Response of adult cat abducens internuclear interneurons to selective removal of their target motoneurons.

The morphological and physiological integrity of abducens internuclear interneurons was evaluated following chemical removal of their almost exclusive target, i.e. the contralateral medial rectus motoneuron population. Motoneuron death was induced by toxic ricin injection into the ipsilateral medial rectus muscle. The main advantage of this method is that target removal is not associated with presynaptic axon damage. The activity of identified abducens internuclear interneurons was recorded before and after target removal during spontaneous eye movements in the chronic cat preparation. Several abnormalities and a remarkable decrease in eye position and velocity sensitivities were observed in the discharge pattern of these neurons for a period of 15-20 days following target removal. After that time all recorded interneurons behaved normally. A parallel morphological study showed the absence of any abducens internuclear interneuron death. These results indicate that, after a critical period, the abducens internuclear interneuron population recovers its normal behavior following target removal with no evidence of neuronal loss.