The Effects of Chronic Regimens of Clorgyline and Pargyline on Monoamine Metabolism in the Rat Brain

The effects of chronic administration of clorgyline and pargyline on rat brain monoamine metabolism have been examined. The inhibitory selectivity of these drugs towards serotonin deamina‐tion (MAO type A) and phenylethylamine deamination (MAO type B) can be maintained over a 21‐day period by proper selection of low doses of these drugs (0.5‐1.0 mg/kg/24h). The results are consistent with MAO type A catalyzing the deamination of serotonin and norepinephrine and with MAO type B having little effect on these monoamines. Dopamine appears to be dcaminated in vivo principally by MAO type A. Clorgyline administration during a 3‐week period was accompanied by persistent elevations in brain norepinephrine concentrations; serotonin levels were also increased during the first 2 weeks, but returned towards control levels by the third week of treatment. Low doses of pargyline did not increase brain monoamine concentrations, but treatment with higher doses for 3 weeks led to elevations in brain norepinephrine and 5‐hydroxytryptamine; at this time significant MAO‐A inhibition had developed. The changes in monoamine metabolism seen at the end of the chronic clorgyline regimen are not due to alterations in tryptophan hydroxylase activity. At this time tyrosine hydroxylase activity was also unaffected.