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

BNN-20, a synthetic microneurotrophin, strongly protects dopaminergic neurons in the "weaver" mouse, a genetic model of dopamine-denervation, acting through the TrkB neurotrophin receptor

Neurotrophic factors are among the most promising treatments aiming at slowing or stopping and even reversing Parkinson's disease (PD). However, in most cases, they cannot readily cross the human blood-brain-barrier (BBB). Herein, we propose as a therapeutic for PD the small molecule 17-beta-spiro-[5-androsten-17,2'-oxiran]-3beta-ol (BNN-20), a synthetic analogue of DHEA, which crosses the BBB and is deprived of endocrine side-effects. Using the "weaver" mouse, a genetic model of PD, which exhibits progressive dopaminergic neurodegeneration in the Substantia Nigra (SN), we have shown that long-term administration (P1-P21) of BNN-20 almost fully protected the dopaminergic neurons and their terminals, via i) a strong anti-apoptotic effect, probably mediated through the Tropomyosin receptor kinase B (TrkB) neurotrophin receptor's PI3K-Akt-NF-κB signaling pathway, ii) by exerting an efficient antioxidant effect, iii) by inducing significant anti-inflammatory activity and iv) by restoring Brain-Derived Neurotrophic Factor (BDNF) levels. By intercrossing "weaver" with NGL mice (dual GFP/luciferase-NF-κΒ reporter mice, NF-κΒ.GFP.Luc), we obtained Weaver/NGL mice that express the NF-κB reporter in all somatic cells. Acute BNN-20 administration to Weaver/NGL mice induced a strong NF-κB-dependent transcriptional response in the brain as detected by bioluminescence imaging, which was abolished by co-administration of the TrkB inhibitor ANA-12. This indicates that BNN-20 exerts its beneficial action (at least in part) through the TrkB-PI3K-Akt-NF-κB signaling pathway. These results could be of clinical relevance, as they suggest BNN-20 as an important neuroprotective agent acting through the TrkB neurotrophin receptor pathway, mimicking the action of the endogenous neurotrophin BDNF. Thus BNN-20 could be proposed for treatment of PD.

Molecular determinants of selective dopaminergic vulnerability in Parkinson's disease: an update

Numerous disorders of the central nervous system (CNS) are attributed to the selective death of distinct neuronal cell populations. Interestingly, in many of these conditions, a specific subset of neurons is extremely prone to degeneration while other, very similar neurons are less affected or even spared for many years. In Parkinson’s disease (PD), the motor manifestations are primarily linked to the selective, progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). In contrast, the very similar DA neurons in the ventral tegmental area (VTA) demonstrate a much lower degree of degeneration. Elucidating the molecular mechanisms underlying the phenomenon of differential DA vulnerability in PD has proven extremely challenging. Moreover, an increasing number of studies demonstrate that considerable molecular and electrophysiologic heterogeneity exists among the DA neurons within the SNpc as well as those within the VTA, adding yet another layer of complexity to the selective DA vulnerability observed in PD. The discovery of key pathways that regulate this differential susceptibility of DA neurons to degeneration holds great potential for the discovery of novel drug targets and the development of promising neuroprotective treatment strategies. This review provides an update on the molecular basis of the differential vulnerability of midbrain DA neurons in PD and highlights the most recent developments in this field.

Expression and phosphorylation of glutamate receptor subunits and CaMKII in a mouse model of Parkinsonism

Dopaminergic deficiency of the weaver mutant mouse is a valuable tool to further our understanding of Parkinson׳s disease (PD) pathogenesis since dopaminergic neurons of the nigrostriatal pathway undergo spontaneous and progressive cell death. In the present study we investigated the changes in protein expression and phosphorylation of glutamate receptor subunits and αCaMKII in weaver striatum at the end of the third and sixth postnatal month. Using immunoblotting, we found increased immunoreactivity levels of both GluN2A and GluN2B subunits of NMDA receptors and GluA1 subunit of AMPA receptors approximately from 75% to 110% in the 3-month-old weaver striatum compared to control. In the 6-month-old weaver striatum, no changes were detected in GluN2A and GluA1 immunoreactivity levels, whereas GluN2B showed a 21% statistically significant increase. Our results also indicated increased phospho-S1303 GluN2B in both 3 and 6 month-olds and increased phospho-S831 and -845 GluA1 in 3 month-old weaver striatum. However, these increases did not exceed the increases observed for total GluN2B and GluA1. Furthermore, our results showed increased immunoreactivity levels for phospho-T286 αCaMKII by approximately 180% in the 6 month-old weaver striatum, while total CaMKII immunoreactivity levels were not altered at either 3- or 6-month-old weaver. Our results suggest that distinct degrees of DA neuron degeneration differentially affect expression and phosphorylation of striatal glutamate receptors and αCaMKII. Findings on this genetic parkinsonian model suggest that striatal glutamatergic signaling may play an important role in synaptic plasticity and motor behavior that follow progressive and chronic dopamine depletion in PD with biochemical consequences beyond those seen in acute toxic models.

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.

Blockade of adenosine A2A receptors downregulates DARPP-32 but increases ERK1/2 activity in striatum of dopamine deficient "weaver" mouse

In the present study we investigated the signal transduction cascade modulated by adenosine A(2A) receptors under chronic dopamine deficiency in the "weaver" mouse. We determined the phosphorylation state of cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) at Thr34 and of Extracellular Signal-regulated Protein Kinases 1/2 (ERK1/2), under basal conditions and after in vivo stimulation of A(2A) receptors by administration of the agonist CGS21680. Our results revealed that the endogenous levels of phospho-DARPPP-32 and phospho-ERK1/2 are elevated in "weaver" striatum probably as an adaptation phenomenon to gradual dopaminergic neurodegeneration appearing in this animal model, characterized as phenocopy of Parkinson's disease. Stimulation of A(2A) receptors by CGS21680 further increases phospho-DARPP-32 but downregulates significantly the elevated phospho-ERK1/2 levels bringing them close to those observed in wild type animals. Consistently, blockade of A(2A) receptors by MSX-3 (A(2A) receptor antagonist) downregulates phospho-DARPP-32 but significantly increases even more the phosphorylation/activation of ERK1/2. These results indicate that under chronic dopamine deficiency (a) the A(2A)/cAMP/PKA/DARPP-32 cascade is overactive due to the elevated endogenous phospho-DARPP-32 levels and (b) the A(2A) receptor modulatory effect on ERK1/2 signaling is dysregulated exerting opposing action compared to that observed in normal animals (Quiroz et al., 2006), i.e. in "weaver" animals A(2A) receptor blockade increases the activity of ERK1/2 cascade. This could be of clinical relevance since A(2A) antagonists are already used in clinical trials for ameliorating Parkinson's disease (PD) symptoms.

MPTP administration in mice changes the ratio of splice isoforms of fosB and rgs9

Most cases of Parkinson's disease (PD) are sporadic, suggesting an environmental influence on individuals affected by this neurodegenerative disorder. Environmental stresses often lead to changes in the regulation of splicing of pre-mRNA transcripts and this may lead to the pathogenesis of the disease. A 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)/probenecid mouse model was used to examine the changes in the splicing of the fosB and rgs9 transcripts. The ratio of DeltafosB/fosB transcript was decreased in the substantia nigra and unchanged in the striatum after acute MPTP treatment. The DeltafosB/fosB transcript ratio decreased initially and then increased in the striatum of chronically MPTP-treated animals due to different degrees of reduction for the splice variants over time, whereas the ratio was unchanged in the substantia nigra. The ratio of rgs9-2/rgs9-1 transcript decreased in the substantia nigra of mice after acute MPTP treatment and increased temporarily in the striatum after chronic MPTP treatment. There was an increase in the DeltaFosB/FosB and RGS9-2/RGS9-1 protein ratios 3 weeks and 3 days post-treatment, respectively, in chronically treated mice. The data indicate that the pattern of splice isoforms of fosB and rgs9 reflects the brain's immediate and long-term responses to the physiological stress associated with Parkinsonism.

Generation and survival of midbrain dopaminergic neurons in weaver mice

Generation and survival of midbrain dopaminergic (DA) neurons were investigated using tyrosine hydroxylase (TH) immunocytochemistry combined with tritiated thymidine autoradiography at appropriate anatomical levels throughout the anteroposterior (A/P) axes of the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA). The wild-type (+/+) and homozygous weaver (wv/wv) mice used here were the offspring of pregnant dams injected with the radioactive precursor when the mesencephalic neurons were being produced (gestational days 11-15). Data reveal that, at postnatal day 90, depletion of TH-stained cells in the wv/wv presented an A/P pattern of increasing severity and, therefore, the DA cells located in posterior parts of the SNc or the VTA appear to be more vulnerable than the settled anterior neurons. When the time of neuron origin is inferred for each level of these cell groups, it is found that the neurogenesis span is similar for both experimental groups, although significant deficits in the frequency of wv/wv late-generated neurons were observed in any level considered. On the other hand, it has been found that TH-positive neurons were settled along the extent of the SNc and the VTA following precise and differential neurogenetic gradients. Thus, the acute rostrocaudal increase in the proportion of late-generated neurons detected in both+/+DA-cell groups is disturbed in the weaver homozygotes due to the indicated A/P depletion.

The weaver gene expression affects neuronal generation patterns depending on age and encephalic region

Cell generation and survival are investigated in three different neuronal populations of weaver mice: Purkinje and fastigial neurons in the cerebellum, and dopaminergic neurons in the substantia nigra pars compacta. Tritiated thymidine was supplied to pregnant females at the time that these neurons were being produced. Autoradiography was then applied on brain sections obtained from the control and weaver offspring at postnatal (P) day 8 and 90. This makes it possible to assess the differential survival of neurons that were born at distinct embryonic times on the basis of the proportion of labeled cells at two postnatal ages. When labeling profiles were measured at P8, the inferred time of origin was similar between +/+ and wv/wv genotypes for each neuronal population considered. The same occurred at P90 for Purkinje or fastigial neurons, but the labeling profiles of midbrain neurons were different between wild type and weaver homozygotes. There is already a substantial reduction in the number of Purkinje and fastigial cells at P8, but loss of dopaminergic neurons was only detected in 90-day-old weavers and, therefore, vulnerability is built into this midbrain neural system during its late postnatal development. Our results show that depletion of Purkinje and fastigial cells is random with respect to the time of their birth, whereas the weaver gene seems to be specifically targeting the late-generated dopaminergic neurons.

Thiol redox state and oxidative stress in midbrain and striatum of weaver mutant mice, a genetic model of nigrostriatal dopamine deficiency

In this study we measured thiol redox state (TRS) and the oxidative stress indicator lipid peroxidation in midbrain and striatum of adult (4 months old) male control (+/+) and weaver (wv/wv) mice in order to relate them with oxidative stress in conditions of progressive and severe (approximately 70%) nigrostriatal dopaminergic neurodegeneration. Specifically, we measured the specific TRS components glutathione (GSH), glutathione disulfide (GSSG), cysteine (CSH), and the general classes of TRS components. The latter are the protein thiols (PSH) and the disulfides between (a) protein (P) and protein thiols (PSSP), (b) protein and non-protein (NP/R) thiols (PSSR, PSSC) and (c) non-protein and non-protein thiols (NPSSR, NPSSC). In addition, the main product of lipid peroxidation malonyl dialdehyde (MDA) was estimated. In the midbrain of wv/wv, GSH and NPSSC levels are decreased (44% and 64%, respectively) and GSSG, NPSSR, CSH, PSH, PSSP, PSSR and MDA levels are increased (23%, 660%, 110%, 51%, 68%, 18% and 44%, respectively). In the striatum of male wv/wv, protein and non-protein thiol/disulfide and MDA levels do not change, possibly due to the high decrease in striatal dopamine level versus midbrain. Our data show that the high degeneration of the dopaminergic nigrostriatal neurons in male adult wv/wv mice is accompanied by significant changes in TRS and an increase in lipid peroxidation in the midbrain, suggesting involvement of oxidative stress in the degeneration of dopaminergic neurons. They also strengthen the possible use of thiol antioxidants for the development of new neuroprotective therapeutic strategies for neurodegenerative diseases, such as Parkinson's disease.

Differential activation of astrocytes and microglia during post-natal development of dopaminergic neuronal death in the weaver mouse

In order to understand the relationship between astrocytes, microglia and injured neurons, we studied the weaver mutant mouse. One of the main characteristics of this mutant is the progressive degeneration of the dopaminergic (DA) nigrostriatal pathway that starts around postnatal day 15 (P15), in the substantia nigra pars compacta (SNpc) and progresses until adult age (P60). In the present paper, we analysed the relationship between astroglial and microglial cells within DA neurons in the nigrostriatal system of homozygous weaver mice, at different postnatal ages corresponding to specific stages of the DA neuronal loss. The activation of astrocytes was found to be an early event in weaver DA denervation, appearing massively at the onset of DA neuronal loss in the SNpc at P15. Astrocytes remained activated in the adult brain even after the slowing down of the neuronal death process. Interestingly, in the ventral tegmental area, where no DA neuronal death could be detected, a profound, permanent astrogliosis was also observed in adult animals. In contrast, an activation of microglial cells was transiently observed in the SNpc but only at the postnatal age when maximal neuronal death was observed (P30). Lastly, in the striatum, where there was a massive loss of DA nerve terminals, neither astrogliosis nor microglial activation was detected. Hence, the reaction of astrocytes and microglial cells to progressive and spontaneous DA neuronal death showed different temporal kinetics, suggesting a different role for these two cell types in the DA neurodegenerative process in the weaver mouse.

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.

Regional differences in the Purkinje cells settled pattern: a comparative autoradiographic study in control and homozygous weaver mice

To determine whether Purkinje cells located in the vermis and the lateral hemispheres of weaver mice homozygotes are distributed according to precise neurogenetic gradients, [3H]thymidine autoradiography was applied on sections of homozygous weaver mice and normal controls on postnatal day 90. The experimental animals were the offspring of pregnant dams injected with [3H]thymidine on embryonic days 11-12, 12-13, 13-14, and 14-15. The results indicate that, at the level of the vermis, neurogenetic gradients were similar for wild-type and homozygous weaver in each lobe studied of the cerebellar cortex. The same was found for the lobulus simplex and for the ansiform and paramedian lobules when the lateral hemisphere was considered. In the vermis of both experimental groups, the anterior and inferior lobes have more late-generated Purkinje cells than the central and posterior lobes, while in the lateral hemisphere, the lobulus simplex and the ansiform lobule present more early generated Purkinje cells than the paramedian lobule. In weaver homozygotes, the most important deficit of Purkinje cells, in the region of the vermis, was observed in the central lobe; depletion was less observable in the anterior lobe and least observable in the posterior and inferior lobes. In the lateral hemispheres, the most important loss of Purkinje cells was observed in the paramedian lobule, followed by the lobulus simplex. The ansiform lobule presented values that showed no statistical difference between control and homozygous weaver. When Purkinje cells were registered in the entire sections, no significant differences were observed between the two experimental groups. This was due to a considerable volume of the weaver homozygote cerebellar tissue, which has no counterpart in the control mice, compensating for the neuronal loss observed in the other studied areas of the lateral hemisphere.

Topology of ionotropic glutamate receptors in brains of heterozygous and homozygous weaver mutant mice

In weaver mice, mutation of a G-protein inwardly rectifying K(+) channel leads to a cerebellar developmental anomaly characterized by granule and Purkinje cell loss and, in addition, degeneration of dopaminergic neurons. To evaluate other deficits, ionotropic glutamate receptors sensitive to N-methyl-D-aspartate (NMDA), amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainic acid (KA) were examined by autoradiography with [(3)H]MK-801, [(3)H]AMPA, and [(3)H]KA. These surveys were carried out in selected areas of cerebral cortex, hippocampus and related limbic regions, basal ganglia, thalamus, hypothalamus, brainstem, and cerebellum from heterozygous (wv/+) and homozygous (wv/wv) weaver mutants, and compared to wild-type (+/+) mice. In wv/+ and wv/wv mutants, NMDA receptor levels were lower in cortical areas, septum, hippocampus, subiculum, neostriatum, nucleus accumbens, superior colliculus, and in the cerebellar granular layer. Densities of KA receptors were lower in cortical areas, hippocampus, limbic system structures, neostriatum, nucleus accumbens, thalamus and hypothalamus, superior and inferior colliculi, and cerebellar cortex of wv/wv mutants. Levels of AMPA receptors in the weaver were higher than in +/+ mice, particularly in somatosensory and piriform cortices and periaqueductal gray of wv/+, and in somatosensory cortex, CA1 field of Ammon's horn and cerebellar granular layer of wv/wv. Abnormal developmental signals, aberrant cellular responses, or a distorted balance between neurotransmitter interactions may underlie such widespread and reciprocal glutamate receptor alterations, while in the case of cerebellar cortex, NMDA receptors are lacking due to a massive disappearance of cerebellar granule cells and some loss of Purkinje neurons.

Evidence that the loss of Purkinje cells and deep cerebellar nuclei neurons in homozygous weaver is not related to neurogenetic patterns

To determine whether the neurogenetic patterns of Purkinje cells and deep cerebellar nuclei neurons were normal in weaver homozygotes and whether the degeneration of those neuronal types was linked to their time of origin, [3H] thymidine autoradiography was applied on sections of homozygous weaver mice and normal controls on postnatal day 90. The experimental animals were the offspring of pregnant dams injected with [3H] thymidine on embryonic days 11-12, 12-13, 13-14 and 14-15. The results show that the onset of neurogenesis, its pattern of peaks and valleys, and its total span were similar between wild type and homozygous weaver in the cerebellar areas analyzed, indicating that the loss of Purkinje cells and deep cerebellar nuclei neurons is not related to neurogenetic patterns. In weaver homozygotes, the loss of Purkinje cells and deep cerebellar nuclei neurons followed a lateral to medial gradient of increasing severity. Thus, the vermis and the fastigial nucleus, which are medially located, presented the most important neuron loss, whereas in the lateral hemisphere and the dentate nucleus, neuron loss was spared.

The weaver mutant mouse: a model to study the ontogeny of dopamine transmission systems and their role in drug addiction

Dopaminergic neurons and their projection-systems are important in some fundamental human activities like locomotion, feeding and sex, essential for survival and procreation, and are relevant to pathologies like Parkinson's disease and drug abuse. Three main dopaminergic projection-systems, namely the nigrostriatal, mesocortical and mesolimbic pathways are the major targets of the neuropharmacological actions of psychomotor stimulants such as cocaine and amphetamine. Studies on knockout mice for dopamine or its receptors provide substantial information but fail to reveal the role of individual dopaminergic projection-systems. Mutant animals with defects specific to one or more projection-systems might be useful for studying the role of individual dopaminergic projection-systems. We propose the weaver mutant mouse, with a defective nigrostriatal dopaminergic projection-system and dopamine depletion in the dorsal striatum but with intact mesocorticolimbic projection-systems, as a suitable model to study the role of individual dopaminergic systems in diverse biological processes including Parkinson's disease and drug abuse.

Distribution of glutamate receptors of the NMDA subtype in brains of heterozygous and homozygous weaver mutant mice

In weaver mice, mutation of an G-protein inwardly rectifying K+ channel leads to a cerebellar developmental anomaly characterized by granule and Purkinje cell loss and, in addition, degeneration of dopaminergic neurons. To evaluate other deficits, glutamate receptors sensitive to N-methyl-D-aspartate (NMDA) were examined by autoradiography with [3H]MK-801 in 36 brain regions from heterozygous (wv/+) and homozygous (wv/wv) weaver mutants, and compared to wild type (+/+) mice. In wv/+ and wv/wv mutants labelling decreased in cortical regions, septum, hippocampus, subiculum, neostriatum, nucleus accumbens, superior colliculus and in the cerebellar granular layer. The reductions in [3H]MK-801 binding were particularly specific in the cerebellar granular layer of wv/wv mutants, but an ubiquitous altered NMDA receptor topology was revealed in other brain regions. Abnormal developmental signals, or aberrant cellular responses, may underlie widespread NMDA receptor reductions, while in cerebellar cortex they could be lacking due to the massive loss of cerebellar granule cells.

Gamma-aminobutyric acid(B) autoreceptors in substantia nigra and neostriatum of the weaver mutant mouse

The weaver mutation causes cell loss in the center of the substantia nigra, pars compacta. We compared the depression of gamma-aminobutyric acid (GABA)(A) synaptic currents by the GABA(B) agonist R-baclofen in pars compacta neurons of weaver mice which were largely spared from cell degeneration and of wild-type mice. In weaver neurons the suppression of GABA(A) synaptic currents by R-baclofen was reduced compared to wild-type neurons. The EC(50) of R-baclofen was 6.3 times higher in weaver than in wild-type mice. In the neostriatum, which is not a target of the mutation, such a difference did not exist. We conclude that in the pars compacta the weaver mutation leads to a reduced presynaptic autoinhibition through GABA(B) receptors which may promote survival of a subset of weaver neurons in the pars compacta.

The weaver mutation reverses the function of dopamine and GABA in mouse dopaminergic neurons

In the present study, we characterized the intrinsic electrophysiological properties and the membrane currents activated by dopamine (DA) D(2) and GABA(B) receptors in midbrain dopaminergic neurons, maintained in vitro in a slice preparation, from wild-type and homozygous weaver (wv/wv) mice. By using patch-clamp techniques, we found that membrane potential, apparent input resistance, and spontaneous firing of wv/wv dopaminergic neurons were similar to those of dopamine-containing cells recorded from nonaffected (+/+) animals. More interestingly, the wv/wv neurons were excited rather than inhibited by dopamine and the GABA(B) agonist baclofen. This neurotransmitter-mediated excitation was attributable to the activation of a G-protein-gated inward current that reversed polarity at a membrane potential of approximately -30 mV. We suggest that the altered behavior of the receptor-operated wv G-protein-gated inwardly rectifying K(+) channel 2 (GIRK2) might be related to the selective degeneration of the dopaminergic neurons. In addition, the wv GIRK2 would not only suppress the autoreceptor-mediated feedback inhibition of DA release but could also establish a feedforward mechanism of DA release in the terminal fields.

The weaver gene continues to target late-generated dopaminergic neurons in midbrain areas at P90

To determine if lethal action of the weaver gene is more intense in late-generated dopaminergic neurons in midbrain areas on postnatal day (P) 90 [3H] thymidine autoradiography and tyrosine hydroxylase immunohistochemistry were combined in the same tissue section in homozygous weaver mice and normal controls. The experimental animals were the offspring of pregnant dams injected with [3H] thymidine on embryonic days (E) 11-12, E12-13, E13-14 and E14-15. Neurogenetic timetables of dopaminergic neurons were different between wild type and homozygous weavers in all midbrain areas analyzed. A substantial number of late-generated neurons in the substantia nigra pars compacta and in the ventral tegmental area are missing at P90, in these dopaminergic areas the loss is greater than at P20 indicating that neuronal loss is progressive. The greatest loss is in the substantia nigra pars compacta, confirming the report of Bayer et al. [Exp. Brain Res. 105 (1995) 200] at P20, while in the retrorubral field and the interfascicular nucleus late-generated neuron loss was less severe. These results furnish more evidence that dopaminergic neuron loss in homozygous weaver midbrain is a phenomenon linked to development.

The weaver gene has no effect on the generation patterns of mesencephalic dopaminergic neurons

To determine if the weaver gene has action on late-generated neurons in midbrain areas on postnatal day (P) 8 [(3)H] thymidine autoradiography and tyrosine hydroxylase immunohistochemistry were combined in the same tissue section in homozygous weaver mice and normal controls. The experimental animals were the offspring of pregnant dams injected with [3H] thymidine on embryonic days (E)11-12, E12-13, E13-14 and E14-15. Both the span of neurogenesis and the neurogenetic timetables of dopaminergic neurons were similar between wild-type and homozygous weavers in all midbrain areas analyzed. No loss of late-generated dopaminergic neurons was observed. The cytoarchitecture of the midbrain dopaminergic cell groups were also the same in both experimental groups indicating that cell migration, settling, and cytodifferentiation proceeds normally in spite of the presence of the weaver gene.

Distribution of dopamine, its metabolites, and D1 and D2 receptors in heterozygous and homozygous weaver mutant mice

In weaver mice, besides a postnatal cerebellar developmental anomaly probably caused by alterations of an inwardly rectifying K+ channel, there is a progressive loss of mesencephalic dopaminergic neurons. To further evaluate this deficit, endogenous dopamine and its metabolites were measured in 22 brain regions from heterozygous (wv/+) and homozygous (wv/wv) mutants, and compared to wild type (+/+) mice. In both wv/+ and wv/wv mutants there were profound dopamine depletions in all regions; these changes were accompanied by decreases in metabolites but with an increase of turnover indexes. Dopamine D1 and D2 receptors were examined by autoradiography, and their distribution was conserved. The results show that the dopaminergic deficit is widespread to all areas of innervation, and is probably compensated for by an increased turnover. Abnormal developmental growth signals, or aberrant cellular responses, may result in defective neurite formation of the midbrain dopaminergic neurons, leading to their postnatal death.

The weaver mouse gain-of-function phenotype of dopaminergic midbrain neurons is determined by coactivation of wvGirk2 and K-ATP channels

The phenotype of substantia nigra (SN) neurons in homozygous weaver (wv/wv) mice was studied by combining patch-clamp and single-cell RT-multiplex PCR techniques in midbrain slices of 14-d-old mice. In contrast to GABAergic SN neurons, which were unaffected in homozygous weaver mice (wv/wv), dopaminergic SN neurons possessed a dramatically altered phenotype with a depolarized membrane potential and complete loss of spontaneous pacemaker activity. The gain-of-function phenotype was mediated by a large, nonselective membrane conductance exclusively present in (wv/wv) dopaminergic SN neurons. This constitutively activated conductance displayed a sensitivity to external QX-314 (IC(50) = 10.6 microM) very similar to that of heterologously expressed wvGirk2 channels and was not further activated by G-protein stimulation. Single-cell Girk1-4 expression profiling suggested that homomeric Girk2 channels were present in most dopaminergic SN neurons, whereas Girk2 was always coexpressed with other Girk family members in GABAergic SN neurons. Surprisingly, acute QX-314 inhibition of wvGirk2 channels did not induce wild-type-like pacemaker activity but instead caused membrane hyperpolarization. Additional application of a blocker of ATP-sensitive potassium channels (100 microM tolbutamide) induced wild-type-like pacemaker activity. We conclude that the gain-of-function weaver phenotype of dopaminergic substantia nigra neurons is mediated by coactivation of wvGirk2 and SUR1/Kir6. 2-mediated ATP-sensitive K(+) channels. We also show that in contrast to wild-type neurons, all (wv/wv) dopaminergic SN neurons expressed calbindin, a calcium-binding protein that marks dopaminergic SN neurons resistant to neurodegeneration. The identification of two ion channels that in concert determine the weaver phenotype of surviving calbindin-positive dopaminergic SN neurons will help to understand the molecular mechanisms of selective neurodegeneration of dopaminergic SN neurons in the weaver mouse and might be important in Parkinson's disease.

Neurons exhibiting dopamine D2 receptor immunoreactivity in the substantia nigra of the mutant weaver mouse

Neurons exhibiting D2 receptor-like immunoreactivity were investigated in the substantia nigra pars compacta of weaver mice at the light and electron microscope levels using immunocytochemical techniques. At the light microscope level, there was significant loss of D2-like immunoreactive cells in weaver mice and the remaining labeled cells exhibited less intense immunoreactivity. At the ultrastructural level, there was a decrease in the number of immunoreactive profiles and fewer synapses were observed abutting labeled dendritic profiles. In addition, degenerative changes were noted in some of the D2 receptor-like immunoreactive profiles. Double labeling with D2 and tyrosine hydroxylase indicated that the majority of the labeled profiles were double labeled. Eight-week-old homozygous weavers were paired with wild-type littermates as controls and perfused with a buffered solution of acrolein/paraformadehyde. Midbrain sections were reacted immunocytochemically either with an antiserum to D2 or with antisera to D2 and tyrosine hydroxylase, using a double-labeling technique. Sections were processed for light and electron microscopy by standard methods. The results of this study confirm the autoreceptor-like activity of D2 receptors on nigral dopamine neurons. The cell degeneration, down-regulation of D2 receptors, and decreased dendritic and synaptic components in the neuropil suggest that the synaptic integrity of the substantia nigra has been compromised, which in turn would affect the functional efficacy of the basal ganglia circuitry. This altered circuity is expressed in the Parkinson-like symptoms displayed by this mutant mouse.

Alternative sulfonylurea receptor expression defines metabolic sensitivity of K-ATP channels in dopaminergic midbrain neurons

ATP-sensitive potassium (K-ATP) channels couple the metabolic state to cellular excitability in various tissues. Several isoforms of the K-ATP channel subunits, the sulfonylurea receptor (SUR) and inwardly rectifying K channel (Kir6.X), have been cloned, but the molecular composition and functional diversity of native neuronal K-ATP channels remain unresolved. We combined functional analysis of K-ATP channels with expression profiling of K-ATP subunits at the level of single substantia nigra (SN) neurons in mouse brain slices using an RT-multiplex PCR protocol. In contrast to GABAergic neurons, single dopaminergic SN neurons displayed alternative co-expression of either SUR1, SUR2B or both SUR isoforms with Kir6.2. Dopaminergic SN neurons expressed alternative K-ATP channel species distinguished by significant differences in sulfonylurea affinity and metabolic sensitivity. In single dopaminergic SN neurons, co-expression of SUR1 + Kir6.2, but not of SUR2B + Kir6.2, correlated with functional K-ATP channels highly sensitive to metabolic inhibition. In contrast to wild-type, surviving dopaminergic SN neurons of homozygous weaver mouse exclusively expressed SUR1 + Kir6.2 during the active period of dopaminergic neurodegeneration. Therefore, alternative expression of K-ATP channel subunits defines the differential response to metabolic stress and constitutes a novel candidate mechanism for the differential vulnerability of dopaminergic neurons in response to respiratory chain dysfunction in Parkinson's disease.

Girk2 expression in the ventral midbrain, cerebellum, and olfactory bulb and its relationship to the murine mutation weaver

The mouse mutant weaver exhibits developmental deficits and cell death in several neuronal classes. weaver is almost certainly a mutation in the potassium channel, Girk2. In some vulnerable neurons, including those in the midbrain, it is not known whether weaver expression is the primary defect, or whether deficits are secondary to weaver expression elsewhere. In wild-type mice, our results point to subsets of dopamine-containing cells of the midbrain as primary targets of weaver. In the midbrain, all Girk2-positive cells examined in A9 (substantia nigra), A10, and A8 (retrorubral nucleus) are tyrosine hydroxylase-positive. The expression of Girk2 varies among and within these regions. Girk2-positive cells are most numerous in the substantia nigra, pars compacta, a region badly affected in homozygous weavers; in this region, Girk2 expression is found in cell somata and dendrites. In addition, in homozygous weavers, the remaining neuronal processes in A9 (as well as A8) are stunted. Within A10, a region largely spared in weaver homozygotes, Girk2 expression is undetectable in the most medially placed nuclei and is present in the nuclei that border A9. In the cerebellum, Girk2 immunoreactivity was also found in somata and dendrites of populations vulnerable to weaver, including the deep cerebellar nuclei. In a region not previously known to be affected, the olfactory bulb, Girk2 protein is detectable only in processes. The expression of mutated Girk2 has consequences for the olfactory bulb where ectopic cells are present in the external plexiform layer of the homozygous weaver. Our results emphasize that the Girk2 mutation may act to alter the development and maintenance of cell processes and that defects may be present in all Girk2-containing regions in weaver mutants.

An immunocytochemical study of a G-protein-gated inward rectifier K+ channel (GIRK2) in the weaver mouse mesencephalon

It has been suggested that a mutation in a G-protein-gated inward rectifier K+ channel (GIRK2) is responsible for inducing cell death in the cerebellum of homozygous weaver (wv/wv) mutant mice. These mice also display a progressive, massive loss of mesencephalic dopaminergic neurones. Using an immunocytochemical method, we detected GIRK2-positive cell bodies and fibres in the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA) of control (+/+) mice. Cell counts of both GIRK2- and tyrosine hydroxylase (TH)-positive neurones demonstrated a marked loss of SNC cell bodies, especially in 12-month-old (12M) wv/wv mice. A considerable proportion of GIRK2-positive cell bodies were preserved, however. In addition, no loss of GIRK2-positive neurones was observed in the VTA of 12M wv/wv mice, despite of a significant reduction in TH-positive cell bodies. These results suggest that expression of the mutated channel is not a sufficient condition to induce cell death in the ventral mesencephalon of the wv/wv mice.

Atrophy and loss of dopaminergic mesencephalic neurons in heterozygous weaver mice

The phenotypic effect of the weaver mutation in the ventral midbrain of homozygous mutants is associated with the progressive loss of dopaminergic neurons. To discover whether the number of mesencephalic dopaminergic cells is altered in weaver heterozygotes (wv/+), we studied mice between 20 and 365 days of age. We counted tyrosine hydroxylase (TH)-immunopositive cells in the substantia nigra (SN), retrorubral nucleus (RRN), and ventral tegmental area (VTA), and measured cross-sectional areas of neuronal somata in the SN of wv/+ and age-matched wild-type controls (+/+). The number of TH-positive cells in the wv/+ ventral midbrain was on average 13% lower than normal. Cell loss was detected selectively in the SN (12%) and VTA (23%). The areas of somatic profiles in the wv/+ nigral neurons were on average reduced by 9.8%. The neuronal losses in the SN and VTA correlated with a 13.8% reduction in dopamine level in the ventral striatum in wv/+ mice at 14-16 months of age. Our findings imply that a single dose of the weaver gene in the mouse is associated with cellular damage leading to a chronic deficiency in the mesostriatal dopaminergic system.

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.

Phenotypic effects of the weaver gene are evident in the embryonic cerebellum but not in the ventral midbrain

Degeneration of neurons in two structures, the cerebellum and the dopaminergic neurons in the ventral midbrain, is a well characterized action of the weaver gene. In order to see whether the gene has effects prenatally, both the cerebellum and the ventral midbrain were examined in mouse embryos genotyped for the weaver gene (wv, Girk2) on day E19. Anatomically matched sections of the midline cerebellar vermis were quantitatively analyzed 2 h after the dams were given a single injection of [3H]thymidine. A gene-dose effect was seen in the retardation of fissure development. This was more pronounced in homozygotes (wv/wv) and less so in heterozygotes (wv/+) when compared with wild type controls (+/+). Quantitative measures of the following features showed stepwise differences between genotypes so that the wv/wv are most affected and wv/+ are somewhat affected compared with +/+: surface length of the midline vermis, area of the entire midline vermis and the external germinal layer (egl), total number of cells in the egl, [3H]thymidine-labeled and -unlabeled egl cells, cells in the Purkinje cell layer, cells in the region of the deep nuclei, [3H]thymidine-labeled cells in the Purkinje cell layer (presumptive proliferating Bergmann glia), and [3H]thymidine-labeled cells in the region of the deep nuclei. In contrast to the obvious phenotypic effects of wv in the embryonic cerebellum, qualitative immunocytochemical examination of tyrosine hydroxylase staining in the ventral midbrains of the same embryos showed that the position and density of the presumptive dopaminergic neurons was similar in all genotypes.

The human homologue of the weaver mouse gene in familial and sporadic Parkinson's disease

The pathological hallmark of Parkinson's disease is cell death of dopaminergic neurons in the substantia nigra, resulting in striatal dopaminergic deficit and a clinical syndrome dominated by disorders of movement. The cause for this cell loss is unknown, but the possibility of a contributing genetic factor is increasingly recognized. Homozygous weaver mice, a mutant mouse strain, display progressive postnatal depletion of dopaminergic cells in the mesencephalon and have thus been proposed as an animal model for Parkinson's disease. Recently, mGIRK2, a putative G-protein inward rectifier K+ channel, has been identified as the causative gene in the weaver mouse and a homozygous mutation has been described in the H5 pore region of this channel. The human homologue of mGIRK2, KCNJ7 or hiGIRK2, has previously been isolated on chromosome 21q22.1. A possible involvement of this gene in the pathogenesis of Parkinson's disease has been discussed. To evaluate the possibility of a shared genetic defect in weaver mouse and Parkinson's disease, we analysed the H5 pore region of hiGIRK2 in familial and sporadic cases of Parkinson's disease. The sequence was normal in all cases examined, suggesting a differing aetiology of nigral cell loss in Parkinson's disease and weaver mice.