Do synaptic rearrangements underlie compensatory reflex enhancement in spinal motoneurons after partial cell loss?

In adult cats, avulsion of a spinal ventral root induces retrograde cell de ath among the corresponding motoneurons and, also, enhanced monosynaptic re flexes ipsilaterally in the adjacent uninjured spinal cord segments. The pr esent study investigates possible mechanisms behind this reflex potentiatio n. At 1-12 weeks after unilateral L7 ventral root avulsion, the L7 dorsal r oot ganglia were bilaterally injected with choleragenoid-HRP to light micro scopically quantify the amount of HRP-labeled terminals in the motor nuclei of the lesioned L7 segment and adjacent intact L6 + S1 segments. In additi on, motoneuron synaptology and individual HRP-labeled boutons were analyzed electron microscopically. In the L7 segment, the loss of motoneurons at 12 weeks after ventral root avulsion was accompanied by a marked loss of HRP- labeled boutons in the corresponding ventral horn. In the L6/S1 segments, t he monosynaptic reflex enhancement found ipsilaterally at 12 weeks postoper atively (mean 212%) was not accompanied by an increased HRP-labeling in the ventral horn (mean 109%), indicating that no sprouting or enlargement of t he monosynaptic boutons had occurred. Ultrastructurally, the values for app osition length, total active site length, cross-sectional area, and mitocho ndrial density of the labeled boutons were also similar between the two sid es. However, ipsilaterally the L6/S1 motoneurons exhibited an increased mem brane covering by presumably excitatory boutons. The present results indica te that after partial cell death in a motoneuron pool the remaining motoneu rons may undergo compensatory synaptic rearrangements leading to increased excitability and enhanced reflexes. (C) 2000 Wiley-Liss, inc.