Hatano high- and low-avoidance rats (HAA and LAA) were selected and bred according to their responses on a shuttlebox task. Previous studies have shown clear strain differences in secretion of gonadotropins (Gns) and steroid hormones during the estrous cycle as well as avoidance behavior. This study was designed to measure circulating inhibin A and B and to correlate Gns, steroid hormones, and ovarian folliculogenesis. Inhibin B was elevated from the day of metestrus and declined gradually through proestrus in both strains. Additionally, inhibin B declined most at 0600 h on the day of estrus. Indeed, LAA showed higher levels of inhibin B than HAA throughout the estrous cycle. Circulating inhibin A, however, was low on the day of metestrus and increased through the afternoon of proestrus in both strains. In addition, LAA showed higher levels of inhibin A than HAA during the late evening of proestrus and early morning of estrus. However, inhibin A and B was inversely correlated with FSH surge in both strains. The presence of small follicles at 1200 h on the day of metestrus and diestrus were greater in LAA, although antral follicles were lower in LAA rats as compared with HAA. These results strongly suggest that inhibin B is secreted from developing small follicles, whereas inhibin A is secreted from mature antral follicles. Thus, the increase in circulating inhibin B and decrease in circulating FSH may be due to acceleration of follicular growth in LAA. In addition, LAA may grow more follicles during the early stage of folliculogenesis, although many may not growth to maturity. Supported in part by a Grant-in-Aid for COE (E-1) and Basic Scientific Research (B-18310044).
