Table 2

Potencies and efficacies of various ligands for inhibiting cAMP accumulation

Drug	n	IC₅₀	% Maximal Inhibition
		nM
Etorphine	7	0.135 ± 0.029	100
Morphine	3	4.92 ± 0.76	101 ± 9
Fentanyl	6	2.06 ± 0.63	99 ± 2
RTI-1a	4	0.128 ± 0.024	103 ± 4
RTI-1b	4	0.247 ± 0.077	104 ± 4
RTI-1c	5	170 ± 20	79 ± 3^2-a
RTI-1d	4	49.2 ± 14.9	64 ± 2^2-a
Buprenorphine	4	1.55 ± 0.36	79 ± 4^2-a

Measurement of the ability of various ligands to inhibit cAMP accumulation was performed as described in Experimental Procedures. The maximal amount of inhibition caused by each ligand is expressed as a percentage of the maximal amount of inhibition caused by etorphine. Curves were fitted to a standard four-parametric logistic equation with SigmaPlot (SPSS Inc., Chicago, IL). Values are mean ± S.E.

↵2-a Significantly less than etorphine-induced inhibition,P < .05 by Student's t test.