In this study, we used the immediate early gene, egr-1, as a marker for neural activation and examined whether egr-1 expression is affected in brain regions associated with the social behavioral network (SBN) when social rank is determined and changed in male medaka fish (Oryzias latipes). Based on the behavioral contest protocol used in this study, we obtained four types of males: social ascending, social descending, dominant, and subordinate. In some brain regions associated with the SBN, we detected higher egr-1 expression in ascending and descending males than in dominant and subordinate males. Social-rank stable males (i.e., dominant and subordinate male fish) showed a similar level of egr-1 expression as the control male fish, which were housed without social stimulus of encountering another conspecific. These findings suggested that the transitioning of social rank could enhance neural activity in some brain regions associated with the SBN in male medaka. The use of medaka fish has many advantages in various fields of research such as genetics, developmental biology, environmental biology, and behavioral neurology. The findings of this study would contribute to future research exploring the roles of the SBN regions in regulating physiological and behavioral events associated with social-rank transition.
Keywords: immediate early gene; male competition; medaka; social behavioral network; social-rank transition.