Table 3

Competition binding of dopaminergic ligands at hD₂ and hD₃ receptors

Ligand	hD₂		hD₃		K_iRatio hD₂/hD₃
Ligand	pK_i/H/L	K_i/H/L	nH	pK_i	K_iRatio hD₂/hD₃	K_i	nH
		nM	% high		nM
Dopamine
pK _i				7.26 ± 0.13	54.9	0.83 ± 0.08
pK _H	7.36 ± 0.08	43.6	0.58 ± 0.02				0.79
pK _L	5.37 ± 0.05	4265	(33.3 ± 2.1)				78
(+)-7-OH-DPAT
pK _i				8.52 ± 0.04	3.02	0.96 ± 0.04
pK _H	8.45 ± 0.11	3.54	0.74 ± 0.01				1.2
pK _L	6.93 ± 0.03	117	(20.3 ± 0.5)				39
PD 128,907
pK _i				8.67 ± 0.11	2.13	0.81 ± 0.03
pK _H	8.13 ± 0.07	7.41	0.62 ± 0.01				3.5
pK _L	6.07 ± 0.02	851	(22.4 ± 1.0)				400
Haloperidol pK _i	9.37 ± 0.04	0.42	0.91 ± 0.05	8.66 ± 0.05	2.18	0.86 ± 0.05	0.2
S 14297 pK _i	6.48 ± 0.04	331	0.87 ± 0.06	7.91 ± 0.02	12.3	0.90 ± 0.03	27
GR 218,231 pK _i	7.17 ± 0.02	67.6	0.96 ± 0.05	8.95 ± 0.1	1.12	1.03 ± 0.02	60

Affinities (pK_i values) at hD₃ and hD₂ receptors stably expressed in CHO cells were determined by competition binding experiments with [¹²⁵I]iodosulpride. Two-site analysis of isotherms at hD₂ receptors is shown if this was significantly superior to a one-site fit (P ≤ 0.05, F test). Percentage of high-affinity binding sites is denoted % high. Isotherms at hD₃ receptors fitted best to a single binding site model. Results are means ± S.E. of mean of at least four independent experiments.K_i/H/L values were calculated from respective mean pK_i/H/L values. hD₂/hD₃affinity ratio was obtained by dividing K_i/H/L value at hD₂ receptors by K_i value at hD₃: for agonist ligands ratios show a wide variation depending on whetherK_i value at hD₃ is compared withK_H or K_L value at hD₂.