High levels of circulating prostaglandin F 2α associated with ovulation stimulate female sexual receptivity and spawning behavior in the goldfish (Carassius auratus).

This study tested the hypothesis that blood-borne prostaglandin F _2α (PGF _2α ) produced at the time of ovulation by female goldfish, a typical scramble-spawning, egg-laying cyprinid fish, functions as a hormone which stimulates female sexual receptivity, behavior, and pheromone release, thereby synchronizing female mating behavior with egg availability. We conducted 5 experiments. First, we tested whether PGF _2α is found in the blood of female fish and if it increases at the time of ovulation. Using gas chromatography-mass spectrometry, we found that circulating PGF _2α was approximately 1 ng/ml prior to ovulation, increased over 50-fold within 3 h of ovulation and returned to preovulatory values after spawning and egg release. Ovulated fish also released over 2 ng/h of PGF _2α and 800 ng/h of 15-keto-PGF _2α , a metabolite of PGF _2α - both compounds with known pheromonal function. Second, we tested how closely levels of circulating PGF _2α tracked the timing of ovulation by sampling fish at the time of ovulation and discovered that PGF _2α increased within 15 min of ovulation, peaked after 9 h, and fell to basal levels as fish spawned and released their eggs. Third, we tested whether an interaction between eggs and the reproductive tract serves as a source of circulating PGF _2α and its relationship with female sexual receptivity by injecting ovulated eggs (or an egg-substitute) into the reproductive tract of females stripped of ovulated eggs. We found both of these treatments elicited measurable increases in plasma PGF _2α as well as female sexual behavior. A fourth experiment showed that indothemacin, a PG synthase inhibitor, blocked both PGF _2α increase and female sexual behavior in egg-substitute-injected fish. Finally, we tested the relationship between the expression of female behavior and PGF _2α in PGF _2α -injected fish and found that circulating PGF _2α levels closely paralleled behavior, rising within 15 min and peaking at 45 min. Together, these experiments establish that PGF _2α functions as a behavioral blood-borne hormone in the goldfish, suggesting it likely has similar activity in other related, externally-fertilizing fishes.
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