Brain oscillations,medium spiny neurons,and dopamine

1. The striatum is part of a multisynaptic loop involved in translating higher order cognitive activity into action. The main striatal computational unit is the medium spiny neuron, which integrates inputs arriving from widely distributed cortical neurons and provides the sole striatal output.2. The membrane potential of medium spiny neurons' displays shifts between a very negative resting state (down state) and depolarizing plateaus (up states) which are driven by the excitatory cortical inputs.3. Because striatal spiny neurons fire action potentials only during the up state, these plateau depolarizations are perceived as enabling events that allow information processing through cerebral cortex – basal ganglia circuits. In vivo intracellular recording techniques allow to investigate simultaneously the subthreshold behavior of the medium spiny neuron membrane potential (which is a reading of distributed patterns of cortical activity) and medium spiny neuron firing (which is an index of striatal output).4. Recent studies combining intracellular recordings of striatal neurons with field potential recordings of the cerebral cortex illustrate how the analysis of the input–output transformations performed by medium spiny neurons may help to unveil some aspects of information processing in cerebral cortex – basal ganglia circuits, and to understand the origin of the clinical manifestations of Parkinson's disease and other neurologic and neuropsychiatric disorders that result from alterations in dopamine-dependent information processing in the cerebral cortex – basal ganglia circuits.     

DRD1 signaling modulates TrkB turnover and BDNF sensitivity in direct pathway striatal medium spiny neurons

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Increased social interaction in mice deficient of the striatal medium spiny neuron-specific phosphodiesterase 10A2

Cyclic nucleotide phosphodiesterase 10A (PDE10A) is a member of phosphodiesterase families that degrade cAMP and/or cGMP in distinct intracellular sites. PDE10A has a dual activity on hydrolysis of both cAMP and cGMP, and is prominently expressed in the striatum and the testis. Previous studies suggested that PDE10A is involved in regulation of locomotor activity and potentially related to psychosis, but concrete physiological roles of PDE10A remains elusive yet. In this study, we genetically inactivated PDE10A2, a prominent isoform of PDE10A in the brain, in mice, and demonstrate that PDE10A2 deficiency results in increased social interaction without any major influence on different other behaviors, along with increased levels of striatal cAMP. We also demonstrate that PDE10A2 is selectively distributed in medium spiny neurons, but not interneurons, of the striatal complex. Thus, our results establish a physiological role for PDE10A2 in regulating cAMP pathway and social interaction, and suggest that cAMP signaling cascade in striatal medium spiny neurons might be involved in regulating social interaction behavior in mice.     

Phosphatidylinositide 3-kinase and protein kinase C zeta mediate retinoic acid induction of DARPP-32 in medium size spiny neurons in vitro

Mature striatal medium size spiny neurons express the dopamine and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32), but little is known about the mechanisms regulating its levels, or the specification of fully differentiated neuronal subtypes. Cell extrinsic molecules that increase DARPP-32 mRNA and/or protein levels include retinoic acid (RA), brain-derived neurotrophic factor, and estrogen (E₂). We now demonstrate that RA regulates DARPP-32 mRNA and protein in primary striatal neuronal cultures. Furthermore, DARPP-32 induction by RA in vitro requires phosphatidylinositide 3-kinase, but is independent of tropomyosin-related kinase B, cyclin-dependent kinase 5, and protein kinase B. Using pharmacologic inhibitors of various isoforms of protein kinase C (PKC), we also demonstrate that DARPP-32 induction by RA in vitro is dependent on PKC zeta (PKCζ). Thus, the signal transduction pathways mediated by RA are very different than those mediating DARPP-32 induction by brain-derived neurotrophic factor. These data support the presence of multiple signal transduction pathways mediating expression of DARPP-32 in vitro , including a novel, important pathway via which phosphatidylinositide 3-kinase regulates the contribution of PKCζ.     

Transcriptional and anatomical diversity of medium spiny neurons in the primate striatum

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Restructuring of basal ganglia circuitry and associated behaviors triggered by low striatal D2 receptor expression: implications for substance use disorders

Dopamine D2 receptors (D2Rs) consistently emerge as a critical substrate for the etiology of some major psychiatric disorders. Indeed, a central theory of substance use disorders (SUDs) postulates that a reduction in D2R levels in the striatum is a determining factor that confers vulnerability to abuse substances. A large number of clinical and preclinical studies strongly support this link between SUDs and D2Rs; however, identifying the mechanism by which low D2Rs facilitate SUDs has been hindered by the complexity of circuit connectivity, the heterogeneity of D2R expression and the multifaceted constellation of phenotypes observed in SUD patient. Animal models are well‐suited for understanding the mechanisms because they allow access to the circuitry and the genetic tools that enable a dissection of the D2R heterogeneity. This review discusses recent findings on the functional role of D2Rs and highlights the distinctive contributions of D2Rs expressed on specific neuronal subpopulations to the behavioral responses to stimulant drugs. A circuit‐wide restructuring of local and long‐range inhibitory connectivity within the basal ganglia is observed in response to manipulation of striatal D2R levels and is accompanied by multiple alterations in dopamine‐dependent behaviors. Collectively, these new findings provide compelling evidence for a critical role of striatal D2Rs in shaping basal ganglia connectivity; even among neurons that do not express D2Rs. These findings from animal models have deep clinical implications for SUD patients with low levels D2R availability where a similar restructuring of basal ganglia circuitry is expected to take place.     

Proteasome inhibition in oxidative stress neurotoxicity: implications for heat shock proteins

Recent studies have demonstrated that inhibition of the proteasome, an enzyme responsible for the majority of intracellular proteolysis, may contribute to the toxicity associated with oxidative stress. In the present study we demonstrate that exposure to oxidative injury (paraquat, H(2)O(2), FeSO(4)) induces a rapid increase in reactive oxygen species (ROS), loss of mitochondrial membrane potential, inhibition of proteasome activity, and induction of cell death in neural SH-SY5Y cells. Application of proteasome inhibitors (MG115, epoxomycin) mimicked the effects of oxidative stressors on mitochondrial membrane potential and cell viability, and increased vulnerability to oxidative injury. Neural SH-SY5Y cells stably transfected with human HDJ-1, a member of the heat shock protein family, were more resistant to the cytotoxicity associated with oxidative stressors. Cells expressing increased levels of HDJ-1 displayed similar degrees of ROS formation following oxidative stressors, but demonstrated a greater preservation of mitochondrial function and proteasomal activity following oxidative injury. Cells transfected with HDJ-1 were also more resistant to the toxicity associated with proteasome inhibitor application. These data support a possible role for proteasome inhibition in the toxicity of oxidative stress, and suggest heat shock proteins may confer resistance to oxidative stress, by preserving proteasome function and attenuating the toxicity of proteasome inhibition.     

Mechanisms of prion-induced modifications in membrane transport properties: implications for signal transduction and neurotoxicity.

Prion-related encephalopathies are associated with the conversion of a normal cellular isoform of prion protein (PrP(c)) to an abnormal pathologic scrapie isoform (PrP(Sc)). The conversion of this single polypeptide chain involves a reduction in the alpha-helices and an increase in beta-sheet content. This change in the content ratio of alpha-helices to beta-sheets may explain the diversity in the proposed mechanisms of action. Many of the pathogenic properties of PrP(Sc), such as neurotoxicity, proteinase-resistant properties and induction of hypertrophy and proliferation of astrocytes, have been attributed to the peptide fragment corresponding to residues 106-126 of prion (PrP[106-126]). In particular, the amyloidogenic and hydrophobic core AGAAAAGA has been implicated in modulation of neurotoxicity and the secondary structure of PrP[106-126]. Because of some similarities between the properties of PrP[106-126] and PrP(Sc), the former is used as a useful tool to characterize the pharmacological and biophysical properties of PrP(Sc) in general and of that domain in particular, by various laboratories. However, it is important to note that by no means can PrP[106-126] be considered a complete equivalent to PrP(Sc) in function. Several hypotheses have been proposed to explain prion-induced neurodegenerative diseases. These non-exclusive hypotheses include: (i) changes in the membrane microviscosity; (ii) changes in the intracellular Ca(2+) homeostasis; (iii) superoxide dismutase and Cu(2+) homeostasis; and (iv) changes in the immune system. The prion-induced modification in Ca(2+) homeostasis is the result of: (1) prion interaction with intrinsic ion transport proteins, e.g. L-type Ca(2+) channels in the surface membrane, and IP(3)-modulated Ca(2+) channels in the internal membranes, and/or (2) formation of cation channels. These two mechanisms of action lead to changes in Ca(2+) homeostasis that further augment the abnormal electrical activity and the distortion of signal transduction causing cell death. It is concluded that the hypothesis of the interaction of PrP[106-126] with membranes and formation of redox-sensitive and pH-modulated heterogeneous ion channels is consistent with: (a) PrP-induced changes in membrane fluidity and viscosity; (b) PrP-induced changes in Ca(2+) homeostasis (and does not exclude changes in endogenous Ca(2+) transport pathways and Cu(2+) homeostasis); (c) PrP role as an antioxidant; and (d) the PrP structural properties, i.e. beta sheets, protein aggregation, hydrophobicity, functional significance of specific amino acids (e.g. methionine, histidine) and regulation with low pH.     

Dopamine D1-D2 receptor heteromer in dual phenotype GABA/glutamate-coexpressing striatal medium spiny neurons: regulation of BDNF, GAD67 and VGLUT1/2

In basal ganglia a significant subset of GABAergic medium spiny neurons (MSNs) coexpress D1 and D2 receptors (D1R and D2R) along with the neuropeptides dynorphin (DYN) and enkephalin (ENK). These coexpressing neurons have been recently shown to have a region-specific distribution throughout the mesolimbic and basal ganglia circuits. While the functional relevance of these MSNs remains relatively unexplored, they have been shown to exhibit the unique property of expressing the dopamine D1-D2 receptor heteromer, a novel receptor complex with distinct pharmacology and cell signaling properties. Here we showed that MSNs coexpressing the D1R and D2R also exhibited a dual GABA/glutamate phenotype. Activation of the D1R-D2R heteromer in these neurons resulted in the simultaneous, but differential regulation of proteins involved in GABA and glutamate production or vesicular uptake in the nucleus accumbens (NAc), ventral tegmental area (VTA), caudate putamen and substantia nigra (SN). Additionally, activation of the D1R-D2R heteromer in NAc shell, but not NAc core, differentially altered protein expression in VTA and SN, regions rich in dopamine cell bodies. The identification of a MSN with dual inhibitory and excitatory intrinsic functions provides new insights into the neuroanatomy of the basal ganglia and demonstrates a novel source of glutamate in this circuit. Furthermore, the demonstration of a dopamine receptor complex with the potential to differentially regulate the expression of proteins directly involved in GABAergic inhibitory or glutamatergic excitatory activation in VTA and SN may potentially provide new insights into the regulation of dopamine neuron activity. This could have broad implications in understanding how dysregulation of neurotransmission within basal ganglia contributes to dopamine neuronal dysfunction.     

Astrocytic production of nerve growth factor in motor neuron apoptosis: implications for amyotrophic lateral sclerosis

Reactive astrocytes frequently surround degenerating motor neurons in patients and transgenic animal models of amyotrophic lateral sclerosis (ALS). We report here that reactive astrocytes in the ventral spinal cord of transgenic ALS-mutant G93A superoxide dismutase (SOD) mice expressed nerve growth factor (NGF) in regions where degenerating motor neurons expressed p75 neurotrophin receptor (p75(NTR)) and were immunoreactive for nitrotyrosine. Cultured spinal cord astrocytes incubated with lipopolysaccharide (LPS) or peroxynitrite became reactive and accumulated NGF in the culture medium. Reactive astrocytes caused apoptosis of embryonic rat motor neurons plated on the top of the monolayer. Such motor neuron apoptosis could be prevented when either NGF or p75(NTR) was inhibited with blocking antibodies. In addition, nitric oxide synthase inhibitors were also protective. Exogenous NGF stimulated motor neuron apoptosis only in the presence of a low steady state concentration of nitric oxide. NGF induced apoptosis in motor neurons from p75(NTR +/+) mouse embryos but had no effect in p75(NTR -/-) knockout embryos. Culture media from reactive astrocytes as well as spinal cord lysates from symptomatic G93A SOD mice-stimulated motor neuron apoptosis, but only when incubated with exogenous nitric oxide. This effect was prevented by either NGF or p75(NTR) blocking-antibodies suggesting that it might be mediated by NGF and/or its precursor forms. Our findings show that NGF secreted by reactive astrocytes induce the death of p75-expressing motor neurons by a mechanism involving nitric oxide and peroxynitrite formation. Thus, reactive astrocytes might contribute to the progressive motor neuron degeneration characterizing ALS.     

Impaired working memory,cognitive flexibility and reward processing in mice genetically lacking Gpr88: Evidence for a key role for Gpr88 in multiple cortico-striatal-thalamic circuits

The GPR88 orphan G protein-coupled receptor is expressed throughout the striatum, being preferentially localised in medium spiny neurons. It is also present in lower densities in frontal cortex and thalamus. Rare mutations in humans suggest a role in cognition and motor function, while common variants are associated with psychosis. Here we evaluate the influence of genetic deletion of GPR88 upon performance in translational tasks interrogating motivation, reward evaluation and cognitive function. In an automated radial arm maze ‘N-back’ working memory task, Gpr88 KO mice showed impaired correct responding, suggesting a role for GPR88 receptors in working memory circuitry. Associative learning performance was similar to wild-type controls in a touchscreen task but performance was impaired at the reversal learning stage, suggesting cognitive inflexibility. Gpr88 KO mice showed higher breakpoints, reduced latencies and lengthened session time in a progressive ratio task consistent with enhanced motivation. Simultaneously, locomotor hyperactivity was apparent in this task, supporting previous findings of actions of GPR88 in a cortico-striatal-thalamic motor loop. Evidence for a role of GPR88 in reward processing was demonstrated in a touchscreen-based equivalent of the Iowa gambling task. Although both Gpr88 KO and wild-type mice showed a preference for an optimum contingency choice, Gpr88 KO mice selected more risky choices at the expense of more advantageous lower risk options. Together these novel data suggest that striatal GPR88 receptors influence activity in a range of procedures integrated by prefrontal, orbitofrontal and anterior cingulate cortico-striatal-thalamic loops leading to altered cognitive, motivational and reward evaluation processes.     

Activation of BDNF mRNA and protein after seizures in hyperbaric oxygen: implications for sensitization to seizures in re-exposures

Previous studies have demonstrated that exposure to convulsive doses of hyperbaric oxygen (HBO) increases sensitivity to seizures in re-exposures. Because brain derived neurotrophic factor (BDNF) is induced after a variety of seizures and increases cell excitability, it may contribute to the mechanism of sensitization. In this study, a fast induction in BDNF mRNA 2 hr after seizures and a temporary increase in BDNF protein 1 day after seizures induced by 100% O₂ at 5 atm (gauge pressure) were demonstrated in the rat cortex. To determine whether an elevation in BDNF protein level can modify sensitivity to the toxic effect of HBO, recombinant BDNF (12 g) was injected into cerebral ventricles 30 min prior to exposure. Administration of exogenous BDNF significantly shortened latent time to seizures in HBO exposures. We propose that upregulation of BDNF expression in the brain after seizures may contribute to sensitization to HBO toxicity.     

Regional cerebral metabolism in mouse under chronic manganese exposure: implications for manganism

Chronic manganese (Mn) exposure in rodents, non-human primates and humans has been linked to Parkinson's disease like condition known as Manganism. Mn being a cofactor for many enzymes in brain has been known to be accumulated in various regions differentially and thus exert toxic effect upon chronic overexposure. In present study, neuropathology of Manganism was investigated by evaluating regional neuronal and astroglial metabolism in mice under chronic Mn exposure. Male C57BL6 mice were treated with MnCl(2) (25 mg/kg, i.p.) for 21 days. Cerebral metabolism was studied by co-infusing [U-(13)C(6)]glucose and [2-(13)C]acetate, and monitoring (13)C labeling of amino acids in brain tissue extract using (1)H-[(13)C] and (13)C-[(1)H]-NMR spectroscopy. Glutamate, choline, N-acetyl aspartate and myo-inositol were found to be reduced in thalamus and hypothalamus indicating a loss in neuronal and astroglial cells due to Mn neurotoxicity. Reduced labeling of Glu(C4) from [U-(13)C(6)]glucose and [2-(13)C]acetate indicates an impairment of glucose oxidation by glutamatergic neurons and glutamate-glutamine neurotransmitter cycle in cortex, striatum, thalamus-hypothalamus and olfactory bulb with chronic Mn exposure. Additionally, reduced labeling of Gln(C4) from [2-(13)C]acetate indicates a decrease in acetate oxidation by astroglia in the same regions. However, GABAergic function was alleviated only in thalamus-hypothalamus. Our findings indicate that chronic Mn impairs excitatory (glutamatergic) function in the majority of regions of brain while inhibitory (GABAergic) activity is perturbed only in basal ganglia.     

Mutant prion protein-mediated aggregation of normal prion protein in the endoplasmic reticulum: implications for prion propagation and neurotoxicity

Familial prion disorders are believed to result from spontaneous conversion of mutant prion protein (PrPM) to the pathogenic isoform (PrPSc). While most familial cases are heterozygous and thus express the normal (PrPC) and mutant alleles of PrP, the role of PrPC in the pathogenic process is unclear. Plaques from affected cases reveal a heterogeneous picture; in some cases only PrPM is detected, whereas in others both PrPC and PrPM are transformed to PrPSc. To understand if the coaggregation of PrPC is governed by PrP mutations or is a consequence of the cellular compartment of PrPM aggregation, we coexpressed PrPM and PrPC in neuroblastoma cells, the latter tagged with green fluorescent protein (PrPC-GFP) for differentiation. Two PrPM forms (PrP231T, PrP217R/231T) that aggregate spontaneously in the endoplasmic reticulum (ER) were generated for this analysis. We report that PrPC-GFP aggregates when coexpressed with PrP231T or PrP217R/231T, regardless of sequence homology between the interacting forms. Furthermore, intracellular aggregates of PrP231T induce the accumulation of a C-terminal fragment of PrP, most likely derived from a potentially neurotoxic transmembrane form of PrP (CtmPrP) in the ER. These findings have implications for prion pathogenesis in familial prion disorders, especially in cases where transport of PrPM from the ER is blocked by the cellular quality control.     

Modulation of SETDB1 activity by APQ ameliorates heterochromatin condensation,motor function,and neuropathology in a Huntington’s disease mouse model

The present study describes evaluation of epigenetic regulation by a small molecule as the therapeutic potential for treatment of Huntington’s disease (HD). We identified 5-allyloxy-2-(pyrrolidin-1-yl)quinoline (APQ) as a novel SETDB1/ESET inhibitor using a combined in silico and in vitro cell based screening system. APQ reduced SETDB1 activity and H3K9me3 levels in a HD cell line model. In particular, not only APQ reduced H3K9me3 levels in the striatum but it also improved motor function and neuropathological symptoms such as neuronal size and activity in HD transgenic (YAC128) mice with minimal toxicity. Using H3K9me3-ChIP and genome-wide sequencing, we also confirmed that APQ modulates H3K9me3-landscaped epigenomes in YAC128 mice. These data provide that APQ, a novel small molecule SETDB1 inhibitor, coordinates H3K9me-dependent heterochromatin remodelling and can be an epigenetic drug for treating HD, leading with hope in clinical trials of HD.