Low-frequency correlations (1/f(α) type) in paint application of metallic colors

We examine trial-to-trial variability of color coordinates in automotive coatings containing effect pigments, which are considered a reference paradigm for engineering angle-dependent color effects. We report the existence of correlations that show 1/f - Fourier spectra at low frequencies in all color coordinates. The scaling exponent was lower at near-specular conditions for lightness variations, suggesting a contribution from the deposition of metal flakes in metallic colors. However, the exponent was lower near the specular for blue-yellow variations, suggesting a contribution from chemical pigments in solid colors. These results were independent of the illuminant spectra. The methods employed are useful in the evaluation of industrial color matching among assembly parts.

HIF-1 and c-Src mediate increased glucose uptake induced by endothelin-1 and connexin43 in astrocytes

In previous work we showed that endothelin-1 (ET-1) increases the rate of glucose uptake in astrocytes, an important aspect of brain function since glucose taken up by astrocytes is used to supply the neurons with metabolic substrates. In the present work we sought to identify the signalling pathway responsible for this process in primary culture of rat astrocytes. Our results show that ET-1 promoted an increase in the transcription factor hypoxia-inducible factor-1α (HIF-1α) in astrocytes, as shown in other cell types. Furthermore, HIF-1α-siRNA experiments revealed that HIF-1α participates in the effects of ET-1 on glucose uptake and on the expression of GLUT-1, GLUT-3, type I and type II hexokinase. We previously reported that these effects of ET-1 are mediated by connexin43 (Cx43), the major gap junction protein in astrocytes. Indeed, our results show that silencing Cx43 increased HIF-1α and reduced the effect of ET-1 on HIF-1α, indicating that the effect of ET-1 on HIF-1α is mediated by Cx43. The activity of oncogenes such as c-Src can up-regulate HIF-1α. Since Cx43 interacts with c-Src, we investigated the participation of c-Src in this pathway. Interestingly, both the treatment with ET-1 and with Cx43-siRNA increased c-Src activity. In addition, when c-Src activity was inhibited neither ET-1 nor silencing Cx43 were able to up-regulate HIF-1α. In conclusion, our results suggest that ET-1 by down-regulating Cx43 activates c-Src, which in turn increases HIF-1α leading to the up-regulation of the machinery required to take up glucose in astrocytes. Cx43 expression can be reduced in response not only to ET-1 but also to various physiological and pathological stimuli. This study contributes to the identification of the signalling pathway evoked after Cx43 down-regulation that results in increased glucose uptake in astrocytes. Interestingly, this is the first evidence linking Cx43 to HIF-1, which is a master regulator of glucose metabolism.

1/f Noise in human color vision: the role of S-cone signals

We examine the functional role of S-cone signals on reaction time (RT) variability in human color vision. Stimuli were selected along red-green and blue-yellow cardinal directions and at random directions in the isoluminant plane of the color space. Trial-to-trial RT variability was not statistically independent but correlated across experimental conditions and exhibited 1/f noise spectra with an exponent close to unity in most of the cases. Regarding contrast coding, 1/f noise for random chromatic stimuli at isoluminance was similar to that for achromatic stimuli, thus suggesting that S-cone signals reduce variability of higher order color mechanisms. If we regard spatial coding, the effect of S-cone density in the retina on RT variability was investigated. The magnitude of 1/f noise at 16 min of arc (S-cone free zone) was higher than at 90 min of arc in the blue-yellow channel, and it was similar for the red-green channel. The results suggest that S-cone signals are beneficial and they modulate 1/f noise spectra at postreceptoral stages. The implications related to random multiplicative processes as a possible source of 1/f noise and the optimal information processing in color vision are discussed.

Scattering characterization of nanopigments in metallic coatings using hyperspectral optical imaging

We have determined the reflectance spectra of colored metallic coatings with high spatial resolution by using a hyperspectral imaging system. Reflectance spectra were converted to color coordinates revealing characteristic color maps in the color space. Principal-component analysis was applied to decorrelate the spatial variability of the reflectance spectra. We found that the eigenvalue spectra follow different power laws. The scaling exponent was analyzed by considering random-walk-type processes. An estimation of the Hurst exponent was done, suggesting anomalous diffusion from multiple light scattering. The results show that hyperspectral imaging combined with principal-component analysis provides a valuable method for nondestructive testing of complex turbid media.

Hyperspectral optical imaging of human iris in vivo: characteristics of reflectance spectra

We report a hyperspectral imaging system to measure the reflectance spectra of real human irises with high spatial resolution. A set of ocular prosthesis was used as the control condition. Reflectance data were decorrelated by the principal-component analysis. The main conclusion is that spectral complexity of the human iris is considerable: between 9 and 11 principal components are necessary to account for 99% of the cumulative variance in human irises. Correcting image misalignments associated with spontaneous ocular movements did not influence this result. The data also suggests a correlation between the first principal component and different levels of melanin present in the irises. It was also found that although the spectral characteristics of the first five principal components were not affected by the radial and angular position of the selected iridal areas, they affect the higher-order ones, suggesting a possible influence of the iris texture. The results show that hyperspectral imaging in the iris, together with adequate spectroscopic analyses provide more information than conventional colorimetric methods, making hyperspectral imaging suitable for the characterization of melanin and the noninvasive diagnosis of ocular diseases and iris color.

S-cone excitation ratios for reaction times to blue-yellow suprathreshold changes at isoluminance

We examined different contrast metrics to scale visual latencies for suprathreshold stimuli modulated along tritan confusion lines. S-cone increments ('blue') and decrements ('yellow') were isolated along two different tritan confusion lines, each one having a different luminance value. Reaction times (RT) were evaluated as a function of the Weber contrast and the S-cone excitation ratio between the test stimulus and the background. RTs were described using a model that generalizes Piéron's law and incorporates the notion of threshold units and power law scaling. Our results show that RTs for S-cone increments and decrements equate better when using the S-cone excitation ratio. However, a single function did not describe all RT data. S-cone RTs are better described by separate functions. We conclude that S-cone increments and decrements do not scale in the same manner. Both Weber contrast and the S-cone excitation ratio are plausible metrics at isoluminance. The implications for the S-cone pathways are discussed.

Connexin43 inhibits the oncogenic activity of c-Src in C6 glioma cells

One of the characteristics of gliomas is a decrease in the expression of connexin43, a protein that forms gap junctions. Restoring connexin43 expression in glioma cells reduces their exacerbated rate of cell growth, although it is not yet known how connexin43 modifies the expression of genes involved in cell proliferation. Here, we show that restoring connexin43 to C6 glioma cells impedes their progression from G0/G1 to the S phase of the cell cycle by reducing retinoblastoma phosphorylation and cyclin E expression through the upregulation of p21 and p27. Interestingly, connexin43 diminishes the oncogenic activity of c-Src exhibited by glioma cells. By studying a Tyr247 and Tyr265 mutant connexin43, we show that these residues are required for connexin43 to inhibit c-Src activity and cell proliferation. In conclusion, by acting as a substrate of c-Src, connexin43 reduces its oncogenic activity and decreases the rate of glioma cell proliferation, potentially an early step in the antiproliferative effects of connexin43. Although c-Src is known to phosphorylate connexin43, this study provides the first evidence that connexin43 can also inhibit c-Src activity.

Inhibition of ATP-sensitive potassium channels increases HSV-tk/GCV bystander effect in U373 human glioma cells by enhancing gap junctional intercellular communication

It is well known that the efficiency of Herpes simplex virus thymidine kinase gene/ganciclovir (HSV-tk/GCV) therapy is improved by the bystander effect, which mainly relies on gap junctional intercellular communication (GJIC). Malignant gliomas communicate poorly through gap junctions, consequently, agents with the ability to increase GJIC are good candidates to improve the efficiency of this therapy. Since we previously showed that the inhibition of ATP-sensitive potassium (KATP) channels promoted by tolbutamide increased GJIC in rat C6 glioma cells, we have investigated whether tolbutamide could increase the bystander effect in HSV-tk/GCV therapy against human glioma cells. We found that tolbutamide increased GJIC in U373 human glioma cells, an effect that was due to the up-regulation of connexin43, a protein that forms gap junctions channels. More interestingly, our results show that tolbutamide increased the efficiency of HSV-tk/GCV in co-cultures containing U373 cells and U373 cells transfected with HSV-tk. This effect was impaired in the presence of carbenoxolone, an inhibitor of GJIC. Furthermore, tolbutamide did not enhance the bystander effect in connexin43-silenced co-cultures. Together our results reveal that the inhibition of KATP channels promoted by tolbutamide enhances the bystander effect in HSV-tk/GCV therapy by increasing connexin43-mediated gap junctional intercellular communication in U373 human glioma cells.

Connexin43 is involved in the effect of endothelin-1 on astrocyte proliferation and glucose uptake

In previous studies, we showed that endothelin-1 increased astrocyte proliferation and glucose uptake. These effects were similar to those observed with other gap junction inhibitors, such as carbenoxolone (CBX). Because 24-h treatment with endothelin-1 or CBX downregulates the expression of connexin43, the main protein forming astrocytic gap junctions, which can also be involved in proliferation, in this study, we addressed the possible role of connexin43 in the effects of endothelin-1. To do so, connexin43 was silenced in astrocytes by siRNA. The knock down of connexin43 increased the rate of glucose uptake, characterized by the upregulation of GLUT-1 and type I hexokinase. Neither endothelin-1 nor CBX were able to further increase the rate of glucose uptake in connexin43-silenced astrocytes. In agreement, no effects of endothelin-1 and CBX on GLUT-1 and type I hexokinase were observed in connexin-43 silenced astrocytes or in astrocytes from connexin43 knock-out (KO) mice. Our previous studies suggested a close relationship between glucose uptake and astrocyte proliferation. Consistent with this, connexin43-silenced astrocytes exhibited an increase in Ki-67, a marker of proliferation. The effects of ET-1 on retinoblastoma phosphorylation on Ser780 and on the upregulation of cyclins D1 and D3 were affected by the levels of connexin43. In conclusion, our results indicate that connexin43 participates in the effects of endothelin-1 on glucose uptake and proliferation in astrocytes. Interestingly, although the rate of growth in connexin43 KO astrocytes has been reported to be reduced, we observed that an acute reduction in connexin43 by siRNA increased proliferation and glucose uptake.

1/falpha noise in reaction times: a proposed model based on Piéron's law and information processing

Piéron's law relates human reaction times to the intensity of a sensory stimulus by a power function. The neural processes responsible for this nonlinear behavior are not understood. A simple neural model based on the Brownian motion of spikes and information theory is presented. The model shows that Piéron's law is a transformation function in time. The shape of Piéron's law is invariant and scales into the intensity-response function of single neurons in a fractal-like process. The model also shows that Piéron's law gives rise to 1/falpha noise together with a high-frequency thermal noise limit. It is proposed that the biophysical origin of reaction time variability is related to a form of noise-induced synchronization in weakly coupled neurons. The implications in visual-motor transduction are discussed.

Linear basis for metallic and iridescent colors

I have examined the correlation structure in goniochromism by principal-component analysis. Reflectance spectra were collected in synthetic samples that reproduce metallic, nacreous, and iridescent effects under different viewing angles. Although three principal components take into account 99% of the variance, between seven and eight are needed to reach 99.99%. The results were also confirmed by analyzing each viewing condition separately. It was found that although the viewing angle does not modify the first three basis functions, it affects the higher-order ones. These angle-dependent effects can be attributed to optical interference flakes. The implications for pigment identification are discussed.

Peroxisome proliferator-activated receptor-alpha is required for the neurotrophic effect of oleic acid in neurons

Oleic acid synthesized by astrocytes behaves as a neurotrophic factor for neurons, up-regulating the molecular markers of axonal and dendritic outgrowth, growth-associated protein 43 and microtubule-associated protein 2. In this work, the nature of the receptor involved in this neurotrophic effect was investigated. As oleic acid has been reported to be a ligand and activator of the peroxisome proliferator-activated receptor (PPAR), we focus on this family of receptors. Our results show that PPARalpha, beta/delta, and gamma are expressed in neurons in culture. However, only the agonists of PPARalpha, Wy14643, GW7647 and oleoylethanolamide, promoted neuronal differentiation, while PPAR beta/delta and gamma agonists did not modify neuronal differentiation. Consequently, we investigated the involvement of PPARalpha (Nr1c1) in oleic acid-induced neuronal differentiation. Our results indicate that oleic acid activates PPARalpha in neurons. In addition, the effect of oleic acid on neuronal morphology, growth-associated protein 43 and microtubule-associated protein 2 expression decreases in neurons after PPARalpha has been silenced by small interfering RNA. Taken together, our results suggest that PPARalpha could be the receptor for oleic acid in neurons, further broadening the range of functions attributed to this family of transcription factors. Although several works have reported that PPARalpha could be involved in neuroprotection, the present work provides the first evidence suggesting a role of PPARalpha in neuronal differentiation.

Colour-luminance interactions in binocular summation

Using a noise-masking paradigm we test the notion of binocular detection mechanisms that combine luminance and colour contrast. Binocular summation was measured for achromatic and red-green isoluminant Gabor stimuli over a range of temporal frequencies and was compared with and without the presence of a two-dimensional, dynamic, luminance noise mask (correlated). While we found that luminance noise reduced binocular luminance summation at all temporal frequencies, binocular red-green summation was reduced only at frequencies of 8 Hz and above. Our results suggest the existence of binocular colour-luminance interactions restricted to high temporal frequencies.

Glucose metabolism and proliferation in glia: role of astrocytic gap junctions

Astrocytes play a well-established role in brain metabolism, being a key element in the capture of energetic compounds from the circulation and in their delivery to active neurons. Their metabolic status is affected in many pathological situations, such as gliomas, which are the most common brain tumors. This proliferative dysfunction is associated with changes in gap junctional communication, a property strongly developed in normal astrocytes studied both in vitro and in vivo. Here, we summarize and discuss the findings that have lead to the identification of a link between gap junctions, glucose uptake, and proliferation. Indeed, the inhibition of gap junctional communication is associated with an increase in glucose uptake due to a rapid change in the localization of both GLUT-1 and type I hexokinase. This effect persists due to the up-regulation of GLUT-1 and type I hexokinase and to the induction of GLUT-3 and type II hexokinase. In addition, cyclins D1 and D3 have been found to act as sensors of the inhibition of gap junctions and have been proposed to play the role of mediators in the mitogenic effect observed. Conversely, in C6 glioma cells, characterized by a low level of intercellular communication, an increase in gap junctional communication reduces glucose uptake by releasing type I and type II hexokinases from the mitochondria and decreases the exacerbated rate of proliferation due to the up-regulation of the Cdk inhibitors p21 and p27. Identification of the molecular actors involved in these pathways should allow the determination of potential therapeutic targets that could lead to the testing of alternative strategies to prevent, or at least slow down, the proliferation of glioma cells.

Tolbutamide reduces glioma cell proliferation by increasing connexin43, which promotes the up-regulation of p21 and p27 and subsequent changes in retinoblastoma phosphorylation

Our previous work has shown that tolbutamide increases gap junctional permeability in poorly coupled C6 glioma cells and that this effect is similar and additive to that found with dbcAMP, a well-known activator of gap junctional communication. Furthermore, the increase in gap junctional communication promoted by tolbutamide or dbcAMP is concurrent with the inhibition of proliferation of C6 glioma cells. In the present work, we show that tolbutamide and dbcAMP increase the synthesis of the tumor suppressor protein Cx43 and that they decrease the level of Ki-67, a protein expressed when cells are proliferating. These effects were accompanied by a reduction in the phosphorylation of pRb, mainly on Ser-795, a residue critical for the control of cell proliferation. The decrease in the phosphorylation of pRb is not likely to be mediated by a reduction in the levels of D-type cyclins, since instead of decreasing the expression of cyclins, D1 and D3 increased slightly after treatment with tolbutamide or dbcAMP. However, the Cdk inhibitors p21 and p27 were up-regulated after treatment with tolbutamide and dbcAMP, suggesting that they would be involved in the decrease in pRb phosphorylation. When Cx43 was silenced by siRNA, neither tolbutamide nor dbcAMP were able to up-regulate p21 and consequently to reduce glioma cell proliferation, as judged by Ki-67 expression. In conclusion, tolbutamide and dbcAMP inhibit C6-glioma cell proliferation by increasing Cx43, which correlates with a reduction in pRb phosphorylation due to the up-regulation of the Cdk inhibitors p21 and p27.

Postreceptoral chromatic-adaptation mechanisms in the red-green and blue-yellow systems using simple reaction times

Simple visual-reaction times (VRT) were measured for a variety of stimuli selected along red-green (L-M axis) and blue-yellow [S-(L + M) axis] directions in the isoluminant plane under different adaptation stimuli. Data were plotted in terms of the RMS cone contrast in contrast-threshold units. For each opponent system, a modified Piéron function was fitted in each experimental configuration and on all adaptation stimuli. A single function did not account for all the data, confirming the existence of separate postreceptoral adaptation mechanisms in each opponent system under suprathreshold conditions. The analysis of the VRT-hazard functions suggested that both color-opponent mechanisms present a well-defined, transient-sustained structure at marked suprathreshold conditions. The influence of signal polarity and chromatic adaptation on each color axis proves the existence of asymmetries in the integrated hazard functions, suggesting separate detection mechanisms for each pole (red, green, blue, and yellow detectors).

Binocular interactions in random chromatic changes at isoluminance

To examine the type of chromatic interactions at isoluminance in the phenomenon of binocular vision, I have determined simple visual reaction times (VRT) under three observational conditions (monocular left, monocular right, and binocular) for different chromatic stimuli along random color axes at isoluminance (simultaneous L-, M-, and S-cone variations). Upper and lower boundaries of probability summation as well as the binocular capacity coefficient were estimated with observed distributions of reaction times. The results were not consistent with the notion of independent chromatic channels between eyes, suggesting the existence of excitatory and inhibitory binocular interactions at suprathreshold isoluminance conditions.

Increased levels of cyclins D1 and D3 after inhibition of gap junctional communication in astrocytes

We showed previously that the inhibition of gap junctional communication in astrocytes increased bromodeoxyuridine (BrdU) incorporation and promoted changes in the metabolic phenotype destined to fulfil the requirements of cell proliferation. In the present study we investigated the changes in the cell cycle of astrocytes promoted by the inhibition of intercellular communication through gap junctions. Thus, the presence of endothelin-1 and carbenoxolone, two gap junction uncouplers, promoted an increase in the percentage of astrocytes found in the S, G2 and M phases of the cell cycle, with a concomitant decrease in G0 and G1 phases. In addition, the levels of Ki-67, a protein present during all active phases of the cell cycle but absent from resting cells, increased after the inhibition of gap junctional communication. These effects were not observed when the inhibition of gap junctions was prevented with tolbutamide, indicating that the inhibition of gap junctional communication promotes the entry of astrocytes into the cell cycle. The passage of the cells from a quiescent state to the cell cycle is ultimately regulated by the degree of retinoblastoma phosphorylation. Inhibition of gap junctions increased the phosphorylation of retinoblastoma at Ser 780 but not at Ser 795 or Ser 807/811. In addition, the levels of cyclins D1 and D3 increased, whereas those of p21 and p27 were not significantly modified. Because D-type cyclins are key regulators of retinoblastoma protein phosphorylation, it is suggested that the phosphorylation of retinoblastoma protein at Ser 780, observed under our experimental conditions, is a consequence of the increase in the levels of cyclins D1 and D3. Our work provides evidence for the involvement of cyclins D1 and D3 as sensors of the inhibition of gap junctional communication in astrocytes.

The increase in gap junctional communication decreases the rate of glucose uptake in C6 glioma cells by releasing hexokinase from mitochondria

We have previously shown that the enhancement of glucose uptake caused by the inhibition of gap junctional communication is a consequence of the increase in astrocyte proliferation. Since C6 glioma cells are highly proliferative and are poorly coupled through gap junctions, we used these cells to investigate the effect of increasing gap junctional communication on the rate of glucose uptake. Previous work by us had shown that tolbutamide increases gap junctional communication in C6 glioma cells, as does dbcAMP, a classical activator of gap junctional communication. In this work, our results show that both tolbutamide and dbcAMP reduce the rate of glucose uptake in C6 glioma cells and that their effects are additive. The main glucose transporters expressed in C6 glioma cells are GLUT-1 and GLUT-3. Neither the expression nor the cellular localization of either GLUT-1 or GLUT-3 were modified by increasing gap junctional communication. The estimation of glucose uptake with 2-deoxyglucose includes not only glucose transport but also glucose phosphorylation, which in C6 glioma cells is mainly catalyzed by type I and type II hexokinase. Our results reveal that the increase in gap junctional communication caused by tolbutamide and dbcAMP is associated with a decrease in the activity of hexokinase. In agreement with this, tolbutamide and dbcAMP caused a rapid change in the localization of both type I and type II hexokinase, which were detached from the mitochondria to the cytosol.

Lactate utilization by brain cells and its role in CNS development

We studied the role played by lactate as an important substrate for the brain during the perinatal period. Under these circumstances, lactate is the main substrate for brain development and is used as a source of energy and carbon skeletons. In fact, lactate is used actively by brain cells in culture. Neurons, astrocytes, and oligodendrocytes use lactate as a preferential substrate for both energy purposes and as precursor of lipids. Astrocytes use lactate and other metabolic substrates for the synthesis of oleic acid, a new neurotrophic factor. Oligodendrocytes mainly use lactate as precursor of lipids, presumably those used to synthesize myelin. Neurons use lactate as a source of energy and as precursor of lipids. During the perinatal period, neurons may use blood lactate directly to meet the need for the energy and carbon skeletons required for proliferation and differentiation. During adult life, however, the lactate used by neurons may come from astrocytes, in which lactate is the final product of glycogen breakdown. It may be concluded that lactate plays an important role in brain development.

Antioxidant defence of the neonatal rat brain against acute hyperammonemia

Oxidative stress associated with the presence of elevated concentrations of ammonia in the brain has been proposed as one possible mechanism involved in ammonia toxicity. In a previous study [Brain Res.973 (2003) 31], we reported that neonatal rats are more resistant to acute ammonia toxicity than adult rats. In the present work, we studied the antioxidant status of the brain in hyperammonemic neonatal rats. Increased activities of the antioxidant enzymes and enhanced glutathione content were found in the brains of the hyperammonemic neonatal rats as compared to the controls. In addition, no changes in brain reactive oxygen species (ROS) levels and lipid peroxidation due to hyperammonemia were found. Therefore, acute ammonia intoxication does not induce oxidative stress in neonatal rats, a fact that may explain the resistance against hyperammonemia shown by neonatal rats.

Endothelin-1 stimulates the translocation and upregulation of both glucose transporter and hexokinase in astrocytes: relationship with gap junctional communication

We have previously shown that endothelin-1 increases glucose uptake in astrocytes. In the present work we investigate the mechanism through which endothelin-1 (ET-1) increases glucose uptake. Our results show that ET-1 activates a short-term and a long-term mechanism. Thus, ET-1 induced a rapid change in the localization of both GLUT-1 and type I hexokinase. These changes are probably aimed at rapidly increasing the entry and phosphorylation of glucose. In addition, ET-1 upregulated GLUT-1 and type I hexokinase and induced the expression of isoforms not normally expressed in astrocytes, such as GLUT-3 and type II hexokinase. These changes provide astrocytes with the machinery required to sustain a high rate of glucose uptake for a longer period of time. Our previous work had suggested that the effect of ET-1 on glucose uptake was associated with the inhibition of gap junctions. In this work, we compare the effect of ET-1 with that of carbenoxolone, a classical inhibitor of gap junction communication. Carbenoxolone increased glucose uptake to the same extent as ET-1 following the same mechanisms. Thus, carbenoxolone induced a rapid change in the localization of both GLUT-1 and type I hexokinase, upregulated GLUT-1 and type I hexokinase and induced the expression of GLUT-3 and type II hexokinase. When the inhibition of gap junction was prevented by tolbutamide, neither ET-1 nor carbenoxolone were able to increase the levels of GLUT-1, GLUT-3, type I hexokinase or type II hexokinase, indicating that these events are closely related to gap junctions.