A 28-year-old woman with bipolar I disorder has been on olanzapine 15 mg nightly for 18 months for mood stabilization. She now presents with amenorrhea for 4 months, galactorrhea, and gynecomastia. Serum prolactin is 85 ng/mL (normal <25 ng/mL). FSH and LH are low-normal. TSH is normal. She is not pregnant. What is the most likely mechanism of olanzapine-induced hyperprolactinemia in this patient?