Ligand-specific induction of endocytosis in taste receptor cells.

We demonstrate a ligand-specific induction of endocytosis in cells of juvenile brown trout taste buds. The process is fast, massive and selective, as only a few cells in each taste buds are stained by exposure of the oral cavity to the taste stimulant l-cysteine together with a dye at 20 degrees C. Low temperature (+2 degrees C) and disruption of microtubules with nocodazole caused a substantial reduction in the number of taste cells stained, indicating endocytic uptake of dye and transport towards the cell soma in vesicles. As endocytosis is evoked by the presence of ligands, it is most likely that the stained cells are the so-called receptor cells, which have taste receptors and the molecular machinery for downstream processing. The number of stained taste cells and taste buds containing stained taste cells increased with the concentration of l-cysteine. Control experiments with different dyes revealed great variability in the ability to induce staining on their own. In particular, Texas Red dextran was efficient and stained many cells within each taste bud. Behavioural experiments demonstrated that Texas Red dextran is a deterrent taste substance for brown trout. In fish first exposed to the stimulant l-cysteine plus a dye and subsequently to a deterrent, either Texas Red, or glycine, the majority of stained cells were found in separate taste receptor cells, indicating that the majority of taste receptors for stimulants and deterrents are expressed in separate taste buds. These results also strengthen the assumption that the stained cells take part in the initiation of taste processes that are related to perception. The functional implication of the induced endocytosis is discussed.