Co-administering tyrosine with glucose potentiates its effect on brain tyrosine levels

A historically controlled trial of tyrosine for cocaine dependence

Cocaine dependence continues to be a major public health problem and efforts to develop pharmacotherapies have been disappointing. Chronic cocaine use is believed to cause catecholamine depletion and similarities exist between cocaine withdrawal and major depression. Tyrosine is the dietary precursor to catecholamines and has yielded positive results in small trials of its antidepressant efficacy. Tyrosine 2 g every 8 hours was administered on an open-label basis to 49 cocaine-dependent individuals, as an adjunct to intensive out-patient drug abuse counseling. Retention in treatment at 90 days was compared to data from a control group of 80 subjects who had received 10 mg of imipramine per day in an earlier trial. Median retention was 17 days in both groups. No side effects were reported by the subjects receiving tyrosine. These results do not support the utility of tyrosine in the treatment of cocaine dependence.

Exogenous tyrosine potentiates the methylphenidate-induced increase in extracellular dopamine in the nucleus accumbens: a microdialysis study

Previous work has shown that the synthesis and release of dopamine may, under certain conditions, be influenced by an increase in the availability of its amino acid precursor, tyrosine. To examine whether exogenous tyrosine could potentiate the methylphenidate-induced increase in extracellular dopamine, male rats were implanted with microdialysis probes aimed at the right nucleus accumbens. Samples were collected from awake animals beginning 22 h after surgery. A repeated measures design was used involving the continuous collection of 20-min samples for a 4-h period once a day for 3 consecutive days. On a given day, an animal was infused with methylphenidate, tyrosine, or methylphenidate plus tyrosine. Periods of infusion with the active compounds were preceded and followed by baseline conditions, and treatments were counterbalanced to control for possible order effects. Methylphenidate plus tyrosine significantly increased extracellular levels of dopamine in comparison to drug alone. This effect was long-lasting, persisting into the post-treatment sampling period and peaking 40 min after the peak induced by methylphenidate alone. Tyrosine alone induced a small but significant increase in extracellular dopamine in the absence of any treatment to accelerate the firing of dopamine cells. These findings may have implications for the treatment of attention deficit hyperactivity disorder.

Catecholamine metabolism in the attention deficit disorder: implications for the use of amino acid precursor therapy

The use of catecholamine precursors and agonists in the attention deficit disorder and various biochemical studies have implicated deficient catecholamine metabolism in the pathogenesis of this illness. The amino acid L-tyrosine, a catecholamine precursor, is capable of augmenting central dopamine norepinephrine. Study of its use in the treatment of the attention deficit disorder as a safer alternative to stimulant therapy is thus warranted.

Food consumption, neurotransmitter synthesis, and human behaviour

Fluctuations in the availability to the brain of tryptophan and tyrosine cause major changes in the rates at which neurons synthesize serotonin and the catecholamines respectively. Such changes occur when the pattern of neutral amino acids in the plasma is altered by what has been eaten, the plasma changes inducing parallel changes in the amounts of tryptophan or tyrosine that are transported from blood to brain. Diet-induced changes in plasma choline levels can produce similar changes in acetylcholine synthesis. The three nutrients, tryptophan, tyrosine and choline, when administered in the pure form or simply ingested in food, can thus act like drugs giving rise to important changes in the chemical composition of structures in the brain. The interactions that relate the amount of a nutrient administered or ingested to its level in the blood plasma, its level in the brain and its effect on nerve neurotransmission, are not simple. The conversion of tryptophan into serotonin is influenced by the proportion of carbohydrate in the diet; the synthesis of serotonin in turn affects the proportion of carbohydrate an individual subsequently chooses to eat. In the case of choline and tyrosine the effect on a neuron of an increased supply of the nutrient varies with the neuron's firing frequency and can lead to changes in that frequency. Choline and tyrosine can hence amplify neurotransmission selectively, increasing it at some synapses but not at others. Taken together, these findings illustrate a novel and hitherto unsuspected aspect of nutrition's effects on the brain and provide the basis for new modes of therapy for patients with some metabolic, neurologic or psychiatric brain diseases.