The endogenous opioid peptides acting at their cognate opioid receptors modulate the effects of many drugs of abuse. The endogenous opioid peptides, beta-endorphin, dynorphin, enkephalin, and orphanin/nociceptin, are derived from precursors encoded by proopiomelanocortin (POMC), prodynorphin (PDYN), proenkephalin (PENK), and nociceptin/orphanin FQ (PNOC), respectively (for figures see LaForge et al. 2000).
