Reexpression of morphine-induced oral stereotypy six months after last morphine sensitizing dose.

We describe three experiments, two of which were designed to determine the duration of the sensitization to morphine-induced oral stereotypy caused by three high doses of morphine (MS) administered 12 h apart (10, 20, and 20 mg/kg). In one of these experiments, we also administered an additional 40 mg/kg. The third experiment was designed to determine the role of mode of drug administration in the development of MS-induced sensitization; drug was delivered by an implanted minipump. By the third high dose, all rats manifested marked repetitive gnawing and biting behavior. The stereotypy was reexpressed up to over 300 days in repeatedly challenged animals and up to at least 180 days in animals receiving only one MS challenge. The development of sensitization was not altered by the mode of drug administration.