Prevention by insulin-like growth factor-I and riluzole in motor neuron death after neonatal axotomy

Transection of the sciatic nerve in neonatal rats results discernable loss of motor neurons in the spinal cord. This neuronal death could be due to lack of retrogradely transported target derived neurotrophic factors, since some of these factors have been shown to be effective in injury induced motor neuron death. Another hypothesis suggests that glutamate and its receptors has been implicated as possible mechanism for motor neuron death, because inhibitor of glutamate release and antagonists of glutamate receptors are effective in preventing axotomized motor neuron death. To investigate the effect of insulin-like growth factor-I (IGF-I) and riluzole, a drug that inhibits glutamate release, on axotomy induced motor neuron death. Newborn rats were anesthetized with hypothermia. Sciatic nerve was cut near the obturator tendon in the left thigh. Animals were then treated daily with different doses of IGF-I and riluzole for 14 days with intraperitoneal injections. Control rats received PBS in the same fashion. After the treatment, the number of surviving motor neurons and the motor neuron diameter in the L(4) was assessed. Both IGF-I (1.0 mg/kg) and riluzole (5.0 mg/kg) rescued motor neuron death in a similar way. Co-administration of IGF-I (1.0 mg/kg) and riluzole (5.0 mg/kg) was more effective than either agent alone and there was a statistically significant difference between co-administration and IGF-I alone. However there was no significant difference between simultaneous treatment and riluzole alone. As for diameter of motor neurons, riluzole (5.0 mg/kg) preserved the motor neuron diameter in the lesion side. Nonetheless, no further increase in motor neuron diameter was seen when riluzole (5 mg/kg) and IGF-I (1.0 mg/kg) were applied in combination. Both agents did not affect diameter of motor neurons in the non-axotomy side. Riluzole is available in amyotrophic lateral sclerosis (ALS) and the positive results of clinical trials with IGF-I suggests that combination treatment of IGF-I and riluzole in ALS remains to be determined.