Novel objects elicit greater activation in the basolateral complex of the amygdala of wild rats compared with laboratory rats

Approach/Avoidance Behavior to Novel Objects is Correlated with the Serotonergic and Dopaminergic Systems in the Brown Rat (Rattus norvegicus)

The brown rat (Rattus norvegicus) is known to show three types of behavioral responses to novel objects. Whereas some rats are indifferent to novel objects, neophobic and neophilic rats show avoidance and approach behavior, respectively. Here, we compared the dopaminergic, serotonergic, and noradrenergic systems immunohistochemically among these rats. Trapped wild rats and laboratory rats were first individually exposed to the novel objects in their home cage. Wild rats were divided into neophobic and indifferent rats depending on their behavioral responses. Similarly, laboratory rats were divided into neophilic and indifferent rats. Consistent with the behavioral differences, in the paraventricular nucleus of the hypothalamus, Fos expression in corticotropin-releasing hormone-containing neurons was higher in the neophobic rats than in the indifferent rats. In the anterior basal amygdala, the neophobic rats showed higher Fos expression than the indifferent rats. In the posterior basal amygdala, the neophobic and neophilic rats showed lower and higher Fos expressions than the indifferent rats, respectively. When we compared the neuromodulatory systems, in the dorsal raphe, the number of serotonergic neurons and Fos expression in serotonergic neurons increased linearly from neophobic to indifferent to neophilic rats. In the ventral tegmental area, Fos expression in dopaminergic neurons was higher in the neophilic rats than in the indifferent rats. These results demonstrate that approach/avoidance behavior to novel objects is correlated with the serotonergic and dopaminergic systems in the brown rat. We propose that the serotonergic system suppresses avoidance behavior while the dopaminergic system enhances approach behavior to novel objects.

An appeasing pheromone ameliorates fear responses in the brown rat (Rattus norvegicus)

The brown rat (Rattus norvegicus) is one of the major animals both in the laboratory and in urban centers. Brown rats communicate various types of information using pheromones, the chemicals that mediate intra-species communication in minute amounts. Therefore, analyses of pheromones would further our understanding of the mode of life of rats. We show that a minute amount of 2-methylbutyric acid (2-MB) released from the neck region can ameliorate fear responses both in laboratory rats and in wild brown rats. Based on these findings, we conclude that 2-MB is an appeasing pheromone in the brown rat. A better understanding of rats themselves would allow us to perform more effective ecologically based research on social skills and pest management campaigns with low animal welfare impacts, which might contribute to furthering the advancement of science and improving public health.

Wild brains: The value of neuroethological approaches in preclinical behavioral neuroscience animal models

For three decades, IBNS has provided a forum for the dissemination of behavioral neuroscience research, broadly defined. Throughout this time, research presented at the annual meetings has reflected representative trends in the field with an emphasis on relevant preclinical animal models. From its inception, IBNS has contributed to my professional development and evolving research interests. Unsurprisingly, throughout the three decades of its existence, IBNS annual programs have reflected research trends that have been thoughtfully evaluated, challenged, and, in some cases, recalibrated. An emphasis in my lab, for example, has slowly navigated toward the inclusion of more diverse species (e.g., nonhuman primate models, wild rats, wild and captive raccoons) assessed in settings that reflect more ethological relevance than typically observed in traditional laboratory settings. Consequently, my research interests are pivoting from laboratory animal model exclusive (L.A.M.E.) endeavors to more natural, diverse, ethoexperimental approaches. As progress toward translational findings for psychiatric and neurological conditions is considered, it is recommended that researchers remain open to nontraditional methodological approaches that incorporate diverse animal models and assessments to inform laboratory-generated findings.

Sex differences in the acute ethanol effects on object recognition memory: influence of estrous cycle

Effects of acute ethanol (EtOH) on memory depend on several factors, including type of behavioral task. Sex differences in EtOH effects have been reported in humans and animals, and recognition memory can be influenced by circulating sex hormones. The aim of this study was to investigate the influence of sex and estrous cycle in the acute effects of EtOH on novel object recognition memory in rats. Male and female Wistar rats were part of one of the groups: control, 0.6-g/kg EtOH and 1.8-g/kg EtOH (administered intraperitoneally before the training session). The estrous cycle was evaluated by vaginal smear. The task was conducted in an open field arena. During training, animals were exposed to two identical objects, and test sessions were performed 1 h (short-term) and 24 h (long-term) later. One of the objects was changed in each test. Increased novel object exploration was shown by male and female controls in the short- and long-term tests, respectively. In the short-term test, females did not show preference for the novel object, and EtOH 1.8 g/kg impaired performance in males. In the long-term test, both sexes showed object discrimination, and 1.8-g/kg EtOH reduced preference for the new object in male rats. The phase of the cycle, the performance on proestrus was worse compared with other phases, and EtOH failed to impair performance mainly on estrous. In conclusion, while male rats displayed ethanol-induced recognition memory deficit, female rats were unaffected by EtOH impairing effects. In addition, the performance of female rats was influenced by the estrous cycle phases.

Divergent Neural and Endocrine Responses in Wild-Caught and Laboratory-Bred Rattus Norvegicus

Although rodents have represented the most intensely studied animals in neurobiological investigations for more than a century, few studies have systematically compared neural and endocrine differences between wild rodents in their natural habitats and laboratory strains raised in traditional laboratory environments. In the current study, male and female Rattus norvegicus rats were trapped in an urban setting and compared to weight-and sex-matched conspecifics living in standard laboratory housing conditions. Brains were extracted for neural assessments and fecal boli were collected for endocrine [corticosterone and dehydroepiandrosterone (DHEA)] assays. Additionally, given their role in immune and stress functions, spleen and adrenal weights were recorded. A separate set of wild rats was trapped at a dairy farm and held in captivity for one month prior to assessments; in these animals, brains were processed but no hormone data were available. The results indicated that wild-trapped rats exhibited 31% heavier brains, including higher densities of cerebellar neurons and glial cells in the bed nucleus of the stria terminalis. The wild rats also had approximately 300% greater spleen and adrenal weights, and more than a six-fold increase in corticosterone levels than observed in laboratory rats. Further research on neurobiological variables in wild vs. lab animals will inform the extensive neurobiological knowledge base derived from laboratory investigations using selectively bred rodents in laboratory environments, knowledge that will enhance the translational value of preclinical laboratory rodent studies.

Existence of wild brown rats (Rattus norvegicus) that are indifferent to novel objects

Exposure to novel objects typically evokes avoidance behavior in wild animals, which is called neophobia. We previously found that wild brown rats (Rattus norvegicus) that were trapped in a park in downtown Tokyo, Japan, exhibited neophobia. We also found that this behavior was accompanied by the activation of the basolateral complex of the amygdala (BLA). Previous studies have suggested that genetic factors are the primary determinants of neophobia. Since rats in cities form populations with distinct genetic characteristics, it is reasonable to assume that wild rats caught at different locations in urban centers will exhibit different levels of neophobia. Here we assessed the intensity of neophobia in wild rats trapped at a wholesale market in Tokyo. Although we performed exactly the same experiment in which neophobia was observed in wild rats trapped at the park, the presence of novel objects did not affect the behaviors of wild rats trapped at the market. Conversely, laboratory rats showed approach and exploratory behaviors as seen in the previous study, suggesting that the experiment was performed appropriately. Compared to the laboratory rats, the lack of behavioral changes in the wild rats was accompanied by fewer Fos immunoreactive cells in the BLA. In addition, the numbers of Fos immunoreactive cells in the bed nucleus of the stria terminalis and ventromedial hypothalamus were similar between the two types of rats. The results demonstrated the existence of wild rats that were indifferent to novel objects.

The Norway rat, from an obnoxious pest to a laboratory pet

The laboratory rat was the first mammal domesticated for research purposes. It is descended from wild Norway rats, Rattus norvegicus, which despite their name likely originated in Asia. Exceptionally adaptable, these rodents now inhabit almost all environments on Earth, especially near human settlements where they are often seen as pests. The laboratory rat thrives in captivity, and its domestication has produced many inbred and outbred lines that are used for different purposes, including medical trials and behavioral studies. Differences between wild Norway rats and their laboratory counterparts were first noted in the early 20^th century and led some researchers to later question its value as a model organism. While these views are probably unjustified, the advanced domestication of the laboratory rat does suggest that resuming studies of wild rats could benefit the wider research community.