Shoaling can be considered a simple form of affective behavior displayed by social fish in which a single fish exhibits an tendency to approach others. In the present study, we adopted a dual approach to investigate shoaling behavior in the medaka fish (Oryzias latipes): a behavioral pharmacological approach to assess mirror approaching behavior and an immunohistochemical approach to examine the neurotransmitter distribution in the medaka telencephalon. In order to gain an insight into shoaling activity, we examined the pharmacological effects of the positive allosteric modulator of the gamma-aminobutyric acid (GABA) anti-anxiety drug, diazepam, and chlorpromazine, a predominantly dopaminergic antagonist on mirror approaching behavior and, in particular, mirror approaching time (MAT; a tendency to shoal) and swimming distance (SWD). Diazepam dose-dependently suppressed MAT, but had no effect on SWD. Conversely, chlorpromazine suppressed SWD without having effects on MAT. The present study demonstrates for the first time that an anti-anxiety drug selectively modifies shoaling behavior in fish. Although the mechanism of this modification remains to be fully identified, immunohistochemical analysis suggests that a positive allosteric modulator of GABA acts on two distinct telencephalic regions. Mirror approaching behavior therefore revealed a close relationship with the action of the GABA-nergic system and not the dopaminergic system in the medaka telencephalon.
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