Regular ArticleMyelinated and unmyelinated axons of the corpus callosum differ in vulnerability and functional recovery following traumatic brain injury
Introduction
Traumatic axonal injury (TAI) is a common feature of traumatic brain injury (TBI) and underlies much of the resulting morbidity and mortality (Gennarelli et al., 1982, Graham et al., 1988, Povlishock, 1992, Smith and Meaney, 2000). Although isolated axons may be torn by shearing forces during severe injuries, TAI is usually manifested as a multiphasic pathology, with axolemmal and cytoskeletal alterations evolving, over the course of hours to days, to result in axonal swelling and/or disconnection (Fitzpatrick et al., 1998, Maxwell et al., 1997, Povlishock et al., 1997). Key features of this pathology have been described, including deleterious proteolysis, ionic dysregulation, and mitochondrial failure (Banik et al., 1987, Buki et al., 1999, Buki et al., 2000, Iwata et al., 2004, Povlishock et al., 1983). However, questions remain regarding whether distinct populations of axons exhibit different responses to injury, and therefore necessitate specific therapeutic strategies.
Most prior studies of in vivo TAI have emphasized the role of larger myelinated axons, with relatively little attention to the pathophysiology of small caliber unmyelinated axons. However, evidence indicates that TAI is not uniformly expressed across axonal populations. In some injured axons, neurofilament compaction has been observed independently of impaired fast transport (Stone et al., 2001). The degree to which these two pathological components were conjointly expressed in individual axons was partly determined by axon caliber and anatomical location, with larger medial lemniscal fibers exhibiting both pathologies concurrently to a greater extent than smaller corticospinal axons. Other workers, using an in vivo optic-nerve stretch model of injury, demonstrated that caliber of the myelinated axons was associated with several TAI parameters, including the degree of neurofilament and microtubule damage (Jafari et al., 1997, Jafari et al., 1998). To date, there have been no in vivo descriptions of traumatic damage in unmyelinated fiber populations: a consequence, most likely, of their small size which has precluded their inclusion in routine analysis.
Despite progress in the structural and molecular characterization of cerebral TAI, few studies have addressed electrophysiological aspects of this pathology. TAI typically occurs diffusely in central white matter, complicating electrophysiological measures based on single units or individual axons. Baker et al. (2002) reported that TBI in rats suppressed compound action potentials (CAPs) evoked in corpus callosum. The current study employed the methodology first reported by Baker and colleagues, now modified to enable separate quantification of CAPs generated by two populations of callosal axons: relatively fast conducting fibers, corresponding to large, myelinated axons, and slower conducting unmyelinated, small-caliber fibers. This electrophysiological assessment was combined with an ultrastructural examination of callosal axons in the same and parallel populations of animals. The current results indicated that these two axonal populations differed in their response to TBI, and in their capacities for functional recovery, with the small fibers exhibiting more dramatic and persistent electrophysiological change. The novel finding, of a preferential vulnerability of the small unmyelinated axon population, distinguishes the current results from prior studies of TAI which have generally emphasized the pathophysiological role of large myelinated axons.
Section snippets
Methods
The procedures for this study followed all national guidelines for the care and use of experimental animals, and the experimental protocol was approved by the Medical College of Virginia Animal Research Committee. Male Sprague–Dawley rats (n = 48) weighing 300–350 g, at the start of the study, were used in these experiments. Animals were housed in individual cages in a temperature- (22°C) and humidity-controlled (50% relative) animal facility on a 12-h light/dark cycle. Rat chow and water were
Results
Consistency in injury severity was assured by including only animals with injury magnitudes in the range 2.0 ± 0.1 atm. In addition, the duration of suppression of the righting reflex, used as an index of traumatic unconsciousness, was compared among the analytical groups used in the study. The overall mean latency to regain the righting reflex, for all animals given fluid percussion injury, was 8.7 ± 0.8 min. None of the injury groups used in the study differed significantly from this overall
Discussion
This is the first study to directly investigate electrophysiological and structural evidence of selective vulnerabilities among CNS axonal populations following traumatic brain injury. We examined injury-induced changes in functional properties of corpus callosum fibers using recording conditions that facilitate separate quantification of two distinct axonal populations: relatively fast-conducting fibers, corresponding to larger, myelinated axons, and a population of slower-conducting
Acknowledgments
The authors wish to thank Susan Walker, Nancy Lee, Tom Coburn, Lesley Harris, and Raiford Black for technical assistance, and Dr. Jonathan Lifshitz for helpful comments on the manuscript. Supported by Virginia Commonwealth Initiative Award 04-091 (TMR) and NS20193 (JTP).
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