Table 1

K_i values of opioid ligands for wild-type MOR and mutant MORD114A receptors expressed in Neuro_2A cells

	K_i
	MOR	MOR D114A
	nm
Morphine	3.7 ± 0.2	234 ± 47
DAMGO	2.6 ± 0.15	707 ± 98
Etorphine	0.31 ± 0.03	120 ± 35
Naloxone	6.0 ± 2	9.5 ± 0.8
Diprenorphine	1.9 ± 0.31	0.9 ± 0.1

K_i values (average ± standard error from three experiments) were calculated from competition binding studies using membranes of Neuro_2A cells expressing wild-type or mutant opioid receptors, 2 nm[³H]diprenorphine, and different concentrations of opioid ligands (1–1000 nm) as described in the text.