A renaissance in trace amines inspired by a novel GPCR family

Lothar Lindemann and Marius C. Hoener

F. Hoffmann-La Roche, Pharmaceuticals Division, Discovery Neuroscience, CH-4070-Basel, Switzerland

Trace amines (TAs) are endogenous compounds that are related to biogenic amine neurotransmitters and are present in the mammalian nervous system in trace amounts. Although their pronounced pharmacological effects and tight link to major human disorders such as depression and schizophrenia have been studied for decades, the understanding of their molecular mode of action remained incomplete because of the apparent absence of specialized receptors. However, the recent discovery of a novel family of G-protein-coupled receptors (GPCRs) that includes individual members that are highly specific for TAs indicates a potential role for TAs as vertebrate neurotransmitters or neuromodulators, although the majority of these GPCRs so far have not been demonstrated to be activated by TAs. The unique pharmacology and expression pattern of these receptors make them prime candidates for targets in drug development in the context of several neurological diseases. Current research focuses on dissecting their molecular pharmacology and on the identification of endogenous ligands for the apparently TA-insensitive members of this receptor family.

Trace amines find their receptors The classical biogenic amines [serotonin (5-HT), noradrenaline, adrenaline, dopamine and histamine] have important roles as neurotransmitters in the central and peripheral nervous systems [1]. Their synthesis and storage, in addition to their degradation and reuptake after release, are tightly regulated, and an imbalance in the levels of biogenic amines is known to be responsible for altered brain function in many pathological conditions [2–5]. A second class of endogenous amine compounds, the so-called trace amines (TAs), overlaps significantly with the classical biogenic amines regarding structure, metabolism and subcellular localization. The TAs include p-tyramine, b-phenylethylamine (b-PEA), tryptamine and octopamine, and are present in the mammalian nervous system at generally lower levels than classical biogenic amines [6]. Their dysregulation has been linked to various psychiatric diseases such as schizophrenia and depression, and potential roles for TAs in other conditions such as attention deficit hyperactivity disorder, migraine

headache, Parkinson’s disease, substance abuse and eating disorders have been suggested [7,8].

For several decades, TA-specific receptors had only been hypothesized based on anatomically discrete high- affinity TA binding sites in the CNS of humans and other mammals [9,10]. Accordingly, the pharmacological effects of TAs were believed to be mediated through the well- known machinery of classical biogenic amines, by either triggering their release, inhibiting their reuptake or by ‘cross-reacting’ with their receptor systems [8,11,12]. This view changed significantly following the recent identification of several members of a novel family of G-protein- coupled receptors (GPCRs) initially termed TA receptors [13,14]; subsequently, the complete receptor family has been identified in human, chimpanzee, rat and mouse (Table 1) [15]. So far, only two members of this receptor family, TA1 and TA2, have been reported to be sensitive to TAs [13,14] whereas all other family members that have been analyzed were found to be unresponsive to TAs [13,15]. This apparent insensitivity to TAs could be due, in part, to insufficient receptor trafficking or missing components of the signal transduction machinery in the employed expression systems [13,16], or these results might indicate that these receptors indeed do not correspond to TAs but rather to other, as yet unidentified, endogenous ligands [13,15,17]. On the basis of this information, a novel receptor nomenclature was proposed, designating the receptor family as trace amine- associated receptors (TAARs) (Table 1) [15], which reflects the fact that at least some members of the receptor family potentially do not respond to TAs. This proposed new nomenclature is based strictly on the sequential order of receptor genes on the chromosomes, includes all vertebrate TA receptors and several new GPCRs previously not recognized as members of the receptor family, and clarifies the interspecies relationship of receptor orthologs. Therefore, in the remainder of this review we will refer to this receptor family using the provisional new abbreviation TAAR and additionally provide the official receptor or gene name in brackets whenever specific receptors are being addressed.

The ongoing efforts in the characterization of this novel GPCR family contribute to a detailed understanding of TA physiology on a molecular level, suggest a role for TAs as neurotransmitters or neuromodulators [18] and might soon pave the way for rational strategies in the development of potential TA-related drugs.

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TRENDS in Pharmacological Sciences Vol.26 No.5 May 2005

Corresponding authors: Lindemann, L. (lothar.lindemann@roche.com), Hoener, M.C. (marius.hoener@roche.com).

Available online 2 April 2005

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Table 1. The new nomenclature for trace amine-associated receptors (TAARs)a

New name Old name Generic Human Chimpanzee Rat Mouse Group 1 TAAR1 TRAR1, TA1, TAR1b NA TRAR1, TA1, TAR1b–d TA1, TAR1b,d TAAR2 GPR58 Pseudogene (NA) NA NA TAAR3 Pseudogene (GPR57P) Pseudogene (NA) NA NA TAAR4 Pseudogene (TA2P, 5-HT4P) Pseudogene (NA) TA2b NA Group 2 TAAR5 PNR NA NA NA Group 3 TAAR6 TRAR4, TA4 TRAR4 TRAR4, TA4 NA TAAR7 Pseudogene (NA) Pseudogene (NA) TAAR7a NA NA TAAR7b TA12 NA TAAR7c NA Pseudogene (NA) TAAR7d TA15 NA TAAR7e TA14 NA TAAR7f Pseudogene (TA13P) NA TAAR7g TA9 TAAR7h TA6 TAAR7i Pseudogene (NA) TAAR8 TRAR5, TA5, TAR5, GPR102 Pseudogene (NA) TAAR8a TA11 NA TAAR8b TA7 NA TAAR8c TA10 NA TAAR9 TRAR3, TA3, TAR3 Pseudogene (NA) TA3 NA aOld gene names are given as far as they had been reported previously in original publications (NA, no published old gene name available). Only the receptors TAAR1 (TA1) and TAAR4 (TA2) have been characterized experimentally as TA receptors. The proposed distinction of three subgroups is based on the phylogenetic relationships (Figure 1a), the pharmacophore similarity analysis of the TAAR family [15] and the available pharmacological data [13–15]. For a comprehensive summary of the available sequence information for human, chimpanzee (Pan troglodytes), rat and mouse TAAR genes, details of the nomenclature system and a discussion of the proposed subgroup distinction see [15]. bSensitivity to TAs was tested by either measuring cAMP elevation in stably transfected cell lines [13–15] or by electrophysiological recordings on Xenopus oocytes [13]. cBunzow et al. [14] reported activation of rat TAAR1 (TA1) by several psychoactive compounds such as amphetamines and ergot alkaloids. d

Scanlan et al. [17] reported an activation of rat and mouse TAAR1 (TA1) by several thyronamine derivatives. [P (suffix), pseudogene.]

www.sciencedirect.com Molecular properties of trace amine-associated

a molecular modeling-driven procedure that provides a receptors

quantitative measure for the similarities in the ligand All mammalian TAARs analyzed to date share several

preferences of different GPCRs and is based on the X-ray molecular properties [15]. All except one TAAR gene

crystal structure of bovine rhodopsin and on data from [TAAR2 (GPR58)] are single-exon encoded, locate to a

mutational and pharmacological studies carried out on a narrow region of w100–200 kb of a single chromosome

wide range of different GPCRs [20]. The complete match of and have coding sequences of w1 kb in length. The total

the three TAAR subgroups defined by either the phyloge- number of genes and the proportion of intact genes

netic relationships or the pharmacophore similarity compared with the proportion of pseudogenes differ

analysis, in agreement with the available pharmacological substantially between species: there are 19 (including 2

data, suggests that receptors belonging to the different pseudogenes) and 16 (including 1 pseudogene) TAAR

subgroups might display different pharmacological pro- genes in rat and mouse genomes, respectively, but only 9

files. The similarity of the predicted ligand binding TAAR genes in human (including 3 pseudogenes) and

pockets throughout the entire receptor family suggests chimpanzee (including 6 pseudogenes) genomes. These

that potential, as yet unidentified, ligands of the appar- remarkable inter-species differences indicate the potential

ently TA-insensitive TAARs might resemble small-mol- importance of TAAR genes in the context of adaptation

ecular-weight compounds that are structurally and processes during evolution, and underscore their link to

chemically similar to TAs. These potential new ligands several diseases, which might be related to the fundamen-

could be, for example: (i) metabolites of amino acids or tally different lifestyles and body functions of human and

neurotransmitters that have so far been considered to be other species [19]. In spite of high overall sequence

biologically inactive; (ii) derivatives of indolamines or identities to, for example, amine receptors, TAARs

phenylethylamines [18,21]; or (iii) iodothyronamines, comprise a well-defined, coherent gene family that is

some of which have been demonstrated recently to act clearly distinct from other GPCR gene families, as

directly on TAAR1 (TA1) [17]. revealed by phylogenetic studies (Figure 1a) [15] and the presence of a TAAR-specific peptide fingerprint motif

Trace amine metabolism and pharmacology (Figure 1b) that is missing in all other known GPCRs.

TAs (b-PEA, p-tyramine, octopamine and tryptamine) are The phylogenetic relationships of the receptor genes

all primary amines generated directly by enzymatic suggest the distinction of three TAAR subgroups (Table 1,

decarboxylation of their respective precursor amino Figure 1a) that overlap exactly with the differences in the

acids or, in the case of octopamine, via additional ligand preferences of the receptors predicted by means of

conversion by dopamine b-hydroxylase (DBH) (Figure 2). pharmacophore similarity analysis [15]. This approach is

TAs are metabolized to biologically inactive degradation

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TRENDS in Pharmacological Sciences Vol.26 No.5 May 2005 (a) (b)

Subgroup 1 Subgroup 2 Subgroup 3

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H u m a n T A A R 4 P

u s

5 R A A T n a m u H

ouse

b 8 R

R 8 c 6

A A T e s u o M

M o u

M s e

T A

ou A

H 2

Hum

TAAR8

Human TAAR2

1 1

9 R A

a m u H

M o u s e T A A R 9

a 7

A T

e s u

o M

M M M o

o u ouse

use e

TA TAAR7e R

A b

o M u e s A T A R 1

Outgroup 'Ancestral TAAR gene'

Figure 1. Phylogeny and membrane topology of TAARs. (a) The phylogenetic relationship of human and mouse TAAR genes. The TAAR family most probably evolved from a common ancestor gene sharing closest similarity to the human gene encoding the 5-HT

receptor (HTR4) by a series of gene duplication and speciation events: (1) gene duplication before the rodent and human lineage split; (2) speciation leading to separate primate and rodent lineages; and (3) gene duplication within the rodent lineage. The phylogenetic relationship suggests the distinction of three TAAR subgroups with potentially different pharmacological profiles. The human TAAR7P pseudogene has been omitted from the phylogenetic tree because only a gene fragment of w210 base pairs is preserved in the human genome. (b) Membrane topology of TAARs, as revealed by an alignment of human, chimpanzee (chimp.) and mouse TAAR1 (TA1). All TAARs share a predictive peptide fingerprint motif (red) that largely overlaps with the seventh transmembrane domain (TM VII) and is absent from all other GPCRs, and have short N- and C-terminal domains of 23–49 and 27–33 amino acids, respectively. The TMs are indicated as predicted for human TAAR1 (TA1), and amino acid positions conserved in all vertebrate TAARs are highlighted by black shading. Abbreviation: P (suffix), pseudogene. Redrawn and modified, with permission, from [15].

products predominantly via monoamine oxidase (MAO) with different selectivities for the MAO-A or MAO-B subtype. As a result of the rapid turnover rate of TAs, the endogenous extracellular levels of TAs in brain tissue are in the low nanomolar range and therefore several hundred-fold below those of the classical biogenic amine neurotransmitters dopamine, noradrenaline and 5-HT [18]. In addition to the main metabolic pathway, TAs can also be converted by nonspecific N-methyltransferase (NMT) [22] and phenylethanolamine N-methyltransfer- ase (PNMT) [23] to the corresponding secondary amines (e.g. synephrine [14], N-methylphenylethylamine and N-methyltyramine [15]), which display similar activities on TAAR1 (TA1) as their primary amine precursors

For a discussion of the in vivo effects of TAs it is important to distinguish the biological activities observed at physiological concentrations from the so-called ‘amphetamine-like’ effects triggered by the high nanomo- lar to low micromolar TA concentrations employed in most in vivo studies (generally in combination with MAO inhibitors). On the molecular level, these ‘amphetamine- like’ effects are reflected mainly by TAs increasing the release of noradrenaline, dopamine and 5-HT and inhibit- ing the reuptake of these biogenic amines [18]. By contrast, lower, more physiological concentrations of TAs appear to have neuromodulatory effects mainly on dopamine-mediated (b-PEA, p-tyramine and tryptamine), noradrenaline-mediated (octopamine) and 5-HT-mediated (tryptamine) neurotransmission [18,24–26]. As summar- ized in Table 2, TAAR1 (TA1) orthologs from human,

M o u s e T A A R 7 d

0.2

mouse and rat are activated by low to submicromolar concentrations of b-PEA and tyramine, whereas trypta- mine activates rodent, but not human, TAAR1 (TA1) with similar potency. The potency of octopamine at human and rodent TAAR1 (TA1) is much lower than that of the other TAs. In contrast to TAs, classical biogenic amines either are unable to activate TAAR1 (TA1) or act as partial agonists on TAAR1 (TA1) from rat (5-HT) or all species tested (dopamine) (Table 2) [13–15].

In an excellent review, Berry [18] recently discussed the possible function of TAs as neuromodulators or neuro- transmitters, with a neuromodulator only able to modify the activity of a coexisting neurotransmitter and a neurotransmitter able to alter the electrical excitability of a postsynaptic neuron on its own. Berry [18] considers TAs as potential neuromodulators rather than neuro- transmitters, mainly because most of them are not released in an activity-dependent manner under physio- logical conditions (particularly b-PEA and tryptamine, which are membrane permeable), and because they do not seem to alter the electrical excitability of a neuron at physiological concentrations in the absence of other neurotransmitters (the neurotransmitter-like properties that are apparent from some ‘amphetamine-like’ activities of TAs were not considered physiological). This view is further supported by a recent study reporting that b-PEA and tyramine specifically depress the GABA

B receptor response in rat dopamine-containing midbrain neurons without affecting postsynaptic potentials on their own [27].

TM I 1 57 Human TAAR1 (1) MMPFCHNIINISCVKNNWSNDVRASLYSLMVLIILTTLVGNLIVIVSISHFKQLHTP Chimp. TAAR1 (1) MMPFCHNIINISCVKNNWSNDVRASLYSLMVLIILTTLVGNLIVIVSISHFKELHTP Mouse TAAR1 (1) -MHLCHAITNISHRNSDWSREVQASLYSLMSLIILATLVGNLIVIISISHFKQLHTP

TM II TM III 58 114 Human TAAR1 (58) TNWLIHSMATVDFLLGCLVMPYSMVRSAEHCWYFGEVFCKIHTSTDIMLSSASIFHL Chimp. TAAR1 (58) TNWLIHSMATVDFLPGCLVMPYSMVRSAEHCWYFGEVFCKIHTSTDIMLSSASIFHL Mouse TAAR1 (57) TNWLLHSMAIVDFLLGCLIMPCSMVRTVERCWYFGEILCKVHTSTDIMLSSASIFHL

TM IV 115 171 Human TAAR1 (115) SFISIDRYYAVCDPLRYKAKMNILVICVMIFISWSVPAVFAFGMIFLELNFKGAEEI Chimp. TAAR1 (115) SFISIDRYYAVCDPLRYKAKINILVICVMIFISWSVPAVFAFGMIFLELNFKGAEEI Mouse TAAR1 (114) AFISIDRYCAVCDPLRYKAKINISTILVMILVSWSLPAVYAFGMIFLELNLKGVEEL

TM V 172 228 Human TAAR1 (172) YYKHVHCRGGCSVFFSKISGVLTFMTSFYIPGSIMLCVYYRIYLIAKEQARLISDAN Chimp. TAAR1 (172) YYKHVHCRGGCSVFFSKISGVLTFMTSFYIPGSIMLCVYYRIYLIAKEQARLINDAN Mouse TAAR1 (171) YRSQVSDLGGCSPFFSKVSGVLAFMTSFYIPGSVMLFVYYRIYFIAKGQARSINRTN

TM VI 229 285 Human TAAR1 (229) QKLQIGLEMKNGISQSKERKAVKTLGIVMGVFLICWCPFFICTVMDPFLHYIIPPTL Chimp. TAAR1 (229) QKLQIGLEMKNGISQSKERKAVKTLGIVMGVFLICWCPFFICTVMDPFLHYIIPPTL Mouse TAAR1 (228) --VQVGLEGKSQAPQSKETKAAKTLGIMVGVFLVCWCPFFLCTVLDPFLGYVIPPSL

TM VII 286 339 Human TAAR1 (286) NDVLIWFGYLNSTFNPMVYAFFYPWFRKALKMMLFGKIFQKDSSRCKLFLELSS Chimp. TAAR1 (286) NDVLIWFGYLNSTFNPMVYAFFYPWFRKALKMMLFGKIFQKDSSRCKLFLELSS Mouse TAAR1 (283) NDALY

WFGYLNSALNPMVYAFFYPWFRRALKMVLLGKIFQKDSSRSKLFL---- TAAR fingerprint:

NSXXNPXXYXXXYXWF H F

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Precursor amino acids

Primary

Active

amines

Secondary amines

NHCH

N-Methylphenylethylamine

NMT

MAO-B

NHCH

N-Methyltyramine

PNMT

MAO-A/B

PNMT

MAO-A

Inactive metabolites

2 AADC COOH

β-Phenylethylamine

-Phenylalanine

COOH

Phenylacetic acid

AADC

COOH

-Tyrosine

p-Tyramine COOH

OH p-Hydroxyphenylacetic acid OH DBH

NHCH

Synephrine

OH TH

PNMT

p-Octopamine

MAO-A/B

COOH

OH p-Hydroxymandelic acid

AADC

COOH

Dopamine

MAO-A/B

COOH -DOPA

COMT

3,4-Dihydroxyphenylacetic acid

COMT

OH 3-Methoxytyramine

MAO-A/B

COOH

OH Homovanillic acid

NHCH

N-Methyltryptamine

N H

COOH

AADC

COOH

-Tryptophan

N H

N H Tryptamine

N H Indoleacetic acid

Figure 2. The main routes of TA metabolism in vertebrates. The TAs b-phenylethylamine (b-PEA), p-tyramine, octopamine and tryptamine (highlighted by white shading) are generated by decarboxylation from the respective precursor amino acids. They are rapidly inactivated predominantly by monoamine oxidase (MAO). To a lesser extent TAs are also N-methylated to the corresponding biologically active secondary amines (N-methylphenylethylamine, N-methyltyramine, synephrine and N-methyltryptamine). Both dopamine and 3-methoxytyramine, which do not undergo further N-methylation, are partial agonists of TAAR1 (TA1). The naturally occurring TA isoform m-tyramine

Review 278 TRENDS in Pharmacological Sciences Vol.26 No.5 May 2005 Table 2. Pharmacology of human, rat and mouse TAAR1 (TA1)a Amine Human TAAR1 Rat TAAR1 Mouse TAAR1

(mM) Trace amine b-Phenylethylamine 0.3b, 0.3c 0.9b, 0.2d 0.7b p-Tyramine 1.1b, 0.2c 0.2b, 0.07d 1.4b Octopamine 10.3b, 4.0c 2.1b, 1.3d 19.7b Tryptamine 46.9b, O6.0c 1.2b, 0.3d 2.0b Biogenic amine Dopamine 15.8b,e, 6.7c,e 5.1b,e, 5.9d,e 11.8b,e 5-HT O50.0b, O10.0c 5.2b,e, O10.0d O50.0b Noradrenaline O50.0b, O5.0c O50.0b O50.0b Histamine O50.0b, O5.0c O50.0b O50.0b aEC

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(mM) EC

values were calculated based on the amount of cAMP produced by cell lines expressing the indicated receptors in response to stimulation with the listed compounds. bStably transfected HEK293 cell lines were used [15]. The human TAAR 1 employed in this study was a chimeric protein, constructed by addition of an N-terminal leader sequence and by replacing selected regions involved in G-protein coupling with the corresponding rat sequence. cTransiently transfected COS-7 cells were used [13]. dStably transfected HEK293 cells were used [14]. eThe maximal elevation of cAMP levels induced by these compounds was 50% compared with p-tyramine, indicating a partial agonist activity of dopamine and 5-HT.

Although the available data suggest that TAs act

dopamine) and 5-HT [35], and therefore might represent mainly as neuromodulators, there are several unresolved

TAergic neurons. Future studies that address these and issues regarding their mode of action at the molecular

other issues of TAAR signal transduction and pharma- level. Most importantly, to date, the effects of TAs on

cology will need to consider several technical peculiarities neurotransmission have not been shown to be mediated

innate to this receptor family (Box 1). exclusively by members of the TAAR family. Even if the binding of TAs to MAO-B might mimic the presence of

TAARs as potential drug targets for the treatment of high-affinity TA binding sites in brain tissue [28], it cannot

psychiatric disorders be excluded that TA-sensitive receptors other than TAARs

The dysregulation of TA levels has been linked to several might mediate the pharmacological effects of TAs, as

diseases, which highlights the corresponding members of supported by the findings that: (i) compounds that are

the TAAR family as potential targets for drug develop- potent activators of TAAR1 (TA1) are unable to displace

ment. In this article, we focus on the relevance of TAs and radiolabeled TAs {i.e. amphetamine activates TAAR1

their receptors to nervous system-related disorders, (TA1) but does not displace [3H]PEA} [29]; (ii) [3H]PEA,

namely schizophrenia and depression; however, TAs [3H]tyramine, [3H]octopamine and [3H]tryptamine are

have also been linked to other diseases such as migraine, potently displaced by several compounds that do not or

attention deficit hyperactivity disorder, substance abuse only weakly activate TAAR1 (TA1) [30]; and (iii) [3H]PEA,

and eating disorders [7,8,36]. [3H]tyramine, [3H]octopamine and [3H]tryptamine are all

Clinical studies report increased b-PEA plasma levels displaced by their respective ligands at low nanomolar

in patients suffering from acute schizophrenia [37] and concentrations (Table 3) [29–32] that are insufficient to

elevated urinary excretion of b-PEA in paranoid schizo- activate TAAR1 (TA1) (Table 2). In addition, the mechan-

phrenics [38], which supports a role of TAs in schizo- isms by which TAs activate TAARs are not fully defined.

phrenia. As a result of these studies, b-PEA has been As suggested by the membrane permeability of some TAs

referred to as the body’s ‘endogenous amphetamine’ [39], in addition to the predominantly intracellular localization

which is consistent with the observation that prolonged of TAAR1 [13,14], which is reminiscent of the subcellular

administration of amphetamine can induce psychiatric distribution reported for other GPCRs such as the 5-HT

symptoms that largely resemble paranoid schizophrenia receptor [33], TAs could trigger TAAR signaling also from

[40]. In addition, a genetic deficiency of MAO-B might intracellular receptor pools. This possibility is supported

trigger psychotic symptoms by resulting in elevated indirectly by the observation that the locomotor-stimulat-

b-PEA levels [41]. Furthermore, linkage studies identified ing activity of b-PEA is absent in transgenic mice that lack

the chromosomal region harboring the TAAR genes as a the dopamine transporter, which is known to also carry

schizophrenia susceptibility locus [42], which sub- b-PEA [34]. The TAAR signaling from intracellular

sequently was narrowed down specifically to the TAAR6 receptor pools could, for example, be realized in the

(TRAR4, TA4) gene [43]. presynaptic structure of dopamine-containing neurons

Taken together, the current evidence that connects where no MAO-B activity has been detected, in presyn-

elevated b-PEA levels or individual TAAR genes to aptic terminals of noradrenaline-containing neurons, or

schizophrenia is suggestive, and TA levels in addition to within so-called D cells. These cells stain positively for

the recently identified single nucleotide polymorphism in aromatic amino acid decarboxylase (an enzyme involved

the TAAR6 (TRAR4, TA4) gene might prove to be valuable in the production of TAs) but lack both tyrosine hydroxyl-

diagnostic parameters. However, a causal link between ase (the rate-limiting enzyme involved in the production of

schizophrenia and TAs or TAARs remains to be

(not shown) has been demonstrated to activate rat TAAR1 (TA1), albeit with w80 times lower potency compared with p-tyramine [14], and has been reported to modulate noradrenaline-mediated neurotransmission [24]. Abbreviations: AADC, aromatic amino acid decarboxylase; COMT, catechol-O-methyltransferase; DBH, dopamine b-hydroxylase; NMT, nonspecific N-methyltransferase; PNMT, phenylethanolamine N-methyltransferase; TH, tyrosine hydroxylase.

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TRENDS in Pharmacological Sciences Vol.26 No.5 May 2005 279

Table 3. Affinity of TAs and biogenic amines to human TAAR1 (TA1) and brain membranes

Amine [3H]Tyramine on human

TAAR1a

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[3H]Tryptamine on human braind K

[3H]Tyramine on rat brainb

[3H]Tryptamine on rat brainc (mM) K

(mM) K

(mM) Trace amine b-Phenylethylamine 0.008 0.425 0.087 0.480 p-Tyramine 0.034 0.016 0.940 0.315 Octopamine 0.493 0.292 – 182.373 Tryptamine 1.084 – 0.002 0.006 Biogenic amine Dopamine 0.422 0.013 0.470 7.953 5-HT O6.000 0.668 0.280 0.682 Noradrenaline O10.000 0.092 22.400 32.835 Histamine 3.107 – – – aTransiently transfected COS-7 cells were used (K

Z20 nM) [13]. bMembrane preparations from rat striatum were used (K

Z1659 fmol mg proteinK1) [30]. cMembrane preparations from rat cortex were used (K

Z11.5 nM, B

max Z2.8 nM, B

max

Z429 fmol mg proteinK1) [32]. d

Membrane preparations from human cortex were used (K

Z2.1 nM, B

max

Z164 fmol mg protein

) [9].

established, which is complicated by the complexity and

display a behavioral profile that resembles animals limited knowledge of the disease mechanism of schizo-

treated with traditional antidepressants [50]. The phar- phrenia [44].

macology of TAs might also contribute to a molecular The strongest argument for a role for TAs in depression

understanding of the well-recognized antidepressant is supported by the so-called ‘PEA hypothesis’, which

effect of physical exercise [51]. In addition to the various suggests that a deficit in the level or turnover of b-PEA

beneficial effects for brain function mainly attributed to an underlies the etiology of endogenous depression, whereas

upregulation of peptide growth factors [52,53], exercise an excess might result in manic episodes [45,46]. The PEA

induces a rapidly enhanced excretion of the main b-PEA hypothesis is supported directly by clinical studies that

metabolite b-phenylacetic acid (b-PAA) by on average 77%, reported a relief from depression symptoms in 60% of

compared with resting control subjects [54], which mirrors depressed patients following administration of b-PEA or

increased b-PEA synthesis in view of its limited endogen- its precursor

-phenylalanine even following prolonged

ous pool half-life of w30 s [18,55]. treatment [47], in addition to reduced b-PEA levels in the

The above-summarized data, mainly focusing on cerebrospinal fluid of depressed patients, compared with

b-PEA, provide substantial evidence for an important healthy control subjects [48]. It is believed that the

role for TAs in the pathophysiology, and potentially the antidepressant effect of MAO inhibitors is mediated in

treatment, of depression. These studies reveal TAs as part by triggering a marked increase of TA brain levels

suitable biomarkers for depression, and TA-related com- [49] by inhibiting the main route of TA catabolism (MAO)

pounds with improved metabolic properties might be (Figure 2) [18]. Moreover, MAO-B-deficient transgenic

promising candidates for the advanced pharmacological mice, in which b-PEA levels in the CNS are increased

treatment of depression, which so far is almost exclusively eightfold compared with wild-type mice whereas the levels

limited to drugs that target the classical biogenic amine of various other monoamines are largely unaffected,

systems.

Box 1. Technical peculiarities of targeting TAARs in drug development

From the perspective of the pharmaceutical industry, TAARs are attractive targets for drug development because of the good chemical tractability inherent to GPCRs, which currently account for w50% of all drugs on the market [58]. The tight link of TAARs to several disease areas, in addition to their high pharmacological potential, evident from the fact that various classes of psychoactive compounds such as amphetamines and ergot alkaloids directly act on TAAR1 (TA1) [14], make them exceptionally promising molecules. However, several technical issues innate to this receptor family will require close attention for the successful development of TA-related drugs:

Species differences The remarkable species differences with regard to the total number of receptors and pseudogenes and to the sensitivity of the receptors to individual TAs [15] require caution for the extrapolation of pharma- cological data between species.

Expression levels and localization Overall, the available data on the tissue distribution of TAARs are few, and detailed expression studies will be needed for a meaningful interpretation of the receptor pharmacology on a systems level. At present, the distribution of TAARs in human and rodents is thought to

be restricted to discrete brain areas [13], and levels of transcripts, but not of protein, are reported to be low.

Receptor trafficking and signaling When expressed in eukaryotic cell lines TAARs show a predominantly intracellular localization [13,14] that seems to closely resemble the in vivo situation in mammalian brain tissue. It is not known if the membrane-permeable TAs b-PEA and tryptamine might trigger signal- ing from an intracellular receptor pool, and, if regulated, membrane insertion in addition to receptor heterodimerization [59,60] might serve to adjust receptor sensitivity and specificity. In contrast to its rodent orthologs, human TAAR1 (TA1) displays a reduced signaling capability in standard heterologous expression systems, which can be restored by replacing either parts of the receptor sequence or the stimulatory G-protein with the corresponding rat counterparts [13,15].

Unknown ligands for most TAARs To date, no functional ligands have been identified for most TAARs. Identification of functional TAAR ligands will support attempts to target these receptors in drug development, and will determine whether TAARs correspond predominantly to TAs or any other class of endogenous ligands.

Review 280 TRENDS in Pharmacological Sciences Vol.26 No.5 May 2005 Outlook and future perspectives The identification of specific receptors has always been key to the understanding of the biological function and pharmacology of any transmitter-like biological com- pound, as the example of histamine illustrates well: when the importance of amine-mediated systems emerged in the 1960s, it was only after the identification of specific receptors that histamine was generally accepted as an established neurotransmitter [56,57]. Likewise, it is only the recent identification of TAARs, some of which have been characterized as specialized TA receptors, that will enable a detailed understanding of TAs as potential vertebrate neuromodulators at the molecular level. For a full understanding of the physiological relevance of TAs and the TAAR family, numerous vital issues need to be resolved. In addition to thorough studies of the tissue distribution and signal transduction mechanisms of the TAARs, the characterization of physiological high-affinity ligands for the apparently TA-insensitive majority of TAARs will be essential. Given the well-established disease relevance of TAs and the TAAR family, in addition to their high pharmacological potency, answering these questions promises to identify TAs and potentially other TA-related compounds as equally important as classical biogenic amines for the understanding of psychiatric conditions such as depression and schizophrenia. The ongoing research on these issues could soon pave the way for an advanced treatment of these highly prevalent diseases.

Acknowledgements We would like to thank our colleagues M. Ebeling, N.A. Kratochwil, J-L. Moreau, H. Stalder, S. Kolczewski, E. Borroni, J.G. Wettstein, A.J. Sleight and D.K. Grandy for many stimulating discussions. We acknowledge the continued support by F. Hoffmann-La Roche.

References

1 Deutch, A.Y. and Roth, R.H. (1999) Neurotransmitters. In Funda- mental Neuroscience (Zigmond, M.J. et al., eds), pp. 193–234, Academic Press 2 Wong, M.L. and Licinio, J. (2001) Research and treatment approaches

to depression. Nat. Rev. Neurosci. 2, 343–351 3 Carlsson, A. et al. (2001) Interactions between monoamines, gluta- mate, and GABA in schizophrenia: new evidence. Annu. Rev. Pharmacol. Toxicol. 41, 237–260 4 Tuite, P. and Riss, J. (2003) Recent developments in the pharmaco- logical treatment of Parkinson’s disease. Expert Opin. Investig. Drugs 12, 1335–1352 5 Castellanos, F.X. and Tannock, R. (2002) Neuroscience of attention- deficit/hyperactivity disorder: the search for endophenotypes. Nat. Rev. Neurosci. 3, 617–628 6 Usdin, E. and Sandler, M., eds (1984) Trace Amines and the brain,

Dekker 7 Branchek, T.A. and Blackburn, T.P. (2003) Trace amine receptors as targets for novel therapeutics: legend, myth and fact. Curr. Opin. Pharmacol. 3, 90–97 8 Premont, R.T. et al. (2001) Following the trace of elusive amines. Proc.

Natl. Acad. Sci. U. S. A. 98, 9474–9475 9 Mousseau, D.D. and Butterworth, R.F. (1995) A high-affinity [3H] tryptamine binding site in human brain. Prog. Brain Res. 106, 285–291 10 McCormack, J.K. et al. (1986) Autoradiographic localization of tryptamine binding sites in the rat and dog central nervous system. J. Neurosci. 6, 94–101

www.sciencedirect.com

11 Dyck, L.E. (1989) Release of some endogenous trace amines from rat striatal slices in the presence and absence of a monoamine oxidase inhibitor. Life Sci. 44, 1149–1156 12 Parker, E.M. and Cubeddu, L.X. (1988) Comparative effects of amphetamine, phenylethylamine and related drugs on dopamine efflux, dopamine uptake and mazindol binding. J. Pharmacol. Exp. Ther. 245, 199–210 13 Borowsky, B. et al. (2001) Trace amines: identification of a family of mammalian G protein-coupled receptors. Proc. Natl. Acad. Sci. U. S. A. 98, 8966–8971 14 Bunzow, J.R. et al. (2001) Amphetamine, 3,4-methylenedioxymetham- phetamine, lysergic acid diethylamide, and metabolites of the catecholamine neurotransmitters are agonists of a rat trace amine receptor. Mol. Pharmacol. 60, 1181–1188 15 Lindemann, L. et al. (2005) Trace amine associated receptors form structurally and functionally distinct subfamilies of novel G protein- coupled receptors. Genomics 85, 372–385 16 Civelli, O. (2005) GPCR deorphanizations: the novel, the known and

the unexpected transmitters. Trends Pharmacol. Sci. 26, 15–19 17 Scanlan, T.S. et al. (2004) 3-Iodothyronamine is an endogenous and

rapid-acting derivative of thyroid hormone. Nat. Med. 10, 638–642 18 Berry, M.D. (2004) Mammalian central nervous system trace amines. Pharmacologic amphetamines, physiologic neuromodulators. J. Neurochem. 90, 257–271 19 Olson, M.V. and Varki, A. (2003) Sequencing the chimpanzee genome: insights into human evolution and disease. Nat. Rev. Genet. 4, 20–28 20 Malherbe, P. et al. (2003) Mutational analysis and molecular modeling of the binding pocket of the metabotropic glutamate 5 receptor negative modulator 2-methyl-6-(phenylethynyl)-pyridine. Mol. Phar- macol. 64, 823–832 21 Saavedra, J.M. (1989) b-Phenylethylamine, phenylethanolamine, tyramine and octopamine. In Catecholamines II (Trendelenburg, U. and Weiner, N., eds), pp. 181–201, Springer 22 Saavedra, J.M. et al. (1973) The distribution and properties of the nonspecific N-methyltransferase in brain. J. Neurochem. 20, 743–752 23 Saavedra, J.M. et al. (1974) Localisation of phenylethanolamine

N-methyl transferase in the rat brain nuclei. Nature 248, 695–696 24 Jones, R.S. and Boulton, A.A. (1980) Tryptamine and 5-hydroxytryp- tamine: actions and interactions of cortical neurones in the rat. Life Sci. 27, 1849–1856 25 Jones, R.S. (1982) Tryptamine modifies cortical neurone responses evoked by stimulation of nucleus raphe medianus. Brain Res. Bull. 8, 435–437 26 Berry, M.D. et al. (1994) The effects of administration of monoamine oxidase-B inhibitors on rat striatal neurone responses to dopamine. Br. J. Pharmacol. 113, 1159–1166 27 Federici, M. et al. Trace amines depress GABAB response in dopaminergic neurons by inhibiting GIRK channels. Mol. Pharmacol. DOI: 10.1124/mol.104.007427 (http://molpharm.aspetjournals.org) 28 Li, X.M. et al. (1992) Absence of 2-phenylethylamine binding after monoamine oxidase inhibition in rat brain. Eur. J. Pharmacol. 210, 189–193 29 Hauger, R.L. et al. (1982) Specific [3H] beta-phenylethylamine binding

sites in rat brain. Eur. J. Pharmacol. 83, 147–148 30 Vaccari, A. (1986) High affinity binding of [3H]-tyramine in the central

nervous system. Br. J. Pharmacol. 89, 15–25 31 Nguyen, T.V. and Juorio, A.V. (1989) Binding sites for brain trace

amines. Cell. Mol. Neurobiol. 9, 297–311 32 Cascio, C.S. and Kellar, K.J. (1983) Characterization of [3H]

tryptamine binding sites in brain. Eur. J. Pharmacol. 95, 31–39 33 Cornea-Hebert, V. et al. (1999) Cellular and subcellular distribution of the serotonin 5-HT2A receptor in the central nervous system of adult rat. J. Comp. Neurol. 409, 187–209 34 Sotnikova, T.D. et al. (2004) Dopamine transporter-dependent and - independent actions of trace amine beta-phenylethylamine. J. Neurochem. 91, 362–373 35 Jaeger, C.B. et al. (1984) Aromatic L-amino acid decarboxylase in the rat brain: immunocytochemical localization in neurons of the brain stem. Neuroscience 11, 691–713 36 Davenport, A.P. (2003) Peptide and trace amine orphan receptors: prospects for new therapeutic targets. Curr. Opin. Pharmacol. 3, 127–134

Review

TRENDS in Pharmacological Sciences Vol.26 No.5 May 2005 281

www.sciencedirect.com 37 Shirkande, S. et al. (1995) Plasma phenylethylamine levels of

49 Dewar, K.M. et al. (1988) Involvement of brain trace amines in the schizophrenic patients. Can. J. Psychiatry 40, 221

behavioural effects of phenelzine. Neurochem. Res. 13, 113–119 38 Potkin, S.G. et al. (1979) Phenylethylamine in paranoid chronic

50 Grimsby, J. et al. (1997) Increased stress response and beta- schizophrenia. Science 206, 470–471

phenylethylamine in MAOB-deficient mice. Nat. Genet. 17, 206–210 39 Janssen, P.A. et al. (1999) Does phenylethylamine act as an

51 Scully, D. et al. (1998) Physical exercise and psychological well being: a endogenous amphetamine in some patients? Int.

critical review. Br. J. Sports Med. 32, 111–120 J. Neuropsychopharmcol. 2, 229–240

52 Russo-Neustadt, A.A. et al. (2000) Physical activity and antidepress- 40 Griffith, J.D. et al. (1970) Psychosis induced by the administration of d-amphetamine to human volunteers. In Psychomimetic Drugs (Efron, D.H., ed.), pp. 287–298, Raven 41 Lenders, J.W. et al. (1996) Specific genetic deficiencies of the A and B isoenzymes of monoamine oxidase are characterized by distinct neurochemical and clinical phenotypes. J. Clin. Invest. 97, 1010–1019 42 Cao, Q. et al. (1997) Suggestive evidence for a schizophrenia susceptibility locus on chromosome 6q and a confirmation in an independent series of pedigrees. Genomics 43, 1–8 43 Duan, J. et al. (2004) Polymorphisms in the trace amine receptor 4 (TRAR4) gene on chromosome 6q23.2 are associated with suscepti- bility to schizophrenia. Am. J. Hum. Genet. 75, 624–638 44 Mueser, K.T. and McGurk, S.R. (2004) Schizophrenia. Lancet 363,

2063–2072 45 Sabelli, H.C. and Mosnaim, A.D. (1974) Phenylethylamine hypothesis

of affective behavior. Am. J. Psychiatry 131, 695–699 46 Davis, B.A. and Boulton, A.A. (1994) The trace amines and their acidic

ant treatment potentiate the expression of specific brain-derived neurotrophic factor transcripts in the rat hippocampus. Neuroscience 101, 305–312 53 Carro, E. et al. (2003) Brain repair and neuroprotection by serum

insulin-like growth factor I. Mol. Neurobiol. 27, 153–162 54 Szabo, A. et al. (2001) Phenylethylamine, a possible link to the antidepressant effects of exercise? Br. J. Sports Med. 35, 342–343 55 Durden, D.A. and Philips, S.R. (1980) Kinetic measurements of the turnover rates of phenylethylamine and tryptamine in vivo in the rat brain. J. Neurochem. 34, 1725–1732 56 Bouthenet, M.L. et al. (1988) A detailed mapping of histamine H1-receptors in guinea-pig central nervous system established by autoradiography with [125I] iodobolpyramine. Neuroscience 26, 553–600 57 Haas, H. and Panula, P. (2003) The role of histamine and the tuberomamillary nucleus in the nervous system. Nat. Rev. Neurosci. 4, 121–130 58 Ma, P. and Zemmel, R. (2002) Value of novelty? Nat. Rev. Drug Discov.1, metabolites in depression - an overview. Prog. Neuropsychopharma-

571–572 col. Biol. Psychiatry 18, 17–45

59 Bouvier, M. (2001) Oligomerization of G protein-coupled transmitter 47 Sabelli, H.C. et al. (1996) Sustained antidepressant effect of PEA

receptors. Nat. Rev. Neurosci. 2, 274–286 replacement. J. Neuropsychiatry Clin. Neurosci. 8, 168–171

60 Agnati, L.F. et al. (2003) Molecular mechanisms and therapeutical 48 Sandler, M. et al. (1979) Decreased cerebrospinal fluid concentration

implications of intramembrane receptor/receptor interactions among of free phenylacetic acid in depressive illness. Clin. Chim. Acta 93,

heptahelical receptors with examples from the striatopallidal GABA 169–171

neurons. Pharmacol. Rev. 55, 509–550

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