Effects of clonidine and some ?-adrenergic antagonists alone and in combination on schedule-controlled behavior in pigeons and mice

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Abstract
Schedule-controlled responding was maintained under multiple fixed-interval, fixed-ratio schedules in pigeons and single fixed-ratio schedules in mice. In pigeons, clonidine, an α2-receptor agonist, produced dose-related decreases in responding under both fixed-interval and fixedratio schedules; fixed-interval responding was decreased at a lower dose than fixed-ratio responding. Low to intermediate doses of yohimbine, an α2-receptor antagonist, increased responding under the fixed-interval schedule without appreciably affecting responding under the fixed-ratio schedule; higher doses decreased responding under both schedules. In mice, both clonidine and yohimbine produced dose-related decreases in responding under fixed-ratio schedules. Decreases in response rates produced by clonidine were antagonized by low to intermediate doses of yohimbine. Decreases in response rates under fixed-ratio schedules produced by yohimbine were antagonized only slightly, if at all, by clonidine. Under the fixed-interval schedule, clonidine potentiated the response-rate increasing effects of intermediate doses of yohimbine and slightly antagonized the rate-decreasing effects. Although some effects of clonidine were antagonized by yohimbine, at no dose combination did performances completely resemble control performances. Prazosin, an α1-receptor antagonist, was ineffective both when administered alone and as an antagonist of the effects of clonidine. The behavioral effects of clonidine appeared to be mediated by α2 rather than α1 receptors. Additionally, yohimbine appears to have significant behavioral effects other than α2-antagonist actions.