Analgesic and anti-hyperalgesic effects of epidural morphine in an equine LPS-induced acute synovitis model

Epidural morphine is widely used in veterinary medicine, but there is no information about the anti-hyperalgesic and anti-inflammatory effects in acute inflammatory joint disease in horses. The analgesic, anti-hyperalgesic and anti-inflammatory effects of epidural morphine (100mg/animal or 0.17±0.02mg/kg) were therefore investigated in horses with acute synovitis. In a cross-over study, synovitis was induced in the talocrural joint by intra-articular lipopolysaccharide (LPS). The effect of epidural morphine was evaluated using physiological, kinematic and behavioural variables. Ranges of motion (ROM) of the metatarsophalangeal and talocrural joints were measured, clinical lameness scores and mechanical nociceptive thresholds (MNTs) were assessed and synovial fluid inflammatory markers were measured. The injection of LPS induced transient synovitis, resulting in clinical lameness, decreased ranges of motion in the talocrural and metatarsophalangeal joints, decreased limb loading at rest and increased composite pain scores. Epidural morphine resulted in a significant improvement in clinical lameness, increased ROM and improved loading of the LPS-injected limb at rest, with no effects on synovial fluid inflammatory markers. Morphine prevented a decrease in MNT and, hence, inhibited the development of hyperalgesia close to the dorsal aspect of inflamed talocrural joints. This study showed that epidural morphine provides analgesic and anti-hyperalgesic effects in horses with acute synovitis, without exerting peripheral anti-inflammatory effects.