Role of RyRs and IP3 Receptors after Traumatic Injury to Spinal Cord White Matter

Calcium influx and elevation of intracellular free calcium (Ca²⁺ _i), with subsequent activation of degenerative enzymes is hypothesized to cause cell injury and death after trauma. We examined the effects of traumatic compressive injury on (Ca²⁺)_i dynamics in spinal cord white matter. We conducted electrophysiological studies with ryanodine and inositol (1,4,5)-triphosphate(IP3) receptor agonists and antagonists in an in vitro model of spinal cord injury (SCI). A 25-30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber (37°C) and injured with a modified clip (2-g closing force) for 15 sec. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential (CAP) with glass microelectrodes. The CAP decreased to 55.2 ± 6.8% of control (p < 0.05) after spinal cord injury (SCI). Chelation of Ca²⁺ _i with BAPTA-AM (a high-affinity calcium chelator) promoted significantly greater recovery of CAP amplitude (83.2 ± 4.2% of control; p < 0.05) after injury. Infusion of caffeine (1 and 10 mM) exacerbated CAP amplitude decline (45.1 ± 5.9% of control; p < 0.05; 44.6 ± 3.1% of control; p < 0.05) postinjury. Blockade of Ca²⁺ _i release through ryanodine-sensitive receptors (RyRs) with dantrolene (10 μM) and ryanodine (50 μM), conferred significant (p < 0.05) improvement in CAP amplitude after injury. On the other hand, blockade of Ca²⁺ _i with inositol (1,4,5)-triphosphate receptor (IP₃Rs) blocker 2APB (10 μM) also conferred significant improvement in CAP amplitude after injury (82.9 ± 7.9%; p < 0.05). In conclusion, the injurious effects of Ca²⁺ _i in traumatic central nervous system (CNS) white matter injury appear to be mediated both by RyRs and through IP₃Rs calcium-induced calcium release receptors (CICRs).