Modulation of peripheral opioid receptors affects the concentration of mu-opioid receptors in rat brain.

Radioligand binding assay was used to evaluate characteristics of central mu-opioid receptors after peripheral administration of mu-opioid receptor agonist loperamide and antagonist methylnaloxone. These substances do not cross the blood-brain barrier. Loperamide and methylnaloxone produced opposite effects on the density of mu-opioid receptors in the frontal cortex of rat brain. These data confirm our hypothesis on reciprocal interactions between central and peripheral compartments of the endogenous opioid system.