Table-top interferometry on extreme time and wavelength scales.

Short-pulse metrology and dynamic studies in the extreme ultraviolet (XUV) spectral range greatly benefit from interferometric measurements. In this contribution a Michelson-type all-reflective split-and-delay autocorrelator operating in a quasi amplitude splitting mode is presented. The autocorrelator works under a grazing incidence angle in a broad spectral range (10 nm - 1 µm) providing collinear propagation of both pulse replicas and thus a constant phase difference across the beam profile. The compact instrument allows for XUV pulse autocorrelation measurements in the time domain with a single-digit attosecond precision and a useful scan length of about 1 ps enabling a decent resolution of E/ΔE = 2000 at 26.6 eV. Its performance for selected spectroscopic applications requiring moderate resolution at short wavelengths is demonstrated by characterizing a sharp electronic transition at 26.6 eV in Ar gas. The absorption of the 11th harmonic of a frequency-doubled Yb-fiber laser leads to the well-known 3s3p64p1P1 Fano resonance of Ar atoms. We benchmark our time-domain interferometry results with a high-resolution XUV grating spectrometer and find an excellent agreement. The common-path interferometer opens up new opportunities for short-wavelength femtosecond and attosecond pulse metrology and dynamic studies on extreme time scales in various research fields.

Bright, high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre.

Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 × 106 photon s-1 eV-1 around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.

Annular beam driven high harmonic generation for high flux coherent XUV and soft X-ray radiation.

Separation of the high average power driving laser beam from the generated XUV to soft-X-ray radiation poses great challenges in collinear HHG setups due to the losses and the limited power handling capabilities of the typically used separating optics. This paper demonstrates the potential of driving HHG with annular beams, which allow for a straightforward and power scalable separation via a simple pinhole, resulting in a measured driving laser suppression of 5⋅10-3. The approach is characterized by an enormous flexibility as it can be applied to a broad range of input parameters and generated photon energies. Phase matching aspects are analyzed in detail and an HHG conversion efficiency that is only 27% lower than using a Gaussian beam under identical conditions is demonstrated, revealing the viability of the annular beam approach for high flux coherent short-wavelength sources and high average power driving lasers.

High resolution XUV Fourier transform holography on a table top.

Today, coherent imaging techniques provide the highest resolution in the extreme ultraviolet (XUV) and X-ray regions. Fourier transform holography (FTH) is particularly unique, providing robust and straightforward image reconstruction at the same time. Here, we combine two important advances: First, our experiment is based on a table-top light source which is compact, scalable and highly accessible. Second, we demonstrate the highest resolution ever achieved with FTH at any light source (34 nm) by utilizing a high photon flux source and cutting-edge nanofabrication technology. The performance, versatility and reliability of our approach allows imaging of complex wavelength-scale structures, including wave guiding effects within these structures, and resolving embedded nanoscale features, which are invisible for electron microscopes. Our work represents an important step towards real-world applications and a broad use of XUV imaging in many areas of science and technology. Even nanoscale studies of ultra-fast dynamics are within reach.

High speed and high resolution table-top nanoscale imaging.

We present a table-top coherent diffractive imaging (CDI) experiment based on high-order harmonics generated at 18 nm by a high average power femtosecond fiber laser system. The high photon flux, narrow spectral bandwidth, and high degree of spatial coherence allow for ultrahigh subwavelength resolution imaging at a high numerical aperture. Our experiments demonstrate a half-pitch resolution of 15 nm, close to the actual Abbe limit of 12 nm, which is the highest resolution achieved from any table-top extreme ultraviolet (XUV) or x-ray microscope. In addition, sub-30 nm resolution was achieved with only 3 s of integration time, bringing live diffractive imaging and three-dimensional tomography on the nanoscale one step closer to reality. The current resolution is solely limited by the wavelength and the detector size. Thus, table-top nanoscopes with only a few-nanometer resolutions are in reach and will find applications in many areas of science and technology.

Inhibition of kynurenine aminotransferase II reduces activity of midbrain dopamine neurons.

Kynurenic acid (KYNA), a neuroactive metabolite of tryptophan, is elevated in the brain of patients with psychotic disorders. Therefore, lowering brain KYNA levels might be a novel approach in the treatment of psychotic disorders. The present in vivo electrophysiological study aimed to investigate the effect of an inhibitor of kynurenine aminotransferase (KAT) II, the primary enzyme for KYNA synthesis, on dopamine (DA) neurons in the ventral tegmental area (VTA). Acute administration of the KAT II inhibitor PF-04859989 (5 or 10 mg/kg) was associated with a short-onset, time-dependent decrease in firing rate and burst activity of DA neurons, both parameters reaching a 50% reduction within 45 min. Furthermore, PF-04859989 reduced the number of spontaneously active DA cells as measured 4-6 after administration. Pretreatment with d-cycloserine (30 mg/kg) or CGP-52432 (10 mg/kg) prevented the inhibitory action of PF-04859989 (5 mg/kg) on firing rate and burst firing activity. In contrast, pretreatment with methyllycaconitine (MLA, 4 mg/kg) did not change the response, whereas picrotoxin (4.5 mg/kg) partially prevented the inhibitory effects of PF-04859989 (5 mg/kg, i.v.). Our results show that a specific inhibition of KAT II is associated with a marked reduction in VTA DA firing activity. This effect appears to be specifically executed by NMDA-receptors and mediated indirectly via a GABA(B)-receptor-induced disinhibition of DA neurons. Our findings are in line with the view that endogenous KYNA, by modulation of the NMDA-receptor, exerts important physiological roles in the brain.

A role for inflammatory metabolites as modulators of the glutamate N-methyl-D-aspartate receptor in depression and suicidality.

BACKGROUND: Patients with depression and suicidality suffer from low-grade neuroinflammation. Pro-inflammatory cytokines activate indoleamine 2,3-dioxygenase, an initial enzyme of the kynurenine pathway. This pathway produces neuroactive metabolites, including quinolinic- and kynurenic acid, binding to the glutamate N-methyl-d-aspartate-receptor, which is hypothesized to be part of the neural mechanisms underlying symptoms of depression. We therefore hypothesized that symptoms of depression and suicidality would fluctuate over time in patients prone to suicidal behavior, depending on the degree of inflammation and kynurenine metabolite levels in the cerebrospinal fluid (CSF). METHODS: We measured cytokines and kynurenine metabolites in CSF, collected from suicide attempters at repeated occasions over 2 years (total patient samples n=143, individuals n=30) and healthy controls (n=36). The association between the markers and psychiatric symptoms was assessed using the Montgomery Asberg Depression Rating Scale and the Suicide Assessment Scale. RESULTS: Quinolinic acid was increased and kynurenic acid decreased over time in suicidal patients versus healthy controls. Furthermore, we found a significant association between low kynurenic acid and severe depressive symptoms, as well as between high interleukin-6 levels and more severe suicidal symptoms. CONCLUSIONS: We demonstrate a long-term dysregulation of the kynurenine pathway in the central nervous system of suicide attempters. An increased load of inflammatory cytokines was coupled to more severe symptoms. We therefore suggest that patients with a dysregulated kynurenine pathway are vulnerable to develop depressive symptoms upon inflammatory conditions, as a result the excess production of the NMDA-receptor agonist quinolinic acid. This study provides a neurobiological framework supporting the use of NMDA-receptor antagonists in the treatment of suicidality and depression.

Connecting inflammation with glutamate agonism in suicidality.

The NMDA-receptor antagonist ketamine has proven efficient in reducing symptoms of suicidality, although the mechanisms explaining this effect have not been detailed in psychiatric patients. Recent evidence points towards a low-grade inflammation in brains of suicide victims. Inflammation leads to production of quinolinic acid (QUIN) and kynurenic acid (KYNA), an agonist and antagonist of the glutamatergic N-methyl-D-aspartate (NMDA) receptor, respectively. We here measured QUIN and KYNA in the cerebrospinal fluid (CSF) of 64 medication-free suicide attempters and 36 controls, using gas chromatography mass spectrometry and high-performance liquid chromatography. We assessed the patients clinically using the Suicide Intent Scale and the Montgomery-Asberg Depression Rating Scale (MADRS). We found that QUIN, but not KYNA, was significantly elevated in the CSF of suicide attempters (P<0.001). As predicted, the increase in QUIN was associated with higher levels of CSF interleukin-6. Moreover, QUIN levels correlated with the total scores on Suicide Intent Scale. There was a significant decrease of QUIN in patients who came for follow-up lumbar punctures within 6 months after the suicide attempt. In summary, we here present clinical evidence of increased QUIN in the CSF of suicide attempters. An increased QUIN/KYNA quotient speaks in favor of an overall NMDA-receptor stimulation. The correlation between QUIN and the Suicide Intent Scale indicates that changes in glutamatergic neurotransmission could be specifically linked to suicidality. Our findings have important implications for the detection and specific treatment of suicidal patients, and might explain the observed remedial effects of ketamine.

Increased levels of kynurenine and kynurenic acid in the CSF of patients with schizophrenia.

BACKGROUND: The kynurenic acid (KYNA) hypothesis for schizophrenia is partly based on studies showing increased brain levels of KYNA in patients. KYNA is an endogenous metabolite of tryptophan (TRP) produced in astrocytes and antagonizes N-methyl-D-aspartate and α7* nicotinic receptors. METHODS: The formation of KYNA is determined by the availability of substrate, and hence, we analyzed KYNA and its precursors, kynurenine (KYN) and TRP, in the cerebrospinal fluid (CSF) of patients with schizophrenia. CSF from male patients with schizophrenia on olanzapine treatment (n = 16) was compared with healthy male volunteers (n = 29). RESULTS: KYN and KYNA concentrations were higher in patients with schizophrenia (60.7 ± 4.37 nM and 2.03 ± 0.23 nM, respectively) compared with healthy volunteers (28.6 ± 1.44 nM and 1.36 ± 0.08 nM, respectively), whereas TRP did not differ between the groups. In all subjects, KYN positively correlated to KYNA. CONCLUSION: Our results demonstrate increased levels of CSF KYN and KYNA in patients with schizophrenia and further support the hypothesis that KYNA is involved in the pathophysiology of schizophrenia.

Elevated levels of kynurenic acid change the dopaminergic response to amphetamine: implications for schizophrenia.

Kynurenic acid (KYNA) is an endogenous compound implicated in the pathophysiology of schizophrenia. This tryptophan metabolite antagonizes both the N-methyl-D-aspartate (NMDA) receptors and the nicotinic alpha7* receptors at micromolar concentrations. In the present study the effects of amphetamine on dopamine (DA) release in the nucleus accumbens and on firing of DA neurons in the ventral tegmental area (VTA) were investigated in rats treated with kynurenine, the precursor of KYNA, in order to elevate brain KYNA levels. In rats subchronically treated with kynurenine (90 mg/kg x d for 6 d via osmotic minipumps, resulting in a 2-fold increase in whole-brain KYNA), the amphetamine-induced (2 mg/kg i.p.) increase in accumbal DA release was clearly enhanced compared to controls. Furthermore, subchronic treatment with kynurenine reduced the inhibitory action of amphetamine (0.2-25.6 mg/kg i.v.) on firing rate and burst firing activity of VTA DA neurons. A single dose of kynurenine (5 mg/kg s.c., 60 min, resulting in a 3-fold increase in whole-brain KYNA) did not alter the amphetamine-induced effects on DA neurotransmission compared to control rats. Present data are in agreement with the increased striatal DA release by amphetamine as observed by brain-imaging studies in patients with schizophrenia. Thus, subchronic elevation of rat brain KYNA, may rationally serve as an animal model similar to a pathophysiological condition of schizophrenia. It is proposed that the reduced responsivity of VTA DA neurons to the inhibitory action of amphetamine observed in rats with subchronically elevated KYNA levels may partly account for the increase in terminal DA release.

Clozapine interacts with the glycine site of the NMDA receptor: electrophysiological studies of dopamine neurons in the rat ventral tegmental area.

Clozapine has a remarkable efficacy in treatment-resistant schizophrenia and is one of the most effective antipsychotic drugs used today. The clinical effects of clozapine are suggested to be related to a unique interaction with a variety of receptor systems, including the glutamatergic receptors. Kynurenic acid (KYNA) is an endogenous blocker of alpha7 nicotinic receptors and a glutamate-receptor antagonist, preferentially blocking N-methyl-D-aspartate (NMDA) receptors. In the present in vivo electrophysiological study, changes in endogenous concentration of brain KYNA were utilized to analyze an interaction between clozapine and the glycine site of NMDA receptors. In control rats intravenously administered clozapine (0.078-10 mg/kg) increased the firing rate and the burst firing activity of dopamine (DA) neurons in the ventral tegmental area (VTA). Pretreatment with indomethacin (50 mg/kg, i.p., 1-3.5 h), a cyclooxygenase (COX)-inhibitor with a preferential selectivity for COX-1, which produced a significant elevation in brain KYNA levels, reversed the excitatory action of clozapine into an inhibitory response. In contrast, pretreatment with the COX-2 selective inhibitor parecoxib (25 mg/kg, i.v., 1-1.5 h) decreased brain KYNA formation and furthermore, clearly potentiated the excitatory effect of clozapine. Our results show that endogenous levels of brain KYNA are of importance for the response of clozapine on VTA DA neurons. On the basis of the present data we propose that clozapine is able to interact with glutamatergic mechanisms, via actions at the NMDA/glycine receptor.

Activation of rat ventral tegmental area dopamine neurons by endogenous kynurenic acid: a pharmacological analysis.

Kynurenic acid (KYNA) is an endogenous NMDA receptor antagonist as well as a blocker of the alpha7* nicotinic receptor and mounting evidence suggests that the compound participates in the pathophysiology of schizophrenia. Previous studies have shown that elevated levels of endogenous KYNA are associated with an increased firing of midbrain dopamine (DA) neurons. In the present study, utilizing extracellular single unit cell recording techniques, the mechanism involved in this excitatory action of the compound was analyzed in male Sprague-Dawley rats. Administration of 4-chlorokynurenine (4-Cl-KYN; 25mg/kg, i.p.), which is converted to the selective NMDA glycine-site antagonist 7-chloro-kynurenic acid (7-Cl-KYNA), was found to increase firing rate and per cent burst firing activity of ventral tegmental area (VTA) DA neurons to the same magnitude as pretreatment of kynurenine (causing a 25-fold elevation in extracellular brain KYNA). Intravenous administration of the selective antagonist at the alpha7* nicotinic receptor methyllycaconitine (MLA; 1-4mg/kg) did not affect firing of VTA DA neurons, whereas intraperitoneal administration of this drug in a high dose (6mg/kg) was associated with a decreased firing rate and per cent burst firing activity. Administration of SDZ 220-581 (10mg/kg, i.v.), a competitive antagonist at the glutamate recognition-site of the NMDA receptor, was found to increase firing rate and per cent burst firing. Present results have potential implications for the treatment of schizophrenia, and indicate that the increased activity of VTA DA neurons following elevation of brain KYNA is mediated through glutamatergic rather than by nicotinergic mechanisms.

Cerebrospinal fluid kynurenic acid in male and female controls - correlation with monoamine metabolites and influences of confounding factors.

The concentrations of the tryptophan metabolite kynurenic acid (KYNA) and the monoamine metabolites homovanillic acid (HVA), 5-hydroxy-indoleacetic acid (5-HIAA) and 4-hydroxy-3-methoxyphenylglycol (HMPG) were determined in the cerebrospinal fluid (CSF) from 43 healthy volunteers (30 males and 13 females). Healthy female controls displayed higher CSF concentration of KYNA (1.91nM+/-0.20) compared to healthy male controls (1.06nM+/-0.07) and lower CSF levels of HMPG (39.2nM+/-2.0 and 43.4+/-1.2, respectively). CSF levels of HVA and 5-HIAA did not differ between females (181.3nM+/-21.9 and 93.7nM+/-11.4, respectively) and males (138.9nM+/-12.6 and 74.8nM+/-5.9, respectively). Positive intercorrelations were found between CSF KYNA, HVA and 5-HIAA while CSF content of HMPG did not correlate with KYNA or the other monoamine metabolites in CSF. A negative correlation was found between back length and CSF concentrations of KYNA, HVA and 5-HIAA and also between CSF KYNA levels and body height. The results of the present study suggest that concentrations of KYNA and the monoamine metabolites in CSF from healthy controls are dependent on gender and back length, which must be taken in consideration when analysing mixed groups of men and women. The higher KYNA concentration found in female controls compared to male might be attributed to a shorter back in women compared to men. Furthermore, these findings suggest that increased KYNA formation is associated with an increased dopamine and serotonin turnover.

Activation of noradrenergic locus coeruleus neurons by clozapine and haloperidol: involvement of glutamatergic mechanisms.

The major brain noradrenergic nucleus locus coeruleus (LC) has long been thought to be involved in states of alertness and cognitive processes. These functional characteristics make this nucleus interesting with regard to the signs of schizophrenia, especially the negative symptoms of the disease. In the present in-vivo electrophysiological study we analyse a putative interaction between endogenous kynurenic acid (KYNA) and the antipsychotic drugs clozapine and haloperidol on noradrenergic LC neurons. Previous studies have shown that systemically administered antipsychotic drugs increase the neuronal activity of LC noradrenaline (NA) neurons. In line with these findings, our results show that clozapine (1.25-10 mg/kg i.v.) and haloperidol (0.05-0.08 mg/kg i.v.) increased the firing rate of LC NA neurons in anaesthetized rats. Pretreatment with PNU 156561A (40 mg/kg i.v., 3 h), a potent inhibitor of kynurenine 3-hydroxylase, produced a 2-fold increase in rat brain KYNA levels. This treatment prevented the increase in firing rate of LC NA neurons induced by haloperidol (0.05-0.08 mg/kg i.v.) and clozapine in high doses (2.5-10 mg/kg i.v.). However, the excitatory action of the lowest dose of clozapine (1.25 mg/kg i.v.) was not abolished by elevated levels of brain KYNA. Furthermore, pretreatment with L-701,324 (4 mg/kg i.v.) a selective antagonist at the glycine site of the NMDA receptor prevented the excitatory effects of both clozapine and haloperidol. The present results suggest that the excitation of LC NA neurons by haloperidol and clozapine involves a glutamatergic component.

Cloning of the human homologue of the metastasis-associated rat C4.4A.

We have previously described a rat metastasis-associated molecule, C4.4A, which has some common features with the uPAR. Because of its restricted expression in non-transformed tissues a search for the human homologue became of interest. Human C4.4A was cloned from a placental cDNA library. As in the rat, the human uPAR and the human C4.4A genes appear to belong to the same family. Both genes are located on chromosome 19q13.1-q13.2 and both molecules have a glycolipid anchor site and are composed of three extracellular domains. Only domains one and two of the human C4.4A and the uPAR protein show a significant degree of identity. Expression of the human C4.4A was observed by RT-PCR and Northern blotting in placental tissue, skin, esophagus and peripheral blood leukocytes, but not in brain, lung, liver, kidney, stomach, colon and lymphoid organs. Yet, tumors derived from the latter tissues frequently contained C4.4A mRNA. As demonstrated for malignant melanoma, C4.4A mRNA expression correlated with tumor progression. While nevi were negative and only a minority of primary malignant melanoma expressed C4.4A, all metastases were C4.4A-positive. Taking into account the high degree of homology between rat and human C4.4A, the conformity of the expression profiles and the association of rat C4.4A with tumor progression, human C4.4A might well become a prognostic marker and possibly a target of therapy.