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Jaeyoung Yang, Jason Myers, Malcolm M. Slaughter; Multiple Sweet Sensing Systems In Rat Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4133.
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© ARVO (1962-2015); The Authors (2016-present)
Sweet taste receptors (STRs) are expressed in various tissues including gut, pancreas, and hypothalamus. We determined the molecular and physiological characteristics of sweet sensing mechanisms in rat retina.
Acutely dissociated or cultured retinal cells from 10 - 16 day Sprague-Dawley rats were treated with various superfused drugs. Internal calcium was monitored using fluo3-based imaging while calcium currents were measured using whole cell patch clamp. Single cell RT-PCR and RACE (rapid amplification of cDNA Ends) were used to determine STR expression. Gene knockdown of T1R2 was done in a retinal cell line, R28.
We found mRNA of T1R1 (umami TR) and T1R2 (sweet TR) expressed in rat retina but T1R3 was not detected. In fluo3 calcium imaging, various sweeteners including aspartame, saccharin, D-amino acids, and glucose raised intracellular calcium in isolated retinal neurons. The full sequences of T1R1 and T1R2 from rat retina were identical to T1R1 and T1R2 of tongue. Yet rat T1R2 in the tongue does not respond to aspartame. In single cell RT-PCR analysis, aspartame responding neurons have both T1R1 and T1R2. But when T1R1 and T1R2 were transfected into HEK293 cells with GIRK and Galpha15, these cells did not respond to sweeteners. In gene knockdown experiments with siRNA against T1R2 in R28 cells, the response to 1mM saccharin was reduced 45% compared to controls, but the response to 1mM aspartame remained. This suggests T1R2 in rat retina is functionally similar to tongue. Part of the aspartame response is mediated by calcium permeable membrane channel s because it could be eliminated by removing extracellular calcium. TRPV1 blockers (10µM SB366791) blocked aspartame’s action and part of the glucose response in rat retina. Cells sensitive to the TRPV1 blocker were not sensitive to capsaicin, suggesting an aspartame-sensitive and capsaicin-insensitive sweet sensing response. Aspartame also altered the ERG b-wave amplitude.
There are multiple sweet sensing pathways in retina, including the canonical T1R2 system found in tongue and a TRPV1 system. Saccharin is detected by the former, aspartame by the latter.
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