Opioid modulation of reflex versus operant responses following stress in the rat

In pre-clinical models intended to evaluate nociceptive processing, acute stress suppresses reflex responses to thermal stimulation, an effect previously described as stress-induced “analgesia.” Suggestions that endogenous opioids mediate this effect are based on demonstrations that stress-induced hyporeflexia is enhanced by high dose morphine (>5 mg/kg) and is reversed by naloxone. However, reflexes and pain sensations can be modulated differentially. Therefore, in the present study direct comparisons were made of opioid agonist and antagonist actions, independently and in combination with acute restraint stress in Long Evans rats, on reflex lick-guard (L/G) and operant escape responses to nociceptive thermal stimulation (44.5 °C). A high dose of morphine (>8 mg/kg) was required to reduce reflex responding, but a moderate dose of morphine (1 mg/kg) significantly reduced escape responding. The same moderate dose (and also 5 mg/kg) of morphine significantly enhanced reflex responding. Naloxone (3 mg/kg) significantly enhanced escape responding but did not affect L/G responding. Restraint stress significantly suppressed L/G reflexes (hyporeflexia) but enhanced escape responses (hyperalgesia). Stress-induced hyperalgesia was significantly reduced by morphine and enhanced by naloxone. In contrast, stress-induced hyporeflexia was blocked by both naloxone and 1 mg/kg of morphine. Thus, stress-induced hyperalgesia was opposed by endogenous opioid release and by administration of morphine. Stress-induced hyporeflexia was dependent upon endogenous opioid release but was counteracted by a moderate dose of morphine. These data demonstrate a differential modulation of reflex and operant outcome measures by stress and by separate or combined opioid antagonism or administration of morphine.