A serotonin receptor subtype found at high levels in the basal ganglia and the frontal cortex. It plays a role as a terminal autoreceptor that regulates the rate of serotonin release from nerve endings. This serotonin receptor subtype is closely related to and has similar drug binding properties as the 5-HT1D receptor. It is particularly sensitive to the agonist sumatriptan and may be involved in mediating the drug's antimigraine effect.