title: Antinociception versus serum concentration relationships following acute administration of intravenous morphine in male and female Sprague-Dawley rats: differences between the tail flick and hot plate nociceptive tests.
abstract: 1. Antinociception versus serum morphine concentration relationships were defined in male and female Sprague-Dawley rats administered single intravenous (i.v.) bolus doses of morphine, using the hot plate (2.1-14 mg/kg) and tail flick tests (1-8 mg/kg). 2. Serum concentrations of morphine and morphine-3-glucuronide (M3G), its major metabolite in the rat, were assayed using high-performance liquid chromatography (HPLC) with electrochemical detection. 3. Significantly higher (P < 0.05) values of peak antinociception (approximately 1.7-fold), as well as the extent and duration of antinociception (approximately fourfold), were observed in male compared with female rats administered 10 mg/kg morphine in the hot plate test. Although there were no significant sex-related differences in the area under the serum morphine concentration versus time curve (AUC) at this dose, systemic exposure to M3G (M3G AUC) was significantly higher (approximately twofold; P < 0.05) in female than male rats. 4. In contrast with most previous studies investigating sex differences in morphine antinociception in rats, the antinociceptive effects of single i.v. doses of morphine (1-8 mg/kg) in the tail flick test did not differ significantly between male and female rats. 5. Morphine ED(50) and EC(50) values (95% confidence intervals) for antinociception in the hot plate test were significantly lower (P < 0.05) in male rats (ED(50) 8.4 mg/kg (7.6-9.2); EC(50) 1.8 nmol/L (1.5-2.1)) compared with female rats (ED(50) 10.6 mg/kg (9.1-12.0); EC(50) 3.7 nmol/L (3.4-4.1)). However, in the tail flick test, there was no significant difference between male and female rats in ED(50) (1.8 (0.4-3.3) and 1.4 mg/kg (0.4-2.5), respectively) or EC(50) (0.5 (0.3-0.6) and 0.4 nmol/L (0.2-0.5), respectively) values. 6. Supraspinal attenuation of morphine antinociception by M3G may account for these differences.
authors.affiliation: School of Pharmacy, The University of Queensland, St Lucia Campus, Brisbane, Queensland, Australia.
authors.forename: Samantha M
authors.initials: SM
authors.lastname: South
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Edwards
authors.forename: Maree T
authors.initials: MT
authors.lastname: Smith
mesh_terms: D000818:Animals; D019540:Area Under Curve; D004305:Dose-Response Relationship, Drug; D004971:Estrus; D005260:Female; D007275:Injections, Intravenous; D008297:Male; D009020:Morphine / Q000008:administration & dosage / Q000097:blood* / Q000493:pharmacokinetics / Q000494:pharmacology*; D010146:Pain / Q000188:drug therapy*; D010147:Pain Measurement; D051381:Rats; D012727:Sex Characteristics; D013739:Testosterone / Q000494:pharmacology
chemical_list: D013739:Testosterone; D009020:Morphine