Presynaptic effects of botulinum toxin type A on the neuronally evoked response of albino and pigmented rabbit iris sphincter and dilator muscles.
OBJECTIVE
To investigate the effects of botulinum toxin type A (botulinum A toxin) on the autonomic and other nonadrenergic, noncholinergic nerve terminals.


METHODS
The effects of botulinum A toxin on twitch contractions evoked by electrical field stimulation (EFS) were studied in isolated albino and pigmented rabbit iris sphincter and dilator muscles using the isometric tension recording method.


RESULTS
Botulinum A toxin inhibited the fast cholinergic and slow substance P-ergic component of the contraction evoked by EFS in the rabbit iris sphincter muscle without affecting the response to carbachol and substance P. These inhibitory effects were more marked in the albino rabbit than in the pigmented rabbit. Botulinum A toxin (150 nmol/L) did not affect the twitch contraction evoked by EFS in the rabbit iris dilator muscle.


CONCLUSIONS
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, the neurotoxin has little effect on the adrenergic nerve terminals of the rabbit iris dilator muscle. Furthermore, the botulinum A toxin binding to the pigment melanin appears to influence the response quantitatively in the two types of irides.
Ishikawa
H
H
Department of Ophthalmology, Kitasato University, School of Medicine, Sagamihara, Japan.
Mitsui
Y
Y
Yoshitomi
T
T
Mashimo
K
K
Aoki
S
S
Mukuno
K
K
Shimizu
K
K
D000417:Albinism / Q000503:physiopathology; D000818:Animals; D019274:Botulinum Toxins, Type A / Q000494:pharmacology*; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007498:Iris / Q000294:innervation*; D007537:Isometric Contraction / Q000187:drug effects; D008297:Male; D009424:Nervous System Physiological Phenomena / Q000187:drug effects; D009465:Neuromuscular Agents / Q000494:pharmacology*; D010858:Pigmentation; D017729:Presynaptic Terminals / Q000187:drug effects*; D011680:Pupil / Q000187:drug effects / Q000502:physiology; D011817:Rabbits
D009465:Neuromuscular Agents; D019274:Botulinum Toxins, Type A