Long-Evans virgin female rats (85 days old) were obtained from Charles River Laboratories (Portage, MI) and housed five per cage under standard vivarium conditions, with room temperature (24-26°C) and lighting (lights-on from 0500 to 1900 h daily) controlled throughout the study. Daily vaginal smears were taken from these rats to determine their estrous cycle patterns. Animals that exhibited at least three consecutive 4- to 5-day-long estrous cycles were considered to be regularly cyclic and were used for the following experiments. Experiments were conducted in accordance with the Guiding Principles for the Care and Use of Research Animals promulgated by the Society for the Study of Reproduction. flovent inhaler
P4 Implant Treatments
Beginning at 100 days and continuing until 8 mo of age, female rats received s.c. Silastic (Dow Corning, Midland, MI) implants containing P4 (P4-implanted; n = 55) or empty capsules (control; n = 59). Implants were in place for 3 wk; this was followed by removal of implants for 1 wk. The P4 implants were prepared by packing Silastic tubing (4 cm in length, 0.333 cm i.d.) with 100% crystalline P4 (Sigma Chemical Co., St. Louis, MO) to attain serum P4 levels of 30-50 ng/ml. Placement and removal of these implants were performed while the rats were under acute, light ether anesthesia. Implant treatments were repeated five times in these same groups of animals until 8 mo of age.