Increase in brain tryptophan and 5-hydroxytryptamine on administration of phenothiazines to rats.

A major factor regulating the synthesis of the neurotransmitter 5-hydroxytryptamine in the brain is the availability of the precursor amino acid, tryptophan. The K, of tryptophan hydroxylase (L-tryptophan mono-oxygenase, EC 1.14.16.4) is of the same order of magnitude as the brain concentration of tryptophan (Gal, 1974). Uptake of tryptophan into the brain is believed to be regulated by that fraction of serum tryptophan that is freely diffusible, rather than by the total serum tryptophan concentration (Etienne et al., 1976) and by the concentrations in serum of those amino acids that compete with tryptophan for the uptake mechanism (Gessa & Tagliamonte, 1974). It therefore seems likely that measurement of difhsible serum tryptophan and the concentration of those amino acids that compete with tryptophan may allow the relative rate of brain 5-hydroxytryptamine synthesis to be predicted. It has been shown previously (Bender, 1976~) that administration of chlorpromazine to rats results in a decrease in the total serum tryptophan concentration, and a decrease in the proportion of tryptophan bound to serum albumin, so that the concentration of diffusible tryptophan remains at or above the control value. At the same time there is a decrease in the total serum amino acid concentration. These serum changes are accompanied by an increase in the uptake of tryptophan into the brain and an increase in the synthesis of 5-hydroxytryptamine. Previous work from this laboratory has also shown that a number of anti-schizophrenia drugs all had effects on serum tryptophan concentrations, albumin binding of tryptophan and total serum amino acid concentrations similar to those shown by chlorpromazine (Bender, 19766). It would therefore be predicted that these drugs would lead to an increase in brain tryptophan uptake and 5-hydroxytryptamine synthesis. Confirmation of this prediction is presented here for three anti-schizophrenia drugs chemically related to chlorpromazine : prochlorperazine, chlorprothixene and thioridazine. At the same time, two further phenothiazines, which have no significant anti-schizophrenia action, trimeprazine and promethazine, have also been tested. Female Courtauld Institute rats, weighing 90-1OOg, were used for these experiments. They were deprived of food, but not water, for 24h, and then the drugs, dissolved in 0. I5 M-NacI, were administered by intraperitoneal injection, at 09: OOh. Control animals received NaCl solution alone. The animals were killed by decapitation between 13:OO and 14:00h. Blood was collected for preparation of serum, and livers and brains were rapidly dissected out and frozen in liquid NZ. They were stored at -20°C until required for analysis. Total serum tryptophan was measured by the method of Denckla & Dewey (I 967), tryptophan binding to albumin by the equilibrium-dialysis method of Bender et al. (1975), total serum amino acids by the method of Maeda &Tsuji (1973); brain tryptophan and 5-hydroxytryptamine were separated and measured by the method described previously (Bender, 1976a) and liver tryptophan oxygenase [L-tryptophanoxygen 2,3-oxidoreductase (decyclizing), EC 1.13.1 1.1 I ] activity was measured by a modification of the method of Knox et al. (1966). Table 1 shows the effects of the drugs on serum tryptophan concentration, albumin binding of tryptophan and total serum amino acid concentrations. As reported previously (Bender, 19766) the anti-schizophrenia phenothiazines led to a decrease in total serum tryptophan and a decrease in the proportion of tryptophan bound to serum albumin, so that the concentration of freely diffusible serum tryptophan was the same as, or slightly higher than, in control animals. The two phenothiazines without antischizophrenia action both led to a slight increase in total serum tryptophan. Trimeprazine, but not promethazine, decreased the percentage of tryptophan freely diffusible.