Opioid receptors are also found in the midbrain periaqueductal gray the nucleus raphe magnus and the rostral ventral medulla that comprise the descending inhibitory system modulating spinal pain transmission 10 opioid receptors bind to three major groups of endogenous opioid peptides including enkephalins endorphins and dynorphins table 10 1 opioid receptors are activated by exogenous . Fully revised and expanded the opiate receptors 2nd edition presents all the advances made in the field in the past twenty years it covers a wide variety of topics including but not limited to the following pharmacology of opioid drugs endogenous opioids molecular biology of mu receptors molecular biology of delta receptors kappa opioid receptor gene and the regulatory mechanisms of its protein expression opioid receptor signal transduction mechanisms mu opioid receptor mediated . An opioid receptor is a g protein coupled receptor where the activation of a protein on the outside of a cell activates a cascade of chemical pathways within the cell opioid receptors are molecules or sites within the body that are activated by opioid substances these sites were discovered in the early 1970s during research into how opiate medications exert their action within the human body opioid receptors are found in abundance in the spinal cord but are also found in in lower