A 28-year-old graduate student presents with complaints of headache. She has had multiple episodes of severe headache over the past three years. She describes the headache as a pounding pain behind her eyes and along the lateral aspects of her head. Prior to the headaches, she almost always sees small flashes of bright light that form enlarging patterns, then clear over time. She often feels nauseated during the headache and occasionally vomits. She has tried multiple over-the-counter pain medications with minimal relief. She has no other medical problems and takes no other medications. She denies fever, weakness, or loss of sensation. Her vital signs are normal. Physical examination, including a full neurologic examination, is normal. The patient's headache is interrupted using sumatriptan. This drug acts by activation of which of the following?