Butorphanol is a morphinan-type synthetic agonist–antagonist opioid analgesic. Butorphanol is most closely structurally related to levorphanol. Butorphanol exhibits partial agonist and antagonist activity at the ÎŒ-opioid receptor, as well as partial agonist activity at the Îș-opioid receptor (Ki = 2.5 nM; EC50 = 57 nM; Emax = 57%). Stimulation of these receptors on central nervous system neurons causes an intracellular inhibition of adenylate cyclase, closing of influx membrane calcium channels, and opening of membrane potassium channels. This leads to hyperpolarization of the cell membrane potential and suppression of action potential transmission of ascending pain pathways. Because of its Îș-agonist activity, at analgesic doses butorphanol increases pulmonary arterial pressure and cardiac work. Additionally, Îș-agonism can cause dysphoria at therapeutic or supertherapeutic doses; this gives butorphanol a lower potential for abuse than other opioid drugs.
