Taste cells in the gut and on the tongue. Their common, paraneuronal features

Chemoreception of foodstuff in the gut is performed by endocrine cells dispersed in the gut epithelium. They are bipolar cells extending an apical process to the gut lumen and releasing their messenger substances from the cell base in response to the apical stimuli. The cells share cell-biological features with neurons and are classified as paraneurons. Noteworthy, the gut paraneurons do not seem to be stimulated by monosodium glutamate (MSG) as our dog experiment indicates. Administration of 50 mM MSG into the duodenal loop of anesthetized dogs did not cause changes in the volume and protein output in the pancreatic juice. The gustatory cells in the taste bud show essentially the same structure and function as the gut endocrine cells. A single gustatory cell type (type III) seems to receive different chemical stimuli, whereas different endocrine cell types in the gut react to different stimuli. The gustatory cells possess numerous peptidergic-type granules besides synapse-associated small clear vesicles. The former granules, in the guinea pig and dog, are abundant in the cytoplasm, giving an endocrine-like appearance to the cell. Peptidic signal substances contained in the granules remain to be identified. Comparison of the gustatory cells in the taste bud with the endocrine cells in the gut and with other paraneurons may put into front certain hitherto unexplored structures and functions of the cells.