ArticlesGender Differences in the Reinforcing Properties of Morphine
Section snippets
Materials
Sprague–Dawley derived rats were purchased from Harlan–Sprague–Dawley, Indianapolis, IN. They were used at 70–90 days of age at the start of all studies described in this article. Morphine-sulfate was purchased from Sigma Chemical Co., St. Louis, MO. The place preference apparatus was custom made to our specifications by MedAssociates (Lafayette, IN), and is described in more detail below. The iodinated morphine radioimmunoassay kit was purchased from Diagnostic Products Corporation (Los
General Considerations Regarding the Place-Conditioning Paradigm
During the acclimation phase both male and female rats displayed a preference for the black chamber, with females showing a significantly greater preference than males (time spent in white − time spent in black): −122.1 (±19.6) s vs. −26.35 (±15.81) s, respectively. On the basis of these findings, morphine injections were generally paired with the nonpreferred white chamber, and saline with the preferred black chamber. However, in eight groups of males and females, the morphine-associated
Discussion
The results of these studies demonstrate significant sex-related differences in the positive reinforcing properties of morphine. Although morphine served as a positive reinforcer in both males and females, the dose–response curves were markedly different. At doses from 0.2 to 10.0 mg/kg, morphine induced a strong preference for the drug-associated chamber in both males and females, but as the dose was increased from 10–17.5 mg/kg, the preference for the drug-associated chamber declined sharply
Acknowledgements
This research was supported in part by USPHS Grants DA03833 (T.J.C.), DA09140 (T.J.C.), and DA09344 (B.N.) from the National Institute on Drug Abuse.
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Justin Ogden was supported by the Department of Molecular Biology and Pharmacology at Washington University School of Medicine.