SB-699,551 is a drug which was the first compound developed to act as a selective antagonist for the serotonin

Serotonin

Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

 receptor 5-HT_5A

5-HT5A receptor

5-hydroxytryptamine receptor 5A, also known as HTR5A, is a protein which in humans is encoded by the HTR5A gene.- Function :...

, with selectivity of around 100x over other serotonin receptor subtypes. The function of the 5-HT_5A receptor remains poorly defined, but SB-699,551 was found to modulate serotonin release from neurons, suggesting it acts as a presynaptic inhibitory autoreceptor similar to the 5-HT_1B

5-HT1B receptor

5-hydroxytryptamine receptor 1B also known as the 5-HT1B receptor is a protein that in humans is encoded by the HTR1B gene. The 5-HT1B receptor is a 5-HT receptor subtype.-Tissue distribution and function:...

 and 5-HT_1D

5-HT1D receptor

5-hydroxytryptamine receptor 1D, also known as HTR1D, is a 5-HT receptor, but also denotes the human gene encoding it. 5-HT1D acts on the central nervous system, and affects locomotion and anxiety...

receptors. Multiple therapeutic roles have been suggested for 5-HT_5A ligands due to the presence of this receptor in several areas of the brain, but research is still at an early stage.