The study objective was to determine whether burrowing behavior is useful as a functional index of pain in both male and female rats, and whether a 'no-training' protocol can be used to increase testing efficiency. Adult Sprague-Dawley rats were injected in one or both hindpaws with oil vehicle or complete Freund's adjuvant (CFA); starting the next day, the amount of gravel each rat burrowed out of a tube in 1鈥塰 was measured daily for 鈮�7鈥塪ays. Without preliminary training on the burrowing procedure, CFA reliably suppressed burrowing for 2-3鈥塪ays compared to controls, in both sexes. However, whereas unilateral CFA completely suppressed burrowing 1-day post-CFA in nearly all males, bilateral CFA was required to do so in females. When administered 30鈥塵in before testing, once daily for 5鈥塪ays post-CFA, the nonsteroidal anti-inflammatory drug ketoprofen (0.01-3.2鈥塵g/kg) and the opioid morphine (0.1-3.2鈥塵g/kg) significantly increased CFA-suppressed burrowing, whereas the purported cannabinoid analgesic 螖9-tetrahydrocannabinol (0.01-2.0鈥塵g/kg) did not. The benzodiazepine chlordiazepoxide (1.25-10鈥塵g/kg), included as a 'true negative' control, also did not restore CFA-suppressed burrowing in either sex. However, in CFA-treated males only, chlordiazepoxide decreased burrowing, suggesting that anxiety may contribute to burrowing in males but not females that are in pain. Overall these results suggest that burrowing is a valid, functional index of inflammatory pain in both sexes, and training on the burrowing procedure is not necessary. However, females are more avid burrowers than males, which should be considered when both sexes are used in inflammatory pain testing.
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