Oxytocin (OXT) is a kind of neuromodulator which is known to be required for many social behaviors such as mother-kin bonding and affiliative behavior towards familiar individuals. In addition, recent studies have reported that OXT treatments improved some social defects in autism spectrum disorder (ASD) patients. Abnormal synapse pruning by microglia in the brain has been proposed as a potential mechanism of ASD.
Previously, our group reported that wild-type (WT) medaka (Oryzias latipes)  females preferred to mate with visually familiarized males, but oxytocin (oxt) mutant females lost this tendency. However, the cause of this behavioral abnormality was remained to be elucidated.
In this study, we report c1qb mutant medaka females lost their sexual preference as the oxt mutant females did. In addition, ionized calcium binding adaptor molecule 1 (Iba1) staining - a microglial/macrophage marker- revealed that the sizes of Iba1+ cell bodies in oxt mutant females were smaller than those in WT females, although the number of Iba1+ cells was not significantly different between them. These results suggested that oxt was required for normal activity of microglia and regulated neural developmental process such as synaptic pruning, which worked via C1q pathway, leading to the female sexual preference in medaka.