Table 1. Opioid Receptor Binding Affinities (K_i, nM) for Ligands in Hartley Guinea Pig Brain Membrane.

	Ki, nMa
ligand	MOR	DOR	KOR
2a	0.24 ± 0.03	1.50 ± 0.57	0.50 ± 0.01
3a	0.2 ± 0.05	6.0 ± 2.05	0.80 ± 0.15
6	0.7 ± 0.15	1.6 ± 0.3	0.5 ± 0.05
4a	2.8 ± 0.15	2.6 ± 0.01	1.9 ± 0.6
5a	0.9 ± 0.05	0.7 ± 0.01	2.8 ± 0.05
4bb	0.9 ± 0.35	1.1 ± 0.35	1.3 ± 0.45
5bb	0.7 ± 0.25	0.7 ± 0.05	2.6 ± 0.01
1a (β-FNA)b	0.4 ± 0.05	7.7 ± 2.4	0.9 ± 0.05
buprenorphinec	1.3 ± 0.15	1.6 ± 0.07	1.5 ± 0.25

^aThe selective radioligands used were [³H]DAMGO (MOR), [³H]Cl-DPDPE (DOR), [³H]U69593 (KOR).

^bData from ref (6).

^cData from ref (14).