Library

Notes: Abstract: A μ-selective opiate receptor agonist, sufentanil, can either increase or decrease the stimulated formation of cyclic AMP (cAMP) in the myenteric plexus. The direction of the opioid modulation of this second messenger depends on the concentration of opioid used. Low doses of opioid enhance, whereas higher concentrations inhibit, the magnitude of cAMP that is formed in response to electrical stimulation. Opioids exert this dual regulation on only stimulated cAMP formation. Basal levels are not affected. Opioid facilitation and inhibition of stimulated cAMP formation are blocked by naloxone, indicating mediation by opiate receptors. Because all experiments were conducted in the presence of a phosphodiesterase inhibitor, it is highly unlikely that opioid regulation of stimulated cAMP formation is due to changes in the rate of its degradation. Positive and negative coupling of μ-opiate receptors to adenylyl cyclase is the most plausible explanation for the bimodal opioid effects on cAMP content. The marked parallel between the current observations and the previously reported bimodal opioid regulation of evoked enkephalin release is consistent with the hypothesis that adenylyl cyclase is one biochemical substrate for the bimodal opiate receptor-coupled regulatory mechanism governing the stimulated release of this opioid peptide.

Notes: Abstract: This laboratory has previously demonstrated that the μ-selective opiate receptor agonist sufentanil can produce a naloxone-reversible increase or decrease in the stimulated formation of cyclic AMP (cAMP) in the myenteric plexus, depending on the concentration of opioid used. On the basis of these results, it was suggested that μ-opiate receptors are positively as well as negatively coupled to adenylyl cyclase. In the present study, the effect of chronic morphine exposure, in vivo, on the magnitude of electrically stimulated formation of cAMP and its modulation by sufentanil was investigated. In chronic morphine-treated preparations, the magnitude of electrically stimulated cAMP formation, while in the presence of an inhibitory (10−6M) concentration of sufentanil, is indistinguishable from the formation that occurs in opiate-naive preparations (in the absence of exogenous opioid). This indicates that the negative modulation of stimulated enteric cAMP formation by sufentanil manifests tolerance. Paradoxically, however, in “addicted tissue” the magnitude of the increase in cAMP formation produced by electrical stimulation in the presence of a previously inhibitory concentration of sufentanil is significantly larger than in its absence. Thus, the equivalence between the magnitude of stimulation-induced increase in cAMP formation observed in naive versus tolerant/dependent tissue, while in the presence of sufentanil, is due to the ability of an originally inhibitory concentration of opioid to enhance or facilitate stimulated formation of cAMP. It is suggested that tolerance/dependence to the opioid inhibition of stimulated cAMP formation results not only from the loss of inhibitory potency but also from its reversal to enhancement.

Notes: We study the effects of a few types of atomic disorder on the electronic and optical properties of AlAs/GaAs (001) and (111) superlattices: (i) atomic intermixing across the interfaces; (ii) replacing a single monolayer in a superlattice by one containing the opposite atomic type (isoelectronic δ doping); and (iii) random layer-thickness fluctuations in superlattices (SL). Type (i) is an example of lateral disorder, while types (ii) and (iii) are examples of vertical disorder. Using three-dimensional empirical pseudopotential theory and a plane-wave basis, we calculate the band gaps, electronic wave functions, and optical matrix elements for systems containing up to 2000 atoms in the computational unit cell. Spin-orbit interactions are omitted. Computationally much less costly effective-mass calculations are used to evaluate the density of states and eigenstates away from the band edges in vertically disordered SLs. Our main findings are: (i) Chemical intermixing across the interface can significantly shift the SL energy levels and even change the identity (e.g., symmetry) of the conduction-band minimum in AlAs/GaAs SLs; (ii) any amount of thickness fluctuations in SLs leads to band-edge wave-function localization; (iii) these fluctuation-induced bound states will emit photons at energies below the "intrinsic'' absorption edge (red shift of photoluminescence); (iv) monolayer fluctuations in thick superlattices create a gap level whose energy is pinned at the value produced by a single δ layer with "wrong'' thickness; (v) (001) AlAs/GaAs SLs with monolayer thickness fluctuations have a direct band gap, while the ideal (001) superlattices are indirect for n〈4; (vi) there is no mobility edge for vertical transport in a disordered superlattice, because all the states are localized; however, the density of states retains some of the features of the ordered-superlattice counterpart. We find quantitative agreement with experiments on intentionally disordered SLs [A. Sasaki, J. Cryst. Growth 115, 490 (1991)], explaining the strong intensity and large red shift of the photoluminescence in the latter system. We provide predictions for the case of unintentional disorder. © 1995 American Institute of Physics.

Notes: This article reports the study on enhancement of copper phthalocyanine (CuPc) for hole injection in blue polymer light-emitting diodes based on three blue emissive polymeric materials with different electronic structures under the circumstances of including or excluding an additional poly(N-vinylcarbazole) (PVK) layer. A thin CuPc layer may effectively enhance the hole injection from the anode to the emissive polymer layer and results in a dramatic decrease of operating voltage of the device. The thickness of the CuPc layer is optimized. The addition of a PVK layer between CuPc and emissive polymer may prevent the potential problem of the undesired electron flow from the emissive layer to the CuPc layer. The efficiency of CuPc for enhancing hole injection is almost independent of the energy levels of the highest occupied molecular orbitals of the polymers which are contacted with it. It is suggested that the higher affinity between the two organic materials in comparison with that between indium–tin–oxide and an organic material and the resulted rough interface play a dominated role in the enhancement for hole injection. © 2001 American Institute of Physics.

Notes: The electronic structure of a GaAsN alloy is calculated using a 4096 atom supercell, with a 70 Ry plane wave basis cutoff and Ga atom 3d electrons as valence electrons. The charge density of this supercell is generated by patching the charge density of a small unit cell with the charge density of bulk GaAs. The local-density-approximation band gap error is corrected by modifying the nonlocal pseudopotentials. A localized nitrogen state [a1(N)] is obtained,and it plays an important role in the band gap reduction of GaAsN. © 2001 American Institute of Physics.