Granulocyte colony-stimulating factor (G-CSF) has the capacity to enhance brain repair following various injuries to brain. G-CSF treatment after TBI in rodents has been reported to promote brain repair, hippocampal neurogenesis, and behavioral recovery. Delta9-THC treatment also enhances brain repair after TBI, and triggers upregulation of G-CSF in brain, raising the question as to whether G-CSF mediates recovery via the eCBs. A recent report revealed that pharmacological blockade of CB1 and CB2 receptors did not impede recovery from CCI. Given that pharmacological blockade of receptors has limitations, studies were conducted in mice with ablated or "knocked out" CB2R (CB2R KO mice). The hypothesis to be tested is that G-CSF enhancement of brain repair does not require activity of CB2 receptors.Results and discussion G-CSF administration for 3 days after CCI did not enhance recovery of balance and coordination measured on the rotometer in CB2R KO mice, unlike the beneficial effects of G-CSF treatment observed in normal control mice. Even before CCI, the CB2R mice were markedly impaired on the rotometer, suggesting that activity of CB2R is important for normal function of neural networks that mediate balance and coordination. Expression of CB2R was increased by G-CSF treatment in normal mice 3 days after CCI but not in CB2R KO mice. Interestingly, the CB1R in the CB2R KO mice was upregulated by G-CSF treatment indicating that "knocking-out" or lowering expression of CB2R did not impact expression of CB1R. Expression of the neurotrophic factors BDNF and GDNF did not change with G-CSF treatment in CB2R KO mice. Levels of the endogenous cannabinoid ligand, 2-AG, were shown to be increased by G-CSF treatment in the CB2R KO mice, but upregulation of 2-AG does not appear to promote recovery of balance and coordination. Additional studies will be required of other components of the eCBs.Conclusion The hypothesis that G-CSF enhancement of brain repair does not require activity of CB2 receptors is disproven by data in this report. The eCBs, in particular activity of the CB2R, is critical for G-CSF promotion of recovery of balance and coordination impaired by CCI.