Advancement of Behavioral Estrus by Subcutaneous Injection of Progesterone in the Four-Day Cyclic Rat

Abstract

Four-day cyclic female Sprague- Dawley rats were each injected once with 2 mg progesterone in 0.25 cc sesame oil, at some time between 1900 hr on diestrus II and 0300 hr on proestrus. The earliest time at which estrous behavior was detected was 0100 hr proestrus, 3–6 hr after injection and 15–18 hr prior to the onset of normal heat. The proportion of subjects responding was low in groups receiving an early injection (30 and 20 % with injection at 1900 and 2130 hr diestrus II, respectively), and increased sharply to 70% and above for all subsequent injection times (2200 and 2330 hr diestrus II, and 0030, 0100, 0130 and 0300 hi- proestrus). In addition to inducing an early heat, progesterone also tended to delay the normal heat at least 1 day, if the injection occurred at or before 0130 hr proestrus. It appears that estrogen priming can be complete 15–18 hr before heat normally begins. The onset of behavior can be advanced with progesterone injections, and is probably triggered by progesterone in the normal...