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 and behavior. Activation of the receptor may play a role in the exit from G0 phase and in promoting DNA synthesis. First expressed in the rostral part of the brain stem at stage 22. At later stages of development, expression is localized to serotonergic neurons. The expression pattern changes in the tadpole of stage 41 where, in addition to serotonergic neurons, expression is also localized to the inner nuclear layer (INL) of the developing retina. This expression pattern continues through to the start of metamorphosis (stage 46). In adults, expressed in the brain, in particular the telencephalon, diencephalon and mesencephalon. In the telencephalic region, expression is localized to the lateral, dorsal and medial pallium, and in the striatum, septum and amygdala. In the mesencephalic region, expression is strongest in the optic tectum and torus semicircularis with moderate levels of expression in tegmental nuclei. In diencephalon, localized to the dorsal and ventral thalamus and the preoptic area of the hypothalamus. Belongs to the G-protein coupled receptor 1 family. 5-hydroxytryptamine receptor subfamily. HTR1A sub-subfamily.