**3.2 Opioid receptors**

Mu receptors are largely thought of as the primary receptor responsible for analgesia with opioids; thus, the term "mu agonist" is often used to describe opioids used for the management of pain. Mu opioid receptors (MOR) are found in high density at the periaqueductal gray (PAG) of the midbrain. Agonism of MOR in this region is thought to eliminate a tonic gamma aminobutyric acid (GABA) tone thus

