Locomotor sensitization to quinpirole in rats: effects of drug abstinence and sex
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RATIONALE: Behavioral sensitization, induced by the chronic administration of psychomotor stimulants, serves as an experimental model for the development of behavioral pathology. Although many factors are known to influence the sensitization produced by indirect dopamine agonists, such as cocaine and the amphetamines, less is known about factors that influence the behavioral sensitization produced by direct dopamine receptor agonists. OBJECTIVE: As the extent to which behavioral sensitization is expressed following the repeated administration of indirect dopamine agonists can depend upon a period of drug abstinence, the present study determined the effects of drug abstinence on the expression of locomotor sensitization to the D2/D3 receptor agonist, quinpirole (QNP). METHODS: Male and female rats were administered ten, twice weekly, injections of 0.5 mg/kg QNP or saline (SAL), and then received one of five QNP doses (0-1.0 mg/kg; n=7-10/dose) in two dose-response tests for locomotor sensitization, conducted at 3 and 15 days following the cessation of chronic treatment. RESULTS: The sensitized locomotor response of QNP-treated animals was similar on the 2 test days in both male and female subjects. Compared to males, female rats displayed greater locomotor responding to QNP, both during chronic treatment and on the dose-response tests for sensitization. CONCLUSIONS: QNP locomotor sensitization is (a) not influenced by 2 weeks of QNP abstinence and (b) can be influenced by the sex of the animal. It is suggested that direct and indirect dopamine agonists produce locomotor sensitization via distinct mechanisms that differ in sensitivity to the passage of time but are both influenced by sex-specific variables.