Electrophysiological evidence for acidic, basic, and neutral amino acid olfactory receptor sites in the catfish.

Abstract

Electrophysiological experiments indicate that olfactory receptors of the channel catfish, Ictalurus punctatus, contain different receptor sites for the acidic (A), basic (B), and neutral amino acids. At least 2 partially interacting neutral sites exist, 1 for the hydrophilic neutral amino acids containing short side chains (SCN), and the 2nd for the hydrophobic amino acids containing long side chains (LCN). The extent of cross-adaptation was determined by comparing the electro-olfactogram (EOG) responses to 20 test amino acids during continuous bathing of the olfactory mucosa with water only (control) to those during each of the 8 adapting amino acid regimes. Both the adapting and test amino acids were adjusted in concentrations to provide approximately equal response magnitudes in the unadapted state. Under all 8 adapting regimes, the test EOG responses were reduced from those obtained in the unadapted state, but substantial quantitative differences resulted, depending upon the molecular structure of the adapting stimulus. Analyses of the patterns of EOG responses to the test stimuli identified and characterized the respective transduction processes, a term used to describe membrane events initiated by a particular subset of amino acid stimuli that are intricately linked to the origin of the olfactory receptor potential. Only when the stimulus compounds interact with different transduction processes are the stimuli assumed to bind to different membrane sites. Four relatively independent L-.alpha.-amino acid transduction processes (and thus at least 4 binding sites) identified include the A process for aspartic and glutamic acids; the B process for arginine and lysine; the SCN process for glycine, alanine, serine, glutamine and possibly cysteine; and the LCN process for methionine, ethionine, valine, norvaline, leucine, norleucine, glutamic acid-.gamma.-methyl ester, histidine, phenylalanine and possibly cysteine. The specificities of these olfactory transduction processes in the catfish are similar to those for the biochemically determined receptor sites for amino acid in other species of fishes and to amino acid transport specificities in tissues of a variety of organisms.