Neurophysiologic detector-a selective and sensitive tool in high-performance liquid chromatography

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.

Single unit responses to skin odorants from conspecifics and heterospecifics in the olfactory bulb of crucian carp Carassius carassius

Injured fish skin leaks alarm substances that induce the fright reaction upon olfactory detection. The skin also contains a multitude of other odorants traditionally related to other behaviors, but to what extent they are detected upon injury is unknown. We have performed single unit recordings in the olfactory bulb (OB) of crucian carp while exposing the olfactory epithelium to skin extracts from conspecifics and three other species of the carp family, common carp, tench and bream. The aims were to investigate whether neural activity may be induced by different types of skin odorants and how well the odorants from injured conspecifics are distinguished from other species. The OB of crucian carp shows a clear chemotopy as units located in different regions respond to either food-related odorants, to pheromones or to alarm odorants respectively. Units in all regions responded to skin extracts, which indicate the detection of odorants usually involved in reproduction and feeding, in addition to the alarm substances. Among OB units responding to only one of the skin extracts, most were sensitive to conspecific skin extract. Furthermore, pair-wise comparisons showed that the discrimination between conspecific skin extract and skin extract from another species was in general better than the discrimination between skin extracts from two heterospecifics. The findings suggest that identification of injured fishes may be based on different groups of odorants and that the crucian carp olfactory system discriminates well between odorants from conspecifics and those from other fish species.