BRAIN TYROSINE HYDROXYLATION - ALTERATION OF OXYGEN-AFFINITY INVIVO BY IMMOBILIZATION OR ELECTROSHOCK IN RAT

The rates of brain tyrosine and tryptophan hydroxylation, estimated in vivo from the accumulation of dopa and 5-hydroxytryptophan after the administration of a decarboxylase inhibitor, appeared dependent on the availability of O2 as a substrate. During 2 types of physical stress, electroshock and curare-immobilization, the rate of brain tyrosine hydroxylation was greater than in unstressed controls and was not significantly decreased when the stressed animals were made hypoxic. The loss of O2 dependence by brain tyrosine hydroxylation during stress was observed in several brain regions and was not associated with alterations in the concentrations of brain tyrosine, tryptophan, serotonin, dopamine or norepinephrine. The rate of brain tryptophan hydroxylation was not affected by stress and remained O2 dependent. The increase in catecholamine synthesis during stress appeared to be the result of increased catecholaminergic nerve impulse flow. During neuronal stimulation an allosteric change in tyrosine hydroxylase may increase the affinity of the enzyme for O2 allowing greater catecholamine synthesis despite limiting concentrations of this substrate.