The spatial distribution of ciliated and microvillous olfactory receptor neurons in the channel catfish is not matched by a differential specificity to amino acid and bile salt stimuli

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Abstract
The distribution of two receptor cell types, stellate microvillous and ciliated, was quantitatively examined in the sensory regions of the olfactory lamellae of the channel catfish, Ictalurus punctatus . Using scanning electron microscopy (SEM), these two cell types were seen to exhibit an incomplete segregation across the sensory epithelium. The density of stellate microvillous cells was greatest in the dorso-medial portion of the sensory region of the lamella and markedly decreased ventrally and laterally. The ciliated cells were more numerous in the ventro-lateral portion of the sensory region and decreased in number dorso-medially. Multi-unit olfactory neural responses to amino acids and bile salts were recorded from the dorso-medial and ventro-lateral portions of the sensory region. In both portions of the sensory region, L-cysteine, L-alanine and L-norleucine were highly stimulatory, while the sodium salts of cholic acid, taurocholic acid and taurolithocholic acid (i.e., bile salts) were the least stimulatory compounds tested. With the exception of L-norleucine, no significant differences were found in the responses from the dorso-medial and ventro-lateral portions of the sensory region for the compounds tested. These results indicate that in I. punctatus the response specificities of the microvillous and ciliated olfactory receptor cells are similar for amino acids and bile salts.