Antinociception in piauçu fish induced by exposure to the conspecific alarm substance.

Defense and antinociceptive responses can be elicited simultaneously by learned or innate danger signals when an organism is in imminent danger. Antinociception blocks the recuperative behavioral reactions following pain perception that could interfere with defensive efforts. Antinociception associated with fearful experiences involving a confrontation with a predator or a predator being in close proximity is well studied in mammals, but very little is known about how fear affects antinociception responses in fish. Antipredator behavior in Ostariophysan fish may be elicited by exposure to conspecific alarm substance (CAS) that can trigger a fear reaction. During the predator versus prey confrontation, the alarm pheromone system is activated and warns conspecifics about the assessment of predation risk. The purpose of the present study was to examine the possible activation of the endogenous analgesic system in Leporinus macrocephalus fish and to evaluate the modification of swimming activity induced by a nociceptive stimulus (i.e., a subcutaneous injection of 3% formalin) in fish that had previously been exposed to the CAS. The results show that formalin-mediated enhancement in swimming activity was significantly reduced after exposure to the CAS. This enhancement was blocked by naloxone (20mg/kg), which suggests that opioid signaling is involved. Therefore, we hypothesized that antinociceptive processes may occur in fish following exposure to a chemical substance that signals predation.

Behavioral characterization of the alarm reaction and anxiolytic-like effect of acute treatment with fluoxetine in piauçu fish.

In Ostariophysan fish, the detection of the alarm substance liberated into the water as a consequence of an attack by a predator elicits an alarm reaction or anti-predatory behavior. In this study, experiments were performed to: (i) describe and quantitatively characterize the behavioral and ventilatory responses in piauçu fish (Leporinus macrocephalus), individually and as part of a school, to conspecific alarm substance (CAS) and; (ii) test the effect of acute fluoxetine treatment on alarm reaction. Histological analysis revealed the presence of club cells in the intermediate and superficial layers of the epidermis. The predominant behavioral response to CAS was freezing for fish held individually, characterized by the cessation of the swimming activity as the animal settles to a bottom corner of the aquarium. Fish exposed to CAS showed decrease in the mean ventilatory frequency (approximately 13%) relative to control. In schools, CAS elicited a biphasic response that was characterized by erratic movements followed by increased school cohesion and immobility, reflected as an increased school cohesion (65.5% vs. -5.8% for controls) and in the number of animals near the bottom of the aquarium (42.0% vs. 6.5% for controls). Animals treated with single i.p. injections of fluoxetine (10 µg/g b.w.) did not exhibit alarm behavior following CAS stimulation. These results show that an alarm pheromone system is present in piauçu fish, evidenced by the presence of epidermal club cells and an alarm reaction induced by CAS and consequently of a chemosensory system to transmit the appropriate information to neural structures responsible for initiating anti-predator behavioral responses. In addition, fluoxetine treatment caused an anxiolytic-like effect following CAS exposure. Thus, the alarm reaction in piauçu can be a useful model for neuroethological and pharmacological studies of anxiety-related states.