Synaptic reorganization in the substantia gelatinosa after peripheral nerve neuroma formation: Aberrant innervation of lamina II neurons by A beta afferents

Intracellular recording and extracellular field potential (FP) recordings w ere obtained from spinal cord dorsal horn neurons (laminae I-IV) in a rat t ransverse slice preparation with attached dorsal roots. To study changes in synaptic inputs after neuroma formation, the sciatic nerve was sectioned a nd ligated 3 weeks before in vitro electrophysiological analysis. Horseradi sh peroxidase labeling of dorsal root axons indicated that A beta fibers sp routed into laminae I-II from deeper laminae after sciatic nerve section. F P recordings from dorsal horns of normal spinal cord slices revealed long-l atency synaptic responses in lamina II and short-latency responses in lamin a III. The latencies of synaptic FPs recorded in lamina II of the dorsal ho rn after sciatic nerve section were reduced. The majority of monosynaptic E PSPs recorded with intracellular microelectrodes from lamina II neurons in control slices were elicited by high-threshold nerve stimulation, whereas t he majority of monosynaptic EPSPs recorded in lamina III were elicited by l ow-threshold nerve stimulation. After sciatic nerve section, 31 of 57 (54%) EPSPs recorded in lamina II were elicited by low-threshold stimulation. Th e majority of low-threshold EPSPs in lamina II neurons after axotomy displa yed properties similar to low-threshold EPSPs in lamina III of control slic es. These results indicate that reoccupation of lamina II synapses by sprou ting Ab fibers normally terminating in lamina III occurs after sciatic nerv e neuroma formation. Furthermore, these observations indicate that the lami na II neurons receive inappropriate sensory information from low-threshold mechanoreceptor after sciatic nerve neuroma formation.