EFFECTS OF TARGET DEPLETION ON ADULT MAMMALIAN CENTRAL NEURONS - MORPHOLOGICAL CORRELATES

The morphological sequelae induced by target removal were studied on a dult cat abducens internuclear neurons at both the somata and terminal axon arborization levels. The neuronal target-the medial rectus moton eurons of the oculomotor nucleus-was selectively destroyed by the inje ction of toxic ricin into the medial rectus muscle. Retrograde labelin g with horseradish peroxidase demonstrated the survival of the entire population of abducens internuclear neurons up to one year after targe t removal. However, soma size was reduced by about 20% three months po stlesion and maintained for one year. At the ultrastructural level, a considerable deafferentation of abducens internuclear neurons was obse rved at short intervals (i.e. 10 days after lesion). Large regions of the plasmalemma appeared devoid of presynaptic boutons but were covere d instead by glial processes. The detachment of synaptic endings was s elective on abducens internuclear neurons since nearby motoneurons alw ays showed a normal synaptic coverage. By one month, abducens internuc lear neurons recovered a normal density of receiving axosomatic synaps es. Anterogradely biocytin-labeled axon terminals of abducens internuc lear neurons remained in place after the lesion of medial rectus moton eurons, although with a progressive decrease in density. Ultrastructur al examination of the oculomotor nucleus 10 days after the lesion reve aled numerous empty spaces left by the dead motoneurons. Targetless bo utons were observed surrounded by large extracellular gaps, still appo sed to remnants of the postsynaptic membrane or, finally, ensheathed b y glial processes. At longer intervals (>one month), the ultrastructur e of the oculomotor nucleus was re-established and labeled boutons wer e observed contacting either unidentified dendrites within the neuropi l or the soma and proximal dendrites of the oculomotor internuclear ne urons that project to the abducens nucleus. Labeled boutons were never found contacting with the oculomotor internuclear neurons either in c ontrol tissue or at short periods after ricin injection. These results indicate that the availability of undamaged neurons close to the lost target motoneurons might support the long-term survival of abducens i nternuclear neurons. Specifically, the oculomotor internuclear neurons , which likely suffer a partial deafferentation after medial rectus mo toneuron loss, constitute a potential new target for the abducens inte rnuclear neurons. The reinnervation of a new target might explain the recovery of synaptic and firing properties of abducens internuclear ne urons after medial rectus motoneuron lesion, which occurred with a sim ilar time course, as described in the accompanying paper [de la Cruz R . R. et al. (1994) Neuroscience 58, 81-97.]