G-protein coupled receptor for 5-hydroxytryptamine (serotonin). Also functions as a receptor for various drugs and psychoactive substances. Ligand binding causes a conformation change that triggers signaling via guanine nucleotide-binding proteins (G proteins) and modulates the activity of down-stream effectors, such as adenylate cyclase. Beta-arrestin family members inhibit signaling via G proteins and mediate activation of alternative signaling pathways. Signaling inhibits adenylate cyclase activity and activates a phosphatidylinositol-calcium second messenger system that regulates the release of Ca(2+) ions from intracellular stores. Plays a role in the regulation of 5-hydroxytryptamine release and in the regulation of dopamine and 5-hydroxytryptamine metabolism. Plays a role in the regulation of dopamine and 5-hydroxytryptamine levels in the brain, and thereby affects neural activity, mood and behavior. Plays a role in the response to anxiogenic stimuli. Heterodimer; heterodimerizes with GPER1 (By similarity). Interacts with YIF1B (By similarity). Most abundantly expressed in midbrain, in dorsal raphe and hippocampus. Detected at lower levels in amygdala and brain cortex. Mutant mice display decreased exploratory behavior and increased fear-related behavior in anxiogenic environments. Mutant mice display altered monoamine metabolism in specific parts of the brain, especially in dorsal and medial raphe nuclei, thalamus and hypothalamus, leading to altered levels of 5-hydroxytryptamine, dopamine and their metabolites, as well as altered noradrenaline levels. Belongs to the G-protein coupled receptor 1 family. 5-hydroxytryptamine receptor subfamily. HTR1A sub-subfamily.