Effect of ganglionic blocking compounds on in-vivo fluid secretion in the rat small intestine

Serotonin involvement in okadaic acid-induced diarrhoea in vivo

The consumption of contaminated shellfish with okadaic acid (OA) group of toxins leads to diarrhoeic shellfish poisoning (DSP) characterized by a set of symptoms including nausea, vomiting and diarrhoea. These phycotoxins are Ser/Thr phosphatase inhibitors, which produce hyperphosphorylation in cellular proteins. However, this inhibition does not fully explain the symptomatology reported and other targets could be relevant to the toxicity. Previous studies have indicated a feasible involvement of the nervous system. We performed a set of in vivo approaches to elucidate whether neuropeptide Y (NPY), Peptide YY (PYY) or serotonin (5-HT) was implicated in the early OA-induced diarrhoea. Fasted Swiss female mice were administered NPY, PYY(3-36) or cyproheptadine intraperitoneal prior to oral OA treatment (250 µg/kg). A non-significant delay in diarrhoea onset was observed for NPY (107 µg/kg) and PYY(3-36) (1 mg/kg) pre-treatment. On the contrary, the serotonin antagonist cyproheptadine was able to block (10 mg/kg) or delay (0.1 and 1 mg/kg) diarrhoea onset suggesting a role of 5-HT. This is the first report of the possible involvement of serotonin in OA-induced poisoning.

Methylisogermabullone isolated from radish roots stimulates small bowel motility via activation of acetylcholinergic receptors

We have previously reported that extract of radish roots exhibits an increase in gastrointestinal motility through the activation of muscarinic acetylcholine (ACh) receptors. Based on the stimulatory activity-guided fractionation on rat ileal segments, this study isolated methylisogermabullone (MIGB, C23H31O5NS, MW 433) from methanol extracts of radish roots. MIGB caused a significant increase of the isolated rat ileal contraction in a concentration-dependent manner (23–693 μm), and the pattern of MIGB-induced ileal contraction was different in the time course to that produced by ACh. The EC50 value of MIGB, to produce 50% maximum ileal contraction, was estimated to be 45.5 μm. MIGB (230 μm)-induced ileal contractions were enhanced by pretreatment of segments with ACh (0.1 μm). Ileal contractions produced by MIGB (230 μm) or ACh (0.1 μm) at submaximal concentration were partially inhibited by pretreatment of hexamethonium (0.1 mm), a ganglionic blocker, whereas they were almost completely abolished by atropine (10 μm). Oral administration of MIGB to mice stimulated the small intestinal transit of charcoal in a dose-dependent manner (10–100 mg kg−1), and MIGB (100 mg kg−1)-induced stimulation of small intestinal transit was significantly attenuated by co-administration of atropine (50 mg kg−1). Taken together, these results demonstrate that MIGB isolated from radish roots stimulates the small bowel motility through the activation of ACh receptors. These findings suggest that MIGB may become a potential regulatory agent for therapeutic intervention in dysfunction of gastrointestinal motility.

An endogenous cannabinoid tone attenuates cholera toxin-induced fluid accumulation in mice

BACKGROUND & AIMS

Cholera toxin (CT) is the most recognizable enterotoxin causing secretory diarrhea, a major cause of infant morbidity and mortality throughout the world. In this study, we investigated the role of the endogenous cannabinoid system (i.e., the cannabinoid receptors and their endogenous ligands) in CT-induced fluid accumulation in the mouse small intestine.

METHODS

Fluid accumulation was evaluated by enteropooling; endocannabinoid levels were measured by isotope-dilution gas chromatography mass spectrometry; CB(1) receptors were localized by immunohistochemistry and their messenger RNA (mRNA) levels were quantified by reverse-transcription polymerase chain reaction (PCR).

RESULTS

Oral administration of CT to mice resulted in an increase in fluid accumulation in the small intestine and in increased levels of the endogenous cannabinoid, anandamide, and increased expression of the cannabinoid CB(1) receptor mRNA. The cannabinoid receptor agonist CP55,940 and the selective cannabinoid CB(1) receptor agonist arachidonoyl-chloro-ethanolamide inhibited CT-induced fluid accumulation, and this effect was counteracted by the CB(1) receptor antagonist SR141716A, but not by the CB(2) receptor antagonist SR144528. SR141716A, per se, but not the vanilloid VR1 receptor antagonist capsazepine, enhanced fluid accumulation induced by CT, whereas the selective inhibitor of anandamide cellular uptake, VDM11, prevented CT-induced fluid accumulation.

CONCLUSIONS

These results indicate that CT, along with enhanced intestinal secretion, causes overstimulation of endocannabinoid signaling with an antisecretory role in the small intestine.

Radish extract stimulates motility of the intestine via the muscarinic receptors

The effects of radish (Brassica oleraceae, Cruciferae) on gastrointestinal motility were examined using rat intestinal segments with myenteric plexus in-vitro and measuring the intestinal transit of charcoal in-vivo. Radish extract (10 microg mL(-1) to 2 mg mL(-1)) caused a dose-dependent increase in contractions of the duodenum, jejunum and ileum, and 1 mg mL(-1) was the maximum effective dose. The largest contraction by the extract was found in ileal segments. The extract-induced (0.5 mg mL(-1)) ileal contraction was remarkably inhibited by pretreatment of segments with atropine (10(-7) M) for 10 min, but not by hexamethonium (0.5 mM). Moreover, antagonists of the muscarinic receptor reduced the radish-induced ileal contraction by a different ratio. The rank order of inhibitory effects was 4-diphenylacetoxy-N-methyl-(2-chloroethyl)-piperidine methiodide (90.5% of control) > tropicamide (67.4%) > pirenzepine (42.8%) > methoctramine (16.7%). Oral administration of radish extract (300-500 mg kg(-1) body weight) to mice remarkably improved the intestinal transit of charcoal, and this was significantly attenuated by co-administration of atropine (50 mg kg(-1)). Taken together, these results suggest that radish extract stimulates gastrointestinal motility through activation of muscarinic pathways.

The chronic infusion of hexamethonium and phenylephrine to effectively clamp sympathetic vasomotor tone. A novel approach

There are several ways to assess the sympathetic nervous system (i.e. , nerve recording, sympathectomy, etc.), each of which has its own limitations. The present study was conducted to establish a standard, testable chronic ganglionic blockade protocol with a fixed level of adrenergic vasomotor tone. Rats were instrumented with radio telemetry pressure transducers and venous catheters for continuous measurement of arterial pressure and infusion of pharmacologic agents, respectively. After 3 days of control measurements, rats were infused for 9 days with a continuous dose of the ganglionic blocking agent, hexamethonium and the alpha-adrenergic agonist, phenylephrine. In this way, sympathetic tone was effectively "clamped," which maintained a normal level of arterial pressure. Control pressure between hexamethonium + phenylephrine (HEX + PE) treated rats (101+/-2 mm Hg) and saline (VEHICLE) treated rats (101+/-2 mmHg) was not different. By day 9 of the infusion, there was no difference in arterial pressure between groups (VEHICLE: 101+/-3 mm Hg, HEX + PE: 103+/-3 mm Hg) or from the control period, although heart rate was significantly less in HEX + PE rats (VEHICLE: 406+/-9 beats/min vs. HEX + PE: 343+/-6 beats/min). The effectiveness of this technique was validated by measuring cardiac baroreceptor reflex sensitivity, as well as the pressor response to the direct ganglionic stimulating agent, 1, 1-dimethyl-4-phenylpiperazinium iodide (DMPP). Compared to VEHICLE rats, HEX + PE rats showed no tachycardic response to depressor stimuli and an absence of a pressor response to DMPP. We conclude that this protocol is a useful technique to chronically, yet reversibly, block the sympathetic nervous system in experimental settings.