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.

Female crucian carp, Carassius carassius, lose predator avoidance behavior when getting ready to mate.

In predator-prey interactions, the prey often have to compromise fitness-related behaviors such as feeding, courting, and territorial defense in order to avoid predators. In these trade-off situations, some behaviors have priority over others. These priorities are not rigid, and may be context-dependent; for instance, many animals show increased risk-taking during courtship behavior by paying less attention to potential predators. We investigated whether the fright reaction, a stereotypical avoidance response to olfactory cues from injured conspecifics, may be affected by reproductive status in a teleost fish, the crucian carp. We demonstrate that among individuals not responding to alarm substances with a fright reaction, the majority were ovulated or spermiated. In females, mean plasma concentrations of 17beta-estradiol and testosterone, gonadal steroids known to decrease during the later stages of sexual maturation, were lower in the individuals not responding with a fright reaction compared to those responding. In males, there were no differences between responsive and non-responsive individuals in mean plasma levels of androgens (testosterone and 11-ketotestosterone) involved in spermatogenesis and male sexual behavior. As the fright reaction in crucian carp consists of behavior incompatible with spawning behavior, we hypothesize that this short-term suppression of the alarm response has evolved so that spawning can occur uninterrupted.

Effects of antalarmin, a CRF receptor 1 antagonist, on fright reaction and endocrine stress response in crucian carp (Carassius carassius).

The corticotrophin-releasing factor (CRF) receptors show striking homogeneity throughout the vertebrate subphylum. In mammals, the CRF(1) receptor (CRFR(1)) plays an important role in mediating behavioral and endocrine responses to fear and stress. The specific roles of this receptor subtype in fear and stress reactions in non-mammalian vertebrates are largely unknown. Crucian carp displays the olfactory-mediated fright reaction, a stereotypic behavioral response to waterborne cues from damaged skin of conspecifics. This reaction shows several similarities to basic components of avoidance behavior in mammals. In the present study, we applied the non-peptide CRFR(1) antagonist, antalarmin, to crucian carp 1 h before exposure to conspecific skin extract. This treatment resulted in a suppression of the fright reaction. After skin extract exposure, antalarmin treatment also lead to lower plasma cortisol values, as compared to vehicle treatment. This suppression of the behavioral fright reaction and the stress induced rise in plasma cortisol in crucian carp suggests that the functions of the CRFR(1) are conserved by evolution.

Seasonal variations in olfactory sensory neurons--fish sensitivity to sex pheromones explained?

Olfactory sensory neurons of vertebrates regenerate throughout the life of the animal. In fishes, crypt cells are a type of olfactory sensory neurons thought to respond to sex pheromones. Here, we demonstrate that the number of crypt cells in the olfactory epithelium of the crucian carp varies dramatically throughout the year. During winter, few crypt cells are observed at any location within the sensory epithelium. In spring, the majority of crypt cells are located deep in the epithelium not yet exposed to the environment. However, during the summer spawning season, crypt cells are positioned at the epithelial surface. These findings may explain previous studies demonstrating a relationship between circulating androgen and olfactory sensitivity to sex pheromones.

Gender distinction in neural discrimination of sex pheromones in the olfactory bulb of crucian carp, Carassius carassius.

Studies on projection of the sensory neurons onto the olfactory bulb in fish have revealed a clear subdivision into spatially different areas that each responded specifically to different classes of odorants. Amino acids induce activity in the lateral part, bile salts induce activity in the medial part, and alarm substances induce activity in the posterior part of the medial olfactory bulb. In the present study, we demonstrate a new feature of the bulbar chemotopy showing that neurons specifically sensitive to sex pheromones are located in a central part of the ventral olfactory bulb in crucian carp. Extensive single-unit recordings were made from these neurons, stimulating with four sex pheromones, 17,20beta-dihydroxy-4-pregnen-3-one, 17,20beta-dihydroxy-4-pregnen-3-one-20-sulfate, androstenedione, and prostaglandin F(2alpha), known to induce specific reproductive behaviors in males of carp fish. All substances were applied separately to the sensory epithelium at a concentration of 10(-9) M. Of the 297 neurons recorded in males, the majority (236 or 79.5%) responded exclusively to one of the four sex pheromones and thus showed a high specificity. Of the 96 neurons recorded from the olfactory bulb in females, only 1 unit showed such a specific activation. These findings reflect remarkable differences between males and females in the discriminatory power of the olfactory neurons toward these sex pheromones. The gender differences are discussed in relation to behavior studies, expression of olfactory receptors, and the convergence of sensory neurons onto the secondary neurons in the olfactory bulb.