Influence of harmaline on the ability of pargyline to alter catecholamine metabolism in rats

The new generation of monoamine oxidase inhibitors

Irreversible and unspecific inhibitors of MAO were the first modern antidepressants, but after an initial success they fell into discredit due to adverse side effects. In the past two decades interest in MAO inhibitors has been renewed because of progress in basic research, a milestone being the finding that there are two subtypes of MAO, MAO-A and MAO-B. These are distinct proteins with high amino acid homology, coded by separate genes both located on the short arm of the human chromosome X. The enzyme subforms show different substrate specificities in vitro and different distributions within the central nervous system and in peripheral organs. In the central nervous system of man MAO-A seems to be mainly involved in the metabolism of 5 HT and noradrenaline, whereas 2-phenylethylamine and probably dopamine are predominantly deaminated by MAO-B. In the intestinal tract tyramine is mainly metabolized by MAO-A. These characteristics indicate distinct physiological functions of the two MAO-subforms. Several irreversible and reversible non-hydrazine inhibitors with relative selectivities for one of the MAO-subforms have been developed. They belong to various chemical classes with different modes of enzyme inhibition. These range from covalent mechanism based interaction (e.g. by propargyl- and allylamine derivatives) to pseudosubstrate inhibition (e.g. by 2-aminoethyl-carboxamides) and non-covalent interaction (e.g. by brofaromine, toloxatone and possibly moclobemide). The most important pharmacological effects of the new types of MAO inhibitors are those observed in neuropsychiatric disorders. The inhibitors of MAO-A show a favorable action in various forms of mental depression. The drugs seem to have about the same activity as other types of antidepressants, including tricyclic and related compounds as well as classical MAO inhibitors. The onset of action of the MAO-A inhibitors is claimed to be relatively fast. Other possible indications of these drugs include disorders with cognitive impairment, e.g. dementia of the Alzheimer type. In subjects with Parkinson's disease the MAO-B inhibitor L-deprenyl exerts a L-dopa-sparing effect, prolongs L-dopa action and seems to have a favorable influence regarding on-off disabilities. The action is in general transitory (months to several years). In addition L-deprenyl has been shown to delay the necessity for L-dopa treatment in patients with early parkinsonism. Whether the drug influence the progression of the disease is still a matter of debate. L-deprenyl also appears to have some antidepressant effect (especially in higher doses) and to exert a beneficial influence in other disorders, e.g. dementia of the Alzheimer type.(ABSTRACT TRUNCATED AT 400 WORDS)

Amine oxidases and their endogenous substrates (with special reference to monoamine oxidase and the brain)

The roles of MAO, BzO, DAO and PAO in the metabolism of endogenous substrates and the functional implications of their action and inhibition is reviewed, the emphasis being on MAO on one hand and on brain on the other. The major issues are the following: There is no discrete subdivision into substrates selective for MAO-A, MAO-B, or mixed ones, but rather a continuum. Tissue differences in substrate specificity are not likely to be due to molecular variability of MAO. For the deamination of DA, 5-HT and PEA at least, the relative participation of either MAO form in a given tissue is primarily determined by the relative abundance of the two forms; only at 10(-5) M and above, substrate concentration begins to matter also. In vivo, compartmentation is of paramount importance: since there seems to be more MAO-A than B inside monoaminergic neurons, DA, 5-HT and NA are predominantly metabolized by MAO-A if metabolism occurs mainly intraneuronally. Conversely, since MAO-B is more abundant extraneuronally, e.g. in glia cells, the relative participation of this form increases if a significant portion of these amines is deaminated outside monoaminergic neurons. In vivo, monoamine deamination is reduced concomitantly with the degree of MAO inhibition, whereas signs of increased transmitter function are only observed if enzyme inhibition is at least 80%. This is likely to be the result of the action of compensatory mechanisms such as feedback inhibition of transmitter release and synthesis. BzO is particularly abundant in vascular tissue, lung and bone. Low levels are found in brain. Endogenous substrates and physiological function are not known. DAO also occurs only in minimal amount in brain, if at all. Its principal substrates are histamine and the polyamines, and the disposal of these amines is probably its main function. Of the PAO's, the type of enzyme found in the rat liver attacks the secondary amino groups and may have a more prominent role in the metabolism of polyamines in the brain than in the periphery. Bovine plasma PAO, which attacks primary amino groups, is only found in the serum of ruminants, but not other species. Its function in the metabolism of polyamines is not known.

Influence of selective, reversible inhibitors of monoamine oxidase on the prolonged depletion of striatal dopamine by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) hydrochloride injected s.c. at 20 mg/kg once daily for four days resulted in marked depletion of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in mouse striatum one week after the last dose. Pretreatment with MD 240928, (R)-[4-((3-chlorophenyl)-methoxy)phenyl]-5-[(methylamino)methyl]-2- oxazolidinone methanesulfonate, prevented the depletion of striatal dopamine, DOPAC and HVA, whereas pretreatment with harmaline did not. MD 240928 selectively inhibited type B not type A monoamine oxidase (MAO), whereas harmaline selectively inhibited type A MAO in mouse striatum. Acutely after injection of harmaline, DOPAC and HVA concentrations were decreased in mouse striatum; these changes were not produced by MD 240928. The acute changes in dopamine metabolites reveal that MAO-A not MAO-B is responsible for the oxidation of dopamine in mouse striatum. Protection against the neurotoxic effects of MPTP by MD 240928 but not by harmaline indicates that prevention of dopamine oxidation is not the mechanism of the protective effect; instead the protection probably is due to prevention of MPTP metabolism by MAO-B, this metabolism having been shown to occur by other workers. The results with these reversible, competitive inhibitors of the two types of MAO are in agreement with previously reported results from studies using irreversible inhibitors of MAO.

Epinephrine accumulation in rat brain after chronic administration of pargyline and LY 51641--comparison with other brain amines

Concentrations of biogenic amines and metabolites were measured in regions of rat brain following administration of monoamine oxidase (MAO) inhibitors for 21 days. Epinephrine concentrations were increased from 350 to 500% following chronic administration of LY 51641, a selective inhibitor of MAO type A. Norepinephrine, dopamine and serotonin showed much less relative accumulation. The marked relative accumulation of epinephrine may be related to the efficacy of inhibitors of MAO type A in the treatment of depression.

N-[2-(o-iodophenoxy)ethyl]cyclopropylamine hydrochloride (LY121768), a potent and selective irreversible inhibitor of type A monoamine oxidase

The effects of N-[2-(o-iodophenoxy)ethyl]cyclopropylamine hydrochloride (LY121768) on types A and B monoamine oxidase (MAO) assayed with radiocarbon-labeled serotonin and phenylethylamine, respectively, were studied in vitro and in vivo. Type A MAO from rat brain was inhibited in vitro by LY121768 with an IC50 of 4 x 10(-10) M, whereas 2500 times higher concentrations of LY121768 (IC50 = 1 x 10(-6) M) were required to inhibit type B MAO. The inhibition of type A MAO increased with time of incubation of LY121768 with enzyme prior to substrate addition and persisted after dialysis, indicative of irreversible inhibition. The irreversible inactivation was prevented by harmaline, a reversible, competitive inhibitor of type A MAO, indicating a requirement for catalytic activity of MAO in the time-dependent inactivation by LY121768. In rats, LY121768 selectively inhibited type A MAO in brain and in liver at low doses. The inhibition of type A MAO persisted for several days after a single 10 mg/kg i.p. dose of LY121768 and was associated with a significant increase in brain dopamine, norepinephrine and epinephrine concentrations and a significant decrease in the concentration of the dopamine metabolites, homovanillic acid and 3,4-dihydroxyphenylacetic acid. The inactivation of type A MAO by LY121768 in vivo was prevented by co-administration of harmaline, indicating a similar mechanism for the in vivo inactivation as for the in vitro inactivation of MAO by LY121768. A reasonable inference from these data and from previous literature is that LY121768 acts as a "suicide substrate" for MAO and inactivates the enzyme by formation of a reactive intermediate which binds covalently to the enzyme. The presence of iodine in the LY121768 molecule not only confers high selectivity for type A MAO but offers a site for radionuclide introduction that might be a useful means of labeling type A MAO in vitro or in vivo for various purposes.

Influence of formamidines on biogenic amine levels in rat brain and plasma

Biogenic amine levels in samples of whole brain and plasma following treatment of rats with chlordimeform (CDF), its two N-demethyl metabolites (DMC and DDC), p-chloroamphetamine (PCA), and harmaline were separated by high performance liquid chromatography equipped with fluorescence or electrochemical detection systems. At 1 hr following s.c. injection, CDF (200 mg/kg) caused a reduction in levels of norepinephrine (NE), 5-hydroxytryptamine (5-HT), and tyramine (TRM), an increase in dopamine (DA), and no change in levels of beta-phenethylamine (PEA) in samples of whole rat brain, whereas DMC (100 mg/kg) and DDC (25 mg/kg) effected reductions of brain levels of NE, 5-HT, TRM, and PEA with no change in DA. The effect of DMC (100 mg/kg) on NE and DA levels in brain was followed periodically for 24 hr. Following a significant decrease at 1 hr, NE levels increased to a maximum at 12 hr and remained higher than controls throughout the remainder of the 24 hr test period. DA levels increased during the initial 12 hr and remained significantly higher than controls for the remaining 12 hr. The influence of s.c. vs i.p. administration of DMC (100 mg/kg) on brain amine levels was examined. Intraperitoneal treatment generally resulted in lower amine levels in DMC and vehicle treated animals. Differences in treatment effects were similar for all amines except for 5-HT in which s.c. injection produced a slight reduction, while i.p. injection resulted in a two-fold increase.(ABSTRACT TRUNCATED AT 250 WORDS)