Interactions between naloxone and narcotic analgesics under three schedules that induce polydipsia.

A pattern of schedule-induced polydipsia was maintained in rats by a fixed-time schedule where food pellets were presented every 90 sec, a fixed-interval schedule where licking the drinking tube produced pellets every 90 sec, or a fixed-interval schedule where lever presses produced pellets every 90 sec. Under all 3 schedules, injections of morphine, methadone, etonitazene and meperidine generally decreased licking rates and amounts of water consumed, as well as rates of lever-pressing under the schedules where lever presses were required. Naloxone (1 mg/kg) almost completely blocked the effects of morphine and etonitazene, but the effects of methadone sometimes were blocked to a lesser degree. Small increases in the rate of licking and amount of water consumed after the lowest dose of meperidine under the schedule requiring lever-presses were blocked by naloxone, but the higher doses of meperidine that decreased licking, lever-pressing and amount of water consumed under the three schedules were not blocked by naloxone. These data suggest that there are important differences in the ability of naloxone to antagonize the behavioral effects of different narcotic analgesics.