Toxicity of pyrethroids to rats after direct injection into the central nervous system.

The administration of very small doses of two synthetic pyrethroids, cismethrin and deltamethrin, into the lateral ventricles of the brain or the subarachnoid space around the spinal cord, produced signs of toxicity in rats that were similar to those observed after iv injection of much larger doses. Intraventricular injection of the radiolabeled pyrethroids demonstrated that the onset of toxicity corresponded to the radiolabel reaching a threshold level in the brain stem, cerebellum or upper spinal cord. The injection of similarly labeled pyrethroid solutions into the lumbar region of the spine indicated that there was very little movement of the pyrethroid up the spinal cord to the brain. This corresponded to the signs of toxicity occurring only caudal to the site of injection. It was concluded that both pyrethroids produce their different syndromes of toxicity predominantly by their action on the spinal cord.