Hormone‐induced ovulation in Ambystoma tigrinum: Influence of prolactin and thyroxine

Treatment of hypophysectomized or intact neotenic tiger salamander larvae (Ambystoma tigrinum) with ovine prolactin (PRL) increased sensitivity of the ovary to the ovulation-inducing hormones, human chorionic gonadotropin and luteinizing hormone (LH). Effects of pretreatment in vivo with PRL or thyroxine (T4) on the responsiveness of ovarian fragments in vitro to ovulation-inducing agents, LH, or progesterone (PROG) in the presence or absence of PRL were observed. In November (prior to normal spawning occurring from January through May) in vivo and in vitro treatment with PRL increased the number of oocytes producing polar bodies and the number of eggs ovulated in vitro. In April addition of PRL in vitro increased polar body formation and ovulations in response to LH or PROG, but in vivo pretreatment with PRL had no effect. Pretreatment in vivo with T4 blocked the in vitro enhancement observed with PRL. Thus, endogenous PRL may play a role in the development of ovarian sensitivity to ovulatory hormones at the level of the oocyte, and T4 may inhibit this action.