Evidence for a role of glutamate as an efferent transmitter in taste buds.
BACKGROUND
Glutamate has been proposed as a transmitter in the peripheral taste system in addition to its well-documented role as an umami taste stimulus. Evidence for a role as a transmitter includes the presence of ionotropic glutamate receptors in nerve fibers and taste cells, as well as the expression of the glutamate transporter GLAST in Type I taste cells. However, the source and targets of glutamate in lingual tissue are unclear. In the present study, we used molecular, physiological and immunohistochemical methods to investigate the origin of glutamate as well as the targeted receptors in taste buds.


RESULTS
Using molecular and immunohistochemical techniques, we show that the vesicular transporters for glutamate, VGLUT 1 and 2, but not VGLUT3, are expressed in the nerve fibers surrounding taste buds but likely not in taste cells themselves. Further, we show that P2X2, a specific marker for gustatory but not trigeminal fibers, co-localizes with VGLUT2, suggesting the VGLUT-expressing nerve fibers are of gustatory origin. Calcium imaging indicates that GAD67-GFP Type III taste cells, but not T1R3-GFP Type II cells, respond to glutamate at concentrations expected for a glutamate transmitter, and further, that these responses are partially blocked by NBQX, a specific AMPA/Kainate receptor antagonist. RT-PCR and immunohistochemistry confirm the presence of the Kainate receptor GluR7 in Type III taste cells, suggesting it may be a target of glutamate released from gustatory nerve fibers.


CONCLUSIONS
Taken together, the results suggest that glutamate may be released from gustatory nerve fibers using a vesicular mechanism to modulate Type III taste cells via GluR7.
Vandenbeuch
Aurelie
A
Department of Otolaryngology, University of Colorado, Denver, CO, USA. aurelie.vandenbeuch@ucdenver.edu
Tizzano
Marco
M
Anderson
Catherine B
CB
Stone
Leslie M
LM
Goldberg
Daniel
D
Kinnamon
Sue C
SC
D000818:Animals; D002118:Calcium / Q000378:metabolism; D018698:Glutamic Acid / Q000378:metabolism* / Q000494:pharmacology; D007150:Immunohistochemistry; D051379:Mice; D008822:Mice, Transgenic; D009476:Neurons, Efferent / Q000187:drug effects / Q000378:metabolism*; D018092:Kainic Acid Receptors / Q000378:metabolism*; D018048:Receptors, Purinergic P2 / Q000378:metabolism; D058476:Receptors, Purinergic P2X2; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013650:Taste Buds / Q000187:drug effects / Q000378:metabolism*; D050598:Vesicular Glutamate Transport Protein 1 / Q000378:metabolism; D050599:Vesicular Glutamate Transport Protein 2 / Q000378:metabolism
D002118:Calcium; D018698:Glutamic Acid; D018092:Kainic Acid Receptors; D018048:Receptors, Purinergic P2; D058476:Receptors, Purinergic P2X2; D050598:Vesicular Glutamate Transport Protein 1; D050599:Vesicular Glutamate Transport Protein 2; C546402:P2rx2 protein, mouse