Table 2

Affinity (K_i) values for the inhibition of [³H]diprenorphine binding by naloxone or naltriben in the absence or presence of GppNHp/NaCl to membranes prepared from opioid naı̈ve, morphine, or DPDPE-pretreated GH₃DORT8 cells

Acute Drug	Chronic Pretreatment	K_i		K_iH/K_iLRatio
Acute Drug	Chronic Pretreatment	−GppNHp/NaCl (High)	⁺GppNHp/NaCl (Low)	K_iH/K_iLRatio
		nM
Naloxone	None	67.1 ± 4.9	12.4 ± 0.97^**	5.5 ± 0.25
	Morphine	69.2 ± 2.7	11.3 ± 2.4^**	6.5 ± 1.0
	DPDPE	61.6 ± 5.3	5.3 ± 1.4^**	13.4 ± 3.5^2-a
Naltriben	None	0.23 ± 0.04	0.15 ± 0.03	1.6 ± 0.09
	Morphine	0.19 ± 0.02	0.11 ± 0.03	1.9 ± 0.3
	DPDPE	0.22 ± 0.07	0.036 ± 0.01^2-150	8.2 ± 2.2^b

The displacement of [³H]diprenorphine binding by nalxone or naltriben to membranes prepared from opioid naı̈ve, and morphine- or DPDPE-pretreated GH₃DORT8 cells was determined. Cells were treated with or without 20 μM morphine or 1 μM DPDPE for 48 h. Membranes were prepared from extensively washed cells as described under Materials and Methods. Competition binding was determined with 1 nM [³H]diprenorphine and increasing concentrations of naloxone or naltriben in the presence or absence of 25 μM GppNHp and 100 mM NaCl. Affinity (K_i) values are expressed as mean ± S.E.M and were calculated from IC₅₀ values derived from complete concentration-effect curves. The K_d values for [³H]diprenorphine in GH₃DORT8 membranes used to calculate the presentedK_i values were obtained from Martin et al. (2002) and are 0.71 ± 0.04 nM (−GppNHp/NaCl) and 1.06 ± 0.14 nM (⁺GppNHp/NaCl). Results are presented as the mean ± S.E.M. from three to six experiments performed in triplicate.

↵2-150 and ** Significantly different from corresponding (high)K_i value (* P < 0.05, ** P < 0.01; Student's t test).
↵2-a and ^b Significantly different from correspondingK_iH/K_iL ratio in cells receiving no chronic pretreatment (^a P < 0.05, ^b P < 0.007; one-way ANOVA plus Dunnett's post hoc test).