[³h]harmaline as a Specific Ligand of Mao A–ii. Measurement of the Turnover Rates of Mao a During Ontogenesis in the Rat Brain

The combined measurement of MAO A activity (using [ 3 H]5‐HT as a specific substrate) and [ 3 H]harmaline binding capacity indicated that the concentration of MAO A in brain was higher in 14‐28 day old rats than in adult animals. The turnover rates of this enzyme in the forebrain and the brain stem of young (14‐28 day old) and adult rats were calculated by following the recovery of MAO A activity and of [ 3 H]harmaline binding capacity after an acute treatment with pargyline (75mg/kg i.p.). Both the fractional rate constant for MAO A degradation and its synthesis rate per g of fresh tissue were significantly higher in young animals. However, the calculation of the absolute synthesis rates of MAO A per brain area gave very similar values in young and adult animals: 1.3‐1.5 × 10 13 molecules of MAO A synthesized per day in the forebrain and 2.3‐2.9 × 10 12 molecules per day in the brain stern. The results illustrate the validity of using [ 3 H]harmaline binding to evaluate possible changes in the turnover rate of MAO A in tissues.