hydroxylase (TH)-expressing neurons appears to be precisely determined in basal conditions within the noradrenergic pontine nucleus locus coeruleus (LC). However, additional neurons exhibiting TH phenotype have been observed in the adult rat LC following a single administration of RU 24722, a potent inducer of TH expression specific to the LC. The neurons acquiring TH phenotype following treatment had a topographical localization similar to that of the neurons, which transiently expressed TH during postnatal development and lost TH phenotype during the third postnatal week. The idea that the fluctuation of TH phenotype in singular subsets of LC neurons during development may be selectively restored in adults is of particular interest. The present study attempted to determine whether the cells in which TH expression was repressed during the third postnatal week could correspond to those which exhibited TH phenotype in response to RU 24722 treatment in adults. We first verified that no massive cell death occurred in the LC during the period ranging from days 13 to 30. Then, we observed that both cell populations exhibited the same altered steady-state concentration of TH-mRNA as compared to cells that permanently expressed TH. Finally, we demonstrated the presence of TH-negative neurons expressing the homeodomain transcription factor Phox2a, specific for the determination of noradrenergic phenotype, providing further evidence that "resting-noradrenergic" neurons exist in the adult rat LC under basal conditions. These neurons provide interesting prospective for gain of noradrenergic function when classical noradrenergic LC neurons are impaired.

Evidence of deprenyl-insensitive apoptosis of nigral dopamine neurons during development.

Apoptosis of dopamine neurons occurs naturally in the substantia nigra during development, culminating in approximately 30% loss of these cells during the perinatal period. Deprenyl, independent of its monoamine oxidase (MAO)-B inhibitory properties, can prevent dopamine neuronal apoptosis in models of neurodegeneration. Our current study demonstrate that apoptotic death of dopamine neurons during development is insensitive to daily treatment of pregnant mothers and then newborns with deprenyl (0.1, 1, or 10 mg/kg). This result is not due to poor crossing of the placental and blood-brain barriers, since deprenyl caused a dose-dependent inhibition of brain MAO-B activity in pups at birth. Determining the pathway(s) leading to deprenyl-insensitive apoptosis of nigral dopamine neurons in development may shed light on mechanisms underlying the premature death of dopamine neurons in neurodegenerative disorders.

In situ examination of tyrosine hydroxylase activity in the rat locus coeruleus using (3',5')-[(3)H(2)]-alpha-fluoromethyl-tyrosine as substrate of the enzyme.

Tyrosine hydroxylase (TH) activity can be modified by changes in the specific activity of the enzyme (SA(TH)) or in the levels of active enzyme. We developed a methodology making it possible to measure with excellent anatomical resolution TH enzymatic activity and TH protein quantity by quantitative autoradiography and immunoautoradiography, respectively, from adjacent sections taken at serial intervals along the longitudinal extent of a same brain. SA(TH) was estimated by the slope of linear regressions established between TH activity and TH quantity measured at each anatomical plane. To evaluate TH activity, we used (3',5')-[(3)H(2)]-(D, L)-alpha-fluoromethyl-tyrosine [(3)H(2)]-MFMT, which is transformed by TH to [(3)H]-MFM-dopa, a potent and irreversible substrate for aromatic amino acid decarboxylase. We found that the SA(TH) in the cell body area of the LC (PKA) was 48% lower than that evaluated in the surrounding pericoerulean neuropil (PCN). In the PCN, 22% only of TH level exhibited a level of enzymatic activity above threshold. We also examined how SA(TH) was distributed in the LC 15 min and 3 days after RU 24722 treatment, a potent phasic and tonic activator of TH enzyme in noradrenergic neurons. Two distinct mechanisms have been observed: the short-term effect was due to an increase in the SA(TH) in the PKA only, while the long-term effect was mainly caused by an increase in the number of active TH proteins in the PCN. These results suggest that the fine regulation of TH activity which occurs in the different compartments of LC neurons may be critical in the functions involving the LC.

Vasoactive intestinal peptide induces both tyrosine hydroxylase activity and tetrahydrobiopterin biosynthesis in PC12 cells.

Vasoactive intestinal peptide plays an important role in the trans-synaptic activation of tyrosine hydroxylase in sympathoadrenal tissues in response to physiological stress. Since tyrosine hydroxylase is thought to be subsaturated with its cofactor, tetrahydrobiopterin, we tested the hypothesis that up-regulation of tyrosine hydroxylase gene expression following vasoactive intestinal peptide treatment is accompanied by a concomitant elevation of intracellular tetrahydrobiopterin biosynthesis. We also investigated the second messenger systems involved in vasoactive intestinal peptide's effects on tetrahydrobiopterin metabolism. Our results demonstrate that treatment of PC12 cells for 24 h with vasoactive intestinal peptide induced intracellular tetrahydrobiopterin levels 3.5-fold. This increase was due to increased expression of the gene encoding GTP cyclohydrolase, the initial and rate-limiting enzyme in tetrahydrobiopterin biosynthesis, which was blocked by the transcriptional inhibitor, actinomycin D. Activation of tyrosine hydroxylase and GTP cyclohydrolase by vasoactive intestinal peptide was mediated by cyclic-AMP. Furthermore, stimulation of cyclic-AMP-mediated responses or protein kinase C activity induced the maximal in vitro activities of both tyrosine hydroxylase and GTP cyclohydrolase; the responses were additive when both treatments were combined. Induction of sphingolipid metabolism had no effect on the activation of tyrosine hydroxylase, while it induced GTP cyclohydrolase in a protein kinase C-independent manner. Our results support the hypothesis that intracellular tetrahydrobiopterin levels are tightly linked to tyrosine hydroxylation and that tetrahydrobiopterin bioavailability modulates catecholamine synthesis.

Reduced lymphoblast neopterin detects GTP cyclohydrolase dysfunction in dopa-responsive dystonia.

OBJECTIVE: To demonstrate that measurement of endogenous neopterin levels in unstimulated lymphoblasts identifies inherited GTP cyclohydrolase 1 (GCH1) dysfunction and can be a diagnostic test for dopa-responsive dystonia (DRD). BACKGROUND: DRD results from decreased dopamine biosynthesis due to dysfunctional GCH1. GCH1 is the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin (BH4), an essential cofactor for catecholamine synthesis. Mutations in the GCH1 coding region are identified in 60 to 70% of DRD cases; in others, the cause of GCH1 dysfunction is unknown. METHODS: Using HPLC, we measured endogenous neopterin, the main byproduct of the GCH1 reaction, in lymphoblasts under basal conditions and following GCH1 stimulation conditions. RESULTS: In a four-generation family, all identified carriers of dysfunctional GCH1 had basal neopterin levels that were below those of controls. The spouse of one carrier had a mutation in exon 6 of GCH1. Although this man's GCH1 function appeared unaffected by this, his daughter, who was a compound heterozygote with her mother's dysfunctional GCH1 and this mutation, had a phenotype that was more severe than that of typical DRD. Cytokine or phytohemagglutinin (PHA) did not induce GCH1 activity in any carrier of dysfunctional GCH1; controls who did not respond to PHA had increased neopterin levels following cytokine induction. CONCLUSIONS: Endogenous neopterin measurement in unstimulated lymphoblasts is an accurate tool to identify dysfunctional GCH1 and a potential specific diagnostic marker for dysfunctional GCH1 in DRD and other neurologic disorders. Not all mutations in GCH1 affect GCH1 enzyme activity. PHA induction alone, previously used by others, may result in incorrect identification of GCH1 dysfunction in DRD.

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.

Tetrahydrobiopterin as a mediator of PC12 cell proliferation induced by EGF and NGF.

Epidermal growth factor and nerve growth factor increased the proliferation of rat phaeochromocytoma PC12 cells through obligatory elevation of intracellular (6R)-tetrahydrobiopterin (BH4). Epidermal growth factor and nerve growth factor increased intracellular BH4 by inducing GTP-cyclohydrolase, the rate-limiting enzyme in BH4 biosynthesis. Specific inhibitors of BH4 biosynthesis prevented growth factor-induced increases in BH4 levels and proliferation. The induction of GTP cyclohydrolase, BH4 and cellular proliferation by nerve growth factor was mediated by cAMP. Elevation of BH4 biosynthesis occurred downstream from cAMP in the cascade used by nerve growth factor to increase proliferation. Thus, intracellular BH4 is an essential mediator of the proliferative effects of epidermal growth factor and nerve growth factor in PC12 cells.

Controlled targeting of tyrosine hydroxylase protein toward processes of locus coeruleus neurons during postnatal development.

Dendrites of locus coeruleus (LC) neurons laying within the pericoerulean neuropil (PCA) organize the major site where tyrosine hydroxylase (TH) is present throughout postnatal development. Those dendrites constitute the neuronal compartment in which TH levels increase beyond postnatal day (P) 21 or after RU24722-induced TH expression. Distal LC dendrites are present in the PCA by at least P20 but are devoid of TH and can rapidly accumulate TH protein when gene induction is triggered. Contrasting with the increase in TH levels within LC perikarya and dendrites, TH-mRNA concentration remains constant in LC perikarya from P4 to P42. Thus, supposing TH synthesis and degradation are also constant, any change in TH levels targeted toward axons might be balanced by a shift in the TH deposition within LC dendrites. This mechanism may be crucial in functions that the different processes of LC neurons have at critical steps of postnatal ontogeny.

Tyrosine hydroxylase expression within Balb/C and C57black/6 mouse locus coeruleus. I. Topological organization and phenotypic plasticity of the enzyme-containing cell population.

Tyrosine hydroxylase phenotype expression was investigated in the catecholaminergic population of the locus coeruleus neurons of two pure inbred mouse strains, Balb/C and C57Black/6. Therefore, we have characterized the precise organization of tyrosine hydroxylase-expressing perikarya population, in control animals and following RU24722 treatment, which is known to induce tyrosine hydroxylase expression. Serial coronal sections were selected along the caudo-rostral extent of the structure and were processed for tyrosine hydroxylase immunocytochemistry. Three days after the treatment, an increase in the number of cells which expressed tyrosine hydroxylase was observed all along the locus coeruleus in the Balb/C strain only. This increase equalized the catecholaminergic neuron populations of the two strains. In the caudal subdivision of the structure, these newly detected perikarya were intermingled with the perikarya which expressed tyrosine hydroxylase in control conditions. In the rostral half, the additional immunoreactive perikarya enlarged the mean coerulean space, defined as the area delimited by the tyrosine hydroxylase-containing perikarya. These results demonstrate a plasticity of the tyrosine hydroxylase phenotype expression, topologically organized and specific to the Balb/C strain.

Tyrosine hydroxylase expression within Balb/C and C57black/6 mouse locus coeruleus. II. Quantitative study of the enzyme level.

The tyrosine hydroxylase phenotype expression was further investigated in the perikarya and pericoerulean areas of the locus coeruleus of two pure inbred mouse strains, Balb/C and C57Black/6, which the topological organization and phenotypic plasticity of the enzyme-containing cell population were previously studied. The tyrosine hydroxylase level and the mean protein quantity provided by each cell were significantly higher within the spaces delimited by the enzyme containing perikarya in the C57Black/6 strain as compared to the Balb/C strain. Three days after RU24722 administration, tyrosine hydroxylase tissue concentration and quantity were significantly increased in both strains. Two strain-dependant mechanisms of this pharmacologically induced protein modulation were demonstrated: the mean tyrosine hydroxylase quantity provided by each cell was increased in the C57Black/6 strain whereas the increase was obviously explained by the subset of additional tyrosine hydroxylase expressing cells previously reported in Balb/C strain. A comparable volume of pericoerulean immunolabeled neuropile, which contains a similar tyrosine hydroxylase level, was measured between the two strains. Within this tissue compartment, an undissociated RU24722 responsiveness was observed between the two strains: a significant increase in the protein level was measured principally resulting from a significant increase in the volume. These results revealed a strain-dependent difference in the response to the RU24722 treatment which may result from a genetic separation of two kinds of tyrosine hydroxylase phenotypic regulations within the perikarya area of the locus coeruleus; whereas the surrounding neuropile seemed to have a different mechanism of the phenotypic protein expression and modulation.

Postnatal development of the tyrosine hydroxylase-containing cell population within the rat locus coeruleus: topological organization andphenotypic plasticity.

The cellular phenotypic characteristics of tyrosine hydroxylase (TH) expression have been studied within the rat locus coeruleus (LC) during postnatal development at six different stages: postnatal day 4 (PND4), PND10, PND14, PND21, PND30, and PND42. Coronal brain sections were selected at intervals of 80 microns along the caudorostral extent of the LC and processed for TH immunohistochemistry. At each anatomical level we (1) reconstructed the mean space of the LC delineated by the TH positive cell bodies, (2) enumerated the mean number of these cell bodies, and (3) determined the mean volume circumscribed by these cell bodies and their density. The topological study revealed a steady remodeling of the structure until the third week, with a progressive reducing of a ventral cellular group in the anterior LC, which was no longer observable at PND21, concomitant to the stretch of the structure toward its caudal limit. We have noted invariant and variant cellular phenotypic characteristics of TH expression. At any stage, the LC could be separated into a posterior and an anterior subregion and its total volume remained quite stable during the studied period. At PND14 and PND21, we observed a transient 33% increase in the total number of TH positive perikarya as compared to PND42. Conjoint analysis of the topological reconstruction and the density of TH positive cells suggested there were three distinct and precisely localized subsets of "quiescent" neurons. TH gene expression in these cells would have lowered between PND14 and PND21 inside two subsets and between PND21 and PND30 inside the last one. So topologically defined populations of cells could be involved in specific functions. If they have not definitively lost their TH expression capacity, they could contribute to increasing TH levels in LC occurring in response to physiological perturbations or pharmacological treatments.

Quantitative study of tyrosine hydroxylase protein levels within the somatic area of the rat locus coeruleus during postnatal development.

To date only global dosages of tyrosine hydroxylase (TH) protein have been realized in the locus coeruleus (LC) without discriminating the enzyme contained in the cell body area from the one in the surrounding neuropil. The preceding immunohistochemical study (Bezin et al., 1994) revealed a dramatic plasticity of the cellular expression of TH in the LC during the postnatal development of the rat. It was therefore necessary to develop a quantitative biochemical approach, strengthened by a great anatomical resolution, to follow the developmental evolution of TH levels exactly in the space containing the coerulean TH-immunoreactive perikarya. In the present work two biochemical parameters necessary for precisely defining the phenotypic characterization of TH expression within the rat LC have been established during the postnatal development at six different stages: postnatal day 4 (PND4), PND10, PND14, PND21, PND30, and PND42. TH tissue concentration and content were precisely determined along the caudorostral extent of the LC within the previously (Bezin et al., 1994) defined spaces delimited by the TH-containing perikarya. TH tissue concentration remained quite stable during the postnatal development. TH quantity exhibited few age-related variations with a transient peak at PND10 and followed the same evolution as the volume containing the TH-expressing perikarya. The mean cell contribution to the total quantity of TH measured in the whole LC showed important age-related fluctuations with a dramatic peak at PND10 followed by a drastic decrease until PND21.(ABSTRACT TRUNCATED AT 250 WORDS)

RU24722 induces spatially organized phenotypic plasticity in the locus coeruleus.

The plasticity of tyrosine hydroxylase (TH) expression in rat locus coeruleus (LC) was evaluated after RU24722 TH induction using, as a new parameter of characterization, the quantitative topology of LC defined by TH-positive cells. This new phenotype was spatially organized into cell subpopulations in the medial LC, dorsal and ventro-lateral to the initial perikaryal space. Reserpine and parachlorophenylalanine, which elicited a similar increase in the TH content, failed to induce a significant change in the number of TH-expressing cells. Activation of TH expression is not sufficient to reveal the existence of such a plasticity and some original but still unknown mechanism(s) of control of TH expression is affected by RU24722.