Actions of N-arachidonyl-glycine in a rat inflammatory pain model

Discovery and Preclinical Evaluation of a Novel Inhibitor of FABP5, ART26.12, Effective in Oxaliplatin-Induced Peripheral Neuropathy

Oxaliplatin-induced peripheral neuropathy (OIPN) is a dose-limiting toxicity characterised by mechanical allodynia and thermal hyperalgesia, without any licensed medications. ART26.12 is a fatty acid-binding protein (FABP) 5 inhibitor with antinociceptive properties, characterised here for the prevention and treatment of OIPN. ART26.12 binds selectively to FABP5 compared to FABP3, FABP4, and FABP7, with minimal off-target liabilities, high oral bioavailability, and a NOAEL of 1,000 mg/kg/day in rats and dogs. In an established preclinical OIPN model, acute oral dosing (25-100 mg/kg) showed a cannabinoid receptor type 1 (CB1)-dependent anti-allodynic effect lasting up to 8 hours (persisting longer than plasma exposure to ART26.12). Antagonists of cannabinoid receptor type 2 (CB2), peroxisome proliferator-activated receptor alpha, and transient receptor potential cation channel subfamily V member 1 (TRPV1) may have also been implicated. Twice daily oral dosing (25 mg/kg bis in die (BID) for 7 days) showed anti-allodynic effects in an established OIPN model without the development of tolerance. In a prevention paradigm, coadministration of ART26.12 (10 and 25 mg/kg BID for 15 days) with oxaliplatin prevented thermal hyperalgesia, mitigated mechanical allodynia, and attenuated OXA-induced weight loss. Multi-scale analyses revealed widespread lipid modulation, particularly among N-acyl amino acids in the spinal cord, including potential analgesic mediators. Additionally, ART26.12 administration led to upregulation of ion channels in the periaqueductal grey, and broad translational upregulation within the plasma proteome. These results show promise that ART26.12 is a safe and well-tolerated candidate for the treatment and prevention of OIPN through lipid modulation. PERSPECTIVE: Inhibition of fatty acid-binding protein 5 (FABP5) is a novel target for reducing pain associated with chemotherapy. ART26.12 is a safe and well-tolerated small molecule FABP5 inhibitor effective at preventing and reducing pain induced with oxaliplatin through lipid modulation and activation of cannabinoid receptors.

Novel Phenylene Lipids That Are Positive Allosteric Modulators of Glycine Receptors and Inhibitors of Glycine Transporter 2

Chronic pain is a complex condition that remains resistant to current therapeutics. We previously synthesized a series of N-acyl amino acids (NAAAs) that inhibit the glycine transporter, GlyT2, some of which are also positive allosteric modulators of glycine receptors (GlyRs). In this study, we have synthesized a library of NAAAs that contain a phenylene ring within the acyl tail with the objective of improving efficacy at both GlyT2 and GlyRs and also identifying compounds that are efficacious as dual-acting modulators to enhance glycine neurotransmission. The most efficacious positive allosteric modulator of GlyRs was 2-[8-(2-octylphenyl)octanoylamino]acetic acid (8-8 OPGly) which potentiates the EC5 for glycine activation of GlyRα1 by 1500% with an EC50 of 664 nM. Phenylene-containing NAAAs with a lysine headgroup were the most potent inhibitors of GlyT2 with (2S)-6-amino-2-[8-(3-octylphenyl)octanoylamino]hexanoic acid (8-8 MPLys) inhibiting GlyT2 with an IC50 of 32 nM. The optimal modulator across both proteins was (2S)-6-amino-2-[8-(2-octylphenyl)octanoylamino]hexanoic acid (8-8 OPLys), which inhibits GlyT2 with an IC50 of 192 nM and potentiates GlyRs by up to 335% at 1 μM. When tested in a dual GlyT2/GlyRα1 expression system, 8-8 OPLys caused the greatest reductions in the EC50 for glycine. This suggests that the synergistic effects of a dual-acting modulator cause greater enhancements in glycinergic activity compared to single-target modulators and may provide an alternate approach to the development of new non-opioid analgesics for the treatment of chronic pain.

Channa striatus in inflammatory conditions: A systematic review

Channa striatus (CS), or snakehead murrel, is an obligate air-breathing freshwater fish. Besides its wound healing properties, CS has also been reported to exhibit anti-inflammatory effects in multiple studies. While there are anti-inflammatory medications such as nonsteroidal anti-inflammatory drugs (NSAIDs), their long-term use is associated with an increased risk of peptic ulcers, acute renal failure, stroke, and myocardial infarction. Thus, it is essential to look at natural methods such as CS extract. While there is an abundant number of investigative studies on the inflammatory properties of CS, the quality of these studies has not been evaluated effectively. Thus, this review aims to summarise, evaluate, and critically appraise currently available literature regarding the anti-inflammatory properties of CS extract. This is done by performing a search using four databases, namely Google Scholar, Embase via Elsevier, Scopus, and Web of Science, with the following terms: Channa striatus AND inflammation. From our review, CS has been experimentally shown to positively affect inflammatory conditions such as gastric ulcers, dermatitis, osteoarthritis, and allergic rhinitis. Beneficial effects were also found on inflammation in the presence of tuberculosis and in situations that involve inflammation, such as wound healing. While CS clearly has potential for treating inflammatory conditions, much work needs to be done on identifying and isolating the active constituents before exact mechanisms of action can be worked out to develop future anti-inflammatory medications.

Positive Allosteric Modulators of Glycine Receptors and Their Potential Use in Pain Therapies

Glycine receptors are ligand-gated ion channels that mediate synaptic inhibition throughout the mammalian spinal cord, brainstem, and higher brain regions. They have recently emerged as promising targets for novel pain therapies due to their ability to produce antinociception by inhibiting nociceptive signals within the dorsal horn of the spinal cord. This has greatly enhanced the interest in developing positive allosteric modulators of glycine receptors. Several pharmaceutical companies and research facilities have attempted to identify new therapeutic leads by conducting large-scale screens of compound libraries, screening new derivatives from natural sources, or synthesizing novel compounds that mimic endogenous compounds with antinociceptive activity. Advances in structural techniques have also led to the publication of multiple high-resolution structures of the receptor, highlighting novel allosteric binding sites and providing additional information for previously identified binding sites. This has greatly enhanced our understanding of the functional properties of glycine receptors and expanded the structure activity relationships of novel pharmacophores. Despite this, glycine receptors are yet to be used as drug targets due to the difficulties in obtaining potent, selective modulators with favorable pharmacokinetic profiles that are devoid of side effects. This review presents a summary of the structural basis for how current compounds cause positive allosteric modulation of glycine receptors and discusses their therapeutic potential as analgesics. SIGNIFICANCE STATEMENT: Chronic pain is a major cause of disability, and in Western societies, this will only increase as the population ages. Despite the high level of prevalence and enormous socioeconomic burden incurred, treatment of chronic pain remains limited as it is often refractory to current analgesics, such as opioids. The National Institute for Drug Abuse has set finding effective, safe, nonaddictive strategies to manage chronic pain as their top priority. Positive allosteric modulators of glycine receptors may provide a therapeutic option.

Advanced Meditation and Vegan Diet Increased Acylglycines and Reduced Lipids Associated with Improved Health: A Prospective Longitudinal Study

Introduction: Samyama is an Isha Yoga 8-day residential meditation/yoga retreat combined with 60 days of preparation with vegan diet. We showed earlier Samyama retreat was associated with lower systemic inflammation and favorable lipid profiles along with other physical and mental health benefits. There is no mechanistic study on the impact of an advanced meditative process on multiple blood lipids and their implications on meditation-related improved physical and mental wellbeing. Methods: Sixty-four Samyama participants on vegan diet had blood sampled immediately before and immediately after the 8-day retreat for lipidomic analysis. The complex plasma lipidome was characterized using high-resolution mass spectrometric analysis and tandem mass spectrometry. Results: Pre- and post-Samyama blood samples of 64 Samyama participants were analyzed. Acylglycines (acetyl, propionyl, butyryl, and valeryl) were increased in the plasma post-Samyama compared with pre-Samyama (p < 0.001). Levels of glycerophosphocholines, glycerophosphoethanolamines, di-unsaturated ethanolamine plasmalogens, cholesterol esters, acylcarnitines, and acylgylcerines (triacylglycerols and diacylglycerols) decreased after the Samyama meditation. Plasma levels of glycerophosphoserines or glycerophosphoinositols were unchanged. Conclusion: An 8-day advanced meditation retreat resulted in increased acylglycines, an endocannabinoid-like fatty acid amide associated with increased cellular anandamide levels, anti-inflammation, analgesia, and vascular relaxation. Other serum lipid levels, including some that are associated with increased risk of atherosclerosis, were reduced following the Samyama program. ClinicalTrials.gov Registration: Identifier: NCT04366544.

Glycine Transporter 2: Mechanism and Allosteric Modulation

Neurotransmitter sodium symporters (NSS) are a subfamily of SLC6 transporters responsible for regulating neurotransmitter signalling. They are a major target for psychoactive substances including antidepressants and drugs of abuse, prompting substantial research into their modulation and structure-function dynamics. Recently, a series of allosteric transport inhibitors have been identified, which may reduce side effect profiles, compared to orthosteric inhibitors. Allosteric inhibitors are also likely to provide different clearance kinetics compared to competitive inhibitors and potentially better clinical outcomes. Crystal structures and homology models have identified several allosteric modulatory sites on NSS including the vestibule allosteric site (VAS), lipid allosteric site (LAS) and cholesterol binding site (CHOL1). Whilst the architecture of eukaryotic NSS is generally well conserved there are differences in regions that form the VAS, LAS, and CHOL1. Here, we describe ligand-protein interactions that stabilize binding in each allosteric site and explore how differences between transporters could be exploited to generate NSS specific compounds with an emphasis on GlyT2 modulation.

Endocannabinoid signaling pathways: beyond CB1R and CB2R

The search for cannabinoid receptors other than CB1R and CB2R has been ongoing for over a decade. A number of orphan receptors have been proposed as potential cannabinoid receptors primarily based on phylogenic arguments and reactivity towards known endocannabinoids and phytocannabinoids. Seven putative cannabinoid receptors are described and discussed, and evidence for and against their inclusion in this category are presented.

Function and therapeutic potential of N-acyl amino acids

N-acyl amino acids (NAAs) are amphiphilic molecules, with different potential fatty acid and head group moieties. NAAs are the largest family of anandamide congener lipids discovered to date. In recent years, several NAAs have been identified as potential ligands, engaging novel binding sites and mechanisms for modulation of membrane proteins such as G-protein coupled receptors (GPRs), nuclear receptors, ion channels, and transporters. NAAs play a key role in a variety of physiological functions as lipid signaling molecules. Understanding the structure, function roles, and pharmacological potential of these NAAs is still in its infancy, and the biochemical roles are also mostly unknown. This review will provide a summary of the literature on NAAs and emphasize their therapeutic potential.

Modulation of Glycinergic Neurotransmission may Contribute to the Analgesic Effects of Propacetamol

Treating neuropathic pain remains challenging, and therefore new pharmacological strategies are urgently required. Here, the enhancement of glycinergic neurotransmission by either facilitating glycine receptors (GlyR) or inhibiting glycine transporter (GlyT) function to increase extracellular glycine concentration appears promising. Propacetamol is a N,N-diethylester of acetaminophen, a non-opioid analgesic used to treat mild pain conditions. In vivo, it is hydrolysed into N,N-diethylglycine (DEG) and acetaminophen. DEG has structural similarities to known alternative GlyT1 substrates. In this study, we analyzed possible effects of propacetamol, or its metabolite N,N-diethylglycine (DEG), on GlyRs or GlyTs function by using a two-electrode voltage clamp approach in Xenopus laevis oocytes. Our data demonstrate that, although propacetamol or acetaminophen had no effect on the function of the analysed glycine-responsive proteins, the propacetamol metabolite DEG acted as a low-affine substrate for both GlyT1 (EC50 > 7.6 mM) and GlyT2 (EC50 > 5.2 mM). It also acted as a mild positive allosteric modulator of GlyRα1 function at intermediate concentrations. Taken together, our data show that DEG influences both glycine transporter and receptor function, and therefore could facilitate glycinergic neurotransmission in a multimodal manner.

A System for Assessing Dual Action Modulators of Glycine Transporters and Glycine Receptors

Reduced inhibitory glycinergic neurotransmission is implicated in a number of neurological conditions such as neuropathic pain, schizophrenia, epilepsy and hyperekplexia. Restoring glycinergic signalling may be an effective method of treating these pathologies. Glycine transporters (GlyTs) control synaptic and extra-synaptic glycine concentrations and slowing the reuptake of glycine using specific GlyT inhibitors will increase glycine extracellular concentrations and increase glycine receptor (GlyR) activation. Glycinergic neurotransmission can also be improved through positive allosteric modulation (PAM) of GlyRs. Despite efforts to manipulate this synapse, no therapeutics currently target it. We propose that dual action modulators of both GlyTs and GlyRs may show greater therapeutic potential than those targeting individual proteins. To show this, we have characterized a co-expression system in Xenopus laevis oocytes consisting of GlyT1 or GlyT2 co-expressed with GlyRα1. We use two electrode voltage clamp recording techniques to measure the impact of GlyTs on GlyRs and the effects of modulators of these proteins. We show that increases in GlyT density in close proximity to GlyRs diminish receptor currents. Reductions in GlyR mediated currents are not observed when non-transportable GlyR agonists are applied or when Na+ is not available. GlyTs reduce glycine concentrations across different concentration ranges, corresponding with their ion-coupling stoichiometry, and full receptor currents can be restored when GlyTs are blocked with selective inhibitors. We show that partial inhibition of GlyT2 and modest GlyRα1 potentiation using a dual action compound, is as useful in restoring GlyR currents as a full and potent single target GlyT2 inhibitor or single target GlyRα1 PAM. The co-expression system developed in this study will provide a robust means for assessing the likely impact of GlyR PAMs and GlyT inhibitors on glycine neurotransmission.

Potential metabolic and behavioural roles of the putative endocannabinoid receptors GPR18, GPR55 and GPR119 in feeding

Endocannabinoids are ancient biomolecules involved in several cellular (e.g., metabolism) and physiological (e.g., eating behaviour) functions. Indeed, eating behaviour alterations in marijuana users have led to investigate the orexigen-ic/anorexigenic effects of cannabinoids in animal/human models. This increasing body of research suggests that the endo-cannabinoid system plays an important role in feeding control. Accordingly, within the endocannabinoid system, canna-binoid receptors, enzymes and genes represent potential therapeutic targets for dealing with multiple metabolic and behav-ioural dysfunctions (e.g., obesity, anorexia, etc.). Paradoxically, our understanding on the endocannabinoid system as a cel-lular mediator is yet limited. For example: (i) only two cannabinoid receptors have been classified, but they are not enough to explain the pharmacological profile of several experimental effects induced by cannabinoids; and (ii) several orphan G pro-tein-coupled receptors (GPCRs) interact with cannabinoids and we do not know how to classify them (e.g., GPR18, GPR55 and GPR119; amongst others).

On this basis, the present review attempts to summarize the lines of evidence supporting the potential role of GPR18, GPR55 and GPR119 in metabolism and feeding control that may explain some of the divergent effects and puzzling data re-lated to cannabinoid research. Moreover, their therapeutic potential in feeding behaviour alterations will be considered.

Identification of an allosteric binding site on the human glycine transporter, GlyT2, for bioactive lipid analgesics

The treatment of chronic pain is poorly managed by current analgesics, and there is a need for new classes of drugs. We recently developed a series of bioactive lipids that inhibit the human glycine transporter GlyT2 (SLC6A5) and provide analgesia in animal models of pain. Here, we have used functional analysis of mutant transporters combined with molecular dynamics simulations of lipid-transporter interactions to understand how these bioactive lipids interact with GlyT2. This study identifies a novel extracellular allosteric modulator site formed by a crevice between transmembrane domains 5, 7, and 8, and extracellular loop 4 of GlyT2. Knowledge of this site could be exploited further in the development of drugs to treat pain, and to identify other allosteric modulators of the SLC6 family of transporters.

Glycine transporter inhibitors: A new avenue for managing neuropathic pain

Interneurons operating with glycine neurotransmitter are involved in the regulation of pain transmission in the dorsal horn of the spinal cord. In addition to interneurons, glycine release also occurs from glial cells neighboring glutamatergic synapses in the spinal cord. Neuronal and glial release of glycine is controlled by glycine transporters (GlyTs). Inhibitors of the two isoforms of GlyTs, the astrocytic type-1 (GlyT-1) and the neuronal type-2 (GlyT-2), decrease pain sensation evoked by injuries of peripheral sensory neurons or inflammation. The function of dorsal horn glycinergic interneurons has been suggested to be reduced in neuropathic pain, which can be reversed by GlyT-2 inhibitors (Org-25543, ALX1393). Several lines of evidence also support that peripheral nerve damage or inflammation may shift glutamatergic neurochemical transmission from N-methyl-D aspartate (NMDA) NR1/NR2A receptor- to NR1/NR2B receptor-mediated events (subunit switch). This pathological overactivation of NR1/NR2B receptors can be reduced by GlyT-1 inhibitors (NFPS, Org-25935), which decrease excessive glycine release from astroglial cells or by selective antagonists of NR2B subunits (ifenprodil, Ro 25-6981). Although several experiments suggest that GlyT inhibitors may represent a novel strategy in the control of neuropathic pain, proving this concept in human beings is hampered by lack of clinically applicable GlyT inhibitors. We also suggest that drugs inhibiting both GlyT-1 and GlyT-2 non-selectively and reversibly, may favorably target neuropathic pain. In this paper we overview inhibitors of the two isoforms of GlyTs as well as the effects of these drugs in experimental models of neuropathic pain. In addition, the possible mechanisms of action of the GlyT inhibitors, i.e. how they affect the neurochemical and pain transmission in the spinal cord, are also discussed. The growing evidence for the possible therapeutic intervention of neuropathic pain by GlyT inhibitors further urges development of drugable compounds, which may beneficially restore impaired pain transmission in various neuropathic conditions.

Purification, characterization, and gene cloning of a novel aminoacylase from Burkholderia sp. strain LP5_18B that efficiently catalyzes the synthesis of N-lauroyl-l-amino acids

An N-lauroyl-l-phenylalanine-producing bacterium, identified as Burkholderia sp. strain LP5_18B, was isolated from a soil sample. The enzyme was purified from the cell-free extract of the strain and shown to catalyze degradation and synthesis activities toward various N-acyl-amino acids. N-lauroyl-l-phenylalanine and N-lauroyl-l-arginine were obtained with especially high yields (51% and 89%, respectively) from lauric acid and l-phenylalanine or l-arginine by the purified enzyme in an aqueous system. The gene encoding the novel aminoacylase was cloned from Burkholderia sp. strain LP5_18B and expressed in Escherichia coli. The gene contains an open reading frame of 1,323 nucleotides. The deduced protein sequence encoded by the gene has approximately 80% amino acid identity to several hydratase of Burkholderia. The addition of zinc sulfate increased the aminoacylase activity of the recombinant E. coli strain.

Lipid-Based Inhibitors Act Directly on GlyT2

The endogenous lipids N-arachidonylglycine and oleoyl-l-carnitine are potential therapeutic leads in the treatment of chronic pain through their inhibition of the glycine transporter GlyT2. However, their mechanism of action is unknown. It has been hypothesized that these "bioactive" lipids either inhibit GlyT2 indirectly, by significantly perturbing the biophysical properties of the membrane, or directly, by binding directly to the transporter (either from a membrane-exposed or solvent-exposed binding site). Here, we used molecular dynamics simulations to study the effects of the lipids anandamide, N-arachidonylglycine, and oleoyl-l-carnitine on (a) the biophysical properties of the bilayer and (b) direct binding interactions with GlyT2. During the simulations, the biophysical properties of the bilayer itself, for example, the area per lipid, bilayer thickness, and order parameters, were not significantly altered by the presence or type of bioactive lipid, regardless of the presence of GlyT2. Our work, together with previous computational and experimental data, suggests that these acyl-inhibitors of GlyT2 inhibit the transporter by directly binding to it. However, these bioactive lipids bound to various parts of GlyT2 and did not prefer a single binding site during 4.5 μs of simulation. We postulate that the binding site is located at the solvent-exposed regions of GlyT2. Understanding the mechanism of action of these and related bioactive lipids is essential in effectively developing high-affinity GlyT2 inhibitors for the treatment of pain.

Protective Effect of N-Arachidonoyl Glycine-GPR18 Signaling after Excitotoxical Lesion in Murine Organotypic Hippocampal Slice Cultures

N-arachidonoyl glycine (NAGly) is an endocannabinoid involved in the regulation of different immune cells. It was shown to activate the GPR18 receptor, which was postulated to switch macrophages from cytotoxic to reparative. To study GPR18 expression and neuroprotection after NAGly treatment we used excitotoxically lesioned organotypic hippocampal slice cultures (OHSC). The effect of NAGly was also tested in isolated microglia and astrocytes as these cells play a crucial role during neuronal injury. In the present study, the GPR18 receptor was found in OHSC at mRNA level and was downregulated after N-Methyl-D-aspartate (NMDA) treatment at a single time point. Furthermore, treatment with NAGly reduced neuronal damage and this effect was abolished by GPR18 and cannabinoid receptor (CB)₂ receptor antagonists. The activation but not motility of primary microglia and astrocytes was influenced when incubated with NAGly. However, NAGly alone reduced the phosphorylation of Akt but no changes in activation of the p44/42 and p38 MAPK and CREB pathways in BV2 cells could be observed. Given NAGly mediated actions we speculate that GPR18 and its ligand NAGly are modulators of glial and neuronal cells during neuronal damage.

Development of an N-Acyl Amino Acid That Selectively Inhibits the Glycine Transporter 2 To Produce Analgesia in a Rat Model of Chronic Pain

Inhibitors that target the glycine transporter 2, GlyT2, show promise as analgesics, but may be limited by their toxicity through complete or irreversible binding. Acyl-glycine inhibitors, however, are selective for GlyT2 and have been shown to provide analgesia in animal models of pain with minimal side effects, but are comparatively weak GlyT2 inhibitors. Here, we modify the simple acyl-glycine by synthesizing lipid analogues with a range of amino acid head groups in both l- and d-configurations, to produce nanomolar affinity, selective GlyT2 inhibitors. The potent inhibitor oleoyl-d-lysine (33) is also resistant to degradation in both human and rat plasma and liver microsomes, and is rapidly absorbed following an intraperitoneal injection to rats and readily crosses the blood-brain barrier. We demonstrate that 33 provides greater analgesia at lower doses, and does not possess the severe side effects of the very slowly reversible GlyT2 inhibitor, ORG25543 (2).

Some Prospective Alternatives for Treating Pain: The Endocannabinoid System and Its Putative Receptors GPR18 and GPR55

Background: Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment. Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB₁ and CB₂) have been deeply studied and classified. Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB₁/CB₂ receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors. Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors. According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration.

Methods: This article reviews select relevant information about the potential role of GPR18 and GPR55 in the pathophysiology of pain.

Results: This work summarized novel data supporting that, besides cannabinoid CB₁ and CB₂ receptors, GPR18 and GPR55 may be useful for pain treatment.

Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.

The role of orphan G protein-coupled receptors in the modulation of pain: A review

G protein-coupled receptors (GPCRs) comprise a large number of receptors. Orphan GPCRs are divided into six families. These groups contain orphan receptors for which the endogenous ligands are unclear. They have various physiological effects in the body and have the potential to be used in the treatment of different diseases. Considering their important role in the central and peripheral nervous system, their role in the treatment of pain has been the subject of some recent studies. At present, there are effective therapeutics for the treatment of pain including opioid medications and non-steroidal anti-inflammatory drugs. However, the side effects of these drugs and the risks of tolerance and dependence remain a major problem. In addition, neuropathic pain is a condition that does not respond to currently available analgesic medications well. In the present review article, we aimed to review the most recent findings regarding the role of orphan GPCRs in the treatment of pain. Accordingly, based on the preclinical findings, the role of GPR3, GPR7, GPR8, GPR18, GPR30, GPR35, GPR40, GPR55, GPR74, and GPR147 in the treatment of pain was discussed. The present study highlights the role of orphan GPCRs in the modulation of pain and implies that these receptors are potential new targets for finding better and more efficient therapeutics for the management of pain particularly neuropathic pain.

Ablation of PM20D1 reveals N-acyl amino acid control of metabolism and nociception

Bioactive lipids control a wide variety of physiologic processes. We have recently identified a branch of bioactive lipid signaling mediated by N-acyl amino acids (NAAs) and the circulating enzyme peptidase M20 domain-containing 1 (PM20D1). Here we generate and characterize mice globally deficient in PM20D1. These PM20D1-KO mice have bidirectional changes in NAA levels in blood and tissues and exhibit a variety of metabolic and nociceptive phenotypes. Our findings elucidate the endogenous physiologic functions for NAA signaling in vivo and suggest PM20D1 inhibitors might be useful for the treatment of pain.

N-acyl amino acids (NAAs) are a structurally diverse class of bioactive signaling lipids whose endogenous functions have largely remained uncharacterized. To clarify the physiologic roles of NAAs, we generated mice deficient in the circulating enzyme peptidase M20 domain-containing 1 (PM20D1). Global PM20D1-KO mice have dramatically reduced NAA hydrolase/synthase activities in tissues and blood with concomitant bidirectional dysregulation of endogenous NAAs. Compared with control animals, PM20D1-KO mice exhibit a variety of metabolic and pain phenotypes, including insulin resistance, altered body temperature in cold, and antinociceptive behaviors. Guided by these phenotypes, we identify N-oleoyl-glutamine (C18:1-Gln) as a key PM20D1-regulated NAA. In addition to its mitochondrial uncoupling bioactivity, C18:1-Gln also antagonizes certain members of the transient receptor potential (TRP) calcium channels including TRPV1. Direct administration of C18:1-Gln to mice is sufficient to recapitulate a subset of phenotypes observed in PM20D1-KO animals. These data demonstrate that PM20D1 is a dominant enzymatic regulator of NAA levels in vivo and elucidate physiologic functions for NAA signaling in metabolism and nociception.

Risk factors for vitamin D deficiency in sickle cell disease

Vitamin D deficiency (VDD), 25-OHD levels <20 ng/ml, is prevalent among patients with sickle cell disease (SCD) and is linked to acute and chronic pain and bone fracture in this population. There is limited literature regarding VDD-associated risk factors for SCD. We examined potential clinical and genomic parameters associated with VDD in 335 adults with SCD in a cross-sectional study. VDD was present in 65% of adult SCD patients, and 25-OHD levels independently and positively correlated with older age (P < 0·001) and vitamin D supplementation (P < 0·001). 25-OHD levels were higher in SCD patients over 40 years of age compared to the general African-American population. Both lower 25-OHD levels and increased pain frequency were associated with increased expression of SLC6A5 encoding glycine transporter-2 (GlyT2), a protein involved in neuronal pain pathways. Lower 25-OHD levels were also associated with increased expression of CYP3A4, and with decreased expression of GC (also termed DBP) and VDR, three genes involved in vitamin D metabolism. We conclude that vitamin D supplementation should be an almost universal feature of the care of young adults with SCD, and that further research is warranted into genomic factors that regulate vitamin D metabolism in SCD.

Activity of novel lipid glycine transporter inhibitors on synaptic signalling in the dorsal horn of the spinal cord

BACKGROUND AND PURPOSE

Inhibitory neurotransmission plays an important role in controlling excitability within nociceptive circuits of the spinal cord dorsal horn. Loss of inhibitory signalling is thought to contribute to the development of pathological pain. Preclinical studies suggest that increasing inhibitory glycinergic signalling is a good therapeutic strategy for treating pain. One approach to increase synaptic glycine is to inhibit the activity of the glycine transporter 2 (GlyT2) on inhibitory nerve terminals. These transporters are involved in regulating glycine concentrations and recycling glycine into presynaptic terminals. Inhibiting activity of GlyT2 increases synaptic glycine, which decreases excitability in nociceptive circuits and provides analgesia in neuropathic and inflammatory pain models.

EXPERIMENTAL APPROACH

We investigated the effects of reversible and irreversible GlyT2 inhibitors on inhibitory glycinergic and NMDA receptor-mediated excitatory neurotransmission in the rat dorsal horn. The effect of these drugs on synaptic signalling was determined using patch-clamp electrophysiology techniques to measure glycine- and NMDA-mediated postsynaptic currents in spinal cord slices in vitro.

KEY RESULTS

We compared activity of four compounds that increase glycinergic tone with a corresponding increase in evoked glycinergic postsynaptic currents. These compounds did not deplete synaptic glycine release over time. Interestingly, none of these compounds increased glycine-mediated excitatory signalling through NMDA receptors. The results suggest that these compounds preferentially inhibit GlyT2 over G1yT1 with no potentiation of the glycine receptor and without inducing spillover from inhibitory to excitatory synapses.

CONCLUSIONS AND IMPLICATIONS

GlyT2 inhibitors increase inhibitory neurotransmission in the dorsal horn and have potential as pain therapeutics.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

N-FATTY ACYLGLYCINES: UNDERAPPRECIATED ENDOCANNABINOID-LIKE FATTY ACID AMIDES?

Long-chain N-fatty acylglycines, R-CO-NH-CH2-COOH (where "R" refers to an unsaturated or saturated alkyl chain of at least 14 carbons) are found in mammals and insects and are structurally related to the cell-signaling, lipid-like, N-fatty acylethanolamines, R-CO-NH-CH2-CH2-OH (where "R" refers to an alkyl chain of at least 14 carbons). Accumulating evidence demonstrates that the N-fatty acylglycines have important cellular functions, but much work remains in order to fully appreciate and understand these biomolecules including: (a) more work on their functions in vivo, (b) measuring their concentrations in the cell, (c) defining the pathways for the biosynthesis and degradation, and (d) understanding the metabolic interconversion(s) between the N-fatty acylglycines and other fatty acid amides. The purpose of reviewing the current state-of-knowledge about the N-fatty acylglycines is to stimulate future research about this intriguing family of biomolecules.

Modulation of Glycine-Mediated Spinal Neurotransmission for the Treatment of Chronic Pain

Chronic pain constitutes a significant and expanding worldwide health crisis. Currently available analgesics poorly serve individuals suffering from chronic pain, and new therapeutic agents that are more effective, safer, and devoid of abuse liabilities are desperately needed. Among the myriad of cellular and molecular processes contributing to chronic pain, spinal disinhibition of pain signaling to higher cortical centers plays a critical role. Accumulating evidence shows that glycinergic inhibitory neurotransmission in the spinal cord dorsal horn gates nociceptive signaling, is essential in maintaining physiological pain sensitivity, and is diminished in pathological pain states. Thus, it is hypothesized that agents capable of enhancing glycinergic tone within the dorsal horn could obtund nociceptor signaling to the brain and serve as analgesics for persistent pain. This Perspective highlights the potential that pharmacotherapies capable of increasing inhibitory spinal glycinergic neurotransmission hold in providing new and transformative analgesic therapies for the treatment of chronic pain.

Synthesis and Characterization of Novel Acyl-Glycine Inhibitors of GlyT2

It has been demonstrated previously that the endogenous compound N-arachidonyl-glycine inhibits the glycine transporter GlyT2, stimulates glycinergic neurotransmission, and provides analgesia in animal models of neuropathic and inflammatory pain. However, it is a relatively weak inhibitor with an IC₅₀ of 9 μM and is subject to oxidation via cyclooxygenase, limiting its therapeutic value. In this paper we describe the synthesis and testing of a novel series of monounsaturated C18 and C16 acyl-glycine molecules as inhibitors of the glycine transporter GlyT2. We demonstrate that they are up to 28 fold more potent that N-arachidonyl-glycine with no activity at the closely related GlyT1 transporter at concentrations up to 30 μM. This novel class of compounds show considerable promise as a first generation of GlyT2 transport inhibitors.

Glycine transporter2 inhibitors: Getting the balance right

Neurotransmitter transporters are targets for a wide range of therapeutically useful drugs. This is because they have the capacity to selectively manipulate the dynamics of neurotransmitter concentrations and thereby enhance or diminish signalling through particular brain pathways. High affinity glycine transporters (GlyTs) regulate extracellular concentrations of glycine and provide novel therapeutic targets for neurological disorders.

G protein coupled receptor 18: A potential role for endocannabinoid signaling in metabolic dysfunction

Endocannabinoids are products of dietary fatty acids that are modulated by an alteration in food intake levels. Overweight and obese individuals have substantially higher circulating levels of the arachidonic acid derived endocannabinoids, anandamide and 2-arachidonoyl glycerol, and show an altered pattern of cannabinoid receptor expression. These cannabinoid receptors are part of a large family of G protein coupled receptors (GPCRs). GPCRs are major therapeutic targets for various diseases within the cardiovascular, neurological, gastrointestinal, and endocrine systems, as well as metabolic disorders such as obesity and type 2 diabetes mellitus. Obesity is considered a state of chronic low-grade inflammation elicited by an immunological response. Interestingly, the newly deorphanized GPCR (GPR18), which is considered to be a putative cannabinoid receptor, is proposed to have an immunological function. In this review, the current scientific knowledge on GPR18 is explored including its localization, signaling pathways, and pharmacology. Importantly, the involvement of nutritional factors and potential dietary regulation of GPR18 and its (patho)physiological roles are described. Further research on this receptor and its regulation will enable a better understanding of the complex mechanisms of GPR18 and its potential as a novel therapeutic target for treating metabolic disorders.

Antinociceptive effect of intracerebroventricular administration of glycine transporter-2 inhibitor ALX1393 in rat models of inflammatory and neuropathic pain

Glycinergic transmission has an important role in regulating nociception in the spinal cord. The glycine transporter-2 (GlyT2) is localized at presynaptic terminals of glycinergic neurons and eliminates glycine from the synaptic cleft to terminate glycinergic transmission. Systemic and intrathecal administration of GlyT2 inhibitors alleviate various types of pain. Although the GlyT2s and glycine receptors are widely distributed in the central nervous system, little is known about the role of glycinergic transmission in pain perception at supraspinal regions. The present study examined the antinociceptive effect of intracerebroventricular (i.c.v.) administration of the selective GlyT2 inhibitor ALX1393 on inflammatory and neuropathic pain in experimental models. For i.c.v. administration, a guide cannula was implanted into the right lateral ventricle of male Sprague-Dawley rats. Normal rats were used to assess inflammatory nociception using the formalin test and motor function using the rotarod test. Chronic constriction injury (CCI) to the sciatic nerve was induced in the rats. The CCI rats were then used to assess mechanical, cold, and thermal hyperalgesia using the electronic von Frey test, cold plate test, and the plantar test, respectively. ALX1393 (25, 50, and 100 μg) was administered i.c.v. to examine its effects on supraspinal antinociception. Supraspinal ALX1393 in normal rats suppressed the late-phase response in the formalin test but did not affect motor performance. In the CCI rats, ALX1393 inhibited mechanical and cold hyperalgesia in a dose-dependent manner. The antihyperalgesic effects of ALX1393 (100 μg) were reversed completely by i.c.v. pretreatment with a glycine receptor antagonist strychnine (10 μg). These results suggest that GlyT2 contributes to nociceptive transmission at supraspinal level and that the selective GlyT2 inhibitor is a promising candidate for the treatment of inflammatory and neuropathic pain without causing motor dysfunction.

Reversible inhibition of the glycine transporter GlyT2 circumvents acute toxicity while preserving efficacy in the treatment of pain

BACKGROUND AND PURPOSE

Available medications for chronic pain provide only partial relief and often cause unacceptable side effects. There is therefore a need for novel molecular targets to develop new therapeutics with improved efficacy and tolerability. Despite encouraging efficacy data in rodents with inhibitors of the neuronal glycine transporter-2 (GlyT2), there are also some reports of toxicity and their development was discontinued.

EXPERIMENTAL APPROACH

In order to clarify the possibility of targeting GlyT2 for the treatment of pain, we have used an integrated approach comprising in vitro pharmacology, selectivity, bioavailability, in vivo efficacy and safety assessment to analyse the properties and efficacy of ALX-1393 and Org-25543, the two published GlyT2 inhibitors from which in vivo data are available.

KEY RESULTS

We report that these compounds have a different set of undesirable properties that limit their usefulness as pharmacological tools. Importantly, we discover that inhibitors of GlyT2 can exert an apparent reversible or irreversible inhibition of the transporter and describe a new class of reversible GlyT2 inhibitors that preserves efficacy while avoiding acute toxicity.

CONCLUSIONS AND IMPLICATIONS

Our pharmacological comparison of two closely related GlyT2 inhibitors with different modes of inhibition provides important insights into their safety and efficacy profiles, uncovering that in the presence of a GlyT2 mechanism-based toxicity, reversible inhibitors might allow a tolerable balance between efficacy and toxicity. These findings shed light into the drawbacks associated with the early GlyT2 inhibitors and describe a new mechanism that might serve as the starting point for new drug development.

Glycine transport inhibitors for the treatment of pain

Opioids, local anesthetics, anticonvulsant drugs, antidepressants, and non-steroidal anti-inflammatory drugs (NSAIDs) are used to provide pain relief but they do not provide adequate pain relief in a large proportion of chronic pain patients and are often associated with unacceptable side effects. Inhibitory glycinergic neurotransmission is impaired in chronic pain states, and this provides a novel target for drug development. Inhibitors of the glycine transporter 2 (GlyT2) enhance inhibitory neurotransmission and show particular promise for the treatment of neuropathic pain. N-arachidonyl-glycine (NAGly) is an endogenous lipid that inhibits glycine transport by GlyT2 and also shows potential as an analgesic, which may be further exploited in drug development. In this review we discuss the role of glycine neurotransmission in chronic pain and future prospects for the use of glycine transport inhibitors in the treatment of pain.

Modulation of T-type calcium channels by bioactive lipids

T-type calcium channels (T-channels/CaV3) have unique biophysical properties allowing a calcium influx at resting membrane potential of most cells. T-channels are ubiquitously expressed in many tissues and contribute to low-threshold spikes and burst firing in central neurons as well as to pacemaker activities in cardiac cells. They also emerged as potential targets to treat cancer and hypertension. Regulation of these channels appears complex, and several studies have indicated that CaV3.1, CaV3.2, and CaV3.3 currents are directly inhibited by multiple endogenous lipids independently of membrane receptors or intracellular pathways. These bioactive lipids include arachidonic acid and ω3 poly-unsaturated fatty acids; the endocannabinoid anandamide and other N-acylethanolamides; the lipoamino-acids and lipo-neurotransmitters; the P450 epoxygenase metabolite 5,6-epoxyeicosatrienoic acid; as well as similar molecules with 18-22 carbons in the alkyl chain. In this review, we summarize evidence for direct effects of these signaling molecules, the molecular mechanisms underlying the current inhibition, and the involved chemical features. The impact of this modulation in physiology and pathophysiology is discussed with a special emphasis on pain aspects and vasodilation. Overall, these data clearly indicate that T-current inhibition is an important mechanism by which bioactive lipids mediate their physiological functions.

Lipid inhibitors of high affinity glycine transporters: identification of a novel class of analgesics

Glycine plays a key role in regulating inhibitory neurotransmission in the spinal cord and concentrations of glycine in the CNS are regulated by two subtypes of high affinity glycine transporters, GlyT1 and GlyT2. In this mini review we will discuss a series of lipid inhibitors of GlyT2 that show promise as analgesics in the treatment of neuropathic and inflammatory pain. N-arachidonyl-glycine inhibits the rate of transport by GlyT2, but has very little or no activity on GlyT1. We will discuss structure-activity studies of the actions of related lipids on GlyT2 and also the characterization of a more potent lipid inhibitor of GlyT2, oleoyl-l-carnitine. Both N-arachidonyl-glycine and oleoyl-l-carnitine show specificity for GlyT2 over GlyT1, which has allowed the use of chimeric GlyT1/GlyT2 transporters to begin characterizing the molecular basis for specificity and mechanism of action of these lipid inhibitors. Although our understanding of the molecular basis for lipid inhibition is still in its infancy, it appears that extracellular loop 4 of GlyT2 plays an important role in the inhibitory mechanism.

Do N-arachidonyl-glycine (NA-glycine) and 2-arachidonoyl glycerol (2-AG) share mode of action and the binding site on the β2 subunit of GABAA receptors?

NA-glycine is an endogenous lipid molecule with analgesic properties, which is structurally similar to the endocannabinoids 2-AG and anandamide but does not interact with cannabinoid receptors. NA-glycine has been suggested to act at the G-protein coupled receptors GPR18 and GPR92. Recently, we have described that NA-glycine can also modulate recombinant α₁β₂γ₂ GABA_A receptors. Here we characterize in more detail this modulation and investigate the relationship of its binding site with that of the endocannabinoid 2-AG.

Oleoyl-L-carnitine inhibits glycine transport by GlyT2

BACKGROUND AND PURPOSE

Concentrations of extracellular glycine in the CNS are regulated by two Na(+)/Cl(-) -dependent glycine transporters, GlyT1 and GlyT2. Selective inhibitors of GlyT1 have been developed for the treatment of schizophrenia, whilst selective inhibitors of GlyT2 are analgesic in animal models of pain. We have assessed a series of endogenous lipids as inhibitors of GlyT1 and GlyT2.

EXPERIMENTAL APPROACH

Human GlyT1 and GlyT2 were expressed in Xenopus laevis oocytes, and the inhibitory actions of a series of acylcarnitines on glycine transport were measured using electrophysiological techniques.

KEY RESULTS

Oleoyl-L-carnitine inhibited glycine transport by GlyT2, with an IC(50) of 340 nM, which is 15-fold more potent than the previously identified lipid inhibitor N-arachidonyl-glycine. Oleoyl-L-carnitine had a slow onset of inhibition and a slow washout. Using a series of chimeric GlyT1/2 transporters and point mutant transporters, we have identified an isoleucine residue in extracellular loop 4 of GlyT2 that conferred differences in sensitivity to oleoyl-L-carnitine between GlyT2 and GlyT1.

CONCLUSIONS AND IMPLICATIONS

Oleoyl-L-carnitine is a potent non-competitive inhibitor of GlyT2. Previously identified GlyT2 inhibitors show potential as analgesics and the identification of oleoyl-L-carnitine as a novel GlyT2 inhibitor may lead to new ways of treating pain.

Inhibition of human recombinant T-type calcium channels by N-arachidonoyl 5-HT

BACKGROUND AND PURPOSE

N-arachidonoyl 5-HT (NA-5HT) has anti-nociceptive effects reported to be mediated by inhibitory actions at the transient receptor potential vanilloid receptor 1 (TRPV1) and fatty acid amide hydrolase (FAAH). Anandamide and N-arachidonoyl dopamine (NA-DA), endocannabinoids that activate TRPV1 or are metabolized by FAAH, also inhibit T-type calcium channels (I(Ca) ). T-type I(Ca) are expressed by many excitable cells, including neurons involved in pain detection and processing. We sought to determine whether NA-5HT also modulates T-type I(Ca) .

EXPERIMENTAL APPROACH

Human recombinant T-type I(Ca) (Ca(V) 3 channels) expressed in HEK 293 cells were examined using standard whole-cell voltage-clamp electrophysiology techniques.

KEY RESULTS

NA-5HT completely inhibited Ca(V) 3 channels with a rank order of potency (pEC(50) ) of Ca(V) 3.1 (7.4) > Ca(V) 3.3 (6.8) ≥ Ca(V) 3.2 (6.6). The effects of NA-5HT were voltage-dependent, and it produced significant hyperpolarizing shifts in Ca(V) 3 steady-state inactivation relationships. NA-5HT selectively affected Ca(V) 3.3 channel kinetics.

CONCLUSIONS AND IMPLICATIONS

NA-5HT increases the steady-state inactivation of Ca(V) 3 channels, reducing the number of channels available to open during depolarization. These effects occur at NA-5HT concentrations at or below those at which NA-5HT affects TRPV1 receptors and FAAH. NA-5HT is one of the most potent inhibitors of T-type I(Ca) described to date, and it is likely to exert some of its biological effects, including anti-nociception, via inhibition of these channels.

Role of cannabinoids in the treatment of pain and (painful) spasticity

Both the discovery of the endocannabinoid system (ECS) and its role in the control of pain and habituation to stress, as well as the significant analgesic and antihyperalgesic effects in animal studies, suggest the usefulness of cannabinoids in pain conditions. However, in human experimental or clinical trials, no convincing reduction of acute pain, which may be caused by a pronociceptive, ECS-triggered mechanism on the level of the spinal cord, has been demonstrated. In contrast, in chronic pain and (painful) spasticity, an increasing number of randomized, double-blind, placebo-controlled studies have shown the efficacy of cannabinoids, which is combined with a narrow therapeutic index. Patients with unsatisfactory response to other methods of pain therapy and who were characterized by failed stress adaptation particularly benefited from treatment with cannabinoids. None of the attempts to overcome the disadvantage of the narrow therapeutic index, either by changing the route of application or by formulating balanced cannabinoid preparations, have resulted in a major breakthrough. Therefore, different methods of administration and other types of cannabinoids, such as endocannabinoid modulators, should be tested in future trials.

N-arachidonyl-glycine modulates synaptic transmission in superficial dorsal horn

BACKGROUND AND PURPOSE

The arachidonyl-amino acid N-arachidonyl-glycine (NAGly) is an endogenous lipid, generated within the spinal cord and producing spinally mediated analgesia via non-cannabinoid mechanisms. In this study we examined the actions of NAGly on neurons within the superficial dorsal horn, a key site for the actions of many analgesic agents.

EXPERIMENTAL APPROACH

Whole cell patch clamp recordings were made from lamina II neurons in rat spinal cord slices to examine the effect of NAGly on glycinergic and NMDA-mediated synaptic transmission.

KEY RESULTS

N-arachidonyl-glycine prolonged the decay of glycine, but not β-alanine induced inward currents and decreased the amplitude of currents induced by both glycine and β-alanine. NAGly and ALX-1393 (inhibitor of the glycine transporter, GLYT2), but not the GLYT1 inhibitor, ALX-5407, produced a strychnine-sensitive inward current. ALX-5407 and ALX-1393, but not NAGly prolonged the decay phase of glycine receptor-mediated miniature inhibitory postsynaptic currents (IPSCs). NAGly prolonged the decay phase of evoked IPSCs, although to a lesser extent than ALX-5407 and ALX-1393. In the presence of ALX-1393, NAGly shortened the decay phase of evoked IPSCs. ALX-5407 increased and NAGly decreased the amplitude of evoked NMDA-mediated excitatory postsynaptic currents.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that NAGly enhanced inhibitory glycinergic synaptic transmission within the superficial dorsal horn by blocking glycine uptake via GLYT2. In addition, NAGly decreased excitatory NMDA-mediated synaptic transmission. Together, these findings provide a cellular explanation for the spinal analgesic actions of NAGly.

N-acyl amino acids and N-acyl neurotransmitter conjugates: neuromodulators and probes for new drug targets

The myriad functions of lipids as signalling molecules is one of the most interesting fields in contemporary pharmacology, with a host of compounds recognized as mediators of communication within and between cells. The N-acyl conjugates of amino acids and neurotransmitters (NAANs) have recently come to prominence because of their potential roles in the nervous system, vasculature and the immune system. NAAN are compounds such as glycine, GABA or dopamine conjugated with long chain fatty acids. More than 70 endogenous NAAN have been reported although their physiological role remains uncertain, with various NAAN interacting with a low affinity at G protein coupled receptors (GPCR) and ion channels. Regardless of their potential physiological function, NAAN are of great interest to pharmacologists because of their potential as flexible tools to probe new sites on GPCRs, transporters and ion channels. NAANs are amphipathic molecules, with a wide variety of potential fatty acid and headgroup moieties, a combination which provides a rich source of potential ligands engaging novel binding sites and mechanisms for modulation of membrane proteins such as GPCRs, ion channels and transporters. The unique actions of subsets of NAAN on voltage-gated calcium channels and glycine transporters indicate that the wide variety of NAAN may provide a readily exploitable resource for defining new pharmacological targets. Investigation of the physiological roles and pharmacological potential of these simple lipid conjugates is in its infancy, and we believe that there is much to be learnt from their careful study.

Fatty acid amide signaling molecules

Key studies leading to the discovery and definition of the role of endogenous fatty acid amide signaling molecules are summarized.

T-type calcium channel inhibition underlies the analgesic effects of the endogenous lipoamino acids

Lipoamino acids are anandamide-related endogenous molecules that induce analgesia via unresolved mechanisms. Here, we provide evidence that the T-type/Cav3 calcium channels are important pharmacological targets underlying their physiological effects. Various lipoamino acids, including N-arachidonoyl glycine (NAGly), reversibly inhibited Cav3.1, Cav3.2, and Cav3.3 currents, with potent effects on Cav3.2 [EC(50) approximately 200 nm for N-arachidonoyl 3-OH-gamma-aminobutyric acid (NAGABA-OH)]. This inhibition involved a large shift in the Cav3.2 steady-state inactivation and persisted during fatty acid amide hydrolase (FAAH) inhibition as well as in cell-free outside-out patch. In contrast, lipoamino acids had weak effects on high-voltage-activated (HVA) Cav1.2 and Cav2.2 calcium currents, on Nav1.7 and Nav1.8 sodium currents, and on anandamide-sensitive TRPV1 and TASK1 currents. Accordingly, lipoamino acids strongly inhibited native Cav3.2 currents in sensory neurons with small effects on sodium and HVA calcium currents. In addition, we demonstrate here that lipoamino acids NAGly and NAGABA-OH produced a strong thermal analgesia and that these effects (but not those of morphine) were abolished in Cav3.2 knock-out mice. Collectively, our data revealed lipoamino acids as a family of endogenous T-type channel inhibitors, suggesting that these ligands can modulate multiple cell functions via this newly evidenced regulation.

Extracellular loops 2 and 4 of GLYT2 are required for N-arachidonylglycine inhibition of glycine transport

Concentrations of extracellular glycine in the central nervous system are regulated by Na(+)/Cl(-)-dependent glycine transporters, GLYT1 and GLYT2. N-Arachidonylglycine (NAGly) is an endogenous inhibitor of GLYT2 with little or no effect on GLYT1 and is analgesic in rat models of neuropathic and inflammatory pain. Understanding the molecular basis of NAGly interactions with GLYT2 may allow for the development of novel therapeutics. In this study, chimeric transporters were used to determine the structural basis for differences in NAGly sensitivity between GLYT1 and GLYT2 and also the actions of a series of related N-arachidonyl amino acids. Extracellular loops 2 and 4 of GLYT2 are important in the selective inhibition of GLYT2 by NAGly and by the related compounds N-arachidonyl-gamma-aminobutyric acid and N-arachidonyl-d-alanine, whereas only the extracellular loop 4 of GLYT2 is required for N-arachidonyl-l-alanine inhibition of transport. These observations suggest that the structure of the head group of these compounds is important in determining how they interact with extracellular loops 2 and 4 of GLYT2. Site-directed mutagenesis of GLYT2 EL4 residues was used to identify the key residues Arg(531), Lys(532), and Ile(545) that contribute to the differences in NAGly sensitivity.

The endocannabinoid anandamide is a precursor for the signaling lipid N-arachidonoyl glycine by two distinct pathways

BACKGROUND

N-arachidonoyl glycine (NAGly) is an endogenous signaling lipid with a wide variety of biological activity whose biosynthesis is poorly understood. Two primary biosynthetic pathways have been proposed. One suggests that NAGly is formed via an enzymatically regulated conjugation of arachidonic acid (AA) and glycine. The other suggests that NAGly is an oxidative metabolite of the endogenous cannabinoid, anandamide (AEA), through an alcohol dehydrogenase. Here using both in vitro and in vivo assays measuring metabolites with LC/MS/MS we test the hypothesis that both pathways are present in mammalian cells.

RESULTS

The metabolic products of deuterium-labeled AEA, D4AEA (deuterium on ethanolamine), indicated that NAGly is formed by the oxidation of the ethanolamine creating a D2NAGly product in both RAW 264.7 and C6 glioma cells. Significantly, D4AEA produced a D0NAGly product only in C6 glioma cells suggesting that the hydrolysis of AEA yielded AA that was used preferentially in a conjugation reaction. Addition of the fatty acid amide (FAAH) inhibitor URB 597 blocked the production of D0NAGly in these cells. Incubation with D8AA in C6 glioma cells likewise produced D8NAGly; however, with significantly less efficacy leading to the hypothesis that FAAH-initiated AEA-released AA conjugation with glycine predominates in these cells. Furthermore, the levels of AEA in the brain were significantly increased, whereas those of NAGly were significantly decreased after systemic injection of URB 597 in rats and in FAAH KO mice further supporting a role for FAAH in endogenous NAGly biosynthesis. Incubations of NAGly and recombinant FAAH demonstrated that NAGly is a significantly less efficacious substrate for FAAH with only ~50% hydrolysis at 30 minutes compared to 100% hydrolysis of AEA. Co-incubations of AEA and glycine with recombinant FAAH did not, however, produce NAGly.

CONCLUSION

These data support the hypothesis that the signaling lipid NAGly is a metabolic product of AEA by both oxidative metabolism of the AEA ethanolamine moiety and through the conjugation of glycine to AA that is released during AEA hydrolysis by FAAH.

Orphan endogenous lipids and orphan GPCRs: a good match

A large and growing family of over 70 endogenous lipids of the basic structure N-acyl amide has been identified during the last 10 years. Only a few of these lipids have been characterized for biological activity, however, those that have shown a wide-range of activity may act at G-protein coupled receptors (GPCRs). Like orphan GPCRs that are identified as being in the genome and expressed in tissue, the majority of these endogenous lipids many produced throughout the body, some predominately in nervous tissue, remain orphaned. Here, we give a brief history of these orphan lipids and highlight the activity of N-arachidonoyl glycine, and farnesyl pyrophosphate at the orphan receptors GPR18 and GPR92, respectively, as well as summarizing the biological and pharmacological data for the recently identified N-palmitoyl glycine that suggests activity at a novel GPCR. Working to deorphanize both lipids and GPCRs together provides a unique opportunity for a greater understanding of cellular signaling and a challenge to find them all a home.

Inhibition of human recombinant T-type calcium channels by the endocannabinoid N-arachidonoyl dopamine

BACKGROUND AND PURPOSE

N-arachidonoyl dopamine (NADA) has complex effects on nociception mediated via cannabinoid CB(1) receptors and the transient receptor potential vanilloid receptor 1 (TRPV1). Anandamide, the prototypic CB(1)/TRPV1 agonist, also inhibits T-type voltage-gated calcium channel currents (I(Ca)). These channels are expressed by many excitable cells, including neurons involved in pain detection and processing. We sought to determine whether NADA and the prototypic arachidonoyl amino acid, N-arachidonoyl glycine (NAGly) modulate T-type I(Ca)

EXPERIMENTAL APPROACH

Human recombinant T-type I(Ca) (Ca(V)3 channels) expressed in HEK 293 cells and native mouse T-type I(Ca) were examined using standard whole-cell voltage clamp electrophysiology techniques.

KEY RESULTS

N-arachidonoyl dopamine completely inhibited Ca(V)3 channels with a rank order of potency (pEC(50)) of Ca(V)3.3 (6.45) > or = Ca(V)3.1 (6.29) > Ca(V)3.2 (5.95). NAGly (10 micromol.L(-1)) inhibited Ca(V)3 I(Ca) by approximately 50% or less. The effects of NADA and NAGly were voltage- but not use-dependent, and both compounds produced significant hyperpolarizing shifts in Ca(V)3 channel steady-state inactivation relationships. By contrast with anandamide, NADA and NAGly had modest effects on Ca(V)3 channel kinetics. Both NAGly and NADA inhibited native T-type I(Ca) in mouse sensory neurons.

CONCLUSIONS AND IMPLICATIONS

N-arachidonoyl dopamine and NAGly increase the steady-state inactivation of Ca(V)3 channels, reducing the number of channels available to open during depolarization. These effects occur at NADA concentrations at or below to those affecting CB(1) and TRPV1 receptors. Together with anandamide, the arachidonoyl neurotransmitter amides, NADA and NAGly, represent a new family of endogenous T-type I(Ca) modulators.

Cannabinoids, endocannabinoids, and related analogs in inflammation

This review covers reports published in the last 5 years on the anti-inflammatory activities of all classes of cannabinoids, including phytocannabinoids such as tetrahydrocannabinol and cannabidiol, synthetic analogs such as ajulemic acid and nabilone, the endogenous cannabinoids anandamide and related compounds, namely, the elmiric acids, and finally, noncannabinoid components of Cannabis that show anti-inflammatory action. It is intended to be an update on the topic of the involvement of cannabinoids in the process of inflammation. A possible mechanism for these actions is suggested involving increased production of eicosanoids that promote the resolution of inflammation. This differentiates these cannabinoids from cyclooxygenase-2 inhibitors that suppress the synthesis of eicosanoids that promote the induction of the inflammatory process.

Biochemical and biological properties of 4-(3-phenyl-[1,2,4] thiadiazol-5-yl)-piperazine-1-carboxylic acid phenylamide, a mechanism-based inhibitor of fatty acid amide hydrolase

Fatty acid amide hydrolase (FAAH) is an integral membrane enzyme within the amidase-signature family. It catalyzes the hydrolysis of several endogenous biologically active lipids, including anandamide (arachidonoyl ethanolamide), oleoyl ethanolamide, and palmitoyl ethanolamide. These endogenous FAAH substrates have been shown to be involved in a variety of physiological and pathological processes, including synaptic regulation, regulation of sleep and feeding, locomotor activity, pain and inflammation. Here we describe the biochemical and biological properties of a potent and selective FAAH inhibitor, 4-(3-phenyl-[1,2,4]thiadiazol-5-yl)-piperazine-1-carboxylic acid phenylamide (JNJ-1661010). The time-dependence of apparent IC(50) values at rat and human recombinant FAAH, dialysis and mass spectrometry data indicate that the acyl piperazinyl fragment of JNJ-1661010 forms a covalent bond with the enzyme. This bond is slowly hydrolyzed, with release of the piperazinyl fragment and recovery of enzyme activity. The lack of inhibition observed in a rat liver esterase assay suggests that JNJ-1661010 is not a general esterase inhibitor. JNJ-1661010 is >100-fold preferentially selective for FAAH-1 when compared to FAAH-2. JNJ-1661010 dose-dependently increases arachidonoyl ethanolamide, oleoyl ethanolamide, and palmitoyl ethanolamide in the rat brain. The compound attenuates tactile allodynia in the rat mild thermal injury model of acute tissue damage and in the rat spinal nerve ligation (Chung) model of neuropathic pain. JNJ-1661010 also diminishes thermal hyperalgesia in the inflammatory rat carrageenan paw model. These data suggest that FAAH inhibitors with modes of action similar to JNJ-1661010 may be useful clinically as broad-spectrum analgesics.

Novel endogenous N-acyl glycines identification and characterization

Discovery of the endogenous cannabinoid and N-acyl amide, anandamide (N-arachidonoyl ethanolamine), paved the way for lipidomics discoveries in the growing family of N-acyl amides. Lipidomics is a field that is broadening our view of the molecular world to include a wide variety of endogenous lipid signaling molecules. Many of these lipids will undoubtedly provide new insights into old questions while others will provide broad platforms for new questions. J Michael Walker's last 8 years were dedicated to this search and he lived long enough to see 54 novel lipids isolated from biological tissues in his laboratory. Here, we summarize the biosynthesis, metabolism and biological activity of two of the family of N-acyl glycines, N-arachidonoyl glycine and N-palmitoyl glycine, and introduce four additional members: N-stearoyl glycine, N-linoleoyl glycine, N-oleoyl glycine, and N-docosahexaenoyl glycine. Each of these compounds is found throughout the body at differing levels suggesting region-specific functionality and at least four of the N-acyl glycines are regulated by the enzyme fatty acid amide hydrolase. The family of N-acyl glycines presented here is merely a sampling of what is to come in the continuing discovery of novel endogenous lipids.