Computational analyses on the intracellular calcium regulation in outer retinal neurons

Abstract

G-protein-coupled receptors are thought to be involved in the detection of umami and l-amino acid taste. These include the heterodimer taste receptor type 1 member 1 (T1r1) + taste receptor type 1 member 3 (T1r3), taste and brain variants of mGluR4 and mGluR1, and calcium sensors. While several studies suggest T1r1 + T1r3 is a broadly tuned l-amino acid receptor, little is known about the function of metabotropic glutamate receptors (mGluRs) in l-amino acid taste transduction. Calcium imaging of isolated taste sensory cells (TSCs) of T1r3-GFP and T1r3 knock-out (T1r3 KO) mice was performed using the ratiometric dye Fura 2 AM to investigate the role of different mGluRs in detecting various l-amino acids and inosine 5′ monophosphate (IMP). Using agonists selective for various mGluRs such as (RS)-3,5-dihydroxyphenylglycine (DHPG) (an mGluR1 agonist) and l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) (an mGluR4 agonist), we evaluated TSCs to determine if they might respond to these agonists, IMP, and three l-amino acids (monopotassium l-glutamate, l-serine and l-arginine). Additionally, we used selective antagonists against different mGluRs such as (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (an mGluR1 antagonist), and (RS)-α-methylserine-O-phosphate (MSOP) (an mGluR4 antagonist) to determine if they can block responses elicited by these l-amino acids and IMP. We found that l-amino acid- and IMP-responsive cells also responded to each agonist. Antagonists for mGluR4 and mGluR1 significantly blocked the responses elicited by IMP and each of the l-amino acids. Collectively, these data provide evidence for the involvement of taste and brain variants of mGluR1 and mGluR4 in l-amino acid and IMP taste responses in mice, and support the concept that multiple receptors contribute to IMP and l-amino acid taste.