Short-term plasticity after partial deafferentation in the oculomotor system

Medial rectus motoneurons are innervated by two main pontine inputs. The specific function of each of these two inputs remains to be fully understood. Indeed, selective partial deafferentation of medial rectus motoneurons, performed by the lesion of either the vestibular or the abducens input, initially induces similar changes in motoneuronal discharge. However, at longer time periods, the responses to both lesions are dissimilar. Alterations on eye movements and motoneuronal discharge induced by vestibular input transection recover completely 2 months post-lesion, whereas changes induced by abducens internuclear lesion are more drastic and permanent. Functional recovery could be due to some kind of plastic process, such as reactive synaptogenesis, developed by the remaining intact input, which would occupy the vacant synaptic spaces left after lesion. Herein, by means of confocal microscopy, immunocytochemistry and retrograde labeling, we attempt to elucidate the possible plastic processes that take place after partial deafferentation of medial rectus motoneuron. 48 h post-injury, both vestibular and abducens internuclear lesions produced a reduced synaptic coverage on these motoneurons. However, 96 h after vestibular lesion, there was a partial recovery in the number of synaptic contacts. This suggests that there was reactive synaptogenesis. This recovery was preceded by an increase in somatic neurotrophin content, suggesting a role of these molecules in presynaptic axonal sprouting. The rise in synaptic coverage might be due to terminal sprouting performed by the remaining main input, i.e., abducens internuclear neurons. The present results may improve the understanding of this apparently redundant input system.