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

Abstract

Wild animals tend to avoid novel objects that do not elicit clear avoidance behaviors in domesticated animals. We previously found that the basolateral complex of the amygdala (BLA) and dorsal bed nucleus of the stria terminalis (dBNST) were larger in trapped wild rats compared with laboratory rats. Based on these findings, we hypothesized that the BLA and/or dBNST would be differentially activated when wild and laboratory rats showed different avoidance behaviors towards novel objects. In this study, we placed novel objects at one end of the home cage. We measured the time spent in that half of the cage and expressed the data as a percentage of the time spent in that region with no object placement. We found that this percentage was lower in the wild rats compared with the laboratory rats. These behavioral differences were accompanied by increased Fos expression in the BLA, but not in the dBNST, of the wild rats. These results suggest that wild rats show greater BLA activation compared with laboratory rats in response to novel objects. We also found increased Fos expression in the paraventricular nucleus of the hypothalamus, ventral BNST, and ventromedial hypothalamus, but not in the central amygdala of wild rats. Taken together, our data represent new information regarding differences in behavioral and neural responses towards novel objects in wild vs. laboratory rats.

Existence of wild brown rats (<i>Rattus norvegicus</i>) that are indifferent to novel objects

“…Based on the findings of the present and previous studies [30], we hypothesize that an individual's response towards novel objects is determined by two variables. Most studies in the literature have attempted to explain the response towards novel objects using only one variable.…”

“…Neophobia could be another representative phenotype that laboratory rats do not have. The importance of BLA was suggested based on our previous studies using wild rats [30]. We consider that examining the underlying physiological and/or neural mechanisms of the phenotypes would be valuable because wild rats showing the same phenotypes would share the Koizumi et al same underlying mechanisms.…”

“…Behavioral tests were conducted in the home cage of each subject between 10:00 to 15:00, as reported in our previous study [ 30 ]. We first transferred the cage to a table in the colony room and inserted a partition into the center of the cage to confine the subject to the non-preferred side.…”

“…Laboratory and wild brown rats are conspecific and are capable of producing hybrids [ 6 ], even though the oldest strain of laboratory rat (Wistar strain) has been maintained in laboratories for more than 100 years [ 32 ]. We previously established an experimental design for observing new-object reaction in home cages [ 30 ]. When we placed novel objects (two pumpkin head plastic dolls or two plastic bear dolls) at one end of the home cage, wild and laboratory rats decreased and increased the time that they spent in the objects’ half of the cage, respectively.…”

“…Consistent with this notion, non-pest rats, such as long-haired rats ( R. villosissimus ) and bush rats ( R. fuscipes ), did not show neophobia, even when trapped individuals were tested in the laboratory [ 10 ]. We previously showed that brown rats caught in a park in a downtown area of Tokyo, Japan, showed high levels of neophobia [ 30 ]. In municipalities, even a small increase in rat populations prompts local residents to report these increases to the local public health center.…”