Buprenorphine is an agonist at the mu opioid receptor (MOR) and antagonist at the kappa (KOR) and delta (DOR) receptors and a nociceptin receptor (NOR) ligand. Buprenorphine has a relatively low intrinsic efficacy for G-proteins and a long brain and MOR dwell time. Buprenorphine ceiling on respiratory depression has theoretically been related multiple factors such as low intrinsic efficacy at MOR, binding to six-transmembrane MOR and interactions in MOR/NOR heterodimers. Buprenorphine reduces analgesic tolerance by acting as a delta opioid receptor (DOR) antagonist. As a kappa opioid receptor (KOR) antagonist, buprenorphine reduces craving associated with addiction. Buprenorphine is a model opioid for the ordinal bifunctional analogs BU10038, BU08028 which have been shown to be potent analgesics in non-human primates without reinforcing effects and little to no respiratory depression.