Effects of amino acid precursors on catecholamine synthesis in the brain

Roles of the tyrosine isomers meta-tyrosine and ortho-tyrosine in oxidative stress

The damage to cellular components by reactive oxygen species, termed oxidative stress, both increases with age and likely contributes to age-related diseases including Alzheimer's disease, atherosclerosis, diabetes, and cataract formation. In the setting of oxidative stress, hydroxyl radicals can oxidize the benzyl ring of the amino acid phenylalanine, which then produces the abnormal tyrosine isomers meta-tyrosine or ortho-tyrosine. While elevations in m-tyrosine and o-tyrosine concentrations have been used as a biological marker of oxidative stress, there is emerging evidence from bacterial, plant, and mammalian studies demonstrating that these isomers, particularly m-tyrosine, directly produce adverse effects to cells and tissues. These new findings suggest that the abnormal tyrosine isomers could in fact represent mediators of the effects of oxidative stress. Consequently the accumulation of m- and o-tyrosine may disrupt cellular homeostasis and contribute to disease pathogenesis, and as result, effective defenses against oxidative stress can encompass not only the elimination of reactive oxygen species but also the metabolism and ultimately the removal of the abnormal tyrosine isomers from the cellular amino acid pool. Future research in this area is needed to clarify the biologic mechanisms by which the tyrosine isomers damage cells and disrupt the function of tissues and organs and to identify the metabolic pathways involved in removing the accumulated isomers after exposure to oxidative stress.

The effects of L-threo-3,4-dihydroxyphenylserine on the total norepinephrine and dopamine concentrations in the cerebrospinal fluid and freezing gait in parkinsonian patients

We studied the effects of L-threo-DOPS (L-DOPS) on the concentrations of total (conjugated and unconjugated) dopamine (DA) and norepinephrine (NE) in the cerebrospinal fluid (CSF) of parkinsonian patients with freezing phenomenon. The NE concentration increased remarkably and dose-dependently after administration of L-DOPS in both L-dopa/carbidopa-pretreated and untreated patients. The DA concentration also increased mildly but significantly in L-dopa/carbidopa-untreated patients. Freezing phenomenon improved in 6 out of 8 patients at Hoehn and Yahr's stage III, and 1 out of 5 patients at stage IV. These results indicate that L-DOPS administration increases the NE concentration dose-dependently, and is effective for freezing of gait of moderate severity.

Further studies on the mechanism of the cardiovascular effects of L-tyrosine

Tyrosine is the precursor amino acid of catecholamines. Low doses of tyrosine produce tachycardia and hypertension, while higher doses induce bradycardia and hypotension in anaesthetised rats. The mechanism and site of action of L-tyrosine are not fully understood. Eight groups of Wistar rats received different pretreatments in order to study the influence of blockade of various receptor mechanisms on the cardiovascular effects of L-tyrosine. The effects mediated by the autonomic nervous system were inhibited by ganglion blockade (hexamethonium), by alpha 1- and beta 1-adrenoceptor blockade (prazosin and atenolol) and by parasympathetic acetylcholine receptor blockade (atropine). The possible role of histamine receptors was studied by inducing H1 and H2-receptor blockade (diphenhydramine and cimetidine, respectively). The effect of inhibition of prostaglandin synthesis by indomethacin was also studied. The L-tyrosine-induced tachycardia was completely blocked by atenolol. Both atenolol and prazosin partly inhibited the hypertensive effects of low doses of tyrosine. The tyrosine-induced bradycardia was not inhibited, and the hypotension was only partly blocked by the pretreatments. Therefore, adrenergic mechanisms seem to mediate the stimulatory cardiovascular effects of tyrosine. The depressant effects of high doses of tyrosine do not appear to be mediated by cholinergic activation, histamine receptor activation, or prostaglandin synthesis.

Cardiovascular effects of L-tyrosine: influence of blockade of tyrosine metabolism

Tyrosine is the precursor of catecholamines. Small doses of tyrosine produce tachycardia and hypertension while higher doses produce bradycardia and hypotension in anaesthetised rats. The mechanism of these effects has not been established. An increased synthesis and release of catecholamines has been suggested to be the mechanism. Various pretreatments were given to anaesthetised Wistar rats to study the influence of a blockade of L-tyrosine metabolism and thus a blockade of catecholamine synthesis, on these cardiovascular effects: valine, which inhibits tyrosine uptake into brain, alpha-methyl-p-tyrosine, which blocks the rate-limiting enzyme, tyrosine hydroxylase, carbidopa and benserazide, which both inhibit dopa decarboxylase, and desipramine, which blocks catecholamine re-uptake. Benserazide and alpha-methyl-p-tyrosine partially blocked the stimulatory effects of tyrosine. None of the pretreatments were able to block effectively the inhibitory effects of L-tyrosine. Therefore, the metabolism of tyrosine to form catecholamines may be involved in the stimulatory but not in the inhibitory cardiovascular effects of L-tyrosine. Valine pretreatment did not antagonize the depressant effects of tyrosine. Since valine blocks the uptake of L-tyrosine into the brain, the depressant effects of L-tyrosine might be peripheral rather than central in origin.

On the mechanism of imipramine's influence in lowering p-hydroxyphenylglycol concentrations in the brain. The role of tyrosine

Administration of imipramine (IMI) to rats was shown to lower after 4.5 hr the brain concentration of the octopamine metabolite p-hydroxyphenylglycol (pHPG) in a dose-dependent manner over the range of 10-40 mg/kg of IMI. Assay of plasma and brain levels of tyrosine revealed that IMI produced a reduction in both but with a shorter time-course than for the depletion in pHPG, with the maximal decreases occurring at 1.5 hr, before there was any loss of pHPG. The reductions in tyrosine and pHPG levels could not be explained by an effect of IMI on food intake, since the levels were diminished even in 24-hr fasted animals. When rats were injected with IMI 4.5 hr before 200 mg/kg of tyrosine and 5.5 hr before being killed, the elevation in brain pHPG levels were attenuated by about 50%, as compared to the animals that received tyrosine alone. These data suggest that the ability of IMI to lower brain pHPG probably involves two distinct mechanisms: (1) a lowering of brain and plasma tyrosine concentrations, and (2) an inhibition of the conversion of tyrosine to pHPG. It is unclear whether these effects are due to IMI itself or to one of its metabolites, such as desmethylimipramine or didesmethylimipramine, which were found in the plasma in amounts equal to or greater than IMI.

Studies on the activity of L-threo-3,4-dihydroxyphenylserine (L-DOPS) as a catecholamine precursor in the brain. Comparison with that of L-dopa

L-Threo-3,4-dihydroxyphenylserine (L-DOPS) was compared with L-3,4-dihydroxyphenylalanine (L-DOPA) with respect to their activities as central amine precursors. The apparent Km value (the substrate affinity) of L-DOPS for aromatic L-amino acid decarboxylase was nearly equal to that of L-DOPA, whereas the vmax value (the rate of decarboxylation) of L-DOPS was much smaller than that of L-DOPA, the penetration of L-DOPS into the brain through the blood-brain barrier was found to be smaller (about one-fourth) than that of L-DOPA but, for an amine precursor, it was still substantial. Unlike L-DOPA, L-DOPS did not cause a marked accumulation of norepinephrine (NE), the corresponding catecholamine in the brain, but nialamide, a monoamine oxidase inhibitor significantly enhanced the L-DOPS-induced rise of NE. Moreover, the brain concentration of 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG), the principal end metabolite of NE, was increased markedly by L-DOPS. These results suggest that L-DOPS may act as an NE precursor in the brain and activate NE neurons by increasing the turnover rate of NE.

Studies on the central action of L-threo-3,4-dihydroxyphenyl-serine (L-threo-DOPS) in FLA-63-treated mice

In order to clarify the central action of L-threo-DOPS, the effect of benserazide on behavioral and biochemical changes by L-threo-DOPS in FLA-63-treated mice was studied. L-threo-DOPS in combination with nialamide markedly increased both the locomotor activity and the concentrations of the brain, heart and kidney norepinephrine (NE) in the FLA-63-treated mice. Benserazide at low doses did not alter either the rise of the brain NE level or the increase in locomotor activity, whereas it significantly inhibited the rise of the heart and kidney NE levels. Benserazide at a high dose significantly inhibited all of them. These results suggested that the increase in locomotor activity might be mediated via activation of the central noradrenergic neurons system by L-threo-DOPS.

Monitoring dopamine metabolism in the brain of the freely moving rat

Determination of DOPAC and HVA in cisternal CSF taken repeatedly from freely moving rats provides a useful means of monitoring central DA metabolism. A large proportion of both metabolites occurs in cisternal CSF as conjugates from which they are liberated by acid hydrolysis. The method enables DA turnover values to be determined for individual rats. Drug experiments indicate that these values reflect brain DA metabolism and that most of this occurs in extrastriatal DA neurons. Concurrent determination of 5HT turnover on the same CSF samples revealed a significant positive correlation between the turnovers of the two transmitters together with considerable inter-individual differences. The turnover method was particularly convenient when investigating daily variations of turnover. Repeated CSF withdrawal also appears to be useful in the analysis of stress-provoked changes of the metabolism of DA and other transmitters. For example, it was used to show that central DA metabolism becomes highly responsive to tyrosine availability if rats are subjected to immobilization stress. The method can also be used to compare the time dependencies of both metabolic and behavioral responses to the stress in the same animal. Preliminary results suggest that the increase of 5HT metabolism during immobilization (rather than that of DA) may oppose the suppression of open field activity that occurs 24 hr later.

Neuropharmacological modification of central catecholamines: effects on pinealectomy-induced convulsions

Removal of the pineal gland produces stereotyped tonic convulsions in parathyroidectomized rats. Inasmuch as central levels of norepinephrine (NE) are decreased in these animals, the purpose of this study was to investigate the effects of alterations in central catecholamine function on convulsions produced by pinealectomy in parathyroidectomized rats. The treatment of rats with alpha-methyl-p-tyrosine or FLA-63 produced large reductions in forebrain levels of both NE and dopamine or NE alone, respectively, which were not associated with facilitation of convulsions. However, the incidence of convulsions was increased by FLA-63 in rats pretreated with the catecholamine precursor L-dihydroxyphenylalanine. Reserpine, a monoamine depleter, had no effect on either the incidence or severity of convulsions. An acute injection of desipramine, an inhibitor of the reuptake of NE, however, significantly lowered the incidence of convulsions. Timolol, a beta-adrenergic receptor antagonist, reduced, in a dose-dependent manner, the average latency to onset of convulsions and increased the average number of convulsions each rat experienced. Clonidine, an alpha 2-adrenergic agonist, did not significantly alter convulsions. Thus presynaptic mechanisms such as synthesis and storage of both NE and DA appear to have little, if any, effect on pinealectomy-induced convulsions, whereas enhancing synaptic levels of NE by blocking its reuptake into adrenergic axons had an anticonvulsant effect. Further evidence suggesting a role for NE in modulating these convulsions is provided by the proconvulsant effect of blocking central beta-adrenergic receptors.

The effects of L-threo-dihydroxyphenylserine on norepinephrine metabolism in rat brain

The effects of L-threo-3,4-dihydroxyphenylserine (DOPS), an artificial precursor of norepinephrine (NE), on NE metabolism in rat brain were investigated. DOPS administration resulted in a significant elevation in cerebral NE and 3-methoxy-4-hydroxyphenylglycol contents, while carbidopa pretreatment completely blocked these increases. After brain NE was depleted by either alpha-methyl-p-tyrosine (AMPT), fusaric acid, FLA-63, or reserpine, NE restoration by DOPS was observed in rats treated with either fusaric acid or chronic reserpine. No NE restoration was observed after pretreatment with AMPT, FLA-63, or acute reserpine. The results suggest that NE formed after DOPS administration is mainly localized in the brain capillaries. In some NE-depleting conditions, however, DOPS can penetrate the brain parenchyma.

The effect of various amino acids and drugs on the para- and meta-hydroxyphenylacetic acid concentrations in the mouse caudate nucleus

Injection of L-p-tyrosine (800 mg/kg, 2 h) increased the mouse striatal para-hydroxyphenylacetic acid (p-HPAA) concentrations. A smaller dose of D,L-m-tyrosine (20 mg/kg, 2 h) produced a larger increase in mouse striatal meta-hydroxyphenylacetic acid (m-HPAA) concentrations. The administration of L-phenylalanine to mice caused a slight increase in the p-HPAA concentration in the corpus striatum after 2 h while a larger dose of L-phenylalanine (800 mg/kg) produced a greater increase. Eight hours following L-phenylalanine injection, p-HPAA concentrations were still elevated. With D-phenylalanine a significant increase was observed at eight hours after drug administration. Two drugs which reduce dopamine synthesis, alpha-methyl-para-tyrosine and apomorphine, decreased m-HPAA striatal concentrations without affecting p-HPAA concentrations. From these results, it is proposed that tyrosine hydroxylase activity determines p-HPAA concentrations by regulating p-tyrosine availability. This enzyme may also synthesize m-tyrosine which is subsequently decarboxylated to form m-tyramine and then oxidatively deaminated to form m-HPAA.

Possible role of octopamine and tyramine in the antihypertensive and antidepressant effects of tyrosine

The administration of a dose of 200 mg/kg of tyrosine (as either the free amino acid or the ethyl ester) increased the 24-hour excretion of p-hydroxyphenethyleneglycol (p-HPG) and p-hydroxyphenylethanol, metabolites of octopamine and tyramine, by 147 and 50%, respectively. One hour after this dose of tyrosine, brain levels of p-HPG and p-hydroxyphenylacetic acid (p-HPA), another metabolite of tyramine, were increased by 82 and 196%, respectively. Pretreatment with Ro4-4602, a peripheral decarboxylase inhibitor, reduced by 50% the tyrosine-induced increases in brain p-HPA levels, suggesting that tyramine was partially formed in the brain parenchyma. Tyrosine caused only slight, but non-significant increases in brain levels of catecholamine metabolites. These results suggest that tyrosine-induced increases in the production of tyramine and octopamine in brain may account for some of the effects of tyrosine, such as its antihypertensive and reported antidepressant properties.