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.) space of the unanesthetized rat, we developed a dialysis catheter for chronic placement. This was accomplished by constructing a loop probe 9 cm in length from 0.3-mm-diameter dialysis tubing that was made impermeable except for the distal loop. This loop catheter was readily inserted though an incision in the cisternal membrane and passed to the lumbar enlargement. The ends of the catheter were then externalized on the top of the head. To permit i.t. injections, an additional i.t. catheter could also be inserted simultaneously by the same route. For dialysis, an external end of the loop catheter was connected to a syringe pump and perfused with artificial CSF (10 microliters/min) and the out flow collected. A series of studies were performed to demonstrate the characteristics and utility of this technique. (1) Stability of resting release: 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 cord compression caused a time-dependent glutamate, aspartate and PGE2 increase. (4) Noxious afferent stimulation induced by the injection of 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 microgram) evoked a significant increase in aspartate and taurine. (6) Continuous 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 provides a robust experimental tool for studying time dependent changes in the concentration of diffusible substances in spinal CSF over an extended post-implantation interval and allows comparison of these changes with concurrently assessed behavioral indices.