Leptin as a modulator of sweet taste sensitivities in mice

Leptin acts as a potent inhibitory factor against obesity by regulating energy expenditure, food intake, and adiposity. The obese diabetic db/db mouse, which has defects in leptin receptor, displays enhanced neural responses and elevated behavioral preference to sweet stimuli. Here, we show the effects of leptin on the peripheral taste system. An administration of leptin into lean mice suppressed responses of peripheral taste nerves (chorda tympani and glossopharyngeal) to sweet substances (sucrose and saccharin) without affecting responses to sour, salty, and bitter substances. Whole-cell patch-clamp recordings of activities of taste receptor cells isolated from circumvallate papillae (innervated by the glossopharyngeal nerve) demonstrated that leptin activated outward K+ currents, which resulted in hyperpolarization of taste cells. The db/db mouse with impaired leptin receptors showed no such leptin suppression. Taste tissue (circumvallate papilla) of lean mice expressed leptin-receptor mRNA and some of the taste cells exhibited immunoreactivities to antibodies of the leptin receptor. Taken together, these observations suggest that the taste organ is a peripheral target for leptin, and that leptin may be a sweet-sensing modulator (suppressor) that may take part in regulation of food intake. Defects in this leptin suppression system in db/db mice may lead to their enhanced peripheral neural responses and enhanced behavioral preferences for sweet substances.

Female preference for swords in Xiphophorus helleri reflects a bias for large apparent size

Swordtail fish (Poeciliidae: genus Xiphophorus) are a paradigmatic case of sexual selection by sensory exploitation. Female preference for males with a conspicuous “sword” ornament is ancestral, suggesting that male morphology has evolved in response to a preexisting bias. The perceptual mechanisms underlying female mate choice have not been identified, complicating efforts to understand the selection pressures acting on ornament design. We consider two alternative models of receiver behavior, each consistent with previous results. Females could respond either to specific characteristics of the sword or to more general cues, such as the apparent size of potential mates. We showed female swordtails a series of computer-altered video sequences depicting a courting male. Footage of an intact male was preferred strongly to otherwise identical sequences in which portions of the sword had been deleted selectively, but a disembodied courting sword was less attractive than an intact male. There was no difference between responses to an isolated sword and to a swordless male of comparable length, or between an isolated sword and a homogenous background. Female preference for a sworded male was abolished by enlarging the image of a swordless male to compensate for the reduction in length caused by removing the ornament. This pattern of results is consistent with mate choice being mediated by a general preference for large males rather than by specific characters. Similar processes may account for the evolution of exaggerated traits in other systems.

Cocaine reward models: Conditioned place preference can be established in dopamine- and in serotonin-transporter knockout mice

Cocaine and methylphenidate block uptake by neuronal plasma membrane transporters for dopamine, serotonin, and norepinephrine. Cocaine also blocks voltage-gated sodium channels, a property not shared by methylphenidate. Several lines of evidence have suggested that cocaine blockade of the dopamine transporter (DAT), perhaps with additional contributions from serotonin transporter (5-HTT) recognition, was key to its rewarding actions. We now report that knockout mice without DAT and mice without 5-HTT establish cocaine-conditioned place preferences. Each strain displays cocaine-conditioned place preference in this major mouse model for assessing drug reward, while methylphenidate-conditioned place preference is also maintained in DAT knockout mice. These results have substantial implications for understanding cocaine actions and for strategies to produce anticocaine medications.

Silent chromatin determines target preference of the Saccharomyces retrotransposon Ty5

The HML and HMR mating loci of Saccharomyces cerevisiae are bound in silent chromatin, which is assembled at the flanking E and I transcriptional silencers. The retrotransposon Ty5 preferentially integrates into regions of silent chromatin, and Ty5 insertions near the HMR-E silencer account for ≈2% of total transposition events. Most Ty5 insertions occur within 800 bp on either side of the autonomously replicating consensus sequence within HMR-E. Ty5 target preference is determined by silent chromatin, because integration near HMR-E is abolished in strains with silencer mutations that alleviate transcriptional repression. The recognition of specific DNA sequences per se does not direct integration, rather, it is the protein complex assembled at the silencers. As demonstrated here for Ty5, recognition of specific chromatin domains may be a general mechanism by which retrotransposons and retroviruses determine integration sites.

A preference for own-subspecies' song guides vocal learning in a song bird

In many song birds, males develop their songs as adults by imitating the songs of one or more tutors, memorized previously during a sensitive phase early in life. Previous work using two assays, the production of imitations by adult males and playback-induced calling by young birds during the sensitive phase for memorization, has shown that song birds can discriminate between their own and other species' songs. Herein I use both assays to show that male mountain white-crowned sparrows, Zonotrichia leucophrys oriantha, must learn to sing but have a genetic predisposition to memorize and learn the songs of their own subspecies. Playback tests to young naive birds before they even begin to sing reveal that birds give begging calls more in response to oriantha song than to songs of another species. After 10 days of tutoring with songs of either their own or another subspecies, birds continue to give stronger call responses to songs of their own subspecies, irrespective of whether they were tutored with them, and are more discriminating in distinguishing between different dialects of their own subspecies. The memory processes that facilitate recognition and discrimination of own-subspecies' song may also mediate the preferential imitation of song of a bird's own subspecies. Such perceptual biases could constrain the direction and rate of cultural evolution of learned songs.

Resveratrol, a polyphenolic compound found in grapes and wine, is an agonist for the estrogen receptor

The phytochemical resveratrol, which is found in grapes and wine, has been reported to have a variety of anti-inflammatory, anti-platelet, and anti-carcinogenic effects. Based on its structural similarity to diethylstilbestrol, a synthetic estrogen, we examined whether resveratrol might be a phytoestrogen. At concentrations (≈3–10 μM) comparable to those required for its other biological effects, resveratrol inhibited the binding of labeled estradiol to the estrogen receptor and it activated transcription of estrogen-responsive reporter genes transfected into human breast cancer cells. This transcriptional activation was estrogen receptor-dependent, required an estrogen response element in the reporter gene, and was inhibited by specific estrogen antagonists. In some cell types (e.g., MCF-7 cells), resveratrol functioned as a superagonist (i.e., produced a greater maximal transcriptional response than estradiol) whereas in others it produced activation equal to or less than that of estradiol. Resveratrol also increased the expression of native estrogen-regulated genes, and it stimulated the proliferation of estrogen-dependent T47D breast cancer cells. We conclude that resveratrol is a phytoestrogen and that it exhibits variable degrees of estrogen receptor agonism in different test systems. The estrogenic actions of resveratrol broaden the spectrum of its biological actions and may be relevant to the reported cardiovascular benefits of drinking wine.

A Novel Alkaline α-Galactosidase from Melon Fruit with a Substrate Preference for Raffinose1

The cucurbits translocate the galactosyl-sucrose oligosaccharides raffinose and stachyose, therefore, α-galactosidase (α-d-galactoside galactohydrolase, EC 3.2.1.22) is expected to function as the initial enzyme of photoassimilate catabolism. However, the previously described alkaline α-galactosidase is specific for the tetrasaccharide stachyose, leaving raffinose catabolism in these tissues as an enigma. In this paper we report the partial purification and characterization of three α-galactosidases, including a novel alkaline α-galactosidase (form I) from melon (Cucumis melo) fruit tissue. The form I enzyme showed preferred activity with raffinose and significant activity with stachyose. Other unique characteristics of this enzyme, such as weak product inhibition by galactose (in contrast to the other α-galactosidases, which show stronger product inhibition), also impart physiological significance. Using raffinose and stachyose as substrates in the assays, the activities of the three α-galactosidases (alkaline form I, alkaline form II, and the acid form) were measured at different stages of fruit development. The form I enzyme activity increased during the early stages of ovary development and fruit set, in contrast to the other α-galactosidase enzymes, both of which declined in activity during this period. In the mature, sucrose-accumulating mesocarp, the alkaline form I enzyme was the major α-galactosidase present. We also observed hydrolysis of raffinose at alkaline conditions in enzyme extracts from other cucurbit sink tissues, as well as from young Coleus blumei leaves. Our results suggest different physiological roles for the α-galactosidase forms in the developing cucurbit fruit, and show that the newly discovered enzyme plays a physiologically significant role in photoassimilate partitioning in cucurbit sink tissue.

Molecular mechanisms of cocaine reward: Combined dopamine and serotonin transporter knockouts eliminate cocaine place preference

Cocaine blocks uptake by neuronal plasma membrane transporters for dopamine (DAT), serotonin (SERT), and norepinephrine (NET). Cocaine reward/reinforcement has been linked to actions at DAT or to blockade of SERT. However, knockouts of neither DAT, SERT, or NET reduce cocaine reward/reinforcement, leaving substantial uncertainty about cocaine's molecular mechanisms for reward. Conceivably, the molecular bases of cocaine reward might display sufficient redundancy that either DAT or SERT might be able to mediate cocaine reward in the other's absence. To test this hypothesis, we examined double knockout mice with deletions of one or both copies of both the DAT and SERT genes. These mice display viability, weight gain, histologic features, neurochemical parameters, and baseline behavioral features that allow tests of cocaine influences. Mice with even a single wild-type DAT gene copy and no SERT copies retain cocaine reward/reinforcement, as measured by conditioned place-preference testing. However, mice with no DAT and either no or one SERT gene copy display no preference for places where they have previously received cocaine. The serotonin dependence of cocaine reward in DAT knockout mice is thus confirmed by the elimination of cocaine place preference in DAT/SERT double knockout mice. These results provide insights into the brain molecular targets necessary for cocaine reward in knockout mice that develop in their absence and suggest novel strategies for anticocaine medication development.

Dominant loss of responsiveness to sweet and bitter compounds caused by a single mutation in α-gustducin

Biochemical and genetic studies have implicated α-gustducin as a key component in the transduction of both bitter or sweet taste. Yet, α-gustducin-null mice are not completely unresponsive to bitter or sweet compounds. To gain insights into how gustducin mediates responses to bitter and sweet compounds, and to elicit the nature of the gustducin-independent pathways, we generated a dominant-negative form of α-gustducin and expressed it as a transgene from the α-gustducin promoter in both wild-type and α-gustducin-null mice. A single mutation, G352P, introduced into the C-terminal region of α-gustducin critical for receptor interaction rendered the mutant protein unresponsive to activation by taste receptor, but left its other functions intact. In control experiments, expression of wild-type α-gustducin as a transgene in α-gustducin-null mice fully restored responsiveness to bitter and sweet compounds, formally proving that the targeted deletion of the α-gustducin gene caused the taste deficits of the null mice. In contrast, transgenic expression of the G352P mutant did not restore responsiveness of the null mice to either bitter or sweet compounds. Furthermore, in the wild-type background, the mutant transgene inhibited endogenous α-gustducin's interactions with taste receptors, i.e., it acted as a dominant-negative. That the mutant transgene further diminished the residual bitter and sweet taste responsiveness of the α-gustducin-null mice suggests that other guanine nucleotide-binding regulatory proteins expressed in the α-gustducin lineage of taste cells mediate these responses.

Female mating preference for bold males in the guppy, Poecilia reticulata.

Although females prefer to mate with brightly colored males in numerous species, the benefits accruing to such females are virtually unknown. According to one hypothesis of sexual selection theory, if the expression of costly preferred traits in males (such as conspicuous colors) is proportional to the male's overall quality or reveals his quality, a well-developed trait should indicate good condition and/or viability for example. A female choosing such a male would therefore stand to gain direct or indirect fitness benefits, or both. Among potential phenotypic indicators of an individual's quality are the amount and brightness of its carotenoid-based colors and its boldness, as measured by its willingness to risk approaching predators without being killed. Here, we show experimentally that in the Trinidadian guppy (Poecilia reticulata) the visual conspicuousness of the color pattern of males correlates positively with boldness toward, and with escape distance from, a cichlid fish predator. Bold individuals are thus more informed about nearby predators and more likely to survive encounters with them. Mate-choice experiments showed that females prefer colorful males as mates, but prefer bolder males irrespective of their coloration when given the opportunity to observe their behavior toward a potential fish predator. By preferentially mating with colorful males, female guppies are thus choosing on average, relatively bold, and perhaps more viable, individuals. In doing so, and to the extent that viability is heritable, they potentially gain indirect fitness benefits by producing more viable offspring than otherwise.