Neuropeptide FF-sensitive confinement of mu opioid receptor does not involve lipid rafts in SH-SY5Y cells

*Mu opioid (MOP) receptor activation can be functionally modulated by stimulation of Neuropeptide FF 2 (NPFF2) G protein-coupled receptors. Fluorescence recovery after photobleaching experiments have shown that activation of the NPFF2 receptor dramatically reduces the fraction of MOP receptors confined in microdomains of the plasma membrane of SH-SY5Y neuroblastoma cells. The aim of the present work was to assess if the direct observation of receptor compartmentation by fluorescence techniques in living cells could be related to indirect estimation of receptor partitioning in lipid rafts after biochemical fractionation of the cell. Our results show that MOP receptor distribution in lipid rafts is highly dependent upon the method of purification, questioning the interpretation of previous data regarding MOP receptor compartmentation. Moreover, the NPFF analogue 1DMe does not modify the distribution profile of MOP receptors, clearly demonstrating that membrane fractionation data do not correlate with direct measurement of receptor compartmentation in living cells.