title: Tolerance to effects of morphine without cross tolerance to effects of clonidine on schedule-controlled behavior of pigeons.
abstract: Schedule-controlled responding was maintained under a multiple fixed-interval, fixed-ratio schedule in pigeons. Dose-related decreases in response rates were produced by clonidine (0.001-0.1 mg/kg) and morphine (0.3-5.6 mg/kg). Chronic administration of morphine produced (1) tolerance to effects of morphine, as evidenced by a decrease in potency of morphine and (2) sensitivity to opioid antagonists, as evidenced by an increase in potency of naloxone. Dose-effect curves for clonidine were not appreciably altered by chronic morphine administration.
mesh_terms: D000818:Animals; D003000:Clonidine / Q000494:pharmacology*; D010856:Columbidae; D003216:Conditioning, Operant / Q000187:drug effects*; D004361:Drug Tolerance; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D012055:Reinforcement Schedule
chemical_list: D009270:Naloxone; D009020:Morphine; D003000:Clonidine