Inhibitory effect of gurmarin on palatal taste responses to amino acids in the rat

Suppression of sweet taste-related responses by plant-derived bioactive compounds and eating. Part II: A systematic review in animals

This article, the second in a two-part series, continues the discussion on the nature of the relationship between the level of sweet taste suppression and eating behaviour, but in animal rather human subjects. In particular, the aim was to review the scientific literature on the impact that bioactive compounds that decrease oral sweet sensations have on intake, preference and physiological status in preclinical studies. This review was registered in the International Prospective Register of Systematic Reviews and conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and the Scottish Intercollegiate Guidelines Network and covered original papers included in Web of Science, PubMed, Scopus, Food Science Source and Food Science and technology abstracts. We identified 28 peer-reviewed English-language studies that fit the topic and met the inclusion criteria. We identified three plant species, Gymnema sylvestre, Hovenia dulcis, and Ziziphus jujuba, that possess acute sweetness-inhibitory properties. When administered orally, these plants reduced neural responses to sweet stimuli and decreased consumption. However, studies on the longer-term effects of antisweet activity remain to be conducted. Translating the valuable insights into the mechanisms underlying the relationship between sweet taste impairment and eating behaviour into practical clinical applications are discussed.

Sweet taste receptor inhibitors: Potential treatment for equine insulin dysregulation

Hyperinsulinemia is a major risk factor for equine laminitis, a debilitating and painful foot condition. Sweet taste receptor (T1R2/3) inhibitors have been used to reduce the insulin and glucose responses to oral carbohydrates in other species. However, their effect in horses has not been investigated. It would be useful to be able to attenuate the large post-prandial insulin response that typically occurs when a carbohydrate-rich meal is fed to insulin-dysregulated horses. Here we have determined the efficacy of two T1R2/3 inhibitors, lactisole and Gymnema sylvestre, for reducing glucose uptake by the equine small intestine in vitro; and post-prandial insulin secretion in ponies in vivo, following a carbohydrate-based meal. We used gas chromatography-mass spectrometry to measure 2-deoxyglucose uptake by explants of small intestine, in the presence and absence of the T1R2/3 inhibitors. Lactisole and G sylvestre reduced 2-deoxyglucose uptake by the intestinal explants by 63% (P = 0.032) and 73% (P = 0.047), respectively, compared to control samples. The study in vivo investigated the effect of the inhibitors on the blood glucose and serum insulin responses to a meal containing D-glucose. Three doses of each inhibitor were tested using a Latin square design, and each dose was compared to a meal with no inhibitor added. Lactisole had no effect on glucose and insulin concentrations, whereas G sylvestre was partially effective at reducing post-prandial blood glucose (by ~10%) and serum insulin concentrations (~25%) in seven ponies, with a most effective dose of 10 mg/kg bodyweight. These data provide preliminary support that T1R2/3 inhibitors may be a useful therapeutic strategy for the management of equine insulin dysregulation and the prevention of laminitis. However, further optimisation of the dose and delivery method for these compounds is required, as well as a direct investigation of their activity on the equine sweet taste receptor.

Hypolipidaemic Effects of Gymnema sylvestre on High Fat Diet Induced Dyslipidaemia in Wistar Rats

INTRODUCTION

Hyperlipidaemia is a well known risk factor for cardiovascular diseases. Lifestyle modification can be the initial step to reduce cholesterol levels. There are various drugs which are used to control dyslipidaemia. Treatment of lipid abnormalities is a lifelong battle. Moreover, the safety and effectiveness of long term lipid lowering treatment are questionable. Gymnema Sylvestre (GS) is a well known herb with various medicinal properties.

AIM

To explore the hypolipidaemic activity of GS leaves extract.

MATERIALS AND METHODS

Adult healthy female wistar rats, 30 in number, divided into five groups, weighing 150- 200 g were used. Dyslipidaemia was induced in rats by feeding them on high fat diet for four weeks. For the next four weeks GS extract was used as test drug while Atorvastatin was used as standard drug. Blood sample was collected for estimation of lipid profile on day 0, week 4 and week 8. Data was recorded as mean±SEM (Standard error of mean). Paired t-test and one way Analysis of Variance (ANOVA) followed by Dunnett's post hoc test was used for comparison. A p-value <0.05 was considered statistically significant. SPSS Statistics 20 (IBM software) was used for the analysis.

RESULTS

Feeding rats with high fat diet for four weeks led to obesity and dyslipidaemia in rats. GS at both the doses (100mg/kg and 200mg/kg) significantly improved the lipid profile. Total Cholesterol (TC), Triglycerides (TG), Very Low Density Lipoprotein (VLDL) and Low Density Lipoprotein (LDL) values reduced significantly while that of High Density Lipoprotein (HDL) increased significantly. GS 200 mg/kg was found more effective than GS 100 mg/kg. GS improved the value of lipid profile significantly but the effect was found inferior to Atorvastatin.

CONCLUSION

From the present study it can be concluded that GS possess an effective hypolipidaemic effect. Hence it can be included as an add on therapy in dyslipidaemia after further confirmatory studies.

Gα-gustducin is extensively coexpressed with sweet and bitter taste receptors in both the soft palate and fungiform papillae but has a different functional significance

To clarify the regional differences in the expression and functional significance of Gα-gustducin in soft palate (SP) and fungiform (FF) taste buds, we examined the coexpression of Gα-gustducin with taste receptors and the impact of Gα-gustducin knockout (gKO) on neural responses to several sweet and bitter compounds. Sweet responses from both the greater superficial petrosal (GSP) and chorda tympani (CT) nerves in gKO mice were markedly depleted, reflecting overlapping expression of Gα-gustducin and Tas1r2. However, although Gα-gustducin was expressed in 87% and 88% of Tas2rs cells in the SP and FF, respectively, there were no statistically significant differences in the CT responses to quinine-HCl (QHCl) and denatonium (Den) between gKO and wild-type (WT) mice. In contrast, GSP responses to these compounds were markedly reduced in gKO mice with an apparent elevation of thresholds (>10-fold). These results suggest that 1) Gα-gustducin plays a critical role in sweet transduction in both the SP and the FF, 2) other Gα subunits coexpressed with Gα-gustducin in the FF are sufficient for responses to QHCl and Den, and 3) robust GSP responses to QHCl and Den occur in the SP by a Gα-gustducin-dependent mechanism, which is absent in the FF.

Greater superficial petrosal nerve transection in rats does not change unconditioned licking responses to putatively sweet taste stimuli

The greater superficial petrosal nerve (GSP), innervating taste buds in the palate, is known to be exceptionally responsive to sucrose, especially compared with the responsiveness of the chorda tympani nerve (CT). However, whereas transection of the CT (CTX) alone has little or no effect on unconditioned licking responses to many "sweet" stimuli, the impact of GSP transection (GSPX) alone is equivocal. To further examine the role of the GSP on licking responses to putatively sweet-tasting substances, brief-access taste tests were conducted in nondeprived rats before and after sham surgery (SHAM) or CTX or GSPX. A range of concentrations of sucrose, L-alanine, glycine, and L-serine, with and without 1.0 mM inosine monophosphate (IMP) added, were used. All groups showed significant concentration-dependent increases in licking to all stimuli presurgically and postsurgically. CTX decreased licking responses relative to SHAM rats in the first sucrose test. There was also a group x concentration interaction for L-alanine, but post hoc tests did not reveal its basis. Other than this, there were no significant differences among the surgical groups. Interestingly, rats with GSPX tended to initiate fewer trials than SHAM rats. Overall, after GSPX, the remaining gustatory nerves are apparently sufficient to maintain concentration-dependent licking responses to all stimuli tested here. The disparity between our results and others in the literature where GSPX reduced licking responses to sucrose is possibly related to differences in surgical technique or test trial duration.

Induction of salivary kallikreins by the diet containing a sweet-suppressive peptide, gurmarin, in the rat

Gymnema sylvestre (gymnema) contains gurmarin that selectively inhibits responses to sweet substances in rodents. The present study investigated possible interaction between gurmarin and the submandibular saliva in rats fed diet containing gymnema. Electrophoretic analyses demonstrated that relative amounts of two proteins in the saliva clearly increased in rats fed the gymnema diet. However, rats previously given section of the bilateral glossopharyngeal nerve showed no such salivary protein induction. Analyses of amino acid sequence indicate that two proteins are rat kallikrein 2 (rK2) and rat kallikrein 9 (rK9). rK2 and rK9, a family of serine proteases, have a striking resemblance of cleavage site in the protein substrates. Interestingly, gurmarin possesses comparable residues with those rK2 and rK9 prefer. The kallikreins significantly inhibited immunoreaction between gurmarin and antigurmarin antiserum. These results suggest that rK2 and rK9 increased by chemosensory information for the gymnema diet via the glossopharyngeal nerve might cleave gurmarin or at least cause specific binding with it.

Mouse Strain Differences in Gurmarin-sensitivity of Sweet Taste Responses Are Not Associated with Polymorphisms of the Sweet Receptor Gene, Tas1r3

Gurmarin (Gur) is a peptide that selectively inhibits responses of the chorda tympani (CT) nerve to sweet compounds in rodents. In mice, the sweet-suppressing effect of Gur differs among strains. The inhibitory effect of Gur is clearly observed in C57BL/6 mice, but only slightly, if at all, in BALB/c mice. These two mouse strains possess different alleles of the sweet receptor gene, Sac (Tas1r3) (taster genotype for C57BL/6 and non-taster genotype for BALB/c mice), suggesting that polymorphisms in the gene may account for differential sensitivity to Gur. To investigate this possibility, we examined the effect of Gur in another Tas1r3 non-taster strain, 129 X 1/Sv mice. The results indicated that unlike non-taster BALB/c mice but similar to taster C57BL/6 mice, 129 X 1/Sv mice exhibited significant inhibition of CT responses to various sweet compounds by Gur. This suggests that the mouse strain difference in the Gur inhibition of sweet responses of the CT nerve may not be associated with polymorphisms of Tas1r3.

In vivo recordings from rat geniculate ganglia: taste response properties of individual greater superficial petrosal and chorda tympani neurones

Coding of gustatory information is complex and unique among sensory systems; information is received by multiple receptor populations located throughout the oral cavity and carried to a single central relay by four separate nerves. The geniculate ganglion is the location of the somata of two of these nerves, the greater superficial petrosal (GSP) and the chorda tympani (CT). The GSP innervates taste buds on the palate and the CT innervates taste buds on the anterior tongue. To obtain requisite taste response profiles of GSP neurones, we recorded neurophysiological responses to taste stimuli of individual geniculate ganglion neurones in vivo in the rat and compared them to those from the CT. GSP neurones had a distinct pattern of responding compared to CT neurones. For example, a small subset of GSP neurones had high response frequencies to sucrose stimulation, whereas no CT neurones had high response frequencies to sucrose. In contrast, NaCl elicited high response frequencies in a small subset of CT neurones and elicited moderate response frequencies in a relatively large proportion of GSP neurones. The robust whole-nerve response to sucrose in the GSP may be attributable to relatively few, narrowly tuned neurones, whereas the response to NaCl in the GSP may relate to proportionately more, widely tuned neurones. These results demonstrate the diversity in the initial stages of sensory coding for two separate gustatory nerves involved in the ingestion or rejection of taste solutions, and may have implications for central coding of gustatory quality and concentration as well as coding of information used in controlling energy, fluid and electrolyte homeostasis.

Differential gurmarin suppression of sweet taste responses in rat solitary nucleus neurons

We examined the effect of the sweet transduction blocker gurmarin on taste responses recorded from neurons in the rat solitary nucleus (NST) to determine how gurmarin sensitivity is distributed across neuronal type. Initially, responses evoked by washing the anterior tongue and palate with 0.5 M sucrose, 0.1 M NaCl, 0.01 M HCl, and 0.01 M quinine-HCl were recorded from 35 neurons. For some cells, responses to a sucrose concentration series (0.01-1.0 M) or an array of sweet-tasting compounds were also measured. Gurmarin (10 microg/ml, 2-4 ml) was then applied to the tongue and palate. Stimuli were reapplied after 10-15 min. Neurons were segregated into groups based on similarities among their initial response profiles using hierarchical cluster analysis (HCA). Results indicated that sucrose responses recorded from neurons representative of each HCA-defined class were suppressed by gurmarin. However, a disproportionate percentage of cells in each group displayed sucrose responses that were substantially attenuated after gurmarin treatment. Postgurmarin sucrose responses recorded from neurons that composed 57% of class S, 40% of class N, and 33% of class H were suppressed by >or=50% relative to control. On average, attenuation was statistically significant only in class S and N neurons. Although the magnitude of gurmarin-induced response suppression did not differ across sucrose concentration, responses to different sweet-tasting compounds were differentially affected. Responses to NaCl, HCl, or quinine were not suppressed by gurmarin. Results suggest that information from gurmarin-sensitive and -insensitive receptor processes converges onto single NST neurons.