Expression of Calcium-Binding Proteins, Calbindin D28k and Calretinin, in the Frog Taste Receptor Structures

Considering that information on the expression of calcium-binding proteins (CaBPs) in different cells of the taste receptors is rather limited, we investigated the distribution of such proteins, calbindin D28k (CB) and calretinin (CR), in the taste disc (TD) of the frog Lithobates catesbeianus. Western blot analysis revealed that CB and CR are expressed in cells of the fungiform papillae. CB-immunoreactive (ir) and CR-ir cell somata were located in the middle layer of the TD. Most CB-ir and CR-ir cells possessed one rod-shaped apical process and one basal process; in some cells there were several extended basal processes. Apical processes of CR-ir cells were thinner than those of CB-ir units, and CR-ir nerve fibers were ramified in the lamina propria directly below the TD. Most CR-ir fiber branches surrounded the TD; however, some penetrated this region, with both types of branches approaching the surface. CB and CR immunoreactivities did not co-occur in TD cells. In the TDs examined, the number of CB-ir cells was significantly greater than that of CR-ir units. Our observations suggest that CB-ir and CR-ir cells in the frog TD correspond to type-II and type-III cells, respectively.

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