A research team led by Hiroshima University and Tokyo University of Agriculture and Technology have proposed a neuroendocrine mechanism in bony fish that signals ovulation from the ovaries to the brain, using the medaka fish as a model; the first step to elucidate the neural circuits for facilitation of sexual receptivity in female teleosts.
Bony fish (teleosts) are one of the most diverse groups of vertebrates, inhabiting a wide variety of aquatic environments. Females of many species are sexually receptive only when eggs have developed in the ovaries and are ready for spawning. In other words, sexual receptivity increases in sync with ovarian status. This neuroendocrine system allows females to accept male courtship at precisely the appropriate moment. However, the details of the neuroendocrine mechanism of how the information of ovarian status is transferred into the brain in fish have remained unclear.
A team of researchers led by Hiroshima University and Tokyo University of Agriculture and Technology, along with Osaka Medical and Pharmaceutical University and The University of Tokyo, used female medaka fish to precisely identify the timing of ovulation and the timing of sexual behavior. They have proposed a neuroendocrine mechanism that signals ovulation from the ovaries to the brain, thereby increasing sexual motivation in females physically capable of spawning.
Their discoveries were published in the Journal of Neuroendocrinology on January 8, 2026.
Medaka fish, also known as the Japanese rice fish, have been a popular aquarium fish in Japan since the 17th century, as well as a model organism for scientific research. The medaka fish used in this study lay eggs on a daily cycle and exhibit stereotypical patterns of sexual behavior, making them well-suited for analyzing sexual behavior. Lots of studies on neuroendocrine mechanisms controlling egg development in their ovaries have been reported, leading the researchers to choose the medaka for this study.
Egg development in the ovaries of teleosts is regulated by neurons in the hypothalamus and hormones secreted from the pituitary gland (Hypothalamic-Pituitary-Gonad axis, HPG axis). The researchers examined whether female medaka responded to courtship from males and whether they ultimately accepted such courtship by using fish whose HPG axis-related genes were knocked out.
Female medaka with a partial deletion in the gene encoding luteinizing hormone (LH) secreted from the pituitary gland—which possessed developed eggs but were unable to ovulate (LH-deficient medaka)—did not accept courtship at all, despite being courted normally by males. Furthermore, by chronologically observing the timing of ovulation and sexual behavior in normal female medaka, the researchers precisely determined that ovulation occurs approximately 2 hours before the lighting in the rearing room turns on, while sexual behavior occurs 1.5 to 1 hour before lighting.
"We found that female sexual receptivity fluctuates in synchrony with the ovulatory cycle," says by Dr. Soma Tomihara , assistant professor at Hiroshima University's Graduate School of Integrated Sciences for Life , first- and one of the corresponding authors of the paper. "Additionally, our results suggest the presence of a neuroendocrine mechanism whereby sex steroid hormones, released in surges around the timing of ovulation, are directly received by specific neurons in the brain and serve as signals that convey the occurrence of ovulation."
When the LH-deficient medaka were administered P4, a progesterone analog, sexual behavior with males was observed, and sexual receptivity was restored—but did not restore ovulation itself. This suggests that P4 restored sexual receptivity not by acting on the ovaries to induce ovulation, but rather by being directly received in the brain. In other words, it is suggested that a pathway exists where hormones secreted from the ovaries during ovulation under natural conditions are directly received in the brain, and these neurons are involved in facilitating sexual receptivity.
"We aim to elucidate the neural circuits that facilitate sexual receptivity in teleost female, specifically focusing on brain neurons that respond to sex steroid hormone released around the timing of ovulation," concluded Dr. Tomihara.
The research team includes Rinko Shimomai & Yoshitaka Oka at The University of Tokyo; Mikoto Nakajo at Osaka Medical and Pharmaceutical University; and Chie Umatani at Tokyo University of Agriculture and Technology.
This study was supported by this work was supported by the Japan Society for the Promotion of Science (JSPS); the MEXT Initiative for Realizing Diversity in the Research Environment (Leadership training for women); the World-Leading Innovative Graduate Study Program for Life Science and Technology (WINGS35 LST) and WISE Program (Doctoral Program for World-Leading Innovative & Smart Education), The University of Tokyo; and MEXT, Japan.
