NTS Prlh overcomes orexigenic stimuli and ameliorates dietary and genetic forms of obesity.

Calcitonin receptor (Calcr)-expressing neurons of the nucleus tractus solitarius (NTS; CalcrNTS cells) contribute to the long-term control of food intake and body weight. Here, we show that Prlh-expressing NTS (PrlhNTS) neurons represent a subset of CalcrNTS cells and that Prlh expression in these cells restrains body weight gain in the face of high fat diet challenge in mice. To understand the relationship of PrlhNTS cells to hypothalamic feeding circuits, we determined the ability of PrlhNTS-mediated signals to overcome enforced activation of AgRP neurons. We found that PrlhNTS neuron activation and Prlh overexpression in PrlhNTS cells abrogates AgRP neuron-driven hyperphagia and ameliorates the obesity of mice deficient in melanocortin signaling or leptin. Thus, enhancing Prlh-mediated neurotransmission from the NTS dampens hypothalamically-driven hyperphagia and obesity, demonstrating that NTS-mediated signals can override the effects of orexigenic hypothalamic signals on long-term energy balance.

Paraventricular Calcitonin Receptor-Expressing Neurons Modulate Energy Homeostasis in Male Mice.

The paraventricular nucleus of the hypothalamus (PVH) is a heterogeneous collection of neurons that play important roles in modulating feeding and energy expenditure. Abnormal development or ablation of the PVH results in hyperphagic obesity and defects in energy expenditure whereas selective activation of defined PVH neuronal populations can suppress feeding and may promote energy expenditure. Here, we characterize the contribution of calcitonin receptor-expressing PVH neurons (CalcRPVH) to energy balance control. We used Cre-dependent viral tools delivered stereotaxically to the PVH of CalcR2Acre mice to activate, silence, and trace CalcRPVH neurons and determine their contribution to body weight regulation. Immunohistochemistry of fluorescently-labeled CalcRPVH neurons demonstrates that CalcRPVH neurons are largely distinct from several PVH neuronal populations involved in energy homeostasis; these neurons project to regions of the hindbrain that are implicated in energy balance control, including the nucleus of the solitary tract and the parabrachial nucleus. Acute activation of CalcRPVH neurons suppresses feeding without appreciably augmenting energy expenditure, whereas their silencing leads to obesity that may be due in part due to loss of PVH melanocortin-4 receptor signaling. These data show that CalcRPVH neurons are an essential component of energy balance neurocircuitry and their function is important for body weight maintenance. A thorough understanding of the mechanisms by which CalcRPVH neurons modulate energy balance might identify novel therapeutic targets for the treatment and prevention of obesity.

Effect of the combinatory mixture of Rubus coreanus Miquel and Astragalus membranaceus Bunge extracts on ovariectomy-induced osteoporosis in mice and anti-RANK signaling effect.

ETHNOPHARMACOLOGICAL RELEVANCE: Postmenopausal osteoporosis is one of the most common disorders in women after menopause, which is linked to an estrogen deficiency and characterized by an excessive loss of trabecular bone. Rubus coreanus and Astragalus membranaceus have been used for their various pharmacological properties in Asia as a traditional medicine. The present study evaluated the anti-osteoporotic effects of the optimal combination of Rubus coreanus and Astragalus membranaceus in 7:3 mixture (RAM) in ovariectomized (OVX) mice by investigating bone biomechanical properties and the serum levels of TNF-α, osteocalcin, RANKL, OPG, and RANK-RANKL signal-related osteoclast differentiation markers. MATERIALS AND METHODS: A total of 36 mature female outbred ICR (Institute of cancer research) strain mice (7 weeks) were divided into 6 groups with 7 mice in each group as follows: (1) Sham-operated control mice (Sham) received daily oral phosphate-buffered-saline (PBS) of equal volumes through gavage. (2) OVX mice received a daily oral gavage of PBS (OVX). (3) OVX mice were treated daily with 50mg/kgb.w./day of RAM (4) with 100mg/kgb.w./day of RAM or (5) with 200mg/kgb.w./day of RAM via oral gavage. (6) OVX mice received i.p. injections of 17ß-estradiol (E2) (0.1mg/kgb.w./day) three times per week for 12 weeks. RESULTS: Micro-CT images showed that oral administration of RAM to OVX mice prevented tibial bone loss, preserved trabecular bone microarchitecture, and improved bone biomechanical properties. RAM administration also showed recovery effects on the levels of TNF-α, OPG and RANKL concentration in OVX-states. Additionally, we found that the mechanism by which RAM elicited anti-osteoporotic effects was by down-regulating the expression of TRAF6 and NFATc1 in RANKL-RANK pathway, a route of osteoclast differentiation, followed by reducing the production of osteoclast differentiation factors, calcitonin receptors and cathepsin K. CONCLUSIONS: Our research strongly suggests that RAM can be clinically used in the prevention and treatment of postmenopausal osteoporosis.

A calcitonin receptor (CALCR) single nucleotide polymorphism is associated with growth performance and bone integrity in response to dietary phosphorus deficiency.

Although concerns over the environmental impact of excess P in the excreta from pig production and governmental regulations have driven research toward reducing dietary supplementation of P to swine diets for over a decade, recent dramatic increases in feed costs have further motivated researchers to identify means to further reduce dietary P supplementation. We have demonstrated that genetic background impacts P utilization in young pigs and have identified genetic polymorphisms in several target genes related to mineral utilization. In this study, we examined the impact of a SNP in the calcitonin receptor gene (CALCR) on P utilization in growing pigs. In Exp. 1, 36 gilts representing the 3 genotypes identified by this CALCR SNP (11, 12, and 22) were fed a P-adequate (PA) or a marginally P-deficient (approximately 20% less available P; PD) diet for 14 wk. As expected, P deficiency reduced plasma P concentration, bone strength, and mineral content (P < 0.05). However, the dietary P deficiency was mild enough to not affect the growth performance of these pigs. A genotype x dietary P interaction (P < 0.05) was observed in measures of bone integrity and mineral content, with the greatest reduction in bone strength and mineral content due to dietary P deficiency being associated with the allele 1. In Exp. 2, 168 pigs from a control line and low residual feed intake (RFI) line were genotyped for the CALCR SNP and fed a PA diet. As expected, pigs from the low RFI line consumed less feed but also gained less BW when compared with the control line (P < 0.05). Although ADFI did not differ between genotypes, pigs having the 11 genotype gained less BW (P < 0.05) than pigs having the 12 or 22 genotypes. Pigs of the 11 and 12 genotypes had bones that tolerated greater load when compared with animals having the 22 genotype (P < 0.05). A similar trend was observed in bone modulus and ash % (P < 0.10). These data are supportive of the association of this CALCR SNP with bone integrity and its response to dietary P restriction. Although the allele 1 is associated with greater bone integrity and mineral content during adequate P nutrition, it is also associated with the greatest loss in bone integrity and mineral content in response to dietary P restriction. Understanding the underlying genetic mechanisms that regulate P utilization may lead to novel strategies to produce more environmentally friendly pigs.

Identification and biological activity of ovine and caprine calcitonin receptor-stimulating peptides 1 and 2.

We have recently reported the isolation of three new members of the calcitonin (CT) gene-related peptide family of peptides, the CT receptor (CT-R)-stimulating peptides (CRSPs). We now report the sequencing and characterization of ovine/caprine CRSP-1 and caprine CRSP-2. Mature ovine and caprine CRSP-1 are identical and have strong structural homology to CRSP-1s identified to date from other species. As with other CRSP-1s, ovine/caprine CRSP-1 binds to and activates the CT-R but not the CT-like receptor (CL-R) in combination with the receptor activity-modifying proteins (RAMPs). By contrast, caprine CRSP-2 does not activate any of these receptor-RAMP complexes. Intravenous infusions of ovine CRSP-1 to normal conscious sheep induced dose-dependent reduction in plasma total Ca levels (P=0.02) and corrected Ca levels (P=0.017) associated with increases in plasma cAMP (P=0.002). CRSP-1 reduced both plasma amino-terminal pro-C-type natriuretic peptide levels (P=0.006) and plasma renin activity (P=0.028). There were no significant effects observed on hemodynamic or renal indices measured. In conclusion, we have sequenced ovine/caprine CRSP-1 and caprine CRSP-2 precursors. This newly identified CRSP-1 has been shown to share the structural and biological features of CRSP-1s known to date. In vivo studies confirm that ovine CRSP-1 reduces plasma Ca levels in sheep, presumably via a cAMP-mediated mechanism. By contrast, caprine CRSP-2 did not stimulate any combination of CT-R, CL-R, and RAMPs. Accession numbers of cDNA determined in this study are caprine CRSP-1, AB364646; caprine CRSP-2, AB364647; and ovine CRSP-1, AB364648.

Water extract of Cordyceps sinensis (WECS) inhibits the RANKL-induced osteoclast differentiation.

It has been reported that Cordyceps sinensis, a traditional Chinese medicine, has various pharmacological effects. The aim of this study was to clarify the effect of water extract of Cordyceps sinensis (WECS) on osteoclast differentiation in vitro. In mouse bone marrow cells and monocyte/macrophage cell line RAW264.7, WECS dose-dependently inhibited the receptor activator of nuclear factor kappa B (NF-kappaB) ligand (RANKL)-induced osteoclast differentiation by tartrate-resistant acid phosphatase (TRAP) staining. In fact, cytotoxic effect was not observed in the RAW264.7 cells treated with WECS. Moreover, the mRNA expression of osteoclast related genes (calcitonin receptor, cathepsin K, matrix metalloprotease 9 and nuclear factor of activated T cells c1) was also inhibited by WECS. Investigation of inhibitory mechanism by using electrophoretic mobility shift assay (EMSA) and Western blot analysis revealed that WECS inhibited the activation of NF-kappaB through the prevention of IkappaBalpha phosphorylation. In conclusion, the present results demonstrate for the first time that WECS is a potent inhibitor of the RANKL-induced osteoclast differentiation through a mechanism involving the NF-kappaB pathway.

Cloning and characterization of osteoclast precursors from the RAW264.7 cell line.

Osteoclasts are bone-resorbing cells that differentiate from macrophage precursors in response to receptor activator of NF-kappaB ligand (RANKL). In vitro models of osteoclast differentiation are principally based on primary cell cultures, which are poorly suited to molecular and transgene studies because of the limitations associated with the use of primary macrophage. RAW264.7 is a transfectable macrophage cell line with the capacity to form osteoclast-like cells. In the present study, we have identified osteoclast precursors among clones of RAW264.7 cells. RAW264.7 cell were cloned by limiting dilution and induced to osteoclast differentiation by treatment with recombinant RANKL. Individual RAW264.7 cell clones formed tartrate resistant acid phosphatase (TRAP)-positive multinuclear cells to various degrees with RANKL treatment. All clones tested expressed the RANKL receptor RANK. Each of the clones expressed the osteoclast marker genes TRAP and cathepsin-K mRNA with RANKL treatment. However, we noted that only select clones were able to form large, well-spread, TRAP-positive multinuclear cells. Clones capable of forming large TRAP-positive multinuclear cells also expressed beta3 integrin and calcitonin receptor mRNAs and were capable of resorbing a mineralized matrix. All clones tested activated NF-kappaB with RANKL treatment. cDNA expression profiling of osteoclast precursor RAW264.7 cell clones demonstrates appropriate expression of a large number of genes before and after osteoclastic differentiation. These osteoclast precursor RAW264.7 cell clones provide a valuable model for dissecting the cellular and molecular regulation of osteoclast differentiation and activation.

Intermedin1-53 protects the heart against isoproterenol-induced ischemic injury in rats.

Intermedin is a novel member of the calcitonin/calcitonin gene-related peptide (CGRP) family peptide, which has vasodilatory and hypotensive actions identical to those of adrenomedullin and CGRP. Cleavage sites located between 2 basic amino acids at Arg93-Arg94 result in the production of prepro-intermedin95-147, namely intermedin1-53. The bioactive action of intermedin1-53 and its physiological significance are unclear. In this work, we aimed to explore the effects of intermedin1-53 on acute myocardial injury induced by isoproterenol. Myocardial ischemia injury in rats was induced by subcutaneous injection of a high dose of isoproterenol, and the therapeutic effect of intermedin1-53 was observed. Plasma lactate dehydrogenase activity, myocardial and plasma malondialdehyde content were higher in the isoproterenol group than that in controls. Isoproterenol-treated rats showed lower maximal rate of increase and decrease of left-ventricle pressure development (+/-left-ventricle dp/dtmax) and higher left-ventricle end-diastolic pressure (all P<0.01), which suggested severe heart failure and myocardial injury. Semi-quantitative RT-PCR analysis showed that the gene expression of calcitonin receptor-like receptor and receptor-activity-modifying protein (RAMP)1, RAMP2 and RAMP3 in ventricular myocardia were up-regulated by 79% (P<0.01), 48% (P<0.01), 31% (P<0.05) and 130% (P<0.01), respectively, compared with controls. In myocardial sarcolemmal membranes, the maximum binding capacity for [125I]-intermedin1-53 was increased by 118% (P<0.01) in the isoproterenol group compared with controls. Rats treated with low dosage intermedin1-53 (5 nmol/kg/day, 2 days) showed 21% (P<0.05) higher myocardial cAMP content, 18% and 31% higher+left-ventricle dp/dtmax and -left-ventricle dp/dtmax respectively, 288% lower left-ventricle end-diastolic pressure (all P<0.01), and attenuated myocardial lactate dehydrogenase leakage and malondialdehyde formation (all P<0.01). Treatment with high dosage intermedin1-53 (20 nmol/kg/day, 2 days) gave better results than that with low dosage intermedin1-53. These results suggest that the intermedin receptor system was up-regulated in isoproterenol-induced myocardial ischemic injury and intermedin1-53 might play a pivotal cardioprotective role in such injury.

Genetic background influences metabolic response to dietary phosphorus restriction.

Dietary phosphorus (P) is essential to bone growth and turnover; however, little research has focused on the genetic mechanisms controlling P utilization. Understanding the interactions between genetics and dietary P that optimize bone integrity could provide novel interventions for osteoporosis. Thirty-six pigs from two sire lines known to differ in bone structure [heavier boned (HB) and lighter boned (LB)] were assigned to one of the three diets (P adequate, P repletion or P deficient). After 14 days, bone marrow and intact radial bones were collected. Differences between these lines in growth rate, bone integrity and gene expression within bone marrow were observed. In HB, but not LB, pigs, the P-deficient diet decreased weight gain (P<.01). For both lines, P deficiency caused a reduction in radial bone strength (P<.01), but HB P-deficient animals had greater (P<.10) bone integrity than P-deficient LB pigs. In HB, but not LB, pigs, dietary treatment affected the expression of CALCR (calcitonin receptor) (P<.05), VDR (vitamin D receptor) (P<.04) and IGFBP3 (insulin-like growth factor binding protein 3) (P<.06). There was also a trend of increased IL6 (interleukin-6), TFIIB (transcription initiation factor IIB) and SOX9 (sex determining region Y-box 9) expression with P deficiency in HB, but not LB, pigs. Both genetic backgrounds responded similarly to P deficiency with an increase in the expression of OXTR (oxytocin receptor) and IGF1 (insulin-like growth factor 1). Differences in growth rate, bone integrity and gene expression within the bone marrow suggest a difference in the homeorhetic control of P utilization between these genetic lines. Understanding these differences could lead to novel treatments for osteoporosis and aid in the development of tests for identifying those at risk for this disease.

Estrogen and phytoestrogen regulate the mRNA expression of adrenomedullin and adrenomedullin receptor components in the rat uterus.

The serum estrogen surge in the uterus triggers precisely-timed physiological and biochemical responses required establishing and maintaining pregnancy. Previous reports have shown that consumption of phytoestrogen-containing plants may disrupt the precise control of pregnancy. To evaluate the effects of phytoestrogens in the uterus, we screened for estradiol (E2)-inducible genes in immature rat uteri. We identified the gene for receptor-activity-modifying protein 2 (Ramp2), known to be a component of the adrenomedullin (ADM) receptor, as responsive to both E2 and the phytoestrogen coumestrol (Cou). We further examined the expression of ADM and ADM signaling components Ramp2, Ramp3, and CRLR in the immature rat uterus and found that both E2 and Cou regulated these genes expression. In addition, treatment with ADM increased uterine weight and edema similar to that observed after Cou treatment. Our findings indicated that the phytoestrogen caused the abnormal induction of vasoactive factors in the uterus.

Molecular cloning of otoconin-22 complementary deoxyribonucleic acid in the bullfrog endolymphatic sac: effect of calcitonin on otoconin-22 messenger ribonucleic acid levels.

Anuran amphibians have a special organ called the endolymphatic sac (ELS), containing many calcium carbonate crystals, which is believed to have a calcium storage function. The major protein of aragonitic otoconia, otoconin-22, which is considered to be involved in the formation of calcium carbonate crystals, has been purified from the saccule of the Xenopus inner ear. In this study, we cloned a cDNA encoding otoconin-22 from the cDNA library constructed for the paravertebral lime sac (PVLS) of the bullfrog, Rana catesbeiana, and sequenced it. The bullfrog otoconin-22 encoded a protein consisting of 147 amino acids, including a signal peptide of 20 amino acids. The protein had cysteine residues identical in a number and position to those conserved among the secretory phospholipase A(2) family. The mRNA of bullfrog otoconin-22 was expressed in the ELS, including the PVLS and inner ear. This study also revealed the presence of calcitonin receptor-like protein in the ELS, with the putative seven-transmembrane domains of the G protein-coupled receptors. The ultimobranchialectomy induced a prominent decrease in the otoconin-22 mRNA levels of the bullfrog PVLS. Supplementation of the ultimobranchialectomized bullfrogs with synthetic salmon calcitonin elicited a significant increase in the mRNA levels of the sac. These findings suggest that calcitonin secreted from the ultimobranchial gland, regulates expression of bullfrog otoconin-22 mRNA via calcitonin receptor-like protein on the ELS, thereby stimulating the formation of calcium carbonate crystals in the lumen of the ELS.

Use of constitutive G protein-coupled receptor activity for drug discovery.

This article describes the behavior of transiently transfected human receptors into melanophores and the potential use of constitutive receptor activity to screen for new drug entities. Specifically, transient transfection of melanophores with different concentrations of receptor cDNA presumably leads to increased levels of receptor expression. This leads to an increased response to agonists (both maxima and potency) and, in some cases, an agonist-independent constitutive receptor activity. Transfections with increasing concentrations of the G(s) protein-coupled human calcitonin receptor type 2 (hCTR2) cDNA produced sufficient levels of constitutively activated receptor to cause elevated basal cellular responses. This was observed as a decrease in the transmittance of light through melanophores (consistent with G(s) protein activation) and increased response to human calcitonin. The receptor-mediated nature of this response was confirmed by its reversal with the hCTR2 peptide inverse agonist AC512. A collection of ligands for hCTR2 either increased or decreased constitutive hCTR2 activity, suggesting that the constitutive system was a sensitive discriminator of positive and negative ligand efficacy. Similar results were obtained with G(i)-protein-coupled receptors. Transient transfection of NPY1, NPY2, NPY4, CXCR4, and CCR5 cDNA produced increased light transmittance through melanophores (consistent with G(i)-protein activation). NPY1 cDNA produced little constitutive response on transfection, whereas maximal levels of constitutive activity ranging from 30 to 45% were observed for the other G(i)-protein-coupled receptors. Responses to agonists for these receptors increased (both maxima and potency) with increasing cDNA transfection. The receptor/G(i)-protein nature of both the constitutive and agonist-mediated responses was confirmed by elimination with pertussis toxin pretreatment. These data are discussed in terms of the theoretical aspects of constitutive receptor activity and the applicability of this approach for the general screening of G protein-coupled orphan receptors.

Constitutive receptor systems for drug discovery.

This paper discusses the use of constitutively active G-protein-coupled receptor systems for drug discovery. Specifically, the ternary complex model is used to define the two major theoretical advantages of constitutive receptor screening-namely, the ability to detect antagonists as well as agonists directly and the fact that constitutive systems are more sensitive to agonists. In experimental studies, transient transfection of Chinese hamster ovary cyclic AMP response element (CRE) luciferase reporter cells with cDNA for human parathyroid hormone receptor, glucagon receptor, and glucagon-like peptide (GLP-1) receptor showed cDNA concentration-dependent constitutive activity with parathyroid hormone (PTH-1) and glucagon. In contrast, no constitutive activity was observed for GLP-1 receptor, yet responses to GLP-1 indicated that receptor expression had taken place. In another functional system, Xenopus laevi melanophores transfected with cDNA for human calcitonin receptor showed constitutive activity. Nine ligands for the calcitonin receptor either increased or decreased constitutive activity in this assay. The sensitivity of the system to human calcitonin increased with increasing constitutive activity. These data indicate that, for those receptors which naturally produce constitutive activity, screening in this mode could be advantageous over other methods.

Expression cloning and receptor pharmacology of human calcitonin receptors from MCF-7 cells and their relationship to amylin receptors.

Human breast cell carcinoma MCF-7 cells were found to bind 125I-labeled rat amylin (rAmylin) and the peptide amylin antagonist radioligand 125I-AC512 with high affinity. This high affinity binding possessed characteristics unique to the already defined high affinity binding site for amylin in the rat nucleus accumbens [Mol. Pharmacol. 44:493-497 (1993); J. Pharmacol. Exp. Ther. 270:779-787 (1994); Eur. J. Pharmacol. 262:133-141 (1994)]. To further define this receptor, we report results of expression cloning studies from an MCF-7 cell library. We isolated two variants of a seven-transmembrane receptor that were identical to two previously described human calcitonin receptors (hCTR1 and hCTR2). These receptors were characterized by expression in different surrogate host cell systems. Transient expression of hCTR1 in COS cells yielded membranes that bound 125I-AC512 and 125I-salmon calcitonin with high affinity, but no high affinity binding was observed with 125I-human calcitonin (hCAL) or 125I-rAmylin. Stable expression of hCTR1 in HEK 293 cells produced similar data. In contrast, expression of hCTR2 in COS cells yielded membranes that bound 125I-AC512, 125I-hCAL, and 125I-rAmylin with high affinity. The agonists 125I-hCAL and 125I-rAmylin bound 65% and 1.5%, respectively, of the sites bound by the antagonist radioligand 125I-AC512 in this expression system. This pattern of binding was repeated in HEK 293 cells stably transfected with hCTR2 (125I-hCAL = 24.8% Bmax, 125I-rAmylin = 8% Bmax). In both expression systems, the agonists hCAL and rAmylin were much more potent in displacing their radioligand counterparts than was the antagonist radioligand 125I-AC512. For example, the pKi value for displacement of 125I-AC512 by rAmylin was 7.2 in HEK 293 cells but rose to 9.1 when displacing 125I-rAmylin. Finally, hCTR2 was expressed in baculovirus-infected Ti ni cells. In this system, only specific binding to the antagonist 125I-AC512 and agonist 125I-hCAL was observed; no binding to 125I-rAmylin could be detected. These data are discussed in terms of two working hypotheses. The first is that amylin is a weak agonist for hCTR2 and that this receptor is unrelated to the amylin receptor found in this cell line. The second is that hCTR2 couples to different G proteins for calcitonin and amylin function in different cells. At present, these data cannot be used to disprove conclusively either hypothesis.

Electrophoretic mobility and glycosylation characteristics of heterogeneously expressed calcitonin receptors.

The translated calcitonin receptor (CTR) complementary DNA sequences contain potential N-linked glycosylation sites within the extracellular N-terminus. We investigated the relative molecular mass (M(r)) and degree of N-linked glycosylation of five cloned CTRs (pig, rat C1a, rat C1b, human I1-ve, and human I1+ve), together with the pig hypothalamic CTR, to analyze the potential contribution of carbohydrate moieties to the molecular identity of these receptors. Receptors were cross-linked to 125I-salmon CT with the homobifunctional reagent bis(sulfosuccinimidyl) suberate. Autoradiographic analysis of the cross-linked receptors, following SDS-PAGE, revealed apparent M(r)S, ranging between 70,000 and 80,000 for the rat, human, and pig hypothalamic receptors. However, the cloned, expressed pig CTR was much smaller (approximately 58,000). The lower M(r) of the cloned pig CTR appeared to be due to absence of N-terminal residues, but this did not impact on ligand-receptor specificity when compared with the hypothalamic pig CTR. Cleavage under nondenaturing conditions of N-linked sugars from the CTRs using endoglycosidase F (Endo F), increased the electrophoretic mobility of all receptors, except the pig CTRs, by approximately 10 kDa. Under denaturing conditions, electrophoretic mobilities increased by approximately 30 kDa for the rat C1a, rat C1b, and humanI1-ve (expressed in human embryonic kidney-293 cells) CTRs and by approximately 20 kDa for the cloned pig, pig hypothalamic, and human CTR isoforms (expressed in baby hamster kidney cells). Competition binding studies using glycosylated and partially deglycosylated (nondenaturing conditions) receptor preparations demonstrated no significant differences in binding affinity or specificity. Thus the CTRs are N-linked glycoproteins whose degree of glycosylation is both cell-type and species dependent.