Electrophysiological evidence for a chemotopy of biologically relevant odors in the olfactory bulb of the channel catfish

Extracellular electrophysiological recordings from single olfactory bulb (O B) neurons in the channel catfish, Ictalurus punctatus, indicated that the OB is divided into different functional zones, each processing a specific c lass of biologically relevant odor. Different OB regions responded preferen tially at slightly above threshold to either a mixture of 1) bile salts (10 (-7) to 10(-5) M Na+ salts of taurocholic, lithocholic, and taurolithocholi c acids), 2) nucleotides [ 10(-6) to 10(-4) M adenosine-5'-triphosphate (AT P), inosine-5'-monophosphate (IMP), and inosine-5'-triphosphate (ITP)], or 3) amino acids (10(-6) to 10(-4) M L-alanine, L-methionine, L-arginine, and L-glutamate). Excitatory responses to bile salts were observed primarily i n a thin, medial strip in both the dorsal (100-450 mum) and ventral (900-1, 200 mum) OB. Excitatory responses to nucleotides were obtained primarily fr om dorsal, caudolateral OB, whereas excitatory responses to amino acids occ urred more rostrally in the dorsolateral OB, but continued more medially in the ventral OB. The chemotopy within the channel catfish OB is more compar able to that previously described by optical imaging studies in zebrafish t han by field potential studies in salmonids. The present results are consis tent with recent studies, suggesting that the specific spatial organization of output neurons in the OB is necessary for the quality coding/decoding o f olfactory information.