Instead, you take the drug systemically (e.g., by mouth or by injection), which means it gets into the bloodstream and has its effect all over the brain. Give someone with schizophrenia a dopamine receptor blocker, and you decrease the abnormally high levels of dopaminergic signaling in the “schizophrenic” part of the brain back to normal; but at the same time, you decrease the normal levels elsewhere to below normal. Give L-DOPA to someone with Parkinson’s, and you raise dopamine signaling in the “Parkinsonian” part of the brain to normal but boost signaling to above-normal levels elsewhere
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