Communication Routes within the Taste Bud by Neurotransmitters and Neuropeptides

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Abstract
Taste receptor cells (TRCs) are located throughout the oral cavity, sequestered into morphological structures known as taste buds. These differentiated epithelial cells relay the presence of tastants to the central nervous system via sensory afferent nerves. Only a minority of TRCs within the bud synapse with these nerve fibers. This anatomical finding has lead to the dichotomization of TRCs into ‘true’ TRCs and supporting cells. However, this original conception—that a single TRC is excited by a tastant molecule and releases neurotransmitter onto its postsynaptic afferent nerve fiber—has been undermined by more recent physiological and molecular findings. For example, a single neurotransmitter, as might be expected from this scheme, has not been observed. Rather, there is evidence for at least five neurotransmitters within the mammalian taste bud: glutamate, serotonin, norepinephrine, acetyl choline and GABA. Additionally, TRCs expressing essential transduction molecules such as gustducin and members of the T2R family are not synaptically connected to the CNS. How then do TRCs equipped to respond to stimuli communicate with the central nervous system? The cloistering of TRCs into the precisely arranged structure of the bud may provide a clue. The bud, a highly conserved morphology of the vertebrate gustatory system, is an obvious substrate for cell to cell communication. Hence, alternative signaling pathways, such as paracrine communication, could explain this paradox. Rather than acting as arbitrarily collected detectors, TRCs within the bud may operate as a unit. Our investigations of neurotransmitters and neuropeptides in rat circumvallate and foliate TRCs have elucidated new pathways of communication among TRCs. Two, serotonin and cholecystokinin, are described here.

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