A 30-year-old male diagnosed with schizophrenia is started on risperidone. After several weeks, he complains of bilateral breast tenderness and notices a milky discharge from his nipples. Which of the following pharmacological actions of risperidone is most likely responsible for this side effect?

Explanation

Risperidone, an atypical antipsychotic, is known for its relatively high potency in blocking dopamine D2 receptors. While all antipsychotics block D2 receptors to some extent, risperidone has a particularly strong D2 blockade, especially at higher doses, which can lead to hyperprolactinemia. Dopamine normally inhibits prolactin release from the pituitary gland. By blocking D2 receptors in the tuberoinfundibular pathway, risperidone reduces this inhibitory effect, leading to increased prolactin levels. Elevated prolactin can cause galactorrhea (milky discharge), gynecomastia (breast tenderness/enlargement), menstrual irregularities, and sexual dysfunction. Serotonin 5-HT2A antagonism is associated with the atypical profile and reduced extrapyramidal side effects. Alpha-1 adrenergic blockade can cause orthostatic hypotension. Histamine H1 blockade can cause sedation and weight gain.