Serotonin Synthesis Rate Measured in Living Dog Brain by Positron Emission Tomography

Abstract

In vivo measurements by positron emission tomography of the brain serotonin synthesis rates in the normal dog, in the dog with increased plasma tryptophan concentration, and in the dog under different arterial oxygen tensions are described. The method described here permits repeated measurements in the same brain for the first time. An increase in the plasma tryptophan concentration from 16.6 to 191.5 and then to 381 μM resulted in close to a linear increase in the brain serotonin synthesis rate. When P_aO₂ was raised from 76 ± 2 to 106 ± 1 mm Hg, the rate of serotonin synthesis in the dog brain increased from 39 ± 8 to 54 ± 10 pmol g⁻¹ min⁻¹. The estimates of the Michaelis-Menten constants, K^app_m and V_max for the transport of tryptophan through the blood-brain barrier are 303 ± 54 μM and 63 ± 10 nmol g⁻¹, min⁻¹, respectively.