A 55-year-old female's medication list shows quetiapine 50 mg at bedtime prescribed by her primary care provider for insomnia. During psychiatric evaluation, she is diagnosed with bipolar I disorder with a current manic episode. The PMHNP plans to increase quetiapine to a therapeutic antipsychotic dose. Which of the following best explains the pharmacological basis for quetiapine's different clinical effects at low versus high doses?