EFFECTS OF PHOSPHOLIPASE A(2) ON LUMBAR NERVE ROOT STRUCTURE AND FUNCTION

Study Design. To investigate the effects of phospholipase A(2) on the neurophysiology and histology of rat lumbar spinal nerves and the corr esponding behavioral changes. Objectives. To study possible mechanisms of sciatica. Summary of Background Data. The pathophysiology of sciat ica is uncertain, although mechanical, chemical, and ischemic factors have been proposed. Methods. Phospholipase A(2) was injected into the rat L4-L5 epidural space, and the rats were observed for 3 or 21 days. Behavioral studies were conducted daily during the survival period. O n the 3rd or 21st day, extracellular nerve recordings were made from d orsal roots, to determine discharge properties and mechanical sensitiv ity. The nerve roots were then sectioned for a right-microscopic exami nation. Results. Motor weakness of hind limbs and altered sensation we re observed. In the 3-day phospholipase A(2) groups, squeezing the dor sal roots at the L4-L5 disc lever (force = 0.8 g) evoked sustained ect opic discharge that lasted approximately 8 minutes. Squeezing the root s distal to the L4-L5 area did not result in sustained discharges. In sham, control, and 21-day phospholipase A(2) groups, squeezing the dor sal roots elicited only a transient firing that lasted approximately 0 .1 second. Loss of myelin was seen in the nerve root cross sections in the 3-day group, and remyelination was observed in the 21-day group. No abnormality was found in the control groups. Conclusions. Based on these studies, it is hypothesized that phospholipase A(2) causes demye lination that results in hypersensitive regions where ectopic discharg e may be elicited by mechanical stimulation. These ectopic discharges may be a source of sciatica. We believe that, as long as these irritat ing factors are present, the hypersensitive nerve root nerve will cont inue to fire, and sciatic pain will persist.