TABLE 2.

Detailed competition studies of selected opioids on [¹²⁵I]IBNtxA binding

Rat brain homogenates were prepared and binding assays performed as described in Materials and Methods. Results are the mean ± S.E.M. of three independent determinations, each performed in triplicate. Competitions were best fit by nonlinear regression analysis.

Drug	Ki High Affinity	Ki Low Affinity		Fraction of High-Affinity Sites	Two-Site versus One-Site Model P Value
	nM
Naltrindole	0.06 ± 0.03		321 ± 160	27 ± 1%	<0.001
Levallorphan	0.12 ± 0.03		119 ± 26	35 ± 5%	<0.001
norBNI	0.08 ± 0.01		184 ± 23	34 ± 7%	<0.001
Levorphanol	0.07 ± 0.01		219 ± 44	42 ± 8%	<0.001