Localization of the specific binding sites of [(3)H]tryptophan in rat brain

The regional and cellular localizations of the previously reported aromatic amino acid binding sites (AABS) in rat brain were investigated using [(3)H]tryptophan as the binding radioligand. An almost 4-fold difference in AABS was observed among the brain regions in the descending order of cerebral cortex ? hippocampus > striatum ? thalamus/hypothalamus ? midbrain ? cerebellum ? brainstem. When rats were injected intracerebroventricularly (i.c.v.) with 5,7-dihydroxytryptamine (0.2 ?mol) after prior administration of desipramine (i.p., 25 mg/kg), [(3)H]tryptophan binding was significantly reduced by 32% in the brainstem but increased by 27% in the midbrain. After injection of 6-hydroxydopamine (i.c.v., 0.2 ?mol), however, binding was significantly reduced in the brainstem by 32%, striatum by 27% and cerebral cortex by 18% but increased by 24% in the midbrain. It is concluded that the AABS are associated with some but not all cells of the three monoaminergic systems in the rat brain. The increases in binding in the midbrain may reflect a localization of the AABS in this region in cells post-synaptic to the degenerating cells.