Identification of AgRP cells in the murine hindbrain that drive feeding

OBJECTIVE

The central melanocortin system is essential for the regulation of food intake and body weight. Agouti-related protein (AgRP) is the sole orexigenic component of the central melanocortin system and is conserved across mammalian species. AgRP is currently known to be expressed exclusively in the mediobasal hypothalamus, and hypothalamic AgRP-expressing neurons are essential for feeding. Here we characterized a previously unknown population of AgRP cells in the mouse hindbrain.

METHODS

Expression of AgRP in the hindbrain was investigated using gene expression analysis, single-cell RNA sequencing, immunofluorescent analysis and multiple transgenic mice with reporter expressions. Activation of AgRP neurons was achieved by Designer Receptors Exclusively Activated by Designer Drugs (DREADD) and by transcranial focal photo-stimulation using a step-function opsin with ultra-high light sensitivity (SOUL).

RESULTS

AgRP expressing cells were present in the area postrema (AP) and the adjacent subpostrema area (SubP) and commissural nucleus of the solitary tract (cNTS) of the mouse hindbrain (termed AgRPHind herein). AgRPHind cells consisted of locally projecting neurons as well as tanycyte-like cells. Food deprivation stimulated hindbrain Agrp expression as well as neuronal activity of subsets of AgRPHind cells. In adult mice that lacked hypothalamic AgRP neurons, chemogenetic activation of AgRP neurons resulted in hyperphagia and weight gain. In addition, transcranial focal photo-stimulation of hindbrain AgRP cells increased food intake in adult mice with or without hypothalamic AgRP neurons.

CONCLUSIONS

Our study indicates that the central melanocortin system in the hindbrain possesses an orexigenic component, and that AgRPHind neurons stimulate feeding independently of hypothalamic AgRP neurons.

Discovery, development, and clinical proof of mechanism of LY3463251, a long-acting GDF15 receptor agonist

GDF15 and its receptor GFRAL/RET form a non-homeostatic system that regulates food intake and body weight in preclinical species. Here, we describe a GDF15 analog, LY3463251, a potent agonist at the GFRAL/RET receptor with prolonged pharmacokinetics. In rodents and obese non-human primates, LY3463251 decreased food intake and body weight with no signs of malaise or emesis. In a first-in-human study in healthy participants, single subcutaneous LY3463251 injections showed a safety and pharmacokinetic profile supporting further clinical development with dose-dependent nausea and emesis in a subset of individuals. A subsequent 12-week multiple ascending dose study in overweight and obese participants showed that LY3463251 induced significant decreases in food intake and appetite scores associated with modest body weight reduction independent of nausea and emesis (clinicaltrials.gov: NCT03764774). These observations demonstrate that agonism of the GFRAL/RET system can modulate energy balance in humans, though the decrease in body weight is surprisingly modest, suggesting challenges in leveraging the GDF15 system for clinical weight-loss applications.

The hypothalamic RFamide, QRFP, increases feeding and locomotor activity: The role of Gpr103 and orexin receptors

Here we show that central administration of pyroglutamylated arginine-phenylamine-amide peptide (QRFP/26RFa) increases both food intake and locomotor activity, without any significant effect on energy expenditure, thermogenesis or reward. Germline knock out of either of the mouse QRFP receptor orthologs, Gpr103a and Gpr103b, did not produce a metabolic phenotype. However, both receptors are required for the effect of centrally administered QRFP to increase feeding and locomotor activity. As central injection of QRFP activated orexin/hypocretin neurons in the lateral hypothalamus, we compared the action of QRFP and orexin on behaviour. Both peptides increased arousal and locomotor activity. However, while orexin increased consummatory behaviour, QRFP also affected other appetitive behaviours. Furthermore, the feeding but not the locomotor response to QRFP, was blocked by co-administration of an orexin receptor 1 antagonist. These results suggest that QRFP agonism induces both appetitive and consummatory behaviour, but only the latter is dependent on orexin/hypocretin receptor signalling.

GIPR Agonism Inhibits PYY-Induced Nausea-Like Behavior

The induction of nausea and emesis is a major barrier to maximizing the weight loss profile of obesity medications, and therefore, identifying mechanisms that improve tolerability could result in added therapeutic benefit. The development of peptide YY (PYY)-based approaches to treat obesity are no exception, as PYY receptor agonism is often accompanied by nausea and vomiting. Here, we sought to determine whether glucose-dependent insulinotropic polypeptide (GIP) receptor (GIPR) agonism reduces PYY-induced nausea-like behavior in mice. We found that central and peripheral administration of a GIPR agonist reduced conditioned taste avoidance (CTA) without affecting hypophagia mediated by a PYY analog. The receptors for GIP and PYY (Gipr and Npy2r) were found to be expressed by the same neurons in the area postrema (AP), a brainstem nucleus involved in detecting aversive stimuli. Peripheral administration of a GIPR agonist induced neuronal activation (cFos) in the AP. Further, whole-brain cFos analyses indicated that PYY-induced CTA was associated with augmented neuronal activity in the parabrachial nucleus (PBN), a brainstem nucleus that relays aversive/emetic signals to brain regions that control feeding behavior. Importantly, GIPR agonism reduced PYY-mediated neuronal activity in the PBN, providing a potential mechanistic explanation for how GIPR agonist treatment reduces PYY-induced nausea-like behavior. Together, the results of our study indicate a novel mechanism by which GIP-based therapeutics may have benefit in improving the tolerability of weight loss agents.

Anorectic and aversive effects of GLP-1 receptor agonism are mediated by brainstem cholecystokinin neurons, and modulated by GIP receptor activation

Objective

Glucagon-like peptide-1 receptor agonists (GLP-1RAs) are effective medications to reduce appetite and body weight. These actions are centrally mediated; however, the neuronal substrates involved are poorly understood.

Methods

We employed a combination of neuroanatomical, genetic, and behavioral approaches in the mouse to investigate the involvement of caudal brainstem cholecystokinin-expressing neurons in the effect of the GLP-1RA exendin-4. We further confirmed key neuroanatomical findings in the non-human primate brain.

Results

We found that cholecystokinin-expressing neurons in the caudal brainstem are required for the anorectic and body weight-lowering effects of GLP-1RAs and for the induction of GLP-1RA-induced conditioned taste avoidance. We further show that, while cholecystokinin-expressing neurons are not a direct target for glucose-dependent insulinotropic peptide (GIP), GIP receptor activation results in a reduced recruitment of these GLP-1RA-responsive neurons and a selective reduction of conditioned taste avoidance.

Conclusions

In addition to disclosing a neuronal population required for the full appetite- and body weight-lowering effect of GLP-1RAs, our data also provide a novel framework for understanding and ameliorating GLP-1RA-induced nausea — a major factor for withdrawal from treatment.

•

CCK^AP/NTS neurons are required for the full anorectic and body weight-lowering effect of GLP-1 receptor agonists.

•

GLP-1 receptor agonists promote the formation of conditioned taste avoidance by activating CCK^AP/NTS neurons.

•

CCK^AP/NTS neurons are not activated in response to GIP receptor agonists.

•

GIP receptor agonists reduce GLP-1 receptor agonist-induced neuronal responses in the caudal brainstem.

•

GIP receptor agonists reduce GLP-1 receptor agonist-induced conditioned taste avoidance.

Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist

Tirzepatide (LY3298176) is a dual GIP and GLP-1 receptor agonist under development for the treatment of type 2 diabetes mellitus (T2DM), obesity, and nonalcoholic steatohepatitis. Early phase trials in T2DM indicate that tirzepatide improves clinical outcomes beyond those achieved by a selective GLP-1 receptor agonist. Therefore, we hypothesized that the integrated potency and signaling properties of tirzepatide provide a unique pharmacological profile tailored for improving broad metabolic control. Here, we establish methodology for calculating occupancy of each receptor for clinically efficacious doses of the drug. This analysis reveals a greater degree of engagement of tirzepatide for the GIP receptor than the GLP-1 receptor, corroborating an imbalanced mechanism of action. Pharmacologically, signaling studies demonstrate that tirzepatide mimics the actions of native GIP at the GIP receptor but shows bias at the GLP-1 receptor to favor cAMP generation over β-arrestin recruitment, coincident with a weaker ability to drive GLP-1 receptor internalization compared with GLP-1. Experiments in primary islets reveal β-arrestin1 limits the insulin response to GLP-1, but not GIP or tirzepatide, suggesting that the biased agonism of tirzepatide enhances insulin secretion. Imbalance toward GIP receptor, combined with distinct signaling properties at the GLP-1 receptor, together may account for the promising efficacy of this investigational agent.

The cytokine GDF15 signals through a population of brainstem cholecystokinin neurons to mediate anorectic signalling

The cytokine, GDF15, is produced in pathological states which cause cellular stress, including cancer. When over expressed, it causes dramatic weight reduction, suggesting a role in disease-related anorexia. Here, we demonstrate that the GDF15 receptor, GFRAL, is located in a subset of cholecystokinin neurons which span the area postrema and the nucleus of the tractus solitarius of the mouse. GDF15 activates GFRAL^AP/NTS neurons and supports conditioned taste and place aversions, while the anorexia it causes can be blocked by a monoclonal antibody directed at GFRAL or by disrupting CCK neuronal signalling. The cancer-therapeutic drug, cisplatin, induces the release of GDF15 and activates GFRAL^AP/NTS neurons, as well as causing significant reductions in food intake and body weight in mice. These metabolic effects of cisplatin are abolished by pre-treatment with the GFRAL monoclonal antibody. Our results suggest that GFRAL neutralising antibodies or antagonists may provide a co-treatment opportunity for patients undergoing chemotherapy.

GDF15 and Growth Control

Growth/differentiation factor-15 (GDF-15) is a distant member of the transforming growth factor β (TGF-β) superfamily and is widely expressed in multiple mammalian tissues. Its expression is highly regulated and is often induced in response to conditions associated with cellular stress. GDF15 serum levels have a strong association with many diseases, including inflammation, cancer, cardiovascular diseases, and obesity, and potentially serve as reliable predictor of disease progression. A functional role for GDF15 has been suggested in cancer, cardiovascular disease, kidney disease and metabolic disease. However, the knowledge of its pathophysiological function at the molecular level is still limited and requires more investigation. Recent identification of the endogenous receptor for GDF15 may provide additional insight in to its' molecular mechanisms and relationship to disease states.

Beyond Glucagon-like Peptide-1: Is G-Protein Coupled Receptor Polypharmacology the Path Forward to Treating Metabolic Diseases?

The glucagon-like peptide-1 receptor (GLP-1R) is a class B G-protein coupled receptor (GPCR) that has proven to be an effective target for developing medicines that treat type 2 diabetes mellitus (T2DM). GLP-1R agonists improve T2DM by enhancing glucose-stimulated insulin secretion, delaying gastric transit, decreasing glucagon levels, and reducing body weight due to anorexigenic actions. The therapeutic successes of these agents helped inspire the design of new multifunctional molecules that are GLP-1R agonists but also activate receptors linked to pathways that enhance insulin sensitization and/or energy expenditure. Herein, these agents are discussed in the context of polypharmacological approaches that may enable even further improvement in treatment outcomes. Moreover, we revisit classical polypharmaceutical GPCR approaches and how they may be utilized for treatment of T2DM. To determine optimal combination regimens, changes in drug discovery practices are likely needed because compensatory mechanisms appear to underlie progression of T2DM and limit the ability of current therapies to induce disease regression or remission.

The current state of GPCR-based drug discovery to treat metabolic disease

One approach of modern drug discovery is to identify agents that enhance or diminish signal transduction cascades in various cell types and tissues by modulating the activity of GPCRs. This strategy has resulted in the development of new medicines to treat many conditions, including cardiovascular disease, psychiatric disorders, HIV/AIDS, certain forms of cancer and Type 2 diabetes mellitus (T2DM). These successes justify further pursuit of GPCRs as disease targets and provide key learning that should help guide identifying future therapeutic agents. This report reviews the current landscape of GPCR drug discovery with emphasis on efforts aimed at developing new molecules for treating T2DM and obesity. We analyse historical efforts to generate GPCR-based drugs to treat metabolic disease in terms of causal factors leading to success and failure in this endeavour.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Antibody-Mediated Targeting of the FGFR1c Isoform Increases Glucose Uptake in White and Brown Adipose Tissue in Male Mice

The increased prevalence of obesity and its cardiometabolic implications demonstrates the imperative to identify novel therapeutic targets able to effect meaningful metabolic changes in this population. Antibody-mediated targeting of fibroblast growth factor receptor 1c isoform (FGFR1c) has been shown to ameliorate hyperglycemia and protect from diet- and genetically-induced obesity in rodents and nonhuman primates. However, it is currently unknown which tissue(s) contribute to this glucose-lowering effect. Thus, to elucidate this effect, we treated euglycemic mice with H7, a monoclonal antibody that selectively targets FGFR1c, and used whole-body positron emission computed tomography with a glucose tracer (18F-fluorodeoxyglucose). Treatment with H7 increased basal glucose uptake in white adipose tissue (WAT), brown adipose tissue (BAT), the brain, and liver but reduced it in the quadriceps muscles. Consequentially, blood glucose was significantly reduced in response to treatment. Under insulin-stimulated conditions, the effects of H7 were maintained in WAT, BAT, liver, and muscle. Treatment with H7 decreased triglyceride (TG) content and increased adipose TG lipase content in white adipose tissue, while increasing activation of acetyl coenzyme A carboxylase, suggesting futile cycling of TGs, albeit favoring net hydrolysis. We demonstrated, in vitro, this is a direct effect of treatment in adipose tissue, as basal cellular respiration and glucose uptake were increased in response to treatment. Taken together, these data suggest that antibody-mediated targeting of FGFR1c exerts its powerful glucose-lowering efficacy primarily due to increased glucose uptake in adipose tissue.

Correlation of disease severity with body weight and high fat diet in the FATZO/Pco mouse

Obesity in many current pre-clinical animal models of obesity and diabetes is mediated by monogenic mutations; these are rarely associated with the development of human obesity. A new mouse model, the FATZO mouse, has been developed to provide polygenic obesity and a metabolic pattern of hyperglycemia and hyperinsulinemia, that support the presence of insulin resistance similar to metabolic disease in patients with insulin resistance/type 2 diabetes. The FATZO mouse resulted from a cross of C57BL/6J and AKR/J mice followed by selective inbreeding for obesity, increased insulin and hyperglycemia. Since many clinical studies have established a close link between higher body weight and the development of type 2 diabetes, we investigated whether time to progression to type 2 diabetes or disease severity in FATZO mice was dependent on weight gain in young animals. Our results indicate that lighter animals developed metabolic disturbances much slower and to a lesser magnitude than their heavier counterparts. Consumption of a diet containing high fat, accelerated weight gain in parallel with disease progression. A naturally occurring and significant variation in the body weight of FATZO offspring enables these mice to be identified as low, mid and high body weight groups at a young age. These weight groups remain into adulthood and correspond to slow, medium and accelerated development of type 2 diabetes. Thus, body weight inclusion criteria can optimize the FATZO model for studies of prevention, stabilization or treatment of type 2 diabetes.

Melanocortin-4 receptor agonists for the treatment of obesity

The melanocortin family of receptors (MC 1-5R) and their endogenous peptide ligands (alpha, beta, gamma- MSH and ACTH) have been implicated in the control of a wide variety of behavioral and physiological functions including the homeostatic control of food intake and body weight. In rodent models, melanocortin agonists including the nonselective peptide MTII have been shown to decrease food intake and body weight while antagonists such as SHU9119 and AGRP have been shown to stimulate food intake and increase body weight. Deletion of either the MC3R or MC4R in mice was found to be associated with obesity although hyperphagia was only observed in the MC4R deficient mice. Similarly in humans, inactivating mutations of the MC4R have been found in as many as six percent of obese individuals. The suggestion from these findings that activation of MC4Rs would have an anorectic effect in humans has resulted in efforts to produce selective agonists for the treatment of obesity. Over the past decade, efforts to develop MC4R selective small molecule and peptide agonists have been met with fractional success. Many small molecule agonists have been identified; however, few have been shown to have activity in vivo. While their use as therapeutics may have limitations, selective and potent peptide agonists have been shown by several investigators to decrease food intake and body weight in rodent models. The subject of the current review is to examine the progress made to date on producing both small molecule and peptide MC4R agonists as potential therapeutics for obesity.

Structure-activity relationship studies of carboxamido-biaryl ethers as opioid receptor antagonists (OpRAs). Part 1

A structurally unique and new class of opioid receptor antagonists (OpRAs) that bear no structural resemblance with morphine or endogenous opioid peptides has been discovered. A series of carboxamido-biaryl ethers were identified as potent receptor antagonists against mu, kappa and delta opioid receptors. The structure-activity relationship indicated para-substituted aryloxyaryl primary carboxamide bearing an amine tether on the distal phenyl ring was optimal for potent in vitro functional antagonism against three opioid receptor subtypes.

Structure-Activity Relationships of β -MSH Derived Melanocortin-4 Receptor Peptide Agonists

The recent emergence of obesity as a major health threat in the industrialized world has intensified the search for novel and effective pharmacologic treatment. The proopiomelanocortin (POMC)-melanocortin 4 receptor (MC4R) axis has been shown to regulate food intake and energy homeostasis and is considered among the most promising antiobesity targets. Our initial efforts in this area have focused on affinity and selectivity directed optimization of the native beta-MSH(5-22) sequence and resulted in the discovery of a potent MC4R agonist: Ac-Tyr-Arg-[Cys-Glu-His-D-Phe-Arg-Trp-Cys]-NH(2) (10). Subcutaneous administration of this peptide produced an excellent in vivo efficacy in reducing food intake and increasing fat metabolism. Additionally, suppression of food intake was observed in wild type but not in MC4R deficient mice, suggesting that the effects observed in the wild type mice were mediated through MC4R signaling. Subsequent optimization efforts led to the identification of a novel series of disulfide constrained hexapeptides as exemplified by Ac-[hCys-His-D-Phe-Arg-Trp-Cys]-NH(2) (100). These cyclic hexapeptides showed a further improved potency in binding MC4R and an enhanced selectivity over MC1R. At a dose of 0.07 mg/kg analog 102 reduced food intake by 38% and increased fat utilization by 58% in rats. These cyclic peptides provide novel and enhanced reagents for the elucidation of melanocortin receptors biology and may find applications in the treatment of obesity and related metabolic disorders.

Potent and selective MC-4 receptor agonists based on a novel disulfide scaffold

Extensive structure-activity relationship studies utilizing a beta-MSH-derived cyclic nonapeptide, Ac-Tyr-Arg-[Cys-Glu-His-D-Phe-Arg-Trp-Cys]-NH(2) (3), led to identification of a series of novel MC-4R selective disulfide-constrained hexapeptide analogs including Ac-[hCys-His-D-Phe-Arg-Trp-Cys]-NH(2) (12). The structural modifications associated with profound influence on MC-4R potency and selectivity were ring size, ring conformation, and the aromatic substitution of the D-Phe7. These cyclic peptide analogs provide novel and enhanced reagents for use in the elucidation of melanocortin-4 receptor-related physiology, and may additionally find application in the treatment of obesity and related metabolic disorders.

Discovery of a beta-MSH-derived MC-4R selective agonist

A series of novel, disulfide-constrained human beta-melanocyte stimulating hormone (beta-MSH)-derived peptides were optimized for in vitro melanocortin-4 receptor (MC-4R) binding affinity, agonist efficacy, and selectivity. The most promising of these, analogue 18, was further studied in vivo using chronic rat food intake and body weight models.

Na+ modulation, inverse agonism, and anorectic potency of 4-phenylpiperidine opioid antagonists

Differences in the anorectic activity of morphinan (e.g., naltrexone) and 3,4-dimethyl-4-(3-hydroxyphenyl)piperidine (4PP) opioid receptor antagonists have been described. In an attempt to explain these differences, the influence of Na(+) on opioid binding affinity and functional activity of 4PP antagonists was compared to other opioid antagonists. The binding affinities of neutral antagonists were unaffected by the addition of Na(+), whereas that for the peptide, inverse agonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI174864) was increased. Similarly, the binding affinities of the 4PP antagonist (3R,4R)-1-((S)-3-hydroxy-3-cyclohexylpropyl)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidine (LY255582) and other 4PP antagonists were increased in the presence of Na(+) with the greatest effects at the delta opioid receptor followed by the mu and kappa opioid receptors, respectively. Similar to ICI174864, 4PP antagonists were found to inhibit basal GTPgamma[(35)S] binding at the delta opioid receptor indicating inverse agonist activity. A correlation was observed between the binding affinities in the presence of Na(+), the inverse agonist potency, and the anorectic potency of 4PP antagonists. These data suggest that 4PP antagonists differ from morphinan antagonists in their inverse agonist activity and suggest a relationship between inverse agonism and anorectic activity.

Synthesis and structure-activity relationships of novel arylpiperazines as potent and selective agonists of the melanocortin subtype-4 receptor

The melanocortin receptors have been implicated as potential targets for a number of important therapeutic indications, including inflammation, sexual dysfunction, and obesity. We identified compound 1, an arylpiperazine attached to the dipeptide H-d-Tic-d-p-Cl-Phe-OH, as a novel melanocortin subtype-4 receptor (MC4R) agonist through iterative directed screening of nonpeptidyl G-protein-coupled receptor biased libraries. Structure-activity relationship (SAR) studies demonstrated that substitutions at the ortho position of the aryl ring improved binding and functional potency. For example, the o-isopropyl-substituted compound 29 (K(i) = 720 nM) possessed 9-fold better binding affinity compared to the unsubstituted aryl ring (K(i) = 6600 nM). Sulfonamide 39 (K(i) = 220 nM) fills this space with a polar substituent, resulting in a further 2-fold improvement in binding affinity. The most potent compounds such as the diethylamine 44 (K(i) = 60 nM) contain a basic group at this position. Basic heterocycles such as the imidazole 50 (K(i) = 110 nM) were similarly effective. We also demonstrated good oral bioavailability for sulfonamide 39.

Na+-dependent high affinity binding of [3H]LY515300, a 3,4-dimethyl-4-(3-hydroxyphenyl)piperidine opioid receptor inverse agonist

Analogues of 3,4-dimethyl-4-(3-hydroxyphenyl)piperidines are high affinity inverse agonists for micro-, delta- and kappa-opioid receptors. To characterize inverse agonist binding, we synthesized a high specific activity radioligand from this series, [3H]LY515300 (3-[1-((3-cyclohexyl-[3,4-3H(2)])-3(R,S)-hydroxypropyl)-3(R),4(R)-dimethylpiperidin-4-yl]phenol). In membranes expressing cloned human opioid receptors, [3H]LY515300 binding was saturable and exhibited low nonspecific binding. [3H]LY515300 bound with high affinity to the micro- (K(d)=0.07 nM), delta- (K(d)=0.92 nM) and kappa-(K(d)=0.45 nM) opioid receptors. High affinity [3H]LY515300 binding to all opioid receptors was Na(+)-dependent, a characteristic of inverse agonists. Displacement by standard opioid compounds yielded K(i) values consistent with their known opioid receptor affinities. Autoradiographic localization of specific [3H]LY515300 binding in rat and guinea pig brain was high in areas known to express high levels of opioid (particularly micro-opioid receptor) binding sites including the caudate, nucleus accumbens, and nucleus tractus solitarius. Thus, [3H]LY515300 is the first radiolabeled opioid receptor inverse agonist useful for the study of opioid receptors in cell lines and native tissues.

Membrane microviscosity modulates mu-opioid receptor conformational transitions and agonist efficacy

The influence of membrane microviscosity on mu-opioid agonist and antagonist binding, as well as agonist efficacy, was examined in membranes prepared from SH-SY5Y cells and from a C6 glioma cell line stably expressing the rat mu-opioid receptor (C6mu). Addition of cholesteryl hemisuccinate (CHS) to cell membranes increased membrane microviscosity and reduced the inhibitory effect of sodium and guanine nucleotides on the affinity of the full agonists sufentanil and [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAMGO) for the mu-opioid receptor. Binding of the antagonists [3H]naltrexone and [3H]diprenorphine and the partial agonist nalbuphine was unaffected by CHS. The effect of CHS on agonist binding was reversed by subsequent addition of cis-vaccenic acid, suggesting that the effect of CHS is the result of increased membrane microviscosity and not a specific sterol-receptor interaction. CHS addition increased the potency of DAMGO to stimulate guanosine-5'-O-(3-[35S]thio)triphosphate binding by fourfold, whereas the potency of nalbuphine was unaffected. However, nalbuphine efficacy relative to that of the full agonist DAMGO was strongly increased in CHS-treated membranes compared with that in control membranes. Membrane rigidification also resulted in an increased efficacy for the partial agonists meperidine, profadol, and butorphanol relative to that of DAMGO as measured by agonist-stimulated GTPase activity in control and CHS-modified membranes. These findings support a regulatory role for membrane microviscosity in receptor-mediated G protein activation.

Pre- and postsynaptic actions of opioid and orphan opioid agonists in the rat arcuate nucleus and ventromedial hypothalamus in vitro

1. Using whole-cell patch clamp recording from neurones in an in vitro slice preparation, we have examined opioid- and orphanin FQ (OFQ)-mediated modulation of synaptic transmission in the rat arcuate nucleus and ventromedial hypothalamus (VMH). 2. Application of OFQ activated a Ba2+-sensitive and inwardly rectifying K+ conductance in approximately 50 % of arcuate nucleus neurones and approximately 95 % of VMH neurones. The OFQ-activated current was blocked by the nociceptin antagonist [Phe1Psi(CH2NH)Gly2]-nociceptin(1-13) NH2 (NCA), a peptide that on its own exhibited only weak agonist activity at high concentrations (> 1 microM). Similar current activation was observed with the mu agonist DAMGO but not delta (DPDPE) or kappa (U69593) agonists. 3. In arcuate nucleus neurones, DAMGO (1 microM), U69593 (1 microM) and OFQ (100 nM to 1 microM) but not DPDPE (1 microM) were found to depress the amplitude of electrically evoked glutamatergic postsynaptic currents (EPSCs) and decrease the magnitude of paired-pulse depression, indicating that opioid receptors were located presynaptically. 4. In VMH neurones, DAMGO strongly depressed the EPSC amplitude in all cells examined. DAMGO decreased the magnitude of paired-pulse depression, indicating that mu receptors were located presynaptically. U69593 weakly depressed the EPSC while OFQ and DPDPE had no effect. 5. In VMH neurones, DAMGO depressed the frequency of miniature EPSCs (-58 %) in the presence of tetrodotoxin and Cd2+ (100 microM), suggesting that the actions of mu receptors could be mediated by an inhibition of the synaptic vesicle release process downstream of Ca2+ entry. 6. The data presented show that presynaptic modulation of excitatory neurotransmission in the arcuate nucleus occurs through mu, kappa and the orphan opioid ORL-1 receptors while in the VMH presynaptic modulation only occurs through mu opioid receptors. Additionally, postsynaptic mu and ORL-1 receptors in both the arcuate nucleus and VMH modulate neuronal excitability through activation of a K+ conductance.

Galanin receptor-mediated inhibition of glutamate release in the arcuate nucleus of the hypothalamus

It is thought that galanin, a 29 amino acid neuropeptide, is involved in various neuronal functions, including the regulation of food intake and hormone release. Consistent with this idea, galanin receptors have been demonstrated throughout the brain, with high levels being observed in the hypothalamus. However, little is known about the mechanisms by which galanin elicits its actions in the brain. Therefore, we studied the effects of galanin and its analogs on synaptic transmission using an in vitro slice preparation of rat hypothalamus. In arcuate nucleus neurons, application of galanin resulted in an inhibition of evoked glutamatergic EPSCs and a decrease in paired-pulse depression, indicating a presynaptic action. The fragments galanin 1-16 and 1-15 produced a robust depression of synaptic transmission, whereas the fragment 3-29 produced a lesser degree of depression. The chimeric peptides C7, M15, M32, and M40, which have been reported to antagonize some actions of galanin, all produced varying degrees of depression of evoked EPSCs. In a minority of cases, C7, M15, and M40 antagonized the actions of galanin. Analysis of mEPSCs in the presence of TTX and Cd2+, or after application of alpha-latrotoxin, indicated a site of action for galanin downstream of Ca2+ entry. Thus, our data suggest that galanin acts via several subtypes of presynaptic receptors to depress synaptic transmission in the rat arcuate nucleus.

Synthesis and characterization of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-labeled fluorescent ligands for the mu opioid receptor

A series of opioid ligands utilizing the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophores 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene++ +-3-propionic acid or 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza- s-indacene-3-propionic acid were synthesized and characterized for their ability to act as a suitable fluorescent label for the mu opioid receptor. All compounds displaced the mu opioid receptor binding of [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol in monkey brain membranes with high affinity. The binding of fluorescent ligands to delta and kappa receptors was highly variable. 5,7-Dimethyl-BODIPY naltrexamine, "6-BNX," displayed subnanomolar affinities for the mu and kappa opioid receptors (Ki 0.07 and 0.43 nM, respectively) and nanomolar affinity at the delta (Ki 1.4 nM) receptor. Using fluorescence spectroscopy, the binding of 6-BNX in membranes from C6 glioma cells transfected with the cloned mu opioid receptor was investigated. In these membranes containing a high receptor density (10-80 pmol/mg protein), 6-BNX labeling was saturable, mu opioid specific, stereoselective (as determined with the isomers dextrorphan and levorphanol), and more than 90% specific. The results describe a series of newly developed fluorescent ligands for the mu opioid receptor and the use of one of these ligands as a label for the cloned mu receptor. These ligands provide a new approach for studying the structural and biophysical nature of opioid receptors.

Opioid efficacy in a C6 glioma cell line stably expressing the delta opioid receptor

A C6 glioma cell line stably transfected with the rat delta opioid receptor (C6delta) was used to characterize receptor binding and G protein activation by both peptide and nonpeptide delta opioid ligands. The ligand binding affinities for [3H]naltrindole and [3H]pCl-[D-Pen2,D-Pen5]enkephalin (DPDPE) were similar to those observed in monkey brain membranes. The nonpeptide agonists, BW373U86 and SNC80, as well as peptide agonist [D-Ser2, L-Leu5]enkephalyl-Thr maximally stimulated [35S]GTPgammaS binding by 640, 654 and 576%, respectively, over basal. The peptide agonists, DPDPE and deltorphin II, both stimulated [35S]GTPgammaS binding by 375%. Etorphine, diprenorphine, oxymorphindole and 7-spiroindanyloxymorphone were also partial agonists in this assay, although they were less efficacious than deltorphin II. Stimulation of [35S]GTPgammaS binding by agonists was blocked completely by pertussis toxin pretreatment. Both delta-1 and delta-2 selective antagonists 7-benzylidenenaltrexone and a benzofuran analog of naltrindole displayed high affinity for the cloned receptor (0.04 and 0.08 nM) and antagonized the stimulation of [35S]GTPgammaS binding by BW373U86 and DPDPE with similar potencies. Other evidence suggesting the lack of receptor subtypes includes the finding that stimulation of [35S]GTPgammaS binding by receptor subtype selective ligands DPDPE and deltorphin II was not additive. BW373U86, SNC80 and DPDPE maximally inhibited forskolin-stimulated adenylyl cyclase. These cells highly express a homogeneous population of delta opioid receptor that couple to inhibitory Go/Gi proteins. Ligand affinity for the delta opioid receptor correlates with ligand EC50 values for stimulation of [35S]GTPgammaS binding.

Regulation of neurotransmission in the arcuate nucleus of the rat by different neuropeptide Y receptors

We examined the effects of peptides of the neuropeptide Y (NPY)/pancreatic polypeptide (PP) family on synaptic transmission in the arcuate nucleus in rat hypothalamic slices. Application of NPY produced two effects. In some cells NPY produced an outward current that had the properties of a K+ current. NPY also inhibited the evoked glutamatergic EPSC recorded in these arcuate neurons by a presynaptic mechanism. Although the effects of NPY on the K+ current reversed within a few minutes of washout of the peptide, its effects on the EPSC frequently were longer lasting (>30 min). Similar effects were observed using peptide YY or the NPY analog [Leu31, Pro34]NPY. Although K+ current activation by [Leu31,Pro34]NPY was blocked by the selective Y1 antagonist BIBP 3226, inhibition of the EPSC was blocked only partially. Other NPY-related peptides such as NPY(13-36), PP, and [D-Trp32]NPY also inhibited the EPSC. However, none of these peptides produced activation of the K+ current. Thus, activation of more than one NPY receptor produces synaptic inhibition in the arcuate nucleus. A Y1 receptor activates a K+ current postsynaptically, and several receptor types appear to inhibit the EPSC by a presynaptic mechanism.

Characterization of opioid agonist efficacy in a C6 glioma cell line expressing the mu opioid receptor

In C6 glioma cells stably expressing a homogeneous population of the cloned rat mu opioid receptor, the binding affinities of opioid agonists and subsequent activation of G protein were examined. Opioid receptor number in membranes of these cells was high (10-30 pmol/mg protein [3H]diprenorphine binding sites). Opioids were found to bind to the receptor with high affinity [Tyr-D-Ala-Gly-(Me)Phe-Gly-ol (DAMGO) 0.23 nM; sufentanil 0.034 nM; morphine 0.16 nM]. Activation of G protein by opioid agonists was examined by measuring the stimulation of guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTP gamma S) binding. Sufentanil increased [35S]GTP gamma S binding by 326% with an EC50 value of 2.39 nM. Agonist stimulation of [35S]GTP gamma S binding was stereoselective, naltrexone-reversible, and pertussis toxin-sensitive. The "intrinsic activity" of opioids at the mu receptor was reflected by the magnitude of agonist-mediated activation of G protein. The rank order of the stimulation of [35S]GTP gamma S binding was etonitazene = sufentanil = DAMGO = PLO17 = fentanyl > morphine > profadol > meperidine > butorphanol = nalbuphine = pentazocine > cyclazocine = nalorphine > levallorphan > naltrexone. High affinity binding of ligands to the mu opioid receptor was reduced by the addition of sodium and guanosine diphosphate at concentrations used in the [35S]GTP gamma S binding assay. Ligand affinity was reduced in a manner correlating with "intrinsic activity". DAMGO, 1229-fold, nalbuphine 35-fold, naltrexone, 3-fold. The results presented show that the stable expression of the rat mu opioid receptor in C6 cells provides an effective tool to examine opioid receptor signal transduction mechanisms and evaluate the activity of novel opioids at the mu receptor.

Binding affinity and selectivity of opioids at mu, delta and kappa receptors in monkey brain membranes

The binding parameters of radiolabeled DAMGO (mu), DPDPE and pCl-DPDPE (delta) and 5 alpha, 7 alpha, 8 beta-N-methyl-N-[7-(1- pyrrolidinyl)-1-oxaspiro(4,5)dec-8-yl]benzeneacetamide (also known as U69593, kappa) and the affinity and selectivity profiles of various opioid agonists and antagonists at the three opioid receptor types were determined in membranes from brain cortex of rhesus monkey. Among the 10 opioids with established mu-selective actions, etonitazene inhibited the binding of [3H]DAMGO with a Ki of 0.02 nM (0.01 nM without sodium) and exhibited mu/delta and mu/kappa selectivities of 8800 and 11,650, respectively. DAMGO had a Ki of 1.23 nM and was about 500-fold more selective at mu receptors compared with delta and kappa sites. Other mu opioids with higher than 100-fold binding selectivity were fentanil and sufentanil. Highly selective delta opioids were DPDPE, deltorphin II and naltrindole. With the exception of N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH, all investigated putative delta opioids bound to delta sites with low Kis, i.e., 0.04 nM, 0.13 nM and 1.4 nM for naltrindole, (+/-)-4-[(alpha-R*)-alpha-((2S*,5R*)-4-allyl-2,5-dimethyl-1-piperazinyl) -3- hydroxybenzyl]-N,N-diethylbenzamide and DPDPE, respectively. In this series, the displacement of [3H]pCl-DPDPE yielded results similar to those obtained with [3H]DPDPE. With nanomolar Kis of 0.70, 0.89, 0.25 and 0.06, respectively, the highest kappa selectivity was displayed by (trans)-(+/-)-3,4-dichloro-N-methyl- N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide and U69593, followed by dynorphin 1-13 and norbinaltorphimine.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipophilicity of opioids determined by a novel micromethod

The lipophilicity of various mu-selective opioids was determined by measuring their distribution between n-octanol and Tris.HCl buffer, pH 7.4, by a procedure requiring submicromolar concentrations (submilligram amounts) of the compounds. After partitioning at 25 degrees C, the concentrations of opioids in the aqueous phase was quantified by their displacement of bound [3H]Tyr-Ala-Gly-(Me)Phe-Gly-ol (DAMGO) from opioid receptor in brain membranes. The obtained distribution coefficients (log Papp) agreed well with respective values determined previously with other, less sensitive or more cumbersome, methods of quantitation. The procedure is precise and versatile, and offers the routine assessment of lipophilicity as part of the in vitro characterization of opioids frequently available in limited quantities. In principle, the method is applicable to any compound whose binding to its receptor is quantifiable.

Synthesis and characterization of 7-nitrobenzo-2-oxa-1,3-diazole (NBD)-labeled fluorescent opioids

Alkylation of sarcosine with 4-chloro-nitrobenzo-2-oxa-1,3-diazole (NBD-chloride) furnished a fluorescent tag that was coupled with a tetrahydrothebaine derivative and beta-naltrexamine, respectively, to yield the fluorescent opioids 7 alpha-(1R)-1-hydroxy-1-methyl-3-(4-hydroxyphenyl)-propyl]-6,14- endoethenotetrahydrothebaine NBD-sarcosinate (ASM-5-10) and N-cyclopropylmethyl-3-hydroxy-14 beta-hydroxy-6 beta-(NBD sarcosinyl)-amino-epoxymorphinan (ASM-5-67). The fluorescence intensity of the novel opioids allowed their detection at subnanomolar concentrations, and was dependent on the polarity of the solvent. Maximum quantum yield was obtained in ethyl acetate and ethanol, and minimal fluorescence in heptane and water. Compounds ASM-5-10 and ASM-5-67 displaced the opioid receptor binding of [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol in monkey brain membranes with IC50 values of 8.4 and 1.5 nM, respectively. Whereas ASM-5-67 bound to mu, delta, and kappa receptors with comparable affinities, ASM-5-10 was mu-selective, with selectivity indices (ratio of respective IC50 values) of 0.04 for both mu/delta and mu/kappa. The sodium response ratio in binding revealed a pronounced agonist property of ASM-5-10. Both opioids were lipophilic, with octanol-water partition coefficients (log Papp) of 2.8 (ASM-5-10) and 1.0 (ASM-5-67). ASM-5-10 exhibited particularly strong membrane retention that was not reversible by four washes. Their favorable characteristics in fluorescence, receptor binding, and membrane interaction make these newly developed ligands useful molecular probes to study opioid receptor mechanisms.