Lack of evidence for the size principle of selective vulnerability of axons in toxic neuropathies. I. The effects of subcutaneous injections of 2,5-hexanedione on behavior and muscle spindle function.

2,5-Hexanedione (HD) produces a central-peripheral distal axonopathy. It has been suggested that agents which produce this type of axonopathy show a predilection to the largest diameter fibers. This has been based primarily on morphological data. However, electrophysiological evidence and some clinical and morphological data suggest that this may not be the case. In particular, in acrylamide neuropathy, muscle spindle primary afferents do not show this selectivity, as well as autonomic fibers. This study was carried out to determine whether the largest diameter axons were selectively vulnerable to HD. We found that subcutaneous injections of HD in cats produced a dose-dependent increase in behavioral deficits such as contact placing, stepping, and locomotion. There was also a dose-dependent decrease in the position sensitivity of muscle spindle primary and secondary endings. However, the secondary endings, which are innervated by smaller axons, were affected prior to the primary endings. Also, the velocity sensitivity of the primary endings was depressed at a similar time frame as the position sensitivity of these same endings. These data are consistent with the hypothesis that the caliber of the axon is not the only determinant of the selective vulnerability of axons in distal axonopathy.