Acetylcholine and Acetylcholine Receptors in Taste Receptor Cells

Extract

Neuroactive substances play important roles as transmitters and neuromodulators. Although many of these substances and/or their receptors are known to be present in taste receptor cells and nerve fibers innervating taste buds, physiological functions of these substances are not well understood (Nagai et al., 1996). Using Ca²⁺ imaging and immunocytochemical techniques, we have examined the physiological responses of taste receptor cells to acetylcholine (ACh), classified the types of ACh receptors, and determined the underlying signaling mechanisms in taste receptor cells. Our results suggest that ACh may be involved in cell-to-cell communication within the taste bud and in neuromodulation of taste transduction mechanisms.

In Ca²⁺-imaging study, freshly isolated taste receptor cells were loaded with Ca²⁺-sensitive fluorescent dye Fura-2 and intracellular Ca²⁺ levels were measured ratiometrically. ACh induced increases in intracellular Ca²⁺ levels ([Ca²⁺]_i) in taste receptor cells of mouse, rat and mudpuppy. The magnitude of the peak Ca²⁺ response to ACh was concentration-dependent with half-maximum responses around 1 µM. To determine which subtypes of ACh receptors and signaling pathway were involved, we examined the effect of receptor antagonists and inhibitors for selective pathway. Atropine (0.5 µM), a muscarinic ACh receptor antagonist, blocked the ACh response, while d-tubocurarine (250 µM), a nicotinic ACh receptor antagonist, had no effect. In addition, the phospholipase C (PLC) inhibitor U73122 (5 µM) and the Ca²⁺-ATPase inhibitor thapsigargin (1 µ M), which depletes intracellular Ca²⁺ stores, blocked the ACh responses. These results suggest that ACh binds to muscarinic ACh receptors, which activates PLC, resulting in the production of IP₃ and the subsequent release of Ca²⁺ from the IP₃-sensitive-intracellular stores. Since it is known that binding of ACh to muscarinic receptor subtypes M1/M3/M5 activates the PLC signaling pathways (Caulfield, 1993), our data indicates the presence of at least one of these receptors in taste receptor cells.