Introduction Ollaetion Morphological basis an overview Peripheral olfactory organ Central projections Registration of olfactory activities Electroolfactogram (EOG) ExtraceUular recordings of action potentials Electroencephalogram (EEG) Cross-adaptation Responses to chemical stimuli Amino acids Steroid hormones Bile salts Prostaglandins (PGs) Other compounds Gustation Morphological basis an overview Taste buds Gustatory nerves and central projections Recordings of gustatory responses Responses to chemical stimuli Amino acids Carboxylic acids Nucleotides Bile salts CO2 and pH Toxins and other compounds Future prospects and research needs Summary and conclusions Olfaction Gustation Acknowledgements References

The diversity of chemical stimulation in fish olfaction and gustation

The characteristics of the electro-olfactogram (EOG): its loss and recovery following olfactory nerve section in rainbow trout (Salmo gairdneri).

1. The specificity and sensitivity of the olfactory organ of adult zebrafish, Danio rerio, to selected amino acid, bile acid, and steroid odorants were characterized using the electro-olfactogram recording technique. The olfactory organ was responsive to 28 of the 29 odorants tested. 2. All of the 100 microM amino acid and bile acid stimulants elicited a negative-going response that was significantly greater than the response to the artificial fresh-water control. The general pattern of relative stimulatory effectiveness established for the amino acid stimuli was neutral amino acids > basic amino acids > acidic amino acids > imino acids. The general pattern of relative stimulatory effectiveness of 100 microM bile acid stimuli was taurine-conjugated bile acids > glycine-conjugated bile acids approximately non-conjugated bile acids. The responses to the most stimulatory bile acid odorants were up to 40% larger than the responses to the most stimulatory amino acid odorants. 3. The response threshold for cysteine and taurocholic acid, the most stimulatory of the amino acid and bile acid stimuli tested, was approximately 10(-8) M. Females are significantly more sensitive to these odorants than males.

Specificity and sensitivity of the olfactory organ of the zebrafish, Danio rerio

Three morphologically distinct classes of receptor-neurons are proposed: (1) type I ciliar cells, (2) microvillar cells and (3) type II ciliar cells. Retrograde transport of horseradish peroxidase by axons in the olfactory nerve to the olfactory organs of goldfish (Carassius auratus) and channel catfish (Ictalurus punctatus) provided evidence that these axon-bearing cells are present in the organs of both species. Goldfish olfactory organs were also studied with scanning electron microscopy, dissociated with papain for isolated cell preparations, and processed for ultrastructural localization of acid phosphatase activity. Type I ciliar cells are similar to ciliar olfactory receptors found in all vertebrate classes. Microvillar cells are present in the olfactory organs of most fishes and in the tetrapod vomeronasal organ. In goldfish and catfish, type I ciliar and microvillar cells are concentrated on the inner third of each lamella, nearest to the median raphe. Type II ciliar cells have often been described as respiratory-type or ciliated nonsensory cells. They are structurally similar to respiratory epithelial cells in the nasal cavities of tetrapods and have motile cilia that beat synchronously, indicative of their role in mediating fluid flow over the olfactory epithelium. In goldfish they occur singly and in aggregates throughout the organ. In catfish they are segregated from type I ciliar and microvillar cells on the outer two-thirds of each lamella. In goldfish and catfish they have axons that pass through the olfactory nerve to the olfactory bulb; hence, they are receptor-neurons as well as analogous to respiratory epithelium.



In addition to the three receptor types described above, cells resembling receptors with rodlike distal processes were seen filled with horseradish peroxidase and observed with scanning and transmission electron microscopy. Cells of similar structure have been documented elsewhere, often called “rod cells,” and sometimes considered a separate receptor type in fishes. In this study, a number of rodlike processes were found with their ciliar or microvillar components partially fused. High levels of acid phosphatase activity were localized to these processes, and examples were found that corresponded to each of the three receptor types. Olfactory receptor turnover is believed to persist through life. The evidence presented supports the hypothesis that fusion of their dendritic apical processes marks an early stage of receptor cell senescence.

Three distinct morphological classes of receptors in fish olfactory organs.

1.

Electrophysiological responses to chemical stimuli were recorded from the spinal nerves innervating the free fin rays of the searobin,Prionotus carolinus. These modified pectoral fin rays are capable of detecting chemical stimuli although they contain neither taste buds nor olfactory receptors.




2.

Squid extract elicited a vigorous response and of the 24 compounds which have been found in squid extract (Mackie 1982) 15 were tested. Ten of these compounds elicited increases in nerve activity and 5 were nonstimulatory (Table 1). Betaine (trimethylglycine) was the most stimulatory compound tested (Table 1) with a threshold of 10−6.2 mol/l.




3.

Response magnitude increased with increasing concentration over approximately 2–3 log units for the most potent stimuli and saturated at higher concentrations (Figs. 4, 6).




4.

Elimination of the methyl groups from betaine led to an increase in threshold concentration and a decrease in relative effectiveness. Substitution of a neutral hydroxymethyl group for the primary carboxyl group of betaine (choline) eliminated the response entirely (Table 1).




5.

Single unit responses to chemical stimuli were similar to the multiunit integrated responses in terms of relative effectiveness, threshold, and concentration-response curves.




6.

Vigorous responses to tactile and proprioceptive stimuli were obtained from all of the animals tested, indicating that the searobin fin rays are sensitive to mechanical as well as chemical stimulation.

Electrophysiological examination of a non-olfactory, non-gustatory chemosense in the searobin,Prionotus carolinus

A comparison of olfactory receptor responses of three species of freshwater catfishes (family Ictaluridae) and one species of marine catfish (family Arridae) was performed. With the exception of the reduced response of the sea catfish (Arius felis) to L-glutamine, no distinctive species specific differences in the electro-olfactogram (EOG) responses were noted. The phasic EOG response increased exponentially with a logarithmic increase in amino acid concentration from threshold to 10−2M. The relative stimulatory effectiveness of the amino acids tested was similar among the four species. The olfactory receptors were highly responsive to L-cysteine and to amino acids containing five carbon atoms having unbranched and uncharged side chains. From phyletic relationships of the species tested, the present data suggest that the olfactory system of the North American catfishes has remained temporally conservative.

Similarity of olfactory receptor responses (EOG) of freshwater and marine catfish to amino acids

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Similarity of olfactory receptor responses (EOG) of freshwater and marine catfish to amino acids