bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electronic structure analysis of (In, Ga, Al) N heterostructures on the nanometre scale using EELS
Authors: LAKNER H. , RAFFERTY B. , BROCKT G. 
Published: March 2003

In this work the local electronic structure of MOVPE‐grown (In, Ga, Al) N heterostructures has been investigated by electron energy loss spectroscopy (EELS). The cold field‐emission scanning transmission electron microscope (VG HB501) used was equipped with a dedicated parallel EELS system which provides high dispersions at an energy resolution of 0.35 eV with the use of subnanometre electron probes the spatial resolution of the measurements depends on the physical localization of the scattering process itself and thus is in the order of nanometres.The low‐loss region of the energy spectra gives information on plasmon excitations and transitions across the bandgap. The main problem on looking at the bandgap region of EELS spectra is to separate the bandgap signal from the fading tail of the zero‐loss peak. High energy resolution and application of suitable deconvolution routines for removal of the zero‐loss peak extract improved information from this energy region.Thus the EEL spectra of different group III nitrides reveal the onset of the bandgap itself and the characteristic shape of the joint density of states. From these results the local optical properties can be deduced via a Kramers–Kronig transformation.The data obtained show detailed structure on the energy scale and are in excellent agreement with optical ellipsometric results. In comparison with these techniques EELS methods yield a superior spatial resolution of better than 10 nm which enables detailed investigation of the effect of local defects and boundaries on the optical properties.
Three‐dimensional analysis of light propagation through uncoated near‐field fibre probes
Authors: Müller R. , Lienau C. 
Published: March 2003

The near‐field emission from uncoated tapered fibre probes is investigated for different probe geometries. The three‐dimensional model calculations are based on Maxwell's curl equations and describe the propagation of a 10 fs optical pulse (λ = 805 nm) through tapers of different lengths and different diameters of the taper exit. The numerical evaluation is done with a finite difference time domain code. Two tapers with cone angles of 50°, with taper lengths of 1.5 µm and 1.0 µm and exit diameters of 100 nm and 520 nm, respectively, are considered. We find that without sample the short taper with large exit diameter optimizes both light transmission and spatial resolution. In the presence of a sample with a high dielectric constant, however, the spatial near‐field distribution changes drastically for both taper geometries. We find a pronounced increase in spatial resolution, down to about 250 nm inside the medium. This collimation of the near‐field distribution arises from interferences between emitted and reflected light from the sample surface and from a collimation effect that the field experiences in the high‐index semiconductor material. The combination of high spatial resolution and transmission and collection efficiencies makes such probes interesting for spectroscopic investigations, as demonstrated by recent experiments.
Comparison of the axial resolution of practical Nipkow‐disk confocal fluorescence microscopy with that of multifocal multiphoton microscopy: theory and experiment
Authors: Egner A. , Andresen V. , Hell S. W. 
Published: March 2003

SummaryWe compare the axial sectioning capability of multifocal confocal and multifocal multiphoton microscopy in theory and in experiment, with particular emphasis on the background arising from the cross‐talk between adjacent imaging channels. We demonstrate that a time‐multiplexed non‐linear excitation microscope exhibits significantly less background and therefore a superior axial resolution as compared to a multifocal single‐photon confocal system. The background becomes irrelevant for thin (< 15 µm) and sparse fluorescent samples, in which case the confocal parallelized system exhibits similar or slightly better sectioning behaviour due to its shorter excitation wavelength. Theoretical and experimental axial responses of practically implemented microscopes are given.
Chemical reactions and morphological stability at the Cu/Al2O3 interface
Authors: Scheu C. , Klein S. , Tomsia A. P. , Rühle M. 
Published: March 2003

SummaryThe microstructures of diffusion‐bonded Cu/(0001)Al2O3 bicrystals annealed at 1000 °C at oxygen partial pressures of 0.02 or 32 Pa have been studied with various microscopy techniques ranging from optical microscopy to high‐resolution transmission electron microscopy. The studies revealed that for both oxygen partial pressures a 20–35 nm thick interfacial CuAlO2 layer formed, which crystallises in the rhombohedral structure. However, the CuAlO2 layer is not continuous, but interrupted by many pores. In the samples annealed in the higher oxygen partial pressure an additional reaction phase with a needle‐like structure was observed. The needles are several millimetres long, ∼10 µm wide and ∼1 µm thick. They consist of CuAlO2 with alternating rhombohedral and hexagonal structures. Solid‐state contact angle measurements were performed to derive values for the work of adhesion. The results show that the adhesion is twice as good for the annealed specimen compared to the as‐bonded sample.
In vivo three‐dimensional imaging of plants with optical coherence microscopy
Authors: Reeves A. , Parsons R. L. , Hettinger J. W. , Medford J. I. 
Published: March 2003

SummaryAchieving the ability to non‐destructively, non‐invasively examine subsurface features of living multicellular organisms at a microscopic level is currently a challenge for biologists. Optical coherence microscopy (OCM) is a new photonics‐based technology that can be used to address this challenge. OCM takes advantage of refractive properties of biological molecules to generate three‐dimensional images that can be viewed with a computer. We describe new data processing techniques and a different visualization algorithm that substantially improve OCM images. We have applied OCM imaging, in conjunction with these improvements, to a variety of structures of plants, including leaves, flowers, ovules and germinating seeds, and describe the visualization of cellular and subcellular structures within intact plants. We present evidence, based on detailed examination of our OCM images, comparisons to classical plant anatomy studies, and current knowledge of light scattering by cells and their components, that we can distinguish nuclei, organelles and vacuoles. Detailed examination of vascular tissue, which contains cells with elaborate wall structure, shows that cell walls produce no significant OCM signal. These improvements to the visualization process, together with the powerful non‐invasive, non‐destructive aspects of the technology, will broaden the application of OCM to questions in studies of plants as well as animals.
A prism combination for near isotropic fluorescence excitation by total internal reflection
Authors: Wakelin S. , Bagshaw C. R. 
Published: March 2003

SummaryTotal internal reflection fluorescence (TIRF) microscopy is finding increasing application for selectively detecting molecules at or near a glass–water surface. As with all fluorescence methods, the efficiency of excitation of a fluorophore is potentially sensitive to the polarization state of the source. In TIRF, s‐polarized excitation produces an evanescent field that is perpendicular to the incident plane (ydirection), whereas p‐polarized light generates a more complex pattern but one dominated by a field that is vertical to the surface (zdirection). Thus, fluorophores whose absorption dipoles are fixed in thexdirection are not favourably aligned for excitation. Here we describe a beam‐splitting prism arrangement that allows excitation by two orthogonal beams, thus giving isotropic excitation in thex–yplane with s‐polarized light. With linearly polarized light at the magic angle, near isotropic excitation in three dimensions should be achieved. This prism design should find application in polarized fluorescence microscopy to investigate the rotational motions of macromolecules or to minimize flickering of fluorescence emission arising from molecular rotations in single molecule studies.
Hypervascularization in the Magnocellular Nuclei of the Rat Hypothalamus: Relationship with the Distribution of Aquaporin‐4 and Markers of Energy Metabolism
Authors: Badaut J. , Nehlig A. , Verbavatz J.‐M. , Stoeckel M.‐E. , Freund‐Mercier M.‐J. , Lasbennes F. 
Published: March 2003

AbstractIn the magnocellular nuclei of the hypothalamus, there is a rich vascular network for which the function remains to be established. In the supraoptic nucleus, the high vascular density may be one element, which together with the water channel aquaporin‐4 expressed in the astrocytes, is related to a role in osmoreception. We tested the osmoreception hypothesis by studying the correlation between vascular and cellular densities in the paraventricular nucleus and the supraoptic nucleus. Whether aquaporin‐4 is likely to contribute to osmoreception was tested by studying the distribution in the magnocellular nuclei of the hypothalamus. The high vascular density may also reflect a high metabolic activity due to the synthesis of vasopressin and oxytocin. This metabolic hypothesis was tested by studying the regional cytochrome oxidase histochemistry, the local cerebral blood flow, and the density of glucose transporter type‐1 in the supraoptic and paraventricular nuclei. All the magnocellular nuclei were characterized by an extended and intense aquaporin‐4 labelling and a weak cytochrome oxidase histochemistry. The highest vascular density was found in the supraoptic nucleus and the magnocellular regions of the paraventricular nucleus. The local cerebral blood flow rates were surprisingly low in the paraventricular nucleus and the supraoptic nucleus in comparison to the cerebral cortex. Furthermore in these nuclei, the antibody for glucose transporter type‐1 revealed two populations of vessels differing by their labelling intensity. The similarities observed between the different nuclei suggest that, in the hypothalamus, all magnocellular regions sense the plasma osmolarity. The low local cerebral blood flow, and the patterns of glucose transporter type‐1 labelling and cytochrome oxidase histochemistry suggest that the high vascularization of these hypothalamic nuclei is not related to a high metabolic capacity in basal conditions.
Agouti Related Protein in the Rat Adrenal Cortex: Implications for Novel Autocrine Mechanisms Modulating the Actions of Pro‐Opiomelanocortin Peptides
Authors: Bicknell A. B. , Lomthaisong K. , Gladwell R. T. , Lowry P. J. 
Published: March 2003

AbstractAgouti related protein (AgRP) is a recently discovered melanocortin receptors (MCR) antagonist implicated in the control of feeding behaviour. Expression of AgRP has been shown to be localized by in situ hybridization to the arcuate nucleus and median eminence of the brain, where it acts as an antagonist to the MC3 and MC4 receptors, while in the periphery the only significant expression was located in the adrenal medulla. As AgRP is only a weak antagonist of the MC2 and MC5 receptors, which are expressed principally by adipocytes and in the adrenal cortex, the question arizes as to the function of peripheral AgRP. In this study, we investigated the expression of AgRP in the rat adrenal and suggest that it is expressed in the adrenal cortex and not as previously described in the medulla. We also show that AgRP mRNA expression is upregulated in the adrenal during fasting and in the contralateral gland following unilateral adrenalectomy but not during chronic stress. These results indicate an as yet undefined role for AgRP in the periphery and are supportive of the suggestion that a further melanocortin receptor exists.
Progestin Regulation of Galanin and Prolactin Gene Expression in Oestrogen‐Induced Pituitary Tumours
Authors: Piroli G. G. , Cassataro J. , Pietranera L. , Grillo C. A. , Ferrini M. , Lux‐Lantos V. , De Nicola A. F. 
Published: March 2003

AbstractGalanin is a peptide widely distributed in the hypothalamic‐pituitary axis. In the female rat pituitary, galanin is mainly present in lactotrophs, where it regulates their secretion and proliferation. Galanin expression is increased in oestrogen‐induced prolactinomas, and it has been proposed that oestrogen effects on lactotroph function and proliferation could be mediated by galanin. Previous studies from our laboratory demonstrated that the synthetic progestin levonorgestrel antagonizes pituitary tumorigenesis of rats given oestrogen, reducing the number of proliferating cells and increasing cell death by nonapoptotic mechanism(s). To elucidate the role of galanin in levonorgestrel effects on the tumours, we examined galanin and prolactin mRNA and peptide expression in prolactinomas of rats receiving the progestin. Levonorgestrel reduced the pituitary weight and serum prolactin concentrations in oestrogen‐treated rats. Galanin mRNA expression (determined by in situ hybridization), and the number of galanin expressing cells (determined by immunocytochemistry) were also reduced by the progestin in tumour‐bearing rats. However, neither prolactin mRNA content, nor the number of prolactin‐expressing cells, were modified by levonorgestrel treatment of oestrogen‐receiving rats. The present study suggests that levonorgestrel controls pituitary growth by diminishing galanin expression. In contrast, changes in serum prolactin concentration seem to be more related to the reduction in tumour size, since the reduction in galanin expression was not large enough to regulate prolactin mRNA expression or the percentage of lactotrophs.
5α‐Reductase Type 2 and Androgen Receptor Expression in Gonadotropin Releasing Hormone GT1‐1 Cells
Authors: Poletti A. , Rampoldi A. , Piccioni F. , Volpi S. , Simeoni S. , Zanisi M. , Martini L. 
Published: March 2003

AbstractGonadal steroids are potent modulators of gonadotropin releasing hormone (GnRH) secretion, and androgen binding sites and 5α‐reductase activity have been found in the immortalized GnRH secreting cell line GT1‐1, suggesting the existence of a direct androgenic control of GnRH dynamics. Two isoforms of the 5α‐reductase have been cloned with very different biochemical/functional properties: 5α‐reductase type 1 (widely distributed in the body) and 5α‐reductase type 2 (confined in androgen target structures). We have analysed whether, in GT1‐1, androgen binding sites are linked to ‘classical’ androgen receptor, and which 5α‐reductase isoform is active. Reverse transcriptase‐polymerase chain reaction analysis showed that the mRNAs coding for androgen receptor and for the two 5α‐reductase isoforms are all expressed in GT1‐1 cells. However, the 5α‐reductase enzymatic reaction showed a peak of activity at a narrow pH around 5.5, the optimum for the 5α‐reductase type 2. The affinity for testosterone, of the enzyme present in GT1‐1 cells, was very similar to that observed for the recombinant type 2 isozyme expressed in yeasts. The data indicate that GT1‐1 cells (i) express a ‘classical’ androgen receptor and (ii) contain the 5α‐reductase type 2 isoform, a specific marker of androgen‐responsiveness.
Neuropeptide‐Y: A Possible Mediator of Prolactin‐Induced Feeding and Regulator of Energy Balance in the Ring Dove (Streptopelia risoria)
Authors: Strader A. D. , Buntin J. D. 
Published: March 2003

AbstractAlthough neuropeptide‐Y (NPY) has been widely reported to be a potent stimulator of feeding activity and regulator of energy homeostasis, most of the supportive evidence for such effects has been gathered in mammalian species. This study characterized the orexigenic potency of NPY in an avian species, the ring dove, and measured changes in hypothalamic NPY‐immunoreactive (NPY‐ir) cell numbers in response to energy state fluctuations or intracranial administration of the potent orexigenic hormone prolactin. Food intake was significantly elevated in male doves at 1 h after intracerebroventricular (i.c.v.) injection of 0.25 and 0.5 µg NPY but not after injection of a higher dose (1.0 µg). In time course studies, food intake was increased at 1 h after i.c.v. injection of 0.5 µg NPY but was not elevated at 2, 3, or 4 h. The number of NPY‐ir cell bodies in the infundibular region of the dove hypothalamus increased two to four‐fold following acute food deprivation, chronic food restriction, or repeated i.c.v. injections of prolactin. No additive effects were observed when food restriction and prolactin treatment were combined. These findings suggest that NPY is involved in energy homeostasis in doves and are consistent with the hypothesis that prolactin‐induced hyperphagia is mediated in part by NPY.
Skeletal Growth Acceleration with Growth Hormone Secretagogues in Transgenic Growth Retarded Rats: Pattern‐Dependent Effects and Mechanisms of Desensitization
Authors: Wells T. , Houston P. A. 
Published: March 2003

AbstractThe transgenic growth retarded (Tgr) rat is the first genetic model of growth hormone (GH) deficiency whose growth can be accelerated with exogenous GH secretagogues (GHSs). In this study, we have demonstrated that GHS‐receptor (GHS‐R) mRNA expression in the arcuate nucleus of Tgr rats was not significantly different to that in wild‐type littermates. We have confirmed that GHS‐induced elevation in body weight gain was accompanied by acceleration of skeletal growth, and that the effects of the GHS, GHRP‐6, were both dose‐ and pattern‐dependent. The growth response with continuous infusion of GHRP‐6 was transient, accompanied by suppression of GH and corticosterone responses to bolus injection of GHRP‐6. This desensitization occurred without downregulation of arcuate GHS‐R mRNA expression, but was accompanied by elevated periventricular somatostatin mRNA expression. In contrast, pulsatile (3‐hourly) infusion of GHRP‐6 produced sustained growth and GH responses, which were accompanied by suppression of corticosterone responses and elevated arcuate GH‐releasing factor (GRF) mRNA expression. Skeletal growth was further accelerated by coinfusion of GRF, but significant depletion of pituitary GH stores suggested that this growth rate may not be sustainable. These experiments confirm the importance of the Tgr rat for investigating the growth promoting potential of the GHSs in the context of GH‐deficient dwarfism, and suggest that elevated somatostatin expression may mediate the suppression of the GRF‐GH and hypothalamo‐pituitary‐adrenal axes following continuous GHRP‐6 treatment.
Changes in Rat Serum Corticosterone After Treatment with Metabotropic Glutamate Receptor Agonists or Antagonists
Authors: Johnson M. P. , Kelly G. , Chamberlain M. 
Published: March 2003

AbstractFrom previous work, it appears that glutamate can activate the hypothalamic‐pituitary‐adrenocortical (HPA) axis by an interaction at either ionotopic or metabotropic (G‐protein coupled) receptors. For example, (1S,3R)‐1‐aminocyclopentane‐1,3‐dicarboxylate (ACPD), a metabotropic glutamate (mGlu) receptor agonist, has been shown to increase the levels of serum corticosterone in rats. The present study was undertaken to further characterize which of the mGlu receptors are substantially involved in control of the HPA axis. The group I mGlu receptor agonists, 3,5‐dihydroxyphenylglycine (DHPG), 1S,3R‐ACPD, and 2‐chloro‐5‐hydroxyphenylglycine (CHPG) but not the inactive isomer 1R,3S‐ACPD were found to dose‐dependently increase serum corticosterone 1 h after intracerebroventricular (i.c.v.) injection in male rats. The relative potency, DHPG (EC50 = 520 nmol) > 1S,3R‐ACPD (1.4 µmol) = CHPG (2.7 µmol) ≫ 1R,3S‐ACPD (≫ 3 µmol) is consistent with activation of group I (mGlu1/5) receptors. The effects of DHPG were long lasting with substantial elevations in corticosterone remaining for at least 3 h. In a similar manner, the group III mGlu receptor agonists, L‐AP4 (4‐phosphono‐2‐aminobutyric acid) and L‐SOP (serine‐O‐phosphate), were found to increase serum corticosterone levels at 1 h. In contrast, the mGlu group II selective agonists LY354740 (10 mg/kg, i.p.) and subtype‐selective doses of the group II antagonist LY341495 (1 mg/kg, i.p.) did not significantly elevate serum corticosterone. Given the group I agonists results, it was surprising to find that group I selective and mGlu1 selective antagonists given alone also increased serum corticosterone. As with the agonists, the rise in serum corticosterone with LY393675 (an mGlu1/5 antagonist, EC50 = 20 nmol, i.c.v.) and LY367385 (an mGlu1 antagonist, 325 nmol, i.c.v.) were dose‐dependent and consistent with their relative affinity for the group I mGlu receptors. The selective mGlu5 antagonist MPEP [2‐methyl‐6‐(phenylethylnyl)pyridine] increased serum cortocosterone but only at high doses (> 30 mg/kg, i.p.). A model involving the high glutamatergic tone on GABAergic interneurons in the paraventricular nucleus of the hypothalamus is discussed as a possible explanation for these results.
Alterations in Hypothalamic Insulin‐Like Growth Factor‐I and its Associations with Gonadotropin Releasing Hormone Neurones During Reproductive Development and Ageing
Authors: Miller B. H. , Gore A. C. 
Published: March 2003

AbstractInsulin‐like growth factor‐I (IGF‐I) is thought to play a role in the onset of reproductive ability at puberty and the control of reproductive function throughout adult life. It is believed that these effects are mediated at least in part by the activation of gonadotropin releasing hormone (GnRH) neurones by IGF‐I, but the interactions of IGF‐I with GnRH neurones in vivo are largely unknown. We first examined the anatomical relationship between GnRH and IGF‐I cells in neuroendocrine regions. Using double‐label immunocytochemistry, we observed that in the preoptic area‐anterior hypothalamus (POA‐AH), the site of GnRH perikarya, the majority (78%) of GnRH cell bodies expressed IGF‐I immunoreactivity. IGF‐I immunoreactivity was also high in the median eminence, the site of GnRH release, and GnRH neuroterminals were seen to interweave among IGF‐I‐immunopositive cells. Due to this substantial overlap of GnRH and IGF‐I immunoreactive elements, we then tested the hypothesis that changes in IGF‐I may regulate the GnRH system. Animals were examined at the two important reproductive life transitions: puberty and reproductive senescence. IGF‐I mRNA levels were measured in POA‐AH and medial basal hypothalamus‐median eminence (MBH‐ME) and effects of IGF‐I treatment on GnRH mRNA levels were quantified by RNase protection assay. Although IGF‐I treatment did not alter GnRH gene expression, there were significant alterations in hypothalamic IGF‐I gene expression at both puberty and reproductive senescence. During puberty, IGF‐I mRNA levels in the MBH‐ME of rats increased from the juvenile stage (P25) to the day of vaginal opening (P35), and from the day of vaginal opening to young adulthood (P45) in the POA‐AH. During reproductive ageing, IGF‐I mRNA levels were significantly lower in middle‐aged than young rats, particularly in the MBH‐ME. At all ages, IGF‐I expression was greater in the MBH‐ME than in the POA‐AH. These experiments demonstrate that: (i) the majority of adult GnRH neurones are immunopositive for the IGF‐I protein; (ii) hypothalamic IGF‐I levels increase at the onset of reproductive function and decrease at reproductive senescence in a regionally specific manner; and (iii) despite the presence of IGF‐I in GnRH perikarya, IGF‐I does not affect GnRH gene expression, suggesting that IGF‐I may act at the level of GnRH release rather than gene expression.