The pharmacological actions of 3,4-dihydroxyphenyl-alpha-methylalanine (alpha-methyldopa), an inhibitor of 5-hydroxytryptophan decarboxylase

Studies on the relationship of 5-hydroxytryptamine and the enterochromaffin cell to anaphylactic shock in mice

Protection against anaphylactic shock in mice by reserpine has been shown to be a delayed phenomenon, probably not dependent upon a direct effect of reserpine. The release and depletion of catechol amines by reserpine show little likelihood of being responsible for protection because these substances are in themselves protective against anaphylactic shock, while β-TM 10, a drug which interferes with their release is not. Since L-α-methyl dopa and reserpine both deplete serotonin, and since both protect against anaphylactic shock, it is proposed that serotonin depletion is responsible for the protection. Enterochromaffin substance is depleted in anaphylactic shock. It is also depleted by reserpine and serotonin, both of which protect against anaphylactic shock when given prior to challenge with antigen. The amount of enterochromaffin substance seems to correlate with susceptibility to anaphylactic shock. The behavior of animals undergoing anaphylactic shock and the effect of shock on body temperature is similar to the effects on behavior and temperature of treatment with reserpine, which is known to release serotonin. The effect of monoamine oxidase inhibition on animals undergoing anaphylactic shock is also similar to the effect of monoamine oxidase inhibition on animals given reserpine. These results are consistent with the views that the release of serotonin is causally related to anaphylactic shock in mice and that serotonin is accumulated in the chromogenic material of the enterochromaffin cell.

Evidence of brain dopamine deficiency in schizophrenia

It is proposed that the increased dopamine function suggested by the dopamine hypothesis of schizophrenia is a dopaminergic postsynaptic receptor supersensitivity resulting from a dopamine deficiency. In support of this, three double-blind controlled studies conducted on drugs which alter brain dopaminergic activity in a manner different from that of classic neuroleptics are reported. 1) alpha-methyldopa-neuroleptic interaction proved efficacious for schizophrenic positive symptoms but only on a short-term basis. 2) Rubidium improved negative symptoms rapidly, and in contrast has a late onset of action on positive symptoms of schizophrenia. 3) Tryptophan-benserazide was efficacious in controlling both negative and positive symptoms of schizophrenia (although less so than chlorpromazine). It is concluded that currently accepted modes of pharmacological therapy (classical neuroleptics) are in the short-term controlling the dopamine supersensitivity secondary to a deficiency, but contributing in the long-term to increase the dopamine deficiency, and so exacerbate the supersensitivity. More effective forms of treatment may involve the use of agents which alter dopamine activity without inducing dopamine supersensitivity.

Uptake of monoamines into central neurones and the blood-brain barrier in the infant rat

1. A fluorescence histochemical study was undertaken to investigate the ability of systemically administered monoamines to penetrate into the central nervous system of infant rats.2. It was found that after subcutaneous injections of large doses, L-noradrenaline, L-alpha-methyl-noradrenaline, DL-alpha-methyl-dopamine, L-alpha-methyl-dopa and 5-hydroxytryptamine could cross the blood-brain barrier of newborn to 2 week old rats and be taken up by neurones normally containing monoamine as well as by several neurones which normally contained no monoamine.3. 5-Hydroxytryptamine containing neurones took up 5-hydroxytryptamine as well as catecholamines and alpha-methyl-dopa. The pinealocytes, which contain large amounts of 5-hydroxytryptamine, were depleted of their endogenous fluorescence by alpha-methyl-dopa and showed no selective uptake of catecholamines.4. The uptake of monoamines was not antagonized by previous reserpine treatment or by inhibition of catecholamine synthesis, and occurred throughout the whole extent of the monoamine-containing neurones (cell body, axon, terminals).5. The monoamines were also taken up by the endothelial cells and pericytes of capillaries in the central nervous system (CNS) which are thought to constitute the physical barrier to these substances in the adult.6. The peripheral sympathetic nerves to pial blood vessels, the pineal and pituitary glands also took up the monoamines and alpha-methyl-dopa; uptake was also noted in cells of the pituitary gland, in epithelial and connective tissue elements of the meninges and choroid plexuses and in the epithelial, connective and muscular tissue of pial blood vessels.7. It is concluded that the blood-brain barrier to monoamines may not be fully developed in infant rats, at least for high levels of circulating monoamines; the central monoamine-containing neurones possess the ;membrane pump' mechanism for uptake of monoamines from the time of birth, even though they are not fully developed morphologically.

Central hypotensive effect of alpha-methyldopa

In cats under chloralose anaesthesia l-α-methyldopa (20 mg/kg) was infused for 1 hr into the left vertebral artery. One to 3 hr after the end of the infusion a gradual and significant lowering of mean arterial blood pressure was observed. The dopamine and noradrenaline contents of the brain were significantly reduced while brain 5-hydroxytryptamine and heart noradrenaline concentrations remained normal. The same low dose of l-α-methyldopa infused into a systemic vein did not affect the blood pressure. However, brain dopamine and noradrenaline were depleted to the same extent as observed after infusion into the vertebral artery. Intravenous infusion of a large dose of l-α-methyldopa (200 mg/kg) did not significantly alter mean arterial blood pressure but lowered brain dopamine, noradrenaline and 5-hydroxytryptamine levels. No effect on heart noradrenaline was observed. Infusion of saline or the d-isomer of α-methyldopa (20 mg/kg) into the vertebral artery had no effect on blood pressure or tissue monoamines.

Decarboxylase Inhibitors Affect Convulsion Thresholds to Hexafluorodiethyl Ether

Alpha-methyl dihydroxyphenylalanine (alpha-MeDOPA) and alpha-methyl meta-tyrosine (alpha-MMT) do not lower significantly the convulsion thresholds to hexafluorodiethyl ether in the mouse, whereas reserpine and tetrabenazine produce marked lowering of these thresholds. Other workers have shown that alpha-MeDOPA and alpha-MMT decrease brain norepinephrine concentrations much more than brain 5-hydroxytryptamine concentrations, while reserpine and tetrabenazine decrease the brain level of these two amines in a parallel fashion. Thus, the fact that alpha-MeDOPA and alpha-MMT, even in very large doses, do not lower convulsion thresholds and in some cases raise them suggests that the decline in 5-hydroxytryptamine may be implicated in the lowering of convulsion thresholds produced by reserpine and other, similar agents.