BOTULINUM NEUROTOXIN ALTERS THE DISCHARGE CHARACTERISTICS OF ABDUCENSMOTONEURONS IN THE ALERT CAT

1. The effects of botulinum neurotoxin (BoTx) injected into the latera l rectus muscle were examined in alert cats by recording the extracell ular activity of abducens motoneurons during spontaneous eye movements . 2. A single high dose (3 ng/kg) of BoTx produced a complete paralysi s of abduction that lasted for more than 2 mo. In addition, changes we re found in the discharge pattern of abducens motoneurons. Motoneurons discharged steadily at a low firing rate (15-50 spikes/s), which in s ome instances showed a complete independence of eye position. Their in creases in activity during ON-directed saccades were markedly reduced with respect to controls. The loss of inhibitory signals for OFF-direc ted saccades was even more evident. 3. A low dose (0.3 ng/kg) of BoTx also produced a paralysis of the lateral rectus muscle that lasted for similar to 1 mo. In this case, only minor modifications in the firing characteristics of abducens motoneurons were observed. 4. The present findings indicate that the effects of BoTx observed in the discharge pattern of abducens motoneurons might be due not only to target discon nection, but also to a central action of the neurotoxin on the motoneu ron.