RESUMEN
Fluctuating sex steroids during the estrous or menstrual cycle of mammalian females make it difficult to determine their role on behaviors and physiology. To avoid this, many investigators ovariectomize their animals and administer progesterone, estradiol or a combination of both. Several different strategies are used to administer estradiol, which confounds interpretation of results. This study compared two methods of estradiol replacement implants: Silastic tubes filled with crystalline estradiol benzoate (E2) and commercially available estradiol benzoate pellets. Implants were placed subcutaneously in adult ovariectomized (OVX) rats and blood samples obtained weekly. Control OVX rats received empty Silastic tubes or placebo pellets. Our data shows that E2 plasma levels from rats with Silastic implants peaked after one week and decreased slowly thereafter. In contrast, plasma E2 from commercial pellets peaked after two weeks, increasing and decreasing over time. To validate hormone release, body weight was monitored. All E2 treated animals maintained a similar body weight over the four weeks period whereas an increase in body weight over time was observed in the OVX group that received empty implants, confirming E2 release and supporting the role of E2 in the regulation of body weight. Furthermore, the effects of E2 on basal locomotor activity were assessed using animal activity cages. Results showed no difference between E2 and control group in several locomotor activities. These results indicate that Silastic implants achieve more stable plasma estradiol levels than pellets and thus are a better alternative for studies of estradiol on brain function and behavior.
RESUMEN
17ß-Estradiol is a multi-active steroid that imparts neuroprotection via diverse mechanisms of action. However, its role as a neuroprotective agent after spinal cord injury (SCI), or the involvement of the estrogen receptor-alpha (ER-α) in locomotor recovery, is still a subject of much debate. In this study, we evaluated the effects of estradiol and of Tamoxifen (an estrogen receptor mixed agonist/antagonist) on locomotor recovery following SCI. To control estradiol cyclical variability, ovariectomized female rats received empty or estradiol filled implants, prior to a moderate contusion to the spinal cord. Estradiol improved locomotor function at 7, 14, 21, and 28 days post injury (DPI), when compared to control groups (measured with the BBB open field test). This effect was ER-α mediated, because functional recovery was blocked with an ER-α antagonist. We also observed that ER-α was up-regulated after SCI. Long-term treatment (28 DPI) with estradiol and Tamoxifen reduced the extent of the lesion cavity, an effect also mediated by ER-α. The antioxidant effects of estradiol were seen acutely at 2 DPI but not at 28 DPI, and this acute effect was not receptor mediated. Rats treated with Tamoxifen recovered some locomotor activity at 21 and 28 DPI, which could be related to the antioxidant protection seen at these time points. These results show that estradiol improves functional outcome, and these protective effects are mediated by the ER-α dependent and independent-mechanisms. Tamoxifen׳s effects during late stages of SCI support the use of this drug as a long-term alternative treatment for this condition.