Explain the current treatments for Parkinson's disease (PD).

Answer: Parkinson's disease involves a progressive deterioration of dopamine neurons from the substantia nigra that innervate the basal ganglia. The standard drug treatment is L-DOPA, which can cross the blood-brain barrier and which is in turn converted into dopamine. Three surgical treatments include transplantation of fetal dopamine neurons into the caudate nucleus; stereotaxically-guided damage to the internal division of globus pallidus (which is inhibitory for movement); the final procedure involves implanting electrodes within the subthalamic nucleus through which electrical stimulation can be delivered to suppress tremors in PD.