ObjectiveTo evaluate the effect of time-related administration of methotrexate (MTX) on neural cell apoptosis in rats after spinal cord injury (SCI) so as to investigate its potential neuroprotective mechanism and appropriate administration time. MethodA total of 120 male Sprague Dawley rats, 247-286 g in weight, were randomly divided into 4 groups (n=30) :sham group (group A), control group (group B), MTX treating group (group C), and MTX prophylaxis group (group D). The SCI model was established in the rats of groups B, C, and D by improved Allen method, and just laminectomy was performed in group A. MTX (0.5 mg/kg) was administered with tail vein injection at 1, 6, 12, 18, and 24 hours after injury in group C, and at 30 minutes before injury and at 6, 12, 18, and 24 hours after injury in group D; the equivalence saline was injected at 1, 6, 12, 18, and 24 hours after injury in groups A and B. Basso-Beattie-Bresnahan (BBB) score was used to evaluate the neural function at 1, 3, 7, 14, and 21 days after injury, HE staining to observe histological changes, immunohistochemical staining and TUNEL method to measure the expression of Caspase-3 and neural cells apoptosis, respectively. ResultsTen rats died during the experiment in groups B, C, and D; 25 rats in each group were included into the experiments at last. BBB score of group A was significantly higher than that of groups B, C, and D at all time points after injury (P<0.05) . BBB score of groups C and D were significantly higher than that of group B at 3, 7, 14, and 21 days (P<0.05) , and BBB score of group D was significantly higher than that of group C at 3, 7, and 14 days (P<0.05) . The histological observation showed normal structure of spinal cord at all time points after injury in group A. While the degree of SCI in group D was lighter than that in groups B and C, and group C was lighter than group B. At 14 days after injury, the degree of SCI in groups B, C, and D tend to keep the same. The number of Caspase-3 and TUNEL positive cells of groups B, C, and D was significantly more than that of group A at all time points after injury (P<0.05) , group B was significantly more than groups C and D (P<0.05) . The number of Caspase-3 positive cells of group C was significantly more than that of group D at 3, 7, and 14 days (P<0.05) . While the number of TUNEL positive cells of group C was significantly more than that of group D at 3 and 7 days (P<0.05) . And the number of Caspase-3 positive cells and TUNEL positive cells was positively correlated in groups B, C, and D (P<0.05) at 1, 3, 7, 14, and 21 days after injury. ConclusionsLow-dose MTX may effectively reduce the degree of the secondary injury of spinal cord by reducing the nerve cell apoptosis. Better effect can be obtained when MTX is used as prevent method than as a way of treatment.