Opioid peptides definition

Similarly opioid peptides are important in nicotine addiction and may have a role in causing nicotine withdrawal symptoms in some smokers [35]. Opioid antagonists such as naltrexone are licensed treatments for dependence syndromes arising from other addictive drugs and could also be of use in some smokers to aid nicotine withdrawal [59] although there is no definitive evidence overall that they are beneficial [60]. 

The endogenous ligands for the opioid receptors are peptides known as the endorphins (endogenous morphine) or opio-peptins. These include the pentapep tides methionine-enkephalin and leucine-enkephalin and a hep tapep tide and octapeptide version of methionine-enkephalin, all derived from preproenkephalin p-endorphin derived from proopiomelanocortin a-and p-dynorphin derived from prodynorphin endomorphin-1 and -2, whose precursor has not been definitively identified and orphanin FQ or nociceptin, derived from OFQ/N precursor protein. These peptides are discussed in more detail in Chapter 34. 

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