A 68-year-old man from Delhi presents with a 3-year history of progressive tremor at rest, rigidity, and bradykinesia. He was started on levodopa-carbidopa 6 months ago with excellent initial response. However, over the past 2 months, he has developed unpredictable fluctuations in motor performance—periods of good mobility alternating with sudden "off" episodes where he becomes nearly immobile. His wife reports he also has involuntary writhing movements of the limbs during his "on" periods. Which of the following pharmacological mechanisms best explains the development of these motor complications?