Effects of type A botulinum toxin on the cholinergic transmission at spinal Renshaw cells and on the inhibitory action at Ia inhibitory interneurones.

1. The central action of botulinum toxin A (BTA) on the cholinergic transmission at Renshaw cells (RCs) and on the RC-induced inhibition of Ia inhibitory interneurones (IaINs) was studied in anaesthetized cats. BTA was administered by application directly into the spinal cord, injection into a ventral root (L7) and/or injection into the triceps surae (GS) muscle. 2. A direct application of BTA into the spinal cord led to a decrease of the early and the late response of RCs. 3. When the neurotoxin was injected into the GS muscle, the RC activity remained unaffected during the test period (33-46 h after application). 4. No effect appeared up to 10 h after an injection into the ventral root L7. 5. The RC-induced inhibition on IaINs, when tested in animals with local botulismus, remained intact during the test period. 6. From the present results it is suggested that on the spinal level the central action of botulinum toxin predominantly passes on the motoneurones.