Transformation of synaptic vesicle phenotype in the intramedullary axonal arbors of cat spinal motoneurons following peripheral nerve injury

Permanent transection of a peripheral motor nerve induces a gradual elimina tion of whole axon collateral systems in the axotomized spinal motoneurons. There is also an initial concurrent decrease in the amount of recurrent in hibition exerted by these arbors in the spinal cord for up to 6 weeks after the injury, whereas the same reflex action returns to normal by the 12-wee k postoperative state. The aim of the present investigation was to study th e fine structure of the intramedullary axonal arbors of axotomized a-motone urons in the adult cat spinal cord following a permanent peripheral motor n erve lesion. For this purpose, single axotomized alpha -motoneurons were la beled intracellularly with horseradish peroxidase at 12 weeks after permane nt transection of their peripheral motor nerve. The intramedullary portions of their motor axon and axon collateral arbors were first reconstructed at the light microscopic level and subsequently studied ultrastructurally. Th is study shows that the synaptic contacts made by the intramedullary axon c ollateral arbors of axotomized motoneurons have under,gone a change in syna ptic vesicle ultrastructure from spherical and clear vesicles to spherical and dense-cored vesicles at 12 weeks after the transection of their periphe ral axons. We suggest that the present transformation in synaptic vesicle f ine structure may also correspond to a change in the contents of these bout ons. This may, in turn, be responsible for the strengthening and recovery o f the recurrent inhibitory reflex action exerted by the axotomized spinal m otoneurons following a prolonged permanent motor nerve injury.