Effects of dietary tyrosine on L-dopa- and amphetamine-induced changes in locomotor activity and neurochemistry in mice

Abstract

Recent findings suggest that intraperitoneal injections of L-tyrosine at high doses (100 mg/kg) alters amphetamine-induced changes in behavior by restoring amphetamine-induced decreases in whole brain norepinephrine (NE). The present study examined the motor effects of L-dihydroxyphenylalanine (L-dopa) and d-amphetamine sulfate in mice after treatment with a basal casein diet supplemented with L-tyrosine. The basal diet supplemented with 1–4% L-tyrosine, or 1–4% L-phenylalanine, produced no changes in motor activity in otherwise untreated mice. Whereas L-dopa (25–100 mg/kg) following inhibition of extracerebral decarboxylase by Ro 4-4602 (25 mg/kg) slightly decreased activity in diet control (casein) animals, this drug treatment enhanced motor activity in a dose-related fashion when L-tyrosine was added to the diet. Increases in motor activity following low doses of amphetamine (0.75–1.5 mg/kg) in casein control mice were antagonized by dietary L-tyrosine, but a higher dose of d-amphetamine (3 mg/kg) interacted with the addition of L-tyrosine producing an increase in motor activity. Neurochemical changes observed in brain concentrations of tyrosine, dopamine (DA), norepinephrine (NE), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), tryptophan, serotonin (5-HT) and 5-hydroxy-indoleacetic acid (5-HIAA) following drug and diet treatment suggest that 5-HT systems, in addition to catecholamine systems, may be involved in mediating these effects.