Behavioral changes following experimentally-induced acute myocardial infarction in rats

Rats with experimentally-induced acute myocardial infarction (AMI) have proven to be a clinically relevant model for visceral pain. As there are no behavioral data available on rats in the postinfarction period, we aimed to identify specific pain-related behavioral changes following AMI to increase the validity of the model. AMI was induced by left coronary artery ligation and pain-related behavior was analyzed using the open field test (OFT) and elevated plus maze (EPM). Morphine was applied following AMI induction to differentiate pain-related changes from those related to nonspecific global changes in responsiveness. AMI was histologically confirmed. Hypolocomotion was consistently evident in all behavioral tests for both the infarcted group and sham group. In the OFT, both AMI and sham rats exhibited less exploratory behavior and less activity. A similar pattern of behavior was observed in EPM, where both surgical groups showed fewer entries to the open arms and spent less time in the open arms. The sham group with an intact pericardium showed the same pattern of activity as control rats. The reduction in activity and rearing observed following AMI was successfully reversed following morphine injection. This effect was abolished after naloxone application allowing us to attribute observed changes specifically to pain.This study demonstrates that pain-related behavior in the acute postinfarction period is generally characterized by reduced mobility and explorative behavior. Our results showed that cardiac ischemia as a consequence of experimentally-induced infarction is a less important source of pain behavior than manipulation of the pericardium.