Uptake and release of neurotransmitter candidates, [³h]serotonin, [³H]glutamate, and [³H]γ-Aminobutyric acid, in taste buds of the mudpuppy, Necturus maculosus

Neurotransmitters in vertebrate taste buds have not yet been identified with confidence. Serotonin, glutamate, and γ-aminobutyric acid (GABA) have been postulated, but the evidence is incomplete. We undertook an autoradiographic study of [³H]serotonin, [³H]glutamate, and [³H]GABA uptake in lingual epithelium from the amphibian, Necturus maculosus, to determine whether taste bud cells would accumulate and release these substances. Lingual epithelium containing taste buds was incubated in low concentrations (0.4-6 μM) of these tritiated transmitter candidates and the tissue was processed for light microscopic autoradiography. Merkel-like basal taste cells accumulated [³H]serotonin. When the tissue was treated with 40 mM K⁺ after incubating the tissue in [³H]serotonin, cells released the radiolabelled transmitter. Furthermore, depolarization (KCl)-induced release of [³H]serotonin was Ca-dependent: if Ca²⁺ was reduced to 0.4 mM and 20 mM Mg²⁺ added to the high K⁺ bathing solution, Merkel-like basal cells did not release [³H]serotonin. In contrast, [³H]glutamate was taken up by several cell types, including non-sensory epithelial cells, Schwann cells, and some taste bud cells. [³H]glutamate was not released by depolarizing the tissue with 40 mM K⁺. [³H]GABA uptake was also widespread, but did not occur in taste bud cells. [³H]GABA accumulated in non-sensory epithelial cells and Schwann cells. These data support the hypothesis that serotonin is a neurotransmitter or neuromodulator released by Merkel-like basal cells in Necturus taste buds. The data do not support (nor rule out) a neurotransmitter role for glutamate or GABA in taste buds.