Taste cell heterogeneity and GABA neurotransmission in facial and vagal nerve innervated taste buds of channel catfish, Ictalurus punctatus

Taste buds are comprised of group of 50-150 elongated taste cells grouped together in a tulip-shaped structure predominantly located on the tongue and soft palate. Tens of thousands of taste buds occupy the lingual epithelium of most vertebrates and provide for the detection, selection and consumption of food. Catfish have a greatly elaborated gustatory system with taste buds distributed over the entire body surface and barbels, as well as the oropharyngeal cavity. These extra-oral taste buds are innervated by the facial nerve and are important in the search for food. By contrast, oropharyngeal taste buds innervated by vagal and glossopharyngeal nerves determine the acceptability of food for consumption. Differences in the sensitivity of facial and vagal nerves to amino acid stimuli have been demonstrated in several electrophysiological studies, which lead us to their detailed examination for the underlying morphological differences. Early studies classified taste cells into two to three types based on electron microscopical properties, while recent studies with functional and histochemical markers point to a more diverse taste cell population. The present project evaluates taste cell heterogeneity using immunochemical staining, image analysis and multispectral classification techniques to determine the distribution of several small molecular weight metabolites including ?-aminobutyric acid (GABA), glutamate, aspartate, alanine, taurine and glutathione. Unique levels of expression are typical for each substrate in the facial (FITBs) and vagal nerve innervated taste buds (VITBs). High levels of GABA occur in a prominent subset of taste cells, which suggests the possibility of GABAergic signaling in catfish FITBs and VITBs. Collectively, the findings confirm the existence of basic cytochemical and morphometric differences between the FITBs and VITBs, which could account for differences in functional sensitivity and behavioral patterns. In particular, the presence of GABA in diverse subsets of taste cells strongly suggests an important role for GABAergic mechanisms in gustatory neuromoldulation of transduction-transmission in the channel catfish.