Peripheral opioid receptor mixed agonists/antagonists

Opioid receptors are distributed in the central nervous system and the digestive tract. Peripheral acting agonist and antagonist are not able to penetrate the blood brain barrier so only affect the peripheral opioid receptors. Opioid receptors can be divided into four types including mu, delta, kappa and the nociceptin receptors, and these can be further divided into subtypes. Agonist and antagonist that act on these receptors cause a range of effects depending on the subtype of receptor. By selectively combining peripheral agonists and antagonists that act on particular subtypes of opioid receptor, medications can be produced to have a specific effect with reduced unwanted effects.