Table 1

Affinity values of the G protein-coupled and noncoupled states of D2L receptors for agonists, partial agonists, and antagonists

	K_i, nM
	[¹²⁵I]-7-OH-PIPAT	[³H]-Spiperone	K_iLow/K_i High
Agonists
Quinpirole	9.5 ± 1.5	634 ± 151	67
Dopamine	17 ± 1.0	576 ± 192	34
Partial agonists
S(−)-PPP	56 ± 4.5	1034 ± 231	18
Terguride	0.16 ± 0.01	0.36 ± 0.04	2
Aripiprizole	0.34 ± 0.02	0.70 ± 0.22	2
Antagonists
Butaclamol	0.43 ± 0.09	0.16 ± 0.01	0.4
Haloperidol	0.30 ± 0.06	0.16 ± 0.02	0.5

K_i values were determined by the method ofCheng and Prusoff (1973) using IC₅₀ values determined by competition for the binding of the agonist [¹²⁵I]-7-OH-PIPAT or the antagonist [³H]spiperone to membranes prepared from CHO cells expressing human recombinant D2L receptors. Binding of [¹²⁵I]-7-OH-PIPAT was performed under conditions favoring formation of the agonist-preferring G protein-coupled state of D2 receptors (K_i high), whereas binding of [³H]spiperone was performed under conditions promoting the noncoupled state of D2 receptors (K_i low). IC₅₀ values were obtained by fitting data to a one-site competitive binding curve using GraphPad Prism version 3.0. Data are the mean ± S.E.M., n= 3 to 4 or the mean ± range, n = 2 experiments.