Effects of naltrexone on learning and performance of conditional fear-induced freezing and opioid analgesia.

In the presence of Pavlovian conditional aversive stimuli rats engage in defensive freezing and exhibit a decreased sensitivity to painful stimulation. Administration of the opioid antagonist naltrexone (7 mg/ml/kg) prior to training in a context fear conditioning procedure using footshock caused an increase in freezing but did not affect suppression of formalin-induced recuperative behaviors relative to controls during a test session 24 hr later. The same dose of naltrexone given during the test session reversed conditional analgesia without altering the level of freezing. These results suggest that full expression of endogenous opioid analgesia during training is not necessary for conditional responding.