A possible role for tyramines in brain function and some mental disorders

Review and Meta-Analyses of TAAR1 Expression in the Immune System and Cancers

Since its discovery in 2001, the major focus of TAAR1 research has been on its role in monoaminergic regulation, drug-induced reward and psychiatric conditions. More recently, TAAR1 expression and functionality in immune system regulation and immune cell activation has become a topic of emerging interest. Here, we review the immunologically-relevant TAAR1 literature and incorporate open-source expression and cancer survival data meta-analyses. We provide strong evidence for TAAR1 expression in the immune system and cancers revealed through NCBI GEO datamining and discuss its regulation in a spectrum of immune cell types as well as in numerous cancers. We discuss connections and logical directions for further study of TAAR1 in immunological function, and its potential role as a mediator or modulator of immune dysregulation, immunological effects of psychostimulant drugs of abuse, and cancer progression.

Aromatic L-amino acid decarboxylase-immunoreactive structures in human midbrain, pons, and medulla

The objective of the present study was to determine with precision the localization of neurons and fibers immunoreactive (ir) for aromatic L-amino acid decarboxylase (AADC), the second-step enzyme responsible for conversion of L-dihydroxyphenylalanine (L-DOPA) to dopamine (DA) and 5-hydroxytryptophan (5-HTP) to serotonin (5-hydroxytryptamine: 5-HT) in the midbrain, pons, and medulla oblongata of the adult human brain. Intense AADC immunoreactivity was observed in a large number of presumptive 5-HT neuronal cell bodies distributed in all of the raphe nuclei, as well as in regions outside the raphe nuclei such as the ventral portions of the pons and medulla. Moderate to strong immunoreaction was observable in presumptive DA cells in the mesencephalic reticular formation, substantia nigra, and ventral tegmental area of Tsai, as well as in presumptive noradrenergic (NA) cells, which were aggregated in the locus coeruleus and dispersed in the subcoeruleus nuclei. In the medulla oblongata, immunoreaction of moderate intensity was distributed in the mid and ventrolateral portions of the intermediate reticular nucleus, which constitutes the oblique plate of A1/C1 presumptive adrenergic and/or NA neurons. The dorsal vagal AADC-ir neurons were fewer in number and stained more weakly than cells immunoreactive for tyrosine hydroxylase (TH). AADC immunoreactivity was not identified in an aggregate of TH-ir neurons lying in the gelatinous subnucleus of the solitary nucleus, a restricted region just rostroventral to the area postrema. Nonaminergic AADC-positive neurons (D neurons), which are abundant in the rat and cat midbrain, pons, and medulla, were hardly detectable in homologous regions in the human brain, although they were clearly distinguishable in the forebrain.

Tyramine-immunoreactive neuronal structures in the rat brain: Abundance in the median eminence of the mediobasal hypothalamus

Immunoreactivity to p-tyramine, one of the natural trace amines, was studied in the rat brain by an anti-p-tyramine antibody. Immunoreactivity to this amine is very weak in the nigrostriatal dopaminergic neurons and terminals, and weak in the locus coeruleus noradrenergic ones. It was intensified in these structures after monoamine oxidase inhibition. On the other hand, this amine was highly concentrated in the median eminence of the mediobasal hypothalamus, in which its physiological function on prolactin release has been demonstrated.

(-)-Deprenyl inhibits tyramine-induced noradrenaline release, but not tyramine-induced dopamine release or potassium-induced noradrenaline release, from rat brain synaptosomes

The effect of (-)-deprenyl (selegiline), a therapeutic agent for Parkinson's disease, on the tyramine-induced release of catecholamine from rat brain synaptosomes was studied using a superfusion system. Tyramine (10(-7) to 10(-5)M) enhanced the release of [3H]noradrenaline (NA) and [3H]dopamine (DA) from forebrain and striatal synaptosomes in a dose-dependent manner. (-)-Deprenyl (5x10(-5)M) had no effect on spontaneous catecholamine release, suggesting that it has no tyramine-like catecholamine releasing effect. Pretreatment with (-)- or (+)-deprenyl (5x10(-5)M) significantly prevented the tyramine (10(-6)M)-induced NA release, but not DA release. The inhibitory action of (-)-deprenyl was not observed on potassium (15mM)-induced NA release. (-)-Desmethyldeprenyl (5x10(-5)M), a metabolite of (-)-deprenyl, and a monoamine oxidase-A (MAO-A) inhibitor, clorgyline (5x10(-5)M), failed to block the tyramine-induced NA and DA release. Although (+)-deprenyl, a potent DA uptake inhibitor, did not inhibit tyramine-induced DA release, a catecholamine uptake inhibitor nomifensine (5x10(-5)M) did. In summary, (-)-deprenyl at a dose inhibiting tyramine-induced NA release did not have any effect on tyramine-induced DA release or potassium-induced NA release.

Dopamine formation from tyramine by CYP2D6

Dopamine is formed form L-tyrosine by tyrosine hydroxylase and aromatic L-amino acid decarboxylase. In addition to this pathway, however, the formation of catecholamines, including dopamine, from trace amines such as tyramine by hepatic microsomes has been demonstrated. In this study, we investigated the formation of dopamine from trace amines, using human hepatic microsomes and human cytochrome P450 (CYP) isoforms expressed in yeast. Among the 11 isoforms of human CYP expressed in yeast, CYP2D6 was the only isoform exhibiting strong ability to convert p-tyramine and m-tyramine to dopamine. In studies with human hepatic microsomes, the hydroxylation of tyramine to dopamine was inhibited by bufuralol, a typical substrate for CYP2D isoforms, and anti-CYP2D1 antiserum. This is the first report showing that CYP2D is capable of converting tyramine to dopamine. The Km values of CYP2D6, expressed in yeast, for p-tyramine and m-tyramine were 190.1 +/- 19.5 microM and 58.2 +/- 13.8 microM, respectively. Tyramine is an endogenous compound which exists in the brain as a trace amine but is also an exogenous compound which is found in foods such as cheese and wine. Our results suggest that dopamine is formed from endogenous and/or exogenous tyramine by this CYP2D isoform.

Regional brain patterns of dopamine, metabolites and D2 receptors in memory

Patterns of dopamine (DA), 3,4-dihydroxyphenyl acetic (DOPAC), and homovanillic (HVA) acids, para-tyramine (p-TA), and D2 receptors for eleven structures of forebrain and midbrain were examined at 1 and 6 days after one trial passive avoidance training of rats, at 1 day after "psychogenic" amnesia production, immediately following training, and at the last day after foot shock of rats with previous "psychogenic" amnesia administration. Essential dopaminergic differences have been found between the groups studied. More significant neurochemical changes were observed in rats from trained and retrieval training groups. Regional DA changes were accompanied by a diminution of DA metabolism, an increase of D2 receptor density and p-TA. These neurochemical alterations differentially characterized the regional neurochemical patterns found in rat's exhibition nonretention (trained), forgetting, and amnesia. It is suggested that the present data reflect the existence of quantitative relationships between D2 receptors, DA, and p-TA, which are probably important in modulation of memory.

Tyramine conjugation deficit in migraine, tension-type headache, and depression

This study was designed to investigate tyramine sulfate conjugation in patients with migraine or tension-type headache, as defined by the newly introduced International Headache Society (IHS) criteria and to examine whether this relationship is mediated by major depression. A total of 62 subjects completed the study: 38 with migraine (22 with aura and 16 without aura), 12 with tension-type headache, and 12 controls. Patients with migraine had significantly lower urinary tyramine sulfate excretion following oral tyramine challenge than normal control. Tension-type headache was also associated with low tyramine conjugation, but only when comorbid with depression. Although mean tyramine sulfate output was lower among subjects with major depression within each of the subtypes of headache, no significant main effect emerged for depression or major subtype thereof. The lower tyramine sulfate excretion values among patients with both migraine and depression compared to those of migraine alone or depression alone in our data and those of others suggests that comorbid migraine with depression may represent a more severe form of migraine than migraine alone. The findings underscore the importance of comorbidity in clinical and epidemiological studies of migraine.

Hydroxylation of phenylethylamine by rat liver preparations. Inhibition studies

1. Rat liver 100,000 g pellet microsomal fraction p-hydroxylate phenylethylamine to tyramine in a relatively slow proceeding, NADPH-requiring reaction; Km 2.1 x 10(-5) M and Vmax 0.32 nmol/mg protein/20 min. 2. This reaction is inhibited, either competitively, noncompetitively or uncompetitively by a number of behaviorally active monomethylated and monohalogenated derivatives of phenylethylamine. 3. Whereas formation of tyramine was not significantly affected by L-phenylalanine or its p-chloro derivative, it was competitively inhibited by imipramine, iprindole and alprazolam. 4. It is suggested that at least some of the effects of these drugs may result from their ability to interfere with phenylethylamine metabolism.

Effects of nutritional stress on brain tyramine concentration and dopamine turnover

This is an investigation of the effect of nutritional stress at various ages on the levels of the p- and m-isomers of tyramine in the caudate nucleus of the rat. For comparison, the effects of nutritional stress on the concentration and turnover of dopamine were also studied. Nutritional stress induced in pre-weaning (3 weeks of age) or post-weaning (up to 9 weeks of age) rats resulted in a decrease in the concentration of p-tyramine and an increase in the concentration of m-tyramine in the caudate nucleus. Dopamine concentration or turnover in the caudate nucleus was not affected by pre-weaning undernutrition; in the olfactory tubercles, however, a significant decrease was observed in dopamine turnover, calculated from the decrease in homovanillic acid levels after monoamine oxidase inhibition. The results suggest the changes observed are dependent on the availability of the amino acid precursors p- and m-tyrosine and their competition towards aromatic-L-amino acid decarboxylase.

Phenylethylamine metabolism to tyramine by postmortem human brain preparations

Human brain preparations obtained from either the putamen, thalamus, hippocampus or lateral occipital gyrus p-hydroxylate phenylethylamine to tyramine, a reaction carried out by a microsomal (100,000 xg pellet) membrane bound, NADPH-requiring enzyme. This is a minor metabolic pathway occurring in chronic psychiatric patients, as well as in age-comparable controls.

High affinity binding of [3H]-tyramine in the central nervous system

Optimum assay conditions for the association of [3H]-para-tyramine [( 3H]-pTA) with rat brain membranes were characterized, and a saturable, reversible, drug-specific, and high affinity binding mechanism for this trace amine was revealed. The binding capacity (Bmax) for [3H]-pTA in the corpus striatum was approximately 30 times higher than that in the cerebellum, with similar dissociation constants (KD). The binding process of [3H]-pTA involved the dopamine system, inasmuch as (a) highest binding capacity was associated with dopamine-rich regions; (b) dopamine and pTA equally displaced specifically bound [3H]-pTA; (c) there was a severe loss in striatal binding capacity for [3H]-pTA and, reportedly, for [3H]-dopamine, following unilateral nigrostriatal lesion; (d) acute in vivo reserpine treatment markedly decreased the density of [3H]-pTA and, reportedly, of [3H]-dopamine binding sites. In competition experiments [3H]-pTA binding sites, though displaying nanomolar affinity for dopamine, revealed micromolar affinities for the dopamine agonists apomorphine and pergolide, and for several dopamine antagonists, while having very high affinity for reserpine, a marker for the catecholamine transporter in synaptic vesicles. The binding process of [3H]-pTA was both energy-dependent (ouabain-sensitive), and ATP-Mg2+-insensitive; furthermore, the potencies of various drugs in competing for [3H]-pTA binding to rat striatal membranes correlated well (r = 0.96) with their reported potencies in inhibiting [3H]-dopamine uptake into striatal synaptosomes. It is concluded that [3H]-pTA binds at a site located on/within synaptic vesicles where it is involved in the transport mechanism of dopamine.

The effect of raphé nuclei lesions on striatal tyramine concentration and dopamine turnover in the rat

This is an investigation of the effects of electrolytic lesions (1 mA, 10s, anodal) on the median and dorsal raphé nuclei of Wistar rats on the striatal concentrations of p-tyrosine, p-tyramine, m-tyramine, DA, DOPAC, and HVA. The extent of the lesions was estimated in terms of the depletion of 5-hydroxytryptamine and 5-hydroxyindole acetic acid as well as histological examination of the lesioned area. The results show that the raphé nuclei lesions increased rat striatal levels of DOPAC and HVA while levels of DA were unaffected, an effect that was observed within the first day after the lesions were made. The increases in DOPAC and HVA were accompanied by a reduction in striatal p-tyramine and an increase in m-tyramine. The results further support the existence of a reciprocal relationship between p- and m-tyramine concentrations and dopamine metabolism. Previous experiments have demonstrated depletion of p-TA following nigral lesions. The present results are, therefore, important in relation to tyramine distribution in brain. The p- and m-tyramine concentrations were not reduced at 7 days after the raphé nuclei lesions indicating that if the striatal tyramine-containing neurons exist, they do not originate in or pass through the dorsal or median raphé nuclei.

The effects of estradiol and progesterone on the concentration of striatal dopamine and tyramine in ovariectomized mice

The concentration of homovanillic acid and p-tyramine in the female corpus striatum were increased after ovariectomy. The increases were reversed by chronic administration of estradiol benzoate and potentiated by progesterone. It is proposed that these changes are the consequence of concomitant variations in p-tyrosine availability.

Cerebral decarboxylation of meta- and para-tyrosine

The decarboxylase inhibitor DL-alpha-monofluoromethyldopa reduces, in a dose dependent manner, the concentration of striatal p-tyramine in the mouse. Homovanillic acid is also significantly reduced. Conversely, this treatment increases the m-tyramine concentration. Administration of m-tyrosine produces large increases in m-tyramine and a slight decrease in p-tyramine; these changes are potentiated in the presence of the decarboxylase inhibitor. Such data along with other recently published results permit the conclusion that m-tyramine arises from phenylalanine via m-tyrosine and that p-tyramine arises by decarboxylation of p-tyrosine. Both these reactions are closely related to the activity of tyrosine hydroxylase and the availability of appropriate substrates.

The effects of amfonelic acid and some other central stimulants on mouse striatal tyramine, dopamine and homovanillic acid

1 The concentrations of p- and m-tyramine, dopamine and homovanillic acid were measured in the mouse striatum following the subcutaneous administration of amfonelic acid, (+)-amphetamine or nomifensine.2 The administration of 2.5-25 mg/kg of amfonelic acid produced a reduction in p-tyramine that lasted at least 8 h. m-Tyramine was significantly increased and this was observed between 2 and 24 h after drug treatment. The levels of homovanillic acid were increased within 4 h after amfonelic acid administration.3 (+)-Amphetamine treatment (5 mg/kg) produced a reduction in p-tyramine observed up to 4 h after its administration and no significant changes in m-tyramine.4 The administration of 10 mg/kg of nomifensine produced no significant changes in p-tyramine, m-tyramine or homovanillic acid. By increasing the dose to 20 mg/kg, nomifensine produced an increase in p-tyramine and homovanillic acid.5 The present results support the view that amfonelic acid and (+)-amphetamine would respectively release granular or newly synthesized dopamine, both actions being accompanied by an increase in tyrosine hydroxylase activity and dopamine turnover which in turn reduces p-tyramine but produces no change or an increase in m-tyramine.6 The effects of nomifensine were observed after the administration of a relatively high dose (20 mg/kg), that was lethal to some mice (about 20%, at 2 h), and more likely to posses unspecific actions.