Neurotoxic lesion of oculomotor neruons: evidence for rearrangement of axon terminals of surviving afferent neurons.

The fate of abducens internuclear neurons was studied after the loss of their target medical rectus motoneurons of the oculomotor nucleus in adult cats. Target motoneurons were killed by injecting ricin, a toxin lectin, into the medial rectus muscle of the eye. The entire population of abducens internuclear neurons survived the loss of target for a long time (1 year), although they exhibited an initial critical period after target loss characterized by a marked reduction of afferent synaptic transmission. One month later, the normal parameters of synaptic transmission resumed. In contrast, the density of axonal terminals of abducens internuclear neurons in the oculomotor nucleus progressively decreased during the year following target loss. Evidence is presented for the reinnervation of a new neuronal target within the oculomotor nucleus that might support both the long-term survival and the recovery of physiological characteristics in the abducens internuclear neurons.