Effect of monoamineoxidase inhibitors on 5‐hydroxytryptamine output from perfused cerebral ventricles of anaesthetized cats

1 . In cats anaesthetized with pentobarbitone sodium, intraperitoneal injections of four inhibitors of monoamine oxidase (MAO) were shown to increase the 5‐hydroxytryptamine (5‐HT) in the effluent from the perfused cerebral ventricles. 2 . Weight for weight, tranylcypromine was found to be about twice as potent as pheniprazine, eight times as potent as nialamide and sixty times as potent as pargyline. 3 . The effect of tranylcypromine was also examined after reserpine had been injected into the cerebral ventricles or after p ‐chlorophenylalanine, given intraperitoneally. In both conditions tranylcypromine retained its ability to increase the 5‐HT output from the perfused cerebral ventricle, but the effect was attenuated, more after p ‐chlorophenylalanine than after reserpine. 4 . Evidence is put forward that in both conditions the brain is not completely depleted of its 5‐HT, but that the 5‐HT is only reduced, more after p ‐chlorophenylalanine than after reserpine.