Ed a 3 or 4 mg Silastic implant. This trend became significantly different by day 21 and disappeared by day 28 (Figure 6).Author Manuscript Author Manuscript Author Manuscript Author ManuscriptDiscussionOur results show that Silastic implants provide a more stable delivery system of estradiol than the commercially available pellets, i.e., delivery by pellets fluctuated significantly from week to week. Although both hormone replacement methods were effective, they differ in their release pattern. Estradiol levels achieved by Silastic implants peaked at our first sampling period (day 7) and decreased slightly MG-132MedChemExpress MG-132 thereafter, following a similar pharmacokinetic profile as previously reported [22]. In contrast, hormone release by pellets increased and decreased from week to week, similar to the pattern observed with their control “placebo” pellets. Our recommendation coincides with that of Strom et al. and Ingberg et al. that Silastic implants are a more reliable and effective method for administering estradiol than pellets [14,23]. Our study also indicates that both methods, and all doses tested, are effective in deterring the increase in body weight observed after ovariectomy. This effect of estradiol on body weight has been extensively documented [12,24,25]. This study also found that after 28 days, the Silastic tube and pellet implants continue to release estradiol. Surprisingly, we found that plasma estradiol in animals that received the placebo pellet (vehicle) were similar to levels observed in gonadally intact females. Gonadally intact female rats go through a 4-5 day estrous cycle, during which plasma estradiol fluctuates almost 100 pg/ml. Thus, the average value of plasma estradiol in gonadally intact rats depends on the number of rats in each stage of the cycle. Our study also found that rats that received a 3 or 4 mg Silastic implant show a trend toward an increase in the time spent in the center of the activity chamber at day 14 which becomes significant at day 21 and is lost by day 28. Estradiol plasma levels of OVX animals that received an empty Silastic implant were no different from OVX rats that didn’t receive an implant, indicating that the Silastic implant does not contain an estrogenic compound that can significantly contribute to detectable estradiol in plasma. Differences in plasma estradiol could account for the variability and sometimes contradictory findings between studies [26,27]. We found that estradiol release peaked at 7 days in rats that received Silastic implants and produced BKT140 biological activity relatively stable estradiol levels that slowly declined until our last time point of 28 days. However, in rats receiving the pellets,J Vet Sci Technol. Author manuscript; available in PMC 2016 March 07.Mosquera et al.Pageestradiol levels peaked at 14 days, decreased at 21 days, and went up again at day 28. These differences in the release pattern of estradiol by the pellets may reflect manufacturing conundrums. It is possible that when the pellet is produced and implanted subcutaneously, some layers are metabolized more easily than others and this may be contributing to differences in the rate of estradiol delivery. This information needs to be taken into consideration in the experimental design of studies that require estradiol replacement. Indeed, recent studies of ischemic brain damage show that the neuroprotective properties of estradiol can vary depending on the method of administration [28]. At first, estradiol plasma levels were meas.Ed a 3 or 4 mg Silastic implant. This trend became significantly different by day 21 and disappeared by day 28 (Figure 6).Author Manuscript Author Manuscript Author Manuscript Author ManuscriptDiscussionOur results show that Silastic implants provide a more stable delivery system of estradiol than the commercially available pellets, i.e., delivery by pellets fluctuated significantly from week to week. Although both hormone replacement methods were effective, they differ in their release pattern. Estradiol levels achieved by Silastic implants peaked at our first sampling period (day 7) and decreased slightly thereafter, following a similar pharmacokinetic profile as previously reported [22]. In contrast, hormone release by pellets increased and decreased from week to week, similar to the pattern observed with their control “placebo” pellets. Our recommendation coincides with that of Strom et al. and Ingberg et al. that Silastic implants are a more reliable and effective method for administering estradiol than pellets [14,23]. Our study also indicates that both methods, and all doses tested, are effective in deterring the increase in body weight observed after ovariectomy. This effect of estradiol on body weight has been extensively documented [12,24,25]. This study also found that after 28 days, the Silastic tube and pellet implants continue to release estradiol. Surprisingly, we found that plasma estradiol in animals that received the placebo pellet (vehicle) were similar to levels observed in gonadally intact females. Gonadally intact female rats go through a 4-5 day estrous cycle, during which plasma estradiol fluctuates almost 100 pg/ml. Thus, the average value of plasma estradiol in gonadally intact rats depends on the number of rats in each stage of the cycle. Our study also found that rats that received a 3 or 4 mg Silastic implant show a trend toward an increase in the time spent in the center of the activity chamber at day 14 which becomes significant at day 21 and is lost by day 28. Estradiol plasma levels of OVX animals that received an empty Silastic implant were no different from OVX rats that didn’t receive an implant, indicating that the Silastic implant does not contain an estrogenic compound that can significantly contribute to detectable estradiol in plasma. Differences in plasma estradiol could account for the variability and sometimes contradictory findings between studies [26,27]. We found that estradiol release peaked at 7 days in rats that received Silastic implants and produced relatively stable estradiol levels that slowly declined until our last time point of 28 days. However, in rats receiving the pellets,J Vet Sci Technol. Author manuscript; available in PMC 2016 March 07.Mosquera et al.Pageestradiol levels peaked at 14 days, decreased at 21 days, and went up again at day 28. These differences in the release pattern of estradiol by the pellets may reflect manufacturing conundrums. It is possible that when the pellet is produced and implanted subcutaneously, some layers are metabolized more easily than others and this may be contributing to differences in the rate of estradiol delivery. This information needs to be taken into consideration in the experimental design of studies that require estradiol replacement. Indeed, recent studies of ischemic brain damage show that the neuroprotective properties of estradiol can vary depending on the method of administration [28]. At first, estradiol plasma levels were meas.