The alarm reaction in fishes--odorants, modulations of responses, neural pathways.

In 1938, von Frisch discovered the alarm reaction ("schreck reaktion") in fishes. He found that substances stored in the skin are released upon injury and detected by the olfactory organ. Several agonists that induce alarm reactions have been identified since then, but the true nature of the alarm substances remains veiled. Behavioral experiments have demonstrated that skin extract of conspecifics is more potent than that of heterospecifics. During development, the alarm reaction is seen in the larval stages in several cyprinid species; the threshold for eliciting the reaction decreases with age. Further, in crucian carp the alarm reaction was shown to be suppressed in the majority of females during the spawning period. The anatomy of the olfactory organ of this species is advantageous for studying underlying mechanisms of olfactory-induced behaviors. Crucian carp have been applied as a model organism in our lab for identification of the neural pathway of the alarm reaction, mainly in primary and secondary olfactory neurons. In the olfactory bulb, there is a distinct region that responds specifically to skin extract. The neural basis for discrimination between extracts of skin from different species has been investigated, suggesting that identification of conspecifics may be based on activation of specific subsets of neurons.