Order of magnitude differences between methods for maintaining physiological 17β‐oestradiol concentrations in ovariectomized rats

The use of animal models, especially the rat, is crucial for elucidating the biological effects and mechanisms of the widely used hormone 17β‐oestradiol. Unfortunately, there is a lack of consensus on optimal means of obtaining and maintaining physiological 17β‐oestradiol concentrations in plasma and this may be the reason for the varying results in several studies, including the disagreement on whether 17β‐oestradiol is neuroprotective or not. Very few studies have been devoted to investigating the characteristics and biological relevance of different methods of 17β‐oestradiol administration. We therefore ovariectomized 75 Sprague‐Dawley rats and, following a 2‐week washout period, administered 17β‐oestradiol using three different methods; daily injections (10 µg 17β‐oestradiol/kg), slow‐release pellets (0.25 mg 60 day‐release pellets, 0.10 mg 90 day‐release pellets) and silastic capsules (with/without washout periods) (silastic laboratory tubing, inner/outer diameter: 1.575/3.175 mm, filled with 20 mm columns of 180 µg 17β‐oestradiol/mL sesame oil). A further 45 animals were used as ovariectomized and native controls studied in different parts of the oestrous cycle. Silastic capsules produced concentrations of 17β‐oestradiol within the physiological range 4–5 weeks independently of whether a prior washout period was included or not. The slow‐release pellets, irrespective of dose or release period, resulted in initial concentrations an order of magnitude above physiological concentrations during the first 2 weeks followed by a substantial decrease. Daily injections resulted in increasing 17β‐oestradiol concentrations, but within physiological levels. Silastic capsules are conveniently manufactured and used and are superior to pellets and injections in reliably producing long‐term 17β‐oestradiol concentrations within the physiological range.