Remembering the opponent: neuronal activation associated with social memory in zebrafish.

Learning and remembering an opponent and the characteristics of a previous encounter may allow individuals to modify their behavior based on that acquired information. In the present study, using a long-term memory paradigm associated with social interactions, we aimed to evaluate the neuronal activation underlying individual recognition and memory of a previous agonistic encounter in male zebrafish. By quantifying the immediate-early gene c-fos, we compared neuronal activation across different telencephalic nuclei of the social decision-making network. Two dorsal nuclei, the medial (Dm) and the lateral (Dl), and two ventral nuclei, the ventral (Vv) and the dorsal (Vd), were evaluated by quantifying c-fos protein by immunohistochemical techniques. Two agonistic encounters between the same pair of opponents were performed. The number of c-fos-immunopositive cells was quantified immediately after the second encounter in fish that either exhibited behavioral evidence of remembering their opponent, behaved as if they did not recognize the opponent due to treatment with an amnesic agent after the first encounter, or belonged to a no-agonistic-encounter group with no physical interaction. We found that the Vv nucleus showed lower activation in individuals who were not exposed to physical interaction than in those who participated in agonistic encounters. The Dl nucleus showed a higher activation only in individuals who would face an unremembered opponent, but not in those who recognized the opponent. Our findings provide a first step toward understanding the neuronal processes underlying opponent recognition and retrieval of prior agonistic experience.