ORIGINAL ARTICLE Subnanomolar dopamine D 3 receptor antagonism coupled to moderate D 2 affinity results in favourable antipsychotic-like activity in rodent models: I. neurochemical characterisation of RG-15 Béla Kiss & István Laszlovszky & Attila Horváth & Zsolt Némethy & Éva Schmidt & Gyula Bugovics & Károly Fazekas & István Gyertyán & Éva Ágai-Csongor & György Domány & Zsolt Szombathelyi Received: 29 February 2008 / Accepted: 1 May 2008 / Published online: 13 June 2008 # Springer-Verlag 2008 Abstract RG-15 ( trans-N-{4-[2-[4-(3-cyano-5-trifluoromethyl- phenyl)-piperazine-1-yl]-ethyl]-cyclohexyl}-3-pyridinesulfonic amide dihydrochloride) displayed subnanomolar affinity to human and rat dopamine D 3 receptors (pKi 10.49 and 9.42, respectively) and nanomolar affinity to human and rat D 2 receptors (pKi 8.23 and 7.62, respectively). No apparent in- teractions were found with the other 44 receptors and four channel sites tested in this study. RG-15 inhibited dopa- mine-stimulated [ 35 S]GTPγS binding in membranes from rat striatum, in murine A9 cells expressing human D 2L receptors and in CHO cells expressing human D 3 receptors (IC 50 values were 21.2, 36.7 and 7.2 nM, respectively). In these tests RG-15 showed the highest affinity toward D 3 receptors when compared to amisulpride, haloperidol and SB-277011. RG-15, similar to haloperidol and amisulpride, dose-dependently inhibited in vivo [ 3 H]raclopride binding in mouse striatum, enhanced dopamine turnover and synthesis rate in mouse and rat striatum and olfactory tubercle. SB- 277011 did not change [ 3 H]raclopride binding in mouse striatum nor biosynthesis or turnover rates in either region in mice or rats. RG-15 and haloperidol, but not SB-277011, antagonised dopamine synthesis inhibition induced by the D 3 /D 2 full agonist 7-OH-DPAT in GBL-treated mice. RG- 15, but not SB-277011, elevated plasma prolactin levels. In vitro receptor binding and functional experiments demon- strated that RG-15 had an antagonist profile on both D 3 and D 2 receptors. with high selectivity for dopamine D 3 receptors over D 2 receptors. However, in vivo, its neurochemical actions were similar to those of D 2 receptor antagonists. Neurochemical comparison of RG-15 with antagonists having a different affinity and selectivity toward D 3 and D 2 receptors indicate that D 3 receptors have little, if any, role in the control of presynaptic dopamine biosynthesis/ release in dopaminergic terminal regions. Keywords Antipsychotic . Dopamine biosynthesis/ turnover . Dopamine D3/D2 antagonist . RG-15 . Schizophrenia Introduction Cerebral dopaminergic systems are implicated in several central nervous system (CNS) functions and the pathophys- iology of CNS diseases, such as psychoses (e.g. schizophre- nia), movement disorders (e.g. Parkinson’ s disease), drug abuse and cognitive deficits. To date, at least five distinct dopamine receptors have been described and characterised by molecular biology and cellular and behavioral pharmacology. One family of dopamine receptors consists of “D 1 -like” receptors (D 1 and D 5 dopamine receptors), which are posi- tively coupled to adenylyl cyclase through stimulatory G- proteins (G s ). The other dopamine receptor family consists Naunyn-Schmiedeberg’ s Arch Pharmacol (2008) 378:515–528 DOI 10.1007/s00210-008-0308-5 B. Kiss : I. Laszlovszky : A. Horváth : Z. Némethy : É. Schmidt : G. Bugovics : K. Fazekas : I. Gyertyán : É. Ágai-Csongor : G. Domány : Z. Szombathelyi Research Division, Gedeon Richter Plc., Budapest 1475, Hungary B. Kiss (*) Department of Molecular Pharmacology, Gedeon Richter Plc., P.O. Box 27 Budapest 1475, Hungary e-mail: b.kiss@richter.hu