Phospholipase A2-induced electrophysiologic and histologic changes in rabbit dorsal lumbar spine tissues.

STUDY DESIGN: The present study was designed to characterize the effect of phospholipase A2 on the discharge of perispinal sensory nerves in the anesthetized New Zealand white rabbit.

OBJECTIVES: To examine the effects of phospholipase A2 on the neural response of somatosensory neurons innervating the lumbar facet joint and surrounding tissues.

SUMMARY OF BACKGROUND DATA: An irritating component of disc tissue may be phospholipase A2, which has been found at extraordinary high levels in herniated and painful discs. Phospholipase A2 has been shown to be inflammatory, but its effect on nerve response has never been shown.

METHODS: Surgically isolated facet joint capsules from rabbits were investigated by means of electrophysiologic and histologic techniques. Phospholipase A2 was injected into the characterized nerve receptive field, and responses were evaluated over time with varying doses.

RESULTS: The injection of phospholipase A2 into the nerve receptive fields produced neurotoxicity with a 1500-U dose, sensitization of the nerves and recruitment of "silent units" with a 750-U dose, and no electrophysiologic effect with a 400-U dose. The tissues injected with phospholipase A2 and control solutions were examined histologically using a hematoxylin and eosin staining technique. In all three doses, the inflammatory changes were observed as soon as 2 hours after the injections. In control subjects, no changes were observed.

CONCLUSIONS: After phospholipase A2 injection, the discharge rate of the units showed dose and time dependent patterns. Regardless of the different doses, histologic changes were observed as soon as 2 hours after the phospholipase A2 injections.