Effect of alpha-methyldopa on dopaminergic transmission in the corpus striatum

Effects of methyldopa metabolites on amine transmitters and adrenergic receptors in rat brain

Studies of catecholamine concentrations in defined nuclei from the anterior hypothalamic-preoptic regions and the medulla oblongata, known to contribute to cardiovascular control, were measured following acute or chronic methyldopa administration. These studies indicated that methyldopa was enzymatically converted to methyldopamine and methylnorepinephrine, and in some areas to methylepinephrine which replaced endogenous epinephrine. The predominant metabolite was methylnorepinephrine, which accumulated in concentrations higher than endogenous norepinephrine levels. (-)Methylnorepinephrine was found to be 6 times more potent and 75 times more selective for alpha 2-adrenergic receptors than (-) norepinephrine, and it is suggested that this alpha 2-adrenergic receptor action, particularly in the nucleus tractus solitarius, contributes to a major part of the antihypertensive effect of methyldopa.

Adrenaline depletion in the hypothalamus of the rat after chronic alpha-methyldopa

1. Individual anterior hypothalamic-preoptic nuclei were dissected from the brains of control and alpha-methyldopa treated (2 x 40 mg/kg, 5 days) rats, and the concentrations of adrenaline and other catecholamines were estimated. 2. By a combination of radioenzymatic assay and paper chromatography the concentrations of adrenaline, noradrenaline, dopamine, alpha-methylnoradrenaline and alpha-methyldopamine were determined concurrently in the same sample. 3. alpha-Methyldopa reduced the adrenaline levels of the hypothalamic nuclei by 47--68% of control concentrations. 4. A differential displacement of the parent catecholamines was observed and the depletions were ranked: adrenaline less than dopamine less than noradrenaline. 5. alpha-Methylnoradrenaline and alpha-methyldopamine were found in all hypothalamic nuclei. 6. The depletion of hypothalamic adrenaline by alpha-methyldopa could represent a pharmacological mechanism contributing to the antihypertensive action of the drug.

Acute and chronic administration of alpha-methyldopa: regional levels of endogenous and alpha-methylated catecholamines in rat brain

Acute administration of alpha-methyldopa (200 mg kg-1 s.c.) produced a short-lived decrease in dopamine levels in 5 regions of the rat brain. Maximal dopamine depletion was 55% in the corpus striatum and over 75% in other regions and occurred 4--6 h after alpha-methyldopa injection; dopamine levels recovered within 12 h. Maximal noradrenaline depletion of over 80% occurred in all regions 4--24 h after alpha-methyldopa and after 24 h noradrenaline levels were still significantly reduced from control values. In these acute studies alpha-methyldopamine accumulated rapidly in amounts equal to or greater than the depleted dopamine in all regions, reaching a peak at 4 h. By contrast, alpha-methylnoradrenaline accumulated more slowly reaching a peak at 6--24 h and was never present in amounts greater than the depleted noradrenaline. Following chronic administration of alpha-methyldopa (40 mg kg-1 s.c., twice daily 5 days) there was a similar depletion of noradrenaline and dopamine to that seen in the acute studies. The depletion was associated with a much smaller accumulation of alpha-methyldopamine. The striking feature of these results, however, was the large accumulation of alpha-methylnoradrenaline in all brain regions. This probably reflects the slow turnover and resistance to degradation by monoamine oxidase of alpha-methylnoradrenaline.