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The Journal of Neuroscience, November 1, 2000, 20(21):7978-7985
In Situ Ca2+ Imaging Reveals
Neurotransmitter Receptors for Glutamate in Taste Receptor
Cells
Alejandro
Caicedo1, 3,
M. Samir
Jafri1, and
Stephen D.
Roper1, 2
1 Department of Physiology and Biophysics and
2 Program in Neuroscience, University of Miami School of
Medicine, Miami, Florida 33136, and 3 Laboratorio de
Biofísica, Centro Internacional de Física, AA 49480 Bogotá, Colombia
The neurotransmitters at synapses in taste buds are not yet known
with confidence. Here we report a new calcium-imaging technique for
taste buds that allowed us to test for the presence of glutamate receptors (GluRs) in living isolated tissue preparations. Taste cells
of rat foliate papillae were loaded with calcium green dextran (CaGD).
Lingual slices containing CaGD-labeled taste cells were imaged with a
scanning confocal microscope and superfused with glutamate (30 µM to 1 mM), kainate (30 and 100 µM), AMPA (30 and 100 µM), or NMDA (100 µM). Responses were observed in 26% of the cells that
were tested with 300 µM glutamate. Responses to glutamate were localized to the basal processes and cell bodies, which are synaptic regions of taste cells. Glutamate responses were
dose-dependent and were induced by concentrations as low as 30 µM. The non-NMDA receptor antagonists CNQX and GYKI 52466 reversibly blocked responses to glutamate. Kainate, but not AMPA, also
elicited Ca2+ responses. NMDA stimulated increases
in [Ca2+]i when the bath medium was
modified to optimize for NMDA receptor activation. The subset of cells
that responded to glutamate was either NMDA-unresponsive (54%) or
NMDA-responsive (46%), suggesting that there are presumably two
populations of glutamate-sensitive taste cells one with NMDA receptors
and the other without NMDA receptors. The function of GluRs in taste
buds is not yet known, but the data suggest that glutamate is a
neurotransmitter there. GluRs in taste cells might be presynaptic
autoreceptors or postsynaptic receptors at afferent or efferent synapses.
Key words:
calcium imaging; taste bud; gustatory system; glutamate
receptors; tongue; foliate papilla; kainate; NMDA; calcium green
dextran
Copyright © 2000 Society for Neuroscience 0270-6474/00/20217978-08$05.00/0
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