Processing of bile salt odor information by single olfactory bulb neurons in the channel catfish

A chemotopic map of biologically relevant odorants (that include amino acids, bile salts, and nucleotides) exists in the olfactory bulb (OB) of channel catfish, Ictalurus punctatus. Neurons processing bile salt odorant information lie medially within this OB map; however, information as to how single neurons process bile salt odorant information is lacking. In the present report, recordings were obtained from 51 OB neurons from 30 channel catfish to determine the excitatory molecular receptive range (EMRR) of bile salt responsive neurons. All recordings were performed in vivo within the medial portions of the OB using extracellular electrophysiological techniques. Excitatory thresholds to bile salts typically ranged between 0.1 and 10 muM. The bile salt specificity of OB neurons were divided into three groups: neurons excited by taurine-conjugated bile salts only (group T), neurons excited by nonconjugated bile salts only (group N), and neurons excited by at least one member of each of the three classes of bile salts tested (group G). In addition to the conjugating group at C24 of the side-chain, OB neurons discriminated bile salts by the molecular features present at three other carbon positions (C3, C7, and C12) along the steroid backbone. These data suggest that OB neurons are selectively excited by combinations of molecular features found on the side-chain and along the steroid nucleus of bile salt molecules.