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 a marker of neuronal activation, the immediate early gene c-fos, we compared the activation in different telencephalic nuclei belonging to 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, and the number of c-fos-immunopositive cells was quantified immediately after the second encounter in fish that had been treated with an amnesic agent after the first encounter (MK-801) and untreated individuals (Water), as well as a Control group with no physical interaction. We found that the Vv nucleus showed lower activation in individuals not exposed to physical interaction than those exposed (MK-801 and Water treatment). The Dl nucleus showed a higher activation only in individuals previously treated with MK-801, who would face an unremembered opponent, but not in those treated with water, which recognized the opponent. Our study provides evidence as a first step to understanding the neuronal processes underlying individual recognition and retrieval of an opponent. Summary statementOur study provides new information about the neuronal activation related to individual recognition and social interaction in zebrafish