Prolonged paralysis, caused by the local injection of botulinum toxin, fails to cause motor nerve terminal sprouting in skeletal muscle of the frog.

Injection of a sublethal dose of botulinum toxin (type D) into the cutaneous pectoris muscle of the frog caused paralysis for about three months, but in contrast to previous studies in the mammal, did not appear to cause axonal sprouting from motor nerve terminals. In frogs in which the cutaneous pectoris had been denervated by crushing its nerve, reinnervation occurred within 2--3 weeks and axonal sprouts beyond the original end-plates were often observed. When the hypoglossal nerve was implanted into the cutaneus pectoris, crushing the original nerve caused profuse axonal growth from the implanted nerve towards the denervated end-plates within one week, whereas injection of botulinum toxin had little effect. Stimulation of the implanted nerve caused contraction of those cutaneus pectoris muscles whose original nerves had been crushed, but no response to stimulation of the implanted nerve was seen in those muscles in which botulinum toxin had been injected. The failure of botulinum toxin to induce nerve sprouting and acceptance of foreign innervation in the frog may be due to the fact that activity may play a less important role in the neural control of the physiological properties of muscle in this species than in the mammal.