Acute fluoxetine treatment increases aggressiveness in juvenile matrinxã (Brycon amazonicus).

Fluoxetine (FLX) is a selective serotonin (5-HT) reuptake inhibitor known for its effects modifying aggressiveness, personality traits, and anxiety-like behaviors. The aim of the present study was to evaluate the influence of the acute treatment, by immersion, with FLX on aggressive behavior of resident Brycon amazonicus fish. Fish pretreated with FLX presented an increase in aggressiveness, evidenced by the increase on the number of bites and chases against the intruder and a decrease in latency for the first attack, when compared to control fish. Together with previous studies, these results show the complexity of the neural modulation of the aggressive behavior in fish by 5-HTergic system.

GABAA-benzodiazepine receptors in the dorsomedial (Dm) telencephalon modulate restraint-induced antinociception in the fish Leporinus macrocephalus.

The possibility that fish experience pain has been denied based on the absence of the neural substrates to support this "experience". In this context, the identification of brain regions involved in nociception modulation could provide important insights regarding the processing of nociceptive information in fish. Our study evaluated the participation of the GABAA-benzodiazepine receptor in the dorsomedial (Dm) telencephalon in restraint-induced antinociception in the fish Leporinus macrocephalus through the microinjection of the anxiolytic drug midazolam. The microinjection of midazolam in the Dm did not alter the nocifensive response; however, this drug did block the inhibition of the nocifensive response to formaldehyde promoted by restraint stress. The fish that received midazolam (40nmol) microinjection prior to restraint (3 or 5min), followed by subcutaneous injection with formaldehyde presented a higher distance traveled than the fish that received saline microinjection. This effect might reflect the specific action of midazolam on benzodiazepine receptors in the Dm telencephalon, as pre-treatment with flumazenil, a benzodiazepine receptor antagonist, inhibited the effects of this drug. In the present study, we present the first evidence demonstrating a role for the dorsomedial telencephalic region in the modulation of stress-induced antinociception in fish, revealing new perspectives in the understanding of nociceptive information processing in this group.

Acute administration of a cannabinoid CB1 receptor antagonist impairs stress-induced antinociception in fish.

This study evaluated the influence of the pre-treatment with AM251 (a cannabinoid type I receptor (CB1) selective antagonist) on the stress-induced antinociception promoted by restraint in the fish Leporinus macrocephalus. The application of 3 and 5 min of restraint stress promoted an inhibition of the behavioural response to the subcutaneous injection of 3% formaldehyde (increase in locomotor activity), suggesting the activation of an antinociceptive system. The acute intraperitoneal administration of AM251 (3 mg·kg(-1)) impaired this antinociceptive response induced by 3 and 5 min of restraint stress. The fish treated with AM251 before the application of restraint stress presented an increase in locomotor activity after the subcutaneous injection of formaldehyde, similar to fish not exposed to restraint, suggesting that the stress-induced antinociception promoted by restraint in fish is probably mediated by cannabinoid CB1 receptors. The results presented in this paper suggest the participation of the endocannabinoid system in nociception modulation in fish, supporting the hypothesis that an endogenous antinociceptive system activated by restraint stress is present in fish and that the modulation of antinociception by the CB1 receptor is evolutionary well-conserved across vertebrates.

Cortisol influences the antipredator behavior induced by chemical alarm cues in the Frillfin goby.

We evaluated the effect of increased plasma cortisol levels on fish antipredator behavior induced by conspecific chemical alarm cues. The experimental model for the study was the Frillfin goby Bathygobius soporator. We first confirmed that the alarm substance induces typical defensive antipredator responses in Frillfin gobies and described their alarm substance cells (epidermal 'club' cells). Second, we confirmed that intraperitoneal cortisol implants increase plasma cortisol levels in this species. We then demonstrated that exogenous cortisol administration and subsequent exposure to an alarm substance decreased swimming activity to a greater extent than the activity prompted by either stimulus alone. In addition, cortisol did not abolish the sheltering response to the alarm chemical cue even though it decreased activity. As predators use prey movements to guide their first contact with the prey, a factor that decreases swimming activity clearly increases the probability of survival. Consequently, this observation indicates that cortisol helps improve the antipredator response in fish.

Stress-induced antinociception in fish reversed by naloxone.

Pain perception in non-mammalian vertebrates such as fish is a controversial issue. We demonstrate that, in the fish Leporinus macrocephalus, an imposed restraint can modulate the behavioral response to a noxious stimulus, specifically the subcutaneous injection of 3% formaldehyde. In the first experiment, formaldehyde was applied immediately after 3 or 5 min of the restraint. Inhibition of the increase in locomotor activity in response to formaldehyde was observed, which suggests a possible restraint-induced antinociception. In the second experiment, the noxious stimulus was applied 0, 5, 10 and 15 min after the restraint, and both 3 and 5 min of restraint promoted short-term antinociception of approximately 5 min. In experiments 3 and 4, an intraperitoneal injection of naloxone (30 mg.kg(-1)) was administered 30 min prior to the restraint. The 3- minute restraint-induced antinociception was blocked by pretreatment with naloxone, but the corresponding 5-minute response was not. One possible explanation for this result is that an opioid and a non-preferential µ-opioid and/or non-opioid mechanism participate in this response modulation. Furthermore, we observed that both the 3- and 5- minutes restraint were severely stressful events for the organism, promoting marked increases in serum cortisol levels. These data indicate that the response to a noxious stimulus can be modulated by an environmental stressor in fish, as is the case in mammals. To our knowledge, this study is the first evidence for the existence of an endogenous antinociceptive system that is activated by an acute standardized stress in fish. Additionally, it characterizes the antinociceptive response induced by stress in terms of its time course and the opioid mediation, providing information for understanding the evolution of nociception modulation.

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.