Regulation of CRH-induced secretion of ACTH and corticosterone by SOM230 in rats.

OBJECTIVE: Adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome is biochemically characterized by increased plasma concentrations of ACTH inducing hypersecretion of cortisol. Somatostatin is known to inhibit ACTH secretion, and in vitro data have shown the inhibition of ACTH secretion by agonists activating sst2 and sst5 receptors. The present study aimed to determine the inhibitory effect of the multireceptor ligand SOM230, compared with the sst2-preferring agonist octreotide, on corticotropin-releasing hormone (CRH)-stimulated secretion of ACTH and corticosterone in rats. METHODS: Secretion of ACTH and corticosterone was induced by i.v. application of CRH (0.5 microg/kg) in rats pretreated 1 h before by i.v. application of SOM230 (1, 3, or 10 microg/kg), octreotide (10 microg/kg) or NaCl 0.9%. RESULTS: SOM230 (3 and 10 microg/kg) inhibited CRH-induced ACTH release by 45+/-3% and 51+/-2%, respectively, and corticosterone release by 43+/-5% and 27+/-16%, respectively. 10 microg/kg of octreotide tended to be less potent at inhibiting ACTH release (34+/-6% inhibition) and did not alter the secretion of corticosterone. CONCLUSION: SOM230 has a stronger inhibitory effect on ACTH and corticosterone secretion than octreotide in rats. This difference can be explained by its higher affinity to sst1, sst3 and especially sst5 receptors compared with octreotide.

SRA880, in vitro characterization of the first non-peptide somatostatin sst(1) receptor antagonist.

This report describes the in vitro features of the first somatostatin sst(1) receptor selective non-peptide antagonist, SRA880 ([3R,4aR,10aR]-1,2,3,4,4a,5,10,10a-Octahydro-6-methoxy-1-methyl-benz[g] quinoline-3-carboxylic-acid-4-(4-nitro-phenyl)-piperazine-amide, hydrogen malonate). SRA was evaluated in a number of in vitro systems of various species, both at native and recombinant receptors, using radioligand binding and second messenger/transduction studies. SRA880 has high affinity for native rat, mouse, monkey and human cerebral cortex somatostatin sst(1) receptors (pK(d) = 7.8-8.6) and for human recombinant sst(1) receptors (pK(d) = 8.0-8.1). SRA880 displayed significantly lower affinity for the other human recombinant somatostatin receptors ( pK(d) < or = 6.0) or a wide range of neurotransmitter receptors, except for the human dopamine D4 receptors. SRA880 was characterized in various transduction assays: somatotropin release inhibiting factor (SRIF) induced inhibition of forskolin-stimulated cAMP accumulation, SRIF stimulated-GTPgammaS binding, and SRIF stimulated luciferase gene expression; in all tests, SRA880 was devoid of intrinsic activity and acted as an apparently surmountable antagonist with pK(B) values of 7.5-7.7. Combined with the data from binding studies, these results suggest that SRA880 acts as a competitive antagonist. Thus, SRA880 is the first non-peptide somatostatin sst(1) receptor antagonist to be reported; SRA880 will be a useful tool for the characterization of somatostatin sst(1) receptor-mediated effects both in vitro and in vivo.

Non-invasive, quantitative assessment of the anatomical phenotype of corticotropin-releasing factor-overexpressing mice by MRI.

High resolution magnetic resonance imaging (MRI) was applied to quantify alterations in thymus and adrenal volumes, as well as body fat in genetically engineered corticotropin-releasing factor (CRF)-overexpressing mice. When compared to the organs in age-matched wild-type animals, the adrenals in CRF-overexpressing male mice were significantly enlarged and the thymus volume in females was significantly smaller. The fat content was significantly larger in CRF-overexpressing mice. The anatomical alterations observed in the MRI studies were in perfect line with post-mortem data (weights of organs). Furthermore, the observed interstrain differences are in agreement with recently published data on (i) the effect of continuous, intraventricular infusion of CRF in rats and (ii) the presence of atrophic adrenals in CRF-knockout mice. The present studies demonstrate that MRI can provide reliable measures of relatively small structures such as the adrenal glands and the thymus in mice. This makes MRI an attractive, non-terminal tool to monitor in laboratory animals, including transgenic mice, the consequence of continuous stress on relevant organs.

Current awareness.

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of NMR in biomedicine. Each bibliography is divided into 9 sections: 1 Books, Reviews ' Symposia; 2 General; 3 Technology; 4 Brain and Nerves; 5 Neuropathology; 6 Cancer; 7 Cardiac, Vascular and Respiratory Systems; 8 Liver, Kidney and Other Organs; 9 Muscle and Orthopaedic. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted.

Cloning, expression, functional coupling and pharmacological characterization of the rat dopamine D4 receptor.

It has been difficult to observe functional coupling of the D4 receptor to second messenger systems and a robust functional assay system for this receptor is still lacking. In the present study, the rat dopamine D4 receptor was cloned from rat retina. Sequence comparison revealed identity with the published sequence of Ashgari and co-workers, including the two amino acid insertions (V-Q) at position 92 which are not present in the published sequence of O'Malley and coworkers. The rat dopamine D4 receptor was stably expressed in Chinese hamster lung fibroblast CCL39 cells. [3H]spiperone saturation binding yielded a Bmax of 2,370+/-546 fmol/mg protein and a pKD of 8.74+/-0.14 (n=4). Forskolin-stimulated cAMP accumulation was inhibited by dopamine (Emax 61+/-1% inhibition of forskolin-stimulated levels, pEC50 7.33+/-0.06, n=23). A similar concentration-dependent inhibition was observed with the dopamine D2-like receptor agonists quinpirole and 7-OH-DPAT which elicited nearly the same Emax as dopamine. By contrast, apomorphine and a number of compounds with reported affinity for human dopamine D4 receptors (PD168077, U-101958, SDZ GLC 756, L-745,870 and NGD 94-1) behaved as partial agonists (Emax ranging between 26% and 56% of that of dopamine). The agonist effect of dopamine was completely blocked by preincubation with pertussis toxin, no further accumulation of cAMP above the forskolin-stimulated levels being observed. Antagonist pKB-values obtained against dopamine in this system were: 8.55+/-0.19 (n=3) for the partial agonist L-745,870, 8.38+/-0.23 (n=5) for spiperone, 7.18+/-0.17 (n=4) for haloperidol, 7.04+/-0.13 (n=4) for clozapine and <6 for raclopride. Other functional assays applicable were stimulation of [35S]GTPgammaS binding, extracellular acidification rate and a serum-responsive element using luciferase expression as a reporter gene. However, the receptor did not couple to phosphatidylinositol turnover or to intracellular Ca2+. Thus, expression of the rat dopamine D4 receptor in CCL39 cells provided several functional assay systems, of which inhibition of cAMP appeared to be the most robust one. These functional models can be used to evaluate the activity of compounds at the rat dopamine D4 receptor.

Receptor density as a factor governing the efficacy of the dopamine D4 receptor ligands, L-745,870 and U-101958 at human recombinant D4.4 receptors expressed in CHO cells.

1. The relationships between the density of dopamine D4.4 receptors and the agonist efficacies of L-745,870 (3-(4-[4-chlorophhenyl]piperazin-1-yl)-methyl-1H-pyrrolo [2, 3-b]pyridine) and U-101958 ((1-benzyl-piperidin-4-yl)-(3-isopropoxy-pyridin-2-yl)-methyl-a min e) were investigated in Chinese hamster ovary (CHO) cells, after treatment with the gene expression enhancer, sodium butyrate. 2. In CHO cells expressing D4.4 receptors (CHO/D4 cells), dopamine inhibited forskolin-stimulated cyclic AMP accumulation (Emax 56+/-1% inhibition, pEC50 7.4+/-0.1, n=10). U-101958 behaved as a partial agonist (39+/-7% the efficacy of dopamine, pEC50 8.1+/-0.3, n=4), whereas L-745,870 had no detectable agonist effect. 3. Receptor density, as estimated by [3H]-spiperone saturation binding was 240+/-30 fmol mg-1 protein (n=8) in CHO/D4 cell homogenates. It reached 560+/-150 (n=6), 1000+/-190 (n=4) and 840+/-120 (n=4) fmol mg-1 protein after treatment with sodium butyrate (5 mM) for 6, 18 and 48 h, respectively. 4. The increase in receptor density was associated with a gradual enhancement of the agonist effects (increased Emax and pEC50 values) of dopamine. The efficacy of U-101958 (relative to dopamine) doubled and L-745,870 was turned into a partial agonist (efficacy 49% relative to dopamine, pEC50 8. 6+/-0.2, n=6, after 48 h treatment with sodium butyrate). These agonist effects of U-101958 and L-745,870 could be antagonized by spiperone (0.1 microM) but not by raclopride (10 microM). 5. The results show that U-101958 and L-745,870 are partial agonists at human dopamine D4.4 receptors expressed in CHO cells. Their efficacy is governed by receptor density. Agonist effects of these two compounds in vivo cannot be excluded under circumstances of increased receptor levels.

Functional, endogenously expressed corticotropin-releasing factor receptor type 1 (CRF1) and CRF1 receptor mRNA expression in human neuroblastoma SH-SY5Y cells.

Corticotropin-releasing factor (CRF) is a hypothalamic 41-amino acid peptide which stimulates corticotropin (ACTH) release from the anterior pituitary and is also involved in the body response to stress. CRF1 receptors represent a potential target for novel antidepressant/anxiolytic drugs. The aim of the present study was to search for a human cell line expressing native, functional CRF1 receptors as a starting material for screening purposes. We identified CRF1 receptors functionally coupled to cAMP formation in human neuroblastoma SH-SY5Y cells. CRF induced concentration-dependent increases in cAMP accumulation in SH-SY5Y cells (maximal increase 6.9 +/- 0.9 fold over basal values, n = 14). This effect was mimicked by related peptides with similar potencies: (mean pEC50 value) human/rat CRF (8.63), rat urocortin (9.32), sauvagine (8.97), urotensin I (8.93), ovine CRF (8.81). The efficacies of these agonists were nearly the same, with the exception of ovine CRF which was slightly less efficacious (75% the Emax of CRF). The responses to CRF were competitively antagonised by the following peptide fragments (mean pKB value): alpha-helical-CRF (9-41) (7.54), [D-Phe12,Nle21,38,C alpha MeLeu37]CRF (12-41) (8.36) and [D-Tyr12]astressin (9.49) and by the selective, non-peptidic CRF1 receptor antagonists, CP-154,526 (7.76) and antalarmin (9.19). Estimation of receptor density by [125I]Tyr0-ovine CRF saturation binding yielded a modest number of binding sites (Bmax 12 fmol/mg protein, KD 0.2 nM). Analysis of mRNA by reverse transcription-polymerase chain reaction clearly revealed the presence of mRNA for CRF1 receptors in SH-SY5Y cells. A slight signal for CRF2 receptor mRNA was also observed. We conclude that neuroblastoma SH-SY5Y cells are endowed with native CRF1 receptors positively coupled to cAMP formation. They therefore constitute a useful functional model for the search of CRF1 selective compounds with potential anxiolytic/antidepressant activity.

NGD 94-1 as an agonist at human recombinant dopamine D4.4 receptors expressed in HEK293 cells.

The atypical antipsychotic, clozapine has some selectivity for dopamine D4 receptors and is a silent antagonist at these receptors. NGD 94-1 (2-phenyl-4(5)-[4-(2-pyrimidinyl)-piperazin-1-yl-methyl]-imidazole ) is a highly selective dopamine D4 receptor ligand recently introduced as a putative antipsychotic mimicking the dopamine D4 receptor antagonist effects of clozapine. We show that NGD 94-1 is not silent. It behaved as an agonist in human embryonic kidney 293 cells expressing human recombinant dopamine D4.4 receptors. This questions the clinical use of NGD 94-1.

Functional alpha2C-adrenoceptors in human neuroblastoma SH-SY5Y cells.

The alpha2-adrenoceptor mediating inhibition of forskolin-stimulated cyclic AMP accumulation in human neuroblastoma SH-SY5Y cells was further characterized. The alpha2-adrenoceptor agonists, UK 14,304 (5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline), oxymetazoline, guanfacine, (-)-noradrenaline and clonidine concentration-dependently decreased cyclic AMP accumulation in this cell line (Emax ca. 50% inhibition). Agonist pEC50 values ranged between 6.7 and 7.8. Clonidine was a partial agonist. The effects of UK 14,304 were blocked after a pertussis toxin treatment. The concentration-response curves of UK 14,304 were shifted to the right in a parallel manner by the following antagonists (mean pK(B) values): yohimbine (8.17), idazoxan (7.63), prazosin (6.66), 2-[2-(4-(2-methoxyphenyl)piperazin-1-yl)ethyl]-4,4-dimethyl-1,3-(2 H,4H) isoquinolindione (ARC 239; 7.12) and 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB-4101; 8.12). The relatively high pKB values of prazosin and ARC 239 point to a non-alpha2A-adrenoceptor-mediated effect. The relatively high pK(B) value of WB-4101 further characterizes the alpha2-adrenoceptor in SH-SY5Y cells as being of the alpha2C subtype. The analysis of the expression of alpha2-adrenoceptor subtypes by reverse transcriptase-polymerase chain reaction (RT-PCR) revealed the exclusive presence of alpha2C-adrenoceptor mRNA in SH-SY5Y cells. We propose that inhibition of forskolin-stimulated cAMP accumulation in SH-SY5Y cells be used as a functional model of human, native alpha2C-adrenoceptors.

The agonist activities of the putative antipsychotic agents, L-745,870 and U-101958 in HEK293 cells expressing the human dopamine D4.4 receptor.

1. Dopamine D4 receptor antagonists are being developed by several pharmaceutical companies as putative novel antipsychotics, possibly with low propensity to side-effects. Two such compounds, L-745,870 and U-101958 have been recently introduced. 2. The radioligand binding and functional activities of L-745,870 and U-101958 were investigated in human embryonic kidney (HEK)293 cells expressing the human recombinant dopamine D4.4 receptor (HEK293/D4 cells). [3H]-spiperone binding experiments were performed and inhibition of forskolin-stimulated cyclic AMP accumulation was used as the functional response. 3. [3H]-spiperone was found to label a homogeneous and saturable population of specific binding sites in HEK293/D4 cell homogenates (Bmax 505+/-90 fmol mg(-1) protein, pK(D) 9.5+/-0.1, n=3). Inhibition of specific [3H]-spiperone binding was observed with spiperone (pKi 9.6+/-0.1, n=3), clozapine (pKi 7.4+/-0.1, n=4), L-745,870 (pKi 8.5+/-0.1, n=3) and U-101958 (pKi 8.9+/-0.1, n=3). By contrast, raclopride was very weak (pKi < 5, n=3). 4. Dopamine inhibited forskolin-stimulated cyclic AMP accumulation in HEK293/D4 cells in a concentration-dependent fashion (Emax 71+/-2% inhibition of forskolin-stimulated levels, pEC50 8.7+/-0.1, n=10). This effect was mimicked by the dopamine D2-like receptor agonists, quinpirole and 7-hydroxy-2-dipropylaminotetralin (7-OH-DPAT). 5. L-745,870 and U-101958 also inhibited forskolin-stimulated cyclic AMP accumulation in HEK293/D4 cells in a concentration-dependent way. L-745,870 was less efficacious than dopamine (71% the efficacy of dopamine), whereas U-101958 behaved as a full agonist compared to dopamine. Potencies (pEC50) values of L-745,870 and U-101958 were 9.0+/-0.2 (n=4) and 8.7+/-0.3 (n=3), consistent with pKi values determined in radioligand binding studies. 6. Dopamine, L-745,870 and U-101958 (up to 1 microM) were devoid of effect on forskolin-stimulated cyclic AMP accumulation in control, non-transfected HEK293 cells. 7. The agonist effects of dopamine, L-745,870 and U-101958 in HEK293/D4 cells could be antagonized by spiperone (pK(B) 8.2-8.8) and clozapine (pK(B) 7.1), but not by raclopride (pK(B) < 5). None of these antagonists had any significant agonist activity at concentrations up to 10 microM. 8. These results show that the putative dopamine D4 receptor antagonists, L-745,870 and U-101958 are not devoid of intrinsic activity at human recombinant dopamine D4.4 receptors. Therefore, they may not represent the most appropriate drugs for testing the benefit of D4 receptor antagonism in schizophrenic patients, if agonism should translate in vivo.

Inhibition of cAMP accumulation via recombinant human serotonin 5-HT1A receptors: considerations on receptor effector coupling across systems.

Inhibition of forskolin-stimulated cyclic AMP accumulation was measured in two stable HeLa cell lines HA6 and HA7 expressing different levels of recombinant human 5-HT1A receptors. These cells were studied previously to characterize another second messenger system activated by 5-HT1A receptors, i.e. calcium mobilization. The pharmacological characterization of the inhibition of cyclic AMP accumulation was made using agonists (5-HT, 8-OH-DPAT, buspirone, MDL 73005) and putative antagonists (SDZ 216-525, NAN-190, WAY-100135, pindolol, propranolol, WAY 100635). It is shown that 5-HT, 8-OH-DPAT, buspirone, MDL 73005 behaved as full (or nearly full) and potent agonists, whereas SDZ 216-525, NAN-190 and WAY-100135 displayed a limited (and similar) degree of intrinsic activity at human 5-HT1A receptors; on the other hand pindolol, propranolol and WAY 100635 behaved as "silent" antagonists. The effects were quantitatively and qualitatively very similar in both cells lines for all drugs tested, suggesting that the coupling between 5-HT1A receptors and inhibition of cyclic AMP accumulation in HeLa cells is very tight. There were, however, significant variations in both the level of agonism and the potency of a number of compounds when calcium mobilization and the inhibition of cyclic AMP accumulation were compared. Especially in HA7 cells which express lower receptor levels, a number of drugs failed to display agonism (e.g. buspirone or MDL 73005), whereas in HA6 cells they acted as partial agonists. Together, the data show that functional responses mediated by the same receptor can vary rather dramatically depending on receptor density and/or on the effector system involved. Interestingly, 5-HT1A receptor-mediated inhibition of adenylate cyclase activity measured in calf hippocampal membranes shows very similar degrees of potency and intrinsic activity for a number of compounds that have been tested on the inhibition of cyclic AMP accumulation in HeLa cells, suggesting that the very tight coupling observed in the recombinant system may apply to native 5-HT1A receptors.

Cloning, distribution and functional expression of the human mGlu6 metabotropic glutamate receptor.

The cDNA encoding the human metabotropic glutamate receptor type 6 (hmGlu6) was isolated from a human retinal cDNA library. The deduced primary sequence (877 amino acids) of the hmGlu6 receptor was 93.5% identical to its rat counterpart and shared 69.8% sequence identity with the related hmGlu4 receptor clone (912 amino acids), isolated in parallel from a human brain cDNA library. In situ hybridization revealed that the hmGlu6 mRNA is highly expressed in cells located in the inner nuclear layer of the human retina, presumably bipolar neurons. Neither PCR analysis nor in situ hybridization could detect hmGlu6 mRNA in human brain. When stably expressed in Chinese hamster ovary cells (CHO-K1) the hmGlu6 receptor inhibited adenylate cyclase through a pertussis toxin-sensitive G-protein, and reduced forskolin-elevated cyclic adenosine monophosphate (cAMP) levels in response to agonists. The rank order of agonist potency was L(+)-2-amino-4-phosphonobutyric acid (L-AP4) > L-serine-O-phosphate > L-glutamate > quisqualate = (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD). (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (L-CCG-I) was a partial agonist at the hmGlu6 receptor, with a potency approaching that of L-serine-O-phosphate.

Functional, endogenously expressed 5-hydroxytryptamine 5-ht7 receptors in human vascular smooth muscle cells.

Human uterine artery smooth muscle cells in culture were shown to express constitutively both 5-ht7 receptor mRNA and 5-ht7-like receptors functionally linked to cyclic AMP formation. 5-Carboxamidotryptamine (5-CT) and 5-HT enhanced forskolin-stimulated cyclic AMP accumulation in these cells, with pEC50 values of 7.12 and 6.25, sumatriptan being very weakly active. Both methiothepin (0.1 microM) and clozapine (1 microM), but not the 5-HT4-receptor antagonist, SDZ 205-557 (10 microM) antagonized the effects of 5-CT. In reverse transcriptase-polymerase chain reaction analysis, the mRNA for 5-ht7, but not for 5-HT4 or 5-ht6 receptors was found to be strongly expressed in the same cells. These findings represent a further step toward the recognition of 5-ht7 receptors as real, functional receptors.

Functional serotonin 5-HT1D receptors and 5-HT1D beta receptor mRNA expression in human umbilical vein endothelial cells.

Pharmacological evidence has suggested the presence of 5-hydroxytryptamine (5-HT, serotonin), 5-HT(1D) receptors on endothelial cells but these receptors have never been identified unambiguously on this type of cells. We now report that human umbilical vein endothelial cells (HUVEC) express 5-HT(1D) receptors coupled to inhibition of cyclic AMP formation. 5-HT and 5-HT(1D) receptor agonists 5-carboxamidotryptamine (5-CT) and sumatriptan were approximately equipotent at inhibiting forskolin-stimulated cyclic AMP accumulation in HUVEC (mean pEC50 7.6-8.2, maximal effect 30% inhibition). The 5-HT(1A) receptor antagonist, 8-OH-DPAT was clearly less potent (pEC50 6.2) and less efficacious. The selective 5-HT(1D) receptor agonist, GR127935 (1 nM) markedly inhibited the effect of 5-HT (apparent pK(B) 10.8). Reverse transcription-polymerase chain reaction analysis showed the mRNA for 5-HT(1D beta) receptors to be expressed in HUVEC. These results demonstrate the presence of functional 5-HT(1D) receptors and the expression of 5-HT(1D beta) receptor mRNA in HUVEC. They support the involvement of 5-HT(1D beta) receptors in endothelial-mediated responses to 5-HT.

5-Hydroxytryptamine 5-HT1D receptors mediating inhibition of cyclic AMP accumulation in Madin-Darby canine kidney (MDCK) cells.

5-Hydroxytryptamine 5-HT1B/5-HT1D receptors are members of the same receptor subfamily, but display a different pharmacology (Hartig et al. (1992) Trends Pharmacol Sci 13: 152-159). Whereas several cell lines have been reported to contain 5-HT1B receptors, none has been described, however, that endogenously expresses well-characterized 5-HT1D receptors. The present study deals with the identification of 5-HT1D receptors inhibiting cyclic AMP accumulation in Madin-Darby canine kidney (MDCK) cells. 5-HT (1 nM-10 microM) induced a concentration-dependent inhibition of the cyclic AMP accumulation stimulated by prostaglandin E1 (1 microM) in MDCK cells. The maximal effect of 5-HT averaged 50% inhibition and was abolished after a pre-treatment of the cells with pertussis toxin. Other agonists mimicked the effects of 5-HT, with the following rank order of potency (pEC50 +/- SEM, n > or = 3): 5-carboxamidotryptamine (8.36 +/- 0.48) > PAPP (p-aminophenylethyl-m-trifluoromethylphenyl piperazine. 7.89 +/- 0.23) > 5-HT (7.35 +/- 0.05) > sumatriptan (6.65 +/- 0.27). PAPP behaved as a partial agonist. 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) was less potent, its maximal effect being not reached at 0.1 mM. Methiothepin. GR127935, (-)propranolol, rauwolscine and ketanserin were all devoid of intrinsic activity (up to 10 microM or 0.1 mM). Methiothepin (10 nM. 0.1 microM and 1 microM) antagonized 5-HT effect (pA2 8.57 +/- 0.44. Schild slope 1.17 +/- 0.21, n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological identity between somatostatin SS-2 binding sites and SSTR-1 receptors.

Somatostatin (SRIF) SS-2 binding sites were originally defined in rat brain cerebral cortex membranes using [125I]Tyr11-SRIF-14 in the presence of 120 mM NaCl. These sites were characterized by their high affinity for SRIF-14 and SRIF-28, but very low affinity for cyclic peptides such as octreotide (SMS 201-995) and seglitide (MK 678). The characteristics of SS-2 sites are reminiscent of 125I]CGP 23996-labelled sites in rat brain which have been termed SRIF-2 sites. In the present study, the pharmacological profile of SS-2 sites was determined in radioligand binding studies performed in rat cortex membranes using [125I]SRIF-14 in the presence of 120 mM NaCl and compared to that of human SSTR-1 receptors expressed in human embryonic kidney (HEK 293) cells, using [125I]SRIF-14. The rank orders of affinity of a variety of SRIF analogues and synthetic peptides for SS-2 binding sites and recombinant human SSTR-1 receptors were very similar and correlated highly significantly (r = 0.99). However, SS-2 binding correlated also with binding to recombinant SSTR-4 receptors (r = 0.91). Autoradiographic studies were performed using the radioligand [125I]CGP 23996 which has been claimed to label selectively SRIF-2 binding sites and compared with the distribution of SSTR-1 receptor mRNA determined using in situ hybridization in rat brain. Although some overlap was observed between the distribution of SSTR-1 mRNA and [125I]CGP 23996 binding sites, the latter were clearly more widespread, suggesting this ligand to label SSTR-1 and other sites. In addition, inhibition of forskolin-stimulated adenylate cyclase was investigated in HEK 293 cells transfected with human SSTR-1 receptors; a variety of SRIF analogues and short synthetic peptides behaved as agonists at adenylate cyclase and displayed a rank order of potency highly similar to that observed for these compounds at SS-2 binding sites. Seglitide acted as an antagonist at SSTR-1 receptor mediated inhibition of adenylate cyclase activity with a pKB of 4.42. It is concluded that the pharmacological profile of SS-2 binding sites resembles most closely that of SSTR-1 receptors (although similarities with SSTR-4 receptors were observed), that [125I]CGP 23996 labels presumably several SRIF receptors in rat brain, and that SSTR-1 receptors are negatively and efficiently coupled to adenylate cyclase activity.

Characterization and distribution of somatostatin SS-1 and SRIF-1 binding sites in rat brain: identity with SSTR-2 receptors.

Somatostatin (SRIF) SS-1 binding sites were initially defined in radioligand binding studies performed in rat brain cerebral cortex membranes using [125I]204-090 (a radiolabelled Tyr3 analogue of SMS 201-995, octreotide). SRIF-1 recognition sites were defined in binding studies performed with [125I]MK 678 (seglitide). Both SS-1 and SRIF-1 sites were characterized by their high affinity for SRIF-14, SRIF-28 and for cyclic peptides such as octreotide and seglitide, in marked contrast to SS-2 and SRIF-2 sites which have very low affinity for these synthetic SRIF analogues. In the present study, SS-1 and SRIF-1 radioligand binding studies were performed in rat cortex membranes and compared to results obtained in cloned Chinese hamster ovary cells expressing human SSTR-2 receptors using [125I]204-090 and/or [125I]MK-678. The rank orders of affinity of a variety of SRIF analogues and synthetic peptides for SS-1/SRIF-1 binding sites and recombinant SSTR-2 receptors were very similar and correlated highly significantly (r = 0.94-0.99); by contrast, correlation between SS-1 and SSTR-5 (r = 0.44) or SSTR-3 binding (r = 0.07) was not significant. Autoradiographic studies were performed in rat brain using both radioligands [125I]204-090 and [125I]MK-678 and compared with the distribution of SSTR-2 receptor mRNA determined using in situ hybridization. A clear overlap was observed between the distribution of SSTR-2 mRNA and binding sites labelled with both radioligands. SSTR-2 receptor-mediated inhibition of forskolin-stimulated adenylate cyclase in Chinese hamster ovary cells by a variety of SRIF analogues and short synthetic peptides displayed a rank order of potency highly similar to their rank order of affinity at SS-1/SRIF-1 binding sites. It is concluded that SS-1 and SRIF-1 binding sites respectively labelled with [125I]204-090 and [125I]MK 678, both display the pharmacological profile of SSTR-2 receptors, that the distribution of [125I]204-090 and [125I]MK-678 binding sites in rat brain is superimposable and largely comparable to that of SSTR-2 mRNA expression. It is also shown that neither [125I]204-090 nor [125I]MK-678 label SSTR-3 or SSTR-5 receptors in rat brain. Finally, it is demonstrated that SSTR-2 receptors can very efficiently couple to adenylate cyclase activity in an inhibitory manner.

(+)-cis-4,5,7a,8,9,10,11,11a-octahydro-7H-10-methylindolo[1,7- bc][2,6]-naphthyridine: a 5-HT2C/2B receptor antagonist with low 5-HT2A receptor affinity.

The indolonaphthyridine 8 is described as a selective 5-HT2C/2B vs 5-HT2A receptor antagonist. The compound was synthesized in seven steps starting from indoline and isonicotinic acid chloride. The key step is a photocyclization of the indolinyl tetrahydropyridinocarbamic acid ethyl ester 4 to the cis-octahydroindolo[1, 7-bc][2,6]naphthyridinecarbamic acid ethyl ester 5. The synthesis was accomplished by reduction with aluminum hydride and racemic resolution. The indolonaphthyridine 8 exerted the binding profile of a selective 5-HT2C receptor ligand (pKD 7.8) and behaved as an antagonist on the 5-HT-induced accumulation of inositol phosphates in pig choroid plexus cells (pKB 7.13). Compound 8 dose-dependently inhibited the ACTH response to MK-212 in rats and the MK-212-induced hypophagic effect with an ID50 value of 0.3 mg/kg sc. Compound 8 acted as a 5-HT2B receptor antagonist at the rat stomach fundus with a pKB value of 7.34.

5-Hydroxytryptamine 5-HT1B receptors inhibiting cyclic AMP accumulation in rat renal mesangial cells.

A clonal cell line derived from rat renal mesangial cells was shown to express endogenous 5-hydroxytryptamine (serotonin, 5-HT) receptors that mediate inhibition of cyclic AMP accumulation. These receptors were characterized as being of the 5-HT1B receptor subtype. 5-HT1 receptor agonists inhibited forskolin-stimulated cyclic AMP accumulation in rat renal mesangial cells (60-70% maximal inhibition) with the following rank order of potency (mean pEC50 values +/- SEM, n > or = 3): ergotamine (9.58 +/- 0.51) > RU 24969 (8.67 +/- 0.23) > or = 5-CT (8.42 +/- 0.06) > or = CP 93129 (8.15 +/- 0.27) > 5-HT (7.75 +/- 0.11) > sumatriptan (6.29 +/- 0.30) > 8-OH-DPAT (4.32 +/- 0.15). 5-HT2 and 5-HT4 receptor agonists were without effect. 5-HT-induced inhibition of cyclic AMP accumulation was abolished by a pre-treatment of the cells with pertussis toxin. (-)Propranolol was a partial agonist (27% maximal inhibition, pEC50 7.19 +/- 0.24, n = 3); when used as an antagonist at 1 microM, it shifted the concentration-response curve of 5-HT to the right (pKB 7.22 +/- 0.35, n = 3). Methiothepin was a competitive antagonist of 5-HT (pA2 8.04 +/- 0.10, Schild slope 0.87 +/- 0.21, n = 3). Rauwolscine (10 microM) had no antagonist activity. There was a significant correlation (r = 0.98, P = 0.0001) between the cyclic AMP data obtained in rat mesangial cells and 5-HT1B binding data reported in rat brain cortex. The same pattern of responses was observed in early passages of primary cultures of rat mesangial cells.(ABSTRACT TRUNCATED AT 250 WORDS)