Synthesis and preliminary screening of novel tryptamines as 5-HT4 receptor ligands.

For the development of novel 5-HT(4) receptor ligands we have designed and synthesized two series of 5-methoxytryptamine derivatives varying the substitution on the primary amine. Their biological activities were evaluated in a receptor binding assay where a subset of compounds showed comparable potency to the agonists serotonin and 5-methoxytryptamine. Structure-activity analyses have highlighted promising avenues for further synthetic work and binding modes were proposed by docking these compounds into a homology model of the 5-HT(4) receptor.

The use of tripeptides for lead discovery of 5-HT4 receptor ligands.

A series of 30 tripeptides were synthesized and tested as novel 5-HT4 receptor ligands. Receptor binding assays showed that a subset of compounds had reasonable potency relative to the agonists serotonin and 5-methoxytryptamine. Structure-activity analyses and molecular docking have highlighted avenues for further synthetic work.

Distribution of serotonin receptors and interacting proteins in the human sigmoid colon.

This study aimed to examine the distribution of 5-HT receptors in the human colon. 5-HT induces desensitization of the circular muscle and as this is facilitated by G-protein coupled receptor kinases (GRKs) and other proteins, we also examined their distribution. Human sigmoid colon samples were dissected into three separate layers (mucosa, taeniae coli and intertaenial strips) and RNA was amplified by RT-PCR. The 5-HT(2B) receptor and all 5-HT(7) receptor splice variants were expressed in all tissues. 5-HT(4) a,b,c and n splice variants were also expressed in all tissues and 5-HT(4d), 5-HT(4g) and 5-HT(4i) were only detected in some samples. The 5-HT(2A) receptor was seen predominantly in the intertaenial strips of the colon. Only one transcript of the serotonin transporter (SERT) was detected in the muscle layers. Variation was seen in GRK expression with GRK2 and 3 predominantly expressed in the mucosa, while GRK5 and 6 were found more commonly in the taeniae coli. PDZ (named after postsynaptic density protein, Drosophila disc large tumour suppressor and tight junction protein ZO-1) domain containing proteins, which may be involved in 5-HT receptor trafficking, were also detected throughout the sigmoid colon. The 5-HT(3A) subunit was expressed in all tissues, whereas the 5-HT(3E) subunit was mainly found in the mucosa layer while the 5-HT(3B) subunit was more common in the muscle layers. Receptor interacting chaperone (RIC-3), which is involved in transporting 5-HT(3) receptor subunits, is expressed less in mucosa compared to muscle layers. In conclusion, these results show that there is variation in distribution of 5-HT receptors and interacting proteins within the sigmoid colon that may contribute to colonic function.

Demonstration of 5-HT(3) receptor function and expression in the mouse bladder.

The aim of this study was to demonstrate the presence of 5-HT(3) receptors in the mouse bladder and to determine their location. Bladder strips from female mice were set up in gassed Krebs-Henseleit solution at 37 degrees C and contractions recorded in response to electrical field stimulation (8 Hz, 60 V, 0.5-ms pulse duration) applied for 2 s every 50 s. The potentiating effects of 5-hydroxytryptamine (5-HT) were recorded (in the presence of 1-microM methysergide and 1-microM GR125487 to isolate the 5-HT(3) receptor response), and contractions were expressed as a percentage of the response to 0.1-M KCl. Responses to (5-HT) were also obtained in the presence of the 5-HT(3) receptor antagonist, ondansetron. RT-PCR was used to detect the expression of the 5-HT(3A) and 5-HT(3B) subunit transcripts of the mouse 5-HT(3) receptor. 5-HT and 5-HT(3) receptor agonists caused concentration-dependent increases in the force of neurogenic contractions without affecting the baseline tone. The rank order of potency was: meta-chloro-phenylbiguanide (m-CPB) = 5-HT > 2-methyl-5-HT (2m5-HT) = 1-phenylbiguanide (1-PBG). The respective pEC(50) values were: 6.42 +/- 0.2 = 5.95 +/- 0.19 > 5.35 +/- 0.12 = 5.14 +/- 0.13. m-CPB acted as a full agonist (E (max) = 40.65 +/- 3.81% KCl), but both 2m5-HT and 1-PBG acted as lower potency partial agonists. Ondansetron (30, 100, 300 nM) caused concentration-related rightward displacements to the concentration-effect curve to 5-HT. Nonlinear regression analysis of the effect of the ondansetron concentrations on the pEC(50) values produced a pK(B) value of 8.29 +/- 0.22. Desensitization of sensory nerves to the contractile effect of capsaicin (10 microM for 60 min) did not alter the ability of 5-HT to potentiate neurogenic contractions. 5-HT (3 microM) inhibited contractions induced by direct muscle stimulation (lignocaine, 300 microM and 10-ms pulse width). m-CPB also caused the same effect with a pIC(50) of 6.62 +/- 0.10 and an E (max) of 48.03 +/- 2.25%. The concentration-response curve to m-CPB was shifted rightwards by ondansetron (1 microM) giving an apparent pK(B) value of 8.15 +/- 0.33. mRNA for both the 5-HT(3A) and 5-HT(3B) receptor subunits was detected in the detrusor as well as the mucosa with a greater relative expression of the 5-HT(3A) subunit in both layers. This study demonstrates that 5-HT mediates enhanced neurogenic contractions of the mouse bladder muscle by an action at 5-HT(3) receptors located prejunctionally on nonsensory nerve elements. Additionally, an inhibitory postjunctional population of the 5-HT(3) receptor was identified. The presence of the 5-HT(3) receptor was confirmed by the expression of both 5-HT(3A) and 5-HT(3B) receptor subunits of the 5-HT(3) receptor.

Comparison of opioid receptor distributions in the rat ileum.

The cellular expression patterns of mu-, delta- and kappa-opioid receptors in the rat ileum were examined using fluorescence immunohistochemistry. Double-labelling was used to examine cellular receptor co-localisation as a pre-requisite for intracellular molecular interactions, such as heterodimerisation. Tissues were stained as whole-mount preparations. Strong, broadly distributed immunoreactivity (ir) was observed for each receptor in the myenteric and submucous plexuses. Although intracellular mu- and delta-ir patterns differed in ganglion neurons, mu/delta co-expression was extensive in these cells. mu/delta co-expression was also observed in interstitial cells, which were diffusely distributed in submucous plexus preparations but generally located adjacent to myenteric plexus structures. Punctate kappa-ir was seen broadly in nerve fibres in both plexuses, suggesting localisation in varicosities. Neuronal mu/kappa co-localisation was not apparent, although kappa-ir fibres were often apposed against mu-ir cells. mu/kappa co-localisation was detected in interstitial cells in submucous plexus preparations. Similarities in mu and delta expression patterns might reflect similar functional properties previously detected for these receptors. This study indicates that the rat gastrointestinal tract might provide a useful tool for the future study of molecular interactions between opioid receptor types.

Influences of gender and region on responses to 5-HT in the rat small intestine.

The aim of this study was to obtain more information regarding the 'atypical' 5-HT7 receptor of the rat jejunum. 5-HT7-induced contractions of the jejunum were elicited by 5-HT in the presence of ondansetron. Maximal responses were slightly larger in tissues from male compared to female rats of comparable age, with Emax values of 97.2 +/- 3.3 and 84.25 +/- 4.3% respectively compared to acetylcholine as an internal standard. However, the pEC50 values for 5-HT were not significantly different. The mRNA expression levels of the 5-HT7 receptor were similar in whole jejunum and longitudinal muscle tissues taken from males and females. It was also shown that the maximal response of the jejunum from male rats was larger than the responses from mid intestine and ileum. However, in female tissues, the Emax of the mid intestine was significantly larger than the ileum, but not different from the jejunum. The results provide further insights into the 'atypical' 5-HT7 receptor of the rat jejunum and are also useful in optimising the preparation for further studies.

Antiinflammatory screening of the medicinal plant Gynura procumbens.

Gynura procumbens is used in Thai folk medicine to treat topical inflammation, rheumatism, and viral ailments. In the present work, attempts were made to verify the folk medicinal claim that the crude ethanolic extract of G. procumbens has antiinflammatory action and to relate the activity to particular fractions using a croton oil-induced mouse ear inflammation model. The original ethanolic extract of G. procumbens was partitioned between water and ethyl acetate. The residues were subjected to antiinflammatory evaluation. While the water extract did not show any antiinflammatory activity, the administration of the original organic extract significantly inhibited the increase in ear thickness in response to croton oil (n = 5). The activity of 0.75 mg/ear original organic extract showed similar antiinflammatory activity (inhibition 65.2%) to that of 6 mg/ear hydrocortisone 21-hemisuccinate sodium salt (inhibition 64.8%). The organic extract was then fractionated with a series of solvents in order of increasing polarity. Each fraction was dried, dissolved in acetone and monitored using the same bioassay. These experiments showed that the hexane and toluene fractions showed significant inhibitions of 44.6% and 34.8%, respectively. These two fractions had similar activities to 4 mg/ear of hydrocortisone (inhibition 35.0%). The possible chemical constituents in the extracts and fractions were investigated using thin layer chromatography and specific color reagents. These tests showed that steroids might be one class of antiinflammatory compounds in this plant.

Expression patterns of 5-HT7 receptor isoforms in the rat digestive tract.

In this study, we determined the expression patterns of the 5-HT7 receptor isoforms at the mRNA level in the rat jejunum and other regions of the rat digestive tract using reverse transcriptase-PCR. In control studies using the rat thalamus, we detected all three known isoforms of the rat 5-HT7 receptor, 5-HT7(a), 5-HT7(b) and 5-HT7(c). In addition, we also detected in the thalamus a new PCR product that we provisionally name 5-HT7(e). All four isoforms of the 5-HT7 receptor were present throughout the rat digestive tract. However, the spatial distribution and levels of expression of the different isoforms varied between regions of the rat digestive tract. All three known isoforms and the provisional isoform were found in the jejunum, ileum, stomach fundus, oesophagus and colon. The level of expression of the isoforms varied with all four being most consistently found in the ileum. The 5-HT7(a) receptor isoform was predominantly expressed in the rat digestive tissue and 5-HT7(b,, 5-HT7(c) and 5-HT7(e) were only sometimes expressed but always accompanying the receptor isoform 5-HT7(a).

Withdrawal and bidirectional cross-withdrawal responses in rats treated with adenosine agonists and morphine.

The aim of this study was to investigate whether the A1/A2 receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA), and the selective A1 agonist, N6-cyclopentyladenosine (CPA), induced physical dependence by quantifying specific antagonist-precipitated withdrawal syndromes in conscious rats. In addition, the presence of bidirectional cross-withdrawal was also investigated. The agonists were administered s.c. to groups of rats at 12 h intervals. Antagonists were administered s.c., 12 hours after the last dose, followed by observation and measurement of faecal output for 20 min. NECA (4 x 0.03 mg kg(-1), s.c) and CPA (4 x 0.03, 0.1 and 0.3 mg kg(-1), s.c.) induced physical dependence, as shown by the expression of a significant withdrawal syndrome when challenged with the adenosine A1/A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX, 0.1 mg kg(-1), s.c.) and the A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPDPX, 0.1 mg kg(-1), s.c.) respectively. The syndromes consisted of teeth chattering and shaking behaviours shown to occur in morphine-dependent animals withdrawn with naloxone viz, paw, body and 'wet-dog' shakes, but with the additional behaviours of head shaking and yawning. In further contrast to the opiate withdrawal syndrome, no diarrhoea occurred in the groups of animals treated with adenosine agonists and withdrawn with their respective antagonists. Bidirectional cross-withdrawal syndromes were also revealed when naloxone (3 mg kg(-1), s.c.) was administered to adenosine agonist pre-treated rats and adenosine antagonists were given to morphine pre-treated rats. This study provides further information illustrating that close links exist between the adenosine and opiate systems.

Lack of evidence for histamine H3 receptor function in rat ileum and human colon.

The effects of histamine and the more selective H3 receptor agonist (R)alpha-methylhistamine were investigated on contractile responses produced by electrical stimulation of the longitudinal and circular muscles of the rat ileum and the circular muscle of the human colon. Histamine (0.1-3.0 microM) and (R)alpha-methylhistamine (0.1-3.0 microM) had no significant effect (P>0.05) on cholinergic nerve stimulation of either the longitudinal or circular muscle of the rat ileum nor the circular muscle of the human colon. Substance P (1 microM) and nicotine (0.1 microM), which both produce a contraction via activation of cholinergic nerves, were also unaffected by histamine (1 microM and 10 microM) or (R)alpha-methylhistamine (1 microM and 10 microM), in either tissue. Preliminary studies using in situ hybridisation histochemistry (ISHH) were performed in rat brain and ileum in an attempt to identify H3 receptor mRNA expression. This was done using 33P-labelled oligonucleotide-specific probes for rat H3 receptor mRNA. Unlike rat brain, where H3 receptor mRNA expression was found to be abundant in several regions, no H3 receptor mRNA expression could be detected in the rat ileum under the conditions used. These findings suggest H3 receptors have no role in the modulation of cholinergic neuronal function in the rat or human intestine unlike those in the guinea-pig. Furthermore, H3 receptors appear to be absent in the rat ileum.

Tachykinin-induced contraction of the guinea-pig isolated oesophageal mucosa is mediated by NK(2) receptors.

1. The tachykinin receptor present in the guinea-pig oesophageal mucosa that mediates contractile responses of the muscularis mucosae has been characterized, using functional in vitro experiments. 2. The NK(1) receptor-selective agonist, [Sar(9)(O(2))Met(11)]SP and the NK(3) receptor-selective agonists, [MePhe(7)]-NKB and senktide, produced no response at submicromolar concentrations. The NK(2) receptor-selective agonists, [Nle(10)]-NKA(4 - 10), and GR 64,349 produced concentration-dependent contractile effects with pD(2) values of 8.20+/-0.16 and 8.30+/-0.15, respectively. 3. The concentration-response curve to the non-selective agonist, NKA (pD(2)=8.13+/-0.04) was shifted significantly rightwards only by the NK(2) receptor-selective antagonist, GR 159,897 and was unaffected by the NK(1) receptor-selective antagonist, SR 140,333 and the NK(3) receptor-selective antagonist, SB 222,200. 4. The NK(2) receptor-selective antagonist, GR 159,897, exhibited an apparent competitive antagonism against the NK(2) receptor-selective agonist, GR 64,349 (apparent pK(B) value=9.29+/-0.16) and against the non-selective agonist, NKA (apparent pK(B) value=8.71+/-0.19). 5. The NK(2) receptor-selective antagonist, SR 48,968 exhibited a non-competitive antagonism against the NK(2) receptor-selective agonist, [Nle(10)]-NKA(4 - 10). The pK(B) value was 10.84+/-0.19.6. It is concluded that the guinea-pig isolated oesophageal mucosa is a useful preparation for studying the effects of NK(2) receptor-selective agonists and antagonists as the contractile responses to various tachykinins are mediated solely by NK(2) receptors.

Tachykinins play a minor role in mediating the third phase of the contractile response to vagal nerve stimulation of the guinea-pig oesophagus.

The aim of this study was to determine whether tachykinin receptors might be involved in the mediation of the atropine- and capsaicin-sensitive third phase of a triphasic contractile response to vagal nerve stimulation of the guinea-pig isolated oesophagus. The third phase was inhibited 23.3 +/- 1.7% (P< 0.001, n = 5) and 30. 8 +/- 9.0% (P< 0.05, n = 5) by the NK(3)receptor antagonist, SR 142 801 (0.1 and 1 microM respectively). SR 142 801 (0.1 and 1 microM) had no significant effect on the response to a submaximal concentration of acetylcholine (0.1 mM, n = 4). The third phase was not significantly affected by NK(1)or NK(2)receptor antagonists. Thus, in the guinea-pig oesophagus, it appears that while NK(1)and NK(2)receptors are not involved, NK(3)receptors play a minor role in mediating a contractile response when afferent neurones are excited by vagal nerve stimulation.

Characterisation of a 5-HT(7) binding site in mouse ileum.

The aim of the present study was to identify 5-hydroxytryptamine(7) (5-HT(7)) binding sites in the mouse ileum, where the presence of mRNA for the receptor has been reported. Studies were performed using [3H]mesulergine, an antagonist with high affinity at 5-HT(7) receptors. In the presence of a combination of masking drugs to inhibit the binding of the radioligand to other receptors at which it has affinity, such as 5-HT(2A), 5-HT(2C) and dopamine D(2) receptors as well as alpha(1)/alpha(2)-adrenoceptors, [3H]mesulergine labelled two sites with pK(D) values of 9.7+/-0.7 and 7.4+/-0.4 and B(max) values of 37.2+/-21.4 and 247.8+/-62.1 fmol mg protein(-1), respectively. Displacement studies also indicated the presence of non-homogenous binding sites, which showed a significant correlation (Pearson correlation factors of 0.91 and 0. 85) with the 5-HT(2C) and 5-HT(7) receptors, respectively. Total binding to the 5-HT(2C) receptor was minimal; <30% of the total specific receptor binding. The antagonist order of affinity at the greater proportion of receptors was: risperidone (pK(i)pindolol (5. 6). This receptor also showed a high affinity for 5-carboxamidotryptamine (5-CT; 10.6) and moderate affinity for (+/-)-2-dipropyl-amino-8-hydroxy-1,2,3,4,-tetrahydronaphthalene (8-OH-DPAT; 7.2), which is typical of the 5-HT(7) receptor profile.

Characterization and tissue location of the neural adenosine receptor in the rat ileum.

1. The aim of the present investigation was to characterize and determine the tissue location of the adenosine receptors present in the rat ileum using a method that detects drug action on the cholinergic nerves innervating the longitudinal and circular muscles. 2. The non-selective adenosine agonist, NECA (10 and 100 nM) caused significant concentration-related reductions in the circular muscle responses to transmural stimulation over the frequency range of 2.5-40 Hz, but did not affect the responses of the longitudinal muscle, nor did it reduce the muscle responses of the guinea-pig ileum. 3. The affinity order of antagonists at inhibiting the effect of NECA on the circular muscle was: CPDPX>8-PT>DMPX with apparent pA2 values of 9.31, 7.54 and 5.63 respectively. CPDPX (10-100 nM) caused parallel displacements of the concentration-effect curves to CPA with a pKb value of 9.15 and Schild slope of 1.03. 4. The agonists previously tested in the rat jejunum peristaltic reflex preparation were also shown to inhibit responses of the rat ileum in the following decreasing order of potency: CPA>NECA>2-CADO>R-PIA>S-PIA>>PAA. In addition, CHA and CCPA were also potent agonists. NECA (100 nM) and CPA (32 nM) did not inhibit carbachol (1 microM)-induced tone of tissues pre-treated with TTX (1 microM). 5. In conclusion, the rat ileum contains inhibitory A1 adenosine receptors situated on cholinergic nerve endings innervating the circular muscle.

[3H]-Mesulergine labels 5-HT7 sites in rat brain and guinea-pig ileum but not rat jejunum.

1. The primary aim of this investigation was to determine whether binding sites corresponding to the 5-HT7 receptor could be detected in smooth muscle of the rat jejunum. Binding studies in rat brain (whole brain minus cerebellum) and guinea-pig ileal longitudinal muscle were also undertaken in order to compare the binding characteristics of these tissues. Studies were performed using [3H]-mesulergine, as it has a high affinity for 5-HT7 receptors. 2. In the rat brain and guinea-pig ileum, pKD values for [3H]-mesulergine of 8.0 +/- 0.04 and 7.9 +/- 0.11 (n = 3) and Bmax values of 9.9 +/- 0.3 and 21.5 +/- 4.9 fmol mg(-1) protein were obtained respectively, but no binding was detected in the rat jejunum. [3H]-mesulergine binding in the rat brain and guinea-pig ileum was displaced with the agonists 5-carboxamidotryptamine (5-CT) > 5-hydroxytryptamine (5-HT) > or = 5-methoxytryptamine (5-MeOT) > sumatriptan and the antagonists risperidone > or = LSD > or = metergoline > ritanserin > > pindolol. 3. Despite the lack of [3H]-mesulergine binding in the rat jejunum, functional studies undertaken revealed a biphasic contractile response to 5-HT which was partly blocked by ondansetron (1 microM). The residual response was present in over 50% of tissues studied and was found to be inhibited by risperidone > LSD > metergoline > mesulergine = ritanserin > pindolol, but was unaffected by RS 102221 (3 microM), cinanserin (30 nM), yohimbine (0.1 microM) and GR 113808 (1 microM). In addition, the agonist order of potency was 5-CT > 5-HT > 5-MeOT > sumatriptan. 4. In conclusion, binding studies performed with [3H]-mesulergine were able to detect 5-HT7 sites in rat brain and guinea-pig ileum, but not in rat jejunum, where a functional 5-HT7-like receptor was present.

Evidence of H3 receptor inhibition by iodoaminopotentidine in the guinea pig ileum.

Activation of histamine H3 receptors by histamine (0.1 to 10 microM), (R)alpha-methylhistamine and N(alpha)-methylhistamine (0.01 to 0.3 microM) was shown to inhibit cholinergic nerve transmission in the guinea-pig ileum. Iodoaminopotentidine (IAP 300 nM), a potent H2 receptor antagonist, was found to decrease this effect but had no significant effect (P>0.05) on contractile responses produced by exogenous acetylcholine (0.2 microM). Dimaprit (0.1 to 10 microM) an H2 receptor agonist/H3 receptor antagonist, produced no significant effect (P>0.05) on the response to cholinergic nerve stimulation but reduced the effect of N(alpha)-methylhistamine. Furthermore, ranitidine (10 microM) an H2 receptor antagonist did not modify the inhibitory effect of histamine. These results suggest that IAP may inhibit H3 receptors in the ileum at similar concentrations reported to inhibit H2 receptors in functional studies.

Investigation into the effect of 5-hydroxytryptamine on fluid transport in the rat small intestine.

1. 5-Hydroxytryptamine (5-HT) has been shown to cause a consistent secretory effect in the rat small intestine only when administered luminally or by close intraarterial infusion. Intraluminal 5-HT-induced secretion is possibly mediated by 5-HT4 receptors. Therefore, it was decided to investigate the effect of 5-HT and selective 5-HT4 receptor agonists (SC 53116 and DAU 6236) on intestinal fluid transport in rat jejunum and ileum. The study also investigated the effect of a selective 5-HT4 receptor antagonist (GR 113808) against the intraluminally administered 5-HT. 2. 5-HT receptor agonists and antagonists were administered intraluminally in pentobarbitone-anesthetized rats. Changes in intestinal fluid transport across the intestinal wall were measured by a single pass technique. 3. Intraluminal 5-HT produced significant antiabsorptive effects is both the jejunum and ileum. The 5-HT-induced responses were blocked by intraluminal administration of the 5-HT4 receptor antagonist GR 113808. The 5-HT4 agonist SC 53116 induced antiabsorptive effects in both regions of the small intestine, but DAU 6236 did not affect the rates of fluid transport. 4. The results indicate that a 5-HT4 receptor has a role in the luminal 5-HT-induced antiabsorptive effect on intestinal fluid transport in the rat.

Characterisation of pre- and post-synaptic alpha-adrenoceptors in modulation of the rat ileum longitudinal and circular muscle activities.

Previous study has shown that alpha2D-adrenoceptors are involved in modulation of peristalsis in the rat ileum. The aim of the present study was to determine the tissue location of alpha-adrenoceptors in the rat ileum by using a recently devised method. The pre-synaptic alpha-adrenoceptors were characterised by measuring the potencies of agonists to inhibit transmurally-evoked (1 ms pulses, 10 Hz, 8-10 s trains) contractions of the longitudinal and circular muscles and the affinities of antagonists. Post synaptic alpha-adrenoceptors were identified by screening agonists and antagonists in carbachol-contracted tissues. In the circular muscle the order of potencies for inhibiting transmurally-induced contraction was: clonidine > or = oxymetazoline > or = UK 14,304 > or = guanfacine > talipexole > phenylephrine > azepexole. The potency ratios relative to clonidine correlated to those previously derived using the rat ileum peristaltic reflex preparation. Most of the alpha-adrenoceptor agonists, however, caused only small inhibitions of the longitudinal muscle contraction in response to transmural stimulation, except phenylephrine and azepexole. RX 821002, yohimbine, rauwolscine, BRL 44408, phentolamine, idazoxan, ARC 239, and prazosin inhibited the effect of clonidine on the circular muscle response with apparent pK(B) values best correlated with pK(B) or pKi values derived from the rat ileum peristaltic reflex preparation and other tissues known to have the alpha2D-subtype. The rank order of potencies at inhibiting carbachol-induced responses of both muscle layers was: phenylephrine > or = oxymetazoline > clonidine > or = talipexole > azepexole >> guanfacine. UK 14,304 was inactive up to 10 microM. The EC50 value of each agonist on the longitudinal muscle was not significantly different to the corresponding value on the circular muscle. Prazosin was more potent than yohimbine at inhibiting the relaxant effect of phenylephrine in both muscle layers of carbachol-contracted tissues. It is concluded that the recently identified alpha2D-adrenoceptors of the rat ileum are located on cholinergic neurons controlling circular muscle contraction. The study also demonstrated the presence of postsynaptic alpha1-adrenoceptors involved in mediating relaxation in both muscle layers.

Studies investigating the possible involvement of adenosine in the antisecretory action of morphine.

1. Fluid secretion was induced in the jejunum of anesthetised rats using vasoactive intestinal peptide. 2. The adenosine antagonist, DPCPX (0.1 mg/kg), suppressed the antisecretory action of morphine (10 mg/kg), but naloxone (80 micrograms/kg) did not inhibit the antisecretory response of the adenosine agonist, NECA (40 micrograms/kg), at a dose previously shown to antagonize the antisecretory response of morphine. 3. NECA (40 (micrograms/kg) reversed secretion in pithed and reserpine-pretreated (5 mg/kg subcutaneously) rats. 4. It is proposed that adenosine acts as a mediator of the morphine antisecretory effect at a site distal to the noradrenergic neurons involved in the action of morphine.

Tachykinin receptors in the guinea-pig isolated oesophagus: a complex system.

1. The tachykinin receptors mediating contraction of isolated longitudinal strips of the guinea-pig oesophageal body were characterized with substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) as well as the analogues, [Sar9,Met(O2)11]SP, [Nle10]NKA(4-10) and [MePhe7]NKB, selective for NK1, NK2 and NK3, receptors, respectively. Experiments were performed both in the absence and presence of a cocktail of peptidase inhibitors, captopril (1 microM), thiorphan (1 microM) and amastatin (20 microM), in order to determine whether membrane bound proteases are important in the metabolism of tachykinins in this preparation. 2. All agonists produced concentration-dependent contractile effects. The presence of the peptidase inhibitors shifted the concentration-response curves of SP, [Nle10]NKA(4-10) and [MePhe7]NKB significantly leftwards and the concentration-response curve of NKB was shifted significantly rightwards. However, the EC50 values were significantly different only for [Nle10]NKA(4-10) and NKB. 3. In the presence of the peptidase inhibitors, the EC50 values of the selective agonists, [MePhe7]NKB (0.6 nM) and [Nle10]NKA(4-10) (66 nM) indicated the presence of both tachykinin NK3 and NK2 receptors. [MePhe7]NKB produced less than 50% of the maximal response obtained with the other agonists. Since [Sar9,Met(O2)11]SP produced a small response in the nanomolar concentration range in about 30% of the preparations tested, it is possible that some NK1 receptors were also present. 4. Assuming competitive antagonism, the NK2-selective antagonist SR 48,968 (30 nM) gave apparent pKH values of 8.13 and 8.65 for [Nle10]NKA(4-10) in the absence and presence of peptidase inhibitors, respectively, supporting the presence of NK2 receptors. 5. The NK3-selective antagonist SR 142,801 (0.1 microM), suppressed responses to low (0.1-10 nM) concentrations of [MePhe7]NKB. These contractile responses to [MePhe7]NKB were also abolished by atropine (0.6 microM) suggesting that this response was mediated via cholinergic nerves. 6. It is concluded that the guinea-pig oesophagus is a complex system which has both NK2 and NK3 receptors and possibly some NK1 receptors as-well.