Sweet sensing mechanisms in the rat retina
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Glucose is the major energy source in most organisms. Glucose level is tightly regulated in the body. Increases in blood glucose can produce a response in pancreatic beta cells that includes an elevation in intracellular free calcium, increased cAMP production, and augmented insulin release (Gerich et al., 1976). There are also glucose sensing neurons in the hypothalamus, which can be divided into Glucose-Excited (GE) or Glucose-Inhibited (GI) based on their firing rate in response to a change in extracellular glucose. These responses depend, respectively, on the closing of a potassium channel or the opening of a chloride channel. In both cases these channels are gated by ATP, and thus indirectly detect glucose by the effects of glucose metabolism on intracellular levels of ATP (Song et al., 2001). In both types of hypothalamic neurons the responses are slow and require large, maintained changes in glucose concentration. In this study we examined whether the nervous system might also possess a receptor based system for detection of extracellular glucose. After the sweet taste receptor (T1R2) was cloned from the tongue, it was suggested that it might be the mediator of responses to extracellular glucose. T1R2 knockout mice are deficient in tasting sweet chemicals such as sucrose, maltose, and saccharin (Zhao et al., 2003). Sweet taste receptor (T1R2) is a member of taste receptor type1 G-protein coupled receptor group consisting of T1R1, T1R2 and T1R3. Functional expression studies in heterologous cells revealed that T1R3 combines with T1R2 to form functional sweet taste receptors (Chandrashekar et al., 2006). It has been reported that sweet taste receptor (T1R2) is expressed in multiple organs such as pancreas, brain, and gut. Artificial sweeteners can induce insulin secretion through T1R2 expressed in pancreatic cells (Nakagawa et al., 2009). Dietary sugar and artificial sweeteners can increase SGLT1 mRNA and protein expression, and glucose absorptive capacity in wild-type mice, but not in knockout mice lacking T1R3 or alpha-gustducin (Margolskee et al., 2007). Food deprivation or low glucose culture media can significantly increase T1R2 expression in brain and in a hypothalamic cell line (Ren et al., 2009).These studies raise the possibility that T1R2 is involved in glucose detection in many different organs. We report that retinal neurons respond to a variety of sweeteners including glucose, aspartame, saccharin, and D-amino acids. We also find that T1R2 mRNA is expressed in retinal neurons. Surprisingly, based on siRNA experiments, T1R2 mediates part of the neuronal response to sacharrin but is not important in the responses to aspartame. We found instead that cellular responses to both glucose and aspartame depend on TRPV1 channels or endocannabinoid receptors. Part of the response to saccharin also depends on the TRPV1 channel. Thus, retinal neurons possess two distinct sweet tastant receptor systems. One involves the canonical T1R2 subunit. The other is an alternate sweet sensing pathway consisting of ionotropic (TRPV1 variant) and metabotropic (CB endocannabinoid) amandamide receptor. This alternate pathway can alter synaptic transmission at the first synapse in the retina.