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

Abstract

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

Dose-Related Neuroprotective versus Neurodamaging Effects of Estrogens in Rat Cerebral Ischemia: A Systematic Analysis

“…However, this hypothesis is not corroborated by the current review because no relationship was found between the effects of estrogens and the washout period. Also, in a previous study in our laboratory, we found that the duration of the washout period did not influence the later induced plasma concentrations (Strom et al, 2008).…”

“…It has been shown that the pellets are unreliable considering what is stated about the dosage in relation to the actual plasma levels produced (Strom et al, 2008). The hypothesis presented in the current paper, that estrogen is related to ischemic damage in a biphasic-or even more complex-manner does not presume that the dose-response curve in any specific interval along the dose axis is perfectly linear.…”

“…Also, the fact that the peak can last for weeks ( Figure 3) makes it overlap the time for ischemic damage (occurring 1 to 2 weeks after hormone administration in the studies reporting neurodamaging effects of estrogens). In conclusion, it can be hypothesized that the drawn-out, very high plasma concentration peak produced by the pellets has a detrimental effect on Figure 3 In a previous study (Strom et al, 2008), the three most well-used methods for administrating 17b-estradiol to ovariectomized rats were tested: slow-release pellets (0.25 mg 60-day release pellets, 0.10 mg 90-day release pellets), silastic capsules (silastic laboratory tubing, inner/outer diameter: 1.575/3.175 mm, filled with 20 mm columns of 180 mg 17b-estradiol per mL sesame oil) and daily subcutaneous injections (10 mg 17b-estradiol per kg bodyweight). Blood samples were obtained at days 2, 7, 14, 21, 28, 35, and 42 after administration, and hormone levels were analyzed.…”

“…In contrast it could be argued that many of the injection regimens adopted in the included articles also have resulted in peaks that are even higher (Green et al, 2001;Liu et al, 2007) than reported for pellets (Strom et al, 2008), but here the peak's width/length of time must also be considered. Although it can take several days or even weeks to return to physiologic levels of 17b-estradiol after administration by slow-release pellets (Strom et al, 2008;, the peak induced by subcutaneous injections is to an end within 24 h (Green et al, 2001;Liu et al, 2007). If very high levels of estrogens contribute to neurodamage, it is reasonable that a long-time peak would exert a more substantial negative impact than a short-time peak.…”

Ovariectomy and 17β-estradiol Replacement in Rats and Mice: A Visual Demonstration

“…This is crucial since the 17β-estradiol level/administration method profoundly influences the experimental results 1-3 . We have in a series of studies characterized the different modes of 17β-estradiol administration, finding that subcutaneous silastic capsules and per-oral nut-cream Nutella are superior to commercially available slow-release pellets (produced by the company Innovative Research of America) and daily injections in terms of producing physiological serum concentrations of 17β-estradiol [4][5][6] . Amongst the advantages of the nut-cream method, that previously has been used for buprenorphine administration 7 , is that when used for estrogen administration it resembles peroral hormone replacement therapy and is non-invasive.…”