Naloxone sensitivity in squirrel monkeys under a schedule of shock titration

In order to explore the conditions under which sensitivity to naloxone develops, naloxone's effects were examined in squirrel monkeys responding under a discrete trial schedule of shock titration. Naloxone's effects were examined prior to and following a chronic naloxone regimen in which 10 mg/kg of naloxone was administered daily for at least 21 days. The level at which monkeys maintained the shock and rates of responding were recorded. Prior to chronic administration, selective doses (1.0, 3.0 mg/kg) of naloxone decreased shock levels slightly. These decreases were accompanied by increases in response rates. Subsequent to chronic administration, shock levels were unaltered; however, rates of responding showed greater increases. These results suggest that, following chronic naloxone exposure, animals responding under a schedule of shock titration became more sensitive to the rate-increasing effects of naloxone, but not to naloxone's effects on shock intensity.