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J. Yang, J. Myers, M. Slaughter; Sweet Taste Receptors in Rat Retinal Neurons. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1023.
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Metabotropic glutamate and GABA receptors are members of a G-protein-coupled family containing more than 50 receptors. Exploring mRNA expression of family members, we found all three type 1 taste receptors expressed in retina. We determined the molecular and physiological characteristics of sweet taste receptors (TR2) 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. RT-PCR and RACE (rapid amplification of cDNA Ends) were used to determine TR expression.
RT-PCR revealed that the TR1, 2, 3 genes are expressed in rat retina. By RT-PCR and RACE, we pulled out the full sequences of TR1 and 2, which were identical to tongue taste receptors. 1mM aspartame raised internal calcium in a subset of isolated retinal neurons. If cells were depolarized by 50 mM KCl, then 1 mM aspartame reduced internal calcium. Aspartame suppressed high-voltage activated calcium currents. Other TR2 agonists (2mM D-tryptophan, 1mM D-phenylalanine, 1mM saccharine, and 50mM glucose) also enhanced intracellular calcium levels. The effects of TR2 agonists could be duplicated by 100uM ACPD, a metabotropic glutamate receptor agonist. The calcium elevation was inhibited by nimodipine or cadmium block of voltage-gated calcium channels or by caffeine depletion of internal calcium stores. There was a developmental switch in the action of TR2 agonists. Aspartame’s suppression of elevated calcium was not evident until postnatal day 14. This time dependence was also observed in retinal neuron in culture. Cultured retinal neurons from postnatal day 10 rats did not exhibit agonist-induced suppression until 2-4 days in culture.
Sweet taste receptors are expressed in retina. Aspartame produced a biphasic modulation of internal calcium: raising calcium in resting neurons and reducing calcium in stimulated neurons. Similar effects were observed using ACPD, indicating this type of modulation exists among many different metabotropic receptors. Bimodal modulation implies that metabotropic receptors can activate opposing biological cascades with single receptors.
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