ICD-10 Specific code G20: Parkinson's disease

Tetrabenazine is a synthetic derivative of benzylquinolizine that causes depletion of dopamine and other monoamines in the central nervous system. The precise mechanism by which tetrabenazine exerts its effects is unknown, but is believed to be related to its effect as a reversible depletor of monoamines (such as dopamine, serotonin, norepinephrine, and histamine) from nerve terminals. Neurotransmitter depletion by a single dose of tetrabenazine is reversible and lasts only a few hours.

Tiapride is an atypical neuroleptic which exhibits selectivity in in-vitro studies for D₂ and D₃ dopamine subtype receptors without any affinity for subtype receptors of the principal central neurotransmitters (including serotonin, noradrenaline and histamine).

Tolcapone is an orally active, selective and reversible catechol-O-methyltransferase (COMT) inhibitor. Administered concomitantly with levodopa and an aromatic amino acid decarboxylase inhibitor (AADC-I), it leads to more stable plasma levels of levodopa by reducing metabolism of levodopa to 3-methoxy-4-hydroxy-L-phenylalanine (3-OMD). High levels of plasma 3-OMD have been associated with poor response to levodopa in Parkinson’s disease patients.

Trihexyphenidyl is an anticholinergic agent. It is an antispasmodic drug which exerts a direct inhibitory effect on the parasympathetic nervous system. It diminishes salivation, increases the heart rate, dilates the pupils and reduces spasm of smooth muscle.