[Presynaptic effects of botulinum toxin type A on the neuronally evoked response of albino and pigmented rabbit iris sphincter and dilator muscles].

Purpose: To investigate the effects of botulinum toxin type A(botulinum A toxin) on the autonomic and other non-adrenergic, non-cholinergic nerve terminals.
Methods: The effects of neurotoxin on twitch contractions evoked by electrical field stimulation (EFS) were studied in isolated rabbit iris sphincter and dilator muscles using isometric tension recording.
Results: Botulinum A toxin(150 nM) inhibited the fast cholinergic and slow substance P-ergic component of contraction evoked by EFS in the rabbit iris sphincter muscle without affecting the response to carbachol and substance P. Botulinum A toxin(150 nM) did not affect the twitch contraction evoked by EFS in the rabbit iris dilator muscle.
Conclusion: These data indicated that botulinum A toxin may inhibit not only the acetylcholine release in the cholinergic nerve terminals, but also substance P release from the trigeminal nerve terminals of the rabbit iris sphincter muscle. However, neurotoxin has little effect on the adrenergic nerve terminals of the rabbit iris dilator muscle.