Botulinum neurotoxin alters the discharge characteristics of abducens motoneurons in the alert cat.

1. The effects of botulinum neurotoxin (BoTx) injected into the lateral rectus muscle were examined in alert cats by recording the extracellular activity of abducens motoneurons during spontaneous eye movements. 2. A single high dose (3 ng/kg) of BoTx produced a complete paralysis of abduction that lasted for more than 2 mo. In addition, changes were found in the discharge pattern of abducens motoneurons. Motoneurons discharged steadily at a low firing rate (15-50 spikes/s), which in some instances showed a complete independence of eye position. Their increases in activity during ON-directed saccades were markedly reduced with respect to controls. The loss of inhibitory signals for OFF-directed 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 approximately 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 disconnection, but also to a central action of the neurotoxin on the motoneuron.