Amyloid-ß dimers in the absence of plaque pathology impair learning and synaptic plasticity.

Despite amyloid plaques, consisting of insoluble, aggregated amyloid-ß peptides, being a defining feature of Alzheimer's disease, their significance has been challenged due to controversial findings regarding the correlation of cognitive impairment in Alzheimer's disease with plaque load. The amyloid cascade hypothesis defines soluble amyloid-ß oligomers, consisting of multiple amyloid-ß monomers, as precursors of insoluble amyloid-ß plaques. Dissecting the biological effects of single amyloid-ß oligomers, for example of amyloid-ß dimers, an abundant amyloid-ß oligomer associated with clinical progression of Alzheimer's disease, has been difficult due to the inability to control the kinetics of amyloid-ß multimerization. For investigating the biological effects of amyloid-ß dimers, we stabilized amyloid-ß dimers by an intermolecular disulphide bridge via a cysteine mutation in the amyloid-ß peptide (Aß-S8C) of the amyloid precursor protein. This construct was expressed as a recombinant protein in cells and in a novel transgenic mouse, termed tgDimer mouse. This mouse formed constant levels of highly synaptotoxic soluble amyloid-ß dimers, but not monomers, amyloid-ß plaques or insoluble amyloid-ß during its lifespan. Accordingly, neither signs of neuroinflammation, tau hyperphosphorylation or cell death were observed. Nevertheless, these tgDimer mice did exhibit deficits in hippocampal long-term potentiation and age-related impairments in learning and memory, similar to what was observed in classical Alzheimer's disease mouse models. Although the amyloid-ß dimers were unable to initiate the formation of insoluble amyloid-ß aggregates in tgDimer mice, after crossbreeding tgDimer mice with the CRND8 mouse, an amyloid-ß plaque generating mouse model, Aß-S8C dimers were sequestered into amyloid-ß plaques, suggesting that amyloid-ß plaques incorporate neurotoxic amyloid-ß dimers that by themselves are unable to self-assemble. Our results suggest that within the fine interplay between different amyloid-ß species, amyloid-ß dimer neurotoxic signalling, in the absence of amyloid-ß plaque pathology, may be involved in causing early deficits in synaptic plasticity, learning and memory that accompany Alzheimer's disease.

Age-related alterations in the expression of genes and synaptic plasticity associated with nitric oxide signaling in the mouse dorsal striatum.

Age-related alterations in the expression of genes and corticostriatal synaptic plasticity were studied in the dorsal striatum of mice of four age groups from young (2-3 months old) to old (18-24 months of age) animals. A significant decrease in transcripts encoding neuronal nitric oxide (NO) synthase and receptors involved in its activation (NR1 subunit of the glutamate NMDA receptor and D1 dopamine receptor) was found in the striatum of old mice using gene array and real-time RT-PCR analysis. The old striatum showed also a significantly higher number of GFAP-expressing astrocytes and an increased expression of astroglial, inflammatory, and oxidative stress markers. Field potential recordings from striatal slices revealed age-related alterations in the magnitude and dynamics of electrically induced long-term depression (LTD) and significant enhancement of electrically induced long-term potentiation in the middle-aged striatum (6-7 and 12-13 months of age). Corticostriatal NO-dependent LTD induced by pharmacological activation of group I metabotropic glutamate receptors underwent significant reduction with aging and could be restored by inhibition of cGMP hydrolysis indicating that its age-related deficit is caused by an altered NO-cGMP signaling cascade. It is suggested that age-related alterations in corticostriatal synaptic plasticity may result from functional alterations in receptor-activated signaling cascades associated with increasing neuroinflammation and a prooxidant state.

Alterations of corticostriatal plasticity by ammonium and rescue by green tea polyphenols.

Hyperammonemia is a major pathophysiological factor in encephalopathies associated with acute and chronic liver failure. On mouse brain slice preparations we analyzed the effects of ammonium on the characteristics of corticostriatal long-term potentiation (LTP) induced by high-frequency electrical stimulation (HFS) of cortical input and the long-lasting effects of pharmacological NMDA receptor (NMDAR) activation. Ammonium chloride exposure enhanced the expression of HFS-induced LTP at the expense of LTD and promoted the generation of NMDA-induced LTD. This treatment did not affect two NMDAR-independent forms of plasticity: taurine-induced LTP and histamine-induced LTD. Alterations in NMDA-induced plasticity were prevented by treatment with green tea polyphenols suggesting the contribution of oxidative stress to the expression of abnormal corticostriatal plasticity.

Ammonia-induced deficit in corticostriatal long-term depression and its amelioration by zaprinast.

Hyperammonemia is a major pathophysiological factor in encephalopathies associated with acute and chronic liver failure. On mouse brain slice preparations, we analyzed the effects of ammonia on the characteristics of corticostriatal long-term depression (LTD) induced by electrical stimulation of cortical input or pharmacological activation of metabotropic glutamate receptors. Long exposure of neostriatal slices to ammonium chloride impaired the induction and/or expression of all studied forms of LTD. This impairment was reversed by the phosphodiesterase inhibitor zaprinast implying lowered cGMP signaling in LTD suppression. Polyphenols from green tea rescued short-term corticostriatal plasticity, but failed to prevent the ammonia-induced deficit of LTD. Zaprinast counteracts the ammonia-induced impairment of long-term corticostriatal plasticity and may thus improve fine motor skills and procedural learning in hepatic encephalopathy.

Histamine-induced plasticity and gene expression in corticostriatal pathway under hyperammonemia.

AIMS: Histamine H3 receptor (H3R) antagonists/inverse agonists increase vigilance. We studied brain histaminergic pathways under hyperammonemia and the transcriptome of receptors and their signaling cascades to provide a rationale for wake-promoting therapies. METHODS: We analyzed histamine-induced long-lasting depression of corticostriatal synaptic transmission (LLDhist). As the expression of dopamine 1 receptors (D1R) is upregulated in LGS-KO striatum where D1R-H3R dimers may exist, we investigated actions of H3R and D1R agonists and antagonists. We analyzed transcription of selected genes in cortex and dorsal striatum in a mouse model of inborn hyperammonemia (liver-specific glutamine synthetase knockout: LGS-KO) and compared it with human hepatic encephalopathy. RESULTS: LGS-KO mice showed significant reduction of the direct depression (DD) but not the long-lasting depression (LLD) by histamine. Neither pharmacological activation nor inhibition of D1R significantly affected DDhist and LLDhist in WT striatum, while in LGS-KO mice D1R activation suppressed LLDhist. Histaminergic signaling was found unchanged at the transcriptional level except for the H2R. A study of cAMP-regulated genes indicated a significant reduction in the molecular signature of wakefulness in the diseased cortex. CONCLUSIONS: Our findings provide a rationale for the development of aminergic wake-promoting therapeutics in hyperammonemic disorders.

Developmental alterations of DHPG-induced long-term depression of corticostriatal synaptic transmission: switch from NMDA receptor-dependent towards CB1 receptor-dependent plasticity.

In animal models of early Parkinson's disease (PD), motor deficits are accompanied by excessive striatal glutamate release. Blockade of group I metabotropic glutamate receptors (mGluRs), endocannabinoid degradation and nitric oxide (NO) synthesis combats PD symptoms. Activation of group I mGluRs with the specific agonist 3,5-dihydroxyphenylglycine (DHPG) induces long-term depression of corticostriatal transmission (LTD(DHPG)) in the adult mouse striatum requiring NO synthesis downstream to cannabinoid CB1 receptor (CB1R) activation suggesting a dual role for LTD(DHPG): neuroprotective by down-regulation of glutamatergic transmission and, under certain circumstances, neurotoxic by release of NO. We report now that LTD(DHPG) undergoes a developmental switch from N-methyl-D-aspartate (NMDA)-receptor-dependent/CB1R-independent to NMDA receptor-independent/CB1R-dependent plasticity with NO playing an essential role for LTD(DHPG) at all developmental stages. The gain in function of CB1R is explained by their developmental up-regulation evaluated with real-time reverse transcription-polymerase chain reaction. These findings are relevant for the pathophysiology and therapy of PD as they link the activation of group I mGluRs, endocannabinoid release, and striatal NO production.

Carbenoxolone impairs LTP and blocks NMDA receptors in murine hippocampus.

Effects of the gap junction blocker carbenoxolone (CBX) on tetanus- and taurine-induced long-term potentiation (LTP) were studied on Schaffer collateral-CA1 field excitatory postsynaptic potentials (fEPSPs) in mouse hippocampal slices. Preincubation with 10 microM CBX reduced the amount of LTP induced by weak theta-burst stimulation (TBS) or a single train of stimuli (HFS; 1s at 100 Hz), but did not affect LTP induced by 30-min perfusion with 10 mM taurine. Incubation with 50-100 microM CBX 15 min before HFS or TBS abolished tetanus-induced LTP. At 100 microM CBX, the concentration that is used for the blockade of gap junctions in vitro, a long-lasting depression of fEPSPs was observed which persisted under the blockade of NMDA receptors, and receptors for corticosteroids. A similar depression was produced by the CBX inactive analogue glycyrrhizic acid and the broad spectrum calcium channel antagonist ruthenium red. Whole-cell patch-clamp recordings from acutely isolated CA1 pyramidal neurons showed that CBX exerts a selective dose-dependent inhibition of NMDA-evoked currents with an IC(50)=104 microM. Thus the widely used gap junction uncoupler CBX acts as an antagonist at NMDA receptors and consequently impairs the induction of LTP.

Impaired novelty acquisition and synaptic plasticity in congenital hyperammonemia caused by hepatic glutamine synthetase deficiency.

Genetic defects in ammonia metabolism can produce irreversible damage of the developing CNS causing an impairment of cognitive and motor functions. We investigated alterations in behavior, synaptic plasticity and gene expression in the hippocampus and dorsal striatum of transgenic mice with systemic hyperammonemia resulting from conditional knockout of hepatic glutamine synthetase (LGS-ko). These mice showed reduced exploratory activity and delayed habituation to a novel environment. Field potential recordings from LGS-ko brain slices revealed significantly reduced magnitude of electrically-induced long-term potentiation (LTP) in both CA3-CA1 hippocampal and corticostriatal synaptic transmission. Corticostriatal but not hippocampal slices from LGS-ko brains demonstrated also significant alterations in long-lasting effects evoked by pharmacological activation of glutamate receptors. Real-time RT-PCR revealed distinct patterns of dysregulated gene expression in the hippocampus and striatum of LGS-ko mice: LGS-ko hippocampus showed significantly modified expression of mRNAs for mGluR1, GluN2B subunit of NMDAR, and A1 adenosine receptors while altered expression of mRNAs for D1 dopamine receptors, the M1 cholinoreceptor and the acetylcholine-synthetizing enzyme choline-acetyltransferase was observed in LGS-ko striatum. Thus, inborn systemic hyperammonemia resulted in significant deficits in novelty acquisition and disturbed synaptic plasticity in corticostriatal and hippocampal pathways involved in learning and goal-directed behavior.

Mechanisms of long-lasting enhancement of corticostriatal neurotransmission by taurine.

The long-lasting enhancement of corticostriatal neurotransmission by taurine, LLE-TAU represents a complex phenomenon requiring concurrent activation of glycine, DA and Ach receptors as well as taurine uptake. The data on the mechanisms of corticostriatal LLE-TAU can be integrated in the following scheme. Taurine interaction with glycine and GABAA receptors causes depolarization of striatal medium spiny cells (Chepkova et al., 2002) which is enhanced by taurine electrogenic uptake by TauT (Sarkar et al., 2003). This depolarization leads to Ca2+ entry via low voltage gated Ca2+ channels. Muscarinic M1 receptors are expressed in medium spiny neurons (Yan et al., 2001) and regulate their excitability mostly via phospholipase C (PLC)/PKC cascade (Lin et al., 2004). Concurrent activation of M1 and PLC-coupled D1 receptors (O'Sullivan et al., 2004) can amplify Ca2+ signal via IP3- stimulated Ca2+ release from intracellular stores and stimulate PKC.

Guanidinoethyl sulphonate is a glycine receptor antagonist in striatum.

1. Guanidinoethyl sulphonate (GES) is an analogue of taurine and an inhibitor of taurine transport. Interactions of GES with GABA(A) and glycine receptors are studied by whole cell recording and fast drug application in isolated striatal neurons of the mouse. 2. We confirm that GES is a weak agonist at GABA(A) receptors, and is able to antagonize GABA-evoked responses. GES did not gate GlyR. 3. GES antagonized glycine responses in a concentration-dependent and surmountable manner. Glycine dose-response curves were shifted to the right by GES (0.5 mM), yielding EC(50)s and Hill coefficients of 62 micro M and 2.5 in control, 154 micro M and 1.3 in the presence of GES. 4. GlyR-mediated taurine responses were competitively antagonized by GES. Taurine dose-response curves, in contrast to the glycine dose-response curves were shifted by GES to the right in a parallel manner. 5. The GlyR-block by GES was not voltage-dependent. 6. In contrast to our findings in the mouse, in rat striatal neurons which lack expression of the alpha3 GlyR subunit, GES shifted the glycine dose-response curve to the right in a parallel way without affecting the maximal response. Subtype-specificity of the GES action at GlyR must await further investigation in artificial expression systems. 7. We conclude that GES is a competitive antagonist at GlyR. The antagonistic action of GES at inhibitory ionotropic receptors can explain its epileptogenic action. Care must be taken with the interpretation of data on GES evoked taurine release.

Cortico-striatal synaptic plasticity in endothelial nitric oxide synthase deficient mice.

Nitric oxide (NO) is a retrograde messenger involved in the processes of learning and memory. The role of the endothelial isoform of nitric oxide synthase (eNOS) in striatal synaptic plasticity was investigated in eNOS-deficient (eNOS(-/-)) and wild type (WT) mice. Tetanic stimulation of cortical afferents in WT mice evoked either long-term potentiation (LTP), or long-term depression (LTD) of cortico-striatal transmission. Both these plasticity related phenomena were NMDA-receptor-dependent; LTD was blocked by sulpiride, a dopamine D2-receptor antagonist. LTP occurrence in slices from eNOS(-/-) mice was significantly reduced when compared with WT mice. The NOS inhibitor NL-ARG reduced the occurrence of LTP and increased the occurrence of LTD in WT mice, resembling the balance of LTP/LTD in eNOS(-/-) mice. Impairment of NO-synthesis thus shifts striatal plasticity towards LTD. This indicates a possible involvement of eNOS from endothelia in neuronal modulation.

Taurine rescues hippocampal long-term potentiation from ammonia-induced impairment.

Hyperammonemia, a major pathophysiological factor in hepatic encephalopathy, impairs long-term potentiation (LTP) of synaptic transmission, a cellular model of learning and memory, in the hippocampus. We have now studied the protective action of taurine on this paradigm by analyzing LTP characteristics in mouse hippocampal slices treated with ammonium chloride (1 mM) in the presence of taurine (1 mM), an ubiquitous osmolyte, antioxidant, and neuromodulator, as well as other substances with such properties. Ammonia-treated slices displayed a significant impairment of LTP maintenance. Taurine and the mitochondrial enhancer l-carnitine, but not the antioxidants (ascorbate, carnosine, and the novel compound GVS-111) or the osmolyte betaine prevented this impairment. The protective effect of taurine was preserved under the blockade of inhibitory GABA(A) and glycine receptors. It is suggested that taurine may rescue the mechanisms of hippocampal synaptic plasticity by improving mitochondrial function under hyperammonemic conditions.

Loss of the limbic mineralocorticoid receptor impairs behavioral plasticity.

Corticosteroid action in the brain is mediated by the mineralocorticoid (MR) and the glucocorticoid (GR) receptor. Disturbances in MR- and GR-mediated effects are thought to impair cognition, behavior, and endocrine control. To assess the function of the limbic MR in these processes, we inactivated the MR gene in the forebrain of the mouse using the Cre/loxP-recombination system. We screened the mice with a limbic MR deficiency in various learning and exploration tests. The mutant mice show impaired learning of the water-maze task and deficits in measures of working memory on the radial maze due to behavioral perseverance and stereotypy. They exhibit a hyperreactivity toward a novel object but normal anxiety-like behavior. The behavioral changes are associated with abnormalities of the mossy fiber projection and an up-regulation of GR expression in the hippocampus. Adult mutant mice show normal corticosterone levels at circadian trough and peak. This genetic model provides important information about the consequences of a permanently altered balance between limbic MR and GR, with implications for stress-related neuroendocrine and neuropsychiatric diseases.