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This study was designed to determine the affinity and binding profile of β-carbolines for imidazoline I₂ receptors and catalytic sites of monoamine oxidase (MAO)-A/B in rat brain and liver. The aim was also directed to assess the in vivo effects of norharman (β-carboline) and LSL 60101 (I₂ ligand) on brain 3,4-dihydroxyphenylalanine (DOPA) synthesis in morphine-dependent rats. Competition experiments against [³H]2-BFI revealed that β-carbolines recognize the high- and low-affinity components of the brain imidazoline I₂ receptor with the rank order of potency (K_iH in nM): noreleagnine (12)>norharman (20)>harmalol (82)>harmaline (177)≫harmine (630)>harman (700)≫FG-7142 (>100,000). In liver, this rank was different: harmine (51)>harmaline (103)=noreleagnine (103)≫harmalol (1290)>harman (2000)≫norharman (12,382)≫FG-7142 (>100,000). In brain and liver, competition curves for β-carbolines …