Photonic time-crystalline behaviour mediated by phonon squeezing in Ta2NiSe5

Photonic time crystals refer to materials whose dielectric properties are periodic in time, analogous to a photonic crystal whose dielectric properties is periodic in space. Here, we theoretically investigate photonic time-crystalline behaviour initiated by optical excitation above the electronic gap of the excitonic insulator candidate Ta2NiSe5. We show that after electron photoexcitation, electron-phonon coupling leads to an unconventional squeezed phonon state, characterised by periodic oscillations of phonon fluctuations. Squeezing oscillations lead to photonic time crystalline behaviour. The key signature of the photonic time crystalline behaviour is terahertz (THz) amplification of reflectivity in a narrow frequency band. The theory is supported by experimental results on Ta2NiSe5 where photoexcitation with short pulses leads to enhanced THz reflectivity with the predicted features. We explain the key mechanism leading to THz amplification in terms of a simplified electron-phonon Hamiltonian motivated by ab-initio DFT calculations. Our theory suggests that the pumped Ta2NiSe5 is a gain medium, demonstrating that squeezed phonon noise may be used to create THz amplifiers in THz communication applications.

GPAW: An open Python package for electronic structure calculations

We review the GPAW open-source Python package for electronic structure calculations. GPAW is based on the projector-augmented wave method and can solve the self-consistent density functional theory (DFT) equations using three different wave-function representations, namely real-space grids, plane waves, and numerical atomic orbitals. The three representations are complementary and mutually independent and can be connected by transformations via the real-space grid. This multi-basis feature renders GPAW highly versatile and unique among similar codes. By virtue of its modular structure, the GPAW code constitutes an ideal platform for the implementation of new features and methodologies. Moreover, it is well integrated with the Atomic Simulation Environment (ASE), providing a flexible and dynamic user interface. In addition to ground-state DFT calculations, GPAW supports many-body GW band structures, optical excitations from the Bethe-Salpeter Equation, variational calculations of excited states in molecules and solids via direct optimization, and real-time propagation of the Kohn-Sham equations within time-dependent DFT. A range of more advanced methods to describe magnetic excitations and non-collinear magnetism in solids are also now available. In addition, GPAW can calculate non-linear optical tensors of solids, charged crystal point defects, and much more. Recently, support for graphics processing unit (GPU) acceleration has been achieved with minor modifications to the GPAW code thanks to the CuPy library. We end the review with an outlook, describing some future plans for GPAW.

Strain-Tunable Hyperbolic Exciton Polaritons in Monolayer Black Arsenic with Two Exciton Resonances

Hyperbolic polaritons have been attracting increasing interest for applications in optoelectronics, biosensing, and super-resolution imaging. Here, we report the in-plane hyperbolic exciton polaritons in monolayer black-arsenic (B-As), where hyperbolicity arises strikingly from two exciton resonant peaks. Remarkably, the presence of two resonances at different momenta makes overall hyperbolicity highly tunable by strain, as the two exciton peaks can be merged into the same frequency to double the strength of hyperbolicity as well as light absorption under a 1.5% biaxial strain. Moreover, the frequency of the merged hyperbolicity can be further tuned from 1.35 to 0.8 eV by an anisotropic biaxial strain. Furthermore, electromagnetic numerical simulation reveals a strain-induced hyperbolicity, as manifested in a topological transition of iso-frequency contour of exciton polaritons. The good tunability, large exciton binding energy, and strong light absorption exhibited in the hyperbolic monolayer B-As make it highly suitable for nanophotonics applications under ambient conditions.

Terahertz parametric amplification as a reporter of exciton condensate dynamics

Condensates are a hallmark of emergence in quantum materials such as superconductors and charge density waves. Excitonic insulators are an intriguing addition to this library, exhibiting spontaneous condensation of electron-hole pairs. However, condensate observables can be obscured through parasitic coupling to the lattice. Here we employ nonlinear terahertz spectroscopy to disentangle such obscurants through measurement of the quantum dynamics. We target Ta2NiSe5, a putative room-temperature excitonic insulator in which electron-lattice coupling dominates the structural transition (Tc = 326 K), hindering identification of excitonic correlations. A pronounced increase in the terahertz reflectivity manifests following photoexcitation and exhibits a Bose-Einstein condensation-like temperature dependence well below the Tc, suggesting an approach to monitor the exciton condensate dynamics. Nonetheless, dynamic condensate-phonon coupling remains as evidenced by peaks in the enhanced reflectivity spectrum at select infrared-active phonon frequencies, indicating that parametric reflectivity enhancement arises from phonon squeezing. Our results highlight that coherent dynamics can drive parametric stimulated emission.

The spontaneous symmetry breaking in Ta2NiSe5 is structural in nature

The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta2NiSe5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material's electronic and crystal structure after light excitation reveals spectroscopic fingerprints that are compatible only with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the gap opening. Our results suggest that the spontaneous symmetry breaking in Ta2NiSe5 is mostly of structural character, hampering the possibility to realize quasi-dissipationless energy transport.

Simulating Terahertz Field-Induced Ferroelectricity in Quantum Paraelectric SrTiO_{3}

Recent experiments have demonstrated that light can induce a transition from the quantum paraelectric to the ferroelectric phase of SrTiO_{3}. Here, we investigate this terahertz field-induced ferroelectric phase transition by solving the time-dependent lattice Schrödinger equation based on first-principles calculations. We find that ferroelectricity originates from a light-induced mixing between ground and first excited lattice states in the quantum paraelectric phase. In agreement with the experimental findings, our study shows that the nonoscillatory second harmonic generation signal can be evidence of ferroelectricity in SrTiO_{3}. We reveal the microscopic details of this exotic phase transition and highlight that this phenomenon is a unique behavior of the quantum paraelectric phase.

Unconventional excitonic states with phonon sidebands in layered silicon diphosphide

Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photoluminescence, optical reflection measurements and ab initio theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP₂), where the bound electron-hole pair is composed of electrons confined within one-dimensional phosphorus-phosphorus chains and holes extended in two-dimensional SiP₂ layers. The excitonic state and emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Our ab initio many-body calculations confirm that the observed phonon sideband results from the correlated interaction between excitons and optical phonons. With these results, we propose layered SiP₂ as a platform for the study of excitonic physics and many-particle effects.

Phonoritons as Hybridized Exciton-Photon-Phonon Excitations in a Monolayer h-BN Optical Cavity

A phonoriton is an elementary excitation that is predicted to emerge from hybridization between exciton, phonon, and photon. Besides the intriguing many-particle structure, phonoritons are of interest as they could serve as functional nodes in devices that utilize electronic, phononic, and photonic elements for energy conversion and thermal transport applications. Although phonoritons are predicted to emerge in an excitonic medium under intense electromagnetic wave irradiation, the stringent condition for their existence has eluded direct observation in solids. In particular, on-resonance, intense pumping schemes have been proposed, but excessive photoexcitation of carriers prevents optical detection. Here, we theoretically predict the appearance of phonoritonic features in monolayer hexagonal boron nitride (h-BN) embedded in an optical cavity. The coherent superposition nature of phonoriton states is evidenced by the hybridization of exciton-polariton branches with phonon replicas that is tunable by the cavity-matter coupling strength. This finding simultaneously provides an experimental pathway for observing the predicted phonoritons and opens a new avenue for tuning materials properties.

Real-time observation of a correlation-driven sub 3 fs charge migration in ionised adenine

Sudden ionisation of a relatively large molecule can initiate a correlation-driven process dubbed charge migration, where the electron density distribution is expected to rapidly move along the molecular backbone. Capturing this few-femtosecond or attosecond charge redistribution would represent the real-time observation of electron correlation in a molecule with the enticing prospect of following the energy flow from a single excited electron to the other coupled electrons in the system. Here, we report a time-resolved study of the correlation-driven charge migration process occurring in the nucleic-acid base adenine after ionisation with a 15-35 eV attosecond pulse. We find that the production of intact doubly charged adenine - via a shortly-delayed laser-induced second ionisation event - represents the signature of a charge inflation mechanism resulting from many-body excitation. This conclusion is supported by first-principles time-dependent simulations. These findings may contribute to the control of molecular reactivity at the electronic, few-femtosecond time scale.

Programmable hyperbolic polaritons in van der Waals semiconductors

Collective electronic modes or lattice vibrations usually prohibit propagation of electromagnetic radiation through the bulk of common materials over a frequency range associated with these oscillations. However, this textbook tenet does not necessarily apply to layered crystals. Highly anisotropic materials often display nonintuitive optical properties and can permit propagation of subdiffractional waveguide modes, with hyperbolic dispersion, throughout their bulk. Here, we report on the observation of optically induced electronic hyperbolicity in the layered transition metal dichalcogenide tungsten diselenide (WSe₂). We used photoexcitation to inject electron-hole pairs in WSe₂ and then visualized, by transient nanoimaging, the hyperbolic rays that traveled along conical trajectories inside of the crystal. We establish here the signatures of programmable hyperbolic electrodynamics and assess the role of quantum transitions of excitons within the Rydberg series in the observed polaritonic response.

Femtosecond exciton dynamics in WSe2 optical waveguides

Van-der Waals (vdW) atomically layered crystals can act as optical waveguides over a broad range of the electromagnetic spectrum ranging from Terahertz to visible. Unlike common Si-based waveguides, vdW semiconductors host strong excitonic resonances that may be controlled using non-thermal stimuli including electrostatic gating and photoexcitation. Here, we utilize waveguide modes to examine photo-induced changes of excitons in the prototypical vdW semiconductor, WSe₂, prompted by femtosecond light pulses. Using time-resolved scanning near-field optical microscopy we visualize the electric field profiles of waveguide modes in real space and time and extract the temporal evolution of the optical constants following femtosecond photoexcitation. By monitoring the phase velocity of the waveguide modes, we detect incoherent A-exciton bleaching along with a coherent optical Stark shift in WSe₂.

The authors use time-resolved scanning near-field optical microscopy to probe the ultrafast excitonic processes and their impact on waveguide operation in transition metal dichalcogenide crystals. They observe significant modulation of the complex index by monitoring waveguide modes on the fs time scale, and identify both coherent and incoherent manipulations of WSe₂ excitonic resonances.

Room Temperature Terahertz Electroabsorption Modulation by Excitons in Monolayer Transition Metal Dichalcogenides

The interaction between off-resonant laser pulses and excitons in monolayer transition metal dichalcogenides is attracting increasing interest as a route for the valley-selective coherent control of the exciton properties. Here, we extend the classification of the known off-resonant phenomena by unveiling the impact of a strong THz field on the excitonic resonances of monolayer MoS₂. We observe that the THz pump pulse causes a selective modification of the coherence lifetime of the excitons, while keeping their oscillator strength and peak energy unchanged. We rationalize these results theoretically by invoking a hitherto unobserved manifestation of the Franz-Keldysh effect on an exciton resonance. As the modulation depth of the optical absorption reaches values as large as 0.05 dB/nm at room temperature, our findings open the way to the use of semiconducting transition metal dichalcogenides as compact and efficient platforms for high-speed electroabsorption devices.

Nature of Symmetry Breaking at the Excitonic Insulator Transition: Ta_{2}NiSe_{5}

Ta_{2}NiSe_{5} is one of the most promising materials for hosting an excitonic insulator ground state. While a number of experimental observations have been interpreted in this way, the precise nature of the symmetry breaking occurring in Ta_{2}NiSe_{5}, the electronic order parameter, and a realistic microscopic description of the transition mechanism are, however, missing. By a symmetry analysis based on first-principles calculations, we uncover the discrete lattice symmetries which are broken at the transition. We identify a purely electronic order parameter of excitonic nature that breaks these discrete crystal symmetries and contributes to the experimentally observed lattice distortion from an orthorombic to a monoclinic phase. Our results provide a theoretical framework to understand and analyze the excitonic transition in Ta_{2}NiSe_{5} and settle the fundamental questions about symmetry breaking governing the spontaneous formation of excitonic insulating phases in solid-state materials.

Contrasting Efficiency of Electron-Induced Reaction at Cu(110) in Aliphatic and Aromatic Bromides

We report a comparative study of the electron-induced reaction of pentyl bromide (PeBr) and phenyl bromide (PhBr) on Cu(110) at 4.6 K, observed by scanning tunneling microscopy (STM). The induced dissociation of the intact adsorbed molecule for both reagents occurred at an energy of 2.0 eV, producing a hydrocarbon radical and a Br atom. Electron-induced C-Br bond dissociation was found to be a single-electron process for both reagents. The impulsive two-state (I2S) model was used to describe the Br atom recoil as due to molecular excitation to a repulsive anti-bonding state, in which recoil of the dissociation products occurred due to C·Br repulsion along the prior C-Br bond direction. The measured reaction yield was 3 orders of magnitude greater for PeBr, 2.0 × 10^-7 reactive events per electron, than for PhBr with a yield of 1.7 × 10^-10. The low yield of dissociation products from the aromatic PhBr was attributed to the presence of two additional anionic states below the 2.0 eV energy limit, absent for the aliphatic PeBr; these additional anionic states for PhBr could provide a pathway for electron transfer to the surface in the case of the aromatic, but not the aliphatic, anion. The consequent shorter lifetime of the repulsive aromatic anion of PhBr is consistent with the observation of shorter mean recoil distance (3.2 Å) of its Br dissociation product, as compared with the markedly longer recoil (8.7 Å) of Br observed from the anion of PeBr.

Cavity Control of Excitons in Two-Dimensional Materials

We propose a robust and efficient way of controlling the optical spectra of two-dimensional materials and van der Waals heterostructures by quantum cavity embedding. The cavity light-matter coupling leads to the formation of exciton-polaritons, a superposition of photons and excitons. Our first-principles study demonstrates a reordering and mixing of bright and dark excitons spectral features and in the case of a type II van-der-Waals heterostructure an inversion of intra- and interlayer excitonic resonances. We further show that the cavity light-matter coupling strongly depends on the dielectric environment and can be controlled by encapsulating the active two-dimensional (2D) crystal in another dielectric material. Our theoretical calculations are based on a newly developed nonperturbative many-body framework to solve the coupled electron-photon Schrödinger equation in a quantum-electrodynamical extension of the Bethe-Salpeter approach. This approach enables the ab initio simulations of exciton-polariton states and their dispersion from weak to strong cavity light-matter coupling regimes. Our method is then extended to treat van der Waals heterostructures and encapsulated 2D materials using a simplified Mott-Wannier description of the excitons that can be applied to very large systems beyond reach for fully ab initio approaches.

Nano-imaging of intersubband transitions in van der Waals quantum wells

The science and applications of electronics and optoelectronics have been driven for decades by progress in the growth of semiconducting heterostructures. Many applications in the infrared and terahertz frequency range exploit transitions between quantized states in semiconductor quantum wells (intersubband transitions). However, current quantum well devices are limited in functionality and versatility by diffusive interfaces and the requirement of lattice-matched growth conditions. Here, we introduce the concept of intersubband transitions in van der Waals quantum wells and report their first experimental observation. Van der Waals quantum wells are naturally formed by two-dimensional materials and hold unexplored potential to overcome the aforementioned limitations-they form atomically sharp interfaces and can easily be combined into heterostructures without lattice-matching restrictions. We employ near-field local probing to spectrally resolve intersubband transitions with a nanometre-scale spatial resolution and electrostatically control the absorption. This work enables the exploitation of intersubband transitions with unmatched design freedom and individual electronic and optical control suitable for photodetectors, light-emitting diodes and lasers.

Dissociation of two-dimensional excitons in monolayer WSe2

Two-dimensional (2D) semiconducting materials are promising building blocks for optoelectronic applications, many of which require efficient dissociation of excitons into free electrons and holes. However, the strongly bound excitons arising from the enhanced Coulomb interaction in these monolayers suppresses the creation of free carriers. Here, we identify the main exciton dissociation mechanism through time and spectrally resolved photocurrent measurements in a monolayer WSe₂p–n junction. We find that under static in-plane electric field, excitons dissociate at a rate corresponding to the one predicted for tunnel ionization of 2D Wannier–Mott excitons. This study is essential for understanding the photoresponse of 2D semiconductors and offers design rules for the realization of efficient photodetectors, valley dependent optoelectronics, and novel quantum coherent phases.

In two-dimensional semiconductors excitons are strongly bound, suppressing the creation of free carriers. Here, the authors investigate the main exciton dissociation pathway in p-n junctions of monolayer WSe₂ by means of time and spectrally resolved photocurrent measurements.

Interlayer Excitons and Band Alignment in MoS2/hBN/WSe2 van der Waals Heterostructures

van der Waals heterostructures (vdWH) are ideal systems for exploring light-matter interactions at the atomic scale. In particular, structures with a type-II band alignment can yield detailed insight into carrier-photon conversion processes, which are central to, for example, solar cells and light-emitting diodes. An important first step in describing such processes is to obtain the energies of the interlayer exciton states existing at the interface. Here we present a general first-principles method to compute the electronic quasi-particle (QP) band structure and excitonic binding energies of incommensurate vdWHs. The method combines our quantum electrostatic heterostructure (QEH) model for obtaining the dielectric function with the many-body GW approximation and a generalized 2D Mott-Wannier exciton model. We calculate the level alignment together with intra- and interlayer exciton binding energies of bilayer MoS₂/WSe₂ with and without intercalated hBN layers, finding excellent agreement with experimental photoluminescence spectra. A comparison to density functional theory calculations demonstrates the crucial role of self-energy and electron-hole interaction effects.

Simple Screened Hydrogen Model of Excitons in Two-Dimensional Materials

We present a generalized hydrogen model for the binding energies (E_{B}) and radii of excitons in two-dimensional (2D) materials that sheds light on the fundamental differences between excitons in two and three dimensions. In contrast to the well-known hydrogen model of three-dimensional (3D) excitons, the description of 2D excitons is complicated by the fact that the screening cannot be assumed to be local. We show that one can consistently define an effective 2D dielectric constant by averaging the screening over the extend of the exciton. For an ideal 2D semiconductor this leads to a simple expression for E_{B} that only depends on the excitonic mass and the 2D polarizability α. The model is shown to produce accurate results for 51 transition metal dichalcogenides. Remarkably, over a wide range of polarizabilities the binding energy becomes independent of the mass and we obtain E_{B}^{2D}≈3/(4πα), which explains the recently observed linear scaling of exciton binding energies with band gap. It is also shown that the model accurately reproduces the nonhydrogenic Rydberg series in WS_{2} and can account for screening from the environment.

Dielectric Genome of van der Waals Heterostructures

Vertical stacking of two-dimensional (2D) crystals, such as graphene and hexagonal boron nitride, has recently lead to a new class of materials known as van der Waals heterostructures (vdWHs) with unique and highly tunable electronic properties. Ab initio calculations should in principle provide a powerful tool for modeling and guiding the design of vdWHs, but in their traditional form such calculations are only feasible for commensurable structures with a few layers. Here we show that the dielectric properties of realistic, incommensurable vdWHs comprising hundreds of layers can be efficiently calculated using a multiscale approach where the dielectric functions of the individual layers (the dielectric building blocks) are computed ab initio and coupled together via the long-range Coulomb interaction. We use the method to illustrate the 2D-3D transition of the dielectric function of multilayer MoS2 crystals, the hybridization of quantum plasmons in thick graphene/hBN heterostructures, and to demonstrate the intricate effect of substrate screening on the non-Rydberg exciton series in supported WS2. The dielectric building blocks for a variety of 2D crystals are available in an open database together with the software for solving the coupled electrodynamic equations.

Interactions among adenosine deaminase, adenosine A(1) receptors and dopamine D(1) receptors in stably cotransfected fibroblast cells and neurons

The role of adenosine deaminase in the interactions between adenosine A(1) and dopamine D(1) receptors was studied in a mouse fibroblast cell line stably cotransfected with human D(1) receptor and A(1) receptor cDNAs (A(1)D(1) cells). Confocal laser microscopy analysis showed a high degree of adenosine deaminase immunoreactivity on the membrane of the A(1)D(1) cells but not of the D(1) cells (only cotransfected with human D(1) receptor cDNAs). In double immunolabelling experiments in A(1)D(1) cells and cortical neurons a marked overlap in the distribution of the A(1) receptor and adenosine deaminase immunoreactivities and of the D(1) receptor and adenosine deaminase immunoreactivities was found. Quantitative analysis of A(1)D(1) cells showed that adenosine deaminase immunoreactivity to a large extent colocalizes with A(1) and D(1) receptor immunoreactivity, respectively. The A(1) receptor agonist caused in A(1)D(1) cells and in cortical neurons coaggregation of A(1) receptors and adenosine deaminase, and of D(1) receptors and adenosine deaminase. The A(1) receptor agonist-induced aggregation was blocked by R-deoxycoformycin, an irreversible adenosine deaminase inhibitor. The competitive binding experiments with the D(1) receptor antagonist [(3)H]SCH-23390 showed that the D(1) receptors had a better fit for two binding sites for dopamine, and treatment with the A(1) receptor agonist produced a disappearance of the high-affinity site for dopamine at the D(1) receptor. R-Deoxycoformycin treatment, which has previously been shown to block the interaction between adenosine deaminase and A(1) receptors, and which is crucial for the high-affinity state of the A(1) receptor, also blocked the A(1) receptor agonist-induced loss of high-affinity D(1) receptor binding. The conclusion of the present studies is that the high-affinity state of the A(1) receptor is essential for the A(1) receptor-mediated antagonistic modulation of D(1) receptors and for the A(1) receptor-induced coaggregates of A(1) and adenosine deaminase, and of D(1) and adenosine deaminase. Thus, the confocal experiments indicate that both A(1) and D(1) receptors form agonist-regulated clusters with adenosine deaminase, where the presence of a structurally intact adenosine deaminase bound to A(1) receptors is important for the A(1)-D(1) receptor-receptor interaction at the level of the D(1) receptor recognition.

Adenosine in the central nervous system: release mechanisms and extracellular concentrations

Adenosine has several functions within the CNS that involve an inhibitory tone of neurotransmission and neuroprotective actions in pathological conditions. The understanding of adenosine production and release in the brain is therefore of fundamental importance and has been extensively studied. Conflicting results are often obtained regarding the cellular source of adenosine, the stimulus that induces release and the mechanism for release, in relation to different experimental approaches used to study adenosine production and release. A neuronal origin of adenosine has been demonstrated through electrophysiological approaches showing that neurones can release significant quantities of adenosine, sufficient to activate adenosine receptors and to modulate synaptic functions. Specific actions of adenosine are mediated by different receptor subtypes (A(1), A(2A), A(2B) and A(3)), which are activated by various ranges of adenosine concentrations. Another important issue is the measurement of adenosine concentrations in the extracellular fluid under different conditions in order to know the degree of receptor stimulation and understand adenosine central actions. For this purpose, several experimental approaches have been used both in vivo and in vitro, which provide an estimation of basal adenosine levels in the range of 50-200 nM. The purpose of this review is to describe pathways of adenosine production and metabolism, and to summarize characteristics of adenosine release in the brain in response to different stimuli. Finally, studies performed to evaluate adenosine concentrations under physiological and hypoxic/ischemic conditions will be described to evaluate the degree of adenosine receptor activation.

Changes in hippocampal adenosine efflux, ATP levels, and synaptic transmission induced by increased temperature

Previous studies have demonstrated that when the temperature of hippocampal brain slices is increased, there is a corresponding depression of synaptic potentials mediated by an increased activation of presynaptic adenosine A(1) receptors. The present experiments demonstrate that when the temperature of hippocampal slices is raised from 32.5 degrees C to either 38.5 degrees C or 40.0 degrees C there is a marked, temperature-dependent increase in the efflux of endogenous adenosine and a corresponding decrease in excitatory synaptic responses. The increase in efflux is rapidly reversible on lowering the slice temperature and the temperature-induced efflux is repeatable. Control experiments suggest that this increased efflux of adenosine is not the result of hypoxia or ischemia secondary to a temperature-induced increase in the metabolic rate of the slice. The increase in adenosine efflux was not accompanied by any significant change in the ATP levels in the brain slice, whereas a hypoxic stimulus sufficient to produce a comparable depression of excitatory transmission produced an approximately 75% decrease in ATP levels. These experiments indicate that changes in brain slice temperature can alter purine metabolism in such a way as to increase the adenosine concentration in the extracellular space, as well as adenosine efflux from hippocampal slices, in the absence of significant changes in ATP levels.

Adenosine extracellular brain concentrations and role of A2A receptors in ischemia

Various experimental approaches have been used to determine the concentration of adenosine in extracellular brain fluid. The cortical cup technique or the microdialysis technique, when adenosine concentrations are evaluated 24 hours after implantation of the microdialysis probe, are able to measure adenosine in the nM range under normoxic conditions and in the microM range under ischemia. In vitro estimation of adenosine show that it can reach 30 microM at the receptor level during ischemia, a concentration able to stimulate all adenosine receptor subtypes so far identified. Although the protective role of A1 receptors in ischemia seems consistent, the protective role of A2A receptors appears to be controversial. Both A2A agonists and antagonists have been shown to be neuroprotective in various in vivo ischemia models. Although A2A agonists may be protective, mainly through peripherally mediated effects, A2A antagonists may be protective through local brain mediated effects. It is possible that A2A receptors are tonically activated following a prolonged increase of adenosine concentration, such as occurs during ischemia. A2A receptor activation desensitizes A1 receptors and reduces A1 mediated effects. Under these conditions A2A receptor antagonists may be protective by potentiating all the neuroprotective A1 mediated effects, including decreased neurotoxicity due to reduced ischemia induced glutamate outflow.

CHS 828, a novel pyridyl cyanoguanidine with potent antitumor activity in vitro and in vivo

A new class of recently discovered antineoplastic agents, the pyridyl cyanoguanidines, exert a potent antitumor activity in rodents after oral administration. Optimization in vitro and in vivo has resulted in the selection of the lead candidate CHS 828 (N-(6-chlorophenoxyhexyl)-N'cyano-N"-4-pyridylguanidine). CHS 828 was found to exert potent cytotoxic effects in human breast and lung cancer cell lines, with lesser effects on normal fibroblasts and endothelial cells. In a study using a panel of cell lines with different resistance patterns, the effects of CHS 828 showed a low correlation with the activity patterns of known anticancer agents, and no sensitivity to known mechanisms of multidrug resistance was observed. In nude mice bearing human tumor xenografts, CHS 828, at doses from 20 to 50 mg/kg/day p.o., inhibited the growth of MCF-7 breast cancer tumors and caused regression of NYH small cell lung cancer tumors. Oral administration of CHS 828 once weekly improved efficacy without increasing toxicity. CHS 828 was found to compare favorably with established chemotherapeutic agents such as cyclophosphamide, etoposide, methotrexate, and paclitaxel. In mice with NYH tumors, long-term survival (>6 months) was observed after treatment with CHS 828 was stopped. In conclusion, CHS 828 is an effective new antitumor agent, with a potentially new mechanism of action. CHS 828 is presently being tested in Phase I clinical trials in collaboration with the European Organization for Research and Treatment of Cancer.

Temporal correlation between adenosine outflow and synaptic potential inhibition in rat hippocampal slices during ischemia-like conditions

The temporal correlation between adenosine outflow and changes in field excitatory post synaptic potentials (fEPSP) occurring during ischemia-like conditions was investigated in rat hippocampal slices. Five-minute long ischemia-like conditions resulted in a 100% depression of fEPSP amplitude, followed by a complete recovery after 6-7 min of reperfusion. By reducing the duration of the ischemic insult to 2 min, fEPSP was depressed by 50%. During both 5 and 2 min of ischemia-like conditions, a significant increase in adenosine outflow was detected. During reperfusion, when fEPSP amplitude recovered completely, the adenosine level in the extracellular fluid returned to basal values. The strict relationship between the increase in adenosine outflow and fEPSP inhibition supports the hypothesis that adenosine is largely responsible for the synaptic transmission depression during cerebral ischemia.

Down-regulation of laminin-binding integrins by 1 alpha,25-dihydroxyvitamin D3 in human melanoma cells in vitro

In the present investigation the effect of 1 alpha,25(OH)2D3 on the expression of the integrin laminin receptor on the melanoma cell line SK-MEL-28 has been examined. The SK-MEL-28 cells were shown to contain high-affinity receptors for 1 alpha,25(OH)2D3 and cell proliferation was found to be inhibited in a dose-dependent manner in response to the hormone. Using monoclonal antibodies against the alpha 6-sub-unit of the integrin laminin receptor, immunocytochemistry demonstrated that exposure of cells to 1 alpha,25(OH)2D3 for 5 days caused a reduced staining intensity. This observation was further confirmed by dot blot analysis, where a dose-dependent decline of alpha 6 expression was obtained after treatment of the cells with 1 alpha,25(OH)2D3 for 6 days. FACS-analysis was performed in order to quantify this decline, and it was found that the level of alpha 6-subunits on the cell surface was reduced by more than 40%. Additional investigations including Northern blot analyses of poly(A)+RNA extracts also showed a dose-dependent reduction of alpha 6 mRNA. Interestingly, the decrease of alpha 6 expression on the surface of SK-MEL-28 melanoma cells was accompanied by a reduced ability of the cells to adhere to an artificial basement membrane. In conclusion, the present investigation shows that besides having an antiproliferative effect on the SK-MEL-28 melanoma cells, 1 alpha,25(OH)2D3 is also able to inhibit the surface expression of the alpha 6-subunit of the integrin laminin receptor. Moreover, the results strongly indicate that 1 alpha,25(OH)2D3 exerts its regulatory effect on the alpha 6-subunit at the transcriptional level rather than at the protein level.

The contribution of different types of calcium channels to electrically-evoked adenosine release from rat hippocampal slices

The role of L-, N- and P-type voltage-dependent calcium channels (VDCCs) in the release of adenosine from rat hippocampal slices was investigated by evaluating the effect of the L-channel activator 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyr idi ne carboxylic acid methyl ester (Bay K 8644) and of three calcium channel antagonists: the L-channel antagonist nifedipine, the N-channel blocker omega-conotoxin GVIA (omega-CgTx) and the P-channel blocker omega-agatoxin IVA (omega-Aga-IVA). Adenosine and inosine release, evoked by 5 min electrical stimulation at 10 Hz of hippocampal slices, was assayed by HPLC with ultraviolet absorbance or fluorimetric detection. Nifedipine (100 nM) did not affect adenosine and inosine release evoked by electrical stimulation. Bay K 8644 (100 nM) brought about a statistically significant increase in adenosine evoked release (70%). At a higher concentration (1 microM) Bay K 8644 had no significant effect either on adenosine or inosine release evoked by electrical stimulation. The increase in adenosine release elicited by 100 nM Bay K 8644 was abolished by nifedipine (100 nM). Both omega-CgTx (10 microM) and omega-Aga-IVA (200 nM) caused a statistically significant reduction (77-78%) in evoked release of adenosine. When the previously demonstrated glutamate-dependent component of the release of adenosine was suppressed in the presence of the NMDA and non-NMDA receptor antagonists, D(-)-2-amino-7-phosphonoheptanoic acid (D-AP7. 100 microM) and 6,7-dinitroquinoxaline-2,3-dione (DNQX, 10 microM), the remaining release of adenosine was again significantly reduced by omega-CgTx (10 microM) (60%) and omega-Aga-IVA (200 nM) (73%). These data suggest that, while L-type VDCCs are involved in the regulation of the evoked release of adenosine only when activated by Bay K 8644, both P- and N-channels play a direct role in the calcium entry involved in the coupling process between electrical stimulation and adenosine release.

Purinoceptors in the Central Nervous System

New exciting developments on the occurrence and functional role of purinoceptors in mammalian brain were presented at the session "Purinoceptors in the central nervous system" chaired by Flaminio Cattabeni and Tom Dunwiddie at the Purines '96 international conference. The focus of the session were topics of recent interest, including the sources and mechanisms involved in ATP and adenosine release during physiological neurotransmission in hippocampus, the brain expression of the recently cloned P2 receptors, and the role of the various adenosine receptor subtypes in brain protection from neurodegeneration associated with trauma-, ischemia-and excessive excitatory amino acid neurotransmission. New important insights into the mechanisms responsible for the formation and release of adenosine into the extracellular space were provided by data obtained by Dunwiddie and coworkers in hippocampal pyramidal neurons. These data may have functional implications for the role of purines in modulation of synaptic plasticity and long-term potentiation in this brain area, and hence in cognitive functions. Buell provided an updated overview on the cloning, molecular characteristics and brain expression of various ligand-gated P2X purinoceptors; although the functional role of these receptors in mammalian brain still awaits elucidation, their widespread distribution in the nervous system strongly suggests that ATP-mediated events are more prevalent and important in brain than expected. Pedata presented data on the functional interrelationships between adenosine and glutamate in the brain, and also provided evidence for alterations of the reciprocal regulation between these two systems in aged brain, which may have important implications for both ischemia-and trauma-associated neurodegenerative events and senescence-associated cognitive impairment. Finally, von Lubitz provided novel data on the molecular mechanisms likely to be at the basis of the brain protective effects associated with the chronic stimulation of the adenosine A3 receptor, further confirming that this receptor represents a crucial target for the development of new antiischemic and antineurodegenerative therapeutic agents.

Adenosine by activating A1 receptors prevents GABAA-mediated actions during hypoxia in the rat hippocampus

The relative contribution of adenosine and gamma-aminobutyric acid (GABA) for the hypoxia-induced depression of field excitatory postsynaptic potentials in the CA1 area of rat hippocampal slices, was investigated. It is concluded that both adenosine and GABA, by activating A1 and GABAA receptors, could be responsible for the inhibition of synaptic transmission during hypoxia, but the action of endogenous GABA becomes evident only when the adenosine A1 receptor action is precluded.

A2 adenosine receptors: their presence and neuromodulatory role in the central nervous system

1. Adenosine is an endogenous neuromodulator that exerts its depressant effect on neurons by acting on the A1 adenosine receptor subtype. Excitatory actions of adenosine, mediated by the activation of the A2 adenosine receptor subtype, have also been shown in the central nervous system. 2. Adenosine A2a receptors are highly localized in the striatum, as demonstrated by the binding assay of the A2a selective agonist, CGS2680, and by analysis of the A2 receptor mRNA localization with in situ hybridization histochemistry. However, adenosine A2a, receptors, albeit at lower levels, are also localized in other brain regions, such as the cortex and the hippocampus. 3. In the striatum, adenosine A2a, receptors are implicated in the control of motor activity. Evidences exists of an antagonistic interaction between adenosine A2a and dopamine D2 receptors. 4. Utilizing selective agonists and antagonists for adenosine A2a receptors, their role in the modulation of the release of several neurotransmitters (acetylcholine, dopamine, glutamate, GABA) has been extensively studied in the brain (striatum, cortex, hippocampus). Controversial results have been obtained and, because the overall effect of endogenous adenosine in the brain is that of an inhibitory tonus, the physiological meaning of the excitatory A2 receptor remains to be clarified.

The source of brain adenosine outflow during ischemia and electrical stimulation

Adenosine outflow and adenosine and adenine nucleotide content of hippocampal slices were evaluated under two different experimental conditions: ischemia-like conditions and electrical stimulation (10 Hz). Five minutes of ischemia-like conditions brought about an 8-fold increase in adenosine outflow in the following 5 min during reperfusion, and a 2-fold increase in adenosine content, a 43% decrease in ATP, a 72% increase in AMP and a 30% decrease in energy charge (E.C.) at the end of the ischemic period. After 10 min of reperfusion ATP, AMP and E.C. returned to control values, while the adenosine content was further increased. Five minutes of electrical stimulation brought about an 8-fold increase in adenosine outflow that peaked 5 min after the end of stimulation, a 4-fold increase in adenosine content and an 18% decrease in tissue E.C. at the end of stimulation. After 10 min of rest conditions the adenosine content and E.C. returned to basal values. The origin of extracellular adenosine from S-adenosylhomocysteine (SAH) was examined under the two different experimental conditions. The SAH hydrolase inhibitor, adenosine-2,3-dialdehyde (10 microM), does not significantly modify the adenosine outflow evoked by electrical stimulation or ischemia-like conditions. This finding excludes a significant contribution by the transmethylation pathway to adenosine extracellular accumulation evoked by an electrical or ischemic stimulus, and confirms that the most likely source of adenosine is from AMP dephosphorylation.

The source of brain adenosine outflow during ischemia and electrical stimulation

Adenosine outflow and adenosine and adenine nucleotide content of hippocampal slices were evaluated under two different experimental conditions: ischemia-like conditions and electrical stimulation (10 Hz). Five minutes of ischemia-like conditions brought about an 8-fold increase in adenosine outflow in the following 5 min during reperfusion, and a 2-fold increase in adenosine content, a 43% decrease in ATP, a 72% increase in AMP and a 30% decrease in energy charge (EC) at the end of the ischemic period. After 10 min of reperfusion ATP, AMP and EC returned to control values, while the adenosine content was further increased. Five minutes of electrical stimulation brought about an 8-fold increase in adenosine outflow that peaked 5 min after the end of stimulation, a 4-fold increase in adenosine content and an 18% decrease in tissue EC at the end of stimulation. After 10 min of rest conditions the adenosine content and EC returned to basal values. The origin of extracellular adenosine from S-adenosylhomocysteine (SAH) was examined under the two different experimental conditions. The SAH hydrolase inhibitor, adenosine-2,3-dialdehyde (10 microM), does not significantly modify the adenosine outflow evoked by electrical stimulation or ischemia-like conditions. This finding excludes a significant contribution by the transmethylation pathway to adenosine extracellular accumulation evoked by an electrical or ischemic stimulus, and confirms that the most likely source of adenosine is from AMP dephosphorylation.

Pharmacokinetic studies of vitamin D analogues: relationship to vitamin D binding protein (DBP)

Vitamin D(3), 25-hydroxyvitamin D(3) (25OHD(3)) and 1α,25-dihydroxyvitamin D(3) (1α,25(OH)(2)D(3)) bind to the vitamin D binding protein (DBP) in the serum. During the development of synthetic vitamin D analogues, it has been shown that the majority of analogues bind to DBP with a low affinity. This modifies their biological activitiesin vitro compared to 1α,25(OH)(2)D(3), since binding to DBP decreases the cellular uptake and access to the vitamin D receptor. It is therefore important to elucidate the possible role played by the binding or lack of binding to DBPin vivo. We have investigated the relationship between the binding affinity for human DBP and the serum level and serum half-life (t(1/2)) in rats of a series of new vitamin D analogues. The binding affinity for DBP was determined by displacement of(3)H-1,25(OH)(2)D(3) from DBP attached to Affi-Gel 10. The serum levels in rats following a single intravenous dose were assessed by HPLC and the serum half-life was determined for each analogue. In the group of vitamin D analogues which showed a low or no affinity for DBP, we have identified compounds with a short t(1/2) and compounds with a long t(1/2), all characterized by low initial serum levels. Compounds with a long t(1/2) were also found in the group with a high affinity for DBP, and they were easily identifiable by their high initial serum level. These results showed that the initial serum level of vitamin D analogues correlated with the affinity for DBP, but that there seemed to be no correlation with the metabolic rate as reflected by measurement of the serum half-life of the analogues.

In vivo regulation of extracellular adenosine levels in the cerebral cortex by NMDA and muscarinic receptors

The adenosine concentration in samples of perfusate was determined 24 h after implantation of microdialysis fibre in the cortex. High performance liquid chromatography coupled with a fluorometric detector was used. K+ (100 mM) depolarization was followed by a 2- to 4-fold increase in adenosine efflux. The addition of tetrodotoxin (1 microM) to the perfusate was followed by a decrease in spontaneous and K(+)-evoked adenosine efflux. The increase induced by high K+ was markedly inhibited by the NMDA receptor antagonist, D(-)-2-amino-7-phosphonoheptanoic acid (1 mM, D-AP7), but not by the muscarinic receptor antagonist, atropine (1.5 microM). The acetylcholine esterase inhibitor, physostigmine (7 microM), and the muscarinic receptor agonist, oxotremorine (100 microM), significantly enhanced the K(+)-evoked increase in adenosine. The spontaneous efflux of adenosine was not modified by any of the drugs tested. A neurotoxic lesion of the cholinergic pathway innervating the cortex, although inducing a marked decrease in cortical choline acetyltransferase activity, did not significantly modify the cortical adenosine efflux. It is concluded that, under K(+)-depolarizing conditions, adenosine efflux is triggered by excitatory amino acids and enhanced by muscarinic activation.

Effect of idebenone on adenosine outflow and adenine nucleotide level in hippocampal slices under ischemia-like conditions

The effect of idebenone on the changes in adenosine and nucleotide metabolism occurring in hippocampal slices after ischemia-like conditions (superfusion with glucose-free Krebs solution gassed with 95% N2-5% CO2) and during reperfusion with normal Krebs solution was investigated by measuring adenosine and inosine outflow, and adenosine and adenine nucleotide levels by HPLC. Five minutes of ischemia-like conditions brought about an 8- and 4-fold increase in adenosine and inosine outflow 10 min after reperfusion and a 75% increase in the tissue level of adenosine, a 40% decrease in ATP, and a 50% increase in AMP at the end of the ischemic period. Ten minutes after reperfusion, ATP and AMP returned to control values. Idebenone (25-100 microM) brought about a concentration-dependent increase in adenosine and inosine outflow evoked by ischemia-like conditions. Idebenone (50 microM) also increased the adenosine content in hippocampal slices after both ischemia (+150%) and reperfusion (+320%). An 82% increase in ADP, 174% in AMP, and 56% in the total sum of nucleotides, 10 min after reperfusion were found in idebenone treated slices. These results suggest that idebenone enhances adenosine formation after ischemia-like conditions from sources other than AMP, and improves phosphorylating activity during reperfusion. Idebenone, by increasing adenosine and total nucleotide levels, may protect brain tissue from ischemic damage.

Investigations into the adenosine outflow from hippocampal slices evoked by ischemia-like conditions

The characteristics of adenosine and inosine outflow evoked by 5 min of ischemia-like conditions in vitro (superfusion with glucose-free Krebs solution gassed with 95% N2/5% CO2) were investigated on rat hippocampal slices. The viability of the slices after "ischemia" was evaluated by extracellular recording of the evoked synaptic responses in the CA1 region. The evoked dendritic field potentials were abolished after 5 min of superfusion under "ischemia" but a complete recovery occurred after 5 min of reperfusion with normal oxygenated Krebs solution. No recovery took place after 10 min of "ischemia." The addition of the adenosine A1 receptor antagonist 8-phenyltheophylline to the superfusate antagonized the depression of the evoked field potentials caused by 5 min of "ischemia." Five minutes of "ischemia" brought about a six- and fivefold increase in adenosine and inosine outflow, respectively, within 10 min. Tetrodotoxin reduced the outflow of adenosine and inosine by 42 and 33%, respectively, whereas the removal of Ca2+ caused a further increase. The NMDA receptor antagonist D(-)-2-amino-7-phosphonoheptanoic acid and the non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione brought about small, not statistically significant decreases of adenosine and inosine outflow. The glutamate uptake inhibitor dihydrokainate did not affect the outflow of adenosine and inosine. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP did not affect basal adenosine outflow but potentiated "ischemia"-evoked adenosine outflow. It is concluded that ischemia-like conditions in vitro evoke a Ca(2+)-independent adenosine and inosine outflow, through a mechanism that partly depends on propagated nervous activity but does not involve excitatory amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)

20-epi-vitamin D3 analogues: a novel class of potent regulators of cell growth and immune responses

The 20-epi-vitamin D3 analogues are a novel class of vitamin D3 derivatives, structurally related to 1 alpha,25-dihydroxycholecalciferol (1 alpha,25(OH)2D3). They are characterized by an altered stereochemistry at carbon 20 in the side-chain. In vitro, these new analogues were found to be considerably more potent as regulators of growth and differentiation in the human histiocytic lymphoma cell line U 937 than 1 alpha,25(OH)2D3, despite a practically unchanged calcemic activity in vivo. The most potent analogue, KH 1060, inhibited cell proliferation by 50% at 10(-12) M (14,000 times more active than 1 alpha,25(OH)2D3). At the same time, KH 1060 induced cell differentiation at concentrations as low as 10(-14)M. In addition, the 20-epi-vitamin D3 analogues were found to be very potent inhibitors of T-lymphocyte proliferation induced by interleukin-1 or alloantigen. In this respect, they were several orders of magnitude more active than the potent immunosuppressive agent cyclosporin A (CyA). KH 1060, the most potent analogue, inhibited interleukin-1-induced mouse thymocyte proliferation by 50% at 3 x 10(-16) M and allogeneic stimulation of mouse spleen lymphocytes at 5 x 10(15) M. These effects were considered to be mediated by inhibition of interleukin-2 release from activated T-lymphocytes. The new analogues are of potential interest in the prevention of graft rejection and in the treatment of psoriasis, cancer and auto-immune diseases.

Numerous proteins phosphorylated on tyrosine and enhanced tyrosine kinase activities in vanadate-treated NIH 3T3 fibroblasts

A monoclonal antibody that can immunoprecipitate proteins containing phosphotyrosine has been isolated and characterized. To identify proteins that can act as substrates for tyrosine kinases in intact cells, extracts of phosphate-labeled NIH cells that had been treated with the phosphotyrosyl phosphatase inhibitor, vanadate, were precipitated with the antibody, and the immunoprecipitates were analyzed by two-dimensional gel electrophoresis. Numerous proteins were specifically precipitated from vanadate-treated NIH 3T3 cells by the antibody. The high level of phosphotyrosine detected in vanadate-treated cells is presumably primarily due to phosphatase inhibition, but approx. 2-fold increased tyrosine kinase activities were also detected in extracts of the cells after treatment with vanadate. The enhanced tyrosine kinase activity may contribute to the generation of the transformed phenotype seen in response to treatment with vanadate.