Alterations in attentional mechanisms in response to acute inflammatory pain and morphine administration

Research indicates that pain negatively impacts attention; however, the extent of this impact and the mechanisms of the effect of pain on normal attentional processing remain unclear. This study 1) examined the impact of acute inflammatory pain on attentional processing, 2) examined the impact of morphine on attentional processing, and 3) determined if an analgesic dose of morphine would return attentional processing to normal levels. Male Sprague–Dawley rats were trained on the 5 choice serial reaction time task (5CSRTT), a test commonly used to assess the attentional mechanisms of rodents. Animals were injected with saline or 1, 3, or 6 mg/kg of morphine. Twenty minutes later, animals received a formalin (or saline) injection into one hind paw to induce an inflammatory condition and were then immediately tested in the 5CSRTT. The results show that the formalin injection significantly impaired performance, as measured by an increase in the number of trials in which the animal failed to attend to the task. Likewise, a high dose of morphine (6 mg/kg) produced similar decrements in task performance. Of primary importance is that 3 mg/kg of morphine produced analgesia with only mild sedation, and performance in the 5CSRTT was improved with this dose. This is the first study to use an animal model of acute pain to demonstrate the negative impact of pain on attention, and provides a novel approach to examine the neural correlates that underlie the disruptive impact of pain on attention.