A principal mechanism of
calcium-mediated neuronal injury is the activation of neutral
proteases known as calpains. Proteolytic substrates for
calpain include receptor and
cytoskeletal proteins, signal transduction
enzymes and
transcription factors. Recently,
calpain inhibitors have been shown to provide benefit in rat models of focal
head injury and focal
cerebral ischemia. The present study sought to investigate, in experiment 1, the time course of
calpain-mediated cytoskeletal injury in a mouse model of diffuse
head injury by measuring the 150- and 145-kDa
alpha-spectrin breakdown products (
SBDP). Secondly, in experiment 2, we examined the effect of early (20 min postinjury) administration of the novel
calpain inhibitor SJA6017 on functional outcome measured 24 h following injury and its effect on posttraumatic
alpha-spectrin degradation. Lastly, in experiment 3, we examined the effect of delayed (4 or 6 h postinjury) administration of
SJA6017 on 24-h postinjury functional outcome. In experiment 1,
isoflurane-anesthetized male CF-1 mice (18-22 g) were subjected to a 750 g-cm weight drop-induced injury and were sacrificed for
SBDP analysis at postinjury times of 30 min, and 1, 2, 6, 24 and 48 h (plus
sham). In experiments 2 and 3, mice were injured as described, and delivered a single tail vein injection of either
SJA6017 (0.3, 1, or 3 mg/kg) or vehicle (administered immediately, 4 or 6 h postinjury [3 mg/kg]). Functional outcome was evaluated in both studies, and, in experiment 2, 24-h postinjury assessment of SBDPs was determined. Following injury, the level of
SBDP 145 was significantly different from
sham at 24 and 48 h in cortical and at 24 h in the hippocampal tissues and at 48 h in the striatum. Immediate postinjury administration of
SJA6017 resulted in a dose-related improvement in 24-h functional outcome (p < 0.05 at 3 mg/kg). Significance was maintained after a 4-h delay of the 3 mg/kg, but was lost after a 6-h delay. Despite improvement in functional outcome at 24 h,
SJA6017 did not reduce
spectrin breakdown in cortical or hippocampal tissues. These results support a role for
calpain-mediated neuronal injury and the potential for a practical therapeutic window for
calpain inhibition following
traumatic brain injury. However, measurements of regional
spectrin degradation may not be the most sensitive marker for determining the effects of
calpain inhibition.