A NEW METHOD TO MEASURE BRAIN-SEROTONIN SYNTHESIS INVIVO .2. A PRACTICAL AUTORADIOGRAPHIC METHOD TESTED IN NORMAL AND LITHIUM-TREATED RATS

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 α-[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 (~100 μm), 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.