Pharmacology of a Novel Selective 5-hydroxytryptamine1B Receptor Antagonist, AR-A000002

The terminal 5-HT(1B) autoreceptors have attracted great pharmacological interest since they are potential targets for compounds modifying serotonergic neurotransmission. In the present work the in vivo biochemical properties of AR-A000002 ((R)-N-[5-methyl-8-(4-methylpiperazin-1-yl)-1,2,3,4-tetrahydro-2-naphthyl]-4-morpholinobenzamide), a novel selective 5-HT(1B) receptor antagonist, are reported. The effects of AR-A000002 on: 5-HT metabolism was measured as the ratio between 5-HIAA and 5-HT concentrations in different brain regions; 5-HT synthesis was measured as the accumulation of 5-HTP after inhibition of the aromatic amino acid decarboxylase activity with m-hydroxybenzylhydrazine (NSD1015); 5-HT release was measured using the microdialysis technique. 5-HT, 5-HIAA and 5-HTP concentrations were analyzed using high power liquid chromatography (HPLC) with electrochemical detection. AR-A000002 significantly enhanced 5-HT metabolism (5-HIAA/5-HT ratio) and 5-HT synthesis in guinea pig brain in the dose range 0.9-18 mg/kg s.c. (ED(50)=1 mg/kg s.c. in the four brain regions examined) with maximal effect seen after 2-4 h. AR-A000002 (9 mg/kg s.c.) significantly increased the extracellular concentrations of 5-HT and 5-HIAA by 20% in the guinea pig frontal cortex, measured with the in vivo microdialysis technique in freely moving guinea pigs. AR-A000002 (9 mg/kg s.c.) in combination with the 5-HT uptake inhibitor citalopram (5 mg/kg s.c.) increased the extracellular 5-HT concentration in guinea pig frontal cortex from 250 to 400% of the basal level. Citalopram alone decreased the extracellular 5-HIAA levels to 70% of the basal value. AR-A000002 counteracted the citalopram-induced decrease in 5-HIAA. Since the basal level of extracellular 5-HIAA was 160 times higher than that of 5-HT the 20% increase in 5-HIAA concentrations indicates that only a few percent of the exocytotically released 5-HT from the nerve terminals reached the extracellular space when the re-uptake mechanism was intact. The results also show that the terminal 5-HT(1B) autoreceptors are tonically activated under drug-free as well as citalopram conditions. The increase in plasma level of cortisol after AR-A000002 administration may indicate stimulation of post-synaptic 5-HT receptors. AR-A000002 also blocked 5-HT(1B) agonist-induced (CP-135,807) decrease in 5-HT metabolism and hypothermia (ED(50)=1 mg/kg s.c.), thus indicating competition between these two drugs. It is concluded that AR-A000002 is a 5-HT(1B) receptor antagonist that enhances the serotonergic neurotransmission in guinea pig brain.