Muscarinic acetylcholine receptors (mAChR) regulate a number of important basic physiologic functions including heart rate, motor and sensory control and more complex behaviors including arousal, memory, and learning. Loss of muscarinic receptor number or function has been implicated in the etiology of several neurological disorders including Alzheimer"s dementia, Down"s syndrome, and Parkinson"s disease. Five subtypes of muscarinic receptors (m1-m5) have been identified by molecular cloning¹ and much has been learned about their distribution, pharmacology, and structure².

Biperiden hydrochloride is a non-selective muscarinic receptor antagonist that displays selectivity for the M₁ subtype (K_i values are 0.48, 2.4, 3.9, 6.3 and 6.3 nM for M₁, M₄, M₃, M₂ and M₅ receptors respectively)^3-4. It is antiparkinsonian⁵ and therefore used for the adjunctive treatment of all forms of Parkinson"s disease (postencephalitic, idiopathic, and arteriosclerotic), also commonly used to improve parkinsonian signs and symptoms related to antipsychotic drug therapy⁶.