Synaptic frequency alteration on rat ventral horn neurons in the first segment proximal to spinal cord hemisection: an ultrastructural statistical study of regenerative capacity.

To study the regenerative capacity of the spinal cord in adult rat, presynaptic boutons were classified as S (spherical vesicles), F (flattened vesicles) and C complexes, and analysed statistically on alpha-motoneuron somata and lamina VII interneurons on the operated side in the first segment rostral to a spinal cord hemisection. Following chloral hydrate anesthesia left spinal cord hemisections were made on twenty adult rats (225 gms) at vertebral level T-2. Animals were prepared for electron microscopy at 7, 14, 30, 45, 60 and 90 DPO and compared with normals. All counts were made on coded material and subjected to statistical analysis. The normal frequency of presynaptic bouton types on alpha-motoneuron somata at 30 DPO. At 45 DPO, massive degeneration with concomitant synaptic remodeling resulted in a return to near normal frequencies of S and F presynaptic boutons. At 60 and 90 DPO a gain in S presynaptic boutons and a concomitant loss in F presynaptic boutons resulted in frequencies different from normal and decreased absolute numbers of presynaptic boutons. The interneuron somata also exhibited alterations over the postoperative period. There was a reversal of frequency of presynaptic boutons at 45 DPO. However unlike on alpha-motoneuron somata the frequency of S and F presynaptic boutons returned to normal at 60 and 90 DPO. The alpha-motoneuron somata appeared to be cyclically and nonselectively reinnervated by ventral horn interneurons over 90 DPO.