Agonist treatment did not affect association of mu opioid receptors with lipid rafts and cholesterol reduction had opposite effects on the receptor-mediated signaling in rat brain and CHO cells

Lipid rafts are small cholesterol- and glycosphingolipid-enriched membrane subdomains. Here we compared the mu opioid receptor (MOR)–lipid rafts relationship in the rat brain, where neurons have non-caveolae rafts, and in CHO cells stably transfected with HA-rat MOR (CHO-HA-rMOR), which are enriched in caveolae. Membranes of rat caudate putamen (CPu) and thalamus or CHO-HA-rMOR cells were homogenized, sonicated in a detergent-free 0.5 M Na2CO3 buffer and fractionated through sucrose density gradients. Western blot and [3H]diprenorphine binding showed that ∼ 70% of MOR in CHO-HA-rMOR was present in low-density (5–20% sucrose) fractions enriched in cholesterol and/or ganglioside M1 (GM1) (lipid rafts) in plasma membranes, whereas about 70% and 45% of MOR in CPu and thalamus, respectively, were associated with lipid rafts. Incubation with a saturating concentration of etorphine or morphine at 37 °C for 30 min failed to change the MOR location in rafts in CHO-HA-rMOR, indicating that the internalized MOR does not move out of rafts, in contrast to the delta opioid receptor. In vivo, rafts association of MOR in CPu and thalamus was not affected significantly in rats implanted with two 75-mg morphine pellets for 72 h. In addition, cholesterol reduction by methyl-β-cyclodextrin (MCD) disrupted rafts and shifted MOR to higher density fractions in both CHO-HA-rMOR and CPu membranes. However, MCD treatment had opposite impacts on MOR signaling in the two tissues: it attenuated MOR-mediated [35S]GTPγS binding in CPu but enhanced it in CHO-HA-rMOR.