Effects of dexamethasone and metopirone on ovulation in the goldfish, Carassius auratus

At 12 ± 1 °C, a temperature at which goldfish will not ovulate, metopirone induced ovulation in gravid fish. Histological analysis revealed marked stimulation of the pituitary–interrenal axis but did not reveal any change in stainability (granulation) or nuclear size of gonadotrops. It is suggested that metopirone induced ovulation in goldfish by stimulating secretion of 11-deoxycortisol and (or) 11-deoxycorticosterone. Other possibilities are discussed.When the ambient temperature was gradually raised to 20 ± 1 °C, dexamethasone treatment blocked the ovulatory response consistently seen in the saline-injected controls. However, fish injected with human chorionic gonadotropin in addition to dexamethasone ovulated like the saline-injected controls. No change in stainability (granulation) was seen in gonadotrops of dexamethasone-treated fish (20 ± 1 °C) compared to saline-injected and uninjected controls held at 12 ± 1 °C, but marked degranulation was observed in saline-injected fish warmed to 20 ± 1 °C. There appeared to be a reduction of gonadotrope nuclear size in some dexamethasone-treated fish. It is suggested that dexamethasone inhibited ovulation in goldfish by suppressing gonadotropin secretion. Action by way of suppression of corticosteriodogenesis, for which there is good histological evidence, is also discussed, as well as the possibility of direct ovarian action.