Analysis of concentration-response relationships by seemingly unrelated nonlinear regression (SUNR) technique

Calcium-antagonistic activity of sumatriptan in the rat anococcygeus muscle

The relaxant effect of sumatriptan, a 5-HT(1B/1D) receptor agonist, on the rat anococcygeus muscle was investigated. Sumatriptan induced concentration-dependent relaxations of the phenylephrine-precontracted rat anococcygeus muscle. N(G)-nitro-L-arginine, methylene blue, glibenclamide, tetrodotoxin, indomethacin, GR 113808, GR 55562, methysergide, ketanserin, ICS 205930, clozapine, methiothepin, metergoline, mesulergine, and ritanserin did not inhibit the relaxations induced by sumatriptan. Sumatriptan converted calcium-induced contractions into relaxations in the preparations depolarized by a calcium-free, high-potassium solution. In membrane preparations obtained from the rat anococcygeus muscle, the basal rate of cAMP production by adenylate cyclase was 4.77 +/- 0.02 pmol/mg protein/min (n = 15). The enzyme activity was increased to 104 +/- 51.7 pmol/mg protein/min by forskolin (10(-4) mol/l), but did not change in the presence of sumatriptan. The mRNA expression of 5-HT(1B), 5-HT(1D), and 5-HT(7) receptors was not observed in the rat anococcygeus muscle. The results indicate that sumatriptan causes relaxation of the precontracted rat anococcygeus muscle by a calcium-antagonistic activity.

Diabetes reduces right atrial β-adrenergic signaling but not agonist stimulation of heart rate in swine

This study assessed the effects of streptozotocin diabetes in swine on the heart rate response to β-adrenergic stimulation the adenylyl cyclase signal transduction pathway. Diabetic animals (n = 9) were hyperglycemic compared to the control group (n = 10) (12.6 ± 1.0 vs. 3.53 ± 0.29 mM). There were no significant differences between the diabetic and nondiabetic groups in the heart rate response to isoproterenol, however, there was a significant reduction (14%) in β-adrenergic receptor density in the right atrium in the diabetic (61 ± 3 fmol/mg protein) versus the nondiabetic group (71 ± 3) (P < 0.05). The content of guanosine triphosphate binding regulatory proteins (Gs and Gi) in the right atrium was not affected by diabetes, nor was adenylyl cyclase activity under unstimulated conditions or with receptor-dependent stimulation with isoproterenol. On the other hand, adenylyl cyclase activity was 34% lower when directly stimulated with forskolin, and it was reduced by 23% when stimulated through Gs with Gpp(NH)p. In conclusion, beta-adrenergic stimulation of heart rate with isoproteronol and the receptor-dependent signal transduction pathway remained intact in the right atrium of diabetic swine despite reduced beta-adrenergic receptor density, G-protein content, and direct stimulation of adenylyl cyclase activity.Key words: diabetes, G-proteins, heart rate, receptors, signal transduction.

Characterization of prostanoid receptor-evoked responses in rat sensory neurones

1. Prostanoid receptor-mediated sensitization, or excitation, of sensory nerve fibres contributes to the generation of hyperalgesia. To characterize the prostanoid receptors present on sensory neurones, biochemical assays were performed on primary cultures of adult rat dorsal root ganglia (DRG) and the F-11 (embryonic rat DRG x neuroblastoma hybrid) cell line. 2. In DRG cultures, the IP receptor agonists, cicaprost and carbaprostacyclin (cPGI2) stimulated cyclic AMP accumulation. Prostaglandin E2 (PGE2) also increased cyclic AMP levels, but to a lesser extent, while carbocyclic thromboxane A2 (cTxA2), PGD2 and PGF2alpha had negligible effects. The rank order of agonist potency was cicaprost>PGE2=BMY45778=cPGI2=PGI2. In the F-11 cells, the rank order of agonist potency for the stimulation of cyclic AMP accumulation was: cicaprost>iloprost=cPGI2=PGI2=BMY45778>PGE2=cTXA2++ +. In DRG cultures, cicaprost induced significantly more accumulation of inositol phosphates than PGE2. 3. To examine the effects of prostanoids on C-fibre activity, extracellular recordings of d.c. potentials from the rat isolated vagus nerve were made with the 'grease-gap' technique. PGI2 (0.1 nM-10 microM) produced the largest depolarizations of the nerve. The rank order of agonist potency was: PGI2=cPGI2=PGE1>cTXA2>PGE2=PGD2=TXB2>PGF2alpha. 4. Prior depolarization of nerves with either forskolin (10 microM) or phorbol dibutyrate (1 microM) alone significantly reduced the response to PGI2 (10 microM), while simultaneous application of both forskolin and phorbol dibutyrate attenuated PGI2 responses almost completely. 5. Putative EP1 and/or TP receptor-selective antagonists had no effect on the responses to PGI2, cPGI2 or PGE2 in the three preparations studied. 6. Collectively, these data are consistent with a positive coupling of IP receptors to both adenylyl cyclase and phospholipase C in sensory neurones. These findings suggest that IP receptors play a major role in the sensitization of rat sensory neurones.

QX-314 inhibits ectopic nerve activity associated with neuropathic pain

In animal models of neuropathic pain, transection or constrictive injury to peripheral nerves produces ectopic discharges originating at both injury sites and related dorsal root ganglia (DRG). In addition, hyperexcitability is observed in associated dorsal horn (DH) neurons of the spinal cord. As ectopic discharges are inhibited by agents that block voltage-sensitive Na+ channels, it has been postulated that accumulation of Na+ channels in the membrane at nerve injury sites may contribute to the development of ectopic nerve activity (ENA). The goal of the present study was to compare the sensitivity of ENA to lidocaine and QX-314, a positively charged lidocaine derivative, which is frequently assumed to be membrane impermeant. Experiments were performed on adult male Sprague-Dawley rats in which the common sciatic nerve had been transected 4-10 days earlier. Extracellular microelectrode recordings were made from DRG and DH neurons, and neuronal activity was measured in fine bundles of microfilaments teased from sciatic nerves in anesthetized and paralyzed rats. Comparative effects on heart rate (HR) and mean blood pressure (MBP) were also studied. To confirm that externally applied QX-314 is able to inhibit high frequency activity in sensory nerves, QX-314 was superfused over isolated rat vagus nerves during stimulation of compound action potentials in C-fibers (C-spikes). As expected, intravenously administered lidocaine inhibited ENA at all three sites. Lidocaine ED50 values (expressed as mg/kg, with 95% confidence limits) were: 10.2 (7.8-13.3), 1.4 (0.8-2.4) and 0.9 (0.4-2.0) for neuromas, DRG and DH neurons, respectively. QX-314 also induced dose-dependent inhibition of ENA at neuromas and DRG, but produced only a small inhibition of DH neuron ENA. QX-314 had the following ED50 values (mg/kg) for neuromas, DRG and DH neurons, respectively: 2.3 (2.0-2.8), 6.9 (4.7-26.5) and 85.7. QX-314-mediated inhibition of DRG ENA had a slow onset and was long-lasting, relative to lidocaine. Lidocaine or QX-314 also significantly reduced HR and MBP in the same dose range as that which reduced ENA in DRG or neuromas. In isolated rat vagus nerve recordings, QX-314 induced marked use-dependent inhibition of C-spike amplitude, with IC50 values (microM) of 9000 (4600-18,000) and 350 (290-420) for low- (0.03 Hz) and high-frequency (30 Hz) C-spikes, respectively. These data support the hypothesis that Na+ channel accumulation contributes to the generation of ectopic discharges in neuromas and DRG, and suggests that intravenous QX-314 can acutely block Na+ channels at these sites.

Functional interactions between muscarinic M2 receptors and 5-hydroxytryptamine (5-HT)4 receptors and beta 3-adrenoceptors in isolated oesophageal muscularis mucosae of the rat

1. Relaxations of isolated oesophageal muscularis mucosae of rat are mediated by 5-hydroxytryptamine (5-HT), acting at 5-HT4 receptors, and isoprenaline, principally acting via beta 3-adrenoceptors. The aim of this study was to investigate the hypothesis that muscarinic M2 receptors, also present in this tissue, functionally oppose 5-HT and beta-adrenoceptor-relaxant effects in this preparation. 2. Contractions of rat oesophageal muscularis mucosae were induced, in a concentration-dependent manner, by the muscarinic receptor agonist, oxotremorine M (pEC50 = 6.7 +/- 0.1). The contractile responses to oxotremorine M were surmountably antagonized by the following compounds, (pKB values in parentheses): atropine (9.1 +/- 0.2), 4-DAMP (4-diphenylacetoxy-N-methyl piperidine methiodide, 8.7 +/- 0.1), p-F-HHSiD (para-fluoro-hexa-hydro-siladifenidol, 7.5 +/- 0.1), zamifenacin (8.6 +/- 0.3), himbacine (7.2 +/- 0.2), pirenzepine (6.8 +/- 0.3) and methoctramine (6.2 +/- 0.2). These data are consistent with a role for muscarinic M3 receptors mediating contractions to oxotremorine M. The contractile response was associated with a low receptor reserve, since the responses were shifted to the right and virtually abolished by the alkylating agent, 4-DAMP mustard (4-diphenylacetoxy-N-(2-chloroethyl) piperidine, 40 nM; 60 min equilibration). 3. In tissues precontracted with U46619 (0.7 microM; approx. EC90), isoprenaline (pEC50 = 8.0 +/- 0.1) and 5-HT (pEC50 = 7.5 +/- 0.2) induced concentration-dependent relaxations. The isoprenaline potency was slightly, but significantly, different in tissues precontracted with oxotremorine M (isoprenaline, pEC50 = 7.4 +/- 0.2). In contrast, the potency of 5-HT (pEC50 = 7.5 +/- 0.2), in tissues that were precontracted with 1 microM (EC90) oxotremorine M, was identical. When these experiments were repeated in the presence of the muscarinic M2 receptor antagonist, methoctramine (1 microM), there was no effect on the relaxant potencies to either 5-HT or isoprenaline. Collectively, these data suggest that muscarinic M2 receptors do not, under these conditions, modulate relaxant potencies to either 5-HT or isoprenaline. 4. In a second protocol, tissues were pre-contracted with U46619 (0.7 microM) and relaxed with either 5-HT (0.1 microM) or isoprenaline (0.1 microM). In these tissues (in which the muscarinic M3 receptor population was extensively depleted by alkylation), oxotremorine M caused concentration-dependent re-contractions (i.e. reversal of relaxations). In tissues relaxed with 5-HT, the potency of oxtremorine M was 5.9 +/- 0.2, while in tissues relaxed with isoprenaline, the potency (pEC50) = 5.6 +/- 0.3. These re-contractions were antagonized, in a surmountable fashion, by methoctramine (1 microM; pKB = 7.6 +/- 0.1). Similar observations were seen when relaxations were induced by isoprenaline (1 microM; pKB = 7.5 +/- 0.2). Under these conditions, therefore, the pKB values are consistent with activation of muscarinic M2 receptors, and inconsistent with activation of M3 receptors. 5. It is concluded that in isolated oesophageal muscularis mucosae of rat, muscarinic M3 receptors mediate direct contractions and are associated with a low receptor reserve. When this population is depleted, and the tissues relaxed via activation of receptors that augment adenylyl cyclase activity, a functional role for muscarinic M2 receptors is revealed.

Comparison of 5-HT4 receptors in guinea-pig colon and rat oesophagus: effects of novel agonists and antagonists

5-HT4 receptors in isolated distal colon myenteric plexus of guinea-pig, mediating contraction of longitudinal smooth muscle, have been further characterized by selective agonists and antagonists. The indole agonists, 5-HT and 5-methoxytryptamine (5-MeOT), were full agonists (relative to 5-HT) with potency values (pEC50) of 8.0 +/- 0.1 (n = 50) and 7.8 +/- 0.1 (n = 12), respectively. 5-HT4 receptor agonists of other structural classes, including benzimidazolones (BIMU 1 and BIMU 8), and benzamides ((S)-zacopride, (R)-zacopride, renzapride, SC 49518) were partial agonists with intrinsic activities less than that of 5-HT. In general, the potencies for these compounds at 5-HT4 receptors in guinea-pig colon were similar to the potencies seen in the rat isolated oesophagus, where 5-HT4 receptors mediate relaxation. GR 113808 ¿[1-[2-[(methylsulfonyl)amino]ethyl]-4-piperidinyl] methyl1-methyl-1H-indole-3-carboxylate¿, RS 39604 ¿1-[4-amino-5-chloro-2-(3, 5-dimethoxybenzyloxy)phenyl]-3[1-[2-[(methylsulfonyl)amino] ethyl]-4-piperidinyl]-1-propanone hydrochloride and SB 204070 ¿(1-n-butyl-4-piperidinyl)methyl 8-amino-7-chloro-1, 4-benzodioxane-5-carboxylate¿ antagonized 5-HT responses with pA2 values of 9.1 +/- 0.1, 9.0 +/- 0.2 and 11.0 +/- 0.1, respectively. These affinity values were similar to those obtained at 5-HT4 receptors in isolated rat oesophagus (9.0+/- 0.4, 9.3 +/- 0.1 and 10.6 +/- 0.1 respectively). Despite these operational similarities between 5-HT4 receptors in guinea-pig colon and rat oesophagus, several novel compounds have revealed important differences between 5-HT4 receptors in the two tissues. For example, the substituted benzoate, RS 23597 ¿3-(piperidine-1-yl) propyl-4-amino-5-chloro-2-methoxybenzoate hydrochloride, acted as a partial agonist (intrinsic activity 0.5) in guinea-pig colon with a potency of 7.6 +/-0.1 (n = 16). In isolated rat oesophagus, however, this compound was a surmountable antagonist (pA2 = 7.8 +/- 0.1) with no intrinsic activity. In contrast, the substituted naphthalimide (S)RS 56532 ¿(S)-6-amino-5-chloro-2-(1-azabicyclo[2, 2, 2]octan-3-yl) 2,3-dihydro-1H-benz[de] isoquinoline-1,3-dione hydrochloride¿, was a potent (pEC50 = 7.9 +/- 0.1), efficacious partial agonist (intrinsic activity = 0.8) in the rat oesophagus. However, in guinea-pig colon, it was a surmountable antagonist with an affinity (pKB) of 9.4 +/- 0.1. Furthermore, several novel, selective, 5-HT4 compounds also showed opposing patterns of intrinsic activities similar to those described for RS 23597 and (S)RS 56532. It is concluded that these differences are inconsistent with differences in 5-HT4 receptor reserves, and may suggest that 5-HT4 receptors in the guinea-pig colon and the rat oesophagus can be operationally distinguished.

Pharmacological characterization of the muscarinic receptors mediating contraction of canine saphenous vein

1. The muscarinic receptor subtype mediating contraction of the canine saphenous vein has been characterized using a range of muscarinic agonists and subtype-selective antagonists. 2. Oxotremorine M and (+)-cis-dioxolane behaved as full agonists, while in comparison L-660,863 ((+/-)-3-(3-amino-1,2,4-oxadiazole-5-yl)quinuclidine) acted as a partial agonist. SDZ ENS 163 (thiopilocarpine), pilocarpine and McN-A-343 (0.1 microM-0.3 mM) did not elicit a response. The profile of agonist potencies suggests a low receptor reserve for contraction. 3. The rank order of antagonist apparent affinities was 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide; 8.41) > pirenzepine (8.10) > himbacine (7.34) > or = p-F-HHSiD (para-fluoro-hexahydrosiladifenidol; 7.15) > methoctramine (6.23). This antagonist apparent affinity profile is consistent with the activation of muscarinic M1 receptors.

Characterization of the interaction of zamifenacin at muscarinic receptors in vitro

The interaction of zamifenacin ((3R)-(+)-diphenylmethoxy-1-(3,4)-methylenedioxyphenethyl)pi peridine) at muscarinic receptor subtypes was studied using radioligand binding and functional techniques, in vitro. In radioligand binding studies, zamifenacin acted as a competitive antagonist, with the following pKi values; rat cerebral cortex (M1) 7.90 +/- 0.08, myocardium (M2) 7.93 +/- 0.13, submaxillary gland (M3) 8.52 +/- 0.04 and rabbit lung (M4) 7.78 +/- 0.04. In functional studies zamifenacin acted as a surmountable antagonist, exhibiting the following apparent affinity values; canine saphenous vein (putative M1) 7.93 +/- 0.09, guinea-pig left atria (M2) 6.60 +/- 0.04, guinea-pig ileum (M3) 9.31 +/- 0.06, guinea-pig oesophageal muscularis mucosae (M3) 8.84 +/- 0.04, guinea-pig trachea (M3) 8.16 +/- 0.04, and guinea-pig urinary bladder (M3) 7.57 +/- 0.15. Therefore, zamifenacin is selective for muscarinic M3 receptors in guinea-pig ileum, oesophageal muscularis mucosae, trachea and bladder over muscarinic M2 receptors in atria. The degree of muscarinic M3/M2 receptor selectivity depends upon the muscarinic M3 receptor preparation studied.

Pharmacological characterization of two novel and potent 5-HT4 receptor agonists, RS 67333 and RS 67506, in vitro and in vivo

1. The pharmacology of two novel 5-HT4 receptor agonists, RS 67333 (1-(4-amino-5-chloro-2-methoxy-phenyl)-3-[1(n-butyl)-4-piperidinyl]-1- propanone HCl) and RS 67506 (1-(4-amino-5-chloro-2-methoxy-phenyl)-3-[1-(2-methyl sulphonylamino)ethyl-4-piperidinyl]-1-propanone HCl) have been assessed in vitro and in vivo. 2. RS 67333 and RS 67506 exhibited affinities (pKi = 8.7 and 8.8, respectively) for the 5-HT4 binding sites, labelled with [3H]-GR 113808, in guinea-pig striatum. The Hill coefficients from these displacement curves were not significantly different from unity. The compounds exhibited lower affinities (< 6.0) at several other receptors including 5-HT1A, 5-HT1D, 5-HT2A, 5-HT2C, dopamine D1, D2 and muscarinic M1-M3 receptors. However, RS 67333 and RS 67506 did exhibit affinities for the sigma 1 (pKi = 8.9 and 7.9, respectively) and sigma 2 (pKi = 8.0 and 7.3, respectively) binding sites. 3. At the 5-HT4 receptor mediating relaxation of the carbachol-precontracted oesophagus, RS 67333 and RS 67506 acted as potent (pEC50 8.4 and 8.6, respectively), partial agonists (intrinsic activities, with respect to 5-HT were 0.5 and 0.6, respectively) with respect to 5-HT. Relaxant responses to RS 67333 or RS 67506 were surmountably antagonized by GR 11308 (10 nM), with apparent affinities (pKB) of 9.1 and 9.0, respectively. RS 67333 and RS 67506 induced dose-dependent increases in heart rate of the anaesthetized micropig (ED50 4.9 and 5.4 micrograms kg-1, i.v.), with maximal increases of 35 and 47 beats min-1, respectively. 4. RS 67333 and RS 67506, therefore, acted as potent, partial 5-HT4 receptor agonists in vitro and in vivo. These compounds, by virtue of their high potency and selectivity, may have some utility in elucidating the physiological role of 5-HT4 receptors.

Characterization of a postjunctional 5-HT receptor mediating relaxation of guinea-pig isolated ileum

The 5-HT receptor mediating postjunctional relaxation of precontracted guinea-pig ileum has been characterized using several agonists and antagonists. Substance P precontracted tissues were potently relaxed by 5-HT (5-hydroxytryptamine, serotonin), 5-CT (5-carboxamidotryptamine) and several other indoles. The rank order of potency, with pEC50 values in parentheses, was 5-CT (7.6) > 5-methoxytryptamine (5.7) > 5-HT (5.5) > alpha-methyl-5-HT (4.7) > 2-methyl-5-HT (< 4.0) = tryptamine (< 4.0) = N,N-dimethyl-tryptamine (< 4.0) = N,N-dimethyl-5-HT (< 4.0) = dipropyl-5-CT (< 4.0) = sumatriptan (< 4.0). 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)-tetralin) acted as a potent (6.3), but partial, agonist with respect to 5-HT. The responses to 5-CT were antagonized by several compounds with the following rank order of affinity, with pKB values in parentheses: LSD (lysergic acid diethylamide; 8.1) = mesulergine (7.8) > methysergide (7.6) = spiperone (7.6) > clozapine (7.3) >> (-)-pindolol (< 6.0) > ketanserin (< 6.0) = ondansetron (< 6.0) = GR 113808 ([1-(2-methane-sulphonamido-ethyl)-piperidin-4-yl]-methyl-in dole-3- carboxylate maleate; < 6.0). The relaxant responses to 5-HT were also resistant to tetrodotoxin. These data are consistent with a functional 5-HT receptor, mediating relaxation of guinea-pig ileum, which exhibits an operational profile similar to that of the cloned guinea-pig 5-ht7 receptor. This study, therefore, provides evidence for a functional correlate of the 5-ht7 gene product.

The interaction of RS 25259-197, a potent and selective antagonist, with 5-HT3 receptors, in vitro

1. A series of isoquinolines have been identified as 5-HT3 receptor antagonists. One of these, RS 25259-197 [(3aS)-2-[(S)-1-azabicyclo[2.2.2]oct-3-yl]-2,3,3a,4,5,6-hexahydro- 1- oxo-1H-benzo[de]isoquinoline-hydrochloride], has two chiral centres. The remaining three enantiomers are denoted as RS 25259-198 (R,R), RS 25233-197 (S,R) and RS 25233-198 (R,S). 2. At 5-HT3 receptors mediating contraction of guinea-pig isolated ileum, RS 25259-197 antagonized contractile responses to 5-HT in an unsurmountable fashion and the apparent affinity (pKB), estimated at 10 nM, was 8.8 +/- 0.2. In this tissue, the -log KB values for the other three enantiomers were 6.7 +/- 0.3 (R,R), 6.7 +/- 0.1 (S,R) and 7.4 +/- 0.1 (R,S), respectively. The apparent affinities of RS 25259-197 and RS 25259-198, RS 25233-197 and RS 25233-198 at 5-HT3 receptors in membranes from NG-108-15 cells were evaluated by a [3H]-quipazine binding assay. The -log Ki values were 10.5 +/- 0.2, 8.4 +/- 0.1, 8.6 +/- 0.1 and 9.5 +/- 0.1, respectively, with Hill coefficients not significantly different from unity. Thus, at these 5-HT3 receptors, the rank order of apparent affinities was (S,S) > (R,S) > (S,R) = (R,R). 3. RS 25259-197 displaced the binding of the selective 5-HT3 receptor ligand, [3H]-RS 42358-197, in membranes from NG-108-15 cells, rat cerebral cortex, rabbit ileal myenteric plexus and guinea-pig ileal myenteric plexus, with affinity (pKi) values of 10.1 +/- 0.1, 10.2 +/- 0.1, 10.1 +/- 0.1 and 8.3 +/- 0.2, respectively. In contrast, it exhibited low affinity (pKi <6.0) at 28 other receptors in binding assays, including adrenoceptors (alpha1A, alpha 1B, alpha2A, alpha 2B ,beta1, beta2), muscarinic (M1-M4), dopamine (D1, D2), opioid and other 5-HT(5-HTlA, 5-HTlD, 5-HT2C, 5-HT4) receptors.4. RS 25259-197 was tritium labelled (specific activity: 70 Ci mmol-1) and evaluated in pharmacological studies. Saturation studies with [3H]-RS 25259-197 in membranes from NG-108-15 and cloned homomeric a subunits of the 5-HT3 receptor from N1E-1 15 cells expressed in human kidney 293E1 cells,revealed an equilibrium dissociation constant (Kd) of 0.05 +/- 0.02 and 0.07 +/- 0.01 nM, and Bmax of610 +/- 60 and 1068 +/- 88 fmol mg-1, respectively. Competition studies in NG-108-15 cells indicated a pharmacological specificity entirely consistent with labelling a 5-HT3 receptor, i.e. RS 25259-197> granisetron> (S)-zacopride> tropisetron> (R)-zacopride> ondansetron> MDL 72222.5. In contrast to the majority of radioligands available to label 5-HT3 receptors, [3H]-RS 25259-197 labelled a high affinity site in hippocampus from human post-mortem tissue with an equilibrium dissociation constant (Kd) of 0.15 +/- 0.07 nM and density (BmaX) of 6.8 +/- 2.4 fmol mg-1 protein. Competition studies in this tissue indicated a pharmacological specificity consistent with labelling of a 5-HT3receptor.6. Quantitative autoradiographic studies in rat brain indicated a differential distribution of 5-HT3receptor sites by [3H]-RS 25259-197. High densities of sites were seen in nuclear tractus solitaris and area postrema, a medium density in spinal trigeminal tract, ventral dentate gyrus and basal medial amygdala,and a low density of sites in hippocampal CAl, parietal cortex, medium raphe and cerebellum.7 In conclusion, the functional, binding and distribution studies undertaken with the radiolabelled and non-radiolabelled RS 25259-197 (S,S enantiomer) established the profile of a highly potent and selective5-HT3 receptor antagonist.

Muscarinic M3 receptors mediate contractions in rabbit, endothelium-denuded aorta in vitro

1. Muscarinic receptors mediating contraction of rabbit endothelium-denuded aorta have been characterized functionally, in vitro, using a range of antagonists (atropine, pirenzepine, methoctramine, himbacine, 4-diphenyl-acetoxy-N-methyl piperidine methiodide (4-DAMP) and para-fluoro-hexahydro-siladifenidol (p-F-HHSiD). 2. The non-selective muscarinic agonist, (+)cis-dioxolane, induced concentration-dependent contractions of endothelium-denuded aortic rings. The potency (EC50) of (+)cis-dioxolane was 1.0 +/- 0.4 microM and the maximal increase in isometric tension was 944 +/- 98 mg (mean +/- SEM, n = 25). The concentration-effect curves to (+)cis-dioxolane were shifted to the right in the presence of antagonists, in a concentration-dependent manner. The following affinities (-log KB) were calculated; atropine, 9.4; pirenzepine, 6.6; methoctramine, 5.9; himbacine, 7.1; 4-DAMP, 9.2; and p-F-HHSiD, 7.7. 3. It is concluded that muscarinic M3 receptors mediate contractions of endothelium-denuded aorta. The low potency of (+)cis-dioxolane, when compared to its potency in other M3 receptor assays, suggests that the efficiency of receptor coupling, associated with contraction of this tissue, is poor.

Characterization of 5-hydroxytryptamine-induced depolarizations in rat isolated vagus nerve

An additional component of the depolarization induced by 5-hydroxytryptamine (5-HT) in the rat isolated vagus nerve has recently been attributed to activation of 5-HT4 receptors. To confirm and extend this finding, extracellular recordings of D.C. potentials were made using the 'grease-gap' technique during continuous superfusion of the isolated nerve. Beginning at 1 nM, 5-HT induced small depolarizations that displayed a slow onset. At concentrations > or = 1 microM, large depolarizations with rapid onset were elicited. In the presence of the 5-HT3 receptor antagonists, granisetron or ondansetron, 5-HT responses were diminished and exhibited an increased latency to peak. These small, slow depolarization were not reduced by 5-HT1 or 5-HT2 receptor antagonists, but were potently inhibited by the 5-HT4 receptor antagonist GR 113808 (pA2 = 9.3), and mimicked by 5-methoxytryptamine (pEC50 = 5.3). 5-HT4-mediated responses were larger at 37 degrees C than at 31 degrees C, but also showed marked diminution with repeated 5-HT applications at concentrations greater than 1 microM. Conversely, 5-HT3 receptor responses were potentiated at lower temperatures (< or = 31 degrees C). Consistent with the reported positive coupling of 5-HT4 receptors to adenylyl cyclase, forskolin and 8-Br-cAMP produced slowly developing depolarizations which were qualitatively similar to 5-HT4 receptor activation. Pre-depolarization of nerves with 10 microM forskolin or 300 microM 8-Br-cAMP diminished the effect of 5-HT4 receptors. This study has confirmed the presence of 5-HT4 receptors on the vagus nerve of the rat and defined some conditions that optimize their pharmacological isolation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of muscarinic M2 and M3 receptor stimulation and antagonism on responses to isoprenaline of guinea-pig trachea in vitro

1. In guinea-pig and canine airway smooth muscle, there is reduced beta-adrenoceptor agonist sensitivity in tissues pre-contracted with muscarinic agonists when compared to tissues pre-contracted with other spasmogens, such as histamine or leukotriene D4. This reduced sensitivity may be the result of an interaction between muscarinic receptors and beta-adrenoceptors. In this study the effects of M2 receptor antagonism and stimulation have been investigated on the relaxant potency of isoprenaline in guinea-pig isolated tracheal smooth muscle. 2. (+)-cis-Dioxolane contracted isolated tracheal strips in a concentration-dependent manner (EC50 = 11.5 +/- 0.9 nM). The rank order of antagonist apparent affinities (with pA2 values in parentheses) was atropine (9.4 +/- 0.1) > zamifenacin (8.2 +/- 0.1) > para-fluoro-hexahydro-siladiphenidol (p-F-HHSiD, 7.2 +/- 0.1) > pirenzepine (6.5 +/- 0.1) > methoctramine (5.5 +/- 0.1). Schild slopes were not significantly different from unity. This was consistent with a role of muscarinic M3 receptors in mediating contraction. 3. In tissues pre-contracted to 3 g isometric tension using (+)-cis-dioxolane (0.2 microM, approximately EC80), the relaxant potency of isoprenaline was significantly (P < 0.05) increased by 0.3 microM methoctramine (control EC50 = 32.2 +/- 4.3 nM, plus methoctramine EC50 = 19.1 +/- 4.5 nM). This concentration of methoctramine had no effect on contractile responses to (+)-cis-dioxolane (control, EC50 = 17.6 +/- 3.2 nM, plus methoctramine, EC50 = 21.0 +/- 4.4 nM). 4 When acetylcholine (non-selective), (+)-cis-dioxolane (non-selective), L-660,863 ((+/- )-3-(3-amino-1,2,4-oxadiazole-5-yl)-quinuclidine, M2-selective) or SDZ ENS 163 (thiopilocarpine, mixed M2 antagonist,partial M3 agonist) were used to achieve isometric tensions of 3 g, the relaxant potency of isoprenaline ranged from 3.7 +/- 0.3 nM (SDZ ENS 163) to 49.4 +/- 3.2 nM ((+)-cis-dioxolane). Reducing the concentration of these agonists (and therefore the level of developed tension to 2 g), significantly(P<0.05) increased the relaxant potency of isoprenaline. In contrast, when histamine was used to pre-contract tissues to either 2 or 3 g (EC50 = 4.2 +/- 0.6 and 3.8 +/- 1.1 nM, respectively), there was no significant effect on the relaxant potency of isoprenaline.5. There was a slight but significant (P<0.05) reduction in the relaxant potency of isoprenaline, in tissues pre-contracted to 3 g using histamine in combination with (+ )-cis-dioxolane (30 nM). This effect was reversed by M2 receptor antagonism, using methoctramine (1 MicroM).6. These data suggest that in guinea-pig isolated trachea, the relaxant potency of isoprenaline may depend not only on the level of developed tension but also, on the level of muscarinic M2 receptor stimulation/blockade of the spasmogen inducing the tension. However, the lack of selective M2 agonist and the low M2/M3 selectivity of antagonists in this tissue do not permit definitive conclusions to be made about the role of these receptors in modulating isoprenaline potency.

(R) and (S) RS 56532: mixed 5-HT3 and 5-HT4 receptor ligands with opposing enantiomeric selectivity

The pharmacological properties of the (R) and (S) enantiomers of RS 56532 have been studied in vitro and in vivo. In radioligand binding studies at 5-HT4 receptors in guinea-pig striatum, (S) RS 56532 exhibited a higher affinity than (R) RS 56532 (-log Ki = 7.6 and 6.5, respectively). (S) RS 56532 acted as a potent agonist at 5-HT4 receptors mediating relaxation of rat oesophageal muscularis mucosae (-log EC50 = 7.9) while (R) RS 56532 acted as a weaker agonist at this receptor (-log EC50 < 6.0). These data suggest that at 5-HT4 receptors, the enantiomeric selectivity of RS 56532 was (S) > (R). In binding studies at 5-HT3 receptors in rat cortex, (R) RS 56532, conversely, exhibited a higher affinity than (R) RS 56532 (-log Ki = 9.1 and 8.0, respectively). At 5-HT3 receptors in guinea-pig isolated ileum, (R) RS 56532 exhibited an affinity (-log KB) of 7.9, whereas (S) RS 56532 (1 nM-1 microM) was inactive. No agonism was observed at ileal 5-HT3 receptors with either enantiomers. These data suggest that at 5-HT3 receptors in rat and guinea-pig, both enantiomers acted as antagonists, with (R) > (S) RS 56532. At the non-5-HT3, high affinity '(R) zacopride' site, (R) RS 56532 exhibited a higher affinity than (S) RS 56532 (-log Ki = 6.1 and 4.9). This site was insensitive to potent 5-HT3 antagonists such as (R) YM 060 or ondansetron. However, it was recognized with relatively high affinity (-log Ki = 7.5) by the (R), but not (S) enantiomer, of RS 42358 (-log Ki = 4.7). Since (S) RS 42358 is a high affinity 5-HT3 receptor antagonist, these data further highlight the dissimilarity between the 5-HT3 receptor and the '(R) zacopride' site. The '(R) zacopride' site also appeared to be pharmacologically distinct from the 5-HT4 receptor, since 5-HT4 ligands such as renzapride, SDZ 205,557 or RS 23597-190 exhibited low affinities. The enantiomeric selectivity of (R) and (S) RS 56532 in vivo was consistent with findings in vitro. At 5-HT4 receptors mediating tachycardia in the pig, 5-HT induced a dose-dependent tachycardia (ED50 = 3 micrograms kg-1, i.v.; maximum response = 90-100 beats min-1). (S) RS 56532 increased heart rate by 88 min-1 with a potency of (ED50) of 3 micrograms kg-1, i.v.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparison of potencies of 5-HT3 receptor antagonists at inhibiting aversive behavior to illumination and the von Bezold-Jarisch reflex in the mouse

The inhibitory effects of ondansetron and R and S zacopride on aversive behavior to light and the von Bezold-Jarisch reflex have been compared in mouse. The potencies (ID50, microgram/kg i.v.) of compounds at inhibiting the von Bezold-Jarisch reflex, elicited by 2-methyl-5-HT (mouse 100 micrograms/kg, i.v.; rat 10-80 micrograms/kg i.v.) were: S zacopride (0.02), granisetron (0.17), R zacopride (0.30) ondansetron (3.16). A similar rank order of ID50 values was observed in rat, i.e. S zacopride (0.02), granisetron (0.36), R zacopride (0.25) and ondansetron (2.65). These data suggest that the activity of compounds at 5-HT3 receptors mediating this effect was similar in both mouse and rat. In mouse behavioral studies, ondansetron and R and S zacopride potently inhibited aversive behavior to light (0.0003-30 micrograms/kg, p.o.), when the amount of time spent in the dark and locomotor activity were measured. Thus, at 0.3 ng/kg, the mean percentage time spent in the dark significantly decreased from 84 to 72, for both R and S zacopride, respectively. The maximal effects of these compounds were modest in comparison to the 'anxiolytic' effects of diazepam (0.3-1.4 mg/kg, i.p.; at 0.3 mg/kg the mean percentage time spent in the dark significantly decreased from 84 to 36) or chlordiazepoxide (3-40 mg/kg, i.p.; at 3 mg/kg, the mean percentage time spent in the dark significantly decreased from 85 to 40). The doses of the 5-HT3 antagonists were approx 1000-fold lower than those effective inhibitory doses determined in the von Bezold-Jarisch reflex studies, in either mouse or rat.(ABSTRACT TRUNCATED AT 250 WORDS)

RS 23597-190: a potent and selective 5-HT4 receptor antagonist

1. The pharmacological properties of RS 23597-190 (3-(piperdine-1-yl)-propyl-4-amino-5-chloro-2-methoxy benzoate hydrochloride) have been studied in vitro and in vivo. 2. RS 23597-190 competitively antagonized 5-HT4 receptor-mediated relaxations of rat, carbachol precontracted oesophageal muscularis mucosae, (pA2 = 7.8 +/- 0.1; Schild slope = 1.2 +/- 0.2). Affinity estimates (-log KB) at 5-HT4 receptors using either renzapride or SC-53116 as agonists yielded a -log KB value of 8.0 +/- 0.01. In contrast, RS 23597-190 failed to antagonize contractile responses to 5-HT of guinea-pig ileal 5-HT3 receptors, even at concentrations up to 10 microM. 3. Increases in short-circuit current, induced by 5-HT, were studied in guinea-pig ileal mucosal sheets. Concentration-response curves to 5-HT were biphasic, with the high potency phase to 5-HT inhibited by RS 23597-190 and mimicked by 5-methoxytryptamine. The -log KB value for RS 23597-190 at the high potency phase was 7.3 confirming that 5-HT4 receptors mediated the high potency phase. 4. In rat isolated vagus nerve, 5-HT elicited a slow, maintained depolarization at low concentrations and a rapid, transient depolarization at higher concentrations. The high potency, slow depolarizing phase to 5-HT was abolished selectively in the presence of 1 microM RS 23597-190 and the low potency phase was abolished selectively in the presence of 1 microM ondansetron. These data confirm that 5-HT4 and 5-HT3 receptors mediated slow and fast depolarization responses, respectively. 5. At 5-HT3 binding sites in membranes from NG 108-15 cells, labelled by [3H]-quipazine, RS 23597-190 exhibited an apparent affinity (- log Ki) of 5.7 +/- 0.1. At 5-HT3 receptors in membranes from rat cerebral cortex, labelled by [3H]-RS 42358-197, the apparent affinity (- log Ki) of RS 23597-190 was also 5.7 +/- 0.1. In both studies, Hill coefficients were not significantly different from unity. At 5-HT1A, 5-HT2,muscarinic M1, M2, M3, M4 and dopamine D1 and D2 receptors, RS 23597-190 exhibited low apparent affinities, with all - log Ki values less than 5.5.6. Intravenous infusion of RS 23597-190 in the conscious, restrained rat antagonized the von Bezold Jarisch reflex induced by 2-methyl 5-HT, with an ID50 of 300 microg kg-1 min-1, i.v. In the anaesthetized,bilaterally vagotomized micropig, RS 23597-190 (6 mg kg-1, i.v.) antagonized 5-HT-induced tachycardia with a half-life of 77 (63-99) min. Transient arrhythmic effects were noted after administration of the compound.7. In conclusion, RS 23597-190 acts as a high affinity, selective competitive antagonist at 5-HT4 receptors. Thus, the compound appears to be a useful tool for 5-HT4 receptor identification in vitro. In vivo, the compound is rapidly metabolized in pigs such that 5-HT4 blockade is not maintained. However,in the rat, when given by infusion, RS 23597-190 antagonizes 5-HT3 mediated responses, at doses consistent with a low affinity 5-HT3 receptor. These data suggest that, under appropriate experimental conditions, RS 23597-190 may also be used in vivo to characterize further 5-HT4 receptor function.