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.