No effect of age and estrogen on aromatic L- amino acid decarboxylase activity in rhesus monkey brain

A variety of studies have shown an effect of estrogen on dopamine function and suggest that estrogen may modulate central dopaminergic activity. Positron emission tomography (PET) and the dopamine metabolism tracer, [18F]6-fluoro-image-m-tyrosine (FMT) were used to evaluate dopaminergic function in the frontal cortex and striatum in six aged, but pre-menopausal, female monkeys before and after ovariectomy (OVX). Dynamic PET brain uptake data and metabolite-corrected blood input functions were fit to a three-compartment model for FMT uptake. Prior to OVX, all animals showed preferential accumulation of the tracer bilaterally in the striatum and less but measurable activity in the frontal cortex. Paired comparisons showed that there were no significant differences in FMT uptake (Ki) in either brain region before and after OVX. In addition, FMT uptake did not differ from a group of young adult female monkeys at either time point. These findings may represent a compensatory up-regulation of aromatic L- amino acid decarboxylase (AADC) activity.