In SituCa²⁺Imaging Reveals Neurotransmitter Receptors for Glutamate in Taste Receptor Cells

Abstract

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 M to 1 mM), kainate (30 and 100 M), AMPA (30 and 100 M), or NMDA (100 M). Responses were observed in 26% of the cells that were tested with 300 M 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 M. The non-NMDA receptor antagonists CNQX and GYKI 52466 reversibly blocked responses to glutamate. Kainate, but not AMPA, also elicited Ca 2ϩ responses. NMDA stimulated increases in [Ca 2ϩ ] 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.

Breadth of Tuning and Taste Coding in Mammalian Taste Buds

“…We prepared lingual slices containing the vallate papilla from adult mice (Ն8 weeks old) and loaded taste cells with a calcium indicator dye (Caicedo et al, 2000(Caicedo et al, , 2002. Briefly, a fluorescent Ca 2ϩ indicator dye was injected iontophoretically through a 40-m-tip glass micropipette into the crypt surrounding the vallate papilla.…”

“…We have presented fluorometric signals as relative fluorescence change: ⌬F/F ϭ (F Ϫ F 0 )/F 0 . Correction for photobleaching was applied when necessary (Caicedo et al, 2000). Taste cells were stimulated repeatedly with a selection of prototypic taste stimuli (see next section).…”

Modulation des perceptions gustatives par les afférences trigéminales : étude électrogustométrique pilote chez l’homme

“…Les réponses gustatives de la CT sont modulées par le CGRP (calcitonin gene-related peptide) et la cholecystokine [32]. Le traitement postnatal par injection de capsaïcine diminue la quantité de peptides tels que la substance P et le CGRP dans les papilles fongiformes et circumvallées de rat [33] et supprime la modulation des réponses gustatives de la CT occasionnées par la stimulation électrique du NL sur les réponses de la CT [17], suggérant ainsi que ces peptides pourraient être impliquées dans la modulation du signal gustatif de la CT. Des travaux ont montré que les fibres glutamatergiques de la CT et du NL pourraient participer à la modulation périphérique du signal gustatif dans des conditions physiologiques au niveau des papilles fongiformes [34,35].…”

Modulation des afférences gustatives linguales par les afférences du nerf alvéolaire inférieure : étude électro-gustomètrique chez l’homme

“…Les réponses gustatives de la CT sont modulées par le CGRP (calcitonin gene-related peptide) et la cholécystokine [32]. Le traitement postnatal par injection de capsaïcine diminue la quantité de peptides tels que la substance P et le CGRP dans les papilles fongiformes et circumvallées de rat [33] et supprime la modulation des réponses gustatives de la CT occasionnées par la stimulation électrique du NL sur les réponses de la CT [17], suggérant ainsi que ces peptides pourraient être impliqués dans la modulation du signal gustatif de la CT. Des travaux ont montré que les fibres glutamatergiques de la CT et du NL pourraient participer à la modulation périphérique du signal gustatif dans des conditions physiologiques au niveau des papilles fongiformes [34,35].…”