Discrete cues paired with naloxone-precipitated withdrawal from acute morphine dependence elicit conditioned...

Acute bolus doses of morphine induce a state of acute opioid dependence as measured by naloxone-precipitated withdrawal. Repeated morphine and precipitated withdrawal experience further enhances naloxone-induced withdrawal severity, partly because of direct neuroadaptation to repeated morphine, and partly because of conditioned associations of context and withdrawal experience. To determine whether a discrete tone/light conditioned stimulus could elicit conditioned withdrawal responses in acute dependence, rats trained on a fixed-ratio-15 operant schedule for food reward received morphine (5.6 mg/kg) 4x at daily or weekly intervals, with each morphine injection followed at 4 h by naloxone (1.0 mg/kg) and an operant session. The conditioned stimulus was presented to a Paired group after each naloxone injection. Separate control groups experienced the conditioned stimulus either at a different time of the day or on a different day of the week than naloxone (Unpaired), received naloxone without any conditioned stimulus exposure [Paired-no conditioned stimulus (Paired-NO CS)] or received vehicle instead of naloxone before conditioned stimulus presentation (NaI-Naive). On the test day, all rats received vehicle before conditioned stimulus exposure. The conditioned stimulus alone reliably suppressed responding in Paired groups relative to control conditions with either daily or weekly intervals between conditioning sessions. The administration of morphine 4 h before conditioned stimulus exposure on the test day was not necessary to observe conditioned withdrawal. Thus, conditioned withdrawal is reliably established to discrete cues associated with naloxone-precipitated withdrawal from acute, infrequent (weekly) opioid exposure.