A group of 40 Wistar female rats was obtained from Charles River Canada (St-Constant, Quebec, Canada) and used to study the effects of hormone replacement on the sexual behavior of aged female rats. The animals were housed in pairs in plexiglas shoebox cages with beta chip as bedding, in a room maintained on a 12-h:12-h light:dark reverse light cycle (lights off at 8:00 AM), with water and commercial rat feed available ad libitum. The animals' weight and general appearance were monitored by the experimenter (S.L.J.) from the time they arrived at the facility.
The study required the rats to be both multiparous and menopausal. The latter characteristic was inferred when females no longer experienced regular 4- to 5-day estrous cycles. To monitor vaginal cycles, vaginal smears were collected by using a cotton swab slightly lubricated in distilled water. Estrous cycle monitoring was done for 25 consecutive days before beginning the experiment and continued until the end of the experiment. When they were approximately 14 months of age, females were separated into four experimental groups and received one of four doses of crystalline testosterone propionate (Steraloids, Newport, RI) administered through an implanted subcutaneous silastic capsule (Dow Corning Corporation, Midland, MI; inner diameter of 0.058 in, outer diameter of 0.077 in) cut to varying lengths (5, 10 or 20 mm). The varying doses of testosterone were administered according to the silastic capsule length; control animals received an empty 5-mm capsule. To implant the capsules, we anesthetized the animals with a mixture of ketamine hydrochloride (50 mg/ml) and xylazine hydrochloride (4 mg/ml) mixed at a ratio of 4:3 and injected intraperitoneally in a volume of 1 ml per kg body weight. A small skin incision was made, the capsule was inserted subcutaneously on the back of the neck between the shoulder blades, and the incision was sutured shut. Postsurgical treatment consisted of local triple antibiotic ointment applied to the incision site.
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