Amantadine and Anticholinergic Agents

Amantadine blocks N-methyl-D-aspartate receptors — glutamate-gated ion channels in the striatum and elsewhere in the brain. In the context of Parkinson's disease, this is relevant for two reasons. First, N-methyl-D-aspartate receptor overactivity in the basal ganglia contributes to the abnormal motor patterns that produce dyskinesias; blocking these receptors dampens this hyperexcitability. Second, N-methyl-D-aspartate antagonism has a modest antiparkinsonian effect in its own right by modulating basal ganglia circuit activity.

Amantadine also enhances dopamine release from surviving presynaptic terminals and may inhibit dopamine reuptake. This dopaminergic effect is modest but contributes to symptomatic improvement in early disease when enough dopaminergic neurons remain to respond.

In early, mild Parkinson's disease, amantadine provides modest symptomatic benefit through its dopaminergic mechanism. It is not as effective as levodopa or dopamine agonists and is generally not used as primary therapy in patients with significant functional impairment.

Its most clinically distinctive use is for levodopa-induced dyskinesias. Amantadine is the only drug with established efficacy for reducing peak-dose dyskinesias without worsening motor control. This application — treating a complication of levodopa therapy rather than Parkinson's disease itself — reflects its N-methyl-D-aspartate antagonist mechanism rather than its dopaminergic effect.

As covered in Module 1, Parkinson's disease creates a relative excess of striatal acetylcholine activity because dopamine — which normally counterbalances acetylcholine interneuron activity — is depleted. Anticholinergic drugs block muscarinic receptors in the striatum, reducing this cholinergic excess and moving the neurotransmitter balance back toward normal. The effect is complementary to dopaminergic therapies: both strategies correct the same imbalance but from opposite directions.

Benztropine and trihexyphenidyl are the two anticholinergic agents used in Parkinson's disease. Both are centrally acting muscarinic antagonists. Benztropine has a longer duration of action and is available in injectable form, which is useful for acute situations such as drug-induced acute dystonia — a use that extends beyond Parkinson's disease management. Trihexyphenidyl is an oral agent used for chronic Parkinson's disease management.

Anticholinergic agents are most effective for tremor and, to a lesser extent, rigidity. They have little effect on bradykinesia — the most disabling feature of Parkinson's disease — which is driven primarily by dopamine deficiency rather than cholinergic excess. The practical result is that anticholinergic drugs are useful mainly in younger patients with tremor-predominant disease who cannot tolerate or do not yet require levodopa, and in patients whose tremor remains inadequately controlled despite optimal dopaminergic therapy.