False Transmitter

Chemotransmitters and receptors in the CNS are similar to those in the periphery, and the drug in this section also has peripheral actions, as is to be expected.

Methyldopa (Aldomet) probably acts primarily in the brain stem vasomotor centres. It is a substrate (in the same manner as L-DOPA) for the enzymes that synthesise noradrenaline. The synthesis of a-methylnoradrenaline results in tonic stimulation of CNS oc2-receptors since a-methylnoradrenaline cannot be metabolised by monoamine oxidase, and selectively stimulates the a2-adrenoceptor. Stimulation of this receptor in the hindbrain nuclei concerned with blood pressure control results in a fall in blood pressure, i.e. methyldopa acts in the same way as clonidine. a-Methylnoradrenaline is also produced at peripheral adrenergic endings, but to a lesser extent and peripheral action is clinically insignificant.

Methyldopa is reliably absorbed from the gastrointestinal tract and readily enters the CNS. The t\ is 1.5 h. Adverse effects, largely expected from its mode of action, include: sedation (frequent), nightmares, depression, involuntary movements, nausea, flatulence, constipation, score or black tongue, positive Coombs test with occasionally haemolytic anaemia, leucopenia, thrombocytopenia, hepatitis.

Gynaecomastia and lactation occur due to interference with dopaminergic suppression of prolactin secretion. Any failure of male sexual function is probably secondary to sedation. Because of its adverse effects methyldopa is no longer a drug of first choice in routine long-term management of hypertension, but remains popular with obstertricians for the hypertension of pregnancy.

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