Shrinkage of ipsilateral taste buds and hyperplasia of contralateral taste buds following chorda tympani nerve transection

The morphological changes that occur in the taste buds after denervation are not well under- stood in rats, especially in the contralateral tongue epithelium. In this study, we investigated the time course of morphological changes in the taste buds following unilateral nerve transection. The role of the trigeminal component of the lingual nerve in maintaining the structural integrity of the taste buds was also examined. Twenty-four Sprague-Dawley rats were randomly divided into three groups： control, unilateral chorda tympani nerve transection and unilateral chorda tympani nerve transection ＋ lingual nerve transection. Rats were allowed up to 42 days of re- covery before being euthanized. The taste buds were visualized using a cytokeratin 8 antibody. Taste bud counts, volumes and taste receptor cell numbers were quantified and compared among groups. No significant difference was detected between the chorda tympani nerve transection and chorda tympani nerve transection ＋ lingual nerve transection groups. Taste bud counts, volumes and taste receptor cell numbers on the ipsilateral side all decreased significantly compared with control. On the contralateral side, the number of taste buds remained unchanged over time, but they were larger, and taste receptor cells were more numerous postoperatively. There was no evidence for a role of the trigeminal branch of the lingual nerve in maintaining the structural integrity of the anterior taste buds.

Docosahexaenoic Acid and Eicosapentaenoic Acid Modulate Calcium Signaling in Murine CD36-Positive Taste Buds Cells, via Phospholipase Pathways

DHA （docosahexaenoic acid） and EPA （eicosapentaenoic acid） induced intracellular calcium increase in taste receptor cells purified from mouse circumvallate papillae. We have demonstrated that DHA and EPA induced calcium increase respectively via cPLA2 （cytosolic phospholipase A2） type lVc and calcium independent iPLA2 （phospholipase A2） type VI. Intracellular calcium increases via PLC （phospholipase C） pathway and leads to a calcium effiux and further SOC （stored operated calcium） channels activation. An abundant increase in intracellular calcium will induce exocytosis of neurotransmitters into the extracellular medium. Our results suggest that DHA and EPA induced calcium signaling, and should be implicated in the transmission of lipid gustatory information to afferent nerve fibers, as shown previously with linoleic acid.

Nutrients Modulate T1r2 Transcript Levels in MIN 6 and Primary Cultured Taste Buds Cells under High Glucose Condition

The sweet taste receptors comprised of T1r2 and T1r3, sense glucose concentrations in the gastrointestine. While hyperglycemia was reported to decrease the T1R2 and T1R3 tanscript levels in healthy subjects, no change was observed in type 2 diabetes patients. We investigated which glucose level and nutrients affect those transcript levels in MIN 6 and primary cultured taste buds cells using quantitative Reverse Trancription Polymerase Chain Reaction. High glucose diminished T1r2 transcript levels in MIN 6 and primary cultured taste buds cells. Resveratrol and its analogue augmented transcript levels of T1r1 and T1r2 above normal levels in MIN 6 cells in the medium with 25 mM glucose. Adenine, but not guanine, augmented T1r2 transcript levels of MIN 6 cells in the medium with 25 mM glucose. These results imply that nutrients in meals could affect sweet taste sensitivity by modulating T1r2 transcript levels in response to blood glucose levels.

Role of BDNF in the taste system

Neurotrophins are a family of proteins that regulate neural survival, development, function and plasticity in the central and the peripheral nervous system. There are four neurotrophins： NGF, BDNF, NT-3 and NT-4. Among them, BDNF is mostly studied in the taste system due to its high expression. Recent studies have shown BDNF play an important role in the developmental and mature taste system, by regulating survival of taste cells and geniculate ganglion neurons, and maintaining and guiding taste nerve innervations. These studies imply BDNF has great potentialities for therapeutic usage to enhance sensory regeneration following nerve injury, with aging, and in some neurodegenerative diseases.