Reversible deafferentation of abducens motoneurons and internuclear neurons with tetanus neurotoxin.

Tetanus neurotoxin (TeNT) is a blocker of synaptic vesicle exocytosis in central synapses with preferential affinity for inhibitory neurotransmission. Following its intramuscular injection, TeNT is retrogradely and trans-synaptically transported towards the premotor terminals. Therefore, we have used TeNT as a tool to study the consequences of functional deafferentation on motoneurons following its peripheral administration. For this, we injected the toxin into the lateral rectus muscle at doses of 5 or 0.5 ng/kg and recorded the discharge activity of abducens motoneurons and internuclear neurons in the alert cat. Our results showed that: (i) TeNT blocked selectively the afferent inhibitory signals on abducens neurons only when used at a low dose, whereas both excitatory and inhibitory synaptic drive was lost after the high dose treatment; (ii) all effects were reversible within one month; and (iii) strikingly, the internuclear neurons of the abducens nucleus showed similar discharge alterations to the motoneurons, suggesting a TeNT action on shared common afferences.