Modulation of the endocannabinoid system by the administration of exogenous agonists and selective antagonists has been shown to have potential to attenuate the contribution of inflammation to secondary injury in the CNS. The two most well-defined receptors are the CB1 and CB2 receptors. CB2, the cannabinoid receptor expressed primarily on hematopoietic cells and activated microglia, mediates the immunoregulatory functions of cannabinoids. The involvement of CB2 in central nervous inflammation has been demonstrated by using both endogenous and exogenous ligands. We showed previously that CB2 selective agonists inhibited leukocyte rolling and adhesion to CNS microvasculature and ameliorate clinical symptom in both chronic and remitting-relapsing EAE models; and our previous studies also demonstrated therapeutic potential of CB2 agonist improving recovery following spinal cord injury in the mouse. The goal of the current investigation was to evaluate the mechanisms through which administration of a selective cannabinoid-2 (CB2) agonist modifies inflammatory responses and helps to improve function following the injury in central nervous system. In the EAE project, we showed that Gp1a, a highly selective CB2 agonist with a four log higher affinity for CB2 than CB1, reduced clinical scores and facilitated recovery in EAE in conjunction with long term reduction in demyelination and axonal loss. We also established that Gp1a affected EAE through at least two different mechanisms, i.e. an early effect on Th1/Th17 differentiation in peripheral immune organs, and a later effect on the accumulation of pathogenic immune cells in the CNS, associated with reductions in the expression of CNS and T cell chemokine receptors, chemokines and adhesion molecules. This is the first report on the in vivo CB2-mediated Gp1a inhibition of Th17/Th1 differentiation. We also confirmed the Gp1a-induced inhibition of Th17/Th1 differentiation in vitro, both in non-polarizing and polarizing conditions. The CB2-induced inhibition of Th17 differentiation is highly relevant in view of recent studies emphasizing the importance of pathogenic self-reactive Th17 cells in EAE/MS. In spinal cord injury project, we showed that spinal cord injury mice CB2 agonist O-1966 (with affinities to the CB1 and CB2 receptors of 5055±984 and 23±2.1 nM, respectively) had improved motor function, autonomic function. They also had significant reductions in CXCL-9, CXCL-11, dramatic reductions in IL-23p19 expression and its receptor IL-23r, and reduction in the number of immunoreactive microglia. The results reported in this thesis, demonstrated that the combined effect on Th17 differentiation and immune cell accumulation into the CNS, may contribute to the usefulness of CB2 selective ligands as potential therapeutic agents in neuroinflammation. / Physiology