Behavioral effects of chronic stress in Carioca high- and low-conditioned freezing rats.

Chronic unpredictable mild stress (CUMS) is a widely used model to study stress-coping strategies in rodents. Different factors have been shown to influence whether animals adopt passive or active coping responses to CUMS. Individual adaptation and susceptibility to the environment seem to play a critical role in this process. To further investigate this relationship, we examined the effects of CUMS on Carioca high- and low-conditioned freezing rats (CHF and CLF, respectively), bidirectional lines of animals selected for high and low freezing in response to contextual cues that were previously associated with footshocks. For this purpose, the behavior of CHF and CLF animals was evaluated in the contextual fear conditioning, open field, elevated T maze, and forced swimming tests before and after 21 days of CUMS. For all tests, CHF rats were more susceptible to the effects of CUMS compared to CLF. CHF animals exposed to CUMS displayed a reduction in freezing behavior, decreased number of entries and time spent in the center of the open field, greater latencies to become immobile, and increased avoidance and escaping behaviors in the elevated T maze. Overall, these findings support the hypothesis that a heightened susceptibility to the environment exerts a strong influence on coping responses to chronic stress.

High-sugar/high-fat diet modulates the effects of chronic stress in cariocas high- and low-conditioned freezing rats.

Stress and eating disorders are closely related and are a topic of major concern due to their burden on human health. Engaging in unhealthy eating habits may come as a result of stress, and it often serves to alleviate the symptoms of anxiety or as a distraction from the stressor itself or self-awareness. However, it can also lead to negative feelings of a person's body figure, guilty, or shame. As diverse as these consequences are in humans, so are the effects of the combined administration of stress and hypercaloric food in animals' models. In this study, we assessed the influence of individual innate behavioral predisposition on the effects of chronic unpredictable mild stress and the dietary supplementation with high-sugar/high-fat food. These conditions were applied to male Carioca low- and high-conditioned freezing (CLF and CHF) rats for 21 days. Behavioral results show that the hypercaloric supplement had a protective effect over the alterations caused by the stress. Notably, it was more strongly observed in CHF rather than CLF animals. As the chronic stress led to an impaired behavior in the contextual fear conditioning and the forced swimming tests in the CLF line, animals fed with the HSHF pellet scored responses similar to their untreated control. On CHF rats, these effects also were seen to a broader extent on the open field test, where the locomotor behavior was also increased. No major effects of the diet were seen in the unstressed groups. Overall, our results show that the influences of both chronic stress and hypercaloric feeding depend on innate differences in fear response traits of male Carioca rats.

Haloperidol and methylphenidate alter motor behavior and responses to conditioned fear of Carioca Low-conditioned Freezing rats.

Animal models are important tools for studying neuropsychological disorders. Considering their limitations, a more extensive translational research must encompass data that are generated from several models. Therefore, a comprehensive characterization of these models is needed in terms of behavior and neurophysiology. The present study evaluated the behavioral responses of Carioca Low-conditioned Freezing (CLF) rats to haloperidol and methylphenidate. The CLF breeding line is characterized by low freezing defensive responses to contextual cues that are associated with aversive stimuli. CLF rats exhibited a delayed response to haloperidol at lower doses, needing higher doses to reach similar levels of catatonia as control randomly bred animals. Methylphenidate increased freezing responses to conditioned fear and induced motor effects in the open field. Thus, CLF rats differ from controls in their responses to both haloperidol and methylphenidate. Because of the dopamine-related molecular targets of these drugs, we hypothesize that dopaminergic alterations related to those of animal models of hyperactivity and attention disorders might underlie the observed phenotypes of the CLF line of rats.

Cued Fear Conditioning in Carioca High- and Low-Conditioned Freezing Rats.

Anxiety disorders (AD) comprise a broad range of psychiatric conditions, including general anxiety (GAD) and specific phobias. For the last decades, the use of animal models of anxiety has offered important insights into the understanding of the association between these psychopathologies. Here, we investigate whether Carioca high- and low-conditioned freezing rats (CHF and CLF, respectively), a GAD animal model of anxiety, show similar high- and low-freezing behavioral phenotypes for cued auditory fear conditioning. Adult CHF (n = 16), CLF (n = 16) and normal age-matched Wistar rats (control, CTL, n = 16) were tested in a classical auditory-cued fear conditioning paradigm over 3 days (Tone + Shock and Tone only groups, n = 8 per treatment). Freezing responses were measured and used as evidence of fear conditioning. Overall, both CHF and CLF rats, as well as CTL animals displayed fear conditioning to the auditory CS. However, CLF animals showed a rapid extinction to the auditory conditioned stimulus compared to CHF and CTL rats. We discuss these findings in the context of the behavioral and neuronal differences observed in rodent lines of high and low anxiety traits.

Effects of N-methyl-D-aspartate-induced amygdala lesion in rats submitted to the elevated T-maze test of anxiety.

The elevated T-maze (ETM) has been used to generate two defensive behaviors in the same rat, inhibitory avoidance and one-way escape, which have been related to generalized anxiety and panic, respectively. In the present study, we investigate the role of the amygdala on the modulation of these two behaviors. Male Wistar rats were tested in the ETM test 2, 7, or 14 days after bilateral N-methyl-D-aspartate (NMDA)-induced lesion of the amygdala. The animals were also tested in an open field for evaluation of motor performance. The results showed that animals tested 7 days after NMDA injection had impairment in the acquisition of inhibitory avoidance, indicating an anxiolytic effect. Lesion of the amygdala did not change one-way escape in any of the tested groups. These results provide further evidence for the involvement of the amygdala in the modulation of defensive behaviors that have been associated to generalized anxiety.