Compensatory Increase in Tyrosine Hydroxylase Activity in Rat Brain After Intraventricular Injections of 6-Hydroxydopamine

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Abstract
The neurotoxin 6-hydroxydopamine produced a permanent loss of endogenous norepinephrine and of 3H-labeled norepinephrine uptake sites in the hippocampus within 5 days. These losses were initially accompanied by parallel decreases in tyrosine hydroxylase activity and synaptosomal norepinephrine synthesis. Within 21 days, however, hippocampal tyrosine hydroxylase activity and norepinephrine synthesis rate increased three- to fivefold. These data suggest a novel form of plasticity in brain-damaged animals characterized by an increase in the capacity for transmitter biosynthesis in residual neurons.