Ultrastructural studies of the dying-back process. IV. Differential vulnerability of pns and cns fibers in experimental central-peripheral distal axonopathies

A companion paper in this issue (46) described the evolution of peripheral nervous system dying-back disease of the giant axonal type in animals chronically intoxicated with the neurotoxic hexacarbons n-hexane (CH₃CH₂CH₂CH₂CH₂CH₃), methyl n.-butyl ketone or MBK (CH₃COCH₂CH₂CH₂CH₃), and 2,5-hexanedione (CH₃COCH₂CH₂COCH₃). The present study compares the distribution and pattern of peripheral (PNS) and central nervous system (CNS) dying-back disease produced by these three neurotoxic hexacarbons with that produced by acrylamide (CH₂CHCONH₂), and, in addition, employs these compounds to address unresolved issues in the dying- back process. In the PNS, large myelinated fibers in tibial nerve branches supplying calf muscles were especially sensitive in rats intoxicated with hexacarbons. These nerve branches and sensory plantar nerves in the hindfeet were equally vulnerable in acrylamide-treated rats. In both conditions, fibers located at these sites commenced degeneration before the distal regions of much longer and smaller diameter nerve fibers in nerve branches supplying the flexor digitorum brevis muscle and, in rats intoxicated with hexacarbons, before equivalent regions of plantar sensory branches to the digits. Pacinian corpuscles sited in the hindfeet of intoxicated cats were much less vulnerable to MBK than to acrylamide. Rats and cats intoxicated with hexacarbons displayed giant axonal swellings in vulnerable regions of the PNS concurrently with similar swellings in sensitive regions of the CNS. PNS degeneration in these animals was accompanied by pronounced endoneurial edema. In the CNS, rostral regions of long, ascending tracts (dorso-spino-cerebellar, gracile and, later, the cuneate) and the caudal end of long, descending tracts (lateral columns, ventrolateral and ventromedial tracts) of hexacarbon-treated animals were especially vulnerable. After prolonged intoxication of cats with MBK, giant axonal swelling was also found in preterminal and terminal axons in Rexed laminae V-VII at spinal levels C₄ through S₃. Neurofilament proliferation without giant axonal swelling was seen in CNS fibers of rats intoxicated with acrylamide. Taken in concert, the findings underline the importance of axon diameter and length in determining the hierarchy of fiber vulnerability and indicate the common sensitivity of selected regions of the PNS and CNS. The term central-peripheral distal axonopathy is introduced to emphasize the wide-spread, distal distribution of disease in these and in similar experimental conditions. It is suggested that certain human neuropathies (toxic, nutritional, uremic, diabetic and some hereditary polyneuropathies, and the neuropathy associated with multiple myeloma) are additional examples of central-peripheral distal axonopathies.