A 28-year-old woman with bipolar I disorder has been on olanzapine 15 mg/day for 10 months. She presents with amenorrhea for 3 months, galactorrhea, and decreased libido. Her serum prolactin is 68 ng/mL (normal <25 ng/mL in women). She denies pregnancy, and thyroid function tests are normal. Which mechanism best explains her hyperprolactinemia and why is this a concern in long-term antipsychotic therapy?