A 67-year-old man with Parkinson disease is started on a medication aimed at reducing tremor and rigidity. Two weeks later, his wife reports that he is moving more slowly and has a shuffling gait that is worse than before starting the new medication. On examination, he has increased tone with cogwheel rigidity and bradykinesia. The neurologist explains that the new medication increased activity of striatal interneurons that promote the basal ganglia circuit that suppresses movement.

Which of the following synaptic effects most directly occurs in the initial step of this movement-suppressing circuit?