NONEXISTENCE OF A TYPE C MONOAMINE OXIDASE IN RAT BRAIN

— The possible existence of type C MAO, distinct from type A and type B, in circumventricular structures of rat brain was examined by histological studies on the inhibitory effects of clorgyline. a preferential type A MAO inhibitor and deprenyl, a preferential type B inhibitor, on enzyme. Brain slices were preincubated with the inhibitors and then incubated with 5‐HT, the substrate for type A MAO, and stained for MAO activity. Deposits of the product formazan were detected in circumventricular structures of slices of brain preincubated with clorgyline and deprenyl at concentrations of 10 ‐7 –10 ‐4 m at room temperature for 5 min. When the slices were preincubated with either of these inhibitors at room temperature for 60 min, strong activity was observed in this region, whereas when they were preincubated with either 10 ‐5 m ‐clorgyline or 10 ‐5 m ‐deprenyl for 20 and 30 min at 37°C, no MAO activity was seen in any region of the brain. Thus, at the higher preincubation temperature, lower concentrations of each inhibitor and a shorter preincubation period were required for inhibition of the enzyme. Preincubation for 60 min at 37°C with a combination of 10 ‐7 m ‐clorgyline and 10 ‐8 m ‐deprenyl did not inhibit the enzyme in the circumventricular region completely, but at the same temperature, concentrations of 10 ‐7 m of both inhibitors inhibited the enzyme completely in 10min, Thus the effects of the inhibitors are synergistic. These results indicate that the inhibitory effects of the two inhibitors on the enzyme in circumventricular structures of the brain is time‐ and temperature‐dependent. Moreover, the activity seems to be sensitive to deprenyl even when 5‐HT is used as substrate. The results do not support the idea of the existence of type C MAO, distinct from type A and type B MAO.