Differential localization of putative amino acid receptors in taste buds of the channel catfish,Ictalurus punctatus

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Abstract
The taste system of catfish, having distinct taste receptor sites for L‐alanine and L‐arginine, is highly sensitive to amino acids. A previously described monoclonal antibody (G‐10), which inhibits L‐alanine binding to a partial membrane fraction (P2) derived from catfish (Ictalurus punctatus) taste epithelium, was found in Western blots to recognize a single band, at apparent MW of 113,000 D. This MW differs from the apparent MW for the presumed arginine receptor identified previously by PHA‐E lectin affinity. In order to test whether PHA‐E lectin actually reacts with the arginine‐receptor, reconstituted membrane proteins partially purified by PHA‐E affinity were used in artificial lipid bilayers. These reconstituted channels exhibited L‐arginine‐activated activity similar to that found in taste cell membranes. Accordingly, we utilized the PHA‐E lectin and G‐10 antibody as probes to differentially localize the L‐alanine and L‐arginine binding sites on the apical surface of catfish taste buds. Each probe labels numerous, small (0.5–1.0 μm) patches within the taste pore of each taste bud. This observation suggests that each bud is not tuned to a single taste substance, but contains putative receptor sites for both L‐arginine and L‐alanine. Further, analysis of double‐labeled tissue reveals that the PHA‐E and G‐10 sites tend to be separate within each taste pore. These findings imply that in catfish, individual taste cells preferentially express receptors to either L‐arginine or L‐alanine. In addition, PHA‐E binds to the apices of solitary chemoreceptor cells in the epithelium, indicating that this independent chemoreceptor system may utilize some receptor sites similar to those in taste buds.