THE SPINAL LOOP DIALYSIS CATHETER - CHARACTERIZATION OF USE IN THE UNANESTHETIZED RAT

To permit long-term measurement of time-dependent changes in levels of dialyzable drugs and transmitters in the spinal intrathecal (i.t.) sp ace of the unanesthetized rat, we developed a dialysis catheter for ch ronic placement. This was accomplished by constructing a loop probe 9 cm in length from 0.3-mm-diameter dialysis tubing that was made imperm eable except for the distal loop. This loop catheter was readily inser ted though an incision in the cisternal membrane and passed to the lum bar enlargement. The ends of the catheter were then externalized on th e top of the head. To permit i.t. injections, an additional i.t. cathe ter could also be inserted simultaneously by the same route, For dialy sis, an external end of the loop catheter was connected to a syringe p ump and perfused with artificial CSF (10 mu l/min) and the out flow co llected. A series of studies were performed to demonstrate the charact eristics and utility of this technique. (1) Stability of resting relea se: glutamate and glucose concentrations in spinal dialysate showed no significant changes from 3 to 10 days after implantation. (2) Spinal cord ischemia: ischemia induced by aortic occlusion or cardiac arrest evoked a time dependent increase in retrieved glutamate. (3) Spinal co rd compression caused a time-dependent glutamate, aspartate and PGE(2) increase. (4) Noxious afferent stimulation induced by the injection o f formalin into the hindpaw resulted in a rapid and transient increase in dialysate glutamate concentration. (5) Direct activation of spinal excitatory amino acids receptors by i.t. injection of kainic acid (1 mu g) evoked a significant increase in aspartate and taurine. (6) Cont inuous delivery of spinal opiate (alfentanil) via dialysis resulted in a maintained, concentration dependent elevation in the thermal escape latencies in the unanesthetized rat. The loop dialysis catheter provi des a robust experimental tool for studying time dependent changes in the concentration of diffusible substances in spinal CSF over an exten ded post-implantation interval and allows comparison of these changes with concurrently assessed behavioral indices.