L-tyrosine potentiates the anorexia induced by mixed-acting sympathomimetic drugs in hyperphagic rats.
 The effects of L-tyrosine (L-TYR) on the anorectic activity of several mixed-acting sympathomimetics were determined during the dark cycle in rats made hyperphagic by food deprivation.
 L-TYR (200 mg/kg) significantly potentiated the anorectic activity of phenylpropanolamine, (-)-ephedrine and (+)-amphetamine by 48, 50 and 37%, respectively.
 When the dose of L-TYR was varied (25-400 mg/kg), a significant dose-dependent relationship was noted.
 The observed potentiation was positively correlated with increases in brain TYR concentrations; blockade of L-TYR uptake into the brain by the coadministration of L-valine prevented this potentiation.
 Various other L-amino acids, as well as D-TYR, failed to mimic the potentiating action of L-TYR.
 As determined by alpha-methyl-p-TYR pretreatment, the L-TYR-induced potentiation was dependent upon increased catecholamine synthesis.
 Although various other mixed-acting sympathomimetic anorexiants were similarly potentiated by L-TYR, the direct-acting beta-2 adrenoceptor anorexiants, salbutamol and methoxyphenamine, were not.
 These results indicate that L-TYR specifically potentiates the anorectic activity of the studied mixed-acting sympathomimetics and are consistent with the requirement of the central conversion of L-TYR to catecholamines via TYR hydroxylase for this response.
 The possibility that the effect of mixed-acting sympathomimetics is normally limited by the availability of L-TYR is suggested.
