Hyperpolarization of myenteric neurons by opioids does not involve cyclic adenosine-3',5'-monophosphate

Involvement of Potassium Channels, Nitric Oxide Synthase, and Guanylate Cyclase in the Spasmolytic Effect of Simaba ferruginea A.St.-Hil on Rat Isolated Ileum

BACKGROUND AND AIM

Simaba ferruginea A.St.-Hil. Popularly known as "calunga," is a typical Brazilian cerrado plant whose rhizomes are popular for treating diarrhea.

AIMS

The aim of this study was to evaluate the spasmolytic activity and the antidiarrheal effect of the ethanolic extract obtained from S. ferruginea (Sf-EtOH).

METHODS

Ileal segments (1-2 cm) from male Wistar rats were mounted in isolated organ baths and connected to a force transducer, and then to an amplifier which was connected to a computer (AVS Projetos/São Paulo-SP). After stabilization for 60 min, under tension (1 gf), two submaximal contractions were induced with KCl 40 mM or carbachol 10^-6 M on ileal segments. During the third tonic and sustained contraction, Sf-EtOH was added in cumulative concentrations to the organ bath. Incubations with L-NAME (10^-4 M), ODQ (10^-5 M), TEA⁺ (5 or 1 mM), glibenclamide (10^-5 M), or apamine (100 nM) were prepared (n = 5), separately and used to verify the involvement of the nitric oxide synthase, guanylate cyclase, and potassium channels in the relaxing effect. The results were expressed as mean ± standard error of the mean and were statistically evaluated using one-way ANOVA followed by Bonferroni test, when necessary *p < 0.05.

RESULTS

Sf-EtOH promotes relaxation on rat isolated ileum pre-contracted with CCh and KCl in a concentration-dependent manner. Sf-EtOH also inhibited ileum contractions against cumulative concentrations of carbachol (CCh), KCl, and CaCl₂, shifting the curves to the right in a non-parallel manner with an E_max reduction. In the presence of potassium channel blockers, Sf-EtOH shifted the curves to the right with a reduction of E_max, suggesting the involvement of BK_Ca, K_ATP, and SK_Ca in its spasmolytic effect. In the presence of L-NAME or ODQ, the relaxation curves were shifted to the right, suggesting the involvement of this pathway in Sf-EtOH spasmolytic effect.

CONCLUSIONS

Sf-EtOH acts in a concentration-dependent manner, involving the positive modulation of K⁺ channels and NO pathway.

Function of opioids in the enteric nervous system

Alterations in gastrointestinal motility and secretion underlie the constipating action of therapeutically administered opiates. The prototype opiate is morphine, which acts to delay gastric emptying and intestinal transit, to suppress intestinal secretion of water and electrolytes and to suppress transport of bile into the duodenum. The effects of opiates, synthetic opioids and endogenously released opioid peptides on these organ-level gastrointestinal functions reflect actions on electrical and synaptic behaviour of neurones in the enteric nervous system. Adverse effects and positive therapeutic effects of administration of opioid-receptor-blocking drugs on the digestive tract must be understood in the context of the neurophysiology of the enteric nervous system and mechanisms of neural control of gastrointestinal smooth muscle, secretory glands and blood-lymphatic vasculature. We review here the integrated systems of physiology and cellular neurobiology that are basic to understanding the actions of opioid agonists and antagonists in the digestive tract.

Na+, K+ ATPase alpha-subunit isoform distribution and abundance in guinea-pig longitudinal muscle/myenteric plexus after exposure to morphine

Previous work in the myenteric plexus has shown that the resting membrane potential of morphine-tolerant guinea-pig myenteric S neurons is significantly depolarized relative to placebo-implanted controls, and that this depolarization is associated with reduced electrogenic Na+, K+ pumping. Identification of the subunits of the sodium pump which are in the myenteric plexus was undertaken in order to facilitate direct qualitative and quantitative measurements of the abundance of sodium pump isoforms after morphine exposure, thereby confirming and extending the electrophysiological data to the molecular level. Seven days prior to the experiments, tolerance was induced by subcutaneous implantation of morphine pellets (one pellet, 75 mg/100 g body weight) while control guinea pigs received placebo pellets. Using immunohistochemistry and confocal microscopy, the distribution of the alpha subunit isoforms of the Na+/K+ -ATPase in placebo and morphine-tolerant guinea-pig ileum was determined. Only the alpha1 and alpha3 subunit isoforms were in sufficient abundance to be observed. The alpha1 subunit isoform was most highly concentrated in the mucosa and in neurons. In contrast, the alpha3 subunit isoform was uniquely localized to neurons. Western and slot blot analyses of longitudinal muscle/myenteric plexus homogenates identified a significant reduction of the alpha3 but not the alpha1 subunit isoform in tolerant preparations. It is concluded that the reduced electrogenic pumping in the S neurons after morphine exposure is associated with a reduction in the alpha3 subunit isoform.

Pharmacological examination of contractile responses of the guinea-pig isolated ileum produced by mu-opioid receptor antagonists in the presence of, and following exposure to, morphine

We have assessed the potential of several mu-opioid receptor antagonists to elicit a response in the guinea-pig isolated ileum in the presence of, and following overnight exposure to, morphine. Naloxone, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), (-)-5, 9alpha-diethyl-2-(3-furyl-methyl)-2'-hydroxy-6,7-benzomorphan (MR2266), but not D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), produced a transient inhibition of electrically-evoked contractions of the guinea-pig ileum. The effect of 1 microM CTOP, but not that to MR2266, was inhibited by 1 microM somatostatin. Naloxone (0.3 microM), CTOP (3 microM), CTAP (3 microM) and MR2266 (0.3 microM) antagonized the inhibitory effect of morphine on electrically-evoked contractions of the guinea-pig to a similar degree and, following 60 min exposure to morphine, produced non-sustained contractions. The response to 3 microM CTOP was significantly smaller than that to 3 microM CTAP. None of the antagonists produced a response in the absence of morphine. Following overnight exposure of the ileum to 0.3 microM morphine (4 degrees C), and repeated washing to remove the agonist, all four antagonists elicited non-sustained contractions. However, the responses to 3 microM CTOP and 0.3 microM MR2266 were significantly smaller than those elicited by 0.3 microM naloxone and 3 microM CTAP. Somatostatin (1 microM) significantly reduced naloxone-induced contractions, but not those to CTAP. While all four mu-opioid antagonists elicited contractions in the presence of, and following prolonged exposure to, morphine, differences between them were noted which may be a consequence of non-opioid actions.

Insights into opioid action in the intestinal tract

Opioids have been used for centuries as antidiarrhoeal drugs. In recent years, their mechanism and sites of action in exerting their antidiarrhoeal effect have been studied intensely. Attempts have been made to propose their general mode of action. Whilst there are numerous similarities in their general effects on motility, fluid secretion, and neuroeffector transmission, the differences between species, in some cases, can be remarkable. We highlight and contrast the similarities and differences in the commonly examined species and compare them to humans. Insights into mechanisms of opioid antidiarrhoeal action now also provide some new perspectives of opioid action in the intestine.

Opioid-controlled adenylate cyclase in the guinea-pig myenteric plexus is confined to nerve somata

Previous studies with the electrically stimulated longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum suggested that opioid control of adenylate cyclase is confined to nerve somata. No indication was found for an opioid effect on the enzyme at nerve terminals of the neuro-muscular junction. The aim of the present investigation was to directly study the effect of opioids on cAMP generation in nerve fragments associated with somata or terminals of the myenteric plexus. Employing the ultracentrifugation technique an enrichment of cell organelles in fractions relating to either somata or terminals was achieved. Opioid binding studies revealed specific mu-receptors which in both fractions were regulated by GTP. Challenge of these fractions with forskolin and prostaglandin E1, respectively, resulted in an increased production of cAMP regardless of their neuroanatomical assignment. Examining the response of neuronal material to the selective mu-opioid DAMGO ([D-Ala2, MePhe4, Gly-ol5]enkephalin) revealed an inhibitory action on cAMP synthesis in somata-enriched fractions. No effect of DAMGO was observed in material linked to nerve terminals, although the presence of mu-opioid receptors and adenylate cyclase has been demonstrated. We conclude that opioid control of adenylate cyclase in the myenteric plexus of the guinea pig is confined to nerve somata.

The electrophysiological effects of [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin on guinea-pig myenteric neurons

The electrophysiological effects of a highly selective mu opioid agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO) were investigated on S and AH myenteric neurons in the guinea-pig ileum. Administration of DAGO (500 nM) caused a mean membrane hyperpolarization (+/- S.E.M.) of 7.8 +/- 0.5 mV in 58 of 138 S neurons, associated with a decrease in input resistance from 158 +/- 18 to 132 +/- 13 m omega. DAGO also produced a significant decrease in the mean amplitude of the cholinergic fast excitatory postsynaptic potentials (EPSPs), from 12.0 +/- 1.1 to 6.6 +/- 1.4 mV in 13 of 27 S neurons. On the other hand, in AH neurons, DAGO did not significantly affect the membrane potential, input resistance, action potential configuration, slow after-hyperpolarization or the antidromic action potential. The experiments indicate that the acute effects of mu opioids in the guinea-pig ileum are confined to a subpopulation of a single electrophysiological class of myenteric neurons.