Leptin's effect on taste bud calcium responses and transmitter secretion

Abstract

Leptin, a peptide hormone released by adipose tissue, acts on the hypothalamus to control cravings and appetite. Leptin also acts to decrease taste responses to sweet substances, though there is little detailed information regarding where leptin acts in the taste transduction cascade. The present study examined the effects of leptin on sweet-evoked responses and neuro transmitter release from isolated taste buds. Our results indicate that leptin moderately decreased sweet-evoked calcium mobilization in isolated mouse taste buds. We also employed Chinese hamster ovary biosensor cells to examine taste transmitter release from isolated taste buds. Leptin reduced ATP and increased serotonin release in response to sweet stimulation. However, leptin has no effect on bitter-evoked transmitter release, further showing that the action of leptin is sweet specific. Our results support those of previous studies, which state that leptin acts on taste tissue via the leptin receptor, most likely on Type II (Receptor) cells, but also possibly on Type III (Presynaptic) cells.

Keywords: ATP; leptin; serotonin; taste bud.

Figure 1

Calcium responses of isolated circumvallate taste buds to sucralose in the absence and presence of leptin. (A) Stimulation of mouse taste buds with 1mM sucralose (arrows) elicits Ca²⁺ responses that are significantly decreased by 1000 ng/mL leptin (bar). (B) Summary of data from 8 taste buds during stimulation with sucralose alone (open circle) and in the presence of leptin (filled circles). Leptin caused a moderate, yet significant decrease in the sucralose-evoked calcium responses (a decrease to 0.82±0.05). Paired t-test, 2-tailed, P < 0.02. *indicates statistical significance. Bar on right shows the mean and 95% confidence interval. Responses for each experiment were normalized to the sucralose-alone response. None of the cells that responded to sucralose were responsive to 50mM KCl.

Figure 2

Sweet-evoked neurotransmitter secretion from taste buds, measured with ATP-biosensors. (A) ATP biosensor responses to sweet taste stimulation with 1mM sucralose + 0.1mM SC45647 (arrows) before and during bath application of 1000ng/mL leptin (bar). (B) Summary of data from 9 taste buds. For each data point, the biosensor response during exposure to leptin was normalized to the corresponding response before adding leptin. The ATP biosensor response during sweet stimulation (open circle) decreased significantly in the presence of 200–1000ng/mL leptin (filled circles). Paired t-test, 2-tailed, P < 0.0001. (C) 1000ng/mL leptin did not affect bitter-evoked (10 µM cycloheximide + 1mM denatonium) ATP secretion from 4 taste buds. Paired t-test, 2-tailed, P = 0.344. Responses were normalized to the stimulus-alone response. Bars on right shows means ± 95% confidence intervals (ns, nonsignificant; *indicates statistical significance.).

Figure 3

Sweet-evoked ATP secretion from isolated CV taste buds measured using biosensors during application of varying concentrations of leptin (100–1000ng/mL). We observed little if any effect at 100ng/mL and unambiguous suppression at 1000ng/mL. Responses were normalized to the stimulus-alone response and fit with an exponential curve.

Figure 4

ATP secretion from isolated CV taste buds in response to sweet, leptin (200ng/mL), and SMLA (5 µg/mL) measured using biosensors. Leptin significantly reduced sweet-evoked ATP secretion, and the addition of SMLA recovered the sweet-evoked response. Data were analyzed with a one-way ANOVA followed by Tukey post hoc test for pairwise comparisons, P < 0.05 for significant differences. *indicates statistical significance. Responses were normalized to the sweet-alone response. Bars indicate the mean and 95% confidence interval (ns, nonsignificant).

Figure 5

Sweet-evoked serotonin (5-HT) secretion from taste buds, measured with biosensors. (A) 5-HT biosensor responses to sweet taste stimulation with sweet (arrows) before and during bath application of 1000ng/mL leptin (bar). 5-HT secretion is significantly enhanced in the presence of leptin. (B) Summary of data from 8 taste buds. The 5-HT biosensor response during sweet stimulation (open circle) increased significantly in the presence of leptin (filled circles). Paired t-test, 2-tailed, P = 0.01. *indicates statistical significance. Bar on right shows the mean and 95% confidence interval. Responses for each experiment were normalized to the sweet-alone response.

Figure 6

Leptin had no effect on bitter- and depolarization-evoked 5-HT secretion from taste buds. (A) There was no significant difference in bitter-evoked (10 μM cycloheximide + 1 mM denatonium) 5-HT secretion before (open circle) and during 1000 ng/mL leptin (filled circles) (n = 6 taste buds). (B) Leptin did not affect depolarization-evoked (50 mM KCl) 5-HT secretion from 11 taste buds. Responses were normalized to the stimulus-alone response. Paired t-test, 2-tailed, P > 0.13. Bar on right shows the mean and 95% confidence intervals (ns, nonsignificant).