ASSESSMENT OF AXONAL DYSFUNCTION IN AN IN-VITRO MODEL OF ACUTE COMPRESSIVE INJURY TO ADULT-RAT SPINAL-CORD AXONS

An in vitro model of spinal cord injury was developed to study the pat hophysiology of posttraumatic axonal dysfunction. A 25 mm length of th oracic spinal cord was removed from the adult male rat (n = 27). A dor sal column segment was isolated and pinned in a recording chamber and superfused with oxygenated (95%O-2/5% CO2) Ringer. The cord was stimul ated with a bipolar electrode, while two point responses were recorded extracellularly. Injury was accomplished by compression with a modifi ed aneurysm clip which applied a 2 g force for 15 s. With injury the c ompound action potential (CAP) amplitude decreased to 53.7 +/- 5.4% (P < 0.001), while the latency increased to 115.6 +/- 3.1% (P < 0.0025) of control values. The absolute refractory period increased with injur y from 1.7 +/- 0.1 ms to 2.1 +/- 0.1 ms (P < 0.001). With train stimul ation (200 and 400 Hz), injured axons showed evidence of high frequenc y conduction failure (P < 0.05). The infusion of 5 mM 4-aminopyridine (4-AP), a blocker of voltage-sensitive 'fast' K channels confined to i nternodal regions, resulted in broadening of the CAP of injured axons to 114.9 +/- 3.1% of control (P < 0.05). Ultrastructural analysis of t he injured dorsal column segments revealed marked axonal and myelin pa thology, including considerable myelin disruption. In conclusion, we h ave developed and characterized an in vitro model of mammalian spinal cord injury which simulates many of the features of in vivo trauma. In jured axons display characteristic changes in physiological function i ncluding a shift in refractory period and high frequency conduction fa ilure. The ultrastructural data and response of injured axons to 4-AP suggest that myelin disruption with exposure of 'fast' K+ channels con tributes to posttraumatic axonal dysfunction.