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

Morpho-Functional Changes of Nigral Dopamine Neurons in an α-Synuclein Model of Parkinson's Disease

BACKGROUND

The accumulation of α-synuclein (α-syn) fibrils in intraneuronal inclusions called Lewy bodies and Lewy neurites is a pathological signature of Parkinson's disease (PD). Although several aspects linked to α-syn-dependent pathology (concerning its spreading, aggregation, and activation of inflammatory and neurodegenerative processes) have been under intense investigation, less attention has been devoted to the real impact of α-syn overexpression on structural and functional properties of substantia nigra pars compacta (SNpc) dopamine (DA) neurons, particularly at tardive stages of α-syn buildup, despite this has obvious relevance to comprehending mechanisms beyond PD progression.

OBJECTIVES

We aimed to determine the consequences of a prolonged α-syn overexpression on somatodendritic morphology and functions of SNpc DA neurons.

METHODS

We performed immunohistochemistry, stereological DA cell counts, analyses of dendritic arborization, ex vivo patch-clamp recordings, and in vivo DA microdialysis measurements in a 12- to 13-month-old transgenic rat model overexpressing the full-length human α-syn (Snca^+/+ ) and age-matched wild-type rats.

RESULTS

Aged Snca^+/+ rats have mild loss of SNpc DA neurons and decreased basal DA levels in the SN. Residual nigral DA neurons display smaller soma and compromised dendritic arborization and, in parallel, increased firing activity, switch in firing mode, and hyperexcitability associated with hypofunction of fast activating/inactivating voltage-gated K⁺ channels and Ca²⁺ - and voltage-activated large conductance K⁺ channels. These intrinsic currents underlie the repolarization/afterhyperpolarization phase of action potentials, thus affecting neuronal excitability.

CONCLUSIONS

Besides clarifying α-syn-induced pathological landmarks, such evidence reveals compensatory functional mechanisms that nigral DA neurons could adopt during PD progression to counteract neurodegeneration. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

From Traumatic Childhood to Cocaine Abuse: The Critical Function of the Immune System

BACKGROUND

Experiencing traumatic childhood is a risk factor for developing substance use disorder, but the mechanisms that underlie this relationship have not been determined. Adverse childhood experiences affect the immune system, and the immune system mediates the effects of psychostimulants. However, whether this system is involved in the etiology of substance use disorder in individuals who have experienced early life stress is unknown.

METHODS

In this study, we performed a series of ex vivo and in vivo experiments in mice and humans to define the function of the immune system in the early life stress-induced susceptibility to the neurobehavioral effects of cocaine.

RESULTS

We provide evidence that exposure to social stress at an early age permanently sensitizes the peripheral (splenocytes) and brain (microglia) immune responses to cocaine in mice. In the brain, microglial activation in the ventral tegmental area of social-stress mice was associated with functional alterations in dopaminergic neurotransmission, as measured by whole-cell voltage clamp recordings in dopamine neurons. Notably, preventing immune activation during the social-stress exposure reverted the effects of dopamine in the ventral tegmental area and the cocaine-induced behavioral phenotype to control levels. In humans, cocaine modulated toll-like receptor 4-mediated innate immunity, an effect that was enhanced in those addicted to cocaine who had experienced a difficult childhood.

CONCLUSIONS

Collectively, our findings demonstrate that sensitization to cocaine in early life-stressed individuals involves brain and peripheral immune responses and that this mechanism is shared between mice and humans.

Tyrosinase mediated oxidative functionalization in the synthesis of DOPA-derived peptidomimetics with anti-Parkinson activity

DOPA-derived peptidomimetics are an attractive therapeutic tool for the treatment of Parkinson's disease.

Functional alterations of the dopaminergic and glutamatergic systems in spontaneous α-synuclein overexpressing rats

The presence of α-synuclein (α-syn) in Lewy bodies and Lewy neurites is an important characteristic of the neurodegenerative processes of substantia nigra pars compacta (SNpc) dopaminergic (DAergic) neurons in Parkinson's disease (PD) and other synucleinopathies. Here we report that Berlin-Druckrey rats carrying a spontaneous mutation in the 3' untranslated region of α-syn mRNA (m/m rats) display a marked accumulation of α-syn in the mesencephalic area, striatum and frontal cortex, accompanied to severe dysfunctions in the dorsolateral striatum. Despite a small reduction in the number of SNpc and ventral tegmental area DAergic cells, the surviving dopaminergic neurons of the m/m rats do not show clear-cut alterations of the spontaneous and evoked firing activity, DA responses and somatic amphetamine-induced firing inhibition. Interestingly, mutant DAergic neurons display diminished whole-cell Ih conductance and a reduced frequency of spontaneous excitatory synaptic currents. By contrast, m/m rats show a severe impairment of DA and glutamate release in the dorsolateral striatum, as revealed by amperometric measure of DA currents and by electrophysiological recordings of glutamatergic synaptic events in striatal medium spiny neurons. These functional impairments are paralleled by a decreased expression of the DA transporter and VGluT1 proteins in the same area. Thus, together with α-syn overload in the mesencephalic region, striatum and frontal cortex, the main functional alterations occur in the DAergic and glutamatergic terminals in the dorsal striatum of the m/m rats.

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.

Distinct mechanisms of presynaptic inhibition at GABAergic synapses of the rat substantia nigra pars compacta

We investigated the mechanisms of presynaptic inhibition of GABAergic neurotransmission by group III metabotropic glutamate receptors (mGluRs) and GABA(B) receptors, in dopamine (DA) neurons of the substantia nigra pars compacta (SNc). Both the group III mGluRs agonist L-(+)-2-amino-4-phosphonobutyric acid (AP4, 100 microM) and the GABA(B) receptor agonist baclofen (10 microM) reversibly depressed the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) to 48.5 +/- 2.7 and 79.3 +/- 1.6% (means +/- SE) of control, respectively. On the contrary, the frequency of action potential-independent miniature IPSCs (mIPSCs), recorded in tetrodotoxin (TTX, 1 microM) and cadmium (100 microM) were insensitive to AP4 but were reduced by baclofen to 49.7 +/- 8.6% of control. When the contribution of voltage-dependent calcium channels (VDCCs) to synaptic transmission was boosted with external barium (1 mM), AP4 became effective in reducing TTX-resistant mIPSCs to 65.4 +/- 3.9% of control, thus confirming a mechanism of presynaptic inhibition involving modulation of VDCCs. Differently from AP4, baclofen inhibited to 58.5 +/- 6.7% of control the frequency mIPSCs recorded in TTX and the calcium ionophore ionomycin (2 microM), which promotes Ca2+-dependent, but VDCC-independent, transmitter release. Moreover, in the presence of alpha-latrotoxin (0.3 nM), to promote a Ca2+-independent vesicular release of GABA, baclofen reduced mIPSC frequency to 48.1 +/- 3.2% of control, while AP4 was ineffective. These results indicate that group III mGluRs depress GABA release to DA neurons of the SNc through inhibition of presynaptic VDCCs, while presynaptic GABA(B) receptors directly impair transmitter exocytosis.

Cell type analysis of functional fetal dopamine cell suspension transplants in the striatum and substantia nigra of patients with Parkinson's disease

We report the first post-mortem analysis of two patients with Parkinson's disease who received fetal midbrain transplants as a cell suspension in the striatum, and in one case also in the substantia nigra. These patients had a favourable clinical evolution and positive 18F-fluorodopa PET scans and did not develop motor complications. The surviving transplanted dopamine neurons were positively identified with phenotypic markers of normal control human substantia nigra (n = 3), such as tyrosine hydroxylase, G-protein-coupled inward rectifying current potassium channel type 2 (Girk2) and calbindin. The grafts restored the cell type that provides specific dopaminergic innervation to the most affected striatal regions in the parkinsonian brain. Such transplants were able to densely reinnervate the host putamen with new dopamine fibres. The patients received only 6 months of standard immune suppression, yet by post-mortem analysis 3-4 years after surgery the transplants appeared only mildly immunogenic to the host brain, by analysis of microglial CD45 and CD68 markers. This study demonstrates that, using these methods, dopamine neuronal replacement cell therapy can be beneficial for patients with advanced disease, and that changing technical approaches could have a favourable impact on efficacy and adverse events following neural transplantation.

Voltage-Gated Calcium Channels Mediate Intracellular Calcium Increase inWeaverDopaminergic Neurons During Stimulation of D2and GABABReceptors

The weaver ( wv) mutation affects the pore-forming region of the inwardly rectifying potassium channel (GIRK) leading to degeneration of cerebellar granule and midbrain dopaminergic neurons. The mutated channel ( wvGIRK) loses its potassium selectivity, allowing sodium (Na+) and possibly calcium ions (Ca2+) to enter the cell. Here we performed whole cell patch-clamp recordings combined with microfluorometry to investigate possible differences in calcium ([Ca2+]i) dynamics in native dopaminergic neurons (expressing the wvGIRK2 subunits) in the midbrain slice preparation from homozygous weaver ( wv/wv) and control (+/+) mice. Under resting conditions, [Ca2+]iwas similar in wv/wv compared with +/+ neurons. Activation of wvGIRK2 channels by D2and GABABreceptors increased [Ca2+]iin wv/wv neurons, whereas activation of wild-type channels decreased [Ca2+]iin +/+ neurons. The calcium rise in wv/wv neurons was abolished by antagonists of the voltage-gated calcium channels (VGCC); voltage clamp of the neuron at −60 mV; and hyperpolarization of the neuron to −80 mV or more, in current clamp, and was unaffected by TTX. Therefore we propose that wvGIRK2 channels in native dopamine neurons are not permeable to Ca2+, and when activated by D2and GABABreceptors they mediate membrane depolarization and an indirect Ca2+influx through VGCC rather than via wvGIRK2 channels. Such calcium influx may be the trigger for calcium-mediated excitotoxicity, responsible for selective neuronal death in weaver mice.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

I(h) channels contribute to the different functional properties of identified dopaminergic subpopulations in the midbrain

Dopaminergic (DA) midbrain neurons in the substantia nigra (SN) and ventral tegmental area (VTA) are involved in various brain functions such as voluntary movement and reward and are targets in disorders such as Parkinson's disease and schizophrenia. To study the functional properties of identified DA neurons in mouse midbrain slices, we combined patch-clamp recordings with either neurobiotin cell-filling and triple labeling confocal immunohistochemistry, or single-cell RT-PCR. We discriminated four DA subpopulations based on anatomical and neurochemical differences: two calbindin D28-k (CB)-expressing DA populations in the substantia nigra (SN/CB+) or ventral tegmental area (VTA/CB+), and respectively, two calbindin D28-k negative DA populations (SN/CB-, VTA/CB-). VTA/CB+ DA neurons displayed significantly faster pacemaker frequencies with smaller afterhyperpolarizations compared with other DA neurons. In contrast, all four DA populations possessed significant differences in I(h) channel densities and I(h) channel-mediated functional properties like sag amplitudes and rebound delays in the following order: SN/CB- --> VTA/CB- --> SN/CB+ --> VTA/CB+. Single-cell RT-multiplex PCR experiments demonstrated that differential calbindin but not calretinin expression is associated with differential I(h) channel densities. Only in SN/CB- DA neurons, however, I(h) channels were actively involved in pacemaker frequency control. In conclusion, diversity within the DA system is not restricted to distinct axonal projections and differences in synaptic connectivity, but also involves differences in postsynaptic conductances between neurochemically and topographically distinct DA neurons.

Confirmation of the retinopetal/centrifugal nature of the tyrosine hydroxylase-immunoreactive fibers of the retina and optic nerve in the weaver mouse

The number of tyrosine hydroxylase-immunoreactive fibers in the nerve fiber layer is increased in the retina of the weaver compared to control mice (Dev. Brain Res. 121 (2000) 113). To confirm the retinopetal/centrifugal nature of these fibers, a newly devised whole-mounted optic nerve technique allowed us to determine, during development, their first appearance within the optic nerve (post-natal day 12) compared to retina (post-natal day 13). One such fiber was also observed looping in the retina of a monkey fetus.

Yeast screen for constitutively active mutant G protein-activated potassium channels

GIRK2 is a major contributor to G protein-activated inward rectifier potassium channels in the mammalian brain. How GIRK channels open upon contact with Gbetagamma remains unknown. Using a yeast genetic screen to select constitutively active mutants from a randomly mutagenized GIRK2 library, we identified five gating mutations at four residues in the transmembrane domain. Further mutagenesis indicates that GIRK channel opening involves a rotation of the transmembrane segments, bringing one of these residues (V188) to a pore-lining position in the open conformation. Combined with double-mutant studies, these findings suggest that GIRK channels gate by moving from the open conformation inferred from our yeast study of Kir2.1 to a closed conformation perhaps resembling the known KcsA structure.

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.

Evidence for a modulatory role of Ih on the firing of a subgroup of midbrain dopamine neurons

A previous investigation has suggested that the hyperpolarization-activated cation current Ih does not contribute to the spontaneous firing of midbrain dopaminergic neurons. This conclusion was reached using Cs(-1). We have re-examined this question with extracellular recordings in slices using the more specific blocker ZD7288. In two-thirds of the cells, low concentrations of ZD7288 induced a decrease of the spontaneous firing. The maximal inhibition was about 40% and the mean IC50 was 1.6 microM. This effect was probably direct because it persisted in the presence of antagonists of various receptors. These concentrations of ZD7288 had no effect in the remaining one third of the examined cells. However, the highest concentration of ZD7288 (300 microM) abolished the firing of all dopaminergic neurons, probably by a mechanism unrelated to the blockade of Ih. We conclude that Ih controls to a certain extent the firing of a majority of midbrain dopaminergic neurons.