Is desensitization of intestinal 5-hydroxytryptamine receptors an in-vitro phenomenon?

Differential regulation of rat peripheral 5-HT(2A) and 5-HT(2B) receptor systems: influence of drug treatment

Most studies of 5-HT(2) receptor regulation have been carried out on the central nervous system (CNS) (which expresses 5-HT(2A) and 5-HT(2C) receptors); very few in vitro studies have addressed the peripheral receptors 5-HT(2A) and 5-HT(2B). The aim of this investigation was to compare the possible short- and long-term processes regulating these peripheral receptors in the rat. The in vitro contractile response elicited by serotonin (5-HT, 10 micro M) in the rat gastric fundus (5-HT(2B) receptor system) was rapid and followed by a partial fade to a steady state, in contrast with the rat thoracic aorta response (5-HT(2A) receptor system), which was more stable, slower and sustained. To characterize drug-receptor interactions, cumulative concentration/response curves (CCRCs) for 5-HT were constructed ex vivo for rat tissues treated with drugs acting at these receptors. Rats were examined 4 or 24 h after a single, i.p. administration of (+/-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(+/-)DOI, 1 or 2.5 mg/kg], clozapine, cyproheptadine or rauwolscine (10 mg/kg), 48 h after a single i.p. administration of (+/-)DOI (2.5 mg/kg), clozapine or cyproheptadine (10 mg/kg) or 24 h after the last of with 15 daily i.p. administrations of (+/-)DOI (1 or 2.5 mg/kg), clozapine, cyproheptadine or rauwolscine (10 mg/kg). In the aorta, E(max) (the maximum response elicited by 5-HT) was unchanged 4 h after a single dose of any of the drugs tested. However, 24 h after a single dose, E(max) was lower in animals treated with (+/-)DOI (2.5 mg/kg), clozapine or cyproheptadine than in controls, whilst 48 h after a single dose of (+/-)DOI (2.5 mg/kg), clozapine or cyproheptadine there was no difference in E(max) between experimental and control animals. After chronic treatment with (+/-)DOI (2.5 mg/kg), clozapine and cyproheptadine, E(max) was lower than in controls. In the gastric fundus, E(max) 4 h after a single dose of each drug was lower than in controls, and the response recovered by 24 or 48 h. Following chronic treatment, E(max) was significantly lower than in controls for each drug used. These findings suggest first, that regulation of peripheral 5-HT(2) receptors (5-HT(2A) and 5-HT(2B)) is a functionally significant phenomenon in vivo, and occurs after administration of both agonists and antagonists. Second, the kinetics of peripheral 5-HT(2) receptor regulation were similar in both in vivo and ex vivo experiments. The 5-HT(2B) receptors in rat gastric fundus are more sensitive to drug-induced regulation than the 5-HT(2A) rat aortic receptors. Finally, long-term regulation of both receptors stabilizes short-term desensitization for longer.

Ursodeoxycholic acid action on the transport function of the small intestine in normal and cystic fibrosis mice

Ursodeoxycholic acid possesses choleretic and cytoprotective properties and in cystic fibrosis (CF) it is used to treat the hepatobiliary symptoms of the disease. This study investigated the effects of this bile acid on the transport function of the small intestine in normal and CF mice. The effects of ursodeoxycholic acid were monitored as changes in short-circuit current (SCC) in stripped sheets of small intestine from normal (Swiss MF1) and transgenic CF (Cftr(tm2Cam)) mice. In ileal sheets from Swiss MF1 mice, mucosal ursodeoxycholic acid caused a biphasic increase in SCC. The first phase was reduced by lowering the mucosal Na+ concentration, while the second phase was inhibited by (Cl-)-free conditions, serosal furosemide or mucosal diphenylamine-2-carboxylic acid (DPC), suggesting an initial Na+-dependent bile acid absorption followed by a stimulation of electrogenic Cl- secretion. Serosal application of ursodeoxycholic acid to the ileum and mucosal or serosal application to the mid-intestine and jejunum elicited a secretory response only. Secretion was Ca2+-dependent, but did not involve neural mechanisms. Mucosal mast cells, histamine and serotonin (5-HT) were implicated in the secretory response. Responses in tissues from transgenic wild-type mice were similar to those obtained with Swiss MF1 mice, but the secretory response to mucosal or serosal application of the bile acid was impaired in CF tissues. In ilea from CF mice the initial absorptive phase of the response to mucosal ursodeoxycholic acid was still observed. It is concluded that ursodeoxycholic acid induces secretion throughout the murine small intestine by a mechanism that involves degranulation of mucosal mast cells. In the ileum Na+-dependent absorption can also be detected. The secretory response is defective in CF intestine, but the absorptive effect is still present.

Taurocholic acid-induced secretion in normal and cystic fibrosis mouse ileum

Bile acids cause secretion throughout the intestinal tract and this process contributes to maintaining the fluidity of intestinal contents. In cystic fibrosis (CF) defective intestinal secretion can lead to excessive dehydration of the luminal contents and the development of clinical symptoms. This study was designed to investigate bile acid-induced secretion in mouse ileum and to determine whether this process was defective in CF. Taurocholic acid-induced secretion was monitored as a rise in short-circuit current (SCC) in ileal sheets from normal (Swiss MF1) and transgenic CF mice. Taurocholic acid increased the SCC in both intact and stripped ileal sheets from Swiss MF1 mice. This effect was due to a stimulation of electrogenic Cl- secretion as it was inhibited by Cl(-)-free conditions, serosal furosemide (frusemide), mucosal diphenylamine-2-carboxylic acid (DPC) and increased serosal K+ concentration, without being affected by reduced mucosal Na+ concentration. Taurocholic acid-induced secretion was inhibited by tetrodotoxin, indicating the involvement of a neural pathway, but this did not include capsaicin-sensitive afferent neurons or muscarinic cholinoreceptors. Mucosal mast cells also contributed to the response. Responses in tissues from transgenic wild-type mice were similar to those obtained with Swiss MF1 animals, but ilea from CF mice exhibited a lower basal SCC with significantly reduced secretory responses to acetylcholine and taurocholic acid. We concluded that taurocholic acid induces ileal secretion by a mechanism that entails activation of enteric nerves and degranulation of mucosal mast cells. Impaired bile acid-induced secretion in CF may contribute to luminal dehydration.

Sensitivity to 5-hydroxytryptamine in different afferent subpopulations within mesenteric nerves supplying the rat jejunum

1. This study was performed to elucidate the type of afferents that mediate the multiple actions of 5-hydroxytryptamine (5-HT) on mesenteric nerve discharge. Electrophysiological recordings were made from mesenteric afferents innervating the mid-jejunum of the urethane-anaesthetized rat. The discharge of single nerves within the whole nerve recording was monitored using waveform discrimination software. 2. Afferents responded to 5-HT in one of two ways: a short latency, transient excitation mediated by 5-HT3 receptors, or a delayed onset, more prolonged effect that was 5-HT2A receptor mediated. Afferents showing the 5-HT3-mediated response did not respond to luminal distension but were sensitive to intraluminal hydrochloric acid (150 mM) in twenty-eight of twenty-nine experiments. In eight experiments, the 5-HT3-mediated response was reversibly abolished by a 2 min exposure to intraluminal application of local anaesthetic (2 % Xylocaine). 3. Mechanosensitive afferents which responded to distension (< 10 cmH2O) did not show a 5-HT3-mediated response (P = 0.92, n = 14), and maintained this mechanosensitivity after luminal anaesthesia. Mechanosensitive afferents did show a secondary response to 5-HT that was significantly attenuated by atropine (100-200 microg kg-1), whereas hexamethonium (8 mg kg-1) had no effect. 4. In animals whose vagal afferent contribution to their mesenteric nerves had been eliminated by chronic truncal vagotomy, the 5-HT3-mediated response was absent in thirty-six of thirty-six nerve bundles. In contrast, mechanosensitivity to distension and the secondary response to 5-HT could still be evoked. 5. These results suggest that 5-HT stimulates mesenteric afferents by a direct action on 5-HT3 receptors that are present on vagal mucosal afferent terminals. The mucosal afferent response to luminal acid, however, was unaffected by treatment with granisetron (0.5 mg kg-1) indicating that endogenous 5-HT from enterochromaffin cells is not essential for transduction of this luminal signal. In contrast, mechanosensitivity in non-vagal afferents was modulated by 5-HT following an intestinal motor response which was influenced by cholinergic tone.

5-Hydroxytryptamine-induced secretion by rat jejunum in-vitro involves several 5-hydroxytryptamine receptor subtypes

The receptors contributing to 5-hydroxytryptamine (5-HT)-induced anion secretion by rat jejunum have been investigated by testing the effects of selective agonists and antagonists in-vitro using both intact and stripped intestinal sheets. In both intact and stripped jejunum 5-HT and 5-methoxytryptamine, an agonist that lacks affinity for 5-HT3 receptors, induced concentration-dependent increases in the short-circuit current (SCC), although 5-methoxytryptamine induced a smaller maximum response. In intact sheets 1-phenylbiguanide, a selective 5-HT3 agonist, induced a response that was similar in magnitude to that of 5-methoxytryptamine, but in stripped preparations it had little effect. Tetrodotoxin inhibited the response of intact jejunum to 5-HT (by 86%) and 5-methoxytryptamine (by 85%) and abolished the response to 1-phenylbiguanide. In stripped sheets inhibition of the 5-HT response by tetrodotoxin was reduced to 27%. Desensitization to 1-phenylbiguanide reduced the response to 5-HT in intact but not stripped sheets whereas, in contrast, desensitization to 5-methoxytryptamine inhibited the 5-HT response in stripped sheets but was without effect in intact sheets. Mianserin, a 5-HT1, 5-HT2 and 5-HT3 antagonist, and renzapride, a 5-HT1 and 5-HT3 antagonist, both reduced the maximum response to 5-HT, but 5-HTP-DP, a 5-HT1 antagonist, was without effect. The 5-HT3 antagonist granisetron reduced the response to 5-HT in intact, but not in stripped sheets. Tropisetron, a 5-HT3 and 5-HT4 antagonist, inhibited the response to 5-methoxytryptamine in both preparations, but did not alter the response to 5-HT. It is concluded that 5-HT-induced jejunal secretion involves more than one 5-HT receptor subtype, with both neural and non-neural mechanisms contributing to the response.

Several receptor subtypes contribute to 5-hydroxytryptamine-induced secretion by rat ileum in-vitro

The receptors contributing to 5-hydroxytryptamine (5-HT)-induced secretion by rat ileum were investigated in-vitro using selective agonists and antagonists. 5-HT induced a dose-dependent increase in the short-circuit current (SCC) generated by both intact and stripped sheets of rat ileum. 1-Phenylbiguanide, a selective 5-HT3 agonist, and 5-methoxytryptamine, an agonist that lacks affinity for 5-HT3 receptors, also increased the SCC. In intact sheets 5-HT was more effective than either 1-phenylbiguanide or 5-methoxytryptamine, whereas in stripped sheets 5-HT and 5-methoxytryptamine were equipotent, with 1-phenylbiguanide having little effect. Tetrodotoxin abolished the response of intact sheets to 1-phenylbiguanide but only reduced the responses to 5-HT and 5-methoxytryptamine by 57% and 54%, respectively. This inhibition was reduced to 25% in stripped sheets. The 5-HT3 antagonist granisetron abolished the response to 1-phenylbiguanide, but did not alter the effects of 5-HT. Ketanserin, a 5-HT2 antagonist, had a small effect on the actions of 5-methoxytryptamine in intact, but not stripped, sheets and no effect on the response to 5-HT in either preparation. Tropisetron, a 5-HT3 and 5-HT4 antagonist, inhibited the response to 5-methoxytryptamine, but had less effect on the response to 5-HT. Desensitization to 1-phenylbiguanide inhibited the response to 5-HT in intact, but not stripped sheets, whereas desensitization to 5-methoxytryptamine abolished the 5-HT response in stripped sheets, but induced only 42% inhibition in intact sheets. Previous exposure to a combination of both 1-phenylbiguanide and 5-methoxytryptamine abolished the 5-HT-induced increase in SCC in both preparations. The findings suggest that 5-HT-induced ileal secretion involves more than one 5-HT receptor subtype and that both neural and non-neural mechanisms contribute to the response.

Changes in sensitivity of 5-HT receptor mediated functional responses in the rat oesophagus, fundus and jejunum following chronic infusion with 5-hydroxytryptamine

The effects of chronic infusion with 5-hydroxytryptamine (5-HT, 75 micrograms/kg per hour) for 5 or 10 days in vivo on the responses of rat oesophagus, fundus and jejunum to 5-HT and partial 5-HT receptor agonists in vitro were investigated. In the rat oesophagus, chronic treatment produced rightward shifts of the 5-HT4 receptor-mediated concentration-effect curves to 5-HT (dose-ratio = 3.8, day 5 and 2.8, day 10) and SC 53116 (1-S,8-S)-4-amino-5-chloro-N-[(hexahydro-1H-pyrrolizin-1-yl) methyl]-2-methoxy-benzamide hydrochloride, dose-ratio = 7.1, day 5 and 8.9, day 10) as compared to control tissues. The maximum effect of 5-HT and SC 53116 in rat oesophagus was reduced following the 10 day treatment. The 5-HT2B receptor-mediated contractile effect of 5-HT on rat fundus from treated animals responded with a significantly reduced potency (dose-ratio = 4.1, 5-day; 4.2, 10-day) compared to control tissues. The maximum response to 5-HT was reduced in tissues from animals treated for 10 days. The concentration-effect curve to the partial agonist 1-(3-chlorophenyl)-piperazine dihydrochloride (mCPP) was shifted to the right in fundic tissue from treated animals (dose-ratio = 2.5, 5-day; 2.8, 10-day) compared to control tissues. The maximum response to mCPP was also reduced in tissues from 5-HT treated animals. It has recently been shown in a preliminary characterisation that in rat jejunum, 5-HT produces a biphasic concentration-effect curve which is mediated by a putative 5-ht7 (first phase) and 5-HT3 (second phase) receptor mechanism. In the present study 5-HT produced a control biphasic concentration-effect curve in rat jejunum with a pEC50-1 value of 8.5 +/- 3.5 and a pEC50-2 value of 5.9 +/- 0.3 and maximum response values of 43.8 (32.5-55.0)% (Emax1) and 65.3 (41.7-88.9)% (Emax 2) of the response to acetylcholine. In jejunal tissues from treated animals, 5-HT produced its contractile effect in vitro with a 3.7 and 2.8 fold (5-day) and a 1.3 and 1.4 fold (10-day) rightward shift of the first and second phase respectively of the control concentration-effect curve to 5-HT. The maximum response produced by 5-HT in the first phase in jejunal tissues from animals treated for 10 days was significantly reduced by 49.1%. These findings represent the first report that chronic infusion of 5-HT produces a residual desensitisation of the 5-HT4, 5-HT2B, and putative 5-ht7/5-HT3 receptor-mediated responses in rat oesophagus, fundus and jejunum respectively when measured in vitro.

Neural involvement in 5-hydroxytryptamine-induced net electrogenic ion secretion in the rat intestine in-vivo

5-Hydroxytryptamine (5-HT) induces active electrogenic anion secretion by both the small intestine and the colon, responses that can be detected from measurements of transmural electrical activity. This approach was adopted to examine the involvement of neural mechanisms in 5-HT-induced secretion in rat proximal jejunum, distal ileum and proximal colon in-vivo. Under control conditions, 5-HT caused maximum rises in transintestinal potential difference of 4.7 +/- 0.3, 3.8 +/- 0.4 and 7.6 +/- 0.3 mV, respectively, with corresponding ED50 values of 28 +/- 3, 38 +/- 4 and 41 +/- 4 nmol kg-1 (n = 12). In each region examined a neural component in the secretory response to 5-HT was identified. Hexamethonium (22 mumol kg-1) reduced the 5-HT response in each region: in the jejunum and colon, it also attenuated the responses to the 5-HT3 agonist, phenylbiguanide and to 5-methoxytryptamine (5-MeOT), an agonist at all 5-HT receptors except 5-HT3, indicating that in these regions the nicotinic pathway can be activated by more than one 5-HT receptor subtype. Atropine (0.27 and 2.7 mumol kg-1) was found to have regional effects on the intestinal responses to 5-HT receptor agonists. In the jejunum, evidence for a pro-secretory muscarinic pathway which could be activated by more than one 5-HT receptor subtype was found. In the ileum and colon no muscarinic pro-secretory pathway was identified, indeed in the colon, an anti-secretory pathway may be present. This muscarinic anti-secretory pathway was observed with phenylbiguanide and 5-MeOT, but not 5-HT. Substance P release does not appear to be involved in mediating the intestinal secretory response to 5-HT. 5-HT-induced intestinal anion secretion may involve a direct secretory action on the enterocyte which can be modified by neurally-mediated pro-secretory and anti-secretory pathways, the balance between these processes varying down the length of the gut.