TABLE 2

Affinities of full and partial opioid agonists and antagonists for binding to the MOR under high- and low-affinity binding conditions

K_low/K_high is the K_i value obtained under low-affinity binding conditions in GP buffer divided by the K_i value obtained under high-affinity Tris-HCl buffer conditions. Data are the mean K_i values ± S.E.M. from three separate experiments performed in triplicate.

Compound	K_low/K_high	K_i
Compound	K_low/K_high	High-Affinity Buffer [³H]DAMGO	Low-Affinity Buffer [³H]Naloxone
		nM ± S.E.M.
Full agonists
Morphine	530	0.32 ± 0.028	170 ± 7.5
Methadone	480	1.3 ± 0.14	630 ± 35
DAMGO	390	0.72 ± 0.084	280 ± 8.2
Fentanyl	380	0.47 ± 0.048	180 ± 7.8
Partial agonists
(–)Pentazocine	37	2.1 ± 0.088	78 ± 5.1
Nalbuphine	19	0.70 ± 0.023	13 ± 0.18
Butorphanol	13	0.24 ± 0.015	3.2 ± 0.054
Nalfurafine	5.5	0.69 ± 0.053	3.8 ± 0.42
BUP	0.40	0.35 ± 0.064	0.14 ± 0.0071
Antagonists
Naloxone	2.2	0.87 ± 0.12	1.9 ± 0.14
Naltrexone	1.4	0.31 ± 0.012	0.42 ± 0.014
Nalmefene	0.92	0.24 ± 0.022	0.22 ± 0.010
SAM	0.49	0.19 ± 0.020	0.094 ± 0.0073