People are different: tyrosine's modulating effect on cognitive control in healthy humans may depend on individual differences related to dopamine function

摘要

The amino-acid tyrosine (TYR) is thought to modulate cognitive functions that are driven by dopamine (DA), as consumption of TYR enhances DA levels in the brain (Gibson and Wurtman, 1977; Cuche et al., 1985). It could therefore reverse decreases in DA level that are detrimental for cognitive performance (Muly et al., 1998; Goldman-Rakic et al., 2000; Nieoullon, 2002). So far, TYR has been considered not so much as an enhancer of healthy cognitive functioning but rather as a means to reduce the negative side-effects of dopamine-related pathologies, such as Parkinson's disease (Growdon et al., 1982; Lemoine et al., 1989), phenylketonuria (van Spronsen et al., 1996), depression (Gelenberg and Gibson, 1984), and attention deficit hyperactivity disorder (Wood et al., 1985). However, the outcomes were mixed: some patients reported significant improvements, while other did not. In clinical samples some variation in response may be explained by impaired processes such as DA synthesis, which would lessen or even completely prevent an effect of TYR. But even healthy samples differ in response to TYR supplementation, which suggests that clinically impaired DA function is not the only source of variation.