Centrally induced hypotension by alpha-methyldopa and alpha-methylnoradrenaline in normotensive and renal hypertensive rats

Correlation between fall in blood pressure and in vivo amine release after alpha-methylDOPA

Blood pressure (BP), hypothalamic tissue concentrations and the in vivo overflow of endogenous and alpha-methylated catecholamines were measured in urethane anaesthetised rats after alpha-methylDOPA (mDOPA) administration (200 mg/kg i.p.). Four hours after mDOPA, BP fell to its lowest value, 60% of control, and slowly returned towards control levels by 24 h. This was closely correlated with the evoked overflow of alpha-methylnoradrenaline (mNA, r = 0.9) and noradrenaline (NA, r = 0.7) but not dopamine (DA) or alpha-methyldopamine (mDA). However, the tissue content of mNA rose much more gradually and was not maximal until after 12 h while mDA content followed the development of the hypotension. The results provide direct evidence for a false transmitter role for mNA in the brain, and suggest that the release of newly synthesised mNA is responsible for the hypotensive effect of mDOPA. Differences in the time course of overflow and storage of NA and mNA suggest the presence of separate transmitter storage and releasable pools.

Effects of clonidine, alpha-methyldopa and hydralazine on met-enkephalinergic neurons in cerebral nuclei of spontaneously hypertensive rats

Immunocytofluorescent and microautoradiographic methods were applied to measure met-enkephalin-like immunoreactivity (MELI) and ME receptor binding (MERB) levels in cerebral nuclei of serial brain slices from SHR which received clonidine, alpha-methyldopa and hydralazine at equivalent hypotensive doses. All three drugs increased both MELI and MERB levels in the caudal part of the n. tractus solitarii and its functionally related dorsomedullary nuclei and decreased them in the n. accumbens septi, in accordance with the correspondent change in glucose utilization rates in these nuclei as reported previously. Both CNS-active agents (not hydralazine) also increased MELI and MERB levels in the n. intercalatus and substantia grisea centralis, and they decreased them in the n. ventromedialis hypothalami. Differences in both CNS-active agents were minor. Vasodilative hydralazine alone decreased these levels both in the n. reticularis medialis and n. tegmenti ventralis, and it increased them slightly in the area lateralis hypothalami. The present studies indicate that ME neurons of these dorsomedullary and supramedullary nuclei may act as direct as well as homeostatic controls of blood pressure.

Antihypertensive metabolites of alpha-methyldopa

alpha-Methyldopa acts through a metabolite in the central nervous system. Of the three metabolites most prominently considered as potential mediators of alpha-methyldopa hypotension--alpha-methyldopamine, alpha-methylnorepinephrine (MNE), and alpha-methylepinephrine (ME)--ME is the most potent depressor agent following intravenous infusion into rats, following injection into the lateral ventricle, and following injection into the solitary tract nucleus (NTS). The depressor effect of ME in the NTS is attenuated as effectively by timolol as by yohimbine, while the combination of both timolol and yohimbine completely abolishes the pharmacological activity of ME in the NTS. ME is approximately eightfold more potent than MNE in the NTS and also has a greater susceptibility to timolol attenuation.

Importance of central serotonin neurons in the hypotensive action of methyldopa in the rat

Normotensive (WKY) and stroke prone spontaneously hypertensive (SHR-SP) rats were given methyldopa (200 mg/kg i.p.) daily for five days and their brains were then sectioned and processed with the Faglu method for catecholamine fluorescence. This treatment with methyldopa induced a green fluorescence not seen in control animals, in cells coinciding with the B1-B9 groups of serotonin neurons in the brainstem. Pretreatment with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT, i.c.v.), which is relatively specific for serotonin neurons, prevented the appearance of this green fluorescence in the serotonin cell groups of rats given methyldopa. Pretreatment with 5,7-DHT, i.c.v. approximately halved the magnitude of the hypotensive response to a single dose of methyldopa (80 mg/kg i.p.). We suggest that central serotonin nerves contribute to the hypotensive action of methyldopa. It is our hypothesis that methyldopa is taken up by these serotonin cells and that the green fluorescence reflects the production of alpha-methyldopamine, as a result of decarboxylation by the ubiquitous enzyme, L-aromatic amino acid decarboxylase.

Influence of alpha-methyldopa treatment on adrenergic receptor binding in rat forebrain

We studied the influence of 6 days treatment with alpha-methyldopa (50 mg/kg s.c.) twice daily on radioligand binding to alpha 1 [3H)prazosin), alpha 2 [3H)clonidine) and beta[3H)dihydroalprenolol (DHA] receptors in rat forebrain. A 27% rise (p less than 0.05) in the Bmax for (3H)prazosin without change in Kd was found. Following alpha-methyldopa, the Kd for (3H)clonidine was increased from 1.78 +/- 19 to 3.03 +/- nM and Bmax fell from 197 +/- 20 to 167 +/- 19 fmoles/mg protein (p less than 0.05). These findings suggest that chronic alpha-methyldopa therapy induces changes in alpha 1 and alpha 2 receptors which may modify the antihypertensive effect of alpha-methyldopa.

alpha-Methylepinephrine, a methyldopa metabolite that binds to alpha-receptors in rat brain

The presence of phenylethanolamine-N-methyl transferase in those brain-stem nuclei where methyldopa is thought to act suggested to us that methylepinephrine could be an active metabolite of methyldopa. We assessed the potential of methylepinephrine to be an active metabolite by determining its capacity to compete for ligands which bind to alpha-receptors in rat brain. Thus, the abilities of epinephrine, norepinephrine, methyldopa, (+/-)-methylnorepinephrine and (+/-)-methylepinephrine to compete for alpha 1-([3H]WB-4101) and alpha 2-([3H]clonidine) binding sites in rat brain wee compared. Order of potency at each binding site was: [3H]WB-4101 -- (-)epinephrine = (-)-norepinephrine greater than (+/-)-methylnorepinephrine greater than (+/-)-methylepinephrine much greater than (-)-methyldopa; [3H]clonidine -- (-)-epinephrine greater than (+/-)-methylnorepinephrine greater than (-)-norepinephrine greater than (+/-)-methylepinephrine much greater than (-)-methyldopa. Since it can compete with a specific ligand for alpha 2-receptors, methylepinephrine could be a centrally active metabolite of methyldopa.