Activation of the PD-1 pathway for Pain therapy: DADLE ([D-Ala2, D-Leu5]-Enkephalin), a synthetic opioid peptide, increases the expression of PD-L1, attenuates acutes and chronic pain, and suppresses mechanical and thermal hypersensitivity and inhibits nociceptive neuron excitability via up-regulation of its target gene, 1/March/2017, 10.56 pm