Studies of tyramine transfer and metabolism using an in vitro intestinal preparation

Gut Bacteria and Neuropsychiatric Disorders

Bacteria in the gut microbiome plays an intrinsic part in immune activation, intestinal permeability, enteric reflex, and entero-endocrine signaling. Apart from physiological and structural changes brought about by gut bacteria on entero-epithelial cells and mucus layers, a vast number of signals generated in the gastro-intestinal tract (GIT) reaches the brain via the vagus nerve. Research on the gut–brain axis (GBA) has mostly been devoted to digestive functions and satiety. Less papers have been published on the role gut microbiota play in mood, cognitive behavior and neuropsychiatric disorders such as autism, depression and schizophrenia. Whether we will be able to fully decipher the connection between gut microbiota and mental health is debatable, especially since the gut microbiome is diverse, everchanging and highly responsive to external stimuli. Nevertheless, the more we discover about the gut microbiome and the more we learn about the GBA, the greater the chance of developing novel therapeutics, probiotics and psychobiotics to treat gastro-intestinal disorders such as inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS), but also improve cognitive functions and prevent or treat mental disorders. In this review we focus on the influence gut bacteria and their metabolites have on neuropsychiatric disorders.

Involvement of Organic Cation Transporter 2 and a Na+-dependent active transporter in p-tyramine transport across Caco-2 intestinal cells

AIMS

We have previously demonstrated that p-tyramine (TYR), an endogenous trace amine-associated receptor 1 agonist, passage across neuronal membranes involves a transporter exhibiting the pharmacological profile of Organic Cation Transporter 2 (OCT2). Since TYR is also a constituent of foodstuffs and produced by the intestinal microbiota, here we have investigated whether similar processes are involved in the passage of 100 nM TYR across apical and basolateral membranes of the Caco-2 human intestinal epithelial cell line.

MATERIALS AND METHODS

[³H]TYR transport across apical and basolateral membranes of Caco-2 cell monolayers was measured in the presence of inhibitors of TYR metabolizing enzymes. Cellular, apical, and basolateral compartments were collected at various timepoints, TYR concentrations calculated, and transport properties pharmacologically characterized.

KEY FINDINGS

Apical transport resulted in equimolar accumulation of TYR within cells. Pentamidine (OCT1/OCT2 inhibitor) decreased apical transport (P = 0.001) while atropine (OCT1 inhibitor) had no effect, suggesting apical transport involved OCT2. In contrast, basolateral transport resulted in 500-1000 nM cellular concentrations (P < 0.0001) indicating the presence of an active transporter. Replacement of Na⁺ on an equimolar basis with choline resulted in loss of TYR transport (P = 0.017). Unexpectedly, this active transport was also atropine-sensitive (P = 0.020). Kinetic analysis of the active transporter revealed V_max = 43.0 nM/s with a K_t = 33.1 nM.

SIGNIFICANCE

We have demonstrated for the first time that TYR is transported across Caco-2 apical membranes via facilitated diffusion by OCT2, whereas transport across basolateral membranes is by a Na⁺-dependent, atropine-sensitive, active transporter.

Trace Amines and Their Receptors

Trace amines are endogenous compounds classically regarded as comprising β-phenylethyalmine, p-tyramine, tryptamine, p-octopamine, and some of their metabolites. They are also abundant in common foodstuffs and can be produced and degraded by the constitutive microbiota. The ability to use trace amines has arisen at least twice during evolution, with distinct receptor families present in invertebrates and vertebrates. The term "trace amine" was coined to reflect the low tissue levels in mammals; however, invertebrates have relatively high levels where they function like mammalian adrenergic systems, involved in "fight-or-flight" responses. Vertebrates express a family of receptors termed trace amine-associated receptors (TAARs). Humans possess six functional isoforms (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9), whereas some fish species express over 100. With the exception of TAAR1, TAARs are expressed in olfactory epithelium neurons, where they detect diverse ethological signals including predators, spoiled food, migratory cues, and pheromones. Outside the olfactory system, TAAR1 is the most thoroughly studied and has both central and peripheral roles. In the brain, TAAR1 acts as a rheostat of dopaminergic, glutamatergic, and serotonergic neurotransmission and has been identified as a novel therapeutic target for schizophrenia, depression, and addiction. In the periphery, TAAR1 regulates nutrient-induced hormone secretion, suggesting its potential as a novel therapeutic target for diabetes and obesity. TAAR1 may also regulate immune responses by regulating leukocyte differentiation and activation. This article provides a comprehensive review of the current state of knowledge of the evolution, physiologic functions, pharmacology, molecular mechanisms, and therapeutic potential of trace amines and their receptors in vertebrates and invertebrates.

Pharmacological characterization of a high-affinity p-tyramine transporter in rat brain synaptosomes

p-Tyramine is an archetypal member of the endogenous family of monoamines known as trace amines, and is one of the endogenous agonists for trace amine-associated receptor (TAAR)1. While much work has focused on the function of TAAR1, very little is known about the regulation of the endogenous agonists. We have previously reported that p-tyramine readily crosses lipid bilayers and that its release from synaptosomes is non-exocytotic. Such release, however, showed characteristics of modification by one or more transporters. Here we provide the first characterization of such a transporter. Using frontal cortical and striatal synaptosomes we show that p-tyramine passage across synaptosome membranes is not modified by selective inhibition of either the dopamine, noradrenaline or 5-HT transporters. In contrast, inhibition of uptake-2 transporters significantly slowed p-tyramine re-uptake. Using inhibitors of varying selectivity, we identify Organic Cation Transporter 2 (OCT2; SLC22A2) as mediating high affinity uptake of p-tyramine at physiologically relevant concentrations. Further, we confirm the presence of OCT2 protein in synaptosomes. These results provide the first identification of a high affinity neuronal transporter for p-tyramine, and also confirm the recently described localization of OCT2 in pre-synaptic terminals.

Membrane permeability of trace amines: evidence for a regulated, activity-dependent, nonexocytotic, synaptic release

Both pre- and post-synaptic effects of trace amines have been demonstrated. The putative intracellular location of Trace Amine-Associated Receptors necessitate that membrane transport processes be present in order for post-synaptic effects to occur. Here we examine the ability of trace amines to cross synthetic (Fluorosomes) and native (synaptosomes) lipid bilayer membranes. Trace amines readily crossed Fluorosome membranes by simple diffusion, p-tyramine (P = 0.01) and tryptamine (P = 0.0004) showing significantly faster diffusion than dopamine and 5-HT, respectively, with diffusion half-lives of 13.5 ± 4.1 (p-tyramine) and 6.8 ± 0.7 seconds (tryptamine). Similarly, release of [(3) H]p-tyramine and [(3) H]2-phenylethylamine from pre-loaded synaptosomes occurred significantly quicker than did [(3) H]dopamine (P = 0.0001), with half lives of 38.9 (p-tyramine), 7.8 (2-phenylethylamine) and 133.6 seconds (dopamine). This was, however, significantly slower than the diffusion mediated passage across Fluorosome membranes (P = 0.0001), suggesting a role for transporters in mediating trace amine release. Further, a pronounced shoulder region was observed in the synaptosome [(3) H]p-tyramine release curve, suggesting that multiple processes regulate release. No such shoulder region was present for [(3) H]dopamine release. Surprisingly, both [(3) H]p-tyramine (P = 0.001) and [(3) H]2-phenylethylamine (P = 0.0001) release from synaptosomes was significantly decreased under depolarizing conditions. As expected, depolarization significantly increased [(3) H]dopamine release. The data presented indicate that the release of p-tyramine and 2-phenylethylamine from neuronal terminals occurs by a different mechanism than dopamine, and does not involve classical exocytosis. The data are consistent with an initial release of trace amines by simple diffusion, followed by an activity-dependent regulation of synaptic levels via one or more transporter proteins.

Luminal regulation of normal and neoplastic human EC cell serotonin release is mediated by bile salts, amines, tastants, and olfactants

Mechanisms by which gut luminal content regulates secretion and motility are ill understood. We evaluated whether neuroendocrine enterochromaffin (EC) cells act as luminal sensors for a wide variety of nutrients and defined the secretory mechanisms of this process. Pure (98-99%) FACS-sorted human EC cells and neoplastic EC cells (KRJ-I) were studied. RT-PCR identified transcripts for T2R1 (bitter), OR1G1 (class II olfactory) and trace amine (TAR1) G protein-coupled receptors (GPCRs) and transporters for glutamine (SNAT1/2), glucose (GLUT1/3/SGLT1), and bile salts (ABST). Glutamine and sodium deoxycholate stimulated 5-HT release (EC(50) = 0.002-0.2 microM; 2-fold release) but were 10-100 times more potent in neoplastic EC cells, which also secreted 6-13 times more 5-HT. Tastants (caffeine, tyramine, octopamine) and olfactants (thymol and eugenol) also stimulated normal and neoplastic EC cell 5-HT secretion (EC(50) = 1.2 nM to 2.1 microM and 0.05 nM to 0.1 microM release, respectively); 2-deoxyglucose and the artificial sweetener sucralose also stimulated (EC(50) = 9.2 and 0.38 nM). 5-HT release was associated with ERK phosphorylation (1.5-fold, P < 0.02) and could be inhibited by a somatostatin analog (IC(50) = 1 pM). Eleven secretory associated genes including the vesicle docking inhibitor STXBP3 were upregulated in response to glutamine and bile salt stimulation in neoplastic EC cells. Targeting STXBP3 expression by use of antisense knockdown significantly (P < 0.05) reduced 5-HT secretion. In conclusion, EC cells express GPCRs and transporters for luminal tastants, olfactants, glutamine, glucose, and bile salts. Activation includes a panel of secretory genes, ERK phosphorylation, and 5-HT secretion. Luminal EC cell regulation is likely to be as important as G cell regulation in gastric acid secretion; development of agents to target EC cell function is therefore a critical therapeutic goal.

[3H]Tyramine uptake by rat liver slices

Rat liver slices were employed as experimental model to characterise the system involved in the transport process which participates in liver tyramine uptake. The uptake of 0.4 micromol l-1of [3H]tyramine by rat liver slices was linear from 5 min up to the end of incubation. At 15 min the uptake was 4.58+/-0.18 pmol mg-1protein. The accumulation of [3H]tyramine was sensitive to temperature (69. 3+/-4.0% inhibition at 0 degrees C, P<0.001), to sodium omission replaced by 150 mmol l-1Tris or 110 mmol l-1Tris+40 mmol l-1choline (27.6+/-6.0%, P<0.01, and 24.6+/-3.8% inhibition, P<0.01, respectively), and the inhibition of Na+-K+-adenosine triphosphatase by 150 micromol l-1ouabain (20.4+/-2.6% decrease, P<0.01). Uptake of [3H]tyramine was cocaine- (10 micromol l-1) and desipramine- (1 micromol l-1) dependent (32.2+/-6.4%, P<0.05, and 31.6+/-4.0% inhibition, P<0.05, respectively). Uptake of [3H]tyramine in rat liver slices was not modified by 30 micromol l-1isoprenaline, 30 micromol l-1corticosterone, 30 micromol l-1normetanephrine and noradrenaline up to 4 micrometers at higher noradrenaline concentrations tyramine transport was diminished (P<0.05). Results achieved by incubation with increasing tyramine concentrations indicate that at the micromolar level hepatic uptake occurs by a combined passive diffusion and transport-mediated mechanism, whereas at greater tyramine concentrations passive transport predominates. These results suggest that both simple diffusion and a transport-mediated mechanism are involved in this uptake from hepatocytes, which presents features similar to those described for type 1 non-neuronal uptake systems.