Functional alterations of cat abducens neurons after peripheral tetanus neurotoxin injection.

Tetanus neurotoxin (TeNT) cleaves synaptobrevin, a protein involved in synaptic vesicle docking and fusion, thereby preventing neurotransmitter release and causing a functional deafferentation. We injected TeNT into the lateral rectus muscle of adult cats at 0.5 or 5 ng/kg (low and high dose, respectively). In the periphery, TeNT slightly slowed motor axon conduction velocity, and at high doses, partially blocked neuromuscular transmission. TeNT peripheral actions displayed time courses different to the more profound and longer-lasting central actions. Central effects were first observed 2 days postinjection and reversed after 1 mo. The low dose induce depression of inhibitory inputs, whereas the high dose produce depression of both inhibitory and excitatory inputs. Simultaneous recordings of eye movement and neuronal firing revealed that low-dose injections specifically reduced inhibition of firing during off-directed saccadic movements, while high-dose injections of TeNT affected both inhibitory and excitatory driven firing patterns. Motoneurons and abducens interneurons were both affected in a similar way. These alterations resulted in modifications in all discharge characteristic analyzed such as background firing, threshold for recruitment, and firing sensitivities to both eye position and velocity during spontaneous movements or vestibulo-ocular reflexes. Removal of inhibition after low-dose injections also altered firing patterns, and although firing activity increased, it did not result in muscle tetanic contractions. Removal of inhibition and excitation by high-dose injections resulted in a decrease in firing modulation with eye movements. Our findings suggest that the distinct behavior of oculomotor and spinal motor output following TeNT intoxication could be explained by their different interneuronal and proprioceptive control.