Burrowing as an Index of Inflammatory Pain in Male vs. Female Rats.

The objective of this study was to determine whether burrowing behavior may be useful as a functional index of pain in both female and male rats, and whether a "no-training" protocol can be used, to increase efficiency of testing. Adult Sprague-Dawley rats were injected in one hindpaw with oil vehicle or complete Freund's adjuvant (CFA), and then placed individually into cages containing a burrowing tube filled with aquarium gravel; the amount of gravel burrowed out of the tube after 1 h was measured each day for up to 7 days. To test the predictive validity of the burrowing test as a screen for analgesic effect, rats were pretreated with the non-steroidal anti-inflammatory drug (NSAID) ketoprofen (0.001-3.2 mg/kg) or the opioid morphine (0.1-3.2 mg/kg) as standard analgesics (true positives). Chlordiazepoxide (1.25-10 mg/kg) was tested as a true negative, and delta-9-tetrahydrocannabinol (THC, 0.01-2.0 mg/kg) was tested as a purported analgesic. CFA reliably suppressed burrowing in both sexes for 2-3 days, using a "no-training" protocol. In regard to predictive validity, ketoprofen restored CFA-suppressed burrowing in both sexes by days 2-3, and was more potent in females than males. In contrast, morphine only restored CFA-suppressed burrowing in males. Chlordiazepoxide further decreased CFA-suppressed burrowing in males, and did not alter burrowing behavior in females. THC did not significantly alter CFA-suppressed burrowing in either sex, up to doses that decreased burrowing in oil-treated controls. These results suggest that in male rats (as others have shown), burrowing is a useful, functional index of inflammatory pain. Furthermore, training/habituation to the burrowing procedure is not necessary. In contrast, although female rats' burrowing behavior is suppressed by CFA to approximately the same extent as males', only the NSAID restored CFA-suppressed burrowing in females. Additional studies are underway to determine the extent to which CFA dose (i.e., pain intensity) and analgesic pretreatment time contribute to the limited efficacy of analgesics in the burrowing test, particularly on the first day after CFA injection.