Injection locking at 2f of spin torque oscillators under influence of thermal noise

Integration of Spin Torque Nano-Oscillators STNO’s in conventional microwave circuits means that the devices have to meet certain specifications. One of the most important criteria is the phase noise, being the key parameter to evaluate the performance and define possible applications. Phase locking several oscillators together has been suggested as a possible means to decrease phase noise and consequently, the linewidth. In this work we present experiments, numerical simulations and an analytic model to describe the effects of thermal noise in the injection locking of a tunnel junction based STNO. The analytics show the relation of the intrinsic parameters of the STNO with the phase noise level, opening the path to tailor the spectral characteristics by the magnetic configuration. Experiments and simulations demonstrate that in the in-plane magnetized structure, while the frequency is locked, much higher reference currents are needed to reduce the noise by phase locking. Moreover, our analysis shows that it is possible to control the phase noise by the reference microwave current (I_RF) and that it can be further reduced by increasing the bias current (I_DC) of the oscillator, keeping the reference current in feasible limits for applications.

Unraveling the complexity of amyotrophic lateral sclerosis: recent advances from the transgenic mutant SOD1 mice

Amyotrophic Lateral Sclerosis (ALS), which accounts for the majority of motor neuron disorders, is a progressive and fatal neurodegenerative disease leading to complete paralysis of skeletal muscles and premature death usually from respiratory failure. About 10% of all ALS cases are inherited, with the responsible gene having been identified in approximately 25% of these individuals. Mutations in the copper-zinc superoxide dismutase (SOD1) gene were the first to be recognized nearly twenty years ago, and since then different animal models, in particular transgenic rodents, have been developed. They replicate many of the clinical, neuropathological and molecular features of ALS patients and have contributed significantly to our understanding of the pathogenic mechanisms of this disease. Although results obtained so far with mutant SOD1 mice have not translated into effective therapies in ALS patients, these models still represent the only experimentally accessible system to study multiple aspects of disease pathogenesis and to provide proof-of-principle for the development of new therapeutic strategies. This review will examine the most recent discoveries obtained from these animal models in an attempt to elucidate the complex mechanisms of the disease. In particular it will focus on the contribution of multiple cell types in governing the disease development and progression.

Inter- and intracellular signaling in amyotrophic lateral sclerosis: role of p38 mitogen-activated protein kinase

The pathogenetic processes underlying the selective motor neuron degeneration in amyotrophic lateral sclerosis (ALS) are complex and still not completely understood even in the cases of inherited disease caused by mutations in the Cu/Zn superoxide dismutase-dependent (SOD1) gene. Recent evidence supports the view that ALS is not a cell-autonomous disease and that glial-neuron cross-talk, throughout cytokines and other toxic factors like the nitric oxide and superoxide, is a crucial determinant for the induction of motor neuron death. This cell-cell interaction may determine the progression of the disease through processes that are likely independent of the initial trigger and that may converge on the activation of intracellular death pathways in the motor neurons. In this review we provide support to the hypothesis that aberrant expression and activity of p38 mitogen protein-activated kinases cascade (p38MAPK) in motor neurons and glial cells may play a role in the development and progression of ALS. Increased activation of p38MAPK may phosphorylate neuron-specific substrates altering their physiological properties and it may turn on responsive genes leading to neurotoxicity.

Activation of the p38MAPK cascade is associated with upregulation of TNF alpha receptors in the spinal motor neurons of mouse models of familial ALS

Phosphorylated p38 mitogen-activated protein kinase (p38MAPK), but not activated c-jun-N-terminal kinase (JNK), increases in the motor neurons of transgenic mice overexpressing ALS-linked SOD1 mutants at different stages of the disease. This effect is associated with a selective increase of phosphorylated MKK3-6, MKK4 and ASK1 and a concomitant upregulation of the TNFalpha receptors (TNFR1 and TNFR2), but not IL1beta and Fas receptors. Activation of both p38 MAPK and JNK occurs in the activated microglial cells of SOD1 mutant mice at the advanced stage of the disease; however, this effect is not accompanied by the concomitant activation of the upstream kinases ASK1 and MKK3,4,6, while both the TNFRs are overexpressed in these cells. No changes of the upstream p38MAPK cascade kinases or TNFRs occur in reactive astrocytes. These findings highlight the activation of a selective intracellular signaling pathway in the motor neurons of SOD1 mutant mice, which is likely implicated in their death.

Persistent activation of p38 mitogen-activated protein kinase in a mouse model of familial amyotrophic lateral sclerosis correlates with disease progression

The p38 mitogen-activated protein kinase (p38MAPK) is activated via phosphorylation in neurones and glial cells by a variety of stimuli including oxidative stress, excitotoxicity, and inflammatory cytokines. Activated p38MAPK can in turn induce phosphorylation of cytoskeletal proteins and activation of cytokines and nitric oxide, thus contributing to neurodegeneration. We investigated the expression and distribution of p38MAPK in the spinal cord of transgenic mice expressing a superoxide dismutase 1 mutation (SOD1G93A), a model of familial amyotrophic lateral sclerosis (ALS). Accumulation of p38MAPK was found by immunoblotting in the spinal cord of G93A mice during the progression of disease, but no changes were detected in its mRNA levels. Immunostaining for phosphorylated p38MAPK in lumbar spinal cord sections of SOD1G93A mice at the presymptomatic and early stages of disease showed an increased labeling in motor neurones that colocalized with phosphorylated neurofilaments in vacuolized perikarya and neurites, as detected by confocal microscopy. As the disease progressed, activated p38MAPK also accumulated in hypertrophic astrocytes and reactive microglia, as demonstrated by colocalization with GFAP and CD11b immunostaining, respectively. These data suggest that activation of p38MAPK in motor neurons and then in reactive glial cells may contribute, respectively, to the development and progression of motor neuron pathology in SOD1G93A mice.

Transgenic SOD1 G93A mice develop reduced GLT-1 in spinal cord without alterations in cerebrospinal fluid glutamate levels

Glutamate-induced excitotoxicity is suggested to play a central role in the development of amyotrophic lateral sclerosis (ALS), although it is still unclear whether it represents a primary cause in the cascade leading to motor neurone death. We used western blotting, immunocytochemistry and in situ hybridization to examine the expression of GLT-1 in transgenic mice carrying a mutated (G93A) human copper-zinc superoxide dismutase (TgSOD1 G93A), which closely mimic the features of ALS. We observed a progressive decrease in the immunoreactivity of the glial glutamate transporter (GLT-1) in the ventral, but not in the dorsal, horn of lumbar spinal cord. This effect was specifically found in 14- and 18-week-old mice that had motor function impairment, motor neurone loss and reactive astrocytosis. No changes in GLT-1 were observed at 8 weeks of age, before the appearance of clinical symptoms. Decreases in GLT-1 were accompanied by increased glial fibrillary acidic protein (GFAP) levels and no change in the levels of GLAST, another glial glutamate transporter. The glutamate concentration in the cerebrospinal fluid (CSF) of TgSOD1 G93A mice was not modified at any of the time points examined, compared with age-matched controls. These findings indicate that the loss of GLT-1 protein in ALS mice selectively occurs in the areas affected by neurodegeneration and reactive astrocytosis and it is not associated with increases of glutamate levels in CSF. The lack of changes in GLT-1 at the presymptomatic stage suggests that glial glutamate transporter reduction is not a primary event leading to motor neurone loss.

Differential expression of S100beta and glial fibrillary acidic protein in the hippocampus after kainic acid-induced lesions and mossy fiber sprouting in adult rat

The expression of S100beta and glial fibrillary acidic protein (GFAP) was analyzed following bilateral injection of kainic acid (KA), a glutamate derivative, into the CA3 region of the adult rat hippocampus. This treatment produces a progressive degeneration of the pyramidal neurons of the hippocampus while sparing the granule cells of the dentate gyrus which undergo sprouting of their axons in the supragranular layer. Messenger RNA and protein levels were measured, by Northern blot and ELISA, in the hippocampus of lesioned and sham-operated rats 1, 7, and 30 days after KA injection. A significant increase of GFAP and its mRNA was demonstrated at each time point, whereas S100beta mRNA levels were significantly enhanced only 30 days after the KA injection and the levels of S100beta protein remained unchanged at all time points. However, when analyzed by immunohistochemistry the S100beta showed clear changes in its expression and distribution depending on the region considered. One month after KA injection, S100beta immunoreactivity was considerably reduced in the stratum radiatum of CA3 region, but there was increased S100beta immunoreactivity in the stratum moleculare. In particular, a notable band of S100beta positive, hypertrophic astrocytes appeared in the supragranular layer of the dentate gyrus where the sprouting of mossy fiber collaterals was detected by Timm's staining. These data show for the first time that an increase in S100beta expression in subpopulations of reactive astrocytes may be involved in the structural reorganization of the hippocampus following KA-induced neurodegeneration.

Trazodone is a potent agonist at 5-HT2C receptors mediating inhibition of the N-methyl-D-aspartate/nitric oxide/cyclic GMP pathway in rat cerebellum

The effects of trazodone on the cyclic GMP elevation elicited by N-methyl-D-aspartate in rat cerebellar slices were analyzed. Trazodone inhibited in a concentration-dependent manner (EC50 = 0.82 nM) the cyclic GMP response evoked by 0.1 microM N-methyl-D-aspartate. The inhibition was near complete at 10 nM trazodone. The effect of 10 nM trazodone was unaffected by 0.3 microM spiperone or rauwolscine, antagonists with selectivity for the 5-HT(serotonin)2A or the 5-HT2B subtype, respectively, but it was totally prevented by 0.01 microM mesulergine, a 5-HT2A/5-HT2B/5-HT2C receptor antagonist. Trazodone was potently counteracted (IC50 = 2.7 nM) by the selective 5-HT2B/5-HT2C receptor antagonist N-(1-methyl-5-indolyl)-N-(3-pyridil) urea HCl and, less potently (IC50 = 95 nM), by ketanserin, a 5-HT2A/5-HT2C receptor blocker. It is concluded that trazodone behaves as a potent full agonist at the 5-HT2C receptor mediating inhibition of the cerebellar N-methyl-D-aspartate/nitric oxide/cyclic GMP system.

Glutamate release in human cerebral cortex and its modulation by 5-hydroxytryptamine acting at h 5-HT1D receptors

1. The release of glutamic acid and its modulation by 5-hydroxytryptamine (5-HT) in the human brain has been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery to reach deeply sited tumours. 2. The Ca2+-dependent K+ (15 mM)-evoked overflow of glutamate was inhibited by 5-HT in a concentration-dependent manner (EC50 = 2.9 nM; maximal effect approximately 50%). The inhibition caused by 5-HT was antagonized by the 5-HT1/5-HT2 receptor antagonist methiothepin. The 5-HT1B/5-HT1D receptor agonist sumatriptan mimicked 5-HT (EC50 = 6.4 nM; maximal effect approximately 50%); the effect of sumatriptan was also methiothepin-sensitive. Selective 5-HT1A receptor antagonists could not prevent the inhibition of glutamate release by 5-HT. 3. The 5-HT1B/5-HT1D receptor ligand GR 127935 and the 5-HT2C/5-HT1B/5-HT1D receptor ligand metergoline were unable to prevent the 5-HT effect; instead they inhibited glutamate release, their effects being abolished by methiothepin. Some 5-HT1A receptor antagonists also displayed intrinsic agonist activity. 4. The effect of sumatriptan was prevented by ketanserin, a drug known to display much higher affinity for recombinant h 5-HT1D than for h 5-HT1B receptors. 5. We propose that neocortical glutamatergic nerve terminals in human brain cortex possess release-inhibiting presynaptic heteroreceptors that appear to belong to the h 5-HT1D subtype.

Serotonin inhibition of the NMDA receptor/nitric oxide/cyclic GMP pathway in rat cerebellum: involvement of 5-hydroxytryptamine2C receptors

Previous studies have shown that 5-hydroxytryptamine (5-HT) can potently inhibit glutamatergic transmission in rat cerebellum through the activation of multiple 5-HT receptors. The aim of this study was to subclassify the 5-HT2 receptor mediating inhibition of the cyclic GMP response elicited by N-methyl-D-aspartate in adult rat cerebellar slices. Seven receptor antagonists, endowed with relative selectivities for the 5-HT2A, 5-HT2B, and 5-HT2C subtypes, differentially affected the inhibition by (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane of the cyclic GMP response, suggesting that the receptor involved belongs to the 5-HT2C subtype.

Plasma cell acid phosphatase as an adjunct in the differential diagnosis of monoclonal immunoglobulinemias

Plasma cell acid phosphatase has been studied in 51 patients, 30 of whom were affected with multiple myeloma (MM) and 21 with nonmyelomatous monoclonal immunoglobulinemias (NMMI). Scoring of results displayed a highly significant difference between the median of the MM and NMMI group (343 vs. 204). The use of plasma cell acid phosphatase as a further criterion in the differential diagnosis of monoclonal immunoglobulinemiasis emphasized.