A taste of TGFβ in Tuscany

The recent FASEB Summer Research Conference entitled 'The TGFβ Superfamily: Signaling in Development and Disease' was held in August, 2011 in the spectacular setting of Il Ciocco, Lucca, amidst the olive trees in Tuscany, Italy. The organizers assembled an amazing forum, which included 53 speakers and 67 poster presentations from laboratories around the world, to showcase recent advances made in our understanding of the transforming growth factor-β (TGFβ) signaling pathway.     

Are there efferent synapses in fish taste buds?

In fish, nerve fibers of taste buds are organized within the bud's nerve fiber plexus. It is located between the sensory epithelium consisting of light and dark elongated cells and the basal cells. It comprises the basal parts and processes of light and dark cells that intermingle with nerve fibers, which are the dendritic endings of the taste sensory neurons belonging to the cranial nerves VII, IX or X. Most of the synapses at the plexus are afferent; they have synaptic vesicles on the light (or dark) cells side, which is presynaptic. In contrast, the presumed efferent synapses may be rich in synaptic vesicles on the nerve fibers (presynaptic) side, whereas the cells (postsynaptic) side may contain a subsynaptic cistern; a flat compartment of the smooth endoplasmic reticulum. This structure is regarded as a prerequisite of a typical efferent synapse, as occurring in cochlear and vestibular hair cells. In fish taste buds, efferent synapses are rare and were found only in a few species that belong to different taxa. The significance of efferent synapses in fish taste buds is not well understood, because efferent connections between the gustatory nuclei of the medulla with taste buds are not yet proved.     

Taste perception in honeybees: just a taste of honey?

The advent of the genomic era has opened new doors to understand the fundamental organization of living organisms and has therefore promoted a fertile field of comparative research that intends to identify similarities and differences between related and unrelated species at the genomic level. One of the organisms whose genome has been recently decoded is that of the honeybee Apis mellifera, enabling a direct comparison with another well-studied insect, the fruit fly Drosophila melanogaster. It was reported that the honeybee has only ten gustatory receptors and thus a very poor taste perception compared to Drosophila, which presents 68 gustatory receptors, and the mosquito Anopheles gambiae, which presents 76 gustatory receptors. In this forum article, we discuss the implications of these findings taking into account previous and new discoveries on honeybee gustation based on behavioral and neurobiological studies by several authors and us. We conclude that the world of taste of a honeybee might not be as poor as proposed and that further studies should integrate molecular, neurobiological, behavioral and ecological approaches to better characterize taste perception in bees. Handling editor: Lars Chittka     

Production of γ-aminobutyric acid in Escherichia coli by engineering MSG pathway

Abstract

The compound γ-aminobutyric acid (GABA) has many important physiological functions. The effect of glutamate decarboxylases and the glutamate/GABA antiporter on GABA production was investigated in Escherichia coli. Three genes, gadA, gadB, and gadC were cloned and ligated alone or in combination into the plasmid pET32a. The constructed plasmids were transformed into Escherichia coli BL21(DE3). Three strains, E. coli BL21(DE3)/pET32a-gadA, E. coli BL21(DE3)/pET32a-gadAB and E. coli BL21(DE3)/pET32a-gadABC were selected and identified. The respective titers of GABA from the three strains grown in shake flasks were 1.25, 2.31, and 3.98?g/L. The optimal titer of the substrate and the optimal pH for GABA production were 40?g/L and 4.2, respectively. The highest titer of GABA was 23.6?g/L at 36?h in batch fermentation and was 31.3?g/L at 57?h in fed-batch fermentation. This study lays a foundation for the development and use of GABA.     

What's in a name? Are MSG and umami the same?

The Japanese word `umami' has a long past. It was already inuse during the Edo period (Tokugawa Shogunate) of Japanese history,which ended in 1868 (Mason, 1993). In Japanese, `umami' oftenconnotes a cognitive category (Yamaguchi and Ninomiya, 1998) oftaste, or perhaps flavor, with definitions that include deliciousness, flavor,relish,     

Is serotonin a chemical transmitter in the frog taste organ?

By artificially perfusing the frog tongue with serotonin (5HT) and its antagonists, the possibility of 5HT as a chemical transmitter from taste cells to nerve terminals in frog taste organ was examined. Although serotonin creatinine sulfate, when perfused through the lingual artery, produced impulse discharges in the glossopharyngeal nerve, creatinine sulfate elicited a similar response. Neural responses to taste stimuli were depressed by perfusion with 5HT. Among many antiserotonergic drugs perfused through the lingual artery, LSD was the only one which modified responses to taste stimuli. LSD suppressed taste responses to NaCl, CaCl₂ and water, while LSD at a high concentration (10^?5 g/ml) enhanced responses to guinine and HCl. When PCPA (DL-p-chlorophenylalanine) was injected intraperitoneally in conbination with reserpine, the agent did not significantly change taste responses. The above results possibly suggest that 5HT would not be a chemical mediator from taste cells to nerve terminals.     

(+)-(S)-alapyridaine--a general taste enhancer?

N-(1-Carboxyethyl)-6-hydroxymethyl-pyridinium-3-ol inner salt (alapyridaine), recently identified in heated sugar/amino acid mixtures as well as in beef bouillon, has been shown to exhibit general taste-enhancing activities, although tasteless on its own. Differing from other taste enhancers reported so far, racemic (R/S)-alapyridaine and, to an even greater extent (+)-(S)-alapyridaine, the physiologically active enantiomer, are able to enhance more than one basic taste quality. The threshold concentrations for the sweet taste of glucose and sucrose, for the umami taste of monosodium L-glutamate (MSG) and guanosine-5'-monophosphate (GMP), as well as the salty taste of NaCl, were significantly decreased when alapyridaine was present. In contrast, perception of the bitter tastes of caffeine and L-phenylalanine, as well as of sour-tasting citric acid, was unaffected. Furthermore, alapyridaine was shown to intensify known taste synergies such as, for example, the enhancing effect of L-arginine on the salty taste of NaCl, as well as that of GMP on the umami taste of MSG. The activity of (+)-(S)-alapyridaine could be observed not only in solutions of single taste compounds, but also in more complex tastant mixtures; for example, the umami, sweet and salty taste of a solution containing MSG, sucrose, NaCl and caffeine was significantly modulated, thus indicating that alapyridaine is a general taste enhancer.     

Is there a relationship between dietary MSG obesity in animals or humans?

The sodium salt of glutamate (monosodium glutamate; MSG) imparts a savory/meaty taste to foods, and has been used as a flavoring agent for millennia. Past research on MSG/glutamate has evaluated its physiologic, metabolic and behavioral actions, and its safety. Ingested MSG has been found to be safe, and to produce no remarkable effects, except on taste. However, some recent epidemiologic and animal studies have associated MSG use with obesity and aberrations in fat metabolism. Reported effects are usually attributed to direct actions of ingested MSG in brain. As these observations conflict with past MSG research findings, a symposium was convened at the 13th International Congress on Amino Acids, Peptides and Proteins to discuss them. The principal conclusions were: (1) the proposed link between MSG intake and weight gain is likely explained by co-varying environmental factors (e.g., diet, physical activity) linked to the “nutrition transition” in developing Asian countries. (2) Controlled intervention studies adding MSG to the diet of animals and humans show no effect on body weight. (3) Hypotheses positing dietary MSG effects on body weight involve results from rodent MSG injection studies that link MSG to actions in brain not applicable to MSG ingestion studies. The fundamental reason is that glutamate is metabolically compartmentalized in the body, and generally does not passively cross biologic membranes. Hence, almost no ingested glutamate/MSG passes from gut into blood, and essentially none transits placenta from maternal to fetal circulation, or crosses the blood–brain barrier. Dietary MSG, therefore, does not gain access to brain. Overall, it appears that normal dietary MSG use is unlikely to influence energy intake, body weight or fat metabolism.     

Control of βAR- and N-methyl-D-aspartate (NMDA) Receptor-Dependent cAMP Dynamics in Hippocampal Neurons

Norepinephrine, a neuromodulator that activates β-adrenergic receptors (βARs), facilitates learning and memory as well as the induction of synaptic plasticity in the hippocampus. Several forms of long-term potentiation (LTP) at the Schaffer collateral CA1 synapse require stimulation of both βARs and N-methyl-D-aspartate receptors (NMDARs). To understand the mechanisms mediating the interactions between βAR and NMDAR signaling pathways, we combined FRET imaging of cAMP in hippocampal neuron cultures with spatial mechanistic modeling of signaling pathways in the CA1 pyramidal neuron. Previous work implied that cAMP is synergistically produced in the presence of the βAR agonist isoproterenol and intracellular calcium. In contrast, we show that when application of isoproterenol precedes application of NMDA by several minutes, as is typical of βAR-facilitated LTP experiments, the average amplitude of the cAMP response to NMDA is attenuated compared with the response to NMDA alone. Models simulations suggest that, although the negative feedback loop formed by cAMP, cAMP-dependent protein kinase (PKA), and type 4 phosphodiesterase may be involved in attenuating the cAMP response to NMDA, it is insufficient to explain the range of experimental observations. Instead, attenuation of the cAMP response requires mechanisms upstream of adenylyl cyclase. Our model demonstrates that Gs-to-Gi switching due to PKA phosphorylation of βARs as well as Gi inhibition of type 1 adenylyl cyclase may underlie the experimental observations. This suggests that signaling by β-adrenergic receptors depends on temporal pattern of stimulation, and that switching may represent a novel mechanism for recruiting kinases involved in synaptic plasticity and memory.     

Do we taste fat?

Sense of taste informs the body about the quality of ingested foods. Five sub-modalities allowing the perception of sweet, salty, sour, bitter, and umami stimuli are classically depicted. However, the inborn attraction of mammals for fatty foods raises the possibility of an additional orosensory modality devoted to fat perception. For a long time, dietary lipids were thought to be detected only by trigeminal (texture perception), retronasal olfactory, and post-ingestive cues. This minireview analyses recent findings showing that gustation also plays a significant role in dietary lipid perception.     

Evidence for human orosensory (taste?) sensitivity to free fatty acids

Accumulating evidence suggests dietary fatty acids (FAs) may be sensed in the oral cavity. However, the effective cues have not been characterized. In particular, influences from other sensory cues have hampered identification of an independent gustatory contribution. Experiment 1 examined techniques to minimize the formation of FA oxidation products and improve the homogeneity of water/lipid emulsions to be used as stimuli in Experiment 2, a psychophysical study to determine FA detection thresholds in humans. Through sonication of chilled samples held in polypropylene labware and the addition of 0.01% ethylenediaminetetraacetic acid, calcium disodium salt, homogenous emulsions of unoxidized linoleic and oleic FAs were obtained. Spectrophotometric analysis revealed no oxidation product formation over a 24-h period. Coupled with these techniques, a masking approach was used to minimize other sensory cues imparted from linoleic, oleic, and stearic FAs. Concentration ranges from 0.00028% to 5% (w/v) were prepared in mixtures with 5% mineral oil (w/v) and 5% gum acacia (w/v) to mask lubricity and viscosity effects, respectively. Testing was conducted under red light with nares blocked to eliminate visual and olfactory cues. Oral rinses with 20 ppm capsaicin were administered to desensitize participants to selected irritation effects prior to remeasuring linoleic acid detection thresholds. To determine if the effective stimulus was an oxidation product, oxidized linoleic acid was included among the test stimuli. Detection thresholds were obtained using a 3-alternative, forced-choice ascending-concentration presentation procedure. The mean detection threshold for linoleic acid pre-desensitization was 0.034 +/- 0.008%, for linoleic acid post-desensitization was 0.032 +/- 0.007%, for oleic 0.022 +/- 0.003%, for stearic 0.032 +/- 0.005%, and oxidized linoleic 0.025 +/- 0.005%. The results suggest that linoleic, oleic, stearic, and oxidized linoleic acids are detectable in the oral cavity of humans with minimal input from the olfactory, capsaicin, and viscosity-assessing tactile systems.     

Electronic sensory systems for taste and odour monitoring in water – Developments and limitations

Taste and odour causingchemicals in drinking water supplies can bedetected and identified using a variety ofanalytical techniques and sensory methods.Currently limitations exist in applying thesetechniques and methods to the continuousmonitoring of taste and odour episodes.Electronic sensory systems so called``electronic noses' using non-specific gassensors could offer a rapid and relative simpletechnique for continuous monitoring of waterquality. Laboratory and field-based continuouswater monitoring showed that introducedpollutants such as 2-chlorophenol and geosmincould be detected by a sensor array, howeverthe detection limits were significant higherthan the odour threshold concentrations (OTC)for the respective compounds. The conditioningof the monitoring system in a temperaturecontrolled environment for on-line headspacegeneration and transfer reduced the impact ofenvironmental fluctuations on the sensorresponse profiles. At present, a sensor arraybased monitoring system could be applied to theintake protection of taste and odour causingcompounds in water supplies with a minimum OTCof 10 ppm.     

Testing for defensive synergy in Caribbean sponges: Bad taste or glass spicules?

Chemical and physical defenses of sessile organisms against consumers are well described for both terrestrial and marine systems. However, previous studies have focused on chemical or physical defenses in isolation, and have not considered their interaction. Marine sponges provide a model system for testing this interaction. Some sponge species produce secondary metabolites that deter predation; they may also contain siliceous spicules, but previous studies have provided little evidence that spicules in isolation offer any defense against generalist fish predators. To determine whether the two components have an additive, antagonistic, or synergistic interaction, crude organic extracts and spicules from individuals of 8 Caribbean sponge species were isolated and tested in laboratory feeding assays. These included one chemically defended reef sponge (Agelas clathrodes) and seven known to be intermediately deterrent: six from reef habitats (Cinachyrella alloclada, Clathria virgultosa, Cribrochalina infundibulum, Niphates digitalis, Svenzea zeai, and Xestospongia muta) and one from mangrove habitats (Tedania ignis). Extracts and spicules were assayed at various concentrations, both individually and in combination, in laboratory feeding assays with the bluehead wrasse, Thalassoma bifasciatum. A SAS based GENMOD procedure based on an isobolographic analysis model was used for statistical comparisons. Four sponges (A. clathrodes, C. alloclada, C. virgultosa, and one of three individuals of X. muta) showed evidence of synergisms. Of these, synergy in C. alloclada, C. virgultosa, and X. muta was caused by approximately natural concentrations of extracts and spicules. The extract of A. clathrodes was deterrent, but combination assays required nearly a 3-fold reduction in extract concentration and an 8-fold increase in spicule concentration to show the synergistic effect. Contrary to previous findings, spicules from C. infundibulum and two of three individuals of X. muta were deterrent at natural concentrations. Sponge spicules may be defensive in isolation, or may enhance chemical defenses against consumers, but the lack of synergisms for individuals in 4 of 7 species with intermediate levels of chemical defense suggests that defensive synergy is not the general rule and, when present, may be an example of an exaptation.     

Does conditioned taste aversion learning in the pond snail Lymnaea stagnalis produce conditioned fear?

In conditioned taste aversion (CTA) training performed on the pond snail Lymnaea stagnalis, a stimulus (the conditional stimulus, CS; e.g., sucrose) that elicits a feeding response is paired with an aversive stimulus (the unconditional stimulus, US) that elicits the whole-body withdrawal response and inhibits feeding. After CTA training and memory formation, the CS no longer elicits feeding. We hypothesize that one reason for this result is that after CTA training the CS now elicits a fear response. Consistent with this hypothesis, we predict the CS will cause (1) the heart to skip a beat and (2) a significant change in the heart rate. Such changes are seen in mammalian preparations exposed to fearful stimuli. We found that in snails exhibiting long-term memory for one-trial CTA (i.e., good learners) the CS significantly increased the probability of a skipped heartbeat, but did not significantly change the heart rate. The probability of a skipped heartbeat was unaltered in control snails given backward conditioning (US followed by CS) or in snails that did not acquire associative learning (i.e., poor learners) after the one-trial CTA training. These results suggest that as a consequence of acquiring CTA, the CS evokes conditioned fear in the conditioned snails, as evidenced by a change in the nervous system control of cardiac activity.     

Modulation of NMDA Receptor Subunit mRNA in Butorphanol-Tolerant and —Withdrawing Rats

The NMDA receptor has been implicated in opioid tolerance and withdrawal. The effects of continuous infusion of butorphanol on the modulation of NMDA receptor subunit NR1, NR2A, NR2B, and NR2C gene expression were investigated by using in situ hybridization technique. Continuous intracerebroventricular (i.c.v.) infusion with butorphanol (26 nmol/l/h) resulted in significant modulations in the NR1, NR2A, and NR2B mRNA levels. The level of NR1 mRNA was significantly decreased in the cerebral cortex, thalamus, and CA1 area of hippocampus in butorphanol tolerant and withdrawal (7 h after stopping the infusion) rats. The NR2A mRNA was significantly decreased in the CA1 and CA3 of hippocampus in tolerant rats and increased in the cerebral cortex and dentate gyrus in butorphanol withdrawal rats. NR2B subunit mRNA was decreased in the cerebral cortex, caudate putamen, thalamus, CA3 of hippocampus in butorphanol withdrawal rats. No changes of NR1, NR2A, NR2C subunit mRNA in the cerebellar granule cell layer were observed in either butorphanol tolerant or withdrawal rats. Using quantitative ligand autoradiography, the binding of NMDA receptor ligand [³H]MK-801 was increased significantly in all brain regions except in the thalamus and hippocampus, at the 7 hr after stopping the butorphanol infusion. These results suggest that region-specific changes of NMDA receptor subunit mRNA (NR 1 and NR2) as well as NMDA receptor binding ([³H]MK-801) are involved in the development of tolerance to and withdrawal from butorphanol.     

竞争性NMDA受体拮抗剂

ＭＮＤＡ受体拮抗剂主要分为二大类,即竞争性拮抗剂和非竞争性拮抗剂,本文综述了竞争性ＮＭＤＡ受体拮抗剂的研究进展。