Trace amine-associated receptors and their ligands

Classical biogenic amines (adrenaline, noradrenaline, dopamine, serotonin and histamine) interact with specific families of G protein-coupled receptors (GPCRs). The term 'trace amines' is used when referring to p-tyramine, beta-phenylethylamine, tryptamine and octopamine, compounds that are present in mammalian tissues at very low (nanomolar) concentrations. The pharmacological effects of trace amines are usually attributed to their interference with the aminergic pathways, but in 2001 a new gene was identified, that codes for a GPCR responding to p-tyramine and beta-phenylethylamine but not to classical biogenic amines. Several closely related genes were subsequently identified and designated as the trace amine-associated receptors (TAARs). Pharmacological investigations in vitro show that many TAAR subtypes may not respond to p-tyramine, beta-phenylethylamine, tryptamine or octopamine, suggesting the existence of additional endogenous ligands. A novel endogenous thyroid hormone derivative, 3-iodothyronamine, has been found to interact with TAAR1 and possibly other TAAR subtypes. In vivo, micromolar concentrations of 3-iodothyronamine determine functional effects which are opposite to those produced on a longer time scale by thyroid hormones, including reduction in body temperature and decrease in cardiac contractility. Expression of all TAAR subtypes except TAAR1 has been reported in mouse olfactory epithelium, and several volatile amines were shown to interact with specific TAAR subtypes. In addition, there is evidence that TAAR1 is targeted by amphetamines and other psychotropic agents, while genetic linkage studies show a significant association between the TAAR gene family locus and susceptibility to schizophrenia or bipolar affective disorder.     

Pharmacological characterization of membrane-expressed human trace amine-associated receptor 1 (TAAR1) by a bioluminescence resonance energy transfer cAMP biosensor

Trace amines are neurotransmitters whose role in regulating invertebrate physiology has been appreciated for many decades. Recent studies indicate that trace amines may also play a role in mammalian physiology by binding to a novel family of G protein-coupled receptors (GPCRs) that are found throughout the central nervous system. A major obstacle impeding the careful pharmacological characterization of trace amine associated receptors (TAARs) is their extremely poor membrane expression in model cell systems, and a molecular basis for this phenomenon has not been determined. In the present study, we show that the addition of an asparagine-linked glycosylation site to the N terminus of the human trace amine associated receptor 1 (TAAR1) is sufficient to enable its plasma membrane expression, and thus its pharmacological characterization with a novel cAMP EPAC (exchange protein directly activated by cAMP) protein based bioluminescence resonance energy transfer (BRET) biosensor. We applied this novel cAMP BRET biosensor to evaluate the activity of putative TAAR1 ligands. This study represents the first comprehensive investigation of the membrane-expressed human TAAR1 pharmacology. Our strategy to express TAARs and to identify their ligands using a cAMP BRET assay could provide a foundation for characterizing the functional role of trace amines in vivo and suggests a strategy to apply to groups of poorly expressing GPCRs that have remained difficult to investigate in model systems.     

A renaissance in trace amines inspired by a novel GPCR family

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.     

Dual excitatory and smooth muscle‐relaxant effect of β‐phenylethylamine on gastric fundus strips in rats

β‐Phenylethylamine (β‐PEA) is a trace amine with chemical proximity to biogenic amines and amphetamines. It is an endogenous agonist of trace amine‐associated receptors (TAARs) that acts as a neuromodulator of classic neurotransmitters in the central nervous system. At high concentrations, β‐PEA contracts smooth muscle, and a role for TAARs in these responses has been postulated. The high dietary intake of trace amines has been associated with such symptoms as hypertension and migraine, especially after the intake of foods containing such compounds. In gastrointestinal tissues, TAAR expression was reported, although the effect of β‐PEA on gastric contractile behaviour is unknown. Here, isolated strips that were obtained from the rat gastric fundus were stimulated with high micromolar concentrations of β‐PEA. Under resting tonus, β‐PEA induced contractions. In contrast, when the strips were previously contracted with KCl, a relaxant response to β‐PEA was observed. The contractile effect of β‐PEA was inhibited by 5‐hydroxytryptamine (5‐HT) receptor antagonists (i.e., cyproheptadine and ketanserin) but not by the TAAR₁ antagonist EPPTB. In gastric fundus strips that were previously contracted with 80 mmol/L KCl, the relaxant effect of β‐PEA intensified in the presence of 5‐HT receptor antagonists, which was inhibited by EPPTB and the adenylyl cyclase inhibitor MDL‐12,330A. The guanylyl cyclase inhibitor ODQ did not alter the relaxant effects of β‐PEA. In conclusion, β‐PEA exerted dual contractile and relaxant effects on rat gastric fundus. The contractile effect appeared to involve the recruitment of 5‐HT receptors, and the relaxant effect of β‐PEA on KCl‐elicited contractions likely involved TAAR₁.     

Cardiac effects of trace amines: pharmacological characterization of trace amine-associated receptors

Trace amine-associated receptors, a novel class of G-protein coupled receptors which respond to trace amines but not to classical biogenic amines, have been found to be expressed in heart. Therefore, we investigated the cardiac effects of the trace amines p-tyramine, beta-phenylethylamine, octopamine, and tryptamine. Isolated rat hearts were perfused in the presence of trace amines, monitoring the hemodynamic variables. In addition, radioligand binding experiments with [3H]-p-tyramine and [125I]-3-iodothyronamine were performed in rat ventricular tissue. Octopamine, beta-phenylethylamine, and tryptamine produced a dose-dependent negative inotropic effect as shown by reduced cardiac output (IC(50)=109 microM, 159 microM, and 242 microM, respectively). In the same preparation a similar effect was produced by thyronamine and 3-iodothyronamine, with IC(50)=94 microM and 27 microM, respectively. The negative inotropic effect of octopamine was confirmed in a papillary muscle preparation. All trace amines except tryptamine increased the heart rate, but this action could be attributed to their sympathomimetic properties, since it was abolished by propranolol. The negative inotropic effect of trace amines was significantly increased by the tyrosine kinase inhibitor genistein. Specific and saturable binding of [(3)H]-p-tyramine and [125I]-3-iodothyronamine was observed in ventricular tissue. While [3H]-p-tyramine was displaced by 3-iodothyronamine, [(125)I]-3-iodothyronamine was not displaced by p-tyramine. In conclusion, trace amines and thyronamines are negative inotropic agents. Their effect appears to be mediated by a subtype of trace amine-associated receptor which is characterized by the rank of potency: 3-iodothyronamine > thyronamine = octopamine = beta-phenylethylamine, while tryptamine and p-tyramine are significantly less active.     

Effects of dietary vanadium exposure on levels of regional brain neurotransmitters and their metabolites

Adult male CD-1 mice were treated with various levels of vanadate in drinking water for 30 days. The levels of catecholamine and indoleamine neurotransmitters and their major metabolites were measured in six different brain regions. Vanadium caused a dose-related decrease in norepinephrine (NE) levels in hypothalamus, the region rich in this biogenic amine. Levels of the NE metabolite, vanillylmandelic acid (VMA), correspondingly decreased in the same region. Although hypothalamic dopamine (DA) also showed a significant decline, vanadium had little effect on DA metabolites. Levels of 5-hydroxytryptamine (5-HT) and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), were not influenced. Levels of DA were not affected in the corpus striatum, where the highest levels of this amine are observed. Effects of vanadium on various biogenic amines and their metabolites were only marginal in other brain regions. Results suggest that vanadium has a selective effect on adrenergic pathways, and effects on other hypothalamic amines appear to be secondary. These observations support the pro-oxidant potential of vanadate ion on catecholamines suggested earlier.     

Effect of sympathomimetic drugs in eliciting hypertensive responses to reserpine in the rat, after pretreatment with monoamineoxidase inhibitors

1. The effects of some rapidly metabolized sympathomimetic amines, such as beta-phenylethylamine and p-tyramine, in eliciting hypertensive responses to reserpine in the anaesthetized rat, have been studied.2. Retardation of metabolism, by pretreatment with the monoamineoxidase inhibitors iproniazid or phenelzine, causes beta-phenylethylamine (which in untreated rats has no effect) to induce hypertensive responses to reserpine. Tyramine and other hydroxy substituted phenylethylamines are much less active in this respect, probably because of relatively poor lipid solubility.3. Hypertensive responses to reserpine are due to catecholamine release, which is believed to be from stores made accessible to indirectly acting sympathomimetic amines with high lipid solubility by an action of reserpine on cell membranes.     

Psychotherapeutic drugs and biogenic amines. Current concepts and therapeutic implications

Over the last 2 decades evidence has continued to accumulate from studies in various model systems that drugs effective in the treatment of major psychiatric disorders alter biogenic amines which function as neurotransmitters. As a result of these findings, various hypotheses have been formulated that there is a fundamental abnormality of one or another of the biogenic amine systems (i.e. the serotonergic, noradrenergic and dopaminergic systems) in the affective disorders and schizophrenia. Numerous attempts have been made to assess these biogenic amine hypotheses, primarily through quantitation of the major metabolites of the amines and recently by assessment of receptor sensitivity or density. The combination of basic pharmacological and clinical research has shown that there are biochemical subtypes, especially among those with affective illness, which might show a preferential response to biochemically specific drugs. If verified, such research constitutes a significant therapeutic advance. The status of these investigations is critically assessed in this review. Finally, many new techniques, challenge tests and biochemically distinct drugs are being introduced. Promising approaches that are likely to produce refinements of the biogenic amine hypotheses, as well as suggesting alternative formulations, are reviewed.     

Pharmacology of cannabinoid receptor ligands.

Mammalian tissues contain at least two types of cannabinoid receptor, CB1 and CB2, both coupled to G proteins. CB1 receptors are expressed mainly by neurones of the central and peripheral nervous system whereas CB2 receptors occur in certain non-neuronal tissues, particularly in immune cells. The existence of endogenous ligands for cannabinoid receptors has also been demonstrated. The discovery of this endogenous cannabinoid system has been paralleled by a renewed interest in possible therapeutic applications of cannabinoids, for example in the management of pain and in the suppression of muscle spasticity/spasm associated with multiple sclerosis or spinal cord injury. It has also prompted the development of a range of novel cannabinoid receptor ligands, including several that show marked selectivity for CB1 or CB2 receptors. This review summarizes current knowledge about the in vitro pharmacological properties of important CB1 and CB2 receptor ligands. Particular attention is paid to the binding properties of these ligands, to the efficacies of cannabinoid receptor agonists, as determined using cyclic AMP or [35S]GTPgammaS binding assays, and to selected examples of how these pharmacological properties can be influenced by chemical structure. The in vitro pharmacological properties of ligands that can potently and selectively oppose the actions of CB1 or CB2 receptor agonists are also described. When administered by themselves, some of these ligands produce effects in certain tissue preparations that are opposite in direction to those produced by cannabinoid receptor agonists and the possibility that the ligands producing such inverse cannabimimetic effects are inverse agonists rather than pure antagonists is discussed.     

The endocannabinoid system: current pharmacological research and therapeutic possibilities

In the relatively short period of time since the discovery of cannabinoid receptors and their endogenous ligands, the endocannabinoids, an intensive research effort has resulted in the identification of agents that affect all aspects of the endocannabinoid system. The cannabinoid(1) receptor antagonist rimonabant is in phase III clinical trials for the treatment of obesity and as an aid to smoking cessation, and cannabinoid(2) receptor agonists are promising in animal models of inflammatory and neuropathic pain. In the present MiniReview, the endocannabinoid system is described from a pharmacological perspective. The main topics covered are: the mechanism of action of cannabinoid(2) receptor agonists; identification of the endocannabinoid(s) involved in retrograde signalling; the elusive mechanism(s) of endocannabinoid uptake; therapeutic possibilities for fatty acid amide hydrolase inhibitors; and the cyclooxygenase-2 and lipoxygenase-derived biologically active metabolites of the endocannabinoids.     

Trace amine-associated receptor 1: a multimodal therapeutic target for neuropsychiatric diseases

Introduction: The trace amines, endogenous amines closely related to the biogenic amine neurotransmitters, have been known to exert physiological and neurological effects for decades. The recent identification of a trace amine-sensitive G protein-coupled receptor, trace amine-associated receptor 1 (TAAR1), and subsequent development of TAAR1-selective small-molecule ligands, has renewed research into the therapeutic possibilities of trace amine signaling.

Areas covered: Recent efforts in elucidating the neuropharmacology of TAAR1, particularly in neuropsychiatric and neurodegenerative disease, addiction, and regulation of arousal state, will be discussed. Focused application of TAAR1 mutants, synthetic TAAR1 ligands, and endogenous biomolecules such as 3-iodothyronamine (T1AM) has yielded a basic functional portrait for TAAR1, despite a complex biochemistry and pharmacology. The close functional relationship between TAAR1 and dopaminergic signaling is likely to underlie many of its CNS effects. However, TAAR1’s influences on serotonin and glutamate neurotransmission will also be highlighted.

Expert opinion: TAAR1 holds great promise as a therapeutic target for mental illness, addiction, and sleep disorders. A combination of preclinical and translationally driven studies has solidified TAAR1 as a key node in the regulation of dopaminergic signaling. Continued focus on the mechanisms underlying TAAR1’s regulation of serotonin and glutamate signaling, as well as dopamine, will yield further disease-relevant insights.     

Effect of long term amineptine treatment on pre- and postsynaptic mechanisms in rat brain.

The effect of amineptine and its two metabolites on monoamine uptake, release and receptor binding was studied in vitro. Amineptine and its two metabolites did not displace labelled ligands for known neurotransmitters and drug receptor sites. Amineptine and its two metabolites did not influence [3H]-5-hydroxytryptamine ([3H]-5-HT) uptake or release by rat brain synaptosomes. Amineptine inhibited [3H]-dopamine and [3H]-noradrenaline ([3H]-NA) accumulation, with IC50 values of 1.4 and 10 microM, respectively. The effect was retained, though with lower efficacy, by the two metabolites. Amineptine released [3H]-dopamine from preloaded synaptosomes. Metabolite 1 had no effect on catecholamine release, and metabolite 2 was about half as active as the parent compound on [3H]-dopamine release. The releasing effect of amineptine on [3H]-dopamine was potentiated by reserpine pretreatment, suggesting that the drug acts on the cytoplasmic neurotransmitter pool. Chronic treatment with amineptine (20 mg kg-1, twice daily for 15 days followed by a 3 days drug withdrawal period) resulted in a decrease of [3H]-spiperone binding sites in striatum, and of [3H]-dihyroalprenolol and [3H]-clonidine in cortex. Chronic treatment with amineptine reduced basal [3H]-dopamine accumulation in striatal synaptosomes, without affecting [3H]-NA or [3H]-5-HT accumulation. The adaptive changes in the pre- and postsynaptic dopamine mechanisms observed after long term treatment with amineptine are consistent with the drug acting as an indirect dopamine agonist. The down regulation of beta- and alpha 2-noradrenoceptors observed after long term amineptine treatment may play a role in the antidepressant activity of the drug.     

The contribution of metabolites to the rapid and potent down-regulation of rat cortical beta-adrenoceptors by the putative antidepressant sibutramine hydrochloride

Sibutramine HCl is an inhibitor of the reuptake of monoamines with a pharmacological profile in rodents indicative of antidepressant activity. The secondary (BTS 54 354) and primary (BTS 54 505) amine metabolites of the tertiary amine sibutramine HCl exhibit similar in vivo pharmacological activity to the parent compound. Thus, each compound displays potent activity in acute behavioural models predictive of antidepressant effects and a comparable ability to inhibit the uptake of monoamines in vivo. In addition, BTS 54 354 and BTS 54 505 induce an equally rapid and potent down-regulation of cortical beta-adrenoceptors in the rat as sibutramine HCl. The secondary and primary amines are, however, considerably more active than sibutramine HCl as inhibitors of the uptake of noradrenaline, dopamine and 5-hydroxytryptamine in vitro. The potent inhibition of the reuptake of noradrenaline by the secondary and primary amine metabolites probably contributes to the rapid and potent down-regulation of beta-adrenoceptors in the rat, induced by the putative antidepressant sibutramine HCl.     

Further Insights Into the Pharmacology of the Human Trace Amine‐Associated Receptors: Discovery of Novel Ligands for TAAR1 by a Virtual Screening Approach

Trace Amine‐Associated Receptor 1 (TAAR1) is a G protein‐coupled receptor that is expressed in brain and periphery and responds to a class of compounds called trace amines, such as β‐phenylethylamine ( β ‐PEA ), tyramine, tryptamine, octopamine. The receptor is known to have a very rich pharmacology and could be also activated by different classes of compounds, including dopaminergic, adrenergic and serotonergic ligands. It is expected that targeting hTAAR1 could provide a novel pharmacological approach for several human disorders, such as schizophrenia, depression, attention deficit hyperactivity disorder, Parkinson's disease and metabolic diseases. Only recently, a small number of selective hTAAR1 agonists (among which RO5166017 and T ₁ AM ) and antagonist ( EPPTB ), have been reported in literature. With the aim to identify new molecular entities able to act as ligands for this target, we used an homology model for the hTAAR1 and performed a virtual screening procedure on an in‐house database of compounds. A number of interesting molecules were selected and by testing them in an in vitro assay we found several agonists and one antagonist, with activities in the low micromolar range. These compounds could represent the starting point for the development of more potent and selective TAAR1 ligands.     

Brain-Specific Overexpression of Trace Amine-Associated Receptor 1 Alters Monoaminergic Neurotransmission and Decreases Sensitivity to Amphetamine

Trace amines (TAs) such as β-phenylethylamine, p-tyramine, or tryptamine are biogenic amines found in the brain at low concentrations that have been implicated in various neuropsychiatric disorders like schizophrenia, depression, or attention deficit hyperactivity disorder. TAs are ligands for the recently identified trace amine-associated receptor 1 (TAAR1), an important modulator of monoamine neurotransmission. Here, we sought to investigate the consequences of TAAR1 hypersignaling by generating a transgenic mouse line overexpressing Taar1 specifically in neurons. Taar1 transgenic mice did not show overt behavioral abnormalities under baseline conditions, despite augmented extracellular levels of dopamine and noradrenaline in the accumbens nucleus (Acb) and of serotonin in the medial prefrontal cortex. In vitro, this was correlated with an elevated spontaneous firing rate of monoaminergic neurons in the ventral tegmental area, dorsal raphe nucleus, and locus coeruleus as the result of ectopic TAAR1 expression. Furthermore, Taar1 transgenic mice were hyposensitive to the psychostimulant effects of amphetamine, as it produced only a weak locomotor activation and failed to alter catecholamine release in the Acb. Attenuating TAAR1 activity with the selective partial agonist RO5073012 restored the stimulating effects of amphetamine on locomotion. Overall, these data show that Taar1 brain overexpression causes hyposensitivity to amphetamine and alterations of monoaminergic neurotransmission. These observations confirm the modulatory role of TAAR1 on monoamine activity and suggest that in vivo the receptor is either constitutively active and/or tonically activated by ambient levels of endogenous agonist(s).     

Recent advances towards the discovery of ORL-1 receptor agonists and antagonists

Since their discovery a decade ago, remarkable progress has been made toward understanding the biological function and significance of the opioid receptor-like-1 (ORL-1) receptor and its endogenous peptide ligand, nociceptin. The human nociceptin receptor, herein referred to as ORL-1, but also known as OP4 (the fourth member of opioid peptide receptor family) or nociceptin/orphanin FQ peptide (NOP) receptor, was first identified as an orphan opioid receptor with close homology to the classical μ-, κ-, and δ-opioid receptors. ORL-1 does not bind endogenous ligands of the other opioid receptors with high affinity, but instead prefers the 17 amino acid peptide nociceptin. The obvious homologies of ORL-1 to opioid receptors, and its ligand nociceptin to opioid peptide ligands, led to a period of intense investigation that resulted in a number of significant reports describing the biology of the receptor and ligand. The emerging pharmacological evidence from these reports suggests that ORL-1 agonists may be clinically useful for treatment of stress, anxiety, substance abuse (opioid and alcohol), anorexia, cachexia, cough, asthma, and possibly neuropathic pain/allodynia. The peripheral effects of nociceptin suggest that agonists may have utility in the treatment of gastrointestinal motility disorders, water retention, and hypertension. ORL-1 antagonists may be useful in enhancing cognitive function and treating locomotor disorders such as Parkinsonism. In addition to research into the fundamental biology of ORL-1 and nociceptin, noteworthy advances have been made in the discovery of new peptide and non-peptide agonists and antagonists of the ORL-1 receptor leading to a better understanding of its involvement in a variety of biological processes. This review highlights the rationale for the development of ORL-1 ligands and recent progress made by different research groups towards the development of peptidic and non-peptidic ORL-1 agonists or antagonists over the last four years. To add perspective on the commercial potential of this research area, the development status of advanced new molecules is addressed together with any pharmacological characterisation of these entities.     

Sigma receptor ligands: possible application as therapeutic drugs and as radiopharmaceuticals

Sigma receptors are classified into sigma(1) and sigma(2) subtypes. These subtypes display a different tissue distribution and a distinct physiological and pharmacological profile in the central and peripheral nervous system. The characterization of these subtypes and the discovery of new specific sigma receptor ligands demonstrated that sigma receptors are novel targets for the therapeutic treatment of neuropsychiatric diseases (schizophrenia, depression, and cognition), brain ischemia, and cocaine addiction. Furthermore, imaging of sigma(1) receptors in the human brain using specific PET radioligands has started. In addition, the two sigma receptor subtypes are also expressed on tumor cells, where they could be of prognostic relevance. The ability of sigma(2) receptor agonists to inhibit tumor cell proliferation through mechanisms that might involve apoptosis, intracellular Ca(2+), and sphingolipids has promoted the development of sigma(2) receptor agonists as novel therapeutic drugs for treating cancer. Consequently, sigma(2) receptor ligands have been demonstrated to be potentially useful tumor imaging ligands. In this article, we focus on the sigma receptor ligands as therapeutic agents and as radiopharmaceuticals.     

Trace amine-associated receptor 1 as a monoaminergic modulator in brain

Brain monoaminergic systems play critical roles in mood, cognition, emotion, reward, learning and attention, and aberrance in brain monoaminergic activity is associated with a variety of neuropsychiatric disorders/diseases. The present commentary focuses on trace amine-associated receptor 1 (TAAR1) and its potential regulatory roles in brain monoaminergic systems. TAAR1 was discovered in 2001 and has been established to be a G-protein-coupled receptor signaling through the cAMP pathway. This receptor is activated by a broad spectrum of agonists, although there are notable species differences in ligand efficacy and potency. TAAR1 is expressed and widely distributed in brain monoaminergic systems and co-localized with the dopamine transporter in a subset of dopaminergic neurons in rhesus monkey and mouse brain substantia nigra. TAAR1 activation by the common biogenic amines, the trace amine β-phenylethylamine and methamphetamine alters the monoamine transporter function in both mouse and rhesus monkey brain synaptosomes, suggesting a modulatory role for this receptor in the presynaptic regulation of monoaminergic activity. However, little is known about other functional roles of TAAR1 in the brain. With a purpose to promote further studies on this receptor, we herein discuss the recent findings that provide insights into the functional significance and biological relevance of this receptor as a modulator in brain monoaminergic systems.     

Insights into the Structure and Pharmacology of the Human Trace Amine‐Associated Receptor 1 (hTAAR1): Homology Modelling and Docking Studies

Trace amine‐associated receptor 1 (TAAR1) is a G protein–coupled receptor that belongs to the family of TAAR receptors and responds to a class of compounds called trace amines, such as β‐phenylethylamine ( β‐PEA ) and 3‐iodothyronamine ( T ₁ AM ). The receptor is known to have a very rich pharmacology and could be also activated by other classes of compounds, including adrenergic and serotonergic ligands. It is expected that targeting TAAR1 could provide a novel pharmacological approach to correct monoaminergic dysfunctions found in several brain disorders, such as schizophrenia, depression, attention deficit hyperactivity disorder and Parkinson’s disease. Only recently, the first selective TAAR1 agonist RO5166017 has been identified. To explore the molecular mechanisms of protein–agonist interaction and speed up the identification of new chemical entities acting on this biomolecular target, we derived a homology model for the hTAAR1. The putative protein‐binding site has been explored by comparing the hTAAR1 model with the β₂‐adrenoreceptor binding site, available by X‐ray crystallization studies, and with the homology modelled 5HT_1A receptor. The obtained results, in tandem with docking studies performed with RO5166017 , β‐PEA and T ₁ AM , provided an opportunity to reasonably identify the hTAAR1 key residues involved in ligand recognition and thus define important starting points to design new agonists.     

Lead generation and lead optimisation approaches in the discovery of selective,non-peptide ORL-1 receptor agonists and antagonists

The discovery of the opioid receptor like-1 (ORL-1) receptor and of its endogenous agonist nociceptin/orphanin FQ has attracted great interest in the scientific community giving rise, in the last five years, to a flurry of biological studies aimed at elucidating the role of this new receptor. Hence, the involvement of the ORL-1 receptor in many important processes, such as antinociception, learning and memory, feeding and anxiety, has been well documented. However, a clear understanding of the potential therapeutic value associated with the modulation of the ORL-1 receptor needs the development of selective non-peptide agonists and antagonists allowing systemic routes of administration. This review addresses the advances made by several research groups in the discovery of such compounds and discusses the medicinal chemistry strategies which, starting from the first non-selective ligands NalBzoH and lofentanil, led to the disclosure of highly potent and selective agonists and antagonists, such as Ro 64-6198, J-113397 and JTC-801. Efforts have also focussed on the pharmacological characterisation of the newly discovered non-peptide tools, which represent a significant step forward in the understanding of the involvement of the ORL-1 receptor in a number of possible pathophysiological conditions.