Abstract

The benzofurane (+)-S 14297, the benzamide nafadotride, the aminoindane U 99194 and the arylpiperazine GR 103,691 have been proposed as “selective” antagonists at dopamine D₃vs.D₂ receptors. Herein, we compared their in vitroaffinities and in vivo actions to those of the aminotetralin D₃ antagonists (+)-AJ 76 and (+)-UH 232. Affinities at recombinant, human (h)D₃ and/or hD₂ sites were determined by employing the mixed D₂/D₃antagonist [¹²⁵I]-iodosulpride and the preferential D₃ ligands [³H]-(+)-PD 128,907 and [³H]-(+)-S 14297. [³H]-(+)-PD 128,907, [³H]-(+)-S 14297 and [¹²⁵I]-iodosulpride yielded an essentially identical pattern of displacement at D₃ sites, which suggests that they recognize the same population of receptors. The rank order of potency (K_i values in nM vs.[³H]-(+)-PD 128,907) was GR 103,691 (0.4) ≈ nafadotride (0.5) > haloperidol (2) ≈ (+)-UH 232 (3) ≈ (+)-S 14297 (5) > (+)-AJ 76 (26) > U 99194 (160). The rank order of preference (K_i ratio, D₂:D₃) for D₃ receptors (labeled by [³H]-PD 128,907)vs. D₂ sites (labeled by [¹²⁵I]-iodosulpride) was (+)-S 14297 (61) ≈ GR 103,691 (60) > U 99194 (14) > nafadotride (9) ≈ (+)-UH 232 (8) ≈ (+)-AJ 76 (6) > haloperidol (0.2). (+)-S 14297 and GR 103,691 also showed greater than 100-fold selectivity at dopamine hD₃vs. hD₄ and hD₁ sites. However, GR 103,691 showed marked affinity for serotonin_1A receptors (5.8 nM) and alpha-1 adrenoceptors (12.6 nM). In vivo, all antagonists except GR 103,691 prevented the induction of hypothermia by (+)-PD 128,907 (0.63 mg/kg s.c.) and a further preferential D₃ agonist, (+)-7-OH-DPAT (0.16 mg/kg s.c.). On the other hand, haloperidol, (+)-AJ 76, (+)-UH 232 and nafadotride all induced catalepsy in rats, whereas (+)-S 14297, U 99194 and GR 103,691 were inactive. Haloperidol, (+)-AJ 76, (+)-UH 232, nafadotride and (weakly) U 99194 also enhanced prolactin secretion and striatal dopamine synthesis, whereas (+)-S 14297 and GR 103,691 were inactive. However, despite its high affinity at 5-HT_1A receptors andalpha-1 adrenoceptors, both of which are present on raphe-localized serotonergic neurons, GR 103,691 (0.5 mg/kg i.v.) failed to influence their basal firing rate or the inhibition of their electrical activity by the 5-HT_1A agonist (±)-8-OH-DPAT (0.005 mg/kg i.v.), a result that casts doubt on its activity in vivo. In conclusion, both (+)-S 14297 and GR 103,691 are markedly selective ligands that permit the characterization of actions at hD₃vs. hD₂ receptors in vitro, but (+)-S 14297 appears to be of greater utility for the evaluation of their functional significance in vivo. Nevertheless, to develop a better understanding of the respective roles of dopamine D₃ and D₂ receptors, we need additional, chemically diverse antagonists of improved potency and selectivity.