5-Hydroxytryptamine-moduline, also known as 5-HT-moduline or serotonin-moduline as well as Leu-Ser-Ala-Leu (LSAL), is an endogenous neuropeptide and high-affinity serotonin 5-HT<sub>1B</sub> and 5-HT<sub>1D</sub> receptor negative allosteric modulator. It is produced in the brain and is co-localized with serotonin 5-HT<sub>1B</sub> receptor-expressing neurons. The compound has been shown to induce desensitization of serotonin 5-HT<sub>1B</sub> receptors. It has been found to increase dopamine release in the striatum in rodents. By inhibiting serotonin 5-HT<sub>1B</sub> autoreceptors, 5-HT-moduline disinhibits serotonin release and may have antidepressant potential. Antibodies and small-molecule antagonists against 5-HT-moduline like HG1 have been found to produce anxiolytic-like effects in rodents. 5-HT-moduline was first described in the scientific literature by 1996. Analogues of 5-HT-moduline have been studied.