Somatostatin receptor subtypes 2 and 5 mediate inhibition of egg yolk-induced gall bladder emptying in mice

BACKGROUND

Somatostatin inhibits gall bladder contraction. Impaired gall bladder emptying is associated with gall bladder stone formation. The incidence of cholecystolithiasis is high in patients treated with a somatostatin agonist octreotide, which predominantly interacts with somatostatin receptor subtype 2 (SSTR2). Therefore, it is believed that SSTR2 regulates gall bladder contraction; however, evidence has not been provided. Here, we evaluate the effects of SSTR1-SSTR5-selective agonists on egg yolk-induced gall bladder contraction in mice.

METHODS

Homozygous deletion of SSTR2 and SSTR5 was generated by cross-mating of SSTR2(-/-) with SSTR5(-/-) mice. Mice of different genotypes were injected with SSTR1-5-selective agonists or octreotide 15 min before induction of gall bladder emptying by egg yolk. One hour later, gall bladders were removed and weighed.

KEY RESULTS

Egg yolk-reduced gall bladder weights in all mice, irrespective of their genotype. Octreotide was the most potent inhibitor of gall bladder emptying in wild-type mice. In contrast, agonists with high selectivity for SSTR2 or SSTR5 inhibited gall bladder emptying by approximately 50-60%, whereas SSTR1-, SSTR3- and SSTR4-selective agonists failed to influence gall bladder contraction. In SSTR2(-/-) mice, octreotide and an SSTR5-selective agonist inhibited gall bladder emptying by approximately 50%, whereas SSTR2-selective agonists were inactive. Octreotide inhibited gall bladder emptying in SSTR5(-/-) mice by approximately 50%, without any effect in SSTR2(-/-)/SSTR5(-/-) mice.

CONCLUSIONS & INFERENCES

Our study provides evidence for the role of SSTR2 and SSTR5 in regulating gall bladder emptying in mice.

The discovery of tetrahydrofluorenones as a new class of estrogen receptor beta-subtype selective ligands

Synthesis and derivatization of a series of substituted tetrahydrofluorenone analogs giving potent, ERbeta subtype selective ligands are described. Several analogs possessing ERbeta binding affinities comparable to 17beta-estradiol but with greater than 75-fold selectivity over ERalpha are reported.

Distinct effects of the antiestrogen Faslodex on the stability of estrogen receptors-alpha and -beta in the breast cancer cell line MCF-7

The effects of estrogen receptor (ER) ligands on the stability and transcriptional activity of ERbeta in the breast cancer cell lines MCF-7 and HeLa were examined. We found that ERbeta was degraded in the presence of 17beta-estradiol. Tamoxifen and Faslodex (ICI 182,780) prevented ERbeta receptor destabilization. In contrast to ERalpha, ERbeta degradation was not abolished by inhibitors of the proteasome-mediated protein degradation pathway. Furthermore, single point mutations in helix 12 of the receptor dramatically affected the stability and subsequent transcriptional activation of ERbeta.

A potent, nonpeptidyl 1H-quinolone antagonist for the gonadotropin-releasing hormone receptor

Extensive development of the structure-activity relationships of a screening lead determined three important pharmacophores for gonadotropin-releasing hormone (GnRH) receptor antagonist activity. Incorporation of the 3,4,5-trimethylphenyl group at the 3-position, 2-(2(S)-azetidinyl)ethoxy group at the 4-position, and N-4-pyrimidinylcarboxamide at the 6-position of the quinolone core resulted in the identification of 4-(2-(azetidin-2(S)-yl)ethoxy)-7-chloro-2-oxo-3-(3,4,5-trimethylphenyl)-1,2-dihydroquinoline-6-carboxylic acid pyrimidin-4-ylamide (1) as a potent antagonist of the GnRH receptor. A 10(4)-fold increase in in vitro binding affinity is observed for the GnRH receptor as compared to the initial screening lead. Compound 1 exhibits nanomolar binding activity and functional antagonism at the human receptor and is 7-fold less active at the rhesus receptor. Intravenous administration of compound 1 to rhesus monkeys results in a significant decrease of the serum levels of downstream hormones, luteinizing hormone (79% decrease in area under the curve) and testosterone (92% decrease in area under the curve), at a dose of 3 mg/kg. Quinolone 1 is a potent nonpeptidyl antagonist for the human GnRH receptor that is efficacious for the suppression of luteinizing hormone and testosterone in primates.

Nipecotic and iso-nipecotic amides as potent and selective somatostatin subtype-2 receptor agonists

N-Substituted nipecotic and iso-nipecotic amides of beta-methylTrpLys tert-butyl ester were found to be novel, selective and potent agonists of the somatostatin subtype-2 receptor in vitro. For example iso-nipecotic amide 8a showed high hsst2 binding affinity (Ki = 0.5 nM) and good selectivity (h5/h2 = 832).

Anticonvulsant and adverse effects of avermectin analogs in mice are mediated through the gamma-aminobutyric acid(A) receptor

Twenty-five avermectin analogs were assessed in a mouse seizure model. The ED(50) against pentylenetetrazole-induced tonic seizures ranged from 0.48 mg/kg (L-676,893) to >160 mg/kg (L-685,869) cf. 0. 26 mg/kg for diazepam. Although avermectins are without acute toxic effects, they have been historically shown to have relative low LD(50) values in mammals. The mechanisms involved in the anticonvulsant effect and the toxicity were investigated. A series of avermectin analogs displaced [(3)H]ivermectin binding to rat brain membranes and recombinant GABA(A) receptors (alpha1beta3gamma2-subtype) with the same affinities, strongly suggesting that [(3)H]ivermectin labels the GABA(A) receptor in rodent brain. Avermectins, which were anticonvulsant, were also potent inhibitors of [(3)H]ivermectin binding in rat brain. However, the rank order for anticonvulsant activity did not parallel the rank order for affinity at the [(3)H]ivermectin site and it was reasoned that avermectins may have differential affinity or efficacy at subtypes of the GABA(A) receptor. All the active compounds tested potentiated the effects of GABA at recombinant GABA(A) receptors in oocytes and at native cortical GABA(A) receptors and the efficacy of avermectins at the GABA(A) receptor correlated best with their anticonvulsant potency. Although avermectins weakly inhibited [(3)H]strychnine binding in rat spinal cord, and inhibited glycine responses on primary cultured cortical neurons, activity at glycine receptors did not correlate with either anticonvulsant activity or toxicity. Because both anticonvulsant activity and toxicity correlated best with activity at GABA(A) receptors, it is unlikely that these effects can be separated, which may contraindicate the potential use of avermectins as anticonvulsants.

Identification of Phe313 of the gonadotropin-releasing hormone (GnRH) receptor as a site critical for the binding of nonpeptide GnRH antagonists

The dog GnRH receptor was cloned to facilitate the identification and characterization of selective nonpeptide GnRH antagonists. The dog receptor is 92% identical to the human GnRH receptor. Despite such high conservation, the quinolone-based nonpeptide GnRH antagonists were clearly differentiated by each receptor species. By contrast, peptide antagonist binding and functional activity were not differentiated by the two receptors. The basis of the differences was investigated by preparing chimeric receptors followed by site-directed mutagenesis. Remarkably, a single substitution of Phe313 to Leu313 in the dog receptor explained the major differences in binding affinities and functional activities. The single amino acid replacement of Phe313 of the human receptor with Leu313 resulted in a 160-fold decrease of binding affinity of the nonpeptide antagonist compound 1. Conversely, the replacement of Leu313 of the dog receptor with Phe313 resulted in a 360-fold increase of affinity for this compound. These results show that Phe313 of the GnRH receptor is critical for the binding of this structural class of GnRH antagonists and that the dog receptor can be "humanized" by substituting Leu for Phe. This study provides the first identification of a critical residue in the binding pocket occupied by nonpeptide GnRH antagonists and reinforces cautious extrapolation of ligand activity across highly conserved receptors.

Identification and characterization of subtype selective somatostatin receptor agonists

High affinity, subtype selective non-peptide agonists of somatostatin receptor subtypes 1-5 were identified in combinatorial libraries constructed based on molecular modeling of known peptide agonists. Simultaneous traditional chemical synthesis yielded an additional series of somatostatin subtype-2 receptor (SSTR2) selective agonists. These compounds have been used to further define the physiological functions of the individual somatostatin receptor subtypes. In vitro experiments demonstrated the role of the SSTR2 in inhibition of glucagon release from mouse pancreatic alpha-cells and the somatostatin subtype-5 receptor (SSTR5) as a mediator of insulin secretion from pancreatic beta-cells. Both SSTR2 and SSTR5 regulated growth hormone release from the rat anterior pituitary gland. In vivo studies performed with SSTR2 receptor selective compounds demonstrated effective inhibition of pulsatile growth hormone release in rats. The SSTR2 selective compounds also lowered plasma glucose levels in normal and diabetic animal models. The availability of high affinity, subtype selective non-peptide agonists for each of the somatostatin receptors provides a direct approach to defining their physiological function both peripherally and in the central nervous system.

Modeling directed design and biological evaluation of quinazolinones as non-peptidic growth hormone secretagogues

Quinazolinone derivatives were synthesized and evaluated as non-peptidic growth hormone secretagogues. Modeling guided design of quinazolinone compound 21 led to a potency enhancement of greater than 200-fold compared to human growth hormone secretagogue affinity of a screening lead 4.

Somatostatin inhibits insulin and glucagon secretion via two receptors subtypes: an in vitro study of pancreatic islets from somatostatin receptor 2 knockout mice

Somatostatin (SST) potently inhibits insulin and glucagon release from pancreatic islets. Five distinct membrane receptors (SSTR1-5) for SST are known, and at least two (SSTR2 and SSTR5) have been proposed to regulate pancreatic endocrine function. Our current understanding of SST physiology is limited by the receptor subtype selectivity of peptidyl SST analogs, making it difficult to assign a physiological function to an identified SST receptor subtype. To better understand the physiology of SSTRs we studied the in vitro effects of potent subtype-selective nonpeptidyl SST analogs on the regulation of pancreatic glucagon and insulin secretion in wild-type (WT) and in somatostatin receptor 2 knockout (SSTR2KO) mice. There was no difference in basal glucagon and insulin secretion between islets isolated from SSTR2KO and WT mice; however, potassium/arginine-stimulated glucagon secretion was approximately 2-fold higher in islets isolated from SSTR2KO mice. Neither SST nor any SSTR-selective agonist inhibited basal glucagon or insulin release. SST-14 potently inhibited stimulated glucagon secretion in islets from WT mice and much less effectively in islets from SSTR2KO mice. The SSTR2 selective analog L-779,976 inhibited glucagon secretion in islets from WT, but was inactive in islets from SSTR2KO mice. L-817,818, an SSTR5 selective analog, slightly reduced glucagon release in both animal groups, whereas SSTR1, -3, and -4 selective analogs were inactive. SST and L-817,818 inhibited glucose stimulated insulin release in islets from WT and SSTR2KO mice. L-779,976 much less potently reduced insulin secretion from WT islets. In conclusion, our data demonstrate that SST inhibition of glucagon release in mouse islets is primarily mediated via SSTR2, whereas insulin secretion is regulated primarily via SSTR5.

Regulation of vascular endothelial growth factor-dependent retinal neovascularization by insulin-like growth factor-1 receptor

Although insulin-like growth factor 1 (IGF-1) has been associated with retinopathy, proof of a direct relationship has been lacking. Here we show that an IGF-1 receptor antagonist suppresses retinal neovascularization in vivo, and infer that interactions between IGF-1 and the IGF-1 receptor are necessary for induction of maximal neovascularization by vascular endothelial growth factor (VEGF). IGF-1 receptor regulation of VEGF action is mediated at least in part through control of VEGF activation of p44/42 mitogen-activated protein kinase, establishing a hierarchical relationship between IGF-1 and VEGF receptors. These findings establish an essential role for IGF-1 in angiogenesis and demonstrate a new target for control of retinopathy. They also explain why diabetic retinopathy initially increases with the onset of insulin treatment. IGF-1 levels, low in untreated diabetes, rise with insulin therapy, permitting VEGF-induced retinopathy.

Nonpeptidyl somatostatin agonists demonstrate that sst2 and sst5 inhibit stimulated growth hormone secretion from rat anterior pituitary cells

Somatostatin (SST) regulates growth hormone (GH) secretion from pituitary somatotrophs by interacting with members of the SST family of G-protein-coupled receptors (sst1-5). We have used potent, nonpeptidyl SST agonists with sst2 and sst5 selectivity to determine whether these receptor subtypes are involved in regulating growth hormone releasing hormone (GHRH) stimulated secretion. GHRH stimulated GH release from pituitary cells in a dose-dependent manner, and this secretion was inhibited by Tyr(11)-SST-14, a nonselective SST analog. A sst2 selective agonist, L-779,976, potently inhibited GHRH-stimulated GH release. In addition, L-817, 818, a potent sst5 receptor selective agonist, also inhibited GH secretion, but was approximately 10-fold less potent (P < 0.01, ANOVA) in inhibiting GH release than either Tyr(11)-SST-14 or L-779, 976. These results show that both sst2 and sst5 receptor subtypes regulate GHRH-stimulated GH release from rat pituitary cells.

A combinatorial approach toward the discovery of non-peptide, subtype-selective somatostatin receptor ligands

The tetradecapeptide somatostatin is widely distributed throughout the body and is thought to be involved with a variety of regulatory functions. Recently, five human somatostatin receptors (hSSTR1-5) have been cloned and characterized. Several selective peptidal agonists of the hSSTR receptors are known, and we sought to apply this information to the design of novel non-peptide small molecule ligands for each receptor. Initial computational methods identified a 200 nM murine SSTR2 active compound via a database search of our sample collection. A combinatorial library was designed around the structural class of the compound with the goal of rapidly developing this initial lead into the desired subtype-selective small molecules in order to characterize the pharmacology of each of the receptor subtypes. The library was synthesized using the resin-archive, iterative deconvolution format. The total number of unique compounds in the library was expected to be 131,670, present in 79 mixtures of 1330 or 2660 compounds per mixture. Through sequences of screening and mixture deconvolution, the components of selective and highly active (Ki = 50 pM to 200 nM) non-peptide small molecule ligands for somatostatin subtypes 1, 2, 4, and 5 were identified. In addition to discovering compounds with the desired activity and selectivity, useful structure/activity information was generated which can be used in the design of new compounds and second-generation combinatorial libraries.

Colocalization of somatostatin receptor sst5 and insulin in rat pancreatic beta-cells

Somatostatin, also known as somatotropin release-inhibiting factor (SRIF), is secreted by pancreatic delta-cells and inhibits the secretion of both insulin and glucagon. SRIF initiates its actions by binding to a family of six G protein-coupled receptors (sst1, -2A, -2B, -3, -4, and -5) encoded by five genes. Messenger RNA for both sst2 and sst5 have been reported in the rat pancreas, and the sst2A receptor protein has been localized to rat pancreatic alpha and pancreatic polypeptide-secreting cells in the islets as well as to pancreatic acinar cells. In this study we have used double immunostaining to show that the sst5 protein is expressed exclusively in the beta-cells of rat pancreatic islets and localizes with insulin-secreting alpha-cells. The sst5 receptor is not colocalized with sst2A. Thus, in the rat SRIF inhibits pancreatic insulin and glucagon secretion via different sst receptor subtypes.

Potent, orally bioavailable somatostatin agonists: good absorption achieved by urea backbone cyclization

Backbone cyclization of urea-based somatostatin agonists resulted in novel, orally bioavailable agonists. Binding assays confirmed that the resulting conformationally constrained cyclic ureas retained the potency of their acyclic counterparts. SAR studies subsequently led to highly potent analogs, selective for receptor subtype 2, and having good oral bioavailability.

Picrotoxin blockade of invertebrate glutamate-gated chloride channels: subunit dependence and evidence for binding within the pore

Glutamate-gated chloride channels have been described in nematodes, insects, crustaceans, and mollusks. Subunits from the nematode and insect channels have been cloned and are phylogenetically related to the GABA and glycine ligand-gated chloride channels. Ligand-gated chloride channels are blocked with variable potency by the nonselective blocker picrotoxin. The first two subunits of the glutamate-gated chloride channel family, GluClalpha and GluClbeta, were cloned from the free living nematode Caenorhabditis elegans. In this study, we analyze the blockade of these novel channels by picrotoxin. In vitro synthesized GluClalpha and GluClbeta RNAs were injected individually or coinjected into Xenopus oocytes. The EC50 values for picrotoxin block of homomeric GluClalpha and GluClbeta were 59 microM and 77 nM, respectively. Picrotoxin block of homomeric GluClbeta channels was promoted during activation of membrane current with glutamate. In addition, recovery from picrotoxin block was faster during current activation by glutamate. A chimeric channel between the N-terminal extracellular domain of GluClalpha and the C-terminal membrane-spanning domain of GluClbeta localized the higher affinity picrotoxin binding site to the membrane-spanning domains of GluClbeta. A point mutation within the M2 membrane-spanning domain of GluClbeta reduced picrotoxin sensitivity >10,000-fold. We conclude that picrotoxin blocks GluCl channels by binding to a site accessible when the channel is open.

Rapid identification of subtype-selective agonists of the somatostatin receptor through combinatorial chemistry

Nonpeptide agonists of each of the five somatostatin receptors were identified in combinatorial libraries constructed on the basis of molecular modeling of known peptide agonists. In vitro experiments using these selective compounds demonstrated the role of the somatostatin subtype-2 receptor in inhibition of glucagon release from mouse pancreatic alpha cells and the somatostatin subtype-5 receptor as a mediator of insulin secretion from pancreatic beta cells. Both receptors regulated growth hormone release from the rat anterior pituitary gland. The availability of high-affinity, subtype-selective agonists for each of the somatostatin receptors provides a direct approach to defining their physiological functions.

Nuclear receptors have distinct affinities for coactivators: characterization by fluorescence resonance energy transfer

Ligand-dependent interactions between nuclear receptors and members of a family of nuclear receptor coactivators are associated with transcriptional activation. Here we used fluorescence resonance energy transfer (FRET) as an approach for detecting and quantitating such interactions. Using the ligand binding domain (LBD) of peroxisome proliferator-activated receptor (PPARgamma) as a model, known agonists (thiazolidinediones and delta12, 14-PGJ2) induced a specific interaction resulting in FRET between the fluorescently labeled LBD and fluorescently labeled coactivators [CREB-binding protein (CBP) or steroid receptor coactivator-1 (SRC-1)]. Specific energy transfer was dose dependent; individual ligands displayed distinct potency and maximal FRET profiles that were identical when results obtained using CBP vs. SRC-1 were compared. In addition, half-maximally effective agonist concentrations (EC59s) correlated well with reported results using cell-based assays. A site-directed AF2 mutant of PPARgamma (E471A) that abrogated ligand-stimulated transcription in transfected cells also failed to induce ligand-mediated FRET between PPARgamma LBD and CBP or SRC-1. Using estrogen receptor (ERalpha) as an alternative system, known agonists induced an interaction between ERalpha LBD and SRC-1, whereas ER antagonists disrupted agonist-induced interaction of ERalpha with SRC-1. In the presence of saturating agonist concentrations, unlabeled CBP or SRC-1 was used to compete with fluorescently labeled coactivators with saturation kinetics. Relative affinities for the individual receptor-coactivator pairs were determined as follows: PPARgamma-CBP = ERalpha-SRC-1 > PPARgamma-SRC-1 >> ERalpha-CBP.

CONCLUSIONS

1) FRET-based coactivator association is a novel approach for characterizing nuclear receptor agonists or antagonists; individual ligands display potencies that are predictive of in vivo effects and distinct profiles of maximal activity that are suggestive of alternative receptor conformations. 2) PPARgamma interacts with both CBP and SRC-1; transcriptional activation and coactivator association are AF2 dependent. 3) Nuclear receptor LBDs have distinct affinities for individual coactivators; thus, PPARgamma has a greater apparent affinity for CBP than for SRC-1, whereas ERalpha interacts preferentially with SRC-1 but very weakly with CBP.

Synthesis and biological activities of potent peptidomimetics selective for somatostatin receptor subtype 2

A series of nonpeptide somatostatin agonists which bind selectively and with high affinity to somatostatin receptor subtype 2 (sst2) have been synthesized. One of these compounds, L-054,522, binds to human sst2 with an apparent dissociation constant of 0.01 nM and at least 3,000-fold selectivity when evaluated against the other somatostatin receptors. L-054,522 is a full agonist based on its inhibition of forskolin-stimulated adenylate cyclase activity in Chinese hamster ovary-K1 cells stably expressing sst2. L-054,522 has a potent inhibitory effect on growth hormone release from rat primary pituitary cells and glucagon release from isolated mouse pancreatic islets. Intravenous infusion of L-054,522 to rats at 50 microgram/kg per hr causes a rapid and sustained reduction in growth hormone to basal levels. The high potency and selectivity of L-054, 522 for sst2 will make it a useful tool to further characterize the physiological functions of this receptor subtype.

High basal gastric acid secretion in somatostatin receptor subtype 2 knockout mice

BACKGROUND & AIMS

Somatostatin receptor subtype 2 (sst2) agonists inhibit gastric secretion. The role of sst2 in the regulation of acid secretion was assessed using sst2 knockout mice and urethane to induce somatostatin release.

METHODS

Acid secretion was monitored every 10 minutes by gastric perfusion and backtitration of perfusates in fasted, urethane-anesthetized C57/129 sst2 (-/-) mice and wild-type (+/+) mice. The ileal vein was cannulated for drug injection. Intragastric pH and serum gastrin were monitored 1 hour after anesthesia without perfusion.

RESULTS

Gastric pH values were lower in sst2 (-/-) mice (3.8 +/- 0.3) than in wild-type mice (7.1 +/- 0.1, P < 0.05), and there was no difference in gastrin levels. Basal acid output per 2 hours was 10-fold higher in sst2 knockout mice compared with wild-type mice. The gastrin antibody abolished the high basal acid secretion in sst2 (-/-) mice and had no effect in wild-type mice. The somatostatin antibody increased basal secretion by 4-fold in wild-type and had no effect in knockout mice. Somatostatin 14 or the sst2 agonist DC 32-87 inhibited pentagastrin-stimulated acid secretion in wild-type mice, but did not alter basal secretion in knockout mice.

CONCLUSIONS

These results indicate that sst2 is the main subtype whereby endogenous somatostatin suppresses gastric acid secretion through inhibition of gastrin action.

Genetic and biochemical evidence for a novel avermectin-sensitive chloride channel in Caenorhabditis elegans. Isolation and characterization

Avermectins are a class of macrocyclic lactones that is widely used in crop protection and to treat helminth infections in man and animals. Two complementary DNAs (GluClalpha and GluClbeta) encoding chloride channels that are gated by avermectin and glutamate, respectively, were isolated from Caenorhabditis elegans. To study the role of these subunits in conferring avermectin sensitivity we isolated a mutant C. elegans strain with a Tc1 transposable element insertion that functionally inactivated the GluClalpha gene (GluClalpha::Tc1). GluClalpha::Tc1 animals exhibit a normal phenotype including typical avermectin sensitivity. Xenopus oocytes expressing GluClalpha::Tc1 strain mRNA elicited reduced amplitude avermectin and glutamate-dependent chloride currents. Avermectin binding assays in GluClalpha::Tc1 strain membranes showed the presence of high affinity binding sites, with a reduced Bmax. These experiments suggest that GluClalpha is a target for avermectin and that additional glutamate-gated and avermectin-sensitive chloride channel subunits exist in C. elegans. We isolated a cDNA (GluClalpha2) encoding a chloride channel that shares 75% amino acid identity with GluClalpha. This subunit forms homomeric channels that are gated irreversibly by avermectin and reversibly by glutamate. GluClalpha2 coassembles with GluClbeta to form heteromeric channels that are gated by both ligands. The presence of subunits related to GluClalpha may explain the low level and rarity of target site involvement in resistance to the avermectin class of compounds.

Somatostatin receptor subtype 2 knockout mice are refractory to growth hormone-negative feedback on arcuate neurons

The pulsatile nature of GH release is apparently regulated by alternating sequential changes in two hypothalamic hormones, GH releasing hormone (GHRH) and somatostatin. Entrainment of this pulsatility appears to involve GH-mediated negative feedback. Recently a new receptor involved in GH release was cloned. Activation of this receptor by GH-releasing peptides and MK-0677 initiates and amplifies GH pulsatility and is associated with increased Fos immunoreactivity and electrical activity in GHRH containing arcuate neurons. We show that pretreating mice with GH blocks activation of these neurons by MK-0677. Similarly, octreotide inhibited the action of MK-0677. To determine whether this GH-mediated negative feedback on GHRH neurons was direct, or by GH stimulation of somatostatin release from periventricular neurons, we selectively inactivated the gene for one of the five specific somatostatin receptor subtypes (subtype 2). In the knockout mice, both GH and octreotide failed to inhibit MK-0677 activation of arcuate neurons. GH did, however, increase Fos immunoreactivity in the periventricular nucleus, consistent with GH stimulation of somatostatin release from periventricular neurons. Thus, GH-mediated negative feedback involves signaling between periventricular and arcuate neurons with the signal being transduced specifically through somatostatin subtype 2 receptors.

Essential role of growth hormone in ischemia-induced retinal neovascularization

Retinal neovascularization is the major cause of untreatable blindness. The role of growth hormone (GH) in ischemia-associated retinal neovascularization was studied in transgenic mice expressing a GH antagonist gene and in normal mice given an inhibitor of GH secretion (MK678). Retinal neovascularization was inhibited in these mice in inverse proportion to serum levels of GH and a downstream effector, insulin-like growth factor-I (IGF-I). Inhibition was reversed with exogenous IGF-I administration. GH inhibition did not diminish hypoxia-stimulated retinal vascular endothelial growth factor (VEGF) or VEGF receptor expression. These data suggest that systemic inhibition of GH or IGF-I, or both, may have therapeutic potential in preventing some forms of retinopathy.

Evolutionary relationship of the ligand-gated ion channels and the avermectin-sensitive, glutamate-gated chloride channels

Two cDNAs, GluClalpha and GluClbeta, encoding glutamate-gated chloride channel subunits that represent targets of the avermectin class of antiparasitic compounds, have recently been cloned from Caenorhabditis elegans (Cully et al., Nature, 371, 707-711, 1994). Expression studies in Xenopus oocytes showed that GluClalpha and GluClbeta have pharmacological profiles distinct from the glutamate-gated cation channels as well as the gamma-aminobutyric acid (GABA)- and glycine-gated chloride channels. Establishing the evolutionary relationship of related proteins can clarify properties and lead to predictions about their structure and function. We have cloned and determined the nucleotide sequence of the GluClalpha and GluClbeta genes. In an attempt to understand the evolutionary relationship of these channels with the members of the ligand-gated ion channel superfamily, we have performed gene structure comparisons and phylogenetic analyses of their nucleotide and predicted amino acid sequences. Gene structure comparisons reveal the presence of several intron positions that are not found in the ligand-gated ion channel superfamily, outlining their distinct evolutionary position. Phylogenetic analyses indicate that GluClalpha and GluClbeta form a monophyletic subbranch in the ligand-gated ion channel superfamily and are related to vertebrate glycine channels/receptors. Glutamate-gated chloride channels, with electrophysiological properties similar to GluClalpha and GluClbeta, have been described in insects and crustaceans, suggesting that the glutamate-gated chloride channel family may be conserved in other invertebrate species. The gene structure and phylogenetic analyses in combination with the distinct pharmacological properties demonstrate that GluClalpha and GluClbeta belong to a discrete ligand-gated ion channel family that may represent genes orthologous to the vertebrate glycine channels.

Solubilization and characterization of a growth hormone secretagogue receptor from porcine anterior pituitary membranes

The discovery of a potential new GH therapy by small molecules that induce GH secretion (GHRP-6, L-692,429, MK-0677), has increased the interest in these GH secretagogues and their receptor and mechanism of action, which is different from the one of GHRH. We report the solubilization of the GH-secretagogue-receptor-ligand-G-protein complex (apparent molecular mass of approximately 255 kDa) from porcine anterior pituitary membranes using digitonin, after labelling the receptor with [35S]MK-0677. The solubilized receptor showed high affinity (KD = 122.2 +/- 14.4 pM) and low capacity (Bmax = 3.8 +/- 0.9 fmol/mg protein). These values and the inhibition constants (Ki) for a series of GH secretagogues were similar to the values determined in membranes isolated from porcine anterior pituitary gland. The solubilization of the GH secretagogue receptor opens up the possibility for further molecular characterization and sequencing of the receptor protein, necessary step prior to the identification of the natural ligand that would act as a GHRH amplifying hormone, and that the GH secretagogues would mimic.

Identification of a Drosophila melanogaster glutamate-gated chloride channel sensitive to the antiparasitic agent avermectin

Glutamate-gated chloride channels, members of the ligand-gated ion channel superfamily, have been shown in nematodes and in insects to be a target of the antiparasitic agent avermectin. Two subunits of the Caenorhabditis elegans glutamate-gated chloride channel have been cloned: GluCl-alpha and GluCl-beta. We report the cloning of a Drosophila melanogaster glutamate-gated chloride channel, DrosGluCl-alpha, which shares 48% amino acid and 60% nucleotide identity with the C. elegans GluCl channels. Expression of DrosGluCl-alpha in Xenopus oocytes produces a homomeric chloride channel that is gated by both glutamate and avermectin. The DrosGluCl-alpha channel has several unique characteristics not observed in C. elegans GluCl: dual gating by avermectin and glutamate, a rapidly desensitizing glutamate response, and a lack of potentiation of the glutamate response by avermectin. The pharmacological data support the hypothesis that the DrosGluCl-alpha channel represents the arthropod H-receptor and an important target for the avermectin class of insecticides.

A receptor in pituitary and hypothalamus that functions in growth hormone release

Small synthetic molecules termed growth hormone secretagogues (GHSs) act on the pituitary gland and the hypothalamus to stimulate and amplify pulsatile growth hormone (GH) release. A heterotrimeric GTP-binding protein (G protein)-coupled receptor (GPC-R) of the pituitary and arcuate ventro-medial and infundibular hypothalamus of swine and humans was cloned and was shown to be the target of the GHSs. On the basis of its pharmacological and molecular characterization, this GPC-R defines a neuroendocrine pathway for the control of pulsatile GH release and supports the notion that the GHSs mimic an undiscovered hormone.

An amino acid substitution in the pore region of a glutamate-gated chloride channel enables the coupling of ligand binding to channel gating

Many of the subunits of ligand-gated ion channels respond poorly, if at all, when expressed as homomeric channels in Xenopus oocytes. This lack of a ligand response has been thought to result from poor surface expression, poor assembly, or lack of an agonist binding domain. The Caenorhabditis elegans glutamate-gated chloride channel subunit GluClbeta responds to glutamate as a homomeric channel while the GluClalpha subunit is insensitive. A chimera between GluClalpha and GluClbeta was used to suggest that major determinants for glutamate binding are present on the GluClalpha N terminus. Amino acid substitutions in the presumed pore of GluClalpha conferred direct glutamate gating indicating that GluClalpha is deficient in coupling of ligand binding to channel gating. Heteromeric channels of GluClalpha+beta may differ from the prototypic muscle nicotinic acetylcholine receptor in that they have the potential to bind ligand to all of the subunits forming the channel.

Ophiobolin M and analogues, noncompetitive inhibitors of ivermectin binding with nematocidal activity

A series of ophiobolins were isolated from a fungal extract based on their nematocidal activity. These compounds are non-competitive inhibitors of ivermectin binding to membranes prepared from the free-living nematode, Caenorhabditis elegans, with an inhibition constant of 15 microM. The ophiobolins which were most potent in the biological assays, ophiobolin C and ophiobolin M, were also the most potent compounds when evaluated in a C. elegans motility assay. These data suggest that the nematocidal activity of the ophiobolins is mediated via an interaction with the ivermectin binding site. The isolation, structure and biological activity of ophiobolins have been described.

The mechanism of action of avermectins in Caenorhabditis elegans: correlation between activation of glutamate-sensitive chloride current, membrane binding, and biological activity

Xenopus laevis oocytes were injected with mRNA isolated from the free-living nematode Caenorhabditis elegans and the activation and potentiation of a glutamate-sensitive chloride current by a series of avermectin analogs and milbemycin D were determined. There was a strong correlation between the EC50 value determined for current activation in oocytes, the LD95 value for nematocidal activity, and also for the Ki value determined in a [3H]ivermectin competition binding assay. Four of the analogs were tested for potentiation of glutamate-sensitive current and the rank order for potentiation correlated with the EC50 for direct activation of current. We conclude that avermectins and milbemycins mediate their nematocidal effects on C. elegans via an interaction with a common receptor molecule, glutamate-gated chloride channels.

Identification of neuron-specific ivermectin binding sites in Drosophila melanogaster and Schistocerca americana

High affinity avermectin binding sites have been identified and partially characterized in membranes from two insect species, Drosophila melanogaster and the locus Schistocerca americana. There is a 10-fold increase in the density of ivermectin binding sites associated with membranes isolated from Drosophila heads (a neuronally enriched tissue source) compared to the bodies (Bmax values were 3.5 and 0.22 pmol/mg, respectively) with only a small difference in the apparent dissociation constant (Kd values of 0.20 and 0.34 nM for heads and bodies, respectively). Membranes prepared from metathoracic ganglia of the locust, Schistocerca americana, were highly enriched in high affinity avermectin binding sites (Kd = 0.2 nM and Bmax = 42 pmol/mg). Using an [125I]arylazido-avermectin analog as a photoaffinity probe, a 45 kDa protein was identified in both the Drosophila head and body tissue preparations. A 45 kDa protein was also specifically labeled with [125I]azido-avermectin in the locust neuronal membranes.

The relationship of age, gender, handedness, and sidedness to the size of the corpus callosum

OBJECTIVES

We correlated the area and size of the corpus callosum, as measured by MR imaging, with the individual's handedness, sidedness, age, and gender.

MATERIALS AND METHODS

A total of 117 patients (59 male, 58 female) aged 15-75 years were selected for this study. This included 12 persons who were either left-handed or ambidextrous. Each patient was tested extensively to determine handedness and sidedness. Callosal areas and thickness were measured and correlated with brain size.

RESULTS

The body of the corpus callosum decreases in size with age and is larger in right-handed persons. The cross-sectional areas of the genu, splenium, and corpus callosum, overall, do not vary significantly with respect to age, gender, sidedness, or handedness.

CONCLUSIONS

The size of the corpus callosum consistently decreases with age. Otherwise, few statistical differences in callosal size relate to gender, sidedness, or handedness.

Cloning of an avermectin-sensitive glutamate-gated chloride channel from Caenorhabditis elegans

The avermectins are a family of macrocyclic lactones used in the control of nematode and arthropod parasites. Ivermectin (22,23-dihydroavermectin B1a) is widely used as an anthelmintic in veterinary medicine and is used to treat onchocerciasis or river blindness in humans. Abamectin (avermectin B1a) is a miticide and insecticide used in crop protection. Avermectins interact with vertebrate and invertebrate GABA receptors and invertebrate glutamate-gated chloride channels. The soil nematode Caenorhabditis elegans has served as a useful model to study the mechanism of action of avermectins. A C. elegans messenger RNA expressed in Xenopus oocytes encodes an avermectin-sensitive glutamate-gated chloride channel. To elucidate the structure and properties of this channel, we used Xenopus oocytes for expression cloning of two functional complementary DNAs encoding an avermectin-sensitive glutamate-gated chloride channel. We find that the electrophysiological and structural properties of these proteins indicate that they are new members of the ligand-gated ion channel superfamily.

Immunoaffinity purification of avermectin-binding proteins from the free-living nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster

Avermectin-binding proteins from the free-living nematode worm Caenorhabditis elegans and from the fruitfly Drosophila melanogaster were purified to homogeneity via a three-step procedure. The binding proteins were covalently labelled using a radioactive photoaffinity probe and then partially purified on a Sephacryl S-300 gel-filtration column. The radiolabelled binding proteins were then purified by immunoaffinity chromatography using a monoclonal antibody to avermectin covalently attached to Protein A-Sepharose beads. Three affinity-labelled Drosophila proteins with molecular masses between 45 and 50 kDa were isolated in this way and then separated from each other by electroelution. This three-step protocol provides a rapid technique for receptor purification which may be of use in the purification of other binding proteins.

Nematocidal activity of MK-801 analogs and related drugs. Structure-activity relationships

A series of dibenzo[a,d]cycloalkenimines were evaluated for their affinity to the (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) binding site in Caenorhabditis elegans membranes and their nematocidal activity. The (+)-MK-801 enantiomer (1) had a higher affinity (Kd = 240 nM) for its specific binding site and was a more potent nematocidal agent than the (-)-MK-801 enantiomer (-1). Ring expansion to form the dibenzo[a,d]cyclooctenimine analogs generally resulted in more potent compounds. The most potent of this series (23) was approximately 7-fold more potent than (+)-MK-801. A good correlation was established between binding affinities and nematocidal activity for all of the analogs that were tested. However, there was no correlation between binding to C. elegans membranes and affinity for mammalian MK-801 binding sites. Other noncompetitive inhibitors of the mammalian N-methyl-D-aspartate site were examined, and a series of diphenylguanidines were identified as potent competitive inhibitors of MK-801 binding to C. elegans membranes, in addition to displaying potent nematocidal activity. The most potent diphenylguanidine analog (24) was approximately 80-fold more potent than (+)-MK-801 in both its affinity for the MK-801 binding site and as a nematocidal agent. Molecular modeling studies support the hypothesis that the diphenylguanidines and MK-801 are binding to the same site and suggest that more potent compounds may be developed by effective modeling of the existing compounds.

Ivermectin binding sites in sensitive and resistant Haemonchus contortus

Membranes from both ivermectin-sensitive and -resistant Haemonchus contortus L3 larvae were examined for the presence of high affinity [3H]ivermectin binding sites. Both tissue preparations displayed high affinity drug binding sites (Kd = 0.13 nM). Receptor density (Bmax = 0.4 pmol/mg) was the same in both the sensitive and resistant nematodes suggesting that target site modification was not involved in the development of drug resistance in this particular strain of H. contortus. The H. contortus ivermectin binding site appeared to be similar to the well characterized Caenorhabditis elegans ivermectin binding site with respect to affinity for ivermectin and receptor density.

Novel nematocidal agents from Curcuma comosa

Curcuma comosa is a member of the economically important plant family, Zingiberaceae. A methanolic extract of C. comosa was shown to be nematocidal when tested against the free-living nematode Caenorhabditis elegans. Five diphenylheptanoids [1-5], one new and four known, have been isolated and shown to be responsible for the activity. This is the first report of three of these compounds [1, 2, 4] being isolated from a natural source.

Affinity probes for the avermectin binding proteins

The design and synthesis of a series of avermectin affinity probes used in the identification and purification of the avermectin binding proteins is described. These modified avermectins fall into two design classes: ligands to covalently modify specific avermectin binding proteins [an 125I-labeled aryl azide photoprobe (15) and a tritiated aziridine analog (6)] and ligands for affinity chromatography applications [three biotinylated compounds (10, 12, and 13) and one resin-bound derivative (9)]. The binding affinities of these compounds for the Caenorhabditis elegans avermectin binding protein is presented as well as their biological activities against C. elegans and Artemia salina.

Expression of a glutamate-activated chloride current in Xenopus oocytes injected with Caenorhabditis elegans RNA: evidence for modulation by avermectin

Membrane currents were recorded from Xenopus laevis oocytes injected with C. elegans poly(A)+ RNA. In such oocytes glutamate activated an inward membrane current that desensitized in the continued presence of glutamate. Glutamate-receptor agonists quisqualate, kainate, and N-methyl-D-aspartate were inactive. The reversal potential of the glutamate-sensitive current was -22 mV, and exhibited a strong dependence on external chloride with a 48 mV change for a 10-fold change in chloride. The chloride channel blockers flufenamate and picrotoxin inhibited the glutamate-sensitive current. Ibotenate, a structural analog of glutamate, also activated a picrotoxin-sensitive chloride current. Ibotenate was inactive when current was partially desensitized with glutamate, and the responses to low concentrations of glutamate and ibotenate were additive. The anthelmintic/insecticide compound avermectin directly activated the glutamate-sensitive current. In addition, avermectin increased the response to submaximal concentrations of glutamate, shifted the glutamate concentration-response curve to lower concentrations, and slowed the desensitization of glutamate-sensitive current. We propose that the glutamate-sensitive chloride current and the avermectin-sensitive chloride current are mediated via the same channel.

The effect of protein binding on ivermectin uptake by bovine brain microvessel endothelial cells

The effect of albumin binding on ivermectin uptake and transfer across the endothelial component of the blood-brain barrier (BBB) was determined with an in vitro model comprised of bovine brain microvessel endothelial cell (BMEC) monolayers. Cellular uptake of ivermectin was limited in the absence of albumin and 90% inhibited in the presence of 10% albumin. Cell membrane association of ivermectin, as followed by fluorescent probe labelling, was observed only at high (micromolar) concentrations of the drug. Membrane association was about 75% inhibited in the presence of albumin. Similarly, transfer across BMEC monolayers was restricted, equivalent to that of BBB impermeant markers. Unlike the uptake studies, however, albumin had little effect on the transfer of ivermectin across BMEC monolayers. These results support recent in vivo findings on the distribution of ivermectin into the brain and suggest that ivermectin has only a limited affinity for the endothelial component of the normal BBB.

Photoaffinity labeling of avermectin binding sites from Caenorhabditis elegans and Drosophila melanogaster

An azido-avermectin analog [4'' alpha-(4-azidosalicylamido-epsilon-caproylamido-beta-alan ylamido)-4''-deoxyavermectin B1a; azido-AVM] was synthesized and used to photoaffinity label avermectin binding sites present in the membranes of Caenorhabditis elegans and Drosophila melanogaster. Azido-AVM was biologically active and behaved like a competitive inhibitor of [3H]ivermectin binding to C. elegans membranes (Ki = 0.2 nM). Radiolabeled azido-AVM bound specifically and with high affinity to C. elegans membranes (Kd = 0.14 nM) and, upon photoactivation, became covalently linked to three C. elegans polypeptides of 53, 47, and 8 kDa. Photoaffinity labeling of a membrane preparation from D. melanogaster heads resulted in labeling of a single major polypeptide of approximately 47 kDa. The proteins that were covalently tagged in these experiments are believed to be associated with avermectin-sensitive chloride channels present in the neuromuscular systems of C. elegans and D. melanogaster. Azido-AVM did not bind to rat brain membranes and therefore was selective for the nematode and insect receptors.

[3H]paraherquamide binding to Caenorhabditis elegans. Studies on a potent new anthelmintic agent

Paraherquamide was identified recently as a potent anthelmintic agent. In this paper we describe the identification and characterization of a specific, high-affinity paraherquamide binding site in a membrane preparation isolated from the free-living nematode, Caenorhabditis elegans. [3H] Paraherquamide bound specifically to C. elegans membranes with an apparent dissociation constant, Kd, of 263 nM. A series of paraherquamide analogs were examined, and their relative affinity for the paraherquamide binding site correlated with their nematocidal activity. Phenothiazines were the only other class of anthelmintics tested which inhibited specific [3H]paraherquamide binding. These results suggest that the anthelmintic activity of paraherquamide and phenothiazine is mediated via an interaction with a common binding site.

Urinary hormonal concentrations and spinal bone densities of premenopausal vegetarian and nonvegetarian women

We evaluated the relationships among nutrition, hormone concentrations, and bone density of the spine in 27 vegetarian and 37 nonvegetarian premenopausal women. The two groups were indistinguishable with respect to age, height, weight, menarche, years of formal education, and medical histories. The frequency of menstrual irregularity was significantly higher in the vegetarian group. The bone densities of the vegetarians and the nonvegetarians were not significantly different. Caffeine intake in both groups had a positive effect on urinary calcium excretion, but no association was observed between bone density and caffeine intake. Total fat, monosaturated fat, and oleic acid intake were positively associated with luteinizing hormone concentrations of the vegetarians only. Overall, the differences in dietary practices of these premenopausal vegetarian and nonvegetarian women did not appear to have major effects on bone biology or reproductive history.

Novel antinematodal and antiparasitic agents from Penicillium charlesii. I. Fermentation, isolation and biological activity

Paraherquamide and six novel analogs were isolated from the fermentation of Penicillium charlesii (ATCC 20841). All seven natural products displayed potent antinematodal activity against Caenorhabditis elegans. None of the novel analogs were more potent than paraherquamide.

Characterization of a high-affinity membrane-associated ornithine decarboxylase from the free-living nematode Caenorhabditis elegans

Ornithine decarboxylase has been identified and characterized in the free-living nematode Caenorhabditis elegans. Unlike previously described ornithine decarboxylases, the enzyme activity is membrane-associated and remains in the membrane fraction after treatment with high salt, detergents or phosphatidylinositol-specific phospholipase C. Ornithine has an apparent Km value of 2.7 microM for ornithine decarboxylase. The enzyme is competitively inhibited by arginine and lysine with Ki values of 4.0 and 24.4 microM respectively. None of the other naturally occurring amino acids inhibited more than 10% of the enzyme activity at concentrations up to 1 mM. Agmatine, putrescine, spermidine and spermine inhibit ornithine decarboxylase in a non-competitive manner with Ki values of 10, 53.5, 59 and 855 microM respectively. A similar ornithine decarboxylase activity was also identified in membrane preparations from the parasitic nematode Haemonchus contortus.

Cochlioquinone A, a nematocidal agent which competes for specific [3H]ivermectin binding sites

Cochlioquinone A, isolated from the fungus Helminthosporium sativum, was found to have nematocidal activity. Cochlioquinone A is a competitive inhibitor of specific [3H]ivermectin binding suggesting that cochlioquinone A and ivermectin interact with the same membrane receptor.

Avermectin binding in Caenorhabditis elegans. A two-state model for the avermectin binding site

Specific binding sites for ivermectin (IVM; 22,23-dihydroavermectin-B1) were identified and characterized in a crude membrane fraction prepared from the nematode, Caenorhabditis elegans (C. elegans). Specific [3H]IVM binding was saturable with an apparent dissociation constant, Kd, of 0.26 nM and a receptor concentration of 3.53 pmol/mg protein. [3H]IVM binding in C. elegans was linear with tissue protein concentration, and optimal binding occurred within a pH range of 7.3 to 7.6. Kinetic analysis of the binding showed that the reaction proceeded by a two-step mechanism. Initially, a rapidly reversible complex was formed and, after additional incubation, this complex was transformed to a much more slowly reversible complex. Stereospecificity of [3H]IVM binding to C. elegans membranes was demonstrated by competition with a series of avermectin derivatives. The in vivo effects of IVM and its derivatives on C. elegans motility were concentration dependent and correlated well with their relative binding affinities. Several putative neurotransmitters including gamma-aminobutyric acid (GABA), carbamyl choline, taurine, glutamate and dopamine were tested and found to have no effect on IVM binding. Specific IVM binding sites were also identified in rat brain; however, the affinity was approximately 100-fold lower than that observed in C. elegans and stereospecificity studies demonstrated structural differences in the two binding sites. These results are the first direct demonstration of a specific IVM binding site in nematodes and thus are important in furthering our understanding of its mode of action.

MK-801 is a potent nematocidal agent. Characterization of MK-801 binding sites in Caenorhabditis elegans

MK-801, an N-methyl-D-aspartate antagonist in mammalian brain tissue, is a potent nematocidal agent. Specific MK-801 binding sites have been identified and characterized in a membrane fraction prepared from the free-living nematode Caenorhabditis elegans. The high-affinity MK-801 binding site has an apparent dissociation constant, Kd, of 225 nM. Unlike the MK-801 binding site in mammalian tissues, the C. elegans binding site is not effected by glutamate or glycine, and polyamines are potent inhibitors of specific MK-801 binding.

A chemiluminescent assay for measuring avermectin binding sites

Avermectins are potent, broad spectrum anthelmintics. We describe the synthesis of a biologically active chemiluminescent compound, 22,23-dihydroavermectin B1a, 4"-[4-[[4-[ethyl(1,2,3,4-tetrahydro-1,4-dioxo-6- phthalazinyl)amino]butyl]amino]-4-oxobutanoate] (ivermectin-luminol). This compound retains its anthelmintic properties and provides an extremely sensitive nonradioactive probe to study avermectin binding sites.