Reorganization of synaptic circuitry has been studied in the upper dorsal horn (Lamina II, substantia gelatinosa Rolandi) of the lumbar spinal cord in the adult rat, by means of electron histochemical visualization of thiamine monophosphatase after transganglionic degenerative atrophy. Thiamine monophosphatase, a highly specific and selective marker of Type C (small) dorsal root neurons, was demonstrated at light and electron microscopic levels by means of a Gömöri-type cytochemical reaction, using thiamine monophosphate chloride (Sigma) as substrate and Pb2+ as a capturing agent. Transganglionic degenerative atrophy, induced by a crush injury of the sciatic nerve, results in partial depletion of thiamine monophosphatase from ipsilateral segments L2-S1. The extent of depletion was determined in a complete series of frozen cross-sections, by means of measuring the projections of active and depleted areas and their distances from the midline. Values were fed into a personal computer and maps demonstrating the distribution of intact and impaired areas were generated. The V-shaped area of depletion starts to shrink due to incipient regeneration on the 23rd postoperative day, in caudorostral and mediolateral gradients. Replenishment of thiamine monophosphatase is completed on the 60th postoperative day. Electron microscopic cytochemistry revealed the presence of the thiamine monophosphatase reaction end product in axonal growth cones, filopodia, young axons and their varicous swellings that are transformed into scallopped en passant terminals in the later course of regeneration. Axonal growth cones and regenerating sprouts undergo Wallerian degeneration and simultaneous redepletion of the marker enzyme after transection of dorsal roots L3, L4 and L5. Thiamine monophosphatase, located initially within the axoplasms of regenerating fibers, is successively translocated to the external axolemmal surfaces. Functional maturity of the terminals is achieved only later. The resulting redundant and transient wiring is thinned out in a following maturation period, in a manner resembling the sequence of events in embryonic development. The regenerative potency of central terminals of primary sensory neurons is not restricted to a single regeneration. By repeatedly crushing the sciatic nerve, four successive degenerative-regenerative cycles have been evoked, resulting in replenishment of the marker enzyme thiamine monophosphatase.
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