Fig. 2. OPRM1 agonists induced rapid OPRM1 desensitization in HEK293 cells.

A, 100nM morphine-pretreatment rapidly reduced future OPRM1 activation. In the first injection, 100nM morphine in HBSS buffer was added into pretreated groups while HBSS was added into control groups. In the second injection, 900nM morphine and 200nM ADP were added into pretreated groups, while 1µM morphine and 200nM ADP were added into control groups; thus the same final concentration of morphine and ADP in both control and pretreated groups was achieved. Then the OPRM1-mediated potentiation of ADP-induced [Ca²⁺]_i release was measured. B, Morphine and DAMGO induced OPRM1 rapid desensitization. The ability of 100nM DAMGO(○) and morphine(■) to induced desensitization of wild type OPRM1 in HEK293 cells was examined as described in Fig. 2A. Total [Ca²⁺]_i response of the second injection was quantitatively analyzed as described in Methods and legend of Figure 1. The total response in control groups of 200nM ADP and 1µM DAMGO-induced in second injection was referred as 100%; and data were expressed as the percentage of the response in the pretreated group as compare to the control group. ** denotes p< 0.01.