124 Section 2/ Drugs Acting on CNSbecome more alert and interested in life
and surroundings.- CVS: The levodopa produces its CVS
effect by being converted into
dopamine. It acts by stimulating alpha
adrenergic receptors in blood and
produce vasoconstriction and may
raise the blood pressure. It also acts
by stimulating betaadrenergic
receptors in heart and produce
tachycardia and increase force of
myocardial contraction (positive
inotropic action).
3. Endocrine system: Dopamine inhibits
prolactin release in human being. It also
acts on somatotrophs to increase
growth hormone release.
4. Miscellaneous actions: Peripherally
formed dopamine (converted peripher-
ally after levodopa therapy) gains access
to the CTZ (chemoreceptor trigger zone)
causing nausea and vomiting.
Pharmacokinetics
Levodopa is rapidly absorbed when
given orally and peak plasma level is
Table 2.10.1: Classification of antiparkinsonism agents.
I. Drug acting on central dopaminergic system
i. Precursors of dopamine
Levodopa (LEVOPA) 0.5-3.0 g/day
ii. Dopaminergic agonists
Bromocriptine (PROCTINAL) 10-40 mg/day
Lisuride 2-5 mg/day
Pergolide 2-4 mg/day
Ropinirole (ROPARK) 0.25 mg TDS, increased each week by 0.25 mg
(max 24 mg/day)
iii.Drugs facilitating dopaminergic transmission
Amantadine 100 mg BD
Selegiline 5-10 mg/day
iv. Peripheral dopa-decarboxylase inhibitors
Carbidopa (used with levodopa; 25 mg carbidopa +
100 mg levodopa; TIDOMET PLUS) 20-100 mg/day
Benserazide (used with levodopa; BENSPAR) 50-250 mg/day
II. Drug acting on central cholinergic system
i. Anticholinergics
Trihexyphenidyl (Benzhexol; PACITANE) 2 mg OD-QID
Procyclidine (KEMADRIN) 2.5-5 mg OD-TDS
Biperiden (DYSKINON) 1-4 mg TDS oral/IM/IV
Benztropine (COGENTIN) 1-2 mg OD-TDS
ii. Antihistaminics
Promethazine (PHENERGAN) 10-25 mg TDS
Orphenadrine (DISIPAL) 50-100 mg BD-TDS
Diphenhydramine (BENADRYL) 50-100 mg/day