Parkinson's disease.

In Parkinson''s disease there is degeneration of neurons in the substantia nigra, with consequent depletion of the neurotransmitter dopamine. The triad of tremor, rigidity and bradykinesia is the clinical hallmark. Drugs currently used for palliative therapy fall into three categories: anticholinergic agents, dopamine precursors (levodopa combined with extracerebral decarboxylase inhibitors) and artificial dopamine agonists. It has been argued, on theoretical grounds, that some drugs slow the progress of Parkinson''s disease, although no firm evidence has supported this. Treatment must be individualized, and more than one type of drug can be given concurrently after a careful build-up in dosage. We review the adverse effects of various drugs and consider new developments such as slow-release preparations, selective D-1 and D-2 agonists and transplants of dopaminergic cells into the brain. The treatment of Parkinson''s disease can be demanding, rewarding and sometimes frustrating, but it remains a most challenging exercise in pharmacotherapy.     

Treatment of Parkinson's disease.

Pharmacotherapy with levodopa for Parkinson''s disease provides symptomatic benefit, but fluctuations in (or loss of) response may eventually occur. Dopamine agonists are also helpful and, when taken with low doses of levodopa, often provide sustained benefit with fewer side effects; novel agonists and new methods for their administration are therefore under study. Other therapeutic strategies are being explored, including the use of type B monoamine oxidase inhibitors to reduce the metabolic breakdown of dopamine, catechol-O-methyltransferase inhibitors to retard the breakdown of levodopa, norepinephrine precursors to compensate for deficiency of this neurotransmitter, glutamate antagonists to counteract the effects of the subthalamic nucleus, and various neurotrophic factors to influence dopaminergic nigrostriatal cells. Surgical procedures involving pallidotomy are sometimes helpful. Those involving cerebral transplantation of adrenal medullary or fetal mesencephalic tissue have yielded mixed results; benefits may relate to the presence of growth factors in the transplanted tissue. The transplantation of genetically engineered cell lines will probably become the optimal transplantation procedure. The cause of Parkinson''s disease may relate to oxidant stress and the generation of free radicals. It is not clear whether treatment with selegiline hydrochloride (a type B monoamine oxidase inhibitor) delays the progression of Parkinson''s disease, because the drug also exerts a mild symptomatic effect. Daily treatment with vitamin E (a scavenger of free radicals) does not influence disease progression, perhaps because of limited penetration into the brain.     

Plasma aminopeptidase activities in Parkinson's disease.

Parkinson's disease (PD) is an age-related neurodegenerative disease characterized by a progressive motor disorder, but frequently is accompanied by autonomic symptoms such as hypotension. Together with the decrease of dopamine, significant decreases in aminopeptidase activities have been reported in PD brains. However, up to date there are no studies about changes of aminopeptidase activities in plasma of PD patients. We studied plasma activities of alanyl-, aspartyl-(AspAP), cystinyl-(CysAP) and glutamyl-aminopeptidase (GluAP) in two groups of subjects: control (n=41) and PD (n=48). Plasma activities of AspAP, CysAP, and GluAP showed significant decreases of 24.9% (p<0.05), 39.4% (p<0.01) and 33.3% (p<0.01), respectively, in PD group. These aminopeptidases are involved in the metabolism of circulating peptides such as the ones of the renin-angiotensin system. The importance of aminopeptidases in striatal dopamine content and in neuroendocrine system in PD is discussed.     

Neurotransmitter receptor alterations in Parkinson's disease.

Neurotransmitter receptor binding for GABA, serotonin, cholinergic muscarinic and dopamine receptors and choline acetyltransferase (ChAc) activity were measured in the frontal cortex, caudate nucleus, putamen and globus pallidus from postmortem brains of 10 Parkinsonian patients and 10 controls. No changes in any of these systems were observed in the frontal cortex. In the caudaye nucleus, only the apparent dopamine receptor binding was altered with a significant 30% decrease in the Parkinsonian brain. Both cholinergic muscarinic and serotonin receptor binding were significantly altered in the putamen, the former increasing and the latter decreasing with respect to controls. In addition, ChAc activity was decreased in the putamen. In the globus pallidus, only ChAc activity was significantly changed, decreasing about 60%, with no change in neurotransmitter receptor binding. The results suggest that a progressive loss of dopaminergic receptors in the caudate nucleus may contribute to the decreased response of Parkinsonian patients to L-dopa and dopamine agonist therapy.     

Drug treatment of Parkinson's disease.

A wide variety of drugs is available for treating Parkinson''s disease, including anticholinergics, amantadine levodopa, dopamine agonists, and selegiline. In younger patients (less than 50) levodopa is usually delayed provided that adequate relief of symptoms can be achieved with other drugs. In older patients (greater than 70) levodopa should be started as soon as symptom relief is required. Between these ages there is no consensus, but at present most such patients should probably be given controlled release levodopa before a dopamine agonist is added. Fluctuations can often be alleviated by giving controlled release preparations of levodopa, by giving small doses at frequent intervals, by adding selegiline or a long acting oral agonist, or by subcutaneous apomorphine. Dyskinesia can be peak dose, diphasic, or "off period." The diphasic form is hardest to alleviate. Psychiatric side effects should initially be managed by changing the antiparkinsonian treatment before resorting to antipsychotic drugs.