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.