Effects of long-term rapamycin treatment on glial scar formation after cryogenic traumatic brain injury in mice

Glial scar impedes axon regeneration and functional recovery following traumatic brain injury (TBI). Although it has been shown that rapamycin (a specific inhibitor of mammalian target of rapamycin) can reduce astrocyte reactivation in the early stage of TBI, its effect on glial scar formation has not been characterized in TBI and other acute brain injury models. To test this, ICR mice received daily administration of rapamycin (0.5 or 1.5 mg/kg, i.p.) beginning at 1 h after cryogenic TBI (cTBI). The results showed that at 3 d post-injury, 1.5 mg/kg rapamycin increased cTBI-induced motor functional deficits and infarct size, and attenuated astrocyte reactivation in the ipsilateral cortex, while 0.5 mg/kg rapamycin did not worsen brain damage and only slightly attenuated astrocyte reactivation. Furthermore, at 7 and 14 d after cTBI, 0.5 mg/kg rapamycin group showed a better motor functional performance than cTBI group. At 14 d post-injury, 0.5 mg/kg rapamycin significantly reduced the area and thickness of glial scar and chondroitin sulfate proteoglycan expression, accompanied by decreased expression of p-S6 and enhanced expression of growth associated protein 43 (an axon regeneration marker) in the region of glial scar. Our data suggest that long-term treatment with rapamycin can inhibit glial scar formation after cTBI, which may be involved in the mechanisms of increased axon regeneration and improved neurological functional recovery, and low-dose rapamycin may be more beneficial for such a therapy.