Burrowing as a non-reflex behavioural readout for analgesic action in a rat model of sub-chronic knee joint inflammation

BackgroundInnate responses against spontaneous pain are proposed to improve the predictive validity of preclinical analgesia models. Therefore, development and validation of novel readouts is necessary. To investigate whether innate rodent burrowing is a useful alternative behavioural readout for assessment of analgesic efficacy, a complete Freund's adjuvant (CFA)-induced model of sub-chronic inflammation was used to compare the effects of naproxen, ibuprofen and pregabalin in weight-bearing (WB), open-field (OF) and burrowing assays. MethodsMale Sprague Dawley rats were injected with 150L of CFA (2mg/mL) into the knee (hind leg) 3days before testing. Naproxen, ibuprofen and pregabalin were administered at different doses 30, 90 and 60min, respectively, before testing. WB was determined using a rat incapacitance tester; horizontal distance moved and vertical rearings were recorded in an OF; and burrowing was measured by the weight of gravel remaining in a hollow tube after 60min. ResultsCFA-induced arthritis reduced WB, OF activity and burrowing. Naproxen, pregabalin and ibuprofen treatment normalized WB; however, horizontal OF activity was not improved by any treatment; rearing behaviour was moderately reinstated by ibuprofen (100mg/kg). In burrowing, naproxen (100mg/kg), ibuprofen (31.6 and 100mg/kg) and pregabalin (10mg/kg) reversed CFA-induced deficits. ConclusionsBurrowing performance is an alternative non-reflex readout relying on innate rodent behaviour that is affected by nociceptive behaviour and can be pharmacologically manipulated. The burrowing assay appears to be more sensitive than OF assays and is as sensitive as WB assays at distinguishing between analgesic doses and doses that impair locomotion.