Abstract
Procedures were developed for determination of labelled 5‐hydroxytryptamine (5‐HT) and 5‐hydroxyindoleacetic acid (5‐HIAA) formedin viuofrom3H‐tryptophan in rat brain. Tryptophan, 5‐HT and 5‐HIA.4 in brain extracts were isolated on columns of Dowex 50W‐X4, Amberlite CG 50, and Sephadex G 10, recoveries being 64 %, 78 % and 66 % respectively. Following i.v. injection of3H‐tryptophan labelled 5‐HT and 5‐HIAA accumulated in brain, reaching maximal levels after approximately 15 and 60 min respectively. The relationship between the specific activity curves for 5‐HT and 5‐HIAA appeared to be consistent with single compartment kinetics for 5‐HT. The effect of probenecid (200 mg/kg) on the amounts of labelled 5‐HT and 5‐HIAA formed from3H‐tryptophan indicates that besides inhibiting 5‐HIAA efflux from brain the drug also enhances 5‐HT synthesis.

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