Patrick I. Garrett, Andrew N. Maloy, Allyson G Barraza, Braxton Tonks, Ashley M. Peterson, Sarah C. Honeycutt, Todd M. Hillhouse (Weber State University)
Current opioid drug treatments for acute and chronic pain have problematic side effects (e.g. addiction and respiratory depression). In general, preclinical studies use assays of pain-stimulated behaviors, which are defined as behaviors that increase in frequency, rate or intensity after delivery of a noxious stimulus. However, pain-stimulated behaviors can be reduced by drugs that suppress motor activity but lack analgesic properties. Novel assays of pain-depressed behaviors are defined as a decrease in frequency, rate or intensity after delivery of a noxious stimulus. Pain-depressed behaviors are used in complement to pain-stimulated behaviors because drugs that suppress motor activity will not produce analgesic properties in these assays. The present study sought to evaluate sex differences in C57BL/6 mice in assays of pain-stimulated (i.e. hot plate and acid-induced stretching) and pain-depressed behaviors (nesting, rearing, and locomotor activity). No sex differences were found in the hot plate test, or acid-induced stretching, as the noxious stimulus increased behavior in both sexes. During control conditions, male mice exhibited greater nesting behavior and rearing activity. However, treatment with 0.32% and 0.56% lactic acid depressed nesting, rearing, and locomotor activity to the same degree in male and female mice. These results suggest that some pain-depressed assays, such as nesting and rearing, might be more sensitive to sex difference; however, these sex differences are not expressed in pain-stimulated behaviors. In recent years the National Institute of Health (NIH) required the use of male and female animals in all studies. This study provides evidence on which pain assays will require separate groups to address behavioral sex differences.