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Vol. 287, Issue 1, 187-197, October 1998
Institut de Recherches Servier, Centre de Recherches de Croissy,
Psychopharmacology Department, Paris, France
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 D3 vs.
D2 receptors. Herein, we compared their in vitro
affinities 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/D3
antagonist [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
(Ki 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
(Ki 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 hD3
vs. hD4 and hD1 sites. However, GR 103,691 showed marked affinity for serotonin1A 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-HT1A receptors and
alpha-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-HT1A 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
hD3 vs. 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.
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