text: Splice variants Three variants of the μ-opioid receptor are well characterized, though reverse transcription polymerase chain reaction has identified up to 10 total splice variants in humans.
  Location
  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). Other areas where they have been located include the external plexiform layer of the olfactory bulb, the nucleus accumbens, in several layers of the cerebral cortex, and in some of the nuclei of the amygdala, as well as the nucleus of the solitary tract.
  Some MORs are also found in the intestinal tract. Activation of these receptors inhibits peristaltic action which causes constipation, a major side effect of μ agonists.
  Activation
  MOR can mediate acute changes in neuronal excitability via suppression of presynaptic release of GABA. Activation of the MOR leads to different effects on dendritic spines depending upon the agonist, and may be an example of functional selectivity at the μ-receptor.  The physiological and pathological roles of these two distinct mechanisms remain to be clarified.  Perhaps, both might be involved in opioid addiction and opioid-induced deficits in cognition.