Estrogen treatment to ovariectomized rats modifies morphine-induced behavior

Two weeks after surgery, ovariectomized (OVX) rats were treated for 3 days with either 17 beta-estradiol (10 or 100 micrograms/kg, SC, per day) or the oil vehicle. They were then tested for morphine-induced hyperactivity (4 mg/kg, IP), analgesia and catalepsy (15 and 20 mg/kg, IP) 24 or 72 hr after the last steroid or oil injection. Estradiol treatment did not affect the locomotion or the sensitivity to nociceptive stimuli of OVX rats and did not induce a cataleptic state in animals. Estradiol- (100 micrograms/kg) treated OVX rats exhibited attenuated morphine-induced hyperlocomotion regardless of the time that had elapsed after estradiol treatment cessation, attenuated morphine-induced catalepsy at 24 hr after estradiol treatment and unaltered morphine-induced analgesia. OVX rats treated with a lower estradiol dose (10 micrograms/kg) exhibited significantly increased morphine-induced analgesia and slightly increased catalepsy. The results show that the sensitivity of brain opiate systems controlling some of the behavioral effects of morphine is modified following estradiol treatment to OVX rats.