Deafferentation-induced alterations in the rat dorsal horn: I. Comparison of peripheral nerve injury vs. rhizotomy effects on presynaptic, postsynaptic, and glial processes.

Light microscopical degeneration and ultrastructural alterations in the rat spinal dorsal horn were studied following either cutting of the sciatic nerve or rhizotomy at L4 and L5; survival time for both procedures was 3 weeks. Fink-Heimer silver methods showed minimal degeneration of afferent central processes after sciatic section, and limited ultrastructural changes were present. Both rhizotomy and nerve section resulted in degenerating terminals. Most were swollen and electron lucent, with loss of vesicles; some electron-dense terminals were present, particularly after rhizotomy. Both procedures also produced significant degeneration of postsynaptic dendrites and soma, evidenced by either increases in electron density, or loss of organelles and cavitation, or accumulation of osmiophilic floccular material. Glial processes frequently were expanded and extended to engulf single degenerating terminals and dendrites, or terminal-dendrite units; in other cases glial tongues separated terminals from their postsynaptic dendrite. Glial processes often wrapped around degenerating profiles or groups of profiles in several layers, sometimes forming complex labyrinths. These results confirm past descriptions of pre- and postsynaptic changes resulting from peripheral nerve section, but newly reveal that dendritic destruction and increased glial activity are also significant following rhizotomy. Documentation of these changes is relevant for studies of reorganization following nerve and spinal cord damage, as well as providing an ultrastructural basis for evaluation of effects of neurotoxins that affect primary afferents, as described in a companion paper.