Evidence for serotonergic system involvement in the effect of morphine on gastrointestinal motility in the rat

Direct effects of morphine but not of fentanyl-type opioids on human 5-HT3A receptors in outside-out patch-clamp studies

BACKGROUND

The alkaloid morphine is historically the oldest opiate, yet still today it has clinically important uses in analgesic therapies. The main analgesic effect of opioids, including synthetic opioids belonging to the family of 4-anilidopiperidines, is mediated via activation of opioid receptors spread throughout the peripheral and central nervous system. However, morphine acting as a 'dirty' drug also exhibits effects on other receptor systems, e.g., the serotonergic system and its 5-HT3 receptor. Therefore, this study focuses on the interaction of morphine and fentanyl-type opioids (alfentanil, remifentanil and sufentanil) with 5-HT3A receptors.

METHODS

Excised outside-out patches from human embryonic kidney-293 cells, stably transfected with the human 5-HT3A receptor cDNA, were used to determine the opioid effects using the patch-clamp technique.

RESULTS

Within clinical concentrations, the effects of morphine are concentration-dependent. Morphine reduced current amplitudes, as well as activation and decay time constants. These effects were not competitive. Contrary to these results, all fentanyl-type opioids only exerted effects far above their clinical concentration ranges. These effects were not homogenous but varying.

CONCLUSIONS

Morphine is an opioid compound exhibiting special antagonistic interaction with 5-HT3A receptors. This interaction is not shared by the newer synthetic derivatives of the fentanyl-type opioids in the clinical relevant concentration range.

Visceral hypersensitivity induced by activation of transient receptor potential vanilloid type 1 is mediated through the serotonin pathway in rat colon

This study aimed to clarify the relationship between TRPV1 activation-induced visceral pain and the serotonin pathway in the colon of rats. The effects of para-chlorophenylalanine (pCPA) on visceral pain threshold pressure were assessed in capsaicin -induced visceral pain of rats. The expression of TRPV1 in the colon was examined by immunohistochemistry and Western blot analysis, and TRPV1 excitability in dorsal root ganglion (DRG) neurons was examined by whole-cell patch-clamp recording in pCPA-treated rats. Calcineurin and Ca(2+)-calmodulin-dependent kinase II (CaMKII), the important proteins in maintaining TRPV1 function in the colon, were also tested by Western blot analysis and immunofluorescence staining. Results showed that pCPA significantly increased the capsaicin-induced visceral pain threshold by 2.3-fold, and the enhanced visceral pain threshold corresponded with decreased 5-HT content (58% depleted) and enterochromaffin cell number (80% reduced). The reduced excitability of TRPV1 in DRG neurons, instead of changed TRPV1 expression, is responsible for the enhanced visceral pain threshold in 5-HT-depleted rats, and the mechanism may be related to the decreased expression of pCaMKII. These results indicate that visceral hypersensitivity induced by TRPV1 activation is modulated through 5-HT pathways and the attenuated function of TRPV1 and decreased protein expression of pCaMKII may play an important role in capsaicin-induced TRPV1 desensitization under 5-HT-depleted condition. The important role of TRPV1 and 5-HT in generating and maintaining visceral hypersensitivity may provide insights for the treatment of visceral hypersensitivity.

Orally administered soymorphins, soy-derived opioid peptides, suppress feeding and intestinal transit via gut mu(1)-receptor coupled to 5-HT(1A), D(2), and GABA(B) systems

We previously reported that soymorphins, mu-opioid agonist peptides derived from soy beta-conglycinin beta-subunit, have anxiolytic-like activity. The aim of this study was to investigate the effects of soymorphins on food intake and gut motility, along with their mechanism. We found that soymorphins decreases food intake after oral administration in fasted mice. Orally administered soymorphins suppressed small intestinal transit at lower dose than that of anorexigenic activity. Suppression of food intake and small intestinal transit after oral administration of soymorphins was inhibited by naloxone or naloxonazine, antagonists of mu- or mu(1)-opioid receptor, respectively, after oral but not intraperitoneal administration. The inhibitory activities of small intestinal transit by soymorphins were also inhibited by WAY100135, raclopride, or saclofen, antagonists for serotonin 5-HT(1A), dopamine D(2), or GABA(B) receptor, respectively. We then examined the order of activation of 5-HT(1A), D(2), and GABA(B) receptors, using their agonists and antagonists. The inhibitory effect of 8-hydroxy-2-dipropylaminotetralin hydrobromide, a 5-HT(1A) agonist, after oral administration on small intestinal transit was blocked by raclopride or saclofen. Bromocriptine, a D(2) agonist-induced small intestinal transit suppression, was inhibited by saclofen, but not by WAY100135. Baclofen, a GABA(B) agonist-induced small intestinal transit suppression, was not blocked by WAY100135 or raclopride. These results suggest that 5-HT(1A) activation elicits D(2) followed by GABA(B) activations in small intestinal motility. We conclude that orally administered soymorphins suppress food intake and small intestinal transit via mu(1)-opioid receptor coupled to 5-HT(1A), D(2), and GABA(B) systems.

Serotonin release and uptake in the gastrointestinal tract

The afferent innervation of the gastrointestinal (GI) tract consists of intrinsic and extrinsic sensory neurons that respond to nutrients, chemicals or mechanical stimuli within the gut lumen. Most stimuli do not interact directly with the afferent nerves but instead activate specialised cells in the epithelium in a process of sensory transduction. It is thought that one of the first steps in this process is the release of serotonin (5-HT) from the enterochromaffin (EC) cells. The EC cells are a sub-type of enteroendocrine (EE) cells which are found among the enterocytes of the intestinal epithelium. The EC cells are responsible for the production and storage of the largest pool of 5 HT in the body. Released 5-HT can act on the intrinsic nerves and vagal endings. This review will focus on the role of 5-HT in sensory transduction and examine how the EC cell produces and releases 5-HT. We will explore recent developments that have helped to elucidate some of the proteins that allow EC cells to sense the luminal environment. Finally, we will highlight some of the findings from new studies using electrochemical techniques which allow the real-time recording of 5-HT concentrations near to the EC cell.

Inhibitory effects of kratom leaf extract (Mitragyna speciosa Korth.) on the rat gastrointestinal tract

Kratom (Mitragyna speciosa Korth.) is an indigenous plant of Thailand used traditionally in folk medicine although it is claimed to cause addiction. It is used to treat diarrhea, however, there is no scientific evidence to support the use. The aim of this study is to investigate the effect of methanolic extract of kratom leaves on the rat gastrointestinal tract. Kratom extract at 50, 100, 200 and 400 mg/kg (p.o.) caused a dose dependent protection against castor oil-induced diarrhea in rats and also inhibited intestinal transit. The antidiarrheal effect was not antagonized by naloxzone. The inhibition of intestinal transit by kratom extract was significantly different from the control when treated with a single dose for 1 day. For longer-term treatments of 15 and 30 days, kratom extract did not decrease the intestinal transit time indicating that adaptation had occurred. Kratom extract at a dose level of 200 and 400 mg/kg for 30 days and morphine at 3 mg/kg (i.p.) caused a decrease in the increment of body weight that was significantly different from the control and kratom extract at lower doses (50 and 100 mg/kg). However it had no effect on the level of plasma cholecystokinin. The results suggested that methanolic kratom extract exhibited its antidiarrheal effect on rat gastrointestinal tract. The effects may occur via pathways in addition to the action on opioid receptors. High does of kratom extract decreased the increment of body weight similar to the effect of morphine.

5-Hydroxytryptamine (5-HT)2 receptor involvement in acute 5-HT-evoked scratching but not in allergic pruritus induced by dinitrofluorobenzene in rats

We investigated the role of serotonin (5-hydroxytryptamine; 5-HT)2 and 5-HT3 receptor subtypes in acute itch-associated scratching behavior as well as in an allergic pruritus model in rats. Intradermal 5-HT evoked hind limb scratching directed toward the injection site in naïve rats. Scratching behavior was significantly reduced by pretreatment with the 5-HT2 receptor antagonist ketanserin. Intradermal injection of alpha-methylserotonin, a 5-HT2 receptor agonist, also elicited scratching behavior in a dose-dependent manner, indicating that acute 5-HT-induced scratching is mediated via peripheral 5-HT2 receptors. To produce a model of allergic pruritus, skin was sensitized by topical application of 5% dinitrofluorobenzene (DNFB). One month later, repeated challenge of the skin with 0.2% DNFB at weekly intervals elicited scratching as part of the immediate allergic response. Scratching was not affected by ketanserin or by the 5-HT3 receptor antagonist ondansetron, indicating that neither 5-HT2 nor 5-HT3 receptors is involved in itch-associated scratching behavior caused by allergic skin dermatitis in rats.