Effects of Tryptophan on Serotonin in Nerve Endings1

Abstract —Preparations of crude synaptosome fractions (P2) from the telencephalon and from the diencephalon plus optic lobes of the pigeon and from the telencephalon of the rat were used to study the effects of l‐tryptophan on (a) the levels of serotonin (5‐HT), norepinephrinc (NE) and dopamine in nerve endings and (b) the release of radioactive 5‐HT, NE and dopamine from nerve endings. The level of 5‐HT was significantly higher (P < 0–05) in the P2 fraction isolated from the telencephalon of pigeons given intramuscular injections of 300mg/kg of l‐tryptophan in comparison to control values (1.11 ± 0.09 vs 0.74 ± 0.13 nmol/g original tissue wt). A smaller but not statistically significant increase in 5‐HT was noted in the P2 fractions isolated from the diencephalon plus optic lobes of pigeons given injections of l‐tryptophan. In vitro studies using preparations of synaptosomes (from both pigeon and rat) labelled with [3H]5–HT demonstrated that 1.0 mm‐l‐tryptophan caused a 30% increase (P < 0.05) in the release of [3H]5‐HT over control values. This effect by l‐tryptophan was blocked when a decarboxylase inhibitor was added to the medium. Tryptophan had no effect on the levels of NE or dopamine in these nerve endings nor did it have any effect on the release of these two amines from these preparations of synaptosomes. The results are discussed in terms of the role of serotonin in producing depression in pigeons working on a certain learned behavioural task.