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Opioid nature of learned helplessness and stress‐induced analgesia observed without re‐exposure to shock

It has been shown that uncontrollable shocks that produce learned helplessness also produce long-term opioid analgesia if the animal is re-exposed to shock immediately before the test. The present study was conducted in order to investigate if this effect can be observed 24 h after the uncontrollable shock treatment without re-exposure to shock, and if it is opioid mediated. Long-term analgesia was found in the absence of re-exposure to shock, and was prevented by an i.p. injection of naloxone (10 mg/kg) administered 10 min before the test. The learned helplessness effect produced by the same shock treatment was prevented by the administration of 10 and 20 mg/kg of naloxone 10 min before the shuttlebox test, but not by a lower naloxone dose (5 mg/kg). These findings suggest that the shock re-exposure requirement proposed in previous studies is not crucial in determining the long-term analgesia, and that both the long-term analgesia and the learned helplessness effect produced by this shock treatment were opioid mediated.