Conduction block of monkey dorsal rootlets by water and hypertonic saline solutions

Abstract

In an attempt to develop an animal model to study chronic pain blockade by intrathecal injection of hypertonic saline, various solutions were applied in vitro to the dorsal rootlets of monkeys, and conduction of A and C fibers was studied by a collision method previously used in the cat. Application of distilled water for 5 min, followed by isotonic saline solution for 5 min, gave a differential block of C fibers with respect to A fibers, as in the cat. Hypertonic saline solution of 1000 mosmole/l. gave similar results. However, at saline osmolalities above 1250 mosmole/l. in contrast to the cat, the differential block was reversed, with the A fibers being blocked more than the C fibers. When infusion of these solutions in intact monkeys was attempted, no persistent effect on conduction of either A or C fibers was found. However, if, in the same animals, distilled water was dripped onto the intact dorsal rootlets with the dura open, then a small differential block, similar to that seen in vitro, was observed. We concluded that, with respect to distilled water, the results in cat and monkey are similar enough to suggest that distilled water might cause a similar differential block in man. On the other hand, the results with hypertonic saline solution varied sufficiently at high osmolalities to prevent us from predicting which animal model would most closely represent conditions prevailing in human subjects during intrathecal injection of hypertonic solutions.