Effect of lithium on serum tryptophan and brain serotonin in rats.

The influence of lithium administration on brain serotonin metabolism and the serum tryptophan level in rats was investigated. The brain tryptophan, serotonin and 5-hydroxyindole acetic acid contents did not change after a single in jection of lithium, but did increase significantly after repeated injections. A single injection of lithium caused a marked decrease in the serum total tryptophan level, but did not change the ratio of free to albumin bound tryptophan. On the contrary, repeated injections of lithium caused a significant increase in serum total tryptophan, and decrease in the ratio of free to bound tryptophan. Addition of lithium ion to normal rat serum in vitro did not change the ratio of free to bound tryptophan. A single injection of lithium produced a marked rise in serum nonesterified fatty acid, which is known to affect the binding of tryptophan by albumin. Lithium salts have been reported to be effective in treatment of mania and are used prophylactically to reduce the frequency of episodes of mania and depression (1). It is not known how lithium exerts the psychotropic effect, although numerous biochemical effects of lithium have been reported, including effects on brain catecholamine metabolism (2, 3), and on the electrolyte balance of the body (4, 5). Conflicting results have been reported on the action of lithium on brain serotonin (5-HT) metabolism (6, 7). However, recently it was reported that chronic administration of lithium increases the content of brain tryptophan and thereby the rate of synthesis of 5-HT in the brain (8). On the other hand, it has been established that the rate of syn thesis of 5-HT in the brain is regulated principally by the level of free tryptophan in the plasma (9). The present study was undertaken to determine the effect of acute or subacute treat ment with lithium on the contents of brain 5-HT and serum total and free tryptophan. MATERIALS AND METHODS Male Sprague-Dawley rats weighing 150 to 170 g and 270 to 300 g were used. Lithi um carbonate was dissolved in water and adjusted to pH 7.3 with N-HC1. As acute treat ment, lithium was injected i.p. as a single dose of 5 mEq/kg. As subacute treatment, in jections of 2 mEq/kg i.p. were given twice daily for 5 days and animals were sacrificed 5 hr after the last injection. Control rats were injected with 5 and 2 mEq/kg of sodium chloride, respectively. Animals were sacrificed by decapitation and the brain quickly removed, homogenized and stored at -20'C until analyzed. Blood was collected in a centrifuge tube and centrifuged and the serum was stored. Brain 5-HT was determined by the method of Bogdanski et a!. (10) and brain 5-hydroxyindole acetic acid (5-HIAA) by the method of Udenfriend et al. (11). Tryptophan in the brain and serum was assayed fluorometrically by the method of Denkla and Dewey (12). Serum free tryptophan was determined by analyzing ultrafiltrates prepared by centrifuging 1 ml of serum in dialysis tubing (size 8/32, Visking Co.) at 3,000 g for 2 hr at 0-4'C. Serum albumin was assayed spectrophotometrically using 2-(4'-hydroxybenzeneazo) benzoic acid (Nakarai Chemicals, Ltd.) (13). Serum nonesterified fatty acid (NEFA) was assayed by the method of Itaya and U 1 (14). RESULTS Effects of a single injection of lithium on the concentration of serum total tryptophan Fig. 1 shows the serum total tryptophan concentrations in 5 and 9 week old rats at intervals after a single injection of lithium. The control level of serum tryptophan was higher in 9 week old (270 to 300 g) than in 5 week old rats (150 to 170 g). In both groups, the concentration decreased significantly within 3 hr after the injection, and the maximum decrease was observed after 6 hr. The decrease was greater in the older rats. Flci. 1. Change in serum total tryptophan after lithium injection. Dose of lithium was 5 mEq,'kg (i.p.). Exp. I : 9 week old rats (270 to 300 g). Exp. 11 : 5 week old rats (150 to 170 g). * Significantly different from control (P-0 .05). ~* Significantly different from control (P<0 .01). Vertical bars express means -I-S.E. of values for 5 to 7 animals. Effects of a single injection of lithium on the brain contents of tryptophan, 5-HT and 5-HIAA The brain content levels of tryptophan, 5-HT and 5-HIAA at intervals after a single injection of lithium are shown in Table 1. A significant change was not observed. Effects of a single injection of lithium on the concentrations of free aid bound tryptophan i n the serum The concentration of serum free tryptophan was 2.51 +0.09 1~g/ml in control rats treated with 5 rnEq/kg of NaCI. Five hr after a single injection of lithium, it had fallen to 65 % of this value (Table 2). The concentration of tryptophan bound to serum pro tein also decreased after the injection, and as a result, the ratio of free to bound trypto phan did not change significantly. TABLE 1. Effects of a single injection of lithium on the brain contents of tryptophan,