Behavior of neurons in the abducens nucleus of the alert cat--III. Axotomized motoneurons.

The effects of peripheral and central VIth nerve axotomy on abducens nucleus synaptic potentials of vestibular origin and the ultrastructure of intracellularly labeled abducens motoneurons were examined in the anesthetized cat. Subsequent experiments explored the activity of identified abducens motoneurons during spontaneous and vestibular induced eye movements in alert cats prepared for chronic recordings of eye movements, single units and field potentials. Following axotomy the typical disynaptic inhibition of abducens motoneurons induced by electrical stimulation of the ipsilateral vestibular nerve either disappeared or was reduced for 5-30 days. Disynaptic activation produced by contralateral VIIIth nerve stimulation was apparently not affected. These changes were accompanied at the ultrastructural level by a decrease of axosomatic pleiomorphic synaptic endings. No changes were observed in either the number or distribution of synaptic endings on proximal and distal dendrites. Although not expected by results obtained in acute experiments, axotomized motoneurons showed a decreased excitability in the behavioral paradigm. Amplitude of the abducens antidromic field potential was significantly reduced 4-6 days following axotomy and frequent failures were observed in the antidromic somadendritic invasion of single motoneurons. Somatic invasion was obtained by the simultaneous presentation of appropriate visual and/or vestibular synaptic activity. Chronic recordings of field potentials showed their amplitude to recover in 30-40 days. The spontaneous and vestibular induced activity of identified axotomized motoneurons during this period of time differed in several aspects from controls. Motoneurons could not maintain tonic activity during eye fixations and they showed short, low frequency, bursts of activity that followed, rather than preceded, on-directed saccades. In some cases axotomized motoneurons fired during horizontal off-directed and vertical saccades. Position and velocity gains of axotomized motoneurons were lower than control values. The effects of central axotomy were always larger and of longer duration than those following peripheral axotomy. Structural and functional properties influenced by axotomy seemed to recover in 2-3 months, but with independent time courses. The present results differ in many aspects from those described after axotomy in spinal and hypoglossal motoneurons. In addition, they point out that behavior or axotomized neurons in chronic preparations are not predictable on the basis of those described in acute experiments.