Ans. is 'b > d' i.e., Buprenorphine > Butorphenol o An opioid is a psychoactive chemical that works by binding to opioid receptors, which are found principally in the central and peripheral nervous system and GIT.o So, opioids can be divided into:-1. Opioid agonists# These have only agonistic action on opioid receptors. Drugs are:1. Natural:- Morphine, codeine.2. Semisynthetic:- diacetylmorphine (Heroin), pholcodeine, hydromorphone, oxymorphone, hydrocodone, oxycodone.3. Synthetic:- Pethidine (mepridine), fentanyl, methadone, dextropropoxyphene, tramadol, sulfentanil, alfentanil, remifentanil, levorphenol, dextromoramide, dipipanone.2. Complex action opioid and opioid antagonists.# Relatively minor changes in the structure of an opioid can convert a drug that is primarily an agonist into one with antagonistic actions at one or more types of opioid receptors. Drugs are:1. Agonist-antagonists (k analgesics) - Nalorphine, Pentazocine, Butorphenol.2. Partial/weak p agonist + k antagonist - Buprenorphine.3. Pure antagonists - Naloxone, Naltrexone, Nalmefene.Nature of interaction of opioid ligands with the three major typesof opioid receptors, along with equivalent analgesic dosesLigandm (mu)k (kappa)d (delta)Analgesic dose (mg)1. MorphineAgo. (St)Ago. (W)Ago. (W)102. NalorphineAnta. (St)Ago. (M)--3. PentazocineP. Ago., Anta. (W)Ago. (M)-30-604. ButorphanolP. Ago. (W)Ago. (St)-1-35. BuprenorphineP. Ago.Anta. (M)-0-3-0-46. NaloxoneAnta. (St)Anta. (M)Anta. (W)-7. NaltrexoneAnta. (St)Anta. (M)Anta. (W)-8. Met/Leu enkephalinAgo. (M)-Ago. (St)-9. b-EndorphinAgo. (St)-Ago. (St)-10. Dynorphin A, BAgo. (W)Ago. (St)Ago. (W)-