title: Dose- and time-dependent estradiol modulation of morphine antinociception in adult female rats.
abstract: To clarify the activational role of ovarian hormones on pain and analgesia, the present study determined whether estradiol (E2) modulation of nociception and morphine antinociception in adult female rats depends on (1) the dose of E2 and (2) the interval between E2 treatment and nociceptive testing. Female rats were ovariectomized (OvX) and either oil vehicle (0), or E2 (0.25, 2.5 or 25 microg/0.1 ml vehicle) was injected s.c. two consecutive days of every four days for five cycles before testing. Either 4, 24, 48 or 96 h after the last injection, nociception was evaluated on the 50 degrees C hotplate and warm water tail withdrawal tests before and after escalating doses of s.c. morphine. Lordosis behavior and uterine weight were assessed in other rats at the same E2 doses and time points. E2 significantly lengthened latency to respond on the hotplate test at 24 h after the last injection, but had no significant effect on tail withdrawal latencies. The lower doses of E2 significantly increased morphine antinociceptive potency at 4-24 h on one or both tests, but the intermediate E2 dose significantly decreased morphine potency at 48 h on the hotplate test. Thus, E2 modulation of morphine antinociception in the adult female rat is bidirectional, and occurs at E2 doses producing cyclic changes in sexual behavior, uterine weight and vaginal cytology that are similar to those observed in gonadally intact, cycling females.
authors.affiliation: Department of Psychology, Washington State University, Pullman, WA 99164-4820, USA. craft@wsu.edu
authors.forename: Rebecca M
authors.initials: RM
authors.lastname: Craft
authors.forename: Catherine
authors.initials: C
authors.lastname: Ulibarri
authors.forename: Michael D
authors.initials: MD
authors.lastname: Leitl
authors.forename: Jean E
authors.initials: JE
authors.lastname: Sumner
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D001835:Body Weight; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004958:Estradiol / Q000494:pharmacology*; D030762:Estrous Cycle / Q000187:drug effects / Q000502:physiology; D005260:Female; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects* / Q000502:physiology; D009929:Organ Size; D010052:Ovariectomy; D017288:Pain Threshold / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012726:Sexual Behavior, Animal / Q000187:drug effects / Q000502:physiology; D014599:Uterus / Q000033:anatomy & histology; D014621:Vagina / Q000166:cytology
chemical_list: D000701:Analgesics, Opioid; D004958:Estradiol; D009020:Morphine