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

Chronic methamphetamine administration causes differential regulation of transcription factors in the rat midbrain

Methamphetamine (METH) is an addictive and neurotoxic psychostimulant widely abused in the USA and throughout the world. When administered in large doses, METH can cause depletion of striatal dopamine terminals, with preservation of midbrain dopaminergic neurons. Because alterations in the expression of transcription factors that regulate the development of dopaminergic neurons might be involved in protecting these neurons after toxic insults, we tested the possibility that their expression might be affected by toxic doses of METH in the adult brain. Male Sprague-Dawley rats pretreated with saline or increasing doses of METH were challenged with toxic doses of the drug and euthanized two weeks later. Animals that received toxic METH challenges showed decreases in dopamine levels and reductions in tyrosine hydroxylase protein concentration in the striatum. METH pretreatment protected against loss of striatal dopamine and tyrosine hydroxylase. In contrast, METH challenges caused decreases in dopamine transporters in both saline- and METH-pretreated animals. Interestingly, METH challenges elicited increases in dopamine transporter mRNA levels in the midbrain in the presence but not in the absence of METH pretreatment. Moreover, toxic METH doses caused decreases in the expression of the dopamine developmental factors, Shh, Lmx1b, and Nurr1, but not in the levels of Otx2 and Pitx3, in saline-pretreated rats. METH pretreatment followed by METH challenges also decreased Nurr1 but increased Otx2 and Pitx3 expression in the midbrain. These findings suggest that, in adult animals, toxic doses of METH can differentially influence the expression of transcription factors involved in the developmental regulation of dopamine neurons. The combined increases in Otx2 and Pitx3 expression after METH preconditioning might represent, in part, some of the mechanisms that served to protect against METH-induced striatal dopamine depletion observed after METH preconditioning.

Altered expression of neuronal nitric oxide synthase in weaver mutant mice

The weaver mouse represents the only genetic animal model of gradual nigrostriatal dopaminergic neurodegeneration which is proposed as a pathophysiological phenotype of Parkinson's disease. The aim of the present study was to analyze the nitric oxide and dopaminergic systems in selected brain regions of homozygous weaver mice at different postnatal ages corresponding to specific stages of the dopamine loss. Structural deficits were evaluated by quantification of tyrosine hydroxylase and neuronal nitric oxide synthase-immunostaining in the cortex, striatum, accumbens nuclei, subthalamic nuclei, ventral tegmental area, and substantia nigra compacta of 10-day, 1- and 2-month-old wild-type and weaver mutant mice. The results confirmed the progressive loss of dopamine during the postnatal development in the adult weaver mainly affecting the substantia nigra pars compacta, striatum, and subthalamic nucleus and slightly affecting the accumbens nuclei and ventral tegmental area. A general decrease in neuronal nitric oxide synthase-immunostaining with age was revealed in both the weaver and wild-type mice, with the decrease being most pronounced in the weaver. In contrast, there was an increase in the substantia nigra pars compacta nitric oxide synthase-immunostaining and a decrease mainly in the subthalamic and accumbens nuclei of the 2-month-old weaver mutant. The decrease in the expression of nNOS may bear functional significance related to the process of aging. DA neurons from the substantia nigra directly modulate the activity of subthalamic nucleus neurons, and their loss may contribute to the abnormal activity of subthalamic nucleus neurons. Although the functional significance of these changes is not clear, it may represent plastic compensating adjustments resulting from the loss of dopamine innervation, highlighting a possible role of nitric oxide in this process.

Regional brain variations of cytochrome oxidase activity and motor coordination in Girk2Wv (Weaver) mutant mice

The Girk2(Wv) (weaver) phenotype, caused by a mutated inward rectifying potassium channel, is characterized by degeneration of cerebellar granule cell population as well as midbrain dopamine-containing cells of the nigrostriatal pathway. To investigate the regional brain metabolic consequences of this combined pathology, cytochrome oxidase (CO) activity was measured by histochemistry from brain regions of wild-type and homozygous Girk2(Wv) mutant mice and correlated with motor performances. CO activity of Girk2(Wv) mutants was abnormal in cerebellar cortex, dentate nucleus, and brainstem regions (medial and lateral vestibular nuclei, prepositus, superior colliculus, lateral cuneiform nucleus, and reticular nuclei) implicated in the gaze system. CO activity increased in midbrain dopaminergic regions after correcting for tissue density, regions with severe depletion of tyrosine hydroxylase activity. Forebrain regions were relatively spared in term of CO activity, except for subthalamic nucleus, lateral geniculate nucleus, and cortical eye field. Similarly to the Rora(sg) cerebellar mutant, metabolic alterations in cerebellar and vestibular regions were linearly correlated with poor motor coordination, underlining the sensitivity of these tests to cerebellar dysfunction.

The effect of GIRK2(wv) on neurite growth, protein expression, and viability in the CNS-derived neuronal cell line, CATH.A-differentiated

Death occurs in the homozygous mutant mouse weaver among several classes of neuron in cerebellum and ventral midbrain, because these neurons carry a mutation in the G protein-gated inwardly rectifying potassium channel, Girk2. GIRK2 is expressed in all neuronal types killed by wv in cerebellum and midbrain as well as in neurons elsewhere that suffer lesser consequences. GIRK2(wv) affects neurons postnatally, after proliferation, at the time of final differentiation. To assess the impact of GIRK2(wv) on neuronal development and viability, we introduced cDNA encoding wild-type and mutant channels into a variant of a CNS derived catecholamine cell line (Cath.a) known as Cath.a-differentiated. When cultured in serum-free medium, Cath.a-differentiated cells cease proliferation and undergo morphological differentiation, growing long neurites. Cath.a-differentiated cells do not express endogenous Girk channels. Transfection of GIRK2(wv) resulted in the death of Cath.a-differentiated cells, in a cDNA-concentration dependent manner. The highest concentration of Girk2(wv) cDNA caused loss of about half the cells, the next highest concentration one-third, and the least had no effect on viability. However, even the lowest concentration resulted in disruption of neurite outgrowth and reduced the protein products of co-transfected genes. High concentrations of MK801, which prevent Na(+) influx through the mutant channel, prevented death induced by GIRK2(wv). Cell death and disruption of neurite outgrowth were counteracted in GIRK2(wv)-expressing cells by the presence of an unrelated inwardly rectifying potassium channel, Kir2.3. These results are consistent with wv being a gain-of-function mutation, causing disruption of cellular homeostasis by mechanisms such as increased Na(+) influx and chronic depolarization which may in turn result in an excessive metabolic burden on the cell.

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.

Effect of chronic treatment with riluzole on the nigrostriatal dopaminergic system in weaver mutant mice

The effects of a chronic treatment with the anti-glutamate and sodium channel modulating neuroprotective agent riluzole on the degeneration of dopamine-containing neurons were studied in the brain of weaver mutant mice. In these animals, as in Parkinson's disease, dopaminergic neurons of the nigro-striatal pathway undergo spontaneous and progressive cell death. Homozygous weaver mice were orally treated twice a day with either 8 mg/kg riluzole or placebo for 2 months. Quantification of tyrosine-hydroxylase and dopamine-transporter axonal immunostaining in the striatum revealed that riluzole significantly increased the density of striatal dopaminergic nerve terminals. These results suggest that riluzole protects dopaminergic processes in the weaver mice and/or promotes their neuroplasticity.

Survival promotion of mesencephalic dopaminergic neurons by depolarizing concentrations of K+ requires concurrent inactivation of NMDA or AMPA/kainate receptors

The death of dopaminergic neurons that occurs spontaneously in mesencephalic cultures was prevented by depolarizing concentrations of K+ (20-50 mM). However, unlike that observed previously in other neuronal populations of the PNS or CNS, promotion of survival required concurrent blockade of either NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptors by the specific antagonists, MK-801 and GYKI-52466, respectively. Rescued neurons appeared to be healthy and functional because the same treatment also dramatically enhanced their capacity to accumulate dopamine. The effects on survival and uptake were rather specific to dopaminergic neurons, rapidly reversible and still observed when treatment was delayed after plating. Glutamate release increased substantially in the presence of elevated concentrations of K+, and chronic treatment with glutamate induced a loss of dopaminergic neurons that was prevented by MK-801 or GYKI-52466 suggesting that an excitotoxic process interfered with survival when only the depolarizing treatment was applied. The effects of the depolarizing stimulus in the presence of MK-801 were mimicked by BAY K-8644 and abolished by nifedipine, suggesting that neuroprotection resulted from Ca(2+) influx through L-type calcium channels. Measurement of intracellular calcium revealed that MK-801 or GYKI-52466 were required to maintain Ca(2+) levels within a trophic range, thus preventing K+-induced excitotoxic stress and Ca(2+) overload. Altogether, our results suggest that dopaminergic neurons may require a finely tuned interplay between glutamatergic receptors and calcium channels for their development and maturation.

Glial cell line-derived neurotrophic factor (GDNF) gene delivery protects dopaminergic terminals from degeneration

Previously, we observed that injection of an adenoviral (Ad) vector expressing glial cell line-derived neurotrophic factor (GDNF) into the striatum, but not the substantia nigra (SN), prior to a partial 6-OHDA lesion protects dopaminergic (DA) neuronal function and prevents the development of behavioral impairment in the aged rat. This suggests that striatal injection of AdGDNF maintains nigrostriatal function either by protecting DA terminals or by stimulating axonal sprouting to the denervated striatum. To distinguish between these possible mechanisms, the present study examines the effect of GDNF gene delivery on molecular markers of DA terminals and neuronal sprouting in the aged (20 month) rat brain. AdGDNF or a control vector coding for beta-galactosidase (AdLacZ) was injected unilaterally into either the striatum or the SN. One week later, rats received a unilateral intrastriatal injection of 6-OHDA on the side of vector injection. Two weeks postlesion, rats injected with AdGDNF into either the striatum or the SN exhibited a reduction in the area of striatal denervation and increased binding of the DA transporter ligand [(125)I]IPCIT in the lesioned striatum compared to control animals. Furthermore, injections of AdGDNF into the striatum, but not the SN, increased levels of tyrosine hydroxylase mRNA in lesioned DA neurons in the SN and prevented the development of amphetamine-induced rotational asymmetry. In contrast, the level of T1 alpha-tubulin mRNA, a marker of neuronal sprouting, was not increased in lesioned DA neurons in the SN following injection of AdGDNF either into the striatum or into the SN. These results suggest that GDNF gene delivery prior to a partial lesion ameliorates damage caused by 6-OHDA in aged rats by inhibiting the degeneration of DA terminals rather than by inducing sprouting of nigrostriatal axons.

Localization and dynamic regulation of biogenic amine transporters in the mammalian central nervous system

The monoamines, serotonin, dopamine, norepinephrine, epinephrine and histamine, play a critical role in the function of the hypothalamic-pituitary-adrenal axis and in the integration of information in sensory, limbic, and motor systems. The primary mechanism for termination of monoaminergic neurotransmission is through reuptake of released neurotransmitter by Na+, CI-dependent plasma membrane transporters. A second family of transporters packages monoamines into synaptic and secretory vesicles by exchange of protons. Identification of those cells which express these two families of neurotransmitter transporters is an initial step in understanding what adaptive strategies cells expressing monoamine transporters use to establish the appropriate level of transport activity and thus attain the appropriate efficiency of monoamine storage and clearance. The most recent advances in this field have yielded several surprises about their function, cellular and subcellular localization, and regulation, suggesting that these molecules are not static and most likely are the most important determinants of extracellular levels of monoamines. Here, information on the localization of mRNAs for these transporters in rodent and human brain is summarized along with immunohistochemical information at the light and electron microscopic levels. Regulation of transporters at the mRNA level by manipulation in rodents and differences in transporter site densities by tomographic techniques as an index of regulation in human disease and addictive states are also reviewed. These studies have highlighted the presence of monoamine neurotransmitter transporters in neurons but not in glia in situ. The norepinephrine transporter is present in all cells which are both tyrosine hydroxylase (TH)- and dopamine beta-hydroxylase-positive but not in those cells which are TH- and phenyl-N-methyltransferase-positive, suggesting that epinephrine cells may have their own, unique transporter. In most dopaminergic cells, dopamine transporter mRNA completely overlaps with TH mRNA-positive neurons. However, there are areas in which there is a lack of one to one correspondence. The serotonin transporter (5-HTT) mRNA is found in all raphe nuclei and in the hypothalamic dorsomedial nucleus where the 5-HTT mRNA is dramatically reduced following immobilization stress. The vesicular monoamine transporter 2 (VMAT2) is present in all monoaminergic neurons including epinephrine- and histamine-synthesizing cells. Immunohistochemistry demonstrates that the plasma membrane transporters are present along axons, soma, and dendrites. Subcellular localization of DAT by electron microscopy suggests that these transporters are not at the synaptic density but are confined to perisynaptic areas, implying that dopamine diffuses away from the synapse and that contribution of diffusion to dopamine signalling may vary between brain regions. Interestingly, the presence of VMAT2 in vesicles underlying dendrites, axons, and soma suggests that monoamines may be released at these cellular domains. An understanding of the regulation of transporter function may have important therapeutic consequences for neuroendocrine function in stress and psychiatric disorders.