Final answer:
Symptoms of Parkinson's disease due to D2 blockade result from decreased dopaminergic signaling in the basal ganglia, as dopaminergic neurons in the substantia nigra pars compacta are lost. This affects the balance between the direct and indirect pathways, leading to motor and cognitive symptoms. L-DOPA is a common treatment that temporarily replenishes dopamine levels.
Step-by-step explanation:
The presence of symptoms of Parkinson's disease produced by D2 blockade in the nigrostriatal pathway is a consequence of disrupted dopaminergic signaling in the basal ganglia. Parkinson's is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNc), leading to motor symptoms such as tremors, bradykinesia, rigidity, and postural instability. The direct pathway increases cortical movement commands, while the indirect pathway, when overactivated due to decreased dopamine, results in the hypokinetic disorder known as Parkinson's disease. Additionally, treatments like L-DOPA aim to increase dopamine levels in the striatum, offsetting the dopamine deficit to reduce symptoms, though effectiveness may decrease over time.