Norbuprenorphine is the primary active metabolite of buprenorphine. Norbuprenorphine acts as a μ-opioid, δ-opioid, and nociceptin receptor full agonist, as well as a κ-opioid receptor partial agonist. Norbuprenorphine crosses the blood-brain-barrier similarly to buprenorphine and likely contributes to its effects. It was observed that Intravenous administration of norbuprenorphine at 1 to 3 mg/kg decreased respiratory rate, whereas buprenorphine had no effect up to 3 mg/kg in rats. 
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