Prolonged-release gonadotrophin-releasing hormone analogue implants enhance oocyte final maturation and ovulation, and increase plasma concentrations of sulphated C21 steroids in North Sea plaice

Reproductively mature female plaice were implanted with or without 50 μg of gonadotrophin‐releasing hormone analogue (GnRHa), suspended in either coconut oil or methacrylate resin. The weight of the GnRHa‐treated fish increased significantly (due to hydration of the oocytes) and reached a peak between 10 and 14 days. The fish produced several batches of eggs, which were consistently bigger than those produced by control fish. Plasma concentrations of free 17β‐oestradiol and glucuronidated testosterone rose briefly (4 days) in response to the GnRHa, but then fell continuously till the end of the experiment (20 days). Plasma concentrations of sulphated 5β‐pregnane‐3α,17,20β‐triol and 5β‐pregnane‐3β,17,20β‐triol (which are putative metabolites of 17,20β‐dihydroxy‐4‐pregnen‐3‐one, the oocyte maturation‐inducing steroid) increased significantly at 4 days and reached a peak between 12 and 16 days. Concentrations were still very elevated on day 20. Plasma concentrations of sulphated 3α,17,21‐trihydroxy‐5β‐pregnan‐20‐one showed a slight increase on day 4 but did not change thereafter. There was a highly significant difference in the amounts of GnRHa released into the bloodstream by the two methods of administration on day 4. However, this was not matched by significant differences in the concentrations of any of the steroids.