 Three variants of the μ-opioid receptor are well characterized, though RT-PCR has identified up to 10 total splice variants in humans.They can exist either presynaptically or postsynaptically depending upon cell types.The μ-opioid receptors exist mostly presynaptically in the periaqueductal gray region, and in the superficial dorsal horn of the spinal cord (specifically the substantia gelatinosa of Rolando).