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Continuous and intermittent levodopa differentially affect rotation induced by D-1 and D-2 dopamine agonists
T M Engber1, Z Susel, J L Juncos
1Experimental Therapeutics Branch, National Institute of Neurological and Communicative Disorders and Stroke, Bethesda, MD 20892.
European Journal of Pharmacology
|September 22, 1989
Summary
Continuous levodopa treatment enhanced responses to D-2 dopamine agonists, while intermittent treatment altered responses to both D-1 and D-2 agonists. Dopamine receptor stimulation intermittence impacts the dopamine system.
Area of Science:
- Neuroscience
- Pharmacology
- Dopamine Pathway Research
Background:
- Parkinson's disease involves the degeneration of dopaminergic neurons.
- Levodopa is a primary treatment, but its long-term effects on dopamine receptor sensitivity are complex.
- Understanding how levodopa administration affects dopamine receptor subtypes is crucial for optimizing treatment.
Purpose of the Study:
- To investigate the differential effects of continuous versus intermittent levodopa administration on dopamine receptor-mediated behaviors.
- To determine if the timing of dopamine stimulation influences the sensitivity of D-1 and D-2 dopamine receptors.
- To elucidate the long-term consequences of different levodopa delivery methods on the nigrostriatal dopamine pathway.
Main Methods:
- Rats with unilateral 6-hydroxydopamine lesions were used to model Parkinson's disease.
- Chronic levodopa administration was delivered via continuous infusion or intermittent injections over 19 days.
- Rotational behavior induced by D-1 (SKF 38393), D-2 (quinpirole), and non-selective (apomorphine) dopamine agonists was measured after a washout period.
Main Results:
- Continuous levodopa enhanced rotation to the D-2 agonist quinpirole but not the D-1 agonist SKF 38393.
- Intermittent levodopa markedly increased quinpirole response, greatly diminished SKF 38393 response, and modestly enhanced apomorphine response.
- Both treatment methods achieved equivalent daily plasma levodopa levels, suggesting administration pattern, not total dose, is key.
Conclusions:
- The intermittence of central dopamine receptor stimulation significantly influences subsequent dopamine system responses.
- D-1 and D-2 dopamine receptor-mediated mechanisms exhibit distinct sensitivities to chronic levodopa treatment.
- Findings highlight the importance of considering levodopa delivery profiles for managing Parkinson's disease symptoms and side effects.