Summary:We describe here an autoradiographic method to measure the in vivo rate of serotonin synthesis in rat brain. The method is based on the use of the L-tryptophan analogue a-methyl-L-tryptophan (a-MTrp), which is con verted in vivo into a-methyl serotonin (a-M5HT). Since a-M5HT is not a substrate for monoamine oxidase, it is accumulated in the brain tissue. Data are presented to confirm time-dependent conversion of a-MTrp into a-M5HT in the dorsal raphe nucleus and also in the pineal body, an organ outside the blood-brain barrier. It has also Several methods have been proposed to measure serotonin synthesis in the brain of laboratory ani mals (for review, see Haber et aI., 198 1; Korf, 1985). Two approaches have been used, i.e., steady state and non-steady state. The rate of serotonin synthesis has usually been estimated on the basis of changes in the concentration of tryptophan metab olites [5-hydroxytryptophan, 5-hydroxytryptamine, or 5-hydroxyindoleacetic acid (5HIAA)] after some kind of pharmacological manipulation. Serotonin (5-hydroxytryptamine) is synthesized from L tryptophan in a two-step enzymatic process. The first step, hydroxylation of L-tryptophan with tryp tophan 5-hydroxylase (EC 1. 14. 16.4), requiring tet rahydrobiopterin as a co-factor and molecular oxy-

We describe here a practical autoradiographic method to estimate the rate of serotonin synthesis in brain. A two-time point method (60 and 150 min after injection of alpha-[14C]methyl-L-tryptophan) was first evaluated in 14 normal rats (7 at each time point). After this the method was tested in lithium-treated rats. In normal rats the rate of serotonin synthesis measured by the two-time point method generally correlated with known concentrations of tryptophan hydroxylase. The rate of synthesis in lithium-treated rats was compared with that in sham-treated rats (NaCl treatment). The results showed a significant increase in the synthesis rate in some cerebral structures. The greatest increases in the serotonin synthesis rate, attributable to the lithium treatment, were observed in the parietal cortex (52%) and caudate nucleus (47%). This is the first investigation to demonstrate, with autoradiographic resolution (approximately 100 microns), the differential changes in the rate of serotonin synthesis in the brain. Lithium had no significant effect on the rate of synthesis in the pineal gland.

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 microM resulted in close to a linear increase in the brain serotonin synthesis rate. When PaO2 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, Kappm and Vmax, for the transport of tryptophan through the blood-brain barrier are 303 +/- 54 microM and 63 +/- 10 nmol g-1 min-1, respectively.

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