Does long term treatment with amitriptyline alter the monoamine oxidase of rat brain?

Effects of long-term treatment with dicyclic, tricyclic, tetracyclic, and noncyclic antidepressant drugs on monoamine oxidase activity in mouse brain

1. The individual long-term effects of the antidepressant drugs zimeldine, viloxazine, imipramine, amitriptyline, nortriptyline, maprotiline, or nomifensine, on brain mitochondrial monoamine oxidase (MAO) activity, were studied in mice that were given daily intraperitoneal injections (30 mg/kg) of these reagents for 4 weeks. 2. Both the A-form (MAO-A) and B-form (MAO-B) of MAO were inhibited after long-term administration of all the drugs except nortriptyline (MAO-A was not affected) and maprotiline (neither MAO-A nor MAO-B were affected). 3. Kinetic analysis showed a significant decrease in Vmax values, and an increase in K(m) values for MAO-B during treatment. 4. All seven drugs are competitive inhibitors of MAO-A, noncompetitive inhibitors of MAO-B, and were more potent in vitro for MAO-B. 5. MAO-A was inhibited by the following drugs (in ascending order of potency) : nortriptyline, amitriptyline, imipramine, maprotiline, zimeldine, nomifensine, and viloxazine. 6. MAO-B was inhibited by the following drugs (in ascending order of potency): nortriptyline, imipramine, maprotiline, amitriptyline, zimeldine, nomifensine, and viloxazine.

Effect of caroxazone, a new antidepressant drug, on monoamine oxidases in healthy volunteers

1 Caroxazone is a new antidepressant drug with a reversible inhibitory effect on monoamine oxidases (MAO) as previously shown experimentally in animals.

2 The effect of caroxazone on MAO was explored in healthy volunteers and compared with that of tranylcypromine and imipramine. Daily urinary excretion of tryptamine and MAO activity in platelets were assayed at various times during and after treatment and the differences from basal values were statistically analysed. In addition, caroxazone plasma levels were determined.

3 Caroxazone administered orally at doses of 300 or 600 mg/day for 12 days induced a significant, dose-dependent increase in urinary tryptamine excretion. Tranylcypromine (20 mg/day for 8 days) was even more active in this respect; imipramine (50 mg/day for 12 days) was completely inactive.

4 MAO activity in platelets was not affected by caroxazone or imipramine, but was completely inhibited by tranylcypromine.

5 Mean steady-state plasma levels of caroxazone were about 6 μg/ml and 11-12 μg/ml with the dose of 300 mg/day and 600 mg/day respectively.

6 It can be concluded that caroxazone is a MAO inhibitor in man too, at a clinically effective dose such as 600 mg/day. The fact that its MAO inhibition is not apparent in platelet preparation may be explained by its reversibility.

7 Tranylcypromine confirmed its potent irreversible MAO inhibitory effect, while imipramine lacked any effect at the tested dose.

Platelet MAO and amitriptyline treatment

Some tricyclic antidepressants have been reported to inhibit monoamine oxidase (MAO) activity in vitro in addition to blocking the reuptake of norepinephrine and/or serotonin. While the inhibition of MAO is reversible, platelet MAO activity in depressed patients responding to amitriptyline treatment has been reported to be reduced after drug treatment. In a reverse design, we measured platelet MAO activity and drug levels in patients chronically being treated with amitriptyline and again 2 and 4 weeks after stopping the medication. Although tricyclic drug concentrations were initially within the therapeutic range and undetectable on placebo treatment, platelet MAO activities were unchanged.

Substrate-typic changes of platelet monoamine oxidase activity in sub-types of schizophrenia

Monoamine oxidase (MAO) activity has been measured in the platelets of controls (n = 42) and schizophrenic patients (n = 49) of three subtypes, using beta-phenylethylamine, p-tyramine, and tryptamine as substrates. Characteristic differences of MAO activity were observed between platelets of patients and controls; the differences were substrate-typic: decreased enzyme activity was found with all three substrates in platelets of the parnaoid subtype. With tryptamine, MAO activity was decreased in the platelets of all three sub-types of schizophrenia. With p-tyramine, MAO was low in patients with affective psychoses and paranoid schizophrenia. The value of MAO activity measurements as a means for distinguishing sub-types of schizophrenic disorders is improved by using two substrates; tryptamine and p-tyramine. Possible mechanisms of the substrate-typic changes of platelet MAO activity in schizophrenia are discussed.